Exhaust sizing race car

[richiev88]

Hello,
I own a 70 Camaro road race car. Its running a 572cu BBC, made 700 RWHP on dyno
(afr 357 cnc, 278/282 @50, 110, .0720 lift, 11.5.1 compression, pro systems 1180cfm carb)
Currently it has 2 1/8 primary tubes into a 3.5 inch collectors. Then 3.5 inch mandrel bent tube to a Flowmaster dual inlet 3.5 inch race muffler.
It feels like it may be restricted at its top end ( 6400rpm)
4 inch would be better, but more difficult to keep under noise limits and we have restricted space
do I need 4 inch system ??

Cheers Richie

[bob460]

Hello,
I own a 70 Camaro road race car. Its running a 572cu BBC, made 700 RWHP on dyno
(afr 357 cnc, 278/282 @50, 110, .0720 lift, 11.5.1 compression, pro systems 1180cfm carb)
Currently it has 2 1/8 primary tubes into a 3.5 inch collectors. Then 3.5 inch mandrel bent tube to a Flowmaster dual inlet 3.5 inch race muffler.
It feels like it may be restricted at its top end ( 6400rpm)
4 inch would be better, but more difficult to keep under noise limits and we have restricted space
do I need 4 inch system ??

Cheers Richie

Well the new 632ci crate engine made 840hp at the rears with a 3 inch exhaust, maybe look at some better mufflers.

[Moparboy440]

[malibuguy79]

What model flowmaster? I do not recall any dual 3.5 inlet Flowmaster mufflers.

[skinny z]

Hello,
I own a 70 Camaro road race car. Its running a 572cu BBC, made 700 RWHP on dyno
(afr 357 cnc, 278/282 @50, 110, .0720 lift, 11.5.1 compression, pro systems 1180cfm carb)
Currently it has 2 1/8 primary tubes into a 3.5 inch collectors. Then 3.5 inch mandrel bent tube to a Flowmaster dual inlet 3.5 inch race muffler.
It feels like it may be restricted at its top end ( 6400rpm)
4 inch would be better, but more difficult to keep under noise limits and we have restricted space
do I need 4 inch system ??

Cheers Richie

Somewhere it was suggested that about 2.2 CFM per HP is required before the exhaust starts to diminish output. The gains are marginal at the top end of the scale.
Although 700 rear wheel might be be difficult to reconcile to engine HP, call it a 20% drivetrain loss. Based on that, engine output could be 840. So, following that guideline, 1680 CFM is required.
As mentioned, gains are small at the top end (as in percentage lost) but even at 1500 CFM, there are only a few mufflers that could flow that much. And I would think that the Db's are through the roof.
Is there room for two?

Muffler CFM.png

Combine that with Moparboy440's post regarding pipe diameter and potential CFM...

Exhaust Pipe Diameter Flow.jpg

The rest is math. And packaging.

[richiev8]

IMG_0071.png

[richiev8]

Thanks for the reply’s,
Muffler is as the picture posted
I’ll drop the exhaust at the header flanges, see what changes next time on the dyno
From the tables supplied the muffler could be choke point

[richiev8]

IMG_5998.jpeg

IMG_6001.jpeg

[Tom68]

Good luck keeping the flat muffler together.
Twin 3.5" should be enough, chassis dyno with some turn downs on the collectors and see.

So much going on, pinion angle, fuel line heat shield, traction bars, rear antiroll bar with leaf springs, is power the only problem ?

[richiev8]

Why would the flat muffler be a problem? I’ve run one for 5 seasons no issue , just not with this motor.
Everything else works fine,all necessary for turning corners
I see u are AU, sometimes we run with your Tourning car masters series

[skinny z]

Thanks for the reply’s,
Muffler is as the picture posted
I’ll drop the exhaust at the header flanges, see what changes next time on the dyno
From the tables supplied the muffler could be choke point

Don't forget that by opening the exhaust at the collectors, you'll change the tuned length.
That in itself would affect the dyno results.
Have you determined what the optimum collector length should be for a mufflerless system and your application? Or what the overall length should be including the mufflers for best torque or HP?

[Tom68]

Why would the flat muffler be a problem? I’ve run one for 5 seasons no issue , just not with this motor.
Everything else works fine,all necessary for turning corners
I see u are AU, sometimes we run with your Tourning car masters series

Normally hard to keep flat mufflers from cracking, but if they're working for you that's good.

[richiev8]

No issue with the mild steel ones as long as you mount them correctly, seen stainless ones split the seems

[Rick!]

Maybe borescope the muffler to see if it is buggered up inside or a baffle has come apart.
I'm with Tom68 on the pinion angle. With multi-leafs and cal-tracs on an f-body set up for road racing, there isn't much droop when jacked up or much jounce when on its tires. Here's a quick measurement using Solidworks on the pic - the perspective may be making look worse but it shouldn't be off much. A quick check wouldn't hurt.

[richiev8]

Maybe borescope the muffler to see if it is buggered up inside or a baffle has come apart.
I'm with Tom68 on the pinion angle. With multi-leafs and cal-tracs on an f-body set up for road racing, there isn't much droop when jacked up or much jounce when on its tires. Here's a quick measurement using Solidworks on the pic - the perspective may be making look worse but it shouldn't be off much. A quick check wouldn't hurt.

It’s ok inside.
Pinion is 2.5 down . Car was originally built for a historic class which limited suspension mods. Springs used to wrap up under hard braking to the point of nearly binding the uj
Running in a different class with a lot more freedoms now, so slowly working through new mods
Under rider bars are solid fabricated ones , think cal tracks slide ?
I’ve attached a pic of the set up some of our cars run, it’s a F body too.