Expert: Dan Liddy - 10/3/2009

Question
Hi Mr. Liddy! I'm busy building a formula vee race car, that uses a 1.4l 4cyl VW golf motor, with twin 40 weber side draughts, and it revs to 7400rpm.

My question is on the exhaust system design. What is the mathematical approach to designing race car exhaust systems? I know some of the guys use different header lengths at altitude and at sea level. My current system seems to follow the 1 swept cylinder volume header design, 4 into 2, into 1, but the lengths for the 2ndary and final pipe don't seem specific.

Answer
Hi Geoff:

Thanks for writing !
Yes, I use the cylinder swept volumn approach too. but since I race sedans, I place an expansion chamber just aft of the 4 into 1 collector.  The size should be about 2 cylinders worth, perhaps a bit more. The large diameter doesn't matter as much as the volumn. Oh and you should have long stacks on the carburetors too !  

I would dump the 4 into 2 system , I've never known anyone able to make it work and I get the same answers from friends. My neighbor races an old Porsche 356 in Vintage and has the typical VW-Porsche header culminatinng into a simple blooie tube out the back, I've seen them bring a larger pipe from the collector into about a 2 1/2" expansion chamber across the back of the car, but they frequently get bumped and you end up with a busted header, and out of the race. The primaries are about 1" diameter but don't hold me to it, he's out of town 'til tomorrow.

Let me ask you - - -you said a Golf motor ? You mean the inline watercooled 1.8L ? or is it the real VW air cooled flat four ?

When You answer with a follow-up I will have asked him about tuned units for those motors. He thinks he know more than I do. ( He's probably right ! )

In the mean time give some thought to the expansion chanmber idea.

Dan Liddy
Sarasota, Florida