m20 cam with turbo set up

[turbo87]

Hi guys,

Thinking about forced induction,now that i have my car set up i.e suspension, brakes and engine rebuild. Iv got alot of time to think this through, want to make a start on it round october time.

The thing is i have a schrick 272/272 cam would this be ok with a turbo set up? i read somewere that after market cams are a no no when it comes to turbod m20's. If it helps its a m20b25 and the year is 1990.

Thanks guys

[stonesie]

I think a few of the turbo lads have kept the standard cam, you don't want too much overlap or some of the boost will just blow straight through and out of the exhaust.


I know a comapny called CatCams do custom work and all thier BMW cam's are made from new BMW blanks, Ant is a dealer for them and specialises in turbo/supercharger conversions, find his details and some pics at www.fastbimmer.co.uk

[Gunni]

I think a few of the turbo lads have kept the standard cam, you don't want too much overlap or some of the boost will just blow straight through and out of the exhaust.

This is a old rumour.
And may only hold true when you select a cam and turbo who do not match.

such as a small small turbo and very high lift cam. That of course is wrong to begin with.

This does not hold true for almost ALL known turbo setups on this forum

[GeoffBob]

Turbo87,

Increased valve overlap due to increased inlet and exhaust cam duration results in reduced torque at lower RPM in any engine (turbo or normally aspirated). Combine this with the reduced compression ratio of a turbocharged engine and you have a recipe for a very grumpy engine at idle and low RPM (specifically while off boost). However, in the same way as longer duration cams provide more power to a normally aspirated engine, a turbocharged engine will see an improvement at higher RPM - that's if you can get it off the line without stalling.

To provide you with some realistic numbers to work with, a road tuned turbo engine would typically use exhaust and inlet cams with a duration of around 200 to 220 degrees, and rally and track turbo cars around 220 to 240 degrees. Turbo drag cars will use cams of around 250 to 288 degree duration, so I think that gives you some idea of where your 272 degree cams fit in! And none of these are particularly high lifting. So drop your compression ratio and fit a big turbo at your own peril - unless of course you specifically want to build a drag car that burbles rather than idles.

A further complication of turbo engines with longer duration cams is the tendency of hot exhaust gas to flow back through the combustion chamber into the inlet tract during the valve overlap period due to the backpressure of the turbine. This both dilutes and heats the fresh air fuel mixture that should be entering the combustion chamber. A larger turbine, ironically, will help with this problem, but will worsen the problem of non-existent low RPM torque.

A supercharged engine, however, does not suffer this problem since there is no turbine to back up the exhaust pressure. Also, a supercharger provides greater lower down torque than a turbocharger and is thus infinitely more compatible with longer duration cams. A supercharger will, however, struggle to provide boost comparable to a turbocharger. But high boost will require a lower compression ratio (unless you run on special high octane fuel), which with 272 degree cams will, once again, reduce your low RPM torque.

I would thus suggest: Lose the cams if you want to go the turbo route, or consider the supercharger route with less boost if you want to keep them.

Hope this helps.

[Gunni]

Bob your thinking about pressure ratios being bad.
My 2.8 only had a 0.7:1 ratio at highest boost and backpressure and the air is already flowing away from the engine it has momentum in that direction so you would have to have sized the turbine rather badly to begin with. As it says in the Corky Bell book ,
"A turbo should be sized to be in full boost when the engine´s cam is coming on"


Much of this blow through, blow back is from very old turbo technology. All mostly gone now.
With the help of better turbines, exhaust manifolds, surge provention on the compressor, bigger flow maps of the compressor and so on. And the general fact that people just don´t care about power at 1500rpm when there is tons of it at 4k.

Also only horrible terribly engine management systems will nowadays let high cam engine idle badly. And nobody is going to stall with a 272cam or a 304cam either turbo or not.

And a supercharger is only good at low end if you size it to be, it´s never a SAID solution that superchargers will give good low end. If they do they give bad high end. While a turbo will on most occasions give a bigger stronger powerband for the same amount of boost

Alot , and I do mean ALOT of people run those 272´s in turbo E30´s with only gains at all rpm´s and improved drivability.
BMW´s cams are in a sense, terrible. They have very low lift and that hurts real bad. Duration means nothing without knowing ramp angles and valve lifts to find mean valve lift and duration.

Almost all the aftermarket cams for M20´s only improve on the engine at all rpms.
And that further helps turbo cars to spool up quicker and make more torque + higher peak power without absolutely no loss in drivability.

The M20 is not a diesel engine ;)
NA engines converted into turbo engines are alot more fun turbo engines then OEM turbo engines.

[turbo87]

So are we saying that if i were to turbo my engine the aftermarket cam would be ok?

[Gunni]

I´m going with YES, it would only improve things.

[GeoffBob]

Bob your thinking about pressure ratios being bad. My 2.8 only had a 0.7:1 ratio at highest boost and backpressure and the air is already flowing away from the engine it has momentum in that direction so you would have to have sized the turbine rather badly to begin with.

I'm glad you're not a rocket scientist Gunni else your rockets would shoot downwards into the ground and aeroplanes wouldn’t be able to measure their airspeed. It's the momentum of the gas that defines the pressure and vice versa. This is stated mathematically in Bernoulli's equation. Quite simply, the momentum of the gas is in the direction of high to low pressure. So if there's greater pressure on the exhaust side than the inlet, and exhaust valves are both open, well then, you do the math...

Much of this blow through, blow back is from very old turbo technology. All mostly gone now. With the help of better turbines, exhaust manifolds, surge provention on the compressor, bigger flow maps of the compressor and so on.

Gunni. What I said is, a small turbo with a high back pressure (in relation to inlet pressure) will tend to blow exhaust gasses back through the combustion chamber if the valve overlap is large. That's irrespective of any of the technology you mention, its basic physics. If you have to run a large turbo that only makes boost at high RPM in order to avoid this problem (as I stated in my last post) then in my book (and Corky Bell's) you're building a drag engine.

And the general fact that people just don’t care about power at 1500rpm when there is tons of it at 4k.

With due respect Gunni, what kind of subjectivist crap is this? Doesn't Turbo87 have traffic lights where he lives?

Turbo87. I'm not saying don't turbo your engine. What I am saying is that your car will have all-round better drivability if you re-fit the stock cams, and your options in terms of which turbocharger will match your engine will be better. What I am advising you here is how to do what you are asking properly. Uwe taught me an important lesson on this forum the other day, and that's not to spoil other members fun in pursuit of perfection. If you want to turbo your engine with 272 degree cams then you can, just be aware that there are other ways that you can achieve an all-round better result.

[Gunni]

Of course the pressure is a by product of the air flow and momentum, but where is this measurement being taken? As it´s not the same everywhere in the exhaust manifold during the valve overlap
AND surtenly not constant at the valve port either. I had the notion that measured backpressure / intake pressure would mean that if there is any higher backpressure it would lead to hot air going into the combustion chamber, but cam timing and air flow behaviour makes it not so simple.

As when the exhaust valve opens and the piston does the pushing to get the mixture out, that will create a momentum and as we know it will take the same force to stop that air flow, meaning for air to go back it will have to hit a wall of pressure and then go back. What the pressure in the exhaust manifolds needs to be so this happens(here the intake valve hasn´t opened yet) I don´t know. And then remember the pressure at the intake valve is not the same as in the intake plenum, as with general pulse tuning as soon as the intake valve opens with lot´s of pressure sitting on it the air rushes in at the right time, the pressure ratio right there can be beneficial so to allow higher overlapþ

The thing with new technology is that larger turbines now spin/spool faster then small older type ones. Thus no need to back up the pressure to get things going.

Now first of all this biggering is getting nowhere unless the original poster explains what
HP he wants
and what kind of driving he´ll be doing.

In my engine setup I get boost right after 2000rpm. And my setup is absolutely terrible compared to what can be done.

From almost all dyno charts I have seen for aftermarket cams for M20´s there is NO loss of low end. So I can´t see how that is in anyway a bad thing to run a cam that will give better results.

My subjective crap comes from having had driven a host of variour turbo systems on E30 325i´s.
very small turbos, medium turbos and large turbos.

The small one had great low end, but horrible horrible high end no matter the boost
We are talking full boost at 2k(13psi).

My driving style tends to be subjective to location.
So at stop lights I do not need boost right from the go. Or in mild traffic as all those places
are the inapproriate places for quick accelaration anyway as there are people around.
I found that enthusiastic drivers to not drive at 2000rpm when they are about to nail it. Rather somewhere in the region of 3500rpm. This is perfect for a turbo car as when you floor it the boost comes in.

Even though I try to drive mildly in traffic boost at 2-2,5k comes on rather easy without full throttle giving very nice accelaration

It´s because of this subjective crap I have come to the conclusion that high ultra low end boost
is just not needed as the cam is not already on(M20 cam comes on no sooner then 4k anyway)
So a strong M20 low end simply means a flat and boring high end as turbos normally do not match both sides of the M20 camshaft.

[GeoffBob]

Gunni, I'm pretty sure that one day, if you make your explantion sound long and complicated enough, you'll actually be able to change the laws of physics.

Sorry, that sounds nasty. Not intended that way. But I'm just not in the mood to debate pseudo-science today.

[Gunni]

Next year I´ll get some pictures up