Best exhaust for 3.8 air cooled beast

[rosenbergendo]

Engine builder is almost done with basically air cooled GT3 motor. Full rebuild with gt rs 4.0 crank, gt3 oil pump, special pistons and cams. Fully rebuilt 915 with all the goodies. Any thoughts on best exhaust setup. Have a M&K 2/2 and was going to pair with these 1 3/4 headers. Any thoughts.

[-Levi-]

I don’t know if this is the “best” setup, but I'm running s-car-go 1 7/8 headers and a factory gt3 exhaust with electric cutouts. Great sound, and I can tone it down a bit if needed.

[Axel84]

Send an email to brian at M&K he Will sort you out

[Bill Verburg]

No such thing

It depends on you, you tastes and usage

I like Levi's a lot but, I would want heat

In general GT3 mufflers are great sounding but still a little loud for street use(JMO yours will differ) even when valved

I had a Ti Gt3 on mine for little while but soon went to a Fabspeed RSR, for heat and very usable and great sounding street use

If you can run unmuffled best power and spread will come from reverse cone megaphones

[rosenbergendo]

Don’t need heat.
I love loud.
One level below cookie cutters!

[Bill Verburg]

Originally Posted by rosenbergendo

Don’t need heat.
I love loud.
One level below cookie cutters!

Then a good set of 1 3/4" headers + GT3 muffler, don't bother w/ the valving just pipe to the lower inlet

or

headers + Burns or Coast Megaphone mufflers

[rosenbergendo]

Have those on my RS and gt3. Don’t like the center exit at all on air cooled.

[snbush67]

Heat;

[Chris78]

Another vote for M&K....beautiful looking and sounding and superlite (...I have the dual in single out sport and its sooo good)

His headers are also incredible

[JoeMag]

Regarding the center vs side exits, the center exits do sound different. …I think better. Looped mine around from side to center.

[winders]

That would be way too loud for any race track here in California....primary lengths seem a bit short too...

[JoeMag]

...Yea, here in the mid-west there is no problem with loud cars. They are a bit on the shorter side; Reid Washbon 1-7/8" 993 RSR headers. Will be re-assessing after seeing where tune comes out on latest motor.

[Turbo_pro]

Originally Posted by JoeMag

...Yea, here in the mid-west there is no problem with loud cars. They are a bit on the shorter side; Reid Washbon 1-7/8" 993 RSR headers. Will be re-assessing after seeing where tune comes out on latest motor.

Short side based on what?
I was of the opinion that primary length was loosely based on cam specs, primary diameter and desired rpm range. There is also an issue of back-pressure desire/requirements.
Given that these engine really only come alive in what most engine builders consider higher RPM ranges, shorted primaries [within reason] make perfect sense.
I never understood how so many Porsche engine builders are mezmerized by headers configurations designed in the 60's for cams with no lift and huge duration.
As for Reid Washbon exhaust, pure track jewelry. I had a few GTP engines with his art work on them and there is no comparison. Even on the IndyCar stuff, his exhaust is hard to match.

[Jeff Alton]

Is this a "short Primary" just asking.......

How long should they be Scott?

joeMag, how long are these?

[winders]

Originally Posted by Jeff Alton

Is this a "short Primary" just asking.......

How long should they be Scott?

joeMag, how long are these?

Yes, I would say that is quite short.

I don't have the exact length for the primaries but when I worked with Mark Robles who makes the headers from SCarGo, this is what they looked like:



William Knight had a set based on those that look like this:



Both have primaries that are quite a bit longer than what is in your photo.

[JoeMag]

My primaries are around 17.5-18"... I say they're short as I did some checking around when I was think'n about changing them several years ago, and did not seem like anyone made them this short. Assuming Scott's are 1.75", they appear to be about 25" long.

...that being said. I have really had no issues with them. In the prior version of my race motor, 3.75 (11.7 static CR, 127mm rod) with untouched 993 heads except bigger valves, the engine made between 250 and 275 ft-lbs torque (dynojet sae5 corr'n) from 4.2k to 7.2k with a 6cup intake on throttles. Granted, I have also not done any back to back testing with longer headers so I do not know effect, and any motor with Xtreme heads will blow those numbers away.

With 993's there is not a wide variety of header options bouncing around out there because they can not go forward of #3 and #6 head like 964 and earlier due to suspension cross brace.

For me, 90% of my WOT time is above 6k... Though it is nice to have a bit of torque down around 4.5k-5k to keep from needing to drop into a lower gear.

It barely ever gets talked about much here on the forum, but you can move your torque curve around a lot by changing intake runner length.

[JoeMag]

Here's better pic and what they looked like in side exit config...

[jeff951nj]

If you want one level lower than cookie cutters call Dawe's Motorsports and get a Yikes pipe.

[Bill Verburg]

Originally Posted by JoeMag

Here's better pic and what they looked like in side exit config...

The shorties aren't that far off if running to ~7500rpm

@7500 each pulse travels ~21in before the next pulse leaves the exhaust port

the lack of bends is a big plus

here's what the exhaust for a 993 RSR w/ sprint cams looks like



there are 2 sources of acoustic resonance to feed off of in the above set up
1" the collector
2) the big one at the exit to atmosphere

stock 993 Cup 39mm aren't too different from yours



The inconel RSR's are also similar

[Jeff Alton]

Originally Posted by winders

Yes, I would say that is quite short.

I don't have the exact length for the primaries but when I worked with Mark Robles who makes the headers from SCarGo, this is what they looked like:



William Knight had a set based on those that look like this:



Both have primaries that are quite a bit longer than what is in your photo.

I guess we see a different angle on the photo.... looking at collector location vs the #1 or #4 cylinder port.... But, I do see the paths/design of the primaries do differ. These may infact be a wee bit shorter...


Cheers