Rocker arms & Cams

[Mr Deagle]

im going to be using a schrick 304 cam in my m10 turbo, with schrick 064 valve springs, and titanium retainers (springs have 50 lbs on seat, 200 at max lift)

redline should be close to 8k

break or no break?

[backtrail69]

I hope you have a fairly large turbine for that cam...and a very free flowing manifold/exhaust.

[Mr Deagle]

see m10 turbo thread

[winstontj]

backpressure is never good, it's exhaust velocity that you are after!

On a car with a cat usually the cat is the single biggest point of restriction but that aside usually you tune the exhaust runners/header for the type of power and flow you want (or match it to the motor you have built). I meant backpressure more in that if I open it up and run a full header with open pipe it'll kill low end more than keeping a closer to stock exhaust.

Hope I explained that correctly. Certanly not trying to get into an exhaust argument regarding a car & motor that I hardly know.

[nando]

yeah, what you are saying is if you open it up too much you lose exhaust velocity. backpressure is a myth ;)

[SA E30]

From what I understand from a bunch of tuners is that backpressure is bad... and then others say it isn't.

From my understandin I would think the least back pressure is best, as the u have exhaust gasses exiting quicker... meaning more space for fresh air 2 enter the cylinder ?

[winstontj]

Starting two different arguments in the same post:

Mr Deagle - I hope you get yourself a set of billet rocker arms & decent conrods for that motor of yours. ANYTHING over 7,500 rpm with stock rockers is risky and you're asking for it with a cam like that. What CR are you going to run? Also speaking of the cam - you do know that with a forced induction (FI) motor you NEVER WANT VALVE OVERLAP right?? If you have valve overlap (think about it) you'll simply push your hard earned compressed intake air right out the open exhaust valve. We do things like 2 into 1 or 3 into 1 headers & manifolds so that the open exhaust valve helps draw the intake air into the cylinder and start the intake stroke more easily. You do not want this with a FI motor. You need a custom cam with lift profiles but make sure you don't have overlap or you're pretty much going to waste your motor. Stock BMW camshafts (e30 m10, e21 m10 or 2002 m10) are fine for turbo applications but Schrick 304 cams are overlap cams - please do your homework before you spend all that money on a part that will only hurt your performance.

Another secret you may already know is that turbo lag is largly based on exhaust restrictions. If you can get away with it run the largest, least restrictive exhaust you can from the turbo back. Side exhaust & straight 3" pipe would be ideal IMO but it would be loud as hell and attract attention when flames shoot out of the side of your car every shift :-)


SA E30 - I think I may have stated "backpressure" incorrectly or maybe I'm being a little misread. I'm no expert but here's my take on things...

Many tuners agree that an ICmotor is an air pump and easy air in & easy air out = a good motor. Since most of us have stock exhausts or at least run catalytic converters I would say that the point of restriction is that converter so anything after that is simply heat & noise management. Anything prior to the cat is open for debate. Torque is very much a function of exhaust back pressure (believe it or not). If you have a car with BIG OPEN headers you'll notice that you will sacrifice low end torque. Also consider that people usually don't put full race headers on a stock motor so there are usually other mods (cam, lighter rotating mass, etc.) but that aside it's a rule of thumb that holds true. If you were to take a stock motor and put different types of exhaust on the motor bigger & better flow = less back pressure & less torque (low end). More restrictive or add back on that cat and all the sudden you'll have your torque back.

We have our X pipe and 2 or 3 into one setup to aid in dynamic gas flow. (read up on it at e30m3performance dot com and other places) With street motors (with cats) it really doesn't matter what we do prior to the cat but if you remove the cat you can actually tune your exhaust with different size header tubes as well as the length before the bend, bend radiuses and runner length between gasket and the split joints (where the pipes join). Just as we tune intake runners common plenum or ITB for torque & power band curves we can also tune exhaust for exhaust gas flow on the other end. It's just managing wavelengths and pulses.

Does any of this make sense or am I WAY off base here??

[Massive Lee]

Hey Winston.

I concure with you on all points mentionned.

[winstontj]

HI LEE!!!

Considering that you & anthony are the ones that taught me most of this stuff on the FAQ that's good to hear!

[nando]

SA E30 - I think I may have stated "backpressure" incorrectly or maybe I'm being a little misread. I'm no expert but here's my take on things...

Many tuners agree that an ICmotor is an air pump and easy air in & easy air out = a good motor. Since most of us have stock exhausts or at least run catalytic converters I would say that the point of restriction is that converter so anything after that is simply heat & noise management. Anything prior to the cat is open for debate. Torque is very much a function of exhaust back pressure (believe it or not). If you have a car with BIG OPEN headers you'll notice that you will sacrifice low end torque. Also consider that people usually don't put full race headers on a stock motor so there are usually other mods (cam, lighter rotating mass, etc.) but that aside it's a rule of thumb that holds true. If you were to take a stock motor and put different types of exhaust on the motor bigger & better flow = less back pressure & less torque (low end). More restrictive or add back on that cat and all the sudden you'll have your torque back.

We have our X pipe and 2 or 3 into one setup to aid in dynamic gas flow. (read up on it at e30m3performance dot com and other places) With street motors (with cats) it really doesn't matter what we do prior to the cat but if you remove the cat you can actually tune your exhaust with different size header tubes as well as the length before the bend, bend radiuses and runner length between gasket and the split joints (where the pipes join). Just as we tune intake runners common plenum or ITB for torque & power band curves we can also tune exhaust for exhaust gas flow on the other end. It's just managing wavelengths and pulses.

Does any of this make sense or am I WAY off base here??

again you are on track but it's not lack of backpressure that decreases torque. the reason you lose torque when you go to a very large exhaust is because the bigger pipes cause the flow of the exhaust to move slower -- thus the term velocity. a pipe that is sized correctly for the engine will keep exhaust velocity as high as possible without having much backpressure (which is never a good thing).

really it's a difference of terms, because you are meaning what I said. myth vs science :p

[Dave]

dammit, you people are talking real science and not spreading misinformation!

this is r3v for heaven's sake!!!!!!!!

[Mr Deagle]

Starting two different arguments in the same post:

Mr Deagle - I hope you get yourself a set of billet rocker arms & decent conrods for that motor of yours. ANYTHING over 7,500 rpm with stock rockers is risky and you're asking for it with a cam like that. What CR are you going to run? Also speaking of the cam - you do know that with a forced induction (FI) motor you NEVER WANT VALVE OVERLAP right?? If you have valve overlap (think about it) you'll simply push your hard earned compressed intake air right out the open exhaust valve. We do things like 2 into 1 or 3 into 1 headers & manifolds so that the open exhaust valve helps draw the intake air into the cylinder and start the intake stroke more easily. You do not want this with a FI motor. You need a custom cam with lift profiles but make sure you don't have overlap or you're pretty much going to waste your motor. Stock BMW camshafts (e30 m10, e21 m10 or 2002 m10) are fine for turbo applications but Schrick 304 cams are overlap cams - please do your homework before you spend all that money on a part that will only hurt your performance.

Another secret you may already know is that turbo lag is largly based on exhaust restrictions. If you can get away with it run the largest, least restrictive exhaust you can from the turbo back. Side exhaust & straight 3" pipe would be ideal IMO but it would be loud as hell and attract attention when flames shoot out of the side of your car every shift :-)

Does any of this make sense or am I WAY off base here??

Thanks for the Blog.

The overlap is actually conservative on that cam, back in the group 5 days, the boys from Porsche and our friends at BMW would run cams in the neighborhood of 5.5-6.5 lift at TDC overlap. The way this works is you have to have the turbine side properly sized. When i dyno my engine (engine dyno) we will be measuring exhaust back pressure, and we will verify the sizing of the turbo by keeping the pressure delta negative on the exhaust side.

Mitch

[winstontj]

Thanks for the Blog.

The way this works is you have to have the turbine side properly sized. When i dyno my engine (engine dyno) we will be measuring exhaust back pressure, and we will verify the sizing of the turbo by keeping the pressure delta negative on the exhaust side.

Mitch

Ummm.... CHECKMATE! You are a little above me here, if you have the time could you explain "pressure delta negative on the exhaust side"? Maybe I know it in a different way but I'm not sure.

[Mr Deagle]

We are talking about less exhaust back pressure than inlet boost pressure.

[Massive Lee]

Mr Deagle, do not forget that if your run a peaky cam, chances are that your turbo will need to run a much higher pressure. The 1.4 litre Formula One 4 banger was running something like 3 bar (that is about 45psi) turbo pressure.

It is true that the 304, being a rally cam, is not that peaky, but it will not easily fill up like a 284. A lot of that good pressure will go thru the exhaust valve(s).

A friend of mine has built a reliable 2 litre M10 to 240HP (rwp) in his 2002. That thing is not the most powerfull thing, but it is torquey and haul ass.