Exhaust backpressure
Discussion
Since we disagree about so many fundamental factors I don't think we have enough common ground to reach an agreement about this any time soon, and this thread is already well past its sell by date. The problem is real, but since it doesn't matter to either of us whether or not you believe that, I'm going to drop the subject. However, next time you come to do a supercharger conversion, I hope you will hear a little warning bell that reminds you to look into this again. 
If you genuinely want to understand the problem then please feel free to contact me by email and we can continue the discussion off line.
If you genuinely want to understand the problem then please feel free to contact me by email and we can continue the discussion off line.
I dont want to sound rude, but I have no need to discuss the matter.
The thread was interesting, and did bring up a few points.
As for future SC projects...yes, I will be supercharging my LS1 engine over the next few months. I had originally intended turbocharging it, but never having used a SC, I though it about time I did. And after hearing the 'Darius 240z' car, I had to have such a beast, even if it uses a relatively small SC, and the old LT1 engine....except mine will have a lot more power...
http://servinitup.com/videos.shtml Check out his 240z....
Mine will have a mild cam, be reasonably fuel efficient, and I will have over 800bhp. Will I have any "blow thru" issues ?? No. The relatively mild cam will see to that, and I will still have power from idle to 7k. Why ?? Because thats all the cam it needs.
And my exhaust will be as free flowing as I can make it, within the confines of my car.
The thread was interesting, and did bring up a few points.
As for future SC projects...yes, I will be supercharging my LS1 engine over the next few months. I had originally intended turbocharging it, but never having used a SC, I though it about time I did. And after hearing the 'Darius 240z' car, I had to have such a beast, even if it uses a relatively small SC, and the old LT1 engine....except mine will have a lot more power...
http://servinitup.com/videos.shtml Check out his 240z....
Mine will have a mild cam, be reasonably fuel efficient, and I will have over 800bhp. Will I have any "blow thru" issues ?? No. The relatively mild cam will see to that, and I will still have power from idle to 7k. Why ?? Because thats all the cam it needs.
And my exhaust will be as free flowing as I can make it, within the confines of my car.
stevieturbo said:
I still dont see why the same blow through problem you state at low rpms, wouldnt be an even bigger one at high rpms.
Inertia of the gases. The gas takes time to get moving when the valve opens. The overlap period is shorter (in terms of time, not degrees of rotation) at high revs, so less gas flow has happened by the time the overlap period ends.
Mr2Mike said:
I think that we are going round in circles. Do you accept that any of the following are true?
A supercharged engine which produces boost at low rpm (i.e. a positive displacement supercharger installation) is vulnerable to blow-through during the overlap period at low rpm.
Depends entirely on the cam and exhaust system.
In degree. It can't be eliminated 100% unless the cam has zero overlap.
Mr2Mike said:
When the amount of blow-through exceeds the unswept chamber volume (i.e. more than 100% scavenging) then any extra blow-through results in unburnt mixture being passed through to the exhaust.
Nope, because you can't guarantee that blowing just the unswept volume into the engine will scavenge it perfectly anyway. In fact it's most unlikely.
For practical purposes it is impossible. The scavenging will always be somewhere between perfect mixing and perfect displacement... and the mixing element more or less guarantees that you'll lose some fresh charge. Also the close proximity of the inlet and exhaust valves more or less guarantees that some of the mixture will short-circuit - pass directly from inlet to exhaust with no scavenging effect.
Mr2Mike said:
Having unburned mixture passed through to the exhaust reduces the amount of power that can be produced by the engine, and can damage components in the exhaust system.
Absolutely incorrect. The amount of unburnt mixture in the exhaust has little or no bearing on the power that can be produced by the engine.
Mixture lost into the exhaust is mixture lost from the charge burnt on the next firing stroke. It has a great bearing on power produced by the engine! It is a particular problem on piston-ported two-strokes and one that two-stroke builders spend a lot of effort to reduce.
Mr2Mike said:
Adding back pressure less than or equal to the boost pressure to the exhaust at low rpm, will reduce or eiminate blow through at low rpm.
Perhaps, but will also increase pumping losses which will mitigate any tiny gain that may be made.
The gain is not necessarily tiny...
Mr2Mike said:
Using a fixed geometry exhaust restriction to produce back pressure at low rpm will produce increased back pressure (approx square with revs) at higher rpm.
Adding back pressure to the exhaust at high rpm (i.e. above the rpm where blow-through is a problem) restricts the amount of power that can be produced by the engine.
Reducing the overlap angle will reduce blow through at low RPM (zero overlap will eliminate blow-through altogether).
Reducing the overlap angle will restrict the engine's breathing ability at high rpm.
A supercharged engine which suffers from restricted breathing at high rpm is vulnerable to thermal runaway at high rpm which reduces the power that the engine can produce and, if uncontrolled, can cause severe damage to the engine.
IMO this whole 'blow through' thing has been 'blown up' out of all proportion. It might be an issue if wild cams with huge duration (and therefore overlap) were used, but these cams are simply not required (or desired) to get big power when you are using forced induction. Forced induction calls for mild cams simply to make the engine as tractable and torquey as possible under no/low boost conditions.
Depends exactly what you're doing, and particularly on how much boost you intend to use. Adding a few psi of supercharge to a Morris Minor is a bolt-on-the-Shorrock job. Sticking 2 bar of boost into a modern engine is a different matter and GreenV8S's point about thermal runaway needs to be heeded.
Pigeon said:
Inertia of the gases. The gas takes time to get moving when the valve opens. The overlap period is shorter (in terms of time, not degrees of rotation) at high revs, so less gas flow has happened by the time the overlap period ends.
This is why your Exup valve works Julian.
I must drop into this debate as it sounds like it applies directly to me.
I have a '94 Porker 993. The engine as standard makes bugger all torque down low, and delivers all the power at high revs.
It's soon to be fitted with a positive displacement supercharger.
I also have a silly loud exhaust fitted which has NO baffles in it, and presumably minimises backpressure.
I understand how blow through can work - is this really only an issue for N/A race cams and high-boost superchargers, or am I going to get molten cats and knackered lamdba sensors with my setup??
Personally I'd expect the backpressure from the cats to be more than enough to prevent blowthrough from the relatively meagre boost the supercharger is going to give (this ain't no hyperbar engine) - but I'd like to know what you think....
I have a '94 Porker 993. The engine as standard makes bugger all torque down low, and delivers all the power at high revs.
It's soon to be fitted with a positive displacement supercharger.
I also have a silly loud exhaust fitted which has NO baffles in it, and presumably minimises backpressure.
I understand how blow through can work - is this really only an issue for N/A race cams and high-boost superchargers, or am I going to get molten cats and knackered lamdba sensors with my setup??
Personally I'd expect the backpressure from the cats to be more than enough to prevent blowthrough from the relatively meagre boost the supercharger is going to give (this ain't no hyperbar engine) - but I'd like to know what you think....
The saga continues......
But edited, becasue this thread appears to be going nowhere.
Some people are intent on blaming free flowing exhausts for some problems a FI engine may have, which are actually down to cam design
Every engine has compromises. Cam design has its compromises. Dont try and make this worse, by restricting the exhaust, and limiting the engines breathing abilities, which in part would defeat the point of Supercharging it in the first place.
You are intent on talking about cams with large amounts of overlap, which are causing the blow thru problems you refer to. If they are as big a problem as you make out, then it shows how badly the cam you have chosen is out for your application. Most modern engines dont need, such wild cams to make power anyway. So why would you want them ???
End of story. Block up your exhausts if you want, I'll build my engines to make power.
>> Edited by stevieturbo on Wednesday 22 September 01:18
But edited, becasue this thread appears to be going nowhere.
Some people are intent on blaming free flowing exhausts for some problems a FI engine may have, which are actually down to cam design
Every engine has compromises. Cam design has its compromises. Dont try and make this worse, by restricting the exhaust, and limiting the engines breathing abilities, which in part would defeat the point of Supercharging it in the first place.
You are intent on talking about cams with large amounts of overlap, which are causing the blow thru problems you refer to. If they are as big a problem as you make out, then it shows how badly the cam you have chosen is out for your application. Most modern engines dont need, such wild cams to make power anyway. So why would you want them ???
End of story. Block up your exhausts if you want, I'll build my engines to make power.
>> Edited by stevieturbo on Wednesday 22 September 01:18
Ultimate turbo engines (in my literature) follow the cam duration progression of normal engines. You would get blowby if using a supercharger. A 1293 mini metro engine made 190hp using a 300deg cam. Not much below 3000rpm.
It ties in with the torque/power/rev properties of a particular engine. If you are after ultimate power, you must decide whether you are going to go for high lift, short duration, low rev torque (like a big V8) or very high boost, high revs and to avoid inertial slowing, big overlaps/duration.
You could have a variable choke in the exhaust but this would be a bit silly really, considering a compromise build would give you a very tractable engine.
IMHO, you need as open an exhaust as possible and tune your desired power with a cam and port sizes for the engines basic characteristics.
s
It ties in with the torque/power/rev properties of a particular engine. If you are after ultimate power, you must decide whether you are going to go for high lift, short duration, low rev torque (like a big V8) or very high boost, high revs and to avoid inertial slowing, big overlaps/duration.
You could have a variable choke in the exhaust but this would be a bit silly really, considering a compromise build would give you a very tractable engine.
IMHO, you need as open an exhaust as possible and tune your desired power with a cam and port sizes for the engines basic characteristics.
s
Pigeon said:
Mixture lost into the exhaust is mixture lost from the charge burnt on the next firing stroke. It has a great bearing on power produced by the engine!
Not if the lost charge has already been replaced with more. It would certainly hurt the fuel efficiency of the engine, but as long as you can still push an arbitrary amount of mixture into the cylinder once the exhaust valve has closed then power will not suffer.
Back pressure is bad for any performance engine.
All these people that reduce back pressure and then complain that the engine is worse after wards have not optimised the engine for the free breathing exhaust.
Back pressure is bad for a turbo engine.
For a turbine to work you require a pressure difference, ie high on one side, low on the other.
For a supercharged engine back pressure is bad. Power is required to push the charge out of the piston.
Back pressure is bad. I position my exhaust outlet in a low pressure area on the car.
Just out of interest for a no loss exhaust system it must flow 2.2 cfm per bhp.
John
All these people that reduce back pressure and then complain that the engine is worse after wards have not optimised the engine for the free breathing exhaust.
Back pressure is bad for a turbo engine.
For a turbine to work you require a pressure difference, ie high on one side, low on the other.
For a supercharged engine back pressure is bad. Power is required to push the charge out of the piston.
Back pressure is bad. I position my exhaust outlet in a low pressure area on the car.
Just out of interest for a no loss exhaust system it must flow 2.2 cfm per bhp.
John
kneegrow said:
You could have a variable choke in the exhaust but this would be a bit silly really, considering a compromise build would give you a very tractable engine.
Why would this be silly?? You could gain the slighlt backpressure needed to combat overlap at low rpms, and still open the exhaust to flow freely at high rpms. I'd say that building a compromise into the engine would be silly.
Mr2Mike said:
Not if the lost charge has already been replaced with more. It would certainly hurt the fuel efficiency of the engine, but as long as you can still push an arbitrary amount of mixture into the cylinder once the exhaust valve has closed then power will not suffer.
The trouble with this is that you don't have a never ending supply of intake charge at a certain boost. As well, pumping fuel into the exhaust can harm cats/sensors/etc.
I believe the difference of opinions is simply down to relative amounts. While blow through at low rpm won't rob top end power, it does rob low to mid-range torque. This may not be of any concern to some, but I like low to mid-range torque. You can eliminate/reduce blow-through by reducing overlap to near nothing, but this DOES affect top-end power, even on a SC engine. So, why build that compromise into the engine? Why not tune (not restrict) the exhaust to reduce the blow through at low rpm while still flowing freely at high rpm, even with an overlap cam? Wouldn't that be the best solution?
Any ideas on how that is achievable?
Gary
>> Edited by gary_tholl on Wednesday 22 September 16:23
gary_tholl said:
Not if the lost charge has already been replaced with more. It would certainly hurt the fuel efficiency of the engine, but as long as you can still push an arbitrary amount of mixture into the cylinder once the exhaust valve has closed then power will not suffer.
The trouble with this is that you don't have a never ending supply of intake charge at a certain boost. As well, pumping fuel into the exhaust can harm cats/sensors/etc.
Yes. The amount of charge is effectively "metered" by the positive displacement supercharger before it gets to the cylinder, so any of that lost out of the exhaust is subtracted from the amount left in the cylinder.
gary_tholl said:
I believe the difference of opinions is simply down to relative amounts. While blow through at low rpm won't rob top end power, it does rob low to mid-range torque. This may not be of any concern to some, but I like low to mid-range torque. You can eliminate/reduce blow-through by reducing overlap to near nothing, but this DOES affect top-end power, even on a SC engine. So, why build that compromise into the engine? Why not tune (not restrict) the exhaust to reduce the blow through at low rpm while still flowing freely at high rpm, even with an overlap cam? Wouldn't that be the best solution?
Any ideas on how that is achievable?
There are plenty of possibilities... one that suggests itself to me as a two-stroke-head is pulse tuning. On a four-cylinder four-stroke you could have a 4 into 1 exhaust with the pipe lengths from head to collector are such that the exhaust pulse from the next cylinder to exhaust causes a positive pressure pulse to arrive at the cylinder which is completing its exhaust stroke during the overlap period, thus effectively "closing" its exhaust valve sooner, and pushing any spilled charge back into the cylinder. At higher revs this pulse will arrive after the exhaust valve has already closed, so will have no effect. An Exup-valve arrangement, considered as a switchable reflector rather than a switchable restriction, is another possibility.
Cylinder filling will be the same, as the exhaust valve has cloased so early, it doesnt matter. The cylinder will still be filled with the pressurised air/fuel. There is no chance boost pressure would drop as a consequence of the small overlap present.
Power will stay the same, fuel economy, and emissions wont.
I think the biggest problem here, is that the people who think the problem is so bad, must be trying to use cams with about 360deg duration, with about 300deg overlap. ( Yes, thats silly, I know )
Do you blame the exhaust on a n/a car with a 300deg cam, that only produces power above 4000rpm, for causing a lack of low down torque ??
Similarly, do you blame the exhaust, on a car with a 240deg cam, that wont make any power above 6000rpm ??
No, its all down to camshaft.
Funniy a 1293 Mini turbo should be mentioned. On my own, I did once try a 286deg cam in mine. Power shifted from about 4000rpm-8000rpm, with a total loss of all driveability below 3000rpm.
Once I swapped back to a more sedate 250 deg cam, it made more power, revved to 7500rpm, was totally tractable, and actually quite torquey from idle, and made around 190bhp with 20psi.
It basically boils down to, Forced induction engines do NOT need wild cams, so why would any idiot try and use them ?????
Power will stay the same, fuel economy, and emissions wont.
I think the biggest problem here, is that the people who think the problem is so bad, must be trying to use cams with about 360deg duration, with about 300deg overlap. ( Yes, thats silly, I know )
Do you blame the exhaust on a n/a car with a 300deg cam, that only produces power above 4000rpm, for causing a lack of low down torque ??
Similarly, do you blame the exhaust, on a car with a 240deg cam, that wont make any power above 6000rpm ??
No, its all down to camshaft.
Funniy a 1293 Mini turbo should be mentioned. On my own, I did once try a 286deg cam in mine. Power shifted from about 4000rpm-8000rpm, with a total loss of all driveability below 3000rpm.
Once I swapped back to a more sedate 250 deg cam, it made more power, revved to 7500rpm, was totally tractable, and actually quite torquey from idle, and made around 190bhp with 20psi.
It basically boils down to, Forced induction engines do NOT need wild cams, so why would any idiot try and use them ?????
stevieturbo said:
Do you blame the exhaust on a n/a car with a 300deg cam, that only produces power above 4000rpm, for causing a lack of low down torque
It basically boils down to, Forced induction engines do NOT need wild cams, so why would any idiot try and use them ?????
You wouldn't blame the exhaust, but you can tune the exhaust to reduce the lack of low-down torque.
Who mentioned anything about wild cams? Even regular cams have overlap. So, if by changing(tuning) the exhaust you were able to use the 286 degree cam in your mini, with it's 8000rpm top end, and still have useability below 3000, that's a bad thing?? I don't understand why you feel you need to compromise on the cam (either top end or low end, not both), when there are other factors that can accomplish a similar outcome.
Pigeon said:
There are plenty of possibilities... one that suggests itself to me as a two-stroke-head is pulse tuning. On a four-cylinder four-stroke you could have a 4 into 1 exhaust with the pipe lengths from head to collector are such that the exhaust pulse from the next cylinder to exhaust causes a positive pressure pulse to arrive at the cylinder which is completing its exhaust stroke during the overlap period, thus effectively "closing" its exhaust valve sooner, and pushing any spilled charge back into the cylinder. At higher revs this pulse will arrive after the exhaust valve has already closed, so will have no effect. An Exup-valve arrangement, considered as a switchable reflector rather than a switchable restriction, is another possibility.
Doesn't a 4-1 collector create a negative pulse travelling back down the other 3 primaries though? That can be used for scavenging purposes, but wouldn't create a 'wall' to stop blow through. You need something for the exhaust pulse to reflect off of (such as the bike's exup valve) to create a positive wave back towards the exhaust valve.
Gary
The 2.2 cfm per bhp came from an article written by David Vizard on exhausts and from his books. He also states that back pressure is bad.
I really respect his writings as his views and opinions are backed up with many hours of testing.
As he says there is no such thing as a bad test only poor interpretation of the results.
John
I really respect his writings as his views and opinions are backed up with many hours of testing.
As he says there is no such thing as a bad test only poor interpretation of the results.
John
While it had an 8000rpm, it made no more power. The power band was simply lifted up.
Totally pointless, and much harder on the engine.
I do understand that virtually all cams have overlap, but I think these guys are blowing any problems way out of proportion.
A camshaft must be chosen to do a particular job. Unless you can have variable cam timing that is. But in a lot of cases, the variable cam timing is also used to create an emissions friendly engine that can also make a bit of power, rather than developing it from a performance point of view outright.
For a tractable road engine, you simply wouldnt install a cam with long duration, and large overlap, as it would be totally wrong for the job, so why would you do it in a SC or Turbo engine ???
Totally pointless, and much harder on the engine.
I do understand that virtually all cams have overlap, but I think these guys are blowing any problems way out of proportion.
A camshaft must be chosen to do a particular job. Unless you can have variable cam timing that is. But in a lot of cases, the variable cam timing is also used to create an emissions friendly engine that can also make a bit of power, rather than developing it from a performance point of view outright.
For a tractable road engine, you simply wouldnt install a cam with long duration, and large overlap, as it would be totally wrong for the job, so why would you do it in a SC or Turbo engine ???
The funny thing about all this is, fundamentally we agree. The difference is that we are talking about degrees of change. You say that blowthrough due to overlap is miniscule and should be discounted, and that a low overlap cam will work at higher rpms.
Peter (and now I) say that, while blowthrough at low rpm won't make or break an engine, why put up with the losses it creates when you can tune the exhaust to reduce the blow through? By doing this, you can run the cam with some (not a race cam) overlap, which is going to work better at higher rpms.
Now, if we could come up with something that would reduce the blow through at low rpm, but flow freely at higher rpm, why wouldn't that be the best of both worlds?
As for the 2.2 cfm bit, if that's the case, why do most diesels run drainpipe exhausts for maybe 200-250 hp, while a gas engine that produces the same can get by with a much smaller exhaust? I've read some of Vizards stuff, and he does seem to know it, but that just seems way to simplistic.
Gary
Peter (and now I) say that, while blowthrough at low rpm won't make or break an engine, why put up with the losses it creates when you can tune the exhaust to reduce the blow through? By doing this, you can run the cam with some (not a race cam) overlap, which is going to work better at higher rpms.
Now, if we could come up with something that would reduce the blow through at low rpm, but flow freely at higher rpm, why wouldn't that be the best of both worlds?
As for the 2.2 cfm bit, if that's the case, why do most diesels run drainpipe exhausts for maybe 200-250 hp, while a gas engine that produces the same can get by with a much smaller exhaust? I've read some of Vizards stuff, and he does seem to know it, but that just seems way to simplistic.
Gary
Turbosteve, that sounds brilliant. Can you elaborate on the spec of the engine? 20 Psi? Bloody hell!
Vizards book has one with 190hp running 17.5 Psi with a 649 cam.
The ramp rates of a 649 are considerable and so with a turbo, it would have caused it to spin up quicker.
I was working through the reasoning (maths) behind why supercharged engines don't just rev to destruction. The whole thing as I could see was to do with charge inertia ultimately forming a self induced power limiter. This would cause the SC to produce less boost and the limit is reached. I suppose on a turbo engine, you can boost until it explodes.
I am building a 1293 engine with an Eaton M45 blower (750ml/R) I hope to run about 12 PSI, I was wondering about the power limiting factors and why I couldn't run more boost. This thread has been great. I'm really interested in your build spec. What books/people did you find useful......stu
Vizards book has one with 190hp running 17.5 Psi with a 649 cam.
The ramp rates of a 649 are considerable and so with a turbo, it would have caused it to spin up quicker.
I was working through the reasoning (maths) behind why supercharged engines don't just rev to destruction. The whole thing as I could see was to do with charge inertia ultimately forming a self induced power limiter. This would cause the SC to produce less boost and the limit is reached. I suppose on a turbo engine, you can boost until it explodes.
I am building a 1293 engine with an Eaton M45 blower (750ml/R) I hope to run about 12 PSI, I was wondering about the power limiting factors and why I couldn't run more boost. This thread has been great. I'm really interested in your build spec. What books/people did you find useful......stu
Its nothing particularly fancy.
std 1293 turbo short motor. 6mm machined from top of pistons to lower CR to approx 7.5:1( this does not intrude into piston crown at all )
Best cylinder head Swiftune offer, cant remember valve sizes, its been 10 years since I built it, but sodium filled exhausts are std. I did have a different head before that, but due to valve sizes causing seat cracking, and other problems, I went to Swiftune. Their reputation for heads was the best IMO.
I tried several cams, eventually settling with a Swiftune SW5, along with high ratios rockers. Turbo motor uses a stock metro cam. MG Metro cam, if you can find one is a good upgrade too,as I used it for a while.
The 286 cam I mention was crap in the turbo engine. Admitedly it was a n/a profile though.
Turbo is T3, 0.36/AR exhaust, with a Cossie T3 compressor housing, using 1.5 bar, blowing through an alloy R5 intercooler ( OE with plastic end tanks replaced with alu.) , into custom plenum, and modified SU.
Larger needle valve, float bowl extension, and larger jet to accept 0.125" needle, along with heavier spring.
Also using a bit of water injection.
Dizzy is just a plain old welded up one, running about 28deg advance.
Engine was built very much on a budget. Money was spent on what makes power. Allowing teh engine to breathe. I have melted a couple of pistons during the project, but the first engine lasted over 40k before that happened. I never had any crank or rod related problems.
Totally tractable, and very torquey, right from idle to 7500rpm, with power coming on very strong from 3500rpm, and a bit silly at 4000rpm.
Box is a 4spd Tran-X, with tran-X 4pin diff ( non lsd ), with 3.1:1 final drive, on 13" wheels.
Traction is a problem below 70mph on dry roads.
And yes, it will pull 7500rpm in 4th......Scary thought really....Actually surprising it doesnt feel as bad as you think, as long as you stay straight ( allowing for torque steer )
Best 1/4, at Santa Pod back in Nov 94 was 14.5 @ 101mph, still in 3rd crossing the line. Full weight mini, + luggage, on 165/65x13 road tyres.
The terminal speed shows there is much more potential on the strip, if there was any grip. On the road, it was excellent fun at the time, with many cossie and Subaru ( new back then ) owners being shown up.
At that time, the Vizard book was probably the only one I read that helped, or even touched on turbocharging.
Everything with the car was trial and error. There was nobody around who could help much, apart from a friend, who had been throwing together DIY turbo motors for a few years. My first trip to Santa Pod in the car was the first Mini showdown. I fully expected there to be many cars there that were much faster, given so many expert tuners around. I only managed 14.7 @96mph that day, but it was enough for 3rd overall and fastest turbo ( by far ), against 2 stripped out race cars with slicks, one with nitrous.
The other car only pipped me by 0.1sec, although my terminal was some 6mph faster.
As I said before, most SC are rather innefficient, especially at higher rpm's, taking large amounts of power to drive them. How many high rpm supercharged engines do you see ??? Boost and airflow is also relative to SC speed, whereas on a turbo, you can control the boost to whatever you want.
Best way to make power, is to allow the engine to breathe. get a good cylinder head. That also allows milder cam profiles without losing so much power, besides, FI engines dont need wild profiles.
I didn for a brief period between cylinder heads run a std one til I got my new head. I had thought back then, that blowing into the engine would have meant that a modified head wouldnt make as much difference. How wrong I was. The difference in power was very noticeable.
std 1293 turbo short motor. 6mm machined from top of pistons to lower CR to approx 7.5:1( this does not intrude into piston crown at all )
Best cylinder head Swiftune offer, cant remember valve sizes, its been 10 years since I built it, but sodium filled exhausts are std. I did have a different head before that, but due to valve sizes causing seat cracking, and other problems, I went to Swiftune. Their reputation for heads was the best IMO.
I tried several cams, eventually settling with a Swiftune SW5, along with high ratios rockers. Turbo motor uses a stock metro cam. MG Metro cam, if you can find one is a good upgrade too,as I used it for a while.
The 286 cam I mention was crap in the turbo engine. Admitedly it was a n/a profile though.
Turbo is T3, 0.36/AR exhaust, with a Cossie T3 compressor housing, using 1.5 bar, blowing through an alloy R5 intercooler ( OE with plastic end tanks replaced with alu.) , into custom plenum, and modified SU.
Larger needle valve, float bowl extension, and larger jet to accept 0.125" needle, along with heavier spring.
Also using a bit of water injection.
Dizzy is just a plain old welded up one, running about 28deg advance.
Engine was built very much on a budget. Money was spent on what makes power. Allowing teh engine to breathe. I have melted a couple of pistons during the project, but the first engine lasted over 40k before that happened. I never had any crank or rod related problems.
Totally tractable, and very torquey, right from idle to 7500rpm, with power coming on very strong from 3500rpm, and a bit silly at 4000rpm.
Box is a 4spd Tran-X, with tran-X 4pin diff ( non lsd ), with 3.1:1 final drive, on 13" wheels.
Traction is a problem below 70mph on dry roads.
And yes, it will pull 7500rpm in 4th......Scary thought really....Actually surprising it doesnt feel as bad as you think, as long as you stay straight ( allowing for torque steer )
Best 1/4, at Santa Pod back in Nov 94 was 14.5 @ 101mph, still in 3rd crossing the line. Full weight mini, + luggage, on 165/65x13 road tyres.
The terminal speed shows there is much more potential on the strip, if there was any grip. On the road, it was excellent fun at the time, with many cossie and Subaru ( new back then ) owners being shown up.
At that time, the Vizard book was probably the only one I read that helped, or even touched on turbocharging.
Everything with the car was trial and error. There was nobody around who could help much, apart from a friend, who had been throwing together DIY turbo motors for a few years. My first trip to Santa Pod in the car was the first Mini showdown. I fully expected there to be many cars there that were much faster, given so many expert tuners around. I only managed 14.7 @96mph that day, but it was enough for 3rd overall and fastest turbo ( by far ), against 2 stripped out race cars with slicks, one with nitrous.
The other car only pipped me by 0.1sec, although my terminal was some 6mph faster.
As I said before, most SC are rather innefficient, especially at higher rpm's, taking large amounts of power to drive them. How many high rpm supercharged engines do you see ??? Boost and airflow is also relative to SC speed, whereas on a turbo, you can control the boost to whatever you want.
Best way to make power, is to allow the engine to breathe. get a good cylinder head. That also allows milder cam profiles without losing so much power, besides, FI engines dont need wild profiles.
I didn for a brief period between cylinder heads run a std one til I got my new head. I had thought back then, that blowing into the engine would have meant that a modified head wouldnt make as much difference. How wrong I was. The difference in power was very noticeable.
gary_tholl said:
There are plenty of possibilities... one that suggests itself to me as a two-stroke-head is pulse tuning. On a four-cylinder four-stroke you could have a 4 into 1 exhaust with the pipe lengths from head to collector are such that the exhaust pulse from the next cylinder to exhaust causes a positive pressure pulse to arrive at the cylinder which is completing its exhaust stroke during the overlap period, thus effectively "closing" its exhaust valve sooner, and pushing any spilled charge back into the cylinder. At higher revs this pulse will arrive after the exhaust valve has already closed, so will have no effect. An Exup-valve arrangement, considered as a switchable reflector rather than a switchable restriction, is another possibility.
Doesn't a 4-1 collector create a negative pulse travelling back down the other 3 primaries though? That can be used for scavenging purposes, but wouldn't create a 'wall' to stop blow through. You need something for the exhaust pulse to reflect off of (such as the bike's exup valve) to create a positive wave back towards the exhaust valve.
Gary
You get both positive and negative pressure pulses sent back up the other headers. Which of these you "use" depends on the length of the headers - the negative pulse comes later, and the scavenging effect is what one is normally trying to optimise. To use the positive pulses to counteract blow-through involves much shorter headers. Multi-cylinder piston-ported two-stroke engines which would run like dogs if they didn't use this effect need such short lengths that a log manifold may be a reasonable approximation. You could argue that if the engine is required to run with a catalyst any attempt at tuning for scavenging may well be stuffed anyway, so tuning to avoid blow-through looks more attractive. Exup valves try and give you the best of both worlds at the expense of more complication.
gary_tholl said:
As for the 2.2 cfm bit, if that's the case, why do most diesels run drainpipe exhausts for maybe 200-250 hp, while a gas engine that produces the same can get by with a much smaller exhaust? I've read some of Vizards stuff, and he does seem to know it, but that just seems way to simplistic.
Gary
Diesels have no throttles, and run with extremely lean air-fuel ratios - the "black smoke limit" on power is at a ratio of around 22:1, ie. around 50% more air than a petrol engine even at full pelt, so the same amount of power involves a much larger gas flow.
I think you are right to say that we are all basically in agreement and it's all a matter of degree. The thing is you hit blow-through at one end of the scale and thermal runaway at the other, and the more extreme your figures for power, boost, revs and swept volume get, the less margin you have to work in.
With a supercharged two-stroke diesel, you're looking to get a *particular amount* of blow-through, which makes things fun
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