Exhaust Options

[Z3_GoCar]

Well, I've finally taken delivery of my Z3 racer. It's got a set of 6-into-2 headers, but there's no exhaust after the two collector pipes. My plan is to fab a two into one pipe and route that back to the rear sub frame and end the exhaust there with a clamp and a down turn/muffler depending on track noise requirements. So what I'm wondering is how long should the two go before they merge and also how big should the merged pipe be. This is on a 2.8l with the power band of 3500-7000 rpm. Thanks,

James

[C. Ludwig]

L= ( (1080-EPD) X 650 / (RPM X RV)

L= Legth of the runners. This is your answer.
EPD= Effective Port Duration (how long they are open for)
RPM= The spot where you want peak horsepower to be. (If you still have the stock gear ratio transmission and this is a streetcar there is absolutely no point in making this anything other than 6500-7500 rpm.)
RV= This stands for Reflective Value. The pressure wave reflects back and forth several times inside the pipe. For the intake the second wave is best so use the numeric value of 2. For a carburated car use 3 or 4 since the manifold may be too long. If you are figuring out exhaust length use 1. this will give you the proper length for a short primary collected system. If you want a long primary system, take the short length and multiply it times 4.

[Z3_GoCar]

Hey Chris,

Thanks, I've got a couple of questions.

On the EPD, is that the exhaust cam duration in degree's from the 0.05 opening to 0.05 closing?

I'd assume that my system will be a long tube system as this will include the 2-1 y-pipe, would you concure?

Thanks,

James

[C. Ludwig]

To be honest I've never applied it to a piston engine. Only rotarys and 2-strokes. With those we always used the degree at which the port first cracked open. With a piston engine and the way cam shaft duration is rated that would be pretty hard.

Also, with your tri-y setup the tuning is going to be different. That formula assumes a single collector. I didn't really think your tri-y setup through when I posted it. Let me think about it. Hopefully a physicist will chime in first. :)

[John Herman]

What can you tell us about the primary pipes? Length and diameter? I'm assuming this is a IT car?

[Z3_GoCar]

Hey Chris,

I think I've seen that equation before, in a book about tuning 2-cycle resonance chambers. Unfortunetly, I got that book from the public library half a state away. It just makes me nervous when I see a number without any indication of units (aka an empirical fudge factor), so that's why I asked about the units. So should the rpm peak be at the projected torque maximum, or somplace between the torque peak and the hp peak? Thanks.

John,

Yes, it's an IT car. The headers are in two parts, first part covers cylinders 1-3 while the second covers 4-6. The primary tubes are ~24" and are 1.375" I.D. (1.5" O.D.) and the two collectors are ~12" and are 2.0" I.D. This type of header actually makes sense since the firing order is 1-5-3-6-2-4 so that each set of three alternate. Thanks.

James

[John Herman]

Try this;
P=((850 x ED)/rpm)-3
where:
P= TOTAL pipe length
ED = 180 deg + the number of degrees the exhaust valve opens before BDC (assume @ 0.050, though the paper I have is not clear on this)
rpm = tuned rpm

There are other formulas to calculate the various diameters for the secondarier/primaries.

I cranked this stuff through the formulas I have. Based on this info, and making some assumptions, the formula says you want a total length of around 30-34 inches, and the secondaries (from the other formulas) at 2 to 2-1/4 id. The formulas I use assume a primary of 15", so yours at 24" may be a little long.(Unfortunately I don't have the reference for these formulas, they came from a co-workers old copy). But formulas will only get you so far. Based on my experiences and your parts you all ready have, I would recommend the following. Build the secondaries out of 2-1/8" od pipe. Make them approximately 18" long. Get them into a straight area as quickly/easily as possible. (Edit: I missed the fact that your collectors are 12" long. I thing I would shorthen them to about 3" if it was easy to do. Otherwise, I would only make the additional secondaries only 6-10") This way, you can experiment with different lengths and diameters more easily/cheaply. There will be power/torque to be found just in tuning the secondaries. Join them into a 2-1/4" collector about 5" long, then join this into a 2-1/2" pipe. Experiment with the length and diameter of the exhaust pipe. I know a lot of people just dump the exhaust in front of the rear axle, because this is "best", but unless you do a lot of testing, you'll never know for sure. I think this will get you close, but if you really want to delve into this, nothing can replace building and testing components.