Exhaust system sizing for big HP

[MadBill]

The flow loss is proportional to the percentage of length affected. A crush might be enough to reduce the flow by 20% if it was over the entire pipe length, but if it's say 10% of the length, the reduction might be only 2%.

[justahoby]

The flow loss is proportional to the percentage of length affected. A crush might be enough to reduce the flow by 20% if it was over the entire pipe length, but if it's say 10% of the length, the reduction might be only 2%.

How much is that compared to a curve, and a mandrel bent curve? ( round) I guess if you don't squash a round too far it's not as bad .
Also consider a bent curve ( not mandrel bent) is a form of a circle crushed to an oval in a way if you cut across the bend with a saw???
Maybe I am talking crazy

[Orr89rocz]

Well i guess i got an option or two here. Install a pair of cutouts where i would plan to put the Ypipe merge for single 5" exhaust. That will drop off roughly 7-8 ft of 3" piping and the 4 mufflers
Dyno it before and after back to back pulls at closer to my peak power numbers but as long as its over 750 hp we can see restriction in 3" pipes. So target 18 psi or so for 800 ish whp.
If it does gain power then we can look into adding the 5" system.

Other option is simpler. Install a pressure tap or 2 in the system hooked up to a gauge. Do a quick boost pull on street and try to see what pressure is left in the exhaust tubing. I guess i would put the tap in the location where the ypipe merge will be because piping before all that will not be changed. If pressure is high it would be reasonable to assume larger pipe area downstream will reduce this pressure and less pressure should mean more power.

[n2omike]

Oval pipe isn't made my simply crushing round pipe into that shape.
You take round pipe and cut it in half lenghtwise... then weld flat pieces between them.
With a little math, you can make the area match the round tubing.... or make it anything you want.

[groberts101]

Oval pipe isn't made my simply crushing round pipe into that shape.
You take round pipe and cut it in half lenghtwise... then weld flat pieces between them.
With a little math, you can make the area match the round tubing.... or make it anything you want.

yep. and a 5" squashed pipe will still easily flow far more than a 3" straight pipe ever will. ESPECIALLY compared to a 3" merge section of x-pipe.

As usual, take it with some salt.. but I see many/maybe even "most" of the big-block single/twin turbo/supercharger street racers running considerably larger pipes on 1000+ RWHP setups. With that kind of volume.. I just can't see 3" pipe allowing full potential.

[Orr89rocz]

Playin with pipemax i noticed bigger cube motors need bigger pipes even if hp and rpm range is same. So i wonder if big block guys at similar power respond better to oversized pipes compared to smaller cube sbc? But everything i understand about flow and exhaust sizing leads me to believe i can gain with more area

[n2omike]

My 306 has a 200hp nitrous plate, and uses dual 3" exhaust. I would not consider using 2.5". 3.0" works great. Horsepower is around 625 on the bottle.

If I were going to double the horsepower, I would want to double the cross sectional area of the exhaust.

Area = pi x r x r

For 3" pipe: Area = pi x 1.5 x 1.5 = 7.06 sq in.

Double that and get 14.12 sq in.

Solving for radius: r = square root of area/pi = 2.12" 2.12 x 2 = 4.25" diameter

In other words, if 3" pipe works good for 600hp... If you are running close to 1000hp, you need at least 4" pipe. It sure won't hurt.

Good Luck!

[900HP]

My 306 has a 200hp nitrous plate, and uses dual 3" exhaust. I would not consider using 2.5". 3.0" works great. Horsepower is around 625 on the bottle.

If I were going to double the horsepower, I would want to double the cross sectional area of the exhaust.

Area = pi x r x r

For 3" pipe: Area = pi x 1.5 x 1.5 = 7.06 sq in.

Double that and get 14.12 sq in.

Solving for radius: r = square root of area/pi = 2.12" 2.12 x 2 = 4.25" diameter

In other words, if 3" pipe works good for 600hp... If you are running close to 1000hp, you need at least 4" pipe. It sure won't hurt.

Good Luck!

Your math is slightly incorrect. 3" exhaust pipe is measured O.D. so if you are using 16gauge (.065") tube, the actual inside diameter is 2.87". so it would be area = pi x 1.435 x 1.435 = 6.48 sq in (approx.) If you use Vizards 2.2 cfm per horsepower for a "zero" loss system and approx. 115 cfm per in2 (for straight tubing) that single 3" O.D. pipe would flow 745 cfm which would support 338 hp or 677 hp for twin 3". I usually round this up to 700 hp which is what I would use twin 3" exhaust up to.

A single 4" pipe would support (using the same formulas) 615 hp or so and probably cause only minor losses to 700 hp.
a single 5" pipe would support (again, same formula) 975 hp or so and probably would be fine up to 1100 hp or so without major losses.
fwiw, twin 3 1/2" exhaust should support some 950 hp or so without trouble.

This does not mean that you can't make 1000 hp with twin 3", it just means that there is some restriction there causing some power loss. Also, exhaust like anything, wants to go straight so the more bends you put in the system the poorer it will flow for a given size.

[MadBill]

One thing to keep in mind: Vizard's formula does not contemplate turbo apps, which are of course known to be very sensitive to back pressure. However, so too are full race engines with long duration cams, so as a result, his target of (AIR) ~ 0.2 psi for zero power loss is probably still good, and in fact at such low B.P.s a turbo might actually suffer less loss for a modest increase in same. (i.e. the world would probably not end if the B.P. crept up to 2 or 3 psi)

[Orr89rocz]

Looking at garrett turbo turbine maps of similar sized wheels, flow increases with increased pressure ratio across wheel. If i can drop backpressure post turbine, and assume preturbine stays similar, i should get higher pressure ratio and increased turbine flow which should come in handy as i approach limits in these small wheels

What really trips me is AMS Alpha series GT-R's make upward of 1600 hp thru twin 3.5" downpipes which appear to neck down to a 94mm single pipe? 3.70". What i cant determine is if their hp ratings are thru open downpipes or thru single 3.7" exhaust. I know its a smaller cube motor at 4L but 1500-1600 hp through a single 3.70" pipe? That doesnt seem possible. Even thru dumped 3.5" downpipes seems like theres alot of restriction there

[user-9613590]

Make a "screamer" i.e. dump the wastegates straight out! That takes lot of flow demand away from the pipes.Just short pipes down from WG's
Of course not if you have to meet some track noise levels (in street 1200 hp car at full boost is not too shy no matter what the exhaust noise..) that don't work

[Orr89rocz]

Surprisingly its not very loud. Didnt need ear plugs in dyno room unlike my 320 whp ls1 car lol. 4 mufflers help but the exhaust thrust coming out was significant. Blowing banners on wall 15 ft behind car with force. I hear mostly turbo noise. I didnt want to dump wastegates but wouldnt be hard to do

[KnightEngines]

On a 600whp subaru we saw a 30 odd Hp gain going from single 3" to single 4" pipe - no other changes, same boost.
Also spooled quicker & made significantly more midrange.

Another option for you is to run screamer pipes - but with mufflers on them, like mini exhaust systems just for the gates.
Going to a setup like that on a 700rwhp 2JZ from a plumbed back gate saw quite a good gain, can't remember exactly, but was in the vicinity of 40rwhp & again spool time was reduced & midrange benefitted.

[johnny5]

Make a "screamer" i.e. dump the wastegates straight out! That takes lot of flow demand away from the pipes.Just short pipes down from WG's
Of course not if you have to meet some track noise levels (in street 1200 hp car at full boost is not too shy no matter what the exhaust noise..) that don't work

That's what i was suggesting.

[jsgarage]

The last engine swap I did was a Banks turbo-Powermax diesel into a formerly gas-engine Chev dually, and had to slot the firewall about a foot long for turbo downpipe clearance. Once cut and a steel patch panel made, the mod wasn't so bad and the firewall bulge inside behind the dash is not noticable. This might allow running a larger exhaust all the way from the turbo back. Just a suggestion-