i currently have hooker shorty headers to 2 1/4 inch pipe to dynomax bullet mufflers dumped under rear seats. is this sufficent enough for a 400hp small block street strip motor? or should i go bigger?

 

Engine size (cubic inches) is less important than the horsepower in determining the appropriate exhaust size. Depending on which chart you believe, the dual 2 1/4" is good for 200-300 HP. Stepping up to dual 2.5" will be good for 350-400hp. So, no. You're dual 2.25" is likely not enough for 400hp and is limiting flow above 300hp.

This thread includes a chart and discussion.
HP Single Dual
200-300 3" 2.25"
300-400 3.5" 2.5"
375-450 4" 3"
450-550 4.5" 3.5"

Here is a similar chart from Magnaflow.
HP Single Dual
100-150 2-1/4" 2"
100-200 2-1/2" 2-1/4"
150-350 3" 2-1/2"
250-550 4" 3"

 

Just as important as the exhaust pipe size is the flow rating of your muffler(s). Just like the carb.
It's been tested that 2.2 cfm of muffler flow per HP is needed or the mufflers will start to limit power output.
Don't get flow confused with inlet and outlet sizes. Many muffler manufactures simply increase these dimensions while leaving the muffler case and it's flow capabilites the same. CFM data is what's needed and it's not as readily availble as it should be.

 

If you go by the 2.2 cfm per hp and David Vizards 115 cfm per sq inch of pipe area,

2" single supports 164 hp
2.25" is 208 hp
2.5" is 257 hp
3" is 369 hp
3.5" is 503 hp
4" is 657 hp
5" is 1026 hp

These are numbers for a zero pressure system, more hp can be made but the pipe size starts to become a restriction. Dual 3" is supposively rated to 739 hp but i make ~1200 hp thru dual 3" and 4 mufflers total. Yes its restrictive but kinda limited in what you can do

So dual 2.25" in your case should support 416 hp.

 

I've wondered about that spec before. I'm assuming it's for a given length of straight pipe. Obviously the flow is reduced in the bends in the pipe but by how much I haven't been able to find other than these charts and lists.

http://www.exhaustvideos.com/faq/how...pipe-diameter/

http://www.titantalk.com/forums/tita...o-numbers.html

There are many more. You have to be careful comparing values as the test parameters change.

 

Yes there are tons of variables. Even exhaust temp plays a role. Tuned lengths and resonances could also change flow. These are just rough guidelines

 

Just reinforces my resolve to improve the exhaust flow in my Coupe.
With a single Flowmaster (possibly a 10 series muffler with less than 300 cfm of flow) in the system, I have a serious restriction. Un-capping the cutout just before the muffler netted 2 mph in the 1/8th.

 

Might be a little restrictive. My 350 is running about 400 hp, I have dual 2.5" dumped just before the rear axle. Fitting anything bigger than that thought would be a pain on these cars, especially if you want to drive it on the road...at all.

 

do a true dual 3in setup. that would be the best for 400 horses

 

No ... that would b good for a 800-1000 hp engine not 400... go single 3.5 or dual 2.25-2.5 I think u could actually tuck dual 2.25's better than a single 3.5

Sent from my XT907 using IB AutoGroup

 

i dont know about that. at least a 2.5 in true dual.

 

ya putting anything bigger then 2.5 inch would be a pain unless run a single to the rear. im going ti look into the 2.5 inch setup

 

2.5 is nice because of available muffler choices and tip choices

 

dual 2.5" will go over the rear axle pretty easy.
the style mufflers your running should flow good.

 

If you have shorties you can run dual 3" down stock location and over axle fairly easily. 2.5" is much easier to work with and you can go under axle to really make it easy and have tons of clearance. I just finished up a dual 2.5" on my 4th gen which basically is same as thirdgen from driveshaft tunnel back. Tons of room with 2.5". Even tho my future build could use 3", there are more choices in 2.5" mufflers and the extra clearance is nice. Also less weight