filmaustin 0 Report post Posted October 8, 2009 Hey, I have just bought some headers for my M20B25, They are stainless three into one (six into two) so two pipes but they have an opening between the two pipes after the bends from the manifold end. I guess this is to help with pressure. MY QUESTION IS... Shall i join them to one pipe or take two pipes right through to the rear muffler then out the back? I would love to hear your thoughts.. Cheers Matt Quote Share this post Link to post Share on other sites
antil33t 90 Report post Posted October 8, 2009 My E30 has two pipes all the way to the rear muffler and out (from factory) most do? I'd assume that would be the best way to go? Quote Share this post Link to post Share on other sites
Deano 1 Report post Posted October 8, 2009 I just removed a twin pipe system from an E30 325 and the weight is huge. Just using a single pipe will save weight and less weight=more hp. The standard BMW exhaust system is efficient. Using headers is not standard so 6 into 2 then into a large diameter single pipe to the rear should provide free flow with no restriction. I am about to investigate this for my E21. My friend has just had headers installed on his E21 and is 6 into 2 then single pipe to the rear cross member where it divides into 2 again with 2 small mufflers either side at the back. Sounds awesome and improved mid range. Quote Share this post Link to post Share on other sites
pureboiracer 0 Report post Posted October 8, 2009 no real difference, Nothing noticable anyway. as long as the joins didnt interupt the flow too much it would be ok. the key to a performance exhaust is having the right amount of flow and backpressure. this can vary depending on motor cc motor mods and where you want your power to be. different size piping and different types of muffers can alter the flow and backpressure. I have a factory manifold running 2 2" pipes with 2 cobys in each pipe, goes well and gives me alot of top end power as the flow is high due to the straight through cobys. but being only 2" for each set of 3 cyl there is adequate back pressure, also the small pipe diameter causes the exhaust gases to flow rather fast which helps with the scavaging efect. Quote Share this post Link to post Share on other sites
Deano 1 Report post Posted October 8, 2009 no real difference, Nothing noticable anyway. as long as the joins didnt interupt the flow too much it would be ok. the key to a performance exhaust is having the right amount of flow and backpressure. this can vary depending on motor cc motor mods and where you want your power to be. different size piping and different types of muffers can alter the flow and backpressure. I have a factory manifold running 2 2" pipes with 2 cobys in each pipe, goes well and gives me alot of top end power as the flow is high due to the straight through cobys. but being only 2" for each set of 3 cyl there is adequate back pressure, also the small pipe diameter causes the exhaust gases to flow rather fast which helps with the scavaging efect. I hear back pressure crop up all the time..what is the meaning. Race cars just have a big straight pipe with a minimal bends and no silencing there fore no back pressure and they are tuned for maximum Hp? Quote Share this post Link to post Share on other sites
bravo 34 Report post Posted October 8, 2009 It's a bit of a misnomer. If your pipe is too small you get backpressure which can "choke" your engine and reduce exhaust scavenging. If it's too big you can lose gas velocity which can cause other problems. People refer to this as being due to not enough backpressure, but that's just a misunderstanding of the principles behind it. Quote Share this post Link to post Share on other sites
|ncary 0 Report post Posted October 8, 2009 The pipe after where the headers all merge doesn't matter that much. It's all in the headers length, diamater and the distance from the engine where the collecter is. Read this actually http://www.e30m3project.com/e30m3performan...pipe/x-pipe.htm Quote Share this post Link to post Share on other sites
Deano 1 Report post Posted October 9, 2009 The pipe after where the headers all merge doesn't matter that much. It's all in the headers length, diamater and the distance from the engine where the collecter is. Read this actually http://www.e30m3project.com/e30m3performan...pipe/x-pipe.htm Thats good reading. When I put the 2.7 together in my E21 I used original the 2 piece 3 branch manifold. I matched it to the 885 head exhaust ports best I could but the original downpipes are smaller diameter than stock 325 as is the rest of the exhaust. The engine makes excellent low to mid power and pulls to redline but there is no surge in power after 4500 like a stock 325 so I am thinking the exhaust is restrictive. I have a stock 325 manifold and down pipe and will get new exhaust made up to the rear and see what happens. Will be interesting to notice and difference. What are the best headers we can source here? Quote Share this post Link to post Share on other sites
charles28 117 Report post Posted October 9, 2009 Thats good reading. When I put the 2.7 together in my E21 I used original the 2 piece 3 branch manifold. I matched it to the 885 head exhaust ports best I could but the original downpipes are smaller diameter than stock 325 as is the rest of the exhaust. The engine makes excellent low to mid power and pulls to redline but there is no surge in power after 4500 like a stock 325 so I am thinking the exhaust is restrictive. I have a stock 325 manifold and down pipe and will get new exhaust made up to the rear and see what happens. Will be interesting to notice and difference. What are the best headers we can source here? From what I've read the surge after 4500 doesn't really happen with the 2.7 stroker engine, I noticed that with mine also and it has nice headers etc on it already, then again mine only had the 731 head. My 2.7 claimed to have a 323i cam with 260degree also, which should help this? Today when the 2.5 engine is running I'll compare to how it was before, the 2.7 was taken out, i'm pretty sure it will instantly pull harder to the redline with everything else left as. Quote Share this post Link to post Share on other sites
Guest FrantiC Report post Posted October 9, 2009 (edited) From what I've read the surge after 4500 doesn't really happen with the 2.7 stroker engine, I noticed that with mine also and it has nice headers etc on it already, then again mine only had the 731 head. My 2.7 claimed to have a 323i cam with 260degree also, which should help this? Today when the 2.5 engine is running I'll compare to how it was before, the 2.7 was taken out, i'm pretty sure it will instantly pull harder to the redline with everything else left as. m20b25 Pull harder from 4500rpm +. The 2.7 will dominate for having much more low end torque and would get to 4500rpm faster than the m20b25. My 327i pulled really hard from 4500+, Mine started a 10.2:1 ETA block though so had 9.5:1 CR with the higher flowing 885 head. Even more now with 12psi of boost being forced in there Edited October 9, 2009 by FrantiC Quote Share this post Link to post Share on other sites
