Free flowing exhaust-loss of power at low RPMs?

[david c.]

Hi guys,

Some of you might recall that I have been fighting with the 2.0 in my 912E all summer.

I think I have solved most of my carb and ignition problems, as the car is idling smooth, not popping anywhere in the rev range (on accel or decel) and has decent power above 3200 RPM or so.

However, there is very noticeable flatness when pulling out up to about 3200 RPM.

For example, when I first autocrossed the sucker last year, it pulled strong out of slow turns while in second. Now, I must downshift to first to get any speed out of it. And, for those of you familiar with the 915 gearbox, downshifting into first while moving "ain't exactly easy"!

This spring, I removed my rusty old Bursch bypass pipes, factory h.exchangers and a Bursch dual in/single out muffler. I installed a pair of exhaust manifolds from a 76 VW bus, with some custom pipes and two glasspacks. The exhaust is completely split. I did this out of necessity, not because I thought it was the best option. I'm still looking for a clean pair of the bypass pipes so that I can reinstall a proper, scavenging system. I also owe a big favor to Doug Robinson at Bursch for helping me out with some custom equal length replacement pipes for the rust factory heat exchangers.

I'm wondering if my problem lies in the exhaust?

Any ideas?

Thanks,

David

[chibone_914]

Hi David,

Typically when you over-do-it on exhaust, you will suffer from a loss of power in the lower RPM range whereas it will have more of a toque band in the higher RPM ranges. How big a primary tube did you go with? Typically, 1 5/8" works well with a mildly built motor and you won't have much of a problem from 1200-3000 RPM range.

[norustscott]

By splitting the system 12-34 you have lost some scavenging effect. Typically this will show itself in the form of lost torque @ mid range rpm. Back pressure may have some affect as well.
If the flow has ben dramatically reduced you may find that opening th main jets will help.

My advise is to put it back as close to a stock configuration or atleast and maybe better, to a 4 into 1 with a merged collector...

[Brad Roberts]

1 5/8 on a 2.0 engine ??

Not unless it spins 7k RPM.

Bigger is not better. Say it with me..

Plain and simple, you lost torque below 3200.

If you buy/build something do it with 1 1/2 which is what the factory used for the most part on everything until the 3.6 engine.

B

[maf 914]

The firing order of a Type 4 is 1-4-3-2. When you split the exhaust into two pipes serving 1-2 and 3-4 you get some really unbalanced pulses in each pipe. This does not work well on t4's. Check out RacerChris's header or even a Bursch, both of which are 4-2-1 types. They both combine 1-3 and 4-2 before bringing these two together to maintain even pulses and obtain an extractor effect.

Mike

[david c.]

thanks guys, it makes sense-I just wasn't sure. Like I said, it was a quick fix so that I could drive it while I locate the needed parts. But, in the meantime, I wanted to eliminate other, non-exhaust related potential problems.

David