3" vs 4" downpipe?

I'm running the standard 3" downpipe that came with the Hellion kit with my T76. I'm thinking it would be a restriction and I'd like to possibly fab up a 4" downpipe but I wanted to hear some opinions on here from people who have actually done it. I'd prefer to hear only from those with real world results, thanks.

 

Re: 3" vs 4" downpipe?

When I was running the T76 I ran a 5" downpipe. I've since switched to a 91mm and I'm thinking I need to switch to 6" piping.

 

Re: 3" vs 4" downpipe?

If 3" was a major restriction concern, hellion would have increased the downpipe with the same kit your using. Although im running a garrett gt-42 with a little bit larger turbine housing than you. I increased my downpipe size from 3.5" to 4" and noticed a substatual increase in throttle responce and it was actually more quiet. Increasing your down pipe size will only make better results

 

Re: 3" vs 4" downpipe?

3" - good to 775 hp.
4" - good to 1,025 hp.
5" - good to 1,230 hp.
6" - good to 1,350 hp.

 

Re: 3" vs 4" downpipe?

Where did you get that chart? A 6" is exactly 4 times larger in surface area than a 3", so I'm a little confused at why you think a 6" is only good to a little over 1.5 times a 3". Something is way wrong here.

 

Re: 3" vs 4" downpipe?

775 for 3" seems optimistic. It has 30% less area than dual 2.5" and I have heard 2x2.5" becomes a restriction at about 500hp.

 

Re: 3" vs 4" downpipe?

Well that is an interesting chart.
why would they make a 88mm t-4 flange turbo if a 3inch exhaust can only support 775 hp?

 

Re: 3" vs 4" downpipe?

They are using 3" on everything from the base 62mm kit to the upgraded 76mm like I have. I was told that they used the 3" for clearance issues and not because it was optimal. I seem to remember an old article in MM&FF from a few years ago saying that Hellion was going to build an optional 4" DP because of the power gains available but it never happened. Someone on modularfords with an 03 Cobra and 78mm turbo said they gained 70rwhp at 20psi going from an open 3" downpipe to an open 4".

 

Re: 3" vs 4" downpipe?

subscribing..
I asked a similar question in the advanced tech section. I think it comes down to length of pipe vs. diameter before there is a definitive advantage. My 3" down pipe is 3' long exiting the housing, I don't think there will be much gain going larger. If it were run to the back of the car then yes.

 

Re: 3" vs 4" downpipe?

You just running an open downpipe?

 

Re: 3" vs 4" downpipe?

No, there is a bullet muffler approx. 18" from the exhaust housing.

 

Re: 3" vs 4" downpipe?

The idea is to keep gas speed in the exhaust pipe, after the turbine housing, down to 250 fps so that restriction is kept to a minimum. This does not take bends into account.

It's very possible that my 5 and 6" hp figures are on the low side. They were quickly extrapolated from a chart and the curve could be off towards the low side by 50 to 100 hp.

 

Re: 3" vs 4" downpipe?

Search for posts by engineermike - I believe he did a test almost just like this. I don't remember hard figures, but I remember it being well worth it. Backpressure went down considerably and he picked up a couple of #s with no other changes.

 

Re: 3" vs 4" downpipe?

Rule of the thumb......

Turbine exducer X 1.25 = ~size needed for DP.

 

Re: 3" vs 4" downpipe?

I tried to search by his username and came with nothing.

 

Re: 3" vs 4" downpipe?

I ran 135.55 at 6000 ft DA with a 3.5" exhaust on a T76 P trim with a .81 a/r tang turbine

 

Re: 3" vs 4" downpipe?

Here you go:

https://www.theturboforums.com/smf/index.php?topic=63459.0

 

Re: 3" vs 4" downpipe?

Diameter - Surface Area
3" - 7.06"
4" - 12.56" (178% of 3")
5" - 19.63" (278% of 3")
6" - 28.28" (400% of 3")

Making the 6" pipe solid for 2500+ HP which sounds about right

 

Re: 3" vs 4" downpipe?

That is more inline with my thinking. People sometimes think that a pipe twice the diameter of another one flows twice as much, when actually it flows four times as much. As your list shows, a 6" is exactly four time bigger than a 3".

 

Re: 3" vs 4" downpipe?

I have a question. If a person were standing on the side lines and heard a turbo car go past at WOT with the exhaust creating a schwishing sound, would you think the exhaust pipe size was too small? What causes the schwishing sound? I always thought that sound was a sign of the pipe size being too small.