Exhaust exit pressure

Ntrprise · 2 Jul 2003, 14:27 (Ref:649727)

I am building a 750 formula car and have been thinking about how to improve air inlet and exhaust system, although am finding it hard as this part is new to me.

What would happen if I made a hole (say 3cm) in the exhaust at about 1 foot away from the exit? Then put a funnel to scoop air and force it through the 3cm gap. This would surly speed up the exhaust and help suck air out from the engine increasing performance.

I thought this would be a simple and effective tweak…and yet I have not seen it on any other car. It is a simple idea and I am sure that if it was true, this would not be standard on most race cars…why not?

Dave Brand · 2 Jul 2003, 16:13 (Ref:649808)

I suspect that ram air won't give you much of a pressure increase over atmospheric at the speeds you're likely to be acieving; back pressure in the exhaust system would probably mean that the pressure in the exhaust system would be higher than you'd get from your scoop.

graham bahr · 2 Jul 2003, 20:05 (Ref:650066)

i remember reading somewhere that the forward facing and very large intakes fitted to tourings cars used to generate a mere couple of millibars pressure at 125mph so i cant see your scoop doing anything, unless it was absolutely enormous and would then probably loose you more performance in increased drag than it gained you.

still on the subject of exhausts, sometimes its even worth fitting a restrictor into it.

a couple of years ago i made a new big bore exhaust for my race car, it actually lost me 10 brake! after fitting a restrictor up it (intended to provide additional silencing if nec) i had the missing power back, strangly but still on the same dyno session we changed the intake trumpet length, the car then gave more power without the restrictor! so obviously the intake and exhaust are closely linked!

boyracer · 3 Jul 2003, 09:32 (Ref:650570)

Good book out, 4get the full name, something like "The scientific design of intake and exhaust systems' have only flicked thru it, but I think would be well worth closer inspection, is quite detailled with a lot of experiments that have been performed and the such so quite practical as well.

Stacy · 3 Jul 2003, 13:53 (Ref:650914)

>I am building a 750 formula car and have been thinking about how to >improve air inlet and exhaust system, although am finding it hard as >this part is new to me.

For what it's worth we rigged up a pressure meter to the airbox on my RX7 last year to test the effectiveness of air scoops. The reality was that the pressure was only worthwile & positive at speeds over 110mph, and at UK circuits you were there for such small periods of time it's pretty neglible. I could find other more lucrative areas of development.

As for exhuausts, I'd imagine you'd have a more lucrative time playing with pulse tuning. Disturbing the air flow at the back of the exhaust may be detrimental to overall flow and power. Our rotary is particularly sensitive to pulses, but obviously your mileage may vary..

Cheers

Stacy

gttouring · 3 Jul 2003, 14:03 (Ref:650919)

to speed up the air flow try working with diameters of exhaust and intakes, before poking holes and funnels.
get three exhausts built up with different lentgh tubes, and each lenth at a difereent diameter say 1.5inches, 1.75, and 2 inches (750? bike engine formula or 750 Kilos-if it is kilos i would thik that the engine is larger so go with 2.0inces-2.25-and 2.5 around)
the intake go with a few diferent length runners and a few diameters, Lots of work and money but 10/10ths'ers are the best so take up a collection.
anyway this test set up can ive you a few different results, adn after tested you can see which pipes give you best sec and you can even geta shorter course set up out of it and a longer more open course set too.

avsfan733 · 3 Jul 2003, 18:01 (Ref:651134)

Anyone ever tried any sort of venturi system on an exhaust?

Dave Brand · 3 Jul 2003, 19:51 (Ref:651257)

Quote:

Originally posted by avsfan733
Anyone ever tried any sort of venturi system on an exhaust?

A sixties 'economy' device made by the Exhaust Ejector Company was a venturi which bolted on to the tailpipe. I don't think it had any discernible effect!

avsfan733 · 4 Jul 2003, 02:21 (Ref:651589)

Darn because we were thinking of trying something like that on our kart. the idea was to fit a tight pipe over our exhaust and then use the same concept as those vacuum generators that use compressed air blowing by the opening

RWC · 17 Jul 2003, 15:15 (Ref:664006)

Do some good research on the subject.Get some good books.
Some years back i was into 2 stroke motorcycle tuning and even an ancient book('72 or so) on the subject was a real eye opener.
EVERYTHING has a purpose in an engine and it's easy to make it less powerfull by fooling with it .

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