Trying to decide what size exhaust to run

[Sam K]

I'm looking to replace the exhaust system on my bug and I'm not sure what size to get. The engine is a 2332cc with Jeff Denham 44 x 37.5 heads, 9.5 to 1 compression, a web-cam with .498 lift and 312 duration, 48 IDA's, etc. The car currently has a 1 3/4 merged system with a large flange and a 2.5 inch Magnaflow muffler. I'm looking to change the exhaust for a set of Flat 4 dual quiet packs that I've recently purchased. I have access to a 1 3/4 system and a 1 5/8 system. I've thought for a while that the current system was a little too big because the car doesn't have a ton of power down low. I've measured the exhaust ports on the heads and they come out to 36.26mm. The ID of the 1 3/4 system is 40.37mm. The ID of the 1 5/8 system 38.92 mm. Both of them have the smaller flange to allow me to ue my new dual QP's. I appreciate any input as I'm not too knowledgeable on this stuff. Thanks!

[andrewlandon67]

I wonder if you could measure the lengths of the primary tubes on each system and use that data as well... also, are your exhaust ports perfectly round or are they a little asymmetrical?

[nicolas]

from what you post, the specs of the engine do lean towards the 1 3/4 exhaust. you are correct that small flange will affect the power. my experience is that with a similar setup the diameter in the flange changes the characteristics of the engine. I would opt for a small flange 13/4 and go from there. a more specific and better solution is to measure it all and even dyne test it.
I went from a 1 5/8 type3 exhaust (SS) to a 1 5/8 type 3 (Turbo Thomas) to a 1 5/8 (csp) and all were different on the same engine. the best compromise (performance, looks, sound) for me was CSP with a selfmade funnel for the flange as it lost too much bottom end.

[TexasTom]

Keep the 1 3/4" system and raise the compression to 10.5:1, you'll get your bottom end!

[Bill Schwimmer]

I was reluctant when I went from 1 5/8 to 1 3/4.  I made the switch & it ran better everywhere. No downside & it felt like it picked up at least 10-15 hp. The ETs dropped  to reflect this.

[Sam K]

Keep the 1 3/4" system and raise the compression to 10.5:1, you'll get your bottom end!

At one point the compression was 10.5 to 1. I also had a close ratio transmission with solid mounts. I was drag racing the car pretty regularly then too. It was fun to drive for short trips but the car sucked to drive for any distance and I missed driving my car. After a couple years I dropped the compression down, put a stock geared transmission back in it with stock mounts and I was much happier with it. Now I race an '89 Mustang.

[Andrew]

CSP recommend a 42mm for my 1956cc, it works well, but I do think I have lost a little on the bottom end when compared to the previous 1.5 inch. I may look into a CSP flange.

[andrewlandon67]

So I'm gonna slightly hijack this thread. I'm not seriously looking at changing anything on my car for the time being, but I'm curious about the Vintage Speed canister exhaust vs the more traditional merged header. I seem to recall some discussion around the differences between the two some time ago, but I can't remember many specifics. I know the merged system is better for high-end power, which is advantageous for drag racing, and is generally cheaper and more compact than the big canister, so I can't help but wonder what the purpose of those really is. I know Porsche used similar mufflers on most of their road cars, so if they're designed right, they probably don't hurt performance too much. I'd like to know what the inside of one of these looks like, and whether or not it aids with the extraction of exhaust pulses from the cylinders like a merged system does.

[Joel Mohr]

1 5/8ths primary 3 step....

[Torben Alstrup]

So I'm gonna slightly hijack this thread. I'm not seriously looking at changing anything on my car for the time being, but I'm curious about the Vintage Speed canister exhaust vs the more traditional merged header. I seem to recall some discussion around the differences between the two some time ago, but I can't remember many specifics. I know the merged system is better for high-end power, which is advantageous for drag racing, and is generally cheaper and more compact than the big canister, so I can't help but wonder what the purpose of those really is. I know Porsche used similar mufflers on most of their road cars, so if they're designed right, they probably don't hurt performance too much. I'd like to know what the inside of one of these looks like, and whether or not it aids with the extraction of exhaust pulses from the cylinders like a merged system does.

The cannister type mufflers like VS and Porsche 356 with shorter (equal length) primaries generally improves lower to mid rpm torque at the cost of torque above peak and hp peaking higher than about 5000 rpm. Dependant on the length of the primaries of course. There is a sports version of the 356 muffler where the pipes from 2 & 4 runs over the muffler and into the back side to make the primaries equal length. Compared to a stock muffler this improves lower rpm up to about 3500 with approx 3% with nothing else done, and released 3-4 hp more on top too. Still on a stock 90 hp SC engine. If you change the tune and/or displacement the difference will be more evident of course.
Same thing with the VS equall length muffler along with the BAS Customsport and the TT stock style muffler. As long as you design the engine to peak around 5500 rpm the mufflers work rather well, and makes superior torque below approx 3300 rpm compared to a fourtuned header. The newer SS143 equall length muffler from VS is actually a very nice muffler. It is a shame though that they did not make an equall length version that can be used with heater boxes. That would have been killer for us northeners.
So, if you like "simplicity" and don´t care for a little less rpm hp these systems are fine. If you want a spirited engine you may want to look for other solutions.

[andrewlandon67]

So I'm gonna slightly hijack this thread. I'm not seriously looking at changing anything on my car for the time being, but I'm curious about the Vintage Speed canister exhaust vs the more traditional merged header. I seem to recall some discussion around the differences between the two some time ago, but I can't remember many specifics. I know the merged system is better for high-end power, which is advantageous for drag racing, and is generally cheaper and more compact than the big canister, so I can't help but wonder what the purpose of those really is. I know Porsche used similar mufflers on most of their road cars, so if they're designed right, they probably don't hurt performance too much. I'd like to know what the inside of one of these looks like, and whether or not it aids with the extraction of exhaust pulses from the cylinders like a merged system does.

The cannister type mufflers like VS and Porsche 356 with shorter (equal length) primaries generally improves lower to mid rpm torque at the cost of torque above peak and hp peaking higher than about 5000 rpm. Dependant on the length of the primaries of course. There is a sports version of the 356 muffler where the pipes from 2 & 4 runs over the muffler and into the back side to make the primaries equal length. Compared to a stock muffler this improves lower rpm up to about 3500 with approx 3% with nothing else done, and released 3-4 hp more on top too. Still on a stock 90 hp SC engine. If you change the tune and/or displacement the difference will be more evident of course.
Same thing with the VS equall length muffler along with the BAS Customsport and the TT stock style muffler. As long as you design the engine to peak around 5500 rpm the mufflers work rather well, and makes superior torque below approx 3300 rpm compared to a fourtuned header. The newer SS143 equall length muffler from VS is actually a very nice muffler. It is a shame though that they did not make an equall length version that can be used with heater boxes. That would have been killer for us northeners.
So, if you like "simplicity" and don´t care for a little less rpm hp these systems are fine. If you want a spirited engine you may want to look for other solutions.

Currently I have a 38mm merged heater box system on my '67 and I'm not particularly interested in changing it at the moment, but I got curious and needed some answers, so your response is perfect! I think it's interesting that we hotrod VW guys always stick with the drag-racing high rpm merged systems, even when we're looking for something more similar in performance to the Porsches with the big canisters. Hopefully someday someone makes something like you're talking about, an equal-length muffler for heater boxes would be awesome, and would definitely help the ground clearance issues presented by the traditional merged systems.

[Sam K]

I'll go ahead and hijack my own thread back. I ended up installing a small flange 1 3/4 system with Flat 4 dual quiet packs in November as well as a set of Gene Berg grooved venturis. Spent a bunch of time retting and tuning the car throughout December, January and February when the weather permitted. Unfortunately the car just didn't pull as hard with the old big flange exhaust and magnaflow muffler. I tried a friend's A1 muffler and that helped a bit but it still wasn't there. A couple weeks ago I put the old exhaust back on and the car ran like a beast again. I guess that engine just needs to breathe. I put the jetting back to where it was before as a baseline and started to rejet for the venturis. I took it out on Saturday and it felt really good up until the point where the transmission blew up. Back to the drawing board.

[brewsy]

I took it out on Saturday and it felt really good up until the point where the transmission blew up. Back to the drawing board.

Sorry to hear that Sam!!

If its any consolation I just found this in my old bookmarks.

http://www.wallaceracing.com/header_length.php

As Im bored and locked down I just did the math (thats a Dad joke BTW. I live in the UK so it should be Maths  )
According to the cam specs Im guessing thats an Web 165 cam?
http://www.webcamshafts.com/pages_vehicles/automobile/volkswagen_install_data/tc_000670_002750.html

Assuming max rpm of 6500 and a VE of 85%
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You get:
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So which exhaust best matches the (calculated) theory then??

Cheers
Marc

[Sam K]

It is indeed a Web 165 camshaft. I'm not sure what the ID of my exhaust tubes are but the collector is 2.5 so that makes sense. As for the transmission, I dug out a "mystery" built swingalxe that I bought a few years ago and I'll probably throw it in soon. I was going to use it in my Notchback but that car is still along way from the road so it will have to go in my bug until I can get the funds together for a new one. I'd love to get a Berg 5 speed.

[andrewlandon67]

That's a bizarre calculator... by that, my 1914 should have primary tubes SMALLER than stock, although the collector diameter is about right. I wonder if it's measuring inner or outer primary diameter...

[richie]

That's a bizarre calculator... by that, my 1914 should have primary tubes SMALLER than stock, although the collector diameter is about right. I wonder if it's measuring inner or outer primary diameter...

Inner, it doesn't know how thick the tube wall is 

[andrewlandon67]

That's a bizarre calculator... by that, my 1914 should have primary tubes SMALLER than stock, although the collector diameter is about right. I wonder if it's measuring inner or outer primary diameter...

Inner, it doesn't know how thick the tube wall is 

Good point! Of course now I want to go break out the micrometer and pull off my header, but that'll have to wait until I can get down to where my car is, which will easily be another month... Odds are though it's bigger than what they recommended, but probably not by much.

[Jimbosmith]

In the interest of not starting a new thread.  Im also trying to decide what size exhaust to run. Im after a setup that i can drive to shows with and be able to use on the strip.

i was thinking of a 1 3/4 into a 2 1/2 muffler but im thinking i might be too big here

Build is 2276

CR is 9.6:1.
fk 8 w/1.4 rockers
Twin 48Idf carbs
Scat super D heads 42x37mm
CB performance 82mm counterweighted crank
Lightend flywheel
All balanced.

Any thoughts?