down pipe and test pipe?

[Cattalbus]

is it possilble to buy larger than a 3" down and test or will I have to get them made? I wanted to hook into my 3.74" (90mm) exhaust or will it be wasted on the car since I cannot get a large enought downand test pipe? if I have to get them made can anyone recomend a place?

thanks

PS this car is going to be all dragster, not a daily driver at all.

[Jim96SC2]

I'm sure you COULD put on a 3.74" pipe as a DP, but you're probably gonna have to watch the clearance issues. Just get a 3''s all your need.

[Jeff240sx]

The principle that the GT Spec exhaust (that u have) is that exhaust gasses expand as they cool.. and that's why the GT Spec is constantly flaring out. Also, IIRC, the GT Spec is 3" inlet. You do more harm than good by getting a 3.74" downpipe and have exhaust gasses hit a .74" wall as they enter the GT Spec system.
Even if I'm wrong about the inlet size, you want 3" downpipe to help the scavenging effect.
-Jeff

[Jim96SC2]

Jeff gases take up LESS volume as they cool down. I think the GT-spec is based more on the "stepped header" idea along with the greater pressure differential equals greater flow.

1) PV=NRT which is equal to V=NRT/P. T and V are directly proportional. Higher temps will lead to higher pressures.

2) Things act on pressure differentials. If you have a 3 inch pipe coming out of the turbo which leads to a larger pipe you have the same volume of gases coming out but they suddenly decrease pressure leading to a increased differential. This differs from the normal turbo setup in that it adds an "extra" differential.

Normal Turbo w/ 3" exhaust (just for example, not definitive)

Combustion Chamber->Manifold->DP->Ambient (outside)

Just remember though that the pressure differential in the DP-> Ambient is a linear one (for the most part). While there will be flow, it won't be as fast as it could be.

Normal Turbo w/ 3" DP and 3.74" Exhaust

Combustion Chamber->Manifold->DP->Exhaust->Ambient (outside)

Now we have an extra gradient. This will promote flow through the DP and create less restriction.

Now all this theory goes on pressure laws (move from high P to a low P) and that they move at greater speeds with increased differentials (sorry forgot the law [Possuiles?]). So heres how the differential actually works.

NORMAL
Downpipe _ E x h a u s t _ Ambient
25 23 21 19 17 15 13 11 9 7 5 3 1

STEPPED FLOW
Downpipe _ E x h a u s t _ Ambient
25 23 21 19 13 12 11 10 9 7 5 3 1

Just basics but you get the idea. There is also velocity that you have to take into account for more on N/A cars for the vaccuum effect on overlap, but on turbos your goal is to create the least restrictive exhaust possible to create a larger gradient across the turbo.

[Tuck&Poke]

holy ****........i understood NOTHING of that post.... can you explain for the stupid people?

[240Stilo]

So it doesn't cause turbulence for the exhaust system when your piping is very large? And even if it did, turbulence is bad right?

[Jim96SC2]

OK.. So here's all that in laymens terms.

Air moves by pressure differential. (from high to low)
Your exhaust works in the same manner.
By "stepping" the exhaust you increase differentials and thereby increase movement (flow).
The GT-spec would create an added differential and therefore get gases out of the downpipe (DP) quicker.


Turbulence shouldn't be a factors because you are not adding any restrictions. It would have high velocity in the DP, but then come out to a larger diameter. If we were to weld some sort of Y-pipe we would have to solve for reynolds number and determine if we are losing laminar flow (assuming we have it in the first place).

[240Stilo]

Ok let me see if i got this right. Gases are moving from higher concentration to lower concentration, like diffusion. Due to the gradual increase of pipe diameter the gases increase velocity with each bigger piping diameter they enter, correct? If you had your piping all one size (the size of the last piping diameter in the stepping exhaust) would it mess up the flow?

[Cattalbus]

thanks so to summarize the 3 inch downpipe and strait pipe will be fine even if I get a 3.74 Gt exhaust. infact it could help in higher boost situations. but a 3inch exhaust will sufice in most cases.