Optimum Exhaust Size

[zentac]

Anyone know what the best diameter is for a VR4 exhaust main pipework. Im guessing 3 inch.

Anyone know what the diameter of the standard exhaust is?

[EdmundVR4]

Stock exhaust pipe is 2.25" diameter which is the same as on the 7G VR4 which as you know has the 6A12TT INSERT INTO post VALUES (2000cc V6) engine. I think 3" mandrel bent pipe would be ideal.
Ed

[Brind]

3 inch is the diameter of the HKS cat back Super Drager.
I'll take Edmunds word for the 2.25" standard as I don't have a clue.

[zentac]

Thanks guys, 3" it will be then.

[Roadrunner]

Be careful. Replacing a complete 2.25" exhaust with a 3" one is likely to change the characteristics of the torque curve significantly. Expect maximum torque to be developed much higher up in the rev range. I did a similar thing with my Legacy RS-B INSERT INTO post VALUES (2.5" standard, replaced with 3") and it moved everything 1000 revs higher - lots of power at the top end, but much less torque at the low end. Probably a bit smoother to drive, but did take away some of the fun of the low-down torque.

As an aside, IIRC, works Evo and Subaru rally cars use a 2.5" exhaust front to back.

Brian

[mook]

That surprises me a little. I was under the impression that back pressure, or lack of it, was not so much of an issue with turbo cars as with normally aspirated, given that the turbo completely messes with the airflow anyway. My understanding was that a bigger bore would allow quicker spool up time and of course make it easier to expell greater amounts of exhaust gases.

Then again, a 12" bore would be over the top, so there must obviously be a peak level. I would have thought 3" was about right, I believe that is standard on a P1. I also know of people who've gone for 3" straight through on scoobies and found it to be of great benefit, although they didn't mention if everything was shifted up the rev range - definately interested to read that as I was thinking of increasing the bore of my exhaust with a custom job, but I do like low down torque so might need to think further on it

[calum]

In theory, for maximum power, after the turbo you want a 'conical' trumpet, increasing in size to as big as you can get under the car.

In practice, fit whatever you can get in. The design of the exhaust INSERT INTO post VALUES (good or bad) has more to do with how it performs on the car than the diameter.

Custom made ones can be a good idea if the shop knows what it's doing INSERT INTO post VALUES (Hayward & Scott, for example) but can be a bad investment if they've not got a clue about good exhaust design.

If you are in doubt, I would suggest going with a good jap exhaust, mook.

I read an excellent article on this somewhere but don't know if I'll be able to find it - if I do I'll post teh link up.

Calum

[mook]

Cheers Calum, would appreciate that

[eavr-4]

Optimum Exhaust Size
Anyone know what the best diameter is for a VR4 exhaust main pipework. Im guessing 3 inch.

Anyone know what the diameter of the standard exhaust is?

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Hi
Wif regards to your query...I have customed front pipe for my VR4 2.5" each bank linked to a 3" main pipe...
Followed by 3" thru' dummy cat

After the cat piping is reduced to 2.75" all the way to the tail muffler which is a straight thru 3". In between is a 22" silencer INSERT INTO post VALUES (internal diameter is 3").

I find this setup is better compared to having 3" all thru INSERT INTO post VALUES (which I orig. done).

Having 3" all thru' is very good for high revs but gives very weak low rev.

my 2 cents//

[Roadrunner]

Having 3" all thru' is very good for high revs but gives very weak low rev

... which was my experience with my Legacy as I said above. It was interesting to note on the Jim Clark Memorial Rally recently that ALL the World Rally Cars INSERT INTO post VALUES (Subaru, Mitsubishi, Ford) and most INSERT INTO post VALUES (can't guarantee is was all) of the top international and national class cars were using relatively narrow guage exhausts INSERT INTO post VALUES (certainly no wider than 2½ inch and may even have been narrower). As I mentioned before, this would give much better low-down torque and, as these cars rarely run flat out for any length of time, good torque would be much more important for acceleration.

Big bore exhausts might look and sound cool, but I don't think they make for a quicker car.

Brian