TeufelHei

I'm looking for some input on my race project.

Here's the stats:
Stock 16v US L-Jet Engine (Will get fresh rebuild/reseal)
Add Headers (TBD)
Add Adjustable RRFPR/Dampner
Long term forced induction plans

My understanding (limited) is that it is generally less restrictive to have a single larger pipe than a pair of smaller pipes due to reductions in contact area and an increase in usable flow diameter. Since I have a choice, and I will be running the absolute minumum restriction allowed for the track (only), which should I go for; a Y-pipe or an X-pipe?

Any advice on the headers? I've got a set of manifolds that look an aweful lot like the one's on my '85 Euro sitting on the shelf. Anyone know if there is a way to tell the difference? I'm not 100% on the car's history so it is not impossible that it had some parts swapped. The current exhaust is the ANSA replacement.

FlyingDog

Read Louis Ott's write up on exhaust sizes. IIRC, 2x2.5 and 1x3.5 flow about the same on the high end, but the duals have more low end torque and sound better.

PorKen

The way I've come to understand it, the duals extract better at lower rpms due to higher core gas speeds. From the center out, the speed of the gas slows down, so the larger pipe has less of a percentage of low speed gas.

The large single pipe may be lighter overall and fit better under the '81.

Lagavulin

I get that sometimes.

mark kibort

Hmmm lets see, i run 3.5" single out of devek level 2 pipes. 293hp to the wheels out of an Ljet 5 liter euro, 243 hp out of a US (tired) 4.7 with only euro intake and cams, and now 335 rear wheel out of a 5 liter S4 with 85US cams.
sounds like that works well. And, there is something to be said for less mufflers and pipes to install and change if you need to change the db output

i think euro 85 manifolds are the same as US 84 and earlier manifolds. JUNK!

MK

Cheburator

Just to confuse you further - my track BMW e30 320iS - 2000cm3 Italian market only 4-cylinder M3 engine - puts out 210bhp on the crank at 7500rpm. The same engine in race trim (Gruppe A) with sharper cams, CF intake and Alpha N puts out 330bhp on the crank at 9000rpm!!! That's 165bhp/litre in NA form...

What is interesting is that the exhaust shape and size stays the same - no matter whether you buy a street car or a Gruppe A monster. I have 2-into-1 and 2-into-1 equal length cast headers, then a H-shaped bridge between them and then a X-over, all original and made by BMW Motorsport GmBH. The diameter is 55mm all the way from the cylinder head to the back box...

Go figure about the presense of the H-shaped bridge, the X-over and the equal diameter of the downpipes? If it worked for the M3 - most wins for a single race car model ever. then...

P.S. You can see the H-shaped bridge on the LHS of the pic. The X-over follows immediately

mark kibort

yes, stop worrying about improving max torque, its meaningless. its all about max hp and again, better said, max hp under the operational range. if you use our cars from 4500rpm to 6400rpm , then it really doesnt matter if you make a little more at 4000rpm. if you want more power at 4000rpm, downshift! driveability is different.
personally, ive never seen a loss in torque at max torque with any of the changes of headers, cat removal, larger exhaust sections, etc. worst ive ever seen is same torque peak, but more hp. usually, with headers for example, i saw 12hp gains and 12 ftlbs of torque gains too. added free flow exhaust, 12 more hp and 12 more ftlbs of torque. so, there goes that theory!.
mk

Vlocity

The other unsung advantage to a single pipe is there are less frictional losses.

Ken

IcemanG17

Single pipe will be lighter than the dual setup as well, which always helps a dedicated track car! For your car I would go with Devek Level 2 headers-custom Y pipe and single 3.5" all the way back, just enough mufflers as you need to reach the sound requirement....that plus a RRFPR should give you a nice HP gain!
Brian

bcdavis

Looking at the exhaust on that BMW, I am reminded also, that my 280Z had a change in the factory heads, that yeilded quite a bit more horsepower. Know what it was? Square exhaust ports, versus round ones... Apparently the turbulence created from square tubing was less than with round... Not a very helpful or practical fact for a 928, but an interesting fact regardless...

Daniel Dudley

For the early US spec cars, Borlas are popular. I hear the early manifolds are poor and that headers make the big gains. What is Jim Bailey running on his car? I have a european cat delete free flow on my 80. Car runs real nice for a US model. Particularly nice, but then that manual tranny has good gearing that makes the car feel real quick from 60 to 90.