3" Downpipe VS 3.5" Downpipe ??

[jmendlik]

any gain moving from a THDP (3") to a TA Performance 3.5" External Gate down pipe??


250 CU Stroker Motor
GN-1 Heads
E85
TA SS/Race Headers & 46MM Gate
6870 3 Bolt DBB Turbo
Pypes 2-1/2 inch exhaust --soon to be replaced with ATR 3" SS Crossflow

 

[jmendlik]

anyone have insight here?

Mike-
I have the 6870 H Cover so 3" Out.

Is the true 3" ATR Crossflow exhaust (single muffler) a free flowing muffler? I had it polished along with full exhaust

I have this setup going on the car in the spring-was going to upgrade DP at same time if benefit to be had,.

 

[ttypewhite]

Do your exhaust first. 3" dual system would be better served with your setup. The 2.5" is a bottle neck. The 6870 comes with 2 versions, 2.5" outlet or 3" outlet. If you have the 3" outlet I would get the 3.5" downpipe, if you have the 2.5" outlet leave the 3" you have.

 

[ttribejr]

I would do it. Not like it’s going to hurt anything. Unless your car is lowered and you have clearance issues.

It depends how hard you run the car. If you are at 20psi or 500ish rwhp you probably won’t notice. 30 psi 700hp you probably will. My numbers are just examples.

 

[Mr.Spool]

any gain moving from a THDP (3") to a TA Performance 3.5" External Gate down pipe??


250 CU Stroker Motor
GN-1 Heads
E85
TA SS/Race Headers & 46MM Gate
6870 3 Bolt DBB Turbo
Pypes 2-1/2 inch exhaust --soon to be replaced with ATR 3" SS Crossflow

The muffler in the 3 Atr will be the choke point before the 3 downpipe.i only saw a gain with the 3.5 downpipe by removing the muffler and going to a 3.5 inch single exhaust.the car leaned down .4 on the wideband and was much more responsive in racing.this was on a very fast buick and I don't think most need more than a 3 inch setup especially if you have a 3 th fp and a nice 3atr crossflow setup which has a sweet sound to it.

 

[earlbrown]

When trying to decide something like this, it's always nice to step back and look at the basics. When trying to increase flow, all you're doing is 'chasing a cork'. If your current DP isn't the cork, going bigger won't benefit you much if any.


The trick with mufflers having little to no pressure drop, is to use a size bigger than your actual pipe with tapered connectors on both sides. If you have a duel 2.5" system with twin mufflers, get a pair of 3" units with four reducers. The science is that a 2.5" muffler can't flow as much as a 2.5" pipe.... but a 3" muffler can.

 

[GNVAIR]

How much of a difference is there from a performance stand point between a 2.5" duals and a 3" duals (or even a 3" single for the matter)?
Are we talking a tenth?, several tenths?, half a second?

 

[Coach]

any gain moving from a THDP (3") to a TA Performance 3.5" External Gate down pipe??


250 CU Stroker Motor
GN-1 Heads
E85
TA SS/Race Headers & 46MM Gate
6870 3 Bolt DBB Turbo
Pypes 2-1/2 inch exhaust --soon to be replaced with ATR 3" SS Crossflow

The general of thought is mid / high 9's is the transition area for going 3.5 over 3. Doesnt gain anything before that. Mid 9's and faster for sure.

 

[Coach]

How much of a difference is there from a performance stand point between a 2.5" duals and a 3" duals (or even a 3" single for the matter)?
Are we talking a tenth?, several tenths?, half a second?

None. 3.5" single will get you mid 9's so to argue that 2.5 vs 3 in a dual system shows any benefit is moot. Either will work just fine.

 

[DSEROVY]

Wouldn't two 2.5" mufflers out flow the single 3" or 2.5" pipe feeding them? I get the reasoning on the reducers though.
I only had the last car at the track once - couldn't get it to hook but went still went 10.80 @ 128 through a Hooker 2.5" cat-back being fed by a 3" downpipe.

 

[earlbrown]

The main benefit of going single is weight. (and balance since the entire system is on the passenger side)

Just to throw some math at you, the cross sectional flow area of a 3" pipe is 7 square inches. The flow area of two 2.5" pipes are 9.8 square inches. It's only a 20% increase in flow area but it comes with a lot more weight and a lot more friction area from the extra walls and bends.

More math... a 10' section of 3" pipe has 7.85 square feet of wall area, 20 feet of 2.5" pipe has 13 square feet (and twice the bends).

So basically 3X the friction area with 20% more flow area. In real world numbers the extra flow area won't do anything unless the 2.5 system is a cork. So any normal street car probably won't see a HP increase worth the cost.

BUT every car, no matter the power level, will see a performance advantage with less weight.


My personal GN has a SS 3" single shot with a Dynamax Race Magnum. I've got a pretty hot roller 4.1 and there's nothing to gain from opening the dump (except noise and turbo whistle). One of these days I'll toss the muffler. It didn't quiet the car down any but it killed off a lot of whistle at cruise.

 

[Nick Micale]

A good friend of mine has a fab shop which specializes in high-end exotics and various performance builds, so we have accumulated lots of data on exhaust systems.

Most turbo Buick headers are made with 1-1/2" primary tubes, but with higher performance builds like 1000 HP and up, we use 1-5/8 inch primary tubes, and at that point a 3-1/2" DP would be a good choice.

For a street car into the low 9's will do fine with a 3" DP, especially when the exhaust is cooling as it travels along the pipes.

A 3" dual exhaust/muffler offers almost no restriction to the exhaust gas traveling along its exit route.

 

[forcefed3.8's]

The 3" crossflow system flows less than a 2.5" dual. 9s is where I would want a 3.5" downpipe.

 

[BobbyBuick]

The 3" crossflow system flows less than a 2.5" dual. 9s is where I would want a 3.5" downpipe.

But a 2.5 dual weighs 40 lbs more.unless the cars running in the 9s a single 3 inch is fine

 

[Coach]

Wouldn't two 2.5" mufflers out flow the single 3" or 2.5" pipe feeding them? I get the reasoning on the reducers though.
I only had the last car at the track once - couldn't get it to hook but went still went 10.80 @ 128 through a Hooker 2.5" cat-back being fed by a 3" downpipe.

There is a difference between out flowing and what the engine is actually pushing thru it. I dont think even an 8 sec engine would out flow a dual 3 like the GN1. I run a 4" single modified to accept the 22" long Borla XR-1 Race muffler. I'll never out flow it and my car makes around 800fwhp.

 

[deezdad]

The main benefit of going single is weight. (and balance since the entire system is on the passenger side)

Just to throw some math at you, the cross sectional flow area of a 3" pipe is 7 square inches. The flow area of two 2.5" pipes are 9.8 square inches. It's only a 20% increase in flow area but it comes with a lot more weight and a lot more friction area from the extra walls and bends.

More math... a 10' section of 3" pipe has 7.85 square feet of wall area, 20 feet of 2.5" pipe has 13 square feet (and twice the bends).

So basically 3X the friction area with 20% more flow area. In real world numbers the extra flow area won't do anything unless the 2.5 system is a cork. So any normal street car probably won't see a HP increase worth the cost.

BUT every car, no matter the power level, will see a performance advantage with less weight.


My personal GN has a SS 3" single shot with a Dynamax Race Magnum. I've got a pretty hot roller 4.1 and there's nothing to gain from opening the dump (except noise and turbo whistle). One of these days I'll toss the muffler. It didn't quiet the car down any but it killed off a lot of whistle at cruise.

Wouldn't that be 30% increase?

 

[JP112_89]

I remember Red saw a gain going from 3" to a 3.5" DP with a 66GTQ if I'm not mistaken

 

[ravege]

To throw in more damn math, gas contracts with temperature drop (PV=nRT). To make up numbers, if it were 1000# at the turbo and 100# at the exhaust tip, that's an order of magnitude less volume the exhaust needs from turbo to tip. You want more diameter where its hottest, not where its coolest.

 

[SloGN]

so whats the flow on the atr crossflow system? I have been in the nines @ 29 psi with a 67 turbo? My current best 1/8 is a 6.28 @ 112 mph @ 27 psi

Am i at the limit of what it can flow?

The downpipe is ATR 3 INCH

 

[Mr.Spool]

so whats the flow on the atr crossflow system? I have been in the nines @ 29 psi with a 67 turbo? My current best 1/8 is a 6.28 @ 112 mph @ 27 psi

Am i at the limit of what it can flow?

The downpipe is ATR 3 INCH

Not yet but close