Board logo

max hp on 2" exhaust
Craigorypeck - 30/1/13 at 06:03 PM

whats the max possible hp via a 2" single exhaust? I know there are a few other factors involved but an approximate fig if possible on a straight thru one box system.... Will this be the same with a boosted engine?

coyoteboy - 30/1/13 at 06:09 PM

Very rough, about 145hp.

rdodger - 30/1/13 at 06:17 PM

Have a look here

clicky

Craigorypeck - 30/1/13 at 06:39 PM

thanks very much! seems well low that....

Paul Turner - 30/1/13 at 06:53 PM

quote:
Originally posted by coyoteboy
Very rough, about 145hp.



Well I had 1 5/8" 4 into 1 primaries into a 1 3/4" silencer and got just over 170 bhp but there was a dip in the power curve that the experienced operator suggested was the size of the pipes. A friend got over 180 bhp with the exact same system. Replaced mine with 1 3/4" primaries into 2 1/4 " silencer and got 175 bhp but it filled in the dip and held onto the power much longer, downside was it robbed power below 4000 rpm.

So 145 bhp is well below what a 2" exhaust can do. But don't go too big otherwise you can loose more than you gain, it all depends where you want the power.

britishtrident - 30/1/13 at 06:59 PM

On a tin top a 2" exhaust would be the norm for about 120ps

SPYDER - 30/1/13 at 07:54 PM

I have a 3SGE Toyota 16V engine in my Spyder Silverstone with a 2 inch "straight thru" system which has a large volume box under the car and a race can tail section. This engine is 178 hp when in a Celica. It gave 168 hp on the rollers at Newark kit car show.
Yes, I know that this might not be accurate.

PeterV - 30/1/13 at 08:05 PM

There are all kinds of odd ball things that mash up the simple volume / flow calcs. Turbos are an obvious one. CATS another. Matching pulse flow from the manifold length. Back pressure from baffles. Vacuum pull from cooling gases further down a long pipe and reduced pressure effects from shorter pipes. Not having a pipe does not always equal max BHP??? Ouch!

Hurts my head when I read the various experts on exhaust design, don’t think I’ll every get it

I’m sure it’s not a black art but the science hurts

MikeRJ - 30/1/13 at 08:43 PM

The EP3 Civic Type R rolls off the production line with a 2.1" exhaust as standard and makes 200bhp, with another 20 or so available through bolt on mods, though the exhaust is holding it back by that stage.


A 2" should be good for at least 170-180bhp, provided it's not got awful non-mandrel bends in it which halve the area of the pipe.

coyoteboy - 30/1/13 at 09:21 PM

Fwiw I think anyone proposing cooling the later half of the exhaust will extract exhaust gasses faster probably hadn't thought it through properly.

Of course the power can be pushed above the theoretical limits when they are based on simple calcs and assumptions. As for a 2.1 inch, that's about 10% more area.

Craigorypeck - 31/1/13 at 12:37 AM

well i'm gonna run twin 2 x 2" systems from a 4 pot YB cos and charger with a Y piece just after the 4-2-1 manifold that has a 2.5" out. was booked for rolling road and mentioned the 2" custom system i currently have and was told I'd be wasting my time and cash. I'm hoping for approx 220 at the very least..
is the complete system actually limited by the out from the 4-2-1 at 2,5" even tho its only gonns be at max about 8" of the entire system???

coyoteboy - 31/1/13 at 01:10 AM

Yes, basically. It's not that simple (it's better to have the narrower bit as close to the head as possible where gases are less dense and hotter, among other things) but ultimately it's limited by its weakest link (smallest diameter).

But 2.5" should be capable of 250ish, so unlikely to affect your goals here I'd have thought, but bear in mind that every time you insert a bend or a split you restrict it further.

cliftyhanger - 31/1/13 at 07:48 AM

A fair bit must be down to the boundry layer of gasses being slowed, fastest gases in the centre?
So 4x1" pipes will not flow as well as a single 2", despite having exactly the same x sectional area.

I also heard that a large expansion chamber near the manifold allows gasses to expand, which cools them. (thermodynamics, can't argue with that) Those cooler gases will then have a smaller volume (bizarre as it sounds) so will flow better.

Me, I fitted whatever would go between he chassis rails. That is 2". As used by the vast majority of kits I think.

rdodger - 31/1/13 at 09:23 AM

Doesn't exhaust length, number of bends and number of silencers make a difference?

Our cars typically have 1 can, maybe a sport cat and a 4 branch manifold.

Production cars have strangled manifold, 1 or 2 cats, 2 or 3 cans and half a dozen pressed bends.

All that has to make a big difference.

My car will be 1.8 MX5 engine, MP62 supercharged,(approx 240 bhp) 4 -1 manifold, 200 cell cat. single can. The exhaust is custom made in 2" by a well respected manufacturer. He assures me that it will be plenty.

Lets hope so! I guess we will find out when it hits the RR.

MikeRJ - 31/1/13 at 12:26 PM

quote:
Originally posted by coyoteboy
Of course the power can be pushed above the theoretical limits when they are based on simple calcs and assumptions. As for a 2.1 inch, that's about 10% more area.


10% more area, yet nearly 40% more power (compared to 145bhp estimate).

coyoteboy - 31/1/13 at 12:55 PM

The estimate is just that. It's 10% more power than the previously noted achievers with a 2inch, so follows the expected error.

Paul Turner - 31/1/13 at 03:13 PM

Just looked on Raceco's site, Ammo knows a thing or two about silencers, extract from the site

"As a rule of thumb 2.25” is suitable for up to 220 bhp, 2.5” for 260 bhp, 2.75” for 280 bhp and 3” for 300 bhp and over."

The site is at http://www.raceco.com/silencers.html

Based on that the 2" system should be good for 195 bhp and the 1 3/4" I had was at the limit at 170 bhp, pretty much my experiences.

MRLuke - 31/1/13 at 04:57 PM

quote:
Originally posted by Paul Turner
Just looked on Raceco's site, Ammo knows a thing or two about silencers, extract from the site

"As a rule of thumb 2.25” is suitable for up to 220 bhp, 2.5” for 260 bhp, 2.75” for 280 bhp and 3” for 300 bhp and over."

The site is at http://www.raceco.com/silencers.html

Based on that the 2" system should be good for 195 bhp and the 1 3/4" I had was at the limit at 170 bhp, pretty much my experiences.


Would 2nd the above.

If the engine has a turbo then exhaust diameter has a big impact, especially downpipe size. You will get better performance from a 2.5" downpipe into a 2" exhaust than the other way around (on a turbo engine anyway) you have to remember the turbo is already a huge restriction to flow.

will121 - 31/1/13 at 05:39 PM

I have a twin silencer 2" exhaust on a fiesta 1.75" manifold ok for 173bhp silenced to 93db

coyoteboy - 31/1/13 at 07:22 PM

quote:

Based on that the 2" system should be good for 195 bhp and the 1 3/4" I had was at the limit at 170 bhp, pretty much my experiences.


Based on 220 out of a 2.25" system, assuming flow is roughly dictated by area:

pi*1.125*1.125 = 3.98

pi*1*1 = 3.14

220*(3.14/3.98)= 174hp.

Approx what's been seen above. Nice to see their figures are more accurate than the original link.

BangedupTiger - 31/1/13 at 09:47 PM

my old 2.25" exhaust was good for 285bhp.

ReMan - 31/1/13 at 09:50 PM

200