Camshaft considerations, Forced induction.
Discussion
I have a 1275 A series engine and I am going to strap a BMW Cooper S supercharger to this, blowing through a Vauxhall TD intercooler. I should be getting around 11 PSI in the intake manifold and I want to have a realistic rev limit at 6500.
There are a fair few turbo cams available for the engine but supercharging is a bit of a grey area, I'm not convinced about some of the logic.
Here's what I think is the truth.
With a forced induction engine, since the inlet charge is compressed, it will take less inlet valve duration to fill the chamber. This means for a given desired performance, the inlet duration can be slightly shortened. As there is a lot of exhaust gas, this can be got rid of by an extended duration on the exhaust valve. So, to gain behaviour (similar) to a 270 deg N/A cam, you could use a 260 inlet and 280 exhaust, having roughly the same overlap. (Without digressing into specific timings).
There are a fair few books and a fair few people using forced induction with symmetrical cams like a 276/276. My logic tells me that there would be blow by at low rpms (at least with a supercharger) with a turbo, I suppose there would be boost lag and it would just contribute to nasty running off boost.
Points so far.
1. For a given overlap, it is better to have a bias of duration to the exhaust valve. The degree of bias being proportional to boost pressure.
2. For turbocharging, the cam is less sensitive to overlap as it will likely be off boost, hence low blowby.
Anyway, I also hear that opening the exhaust valve earlier gives a better exhaust pulse which can spin the turbo up. I assume that it isn't really that significant for a supercharger, just that the overlap is minimised and the exhaust is open for as much as possible.
I also hear that tighter Lobe Centre Angles are a good thing for superchargers as this reduces the significance of overlap and allows greater durations to be used for a given power/revs spread.
I assume that for turbocharged engines, a tighter LCA would increase the exhaust pulse and make for a better spool up as well.
I expect the downside is increased timing gear wear.
More points.
3. Forced induction favours tighter LCA's.
4. Overlap must be minimised but duration and lift must be maximised.
What do you think fellas?
I am convinced that my logic is right and a N/A fast road cam for a supercharger is a bad idea. Am I right?
Can your arguments have as many figures as possible please?
stu
There are a fair few turbo cams available for the engine but supercharging is a bit of a grey area, I'm not convinced about some of the logic.
Here's what I think is the truth.
With a forced induction engine, since the inlet charge is compressed, it will take less inlet valve duration to fill the chamber. This means for a given desired performance, the inlet duration can be slightly shortened. As there is a lot of exhaust gas, this can be got rid of by an extended duration on the exhaust valve. So, to gain behaviour (similar) to a 270 deg N/A cam, you could use a 260 inlet and 280 exhaust, having roughly the same overlap. (Without digressing into specific timings).
There are a fair few books and a fair few people using forced induction with symmetrical cams like a 276/276. My logic tells me that there would be blow by at low rpms (at least with a supercharger) with a turbo, I suppose there would be boost lag and it would just contribute to nasty running off boost.
Points so far.
1. For a given overlap, it is better to have a bias of duration to the exhaust valve. The degree of bias being proportional to boost pressure.
2. For turbocharging, the cam is less sensitive to overlap as it will likely be off boost, hence low blowby.
Anyway, I also hear that opening the exhaust valve earlier gives a better exhaust pulse which can spin the turbo up. I assume that it isn't really that significant for a supercharger, just that the overlap is minimised and the exhaust is open for as much as possible.
I also hear that tighter Lobe Centre Angles are a good thing for superchargers as this reduces the significance of overlap and allows greater durations to be used for a given power/revs spread.
I assume that for turbocharged engines, a tighter LCA would increase the exhaust pulse and make for a better spool up as well.
I expect the downside is increased timing gear wear.
More points.
3. Forced induction favours tighter LCA's.
4. Overlap must be minimised but duration and lift must be maximised.
What do you think fellas?
I am convinced that my logic is right and a N/A fast road cam for a supercharger is a bad idea. Am I right?
Can your arguments have as many figures as possible please?
stu
I'd basically agree with your logic... there was a big thread on this a while ago:
www.pistonheads.com/gassing/topic.asp?t=124140&f=66&h=0
The standard cam in an A-series does pretty well when supercharged, though that's not to say there isn't room for improvement.
www.pistonheads.com/gassing/topic.asp?t=124140&f=66&h=0
The standard cam in an A-series does pretty well when supercharged, though that's not to say there isn't room for improvement.
A tighter/lower LCA will increase overlap, not reduce it.
So generally you want higher numbers for a FI camshaft, but they may not always help things. Its a compromise, and a lot depends on how well the heads flow.
Im using a Swiftune SW5 in my turbo mini, and it performs very well. I had previously tried a few cams, incl std metro turbo ( same as std metro ), MG metro, and Kent 286, all with high lift rockers.
The SW5 is the best of the bunch, with the MG metro cam close by.
The Kent 286 in my turbo application was totally crap, and I replaced it after 4 days.
With any FI engine, stick with a mild cam, and you it wil work. Dont be tempted to go for a racy cam. you will regret it.
So generally you want higher numbers for a FI camshaft, but they may not always help things. Its a compromise, and a lot depends on how well the heads flow.
Im using a Swiftune SW5 in my turbo mini, and it performs very well. I had previously tried a few cams, incl std metro turbo ( same as std metro ), MG metro, and Kent 286, all with high lift rockers.
The SW5 is the best of the bunch, with the MG metro cam close by.
The Kent 286 in my turbo application was totally crap, and I replaced it after 4 days.
With any FI engine, stick with a mild cam, and you it wil work. Dont be tempted to go for a racy cam. you will regret it.
stevieturbo said:
With any FI engine, stick with a mild cam, and you it wil work.
Um, I'm thinking that any cam will work, but I'm also gonna guess that he is looking to optimize things a little beyond 'will work'.
I'd go and read that thread that was mentioned, there is quite a bit on cams and forced induction in there.
Gary
work as in, produce power, without sacrificing driveability.
IMO, racy cams, with big overlap, do not work, period. They can have potential to make power up top, but what they lose low down, makes them a failure, unless its for an out and out race engine.
Yes, there is a lot of info in that thread, but there are also some very conflicting schools of thought, so wether he may find it useful or not is hard to say.
IMO, racy cams, with big overlap, do not work, period. They can have potential to make power up top, but what they lose low down, makes them a failure, unless its for an out and out race engine.
Yes, there is a lot of info in that thread, but there are also some very conflicting schools of thought, so wether he may find it useful or not is hard to say.
I was led to believe the turbo metro had a standard 1275cc cam in it.An engine builder put a MG one in an old girlfriends Turbo engine when it was rebuilt.Said it would give a good basis for other tuning and not needing to pull the engine out for anything else which he didn't recommend.
I've found that turbo's work great in a street application with a street type of camshaft. Anything sensible (wide LCA) will allow huge gains in torque in the mid range. You can optimise the overlap for even more mid range grunt but you don't need to. A turbo just magnifies the camshaft characteristics which you already have imho. If you go for a racier cam you will get greater bhp gains up top but lots of blow by everywhere else due to the narrowier LCA.
Boosted.
Boosted.
according to the Vizard book in front of me,the standard metro cam was also used in the Turbo,marina,allegro.It has 26 deg overlap as opposed to 45 deg for the MG.The cooper's' has 31 deg overlap .They all have the same valve lift of .250 on inlet and exhaust.All later cams had 1/2 inch width lobes and early cams had 3/8.He advises using the wider for re-profiling.As said there isn't any markings on the cam to indentify it,so best maybe to put a mild uprated cam in.Vizard did say that off set rockers did work weel with the standard cam.
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