Downpipe sizing

[grape of wrath]

I found this link and was wondering if there is some kind of formula or experienced advice into using the correct size downpipe. Is there a HP level or boost level that you should change to a larger downpipe?
This article recommends 250 hp=2.5'' 300 hp =3'' 400-450 hp=3.5'' and up.

Turbo Exhaust Theory

[DCVING 6]

Doubt that's really accurate.

IIRC, Steve Hughes did a back to back track test on a car that ran 9.40's in the 1/4 mile with a 3" and 3.5" downpipe and found little to no gain going from 3 to 3.5"

[Quik Six]

In an article in GMHTP, Red Armstrong attributed a 25 HP gain in Jane's car to a 3.5" DP over a 3" DP.

Let's do the math and see how much more surface area there is in a 3.5" pipe over a 3" pipe. The area of a 3.00 inch pipe is 7.07 square inches and the area of a 3.5 inch pipe is 9.62 square inches. The 3.5 inch pipe is 2.55 square inches larger in area, a 36% gain over the 3 inch.

[qwks6]

Originally Posted by Quik Six

In an article in GMHTP, Red Armstrong attributed a 25 HP gain in Jane's car to a 3.5" DP over a 3" DP.

Let's do the math and see how much more surface area there is in a 3.5" pipe over a 3" pipe. The area of a 3.00 inch pipe is 7.07 square inches and the area of a 3.5 inch pipe is 9.62 square inches. The 3.5 inch pipe is 2.55 square inches larger in area, a 36% gain over the 3 inch.

That means that a 3.5 inch would support 36% more exhaust gasses, right? Does that mean to see the gain you'd have to make 36% more gasses? I could see the advantage on a car with a huge turbo and custom headers, but for most of our cars, especially with a 3 bolt turbo, I don't see it making any difference because of the restrictions pre-downpipe.

That being said, the quicker you dump that exhaust the better. Bigger is better.

HP figures in the first post must refer to slower cars

[Quik Six]

I believe Jason Cramer does a lot of research on these cars before he comes out with a product and look at some of the newer DP's he has available. His 5" to 4" DP makes my 3.5" DP look skinny.

FYI, I picked up 39 RWHP and 50#'s of torque by opening my dump cap on the 2nd run on a dyno. I always open it up at the track and have to add fuel to compensate for increased air flow. I am replacing the 2.5" Hooker exhaust with a 3" dual set-up from Jack Cotton and that should improve the air flow through the exhaust system significantly.

[SPOOLN]

I think DPs are over rated. I went from the stock 2 1/4 neck to a Hooker 2 1/2 true smooth radiaus bent delta and noticed a big difference. Turbo spooled way faster and you could hear the turbine whistling. Throttle response was instant. I next went to a 3 inch THDP and noticed lag and the turbine whistle was no longer as noticeable. Bottom end was not as good, but then power came on. I even uncapped it and it felt even more turbo lag. You need exhaust velocity to scavenge the turbo and headers. Terry Houstons car went high 8s in the 1/4 mile and he was still using an original 3 inch DP. My car is far from his times! I have a street driver and use an ATR true 2 1/2 stock replacement SS DP and its fun to drive.. I might build a SS 2 1/2 DP with a delta like the Hooker dp but one that fits better. I think if the exhaust turbine is at least 3 inch could benefit from a larger 3 inch dp but mine only measures 2.25 so its smaller then 2 1/2inch dp. This is just my opinion and what i have noticed from spending lots of cash

You guys running the dyno and picking up additional HP and TQ where is it showing on the graph ? Are you just moving the HP and TQ in different areas of the RPM range? I am curious to see.

[Quik Six]

Sounds like your combo works for you and how you want your car to perform.

I don't agree that DP's are overrated.

All my max HP and torque readings on a dyno were at the same RPM, 4000. Power dropped quickly after that most likely because of tired valve springs.

[qwks6]

Originally Posted by Quik Six

I believe Jason Cramer does a lot of research on these cars before he comes out with a product and look at some of the newer DP's he has available. His 5" to 4" DP makes my 3.5" DP look skinny.

FYI, I picked up 39 RWHP and 50#'s of torque by opening my dump cap on the 2nd run on a dyno. I always open it up at the track and have to add fuel to compensate for increased air flow. I am replacing the 2.5" Hooker exhaust with a 3" dual set-up from Jack Cotton and that should improve the air flow through the exhaust system significantly.

If you're gaining 39RWHP by opening the dump that means it's the cat-back system that's holding you up, not the dp. Good call with replacing the Hooker exhaust. I've seen many people crimp them while trying to bend pipe, ending up with a thin pipe. That's the way mine was. Some crappy muffler shop had put it on.

[bison]

Originally Posted by grape of wrath

I found this link and was wondering if there is some kind of formula or experienced advice into using the correct size downpipe. Is there a HP level or boost level that you should change to a larger downpipe?
This article recommends 250 hp=2.5'' 300 hp =3'' 400-450 hp=3.5'' and up.

Turbo Exhaust Theory

More like 2.5" up to 500hp, 3" up to 750hp, and 3.5" over 750hp on a 3 bolt application assuming the dp flange isnt covering part of the ex outlet when bolted up. If you run the largest ex wheel you can in a 3 bolt you will need to grind out some material or run a 3.5". Even on a nearly stock car a bigger dp wont do much. Less than 10hp gain in a car running less than 110mph in the quarter. The big gain after the replacement is getting rid of the 90* elbow and putting on a pipe with a smooth radius bend.

[1BADDAM]

Originally Posted by grape of wrath

I found this link and was wondering if there is some kind of formula or experienced advice into using the correct size downpipe. Is there a HP level or boost level that you should change to a larger downpipe?
This article recommends 250 hp=2.5'' 300 hp =3'' 400-450 hp=3.5'' and up.

Turbo Exhaust Theory

I have the stock DP on my TTA and it is far more choked off than the stock DP's on the buicks and I have trapped 117 mph and have dynoed at 411 rwhp / 450 rwtq on an unopened motor. So I would have to disagree with the article. Bison's #'s appear to be far more aligned with reality.

[Grumpy]

Originally Posted by 1BADDAM

I have the stock DP on my TTA and it is far more choked off than the stock DP's on the buicks and I have trapped 117 mph and have dynoed at 411 rwhp / 450 rwtq on an unopened motor. So I would have to disagree with the article. Bison's #'s appear to be far more aligned with reality.

I remember lookin at the TTA downpipe way back in the 90s.. what a POS !!! crushed and weird bends in it.. Had a few friends buy the 3" one an it did NOTHING !! these were fairly stock cars. We ran 114/115 mph back then with the stocker. SOOOO bigger isn't always better BUT a lot of guys need the "cool guy" big parts for their 12 sec car