Ok. Say you've got a b18c5 with significant motor mods (cams, cam gears, bumped up compression 11.1-11.5, stand alone ecu, JDM-style header, high flow cat, etc. etc.).

What effect (if any) will the exhaust piping diameter (2.25" vs. 2.5") play in hp and tq output? Would a 2.5" exhaust make that much of difference over the 2.25" in terms of power? Give me your thoughts.

Sorry if this has been covered already. I'm just trying to get some ideas for the future of my car.

Tony

 

That's what I'm wondering too. will .25 inches make that much of a difference to justify a ~500 dollar cost?

 

One is .25 inches bigger than the other.



If you're gonn upgrade, you've gotta do the whole system so there aren't any bottlenecks left...header, cat, exhaust. Probably give you a higher horsepower potential, might loose some low-end with the 2.5...depends on tuning as well.


[Modified by Aleph, 4:22 PM 9/13/2001]

 

That's my point exactly. I don't want to buy parts twice (i.e., a 2.25" exhaust being too small for an N/A build-up).

Tony

 

500 dollars? cant you just call up SMSP and order yourself some custom piping for a significantly cheaper price than 500?

 

imho:

the stock piping is perfect for the stock system (i/h/ cams, etc...). When you are putting out more power, the stock system is a bottleneck. You want the biggest piping that will not be a bottleneck, but not too big to impede exhaust scavenging or taking away from exhaust gas velocity...

.25" is significant in terms of flow - just ask dave @ sms...

right now the general consensus is that 2.5" is the appropriate size for a na motor putting out ~200whp... Any smaller and the exhaust is a bottleneck and any bigger and the exhaust is slowing exhaust gas velocity.

hth

 

2.00" diameter = 3.14 inches squared
2.25" diameter = 3.97 inches squared
2.50" diameter = 4.91 inches squared
3.00" diameter = 7.07 inches squared

as you can see the 2.5" is an increase of 1 cubic inch in area of the tubing from the 2.25"...the 3" is another matter, which is why the turbo guys like 3"
Is it worth the extra flow...I would think so.

 

You need to calculate the air flow (in cubic feet per second) that the engine will create will all the mods you have specified at WOT. Ideally the exhaust system should match the flow demands of the motor. However, with an N/A motor some backpressure is desirable which is very different from a turbo motor.


[Modified by Big Phat R, 3:00 PM 9/13/2001]

 

Could you please explain "backpressure" here? Or at least direct me to some informative sites? I have heard this before and others have stoutheartedly refuted such notions. I am simply in search of facts.

Thanks!

 

You really need to look at the inside area. 16g 2-1/2" OD tubing has an inside area 25% greater than 16g 2-1/4" tubing. Or 13% more than a 60mm system. General rule is that a 2-1/4" system should be used for up to 210-215 crank HP.

 

From what I understand - some backpressure is desirable in order that you can keep the decibel level of the exhaust down to an acceptable level - but backpressure needs to be distinguished from restriction here. I would agree with the above that 2.25" exhaust is good up to a 210 hp in a 1.8L NA motor. Anymore power would ideally get 2.5". If you're interested in all out power then you should go for a straight-through 2.5" design. The exhaust should match the header/cat i.d. btw.

 

Thanks guys. The info. you've provided has been very helpful. If anyone else has anything to add, please feel free. Thanks.

Tony

 

Porsches have much larger displacement engines and their exhaust systems are very short. They also don't rev as high. Apples/oranges etc.

 

I don't know the answer to that question. I guess you would have to compare torque values of an open header (i.e. infinite diameter exhaust) to a complete exhaust system (defined diameter exhaust) to find out. Torque depends on a lot of things though. All of the Porsche guys I know (993/996) run big bore exhausts because they feel they are getting <u>more</u> power. I think you would agree that my main point is that the Porsche engine (flat 6 boxer) is waaaay different from an inline 4 in terms of power characteristics.


[Modified by Big Phat R, 7:41 PM 9/13/2001]

 

2.25" exhaust (depends on muffler) will be the best setup for light tune engines like 10.6-11.5 CR, mild cams...etc.

2.5" (depends on muffler) will actually will be better for big cams and Hard tune engines.

2.25" (all the way w/o bottle neck) exhaust will be better for street since you won't suffer too much of LOW torque or LOW-MID rpm and ON/OFF throttle response. On the other hand 2.5" will be much better for DRAG setup since you don't need any throttle response for straight away.

A lot of Japanese exhaust manufacturers make their systems with bottle neck is becuz they wanted to have a little exhaust restriction for N.A.engines "not just power, they need more response."

I have my Amuse Titanium exhaust 50mm (2") underaxle exhaust and actually the exhaust efficiency is pretty much a 2.5" (factory style) exhaust. Just all depends on the exhaust efficency . 2.5" sure will have a better efficiency BUT depends on what kind of muffler is combinated?? Dynomax,Flowmaster,straight pipe or thermal??

 

ok so now im even more confused. For the occasional drag/autoxer who daily drives, with Toda B's and mid 11 CR in the future, 2.25" w/o bottle necks ?

 

I think 2.5"w/bottleneck or 2.25 all the way may be your choice.

 

o...the amuse is only 50mm??
i thought is 60mm

 

Why would you want a bottleneck after the cat or test pipe? JDM headers have 65mm collectors and most of the custom US built pieces are 2-1/2", cats and test pipes are 2-1/2". Why restrict the flow after these pieces?

Note: Ensure the IDs of all pieces all equa to prevent mismatches.

 

The bottlenect I mention is between the muffler and the B-pipe. I have tried all the way 2.5" with tuning my camgears on the dyno it still loses torque low-mid. Ofcuz the peak torque is higher but it doesn't seem working very good for me since I love to have low-mid.

Now , I am having 2.5" (bottleneck between B-pipe and muffler) and I am pretty happy low-high rpm go more smoother instead of just getting that hump after VTEC kicks it.

SMSP : You are rite that 2.5" will make more power and I have no offence to that. Just for my application (uphill and downhill) so I needed more response.

 

I have a JDM HKS sport exhaust for my G2 and the piple that goes from the cat til about the bend just before the rear wheel is 2" and the pipe and muffler (axle back) is 2.25". Is this the japanese bottleneck design you were talking about cause i have no clue?

BTW, it says it's for a B16 engine since that's the engine G2 tegs have in japan.


[Modified by jonsteR, 6:23 PM 9/14/2001]

 

ttt

 

ok so can you get lets say 2.5" piping for the greddy evo system from greddy or do you need to get the muffler and have a shop custom make the pipe??? caus i the website they say 60-70mm for the pipe.