Is there a formula or online calculator to help choose exhaust sizing for a motor.

GMPP crate motor, 426hp, 465tq, Hooker LT headers, Holley carb, MSD carb, coil, 6AL box, 8.75:1 comp ratio, 224 @.050", .510"/.540"

Trying to decide on exhaust pipe sizing. Either 2.5" or try to squeeze 3" pipes through the frame... What size will fit through the frame rails? ('72 4-spd car.)

GM recomended rev limit, 5,500 rpm.

 

I don't think you'll need larger than 2.5 for that. Maybe 2.75. 3" is a bit overkill for that.

 

Thanks for the opinion... I measured the size of the frame rails when I was installing the tranny yesterday and they seemed to be 3.5" diameter. Pretty good sized.

I figured a LOT of SBC guys run 3" but they also see more rpm... but much smaller cubes.

I'd really like to find a calculator to help figure this out.

 

Here you go...

Auto Exhaust Science

 

Thanks, good info to read, especially since I am building a "race" exhaust for my '96 Camaro drag car...

But that still didn't help me figure out or decide on exhaust size for this "cruiser"... It doesn't have to be perfect, but I'd rather not leave any HP/TQ on the table with a poor decision.

That article did show me that choosing a muffler is a LOT harder, as not many companies advertise CFM...

 

You'll find that the best flowing mufflers are generally made by dynomax. Of course you could always go without mufflers, if you want.

 

I don't believe cid, or rpm, or anything other than the HP number is what matters. Lingenfeltner said 2.5" was fine for 400 HP and you are not much over that. I also believe he said up to 3 psi in the pipes is nothing to wory about. Bigger pipes may be awfully hard to fit and may hit the frame holes when the engine rocks.

 

CID and RPM are directly linked to CFM, which is how you size a carb and I would assume how you size exhaust.

2.5" is sounding fine, maybe 2.75" to make sure but not go overkill with 3" which will likely be a PITA to run and make noise... Although with the Energy Suspension motor mounts it probably won't move around a whole lot.

 

I used to have specific range pipe size info. I'll try to find it. In the meantime...

Muffler flow rates:
OEM..................................... ........................................ ...225
Gibson SuperFlow 788200.................................. ............311
TTS Bullet Cat Converter............................... ..................324
Flowmaster 40 series 42540................................... .......352
Flowmaster 50 Series.................................. ...................362
Flowmaster 40 series 43040................................... .......392
Dynomax Super Turbo................................... .................410
Flowmaster 62631................................... .......................435
Carsound Cat Converter............................... ..................436
FLP Cat Converter............................... ............................440
Edelbrock Victor 5535.................................... ..................562
Flowmaster 40 series 435409.................................. ........576
Edelbrock RPM series 5511.................................... ..........579
Flowmaster Delta Force Racing 54040-10........................634
Edelbrock 304 series 5560.................................... ...........640
Edelbrock Gen App 5505.................................... ..............708
Dynomax Race Magnum Welded 17216...........................711
Borla XR-1 40600................................... ..........................836
Borla XR-1 40615................................... ..........................854
Dynomax UltraFlow SS 17263................................... .......1000
Dynomax Race Magnum Welded 17218...........................1000
Borla XR-1 40575................................... ..........................1100
Dynomax UltraFlow............................... ...........................1133
Edelbrock Victor 5537.................................... ..................1400
Borla XR-1 40450................................... ..........................1400
Borla XR-1 40741................................... ..........................1450
Dynomax UltraFlow SS 17296, 17268..............................2200
Dynomax Race Magnum Welded 17220, 17224...............2200
Dynomax Race Magnum Welded 17225...........................2600

 

Some ex. pipe size guidelines:
Racing Head Service..... Dynomax
DUALS
2" .... 200 BHP...............100-200
2.25" 320 BHP................250
2.5 .. 400 BHP................300-400
3.0 .. 600 BHP................450-500
3.5...............................500-750+