What factors dictate rev limit in an RV8..?
Discussion
Hey guys -
I have had my 4.6l RV8 re-built, using standard components in regards to crank, pistons, rods - although many other parts aftermarket (please see the full spec in my profile for details on cam, BV heads, etc)...
I have run it in for nearly 3000miles now, and have been told that I should get it tuned for the set-up (ie. ignition/fuelling) - so thinking Mark Adams.
It seems to have the rev limiter set at 6000 (although I seem to remember it being ~6250 prior to rebuild? Odd) - and I want to get the rev limiter raised if possible.
Crank, pistons and rods are all OE (sourced from Rimmers anyway) - but the whole crank/fly/clutch-cover assembly has been dynamically balanced to 7000rpm at Basset Down balancing...
So my question is: what factors are involved with calculating the max RPM safely *regularly* possible, and what sort of rpm limit should I be looking at...
Cheers,
Dom
I have had my 4.6l RV8 re-built, using standard components in regards to crank, pistons, rods - although many other parts aftermarket (please see the full spec in my profile for details on cam, BV heads, etc)...
I have run it in for nearly 3000miles now, and have been told that I should get it tuned for the set-up (ie. ignition/fuelling) - so thinking Mark Adams.
It seems to have the rev limiter set at 6000 (although I seem to remember it being ~6250 prior to rebuild? Odd) - and I want to get the rev limiter raised if possible.
Crank, pistons and rods are all OE (sourced from Rimmers anyway) - but the whole crank/fly/clutch-cover assembly has been dynamically balanced to 7000rpm at Basset Down balancing...
So my question is: what factors are involved with calculating the max RPM safely *regularly* possible, and what sort of rpm limit should I be looking at...
Cheers,
Dom
Edited by domV8 on Tuesday 17th November 23:20
There is no point in having a rev limit far in excess of the point maximum power is produced. All you will achieve is rapid wear.
Put it on the rollers and find out at what rpm peak power is produced and set the limitter a little past that. I doubt it will be higher than the limit already set.
Put it on the rollers and find out at what rpm peak power is produced and set the limitter a little past that. I doubt it will be higher than the limit already set.
Edited by Uncle Fester on Wednesday 18th November 09:48
The weight and strength of the rotating parts will determine your rev limit. To get to 7 or 8k, you'll need a steel crank, steel rods, stronger pins and all sorts. Big money, and even then, going up there regularly will shorten the life of your engine I would have thought. As I understand it, dynamic balancing of the crank, flywheel and clutch will only make your engine run more smoothly, not higher rpm.
As has been said though, a fairly standard RV8 will run out of puff before 6k anyway, so 6-7 or 6-8k would be pointless. You'd basically need to re-spec the whole engine to race spec to be strong enough to rev that high and make any power when it got there. I'm of course not an expert though so I'm sure someone with a greater depth of knowledge will be here shortly to fill in the details!
As has been said though, a fairly standard RV8 will run out of puff before 6k anyway, so 6-7 or 6-8k would be pointless. You'd basically need to re-spec the whole engine to race spec to be strong enough to rev that high and make any power when it got there. I'm of course not an expert though so I'm sure someone with a greater depth of knowledge will be here shortly to fill in the details!
Please see profile for spec with relation to engine set up...
I am just looking to get to somewhere between 6.5-7k revs, as spend says, not much higher than some engines standard limiter.
My limiter cutting in at 6k is starting to pee me off, because it feels like the engine has loads more to give...
The TVR885 cam and BV heads should mean that it certainly won't be "running out of puff" at 6k, it will surely be mid-stride, and another 750rpm or so would be just the ticket...
anyone know the safe limit for the standard components mentioned above, rather than the peak power limits of cam profiles etc
many thanks,
Dom
I am just looking to get to somewhere between 6.5-7k revs, as spend says, not much higher than some engines standard limiter.
My limiter cutting in at 6k is starting to pee me off, because it feels like the engine has loads more to give...
The TVR885 cam and BV heads should mean that it certainly won't be "running out of puff" at 6k, it will surely be mid-stride, and another 750rpm or so would be just the ticket...
anyone know the safe limit for the standard components mentioned above, rather than the peak power limits of cam profiles etc
many thanks,
Dom
domV8 said:
Please see profile for spec with relation to engine set up...
I am just looking to get to somewhere between 6.5-7k revs, as spend says, not much higher than some engines standard limiter.
My limiter cutting in at 6k is starting to pee me off, because it feels like the engine has loads more to give...
The TVR885 cam and BV heads should mean that it certainly won't be "running out of puff" at 6k, it will surely be mid-stride, and another 750rpm or so would be just the ticket...
anyone know the safe limit for the standard components mentioned above, rather than the peak power limits of cam profiles etc
many thanks,
Dom
I'm interested in what you are doing and would appreciate being updated. I've got to say, though, that I can't be of any help. I would have suggested that the extra 750 revs you are after might be a bit on the dangerous side. That's pushing the revs up by an eigth on an engine designed some weeks ago now.I am just looking to get to somewhere between 6.5-7k revs, as spend says, not much higher than some engines standard limiter.
My limiter cutting in at 6k is starting to pee me off, because it feels like the engine has loads more to give...
The TVR885 cam and BV heads should mean that it certainly won't be "running out of puff" at 6k, it will surely be mid-stride, and another 750rpm or so would be just the ticket...
anyone know the safe limit for the standard components mentioned above, rather than the peak power limits of cam profiles etc
many thanks,
Dom
Regards,
Derek
As others have said above, plus;
The TVR std cam and most mild aftermarket cams produce peak torque at around 4.5k rpm so by 5.5k rpms your max power will have levelled out and indead start dropping off much over 6k rpm...without significant changes to the stock induction setup and cam profile.
If you want high rpms you will also need to uprate the valve train as you'll start to get valve float above 7k rpm
The TVR std cam and most mild aftermarket cams produce peak torque at around 4.5k rpm so by 5.5k rpms your max power will have levelled out and indead start dropping off much over 6k rpm...without significant changes to the stock induction setup and cam profile.
If you want high rpms you will also need to uprate the valve train as you'll start to get valve float above 7k rpm
To get the power curve to produce peak power at about 7k rpm you'd have to have a cam that produces it there, a bottom end that is safe up there, solid lifters, & a big enough inlet track to flow enough air to do all the other stuff justice & get the fueling improved to get the AFR in the right place up there, (bigger pump/injectors etc). If you have a standardish cam with a standard but balanced bottom end the peak power will be around 5.5k rpm as said. If you feel it has more to give then it would probably benefit from a session on the rollers to get what you have, to perform to the best of it's abilities. 
All IMHO of course.
Edited to add, the rollers session will also determine whether your rev counter is accurate. My old Chim was way out. (showing 6k rpm at 4k.)
All IMHO of course.
Edited to add, the rollers session will also determine whether your rev counter is accurate. My old Chim was way out. (showing 6k rpm at 4k.)
Edited by pwd95 on Wednesday 18th November 11:05
"Blueprinting and balancing" are not very specific and can be done to a pretty wide range of tolerances. Yet if you have had it done, ask the engine builder what he considers the safe rpm limit for the work he did, simple really.
You can get hydraulic cam / valve gear to rev high, but TBH it's easier/better to convert to solids & a mechanical cam IMHO.
Problem is you can't get a cam that will work at 7000rpm as well as down at 1000rpm. You can do various things to 'tame' those characteristics, depending on your cam choice. Really a good fuel & spark ecu is the starting point and if you want to go farther possibly throttle bodies....
You can get hydraulic cam / valve gear to rev high, but TBH it's easier/better to convert to solids & a mechanical cam IMHO.
Problem is you can't get a cam that will work at 7000rpm as well as down at 1000rpm. You can do various things to 'tame' those characteristics, depending on your cam choice. Really a good fuel & spark ecu is the starting point and if you want to go farther possibly throttle bodies....
rev-erend said:
Nice spec you have there.
My guess is the hydraulic lifters do not allow you to get past 6.5K .. I'm not aware of a rev limiter in the 14CUX but as I had a Mark Adams special it may have been removed.
They do My guess is the hydraulic lifters do not allow you to get past 6.5K .. I'm not aware of a rev limiter in the 14CUX but as I had a Mark Adams special it may have been removed.
Mark will turn it off during the power runs and then resets it where he thinks it's safe optimum is. Can't remember off hand exactly how it works but i think it just shuts off the injectors at the pre defined rev limit.My engine was still pulling at 7k rpm but its well beyond the safe limit for the charger so mine is set at 6.5k rpm
Speaking to Dom at TVR power - he said the cam was designed to run from somewhere low down (can't remember!) to around 6250ish...
Now granted, peak BHP etc will be lower than that - but at the moment, the rev-limiter hits like a wall when the engine is in full flood...
I have to drive it changing gear well before the power delivery slows - because the line between full-on power and bang! rev-limiter cutting-in is infitesimily small...
I find myself changing at around 5500 at the moment to stop any chance of hitting the rev-limiter (as it is not a pleasant experience in my car) - and losing out on the last 500rpm of rev-range (which is very powerful and goes very quickly)...
Having a rev limiter reset to say 6600 means I would be able to rev the engine to, say 6100-6200 where the torque is beginning to drop, without the fear that I was a split second away from having myself thrown out of my seat into the windscreen by the rev-limiter cutting in...
Many of the TVRs have a standard rev-limit of 6250 rather than my 6000 - and I'm sure mine did too before the re-build, which is fairly bizarre (but I could be wrong) - so I can't see there should be any problem with this limit at least.
Crank assembly was dynamically balanced to 7000rpm by Basset Down balancing, who were recommended to me as being very good - and so I am wondering how far beyond the "standard" 6250 I can push it safely, as obviously raising the limit to 6500 for example means you can rev to 6250 (within the limit of cams & heads) without worrying about hitting the limiter and it's arrhythmia-inducing bang...
Dom
Now granted, peak BHP etc will be lower than that - but at the moment, the rev-limiter hits like a wall when the engine is in full flood...
I have to drive it changing gear well before the power delivery slows - because the line between full-on power and bang! rev-limiter cutting-in is infitesimily small...
I find myself changing at around 5500 at the moment to stop any chance of hitting the rev-limiter (as it is not a pleasant experience in my car) - and losing out on the last 500rpm of rev-range (which is very powerful and goes very quickly)...
Having a rev limiter reset to say 6600 means I would be able to rev the engine to, say 6100-6200 where the torque is beginning to drop, without the fear that I was a split second away from having myself thrown out of my seat into the windscreen by the rev-limiter cutting in...
Many of the TVRs have a standard rev-limit of 6250 rather than my 6000 - and I'm sure mine did too before the re-build, which is fairly bizarre (but I could be wrong) - so I can't see there should be any problem with this limit at least.
Crank assembly was dynamically balanced to 7000rpm by Basset Down balancing, who were recommended to me as being very good - and so I am wondering how far beyond the "standard" 6250 I can push it safely, as obviously raising the limit to 6500 for example means you can rev to 6250 (within the limit of cams & heads) without worrying about hitting the limiter and it's arrhythmia-inducing bang...
Dom
Are you still running a factory ecu chip Dom?
If so, they do very from car to car where the limiter is set (anywhere between 5750-6250 rpm) my car was originally set at 5750 rpm. So if you have had a total rebuild then I can understand your issue and would suggest an after market chip and r/r tune.
If so, they do very from car to car where the limiter is set (anywhere between 5750-6250 rpm) my car was originally set at 5750 rpm. So if you have had a total rebuild then I can understand your issue and would suggest an after market chip and r/r tune.
Bluebottle said:
rev-erend said:
Nice spec you have there.
My guess is the hydraulic lifters do not allow you to get past 6.5K .. I'm not aware of a rev limiter in the 14CUX but as I had a Mark Adams special it may have been removed.
Can't remember off hand exactly how it works but i think it just shuts off the injectors at the pre defined rev limit.My guess is the hydraulic lifters do not allow you to get past 6.5K .. I'm not aware of a rev limiter in the 14CUX but as I had a Mark Adams special it may have been removed.
With spark control they can choose to trim back the sparks on individual or pairs of cylinders for a smooth cut limit then a final cut of the fuel a bit higher up..
Engine has to be built for the rev range.
Strength at the bottom end, weight in the rods/pistons, cam and timing.
Ran mine to 8k for 20k miles...ran fine but was built to do so. Should still have blown up in that time though...I was lucky. And without the rev limiter it would have revved to destruction.
Hyd lifters will pump up over 6.5 especially with the high lift you'll need (BAD), and even though it feels fast...with std induction it's all over by 5.8k anyway.
Get a shift light (cheapest £50)...so you can see it coming. Or omex etc so that the rev limit can be reached more sympathetically.
Andy
Strength at the bottom end, weight in the rods/pistons, cam and timing.
Ran mine to 8k for 20k miles...ran fine but was built to do so. Should still have blown up in that time though...I was lucky. And without the rev limiter it would have revved to destruction.
Hyd lifters will pump up over 6.5 especially with the high lift you'll need (BAD), and even though it feels fast...with std induction it's all over by 5.8k anyway.
Get a shift light (cheapest £50)...so you can see it coming. Or omex etc so that the rev limit can be reached more sympathetically.
Andy
Bluebottle said:
Are you still running a factory ecu chip Dom?
Yes.rev-erend said:
As the 14CUX does not control the spark - there is no other option .. it's will cut very suddenly.
As I understand it - rev-limiter is fuel cut-out, controlled by 14CUX. So I was going to get an MA to map it and extend the map for a higher rev-range, and possibly any induction changes he thinks necessary, and raise the limit so I can use the entire range of the engine with out hitting the cut-out (although I appreciate there will be no "soft-cut" option and there will still be a "hard-cut", just at
higher up the rev-range).
Not as good as aftermarket ECU, but hey-ho - the map needs tuning to suit all the mods anyway, so why not tailor other areas while I'm at it..?
Dom
Guillotine said:
Hyd lifters will pump up over 6.5 especially with the high lift you'll need (BAD), and even though it feels fast...with std induction it's all over by 5.8k anyway.
Hey Andy, can you elaborate on the hydraulic lifters issue, I don't understand the way you have phrased it. And what is the restriction imposed by the standard induction..?I am not looking for super high revs like yours - just hopefully around 6700ish... The standard engine is good for 6250, as demonstrated by most other peeps rev limits - so I'm wondering if a balanced engine can be reliably pushed to 6700...?
That would allow the engine to rev to 6250-6500 without hitting the limiter - but what is the restriction imposed by "standard induction"..? I was considering a 45mm manifold and plenum from a 5.0l, might this be a budget solution to the issue you mention..?
Cheers,
Dom
I used to rev the knackers off my old 4.0. I converted it to megasquirt with spark control and set the limiter at 6800 and it used to scream its tits off at this and seem to love it. I had a real steel cam in there but after the ecu it made it want to rev so much more. And holding these revs on these engines sounds brilliant.
On my last night before it was coming off the road for a body off I tried to kill it literally and it wouldnt let go. I think these engines are stronger than most people think.
On my last night before it was coming off the road for a body off I tried to kill it literally and it wouldnt let go. I think these engines are stronger than most people think.
My 400SE('Real steel' cam) would also merrily rev its knackers off and sound fabulous in the process, but still made max power at 5,350 and max torque at 4,000rpm (R/Road), so a pointless exercise really, except for the noise and excessive fuel consumption.
Even as a comparative layman I realise that revving a V8 over 6,000rpm on a regular basis requires a bit more than a bottom end balance.
What about all the other reciprocating parts...conrods, pistons, valvegear, etc ?
Even as a comparative layman I realise that revving a V8 over 6,000rpm on a regular basis requires a bit more than a bottom end balance.
What about all the other reciprocating parts...conrods, pistons, valvegear, etc ?
Edited by grahamw48 on Thursday 19th November 00:12
Just a little technical note about the 14CUX - most of which has been said here already.
The ECU does control the rev limit by means of a fuel cut, which is programmable in terms of speed and severity. There are programmable soft and hard cuts, and hysteresis. TVR set the rev limit at 6000 RPM on all 5.0 Litre cat cars that I have seen.
Sadly the 14CUX doesn't control the ignition, so fuel cut is the only option. Actually the choice of rev limiter implementation with a catalyst car is not totally simple. If you use an ignition cut only, unburnt fuel goes straight down the exhaust and into the catalyst (where it is burnt) on the limit. Worse still when the sparks resume, any unburnt fuel in the exhaust is lit up violently.
Fuel-only rev limiters on a non-sequential system will cause the mixture in a number of cylinders to be too weak to burn properly, so although some fuel enters the exhaust it is much less than with an ignition-only cut. A fully sequential fuel cut doesn't have these problems, although most sequential systems operate semi-sequentially at high speed.
There is another issue which affects how well 14CUX equipped engines will rev. The standard fuel map only goes up to 5500RPM, so after this point the fueling usually leans off anyway. However this can be fixed, because the speed breakpoints are also programmable in the 14CUX.
Just to show what the 14CUX can do, a few years ago John Eales built some ultra-short stroke 3.5 litre engines for a specific off-road class. These had to run 14CUX, and we ran them up to 8500RPM perfectly (although they used to eat valve gear).
Sadly it has become fashionable to knock the 14CUX, but how can you criticise something if you don't know how it works? The 14CUX is far more sophisticated (in fuelling terms) than the vast majority of aftermarket systems available today. Although the system is now thirty years old, most aftermarket systems have still not caught up with it in fuelling terms.
Due to the huge variety of systems I use, I get to compare them all. Currently I am developing a performance package for the supercharged Range Rover, and that ECU contains over 76000 maps and variables. Thank goodness most of them can be left alone!
Frankly nearly all aftermarket ECUs are quite simple by comparison to manufacturer's systems (yes - including the 14CUX). Manufacturers spend millions of pounds on developing them, and have large teams with equipment we can only dream of. They have to be excellent in order to meet emission legislation, otherwise they cannot sell the cars.
So if you have a 14CUX, you certainly are not running anything second class. However you are largely stuck with me to map it, although I see Austec have their own offering these days (nothing to do with me). However you could certainly benefit from adding mapped ignition.
Normally when I remap 14CUX systems for TVRs, I extend the map to 6500 RPM to give full control across the entire operating range. During mapping it is desirable to rev the engine past peak power (within the confines of the mechanical specification of the engine) to check the fuelling. After this has been done, the rev limit can be set with reference to the power and torque curves.
I had a really sweet 4.3 from V8 Developments that was built to rev to 6800 RPM eagerly and made 290 BHP. That lasted for 180K Miles (with a new camshaft at 90K Miles) before cracking the block, and did literally dozens of dyno sessions. As you can imagine I was outraged at such an early failure and demanded my money back - not!
The ECU does control the rev limit by means of a fuel cut, which is programmable in terms of speed and severity. There are programmable soft and hard cuts, and hysteresis. TVR set the rev limit at 6000 RPM on all 5.0 Litre cat cars that I have seen.
Sadly the 14CUX doesn't control the ignition, so fuel cut is the only option. Actually the choice of rev limiter implementation with a catalyst car is not totally simple. If you use an ignition cut only, unburnt fuel goes straight down the exhaust and into the catalyst (where it is burnt) on the limit. Worse still when the sparks resume, any unburnt fuel in the exhaust is lit up violently.
Fuel-only rev limiters on a non-sequential system will cause the mixture in a number of cylinders to be too weak to burn properly, so although some fuel enters the exhaust it is much less than with an ignition-only cut. A fully sequential fuel cut doesn't have these problems, although most sequential systems operate semi-sequentially at high speed.
There is another issue which affects how well 14CUX equipped engines will rev. The standard fuel map only goes up to 5500RPM, so after this point the fueling usually leans off anyway. However this can be fixed, because the speed breakpoints are also programmable in the 14CUX.
Just to show what the 14CUX can do, a few years ago John Eales built some ultra-short stroke 3.5 litre engines for a specific off-road class. These had to run 14CUX, and we ran them up to 8500RPM perfectly (although they used to eat valve gear).
Sadly it has become fashionable to knock the 14CUX, but how can you criticise something if you don't know how it works? The 14CUX is far more sophisticated (in fuelling terms) than the vast majority of aftermarket systems available today. Although the system is now thirty years old, most aftermarket systems have still not caught up with it in fuelling terms.
Due to the huge variety of systems I use, I get to compare them all. Currently I am developing a performance package for the supercharged Range Rover, and that ECU contains over 76000 maps and variables. Thank goodness most of them can be left alone!
Frankly nearly all aftermarket ECUs are quite simple by comparison to manufacturer's systems (yes - including the 14CUX). Manufacturers spend millions of pounds on developing them, and have large teams with equipment we can only dream of. They have to be excellent in order to meet emission legislation, otherwise they cannot sell the cars.
So if you have a 14CUX, you certainly are not running anything second class. However you are largely stuck with me to map it, although I see Austec have their own offering these days (nothing to do with me). However you could certainly benefit from adding mapped ignition.
Normally when I remap 14CUX systems for TVRs, I extend the map to 6500 RPM to give full control across the entire operating range. During mapping it is desirable to rev the engine past peak power (within the confines of the mechanical specification of the engine) to check the fuelling. After this has been done, the rev limit can be set with reference to the power and torque curves.
I had a really sweet 4.3 from V8 Developments that was built to rev to 6800 RPM eagerly and made 290 BHP. That lasted for 180K Miles (with a new camshaft at 90K Miles) before cracking the block, and did literally dozens of dyno sessions. As you can imagine I was outraged at such an early failure and demanded my money back - not!
Edited by Mark Adams on Friday 20th November 21:33
Gassing Station | General TVR Stuff & Gossip | Top of Page | What's New | My Stuff