[Chris-Austria]

Hi,

I just calculated how much cfm is needed for my exhaust system and think, that my Flowmaster 40s are not sufficient.
Engine is a 462cui, E-heads, 10.25:1, HR cam, 750 Holley, power up to about 5800rpm (street engine) with headers and a 2.5" Flowmaster exhaust system (h-pipe and 40 series mufflers).

My first thought was that I need at least the cfm ratings I calculated on the intake side (carb size) for my exhaust system, but found a simple forumla that says "hp x 2.2 = cfm needed". So thats about a 1000cfm for my engine. On the intake side the 750cfm Holley is sufficient, so maybe the exhaust in general may need more cfm than the intake.

Now to what I found out.. a Flowmaster 40 with 2.5" will allow about 250cfm... so my complete exhaust system can only support 500cfm because it is restricted by the mufflers.
The 2.5" Pypes Race Pro or Violator flow about 490cfm, so they would be sufficient for my engine.

Now the 3 big questions:

1. Are my calculations realistic?
2. How much would I benefit if you consider that my exhaust flows only half the cfm that the engine needs?
3. Will the Pypes Violators be louder/deeper than the Flowmasters? (The sound clips I found are not as good as I hoped and I never found Violators with a H-Pipe system)

Thank you!!
Chris

[Ccass]

For a motor of your size, a straight through muffler like the Violator or Race Pro will give you an extra 5-6 hp. So you might be over analyzing the muffler decision based on performance. The FM muffler sound great but is 40 year old technology. The big problem with the FM is that at highway speeds, they are very annoying.
If you are going to the track and wanting to shave time, get the Race Pro. if you want great sound but not too obnoxious at highway speed, get the Street Pro. If you want to make a big statement pulling into a parking lot, want the best performance, then pick the Violator.
I have a 462 in my 67 GTO and have tried all the mufflers. I currently have the Violators and love that sound best. However, I don't drive the car on the highway at all.

[Chris-Austria]

I hoped it would make a bigger difference.. but thank you anyway
How would you compare the Flowmaster 40s to the Violators (sound)? I don't really care if they are annoying at higher speeds.. but I don't want to quieten down my car.. that's why I'd rather try the Violators than the Race Pros.
I guess your car should sound very like mine would with the Violators..

[goatless]

I run the Race Pros on my Lemans with a 462. They are not a "quiet" muffler. They are relatively mellow at idle, but their straight through design gives them plenty of bark at full throttle!

[Chris-Austria]

I like the sound and loudness of the Flowmasters, but I also think a 250cfm muffler isn't the best choice on my car.. if the Violators are a little louder or at least have the same level as the Flowmasters that would be nice... (but better flowing)

[pfilean]

If 750 cfm on the input side is sufficient it still would not be directly equitable to what you need on the exhaust side. You are putting in air and fuel at a low temperature. But then you burn the fuel into a different molecular structure. Without getting into a full gas dynamics calculation (pv=RT and all that stuff), remember the exhaust goes out at a much higher temperature so it will be at an expanded volume. I have no idea if that HP x 2.2 = cfm is a valid formula or not but at least it gave you a bigger number than 750.

[Old Man Taylor]

I think the formula is HP/2.2*CFM. Or, 2.2 HP for every 1 CFM. That's with a pretty efficient engine.

[Chris-Austria]

I now have the Violators and the sound from idle to lets say 3500rpm is really great, a lot better than with the Flowmasters (less drone), but at high rpm's it sounds a little too "brutal".
It's loud but not too loud (for me).. a little louder than before I'd say, but not much. Idle sounds a lot more like a muscle car should

[Lee]

I made a somewhat similar swap a few years ago, and tested before and after on my chassis dyno. '72 Cutlass convertible, mild 455 Olds engine

Condition initial: 2.5" Flowmasters, 2.5" pipe reduced from 3" collectors to mufflers, and 2.5" tailpipes. The Flowmasters had about 40,000 miles on them, but they were old. They sounded the same, and no rattles, so no obvious signs of deterioration. Additionally, the car had felt "soft" at higher rpm for quite awhile, although it launched rather strongly.

Changes: Retained 2.5" tail pipes and existing headers. Installed 3" Pypes RacePros, and 3" pipe from collectors to the mufflers.

Results: Obviously, from the dyno results, SOMETHING was wrong with the Flowmasters. My previous guess that power was dropping at upper rpm is verified by the dyno results. I suspect some internal baffle rusted through, or moved, or SOMETHING (manufacturing defect?) to cause them to perform so poorly. The Pypes provided a 15rwhp gain at the point of least difference, 20rwhp gain comparing peak-to-peak results, and at 5200rpm you can extrapolate the difference would be greater than 40rwhp. No other changes between the tests, and same dyno.

Impressions: I've since changed the motor in the Cutlass, the current motor puts close to 400hp to the wheels, but still have the same 3" pipes to the same RacePros, and same 2.5" tail pipes. I also have recently acquired a '73 Firebird with a stock 350 Pontiac, and with 2.5" FlowMasters. I recently had the Cutlass and the Firebird idling, side by side, as the same time - the Firebird is NOTICABLY louder at idle! Half the horsepower, .5" smaller mufflers, but definitely louder.

That changes once the throttle is pressed, though. For me, the RacePros are way too raspy. At anything more than light throttle, they sound WAAYYY too loud & raspy for me, and flat out obnoxious at WOT. I'll change them out eventually, but I don't yet know what with.


On an additional note, back in 2005/06 I changed the 3" Flowmasters on my Firebird, installing Georlich (now AP Exhaust) Xlerator mufflers. The new mufflers were shockingly more quiet, so much so that my brain thought that I MUST have lost at least 10hp. First trip to the track, though, and I ran .08 quicker and over 1mph faster than my previous best with the Flowmasters.

I'll be chaning the exhaust on the '73 Firebird in the very near future, getting rid of the Flowmasters.

[Formulajones]

Lee,

If you are open to suggestions for your 73 and getting rid of the flowmasters, I'd highly recommend the Pypes Transverse system.

For years I ran the Flowmaster 2 1/2" Transverse on my 70 Formula, raced the car a lot as a pure stocker. I got tired of it and wanted to try an "X" system so I made the switch to the Pypes transverse setup which uses their race pro design for the large single muffler. However it's not as loud and raspy as the race pro dual muffler setups. Matter of fact it's quieter than the flowmaster I pulled off, it has no drone at all, my wife is happier, and the car picked up not quite a tenth and 1 mph at the track with it.

On other cars here I've run dual muffler systems with both Pypes Race Pro and Street Pro mufflers. I have to agree with you, I'm not as fond of the Race Pro sound. However the Street Pros I have on my truck have a very tolerable sound level that pleases the wife.
But my favorite dual muffler setups lately are the Dynomax Ultraflows. I have one car here with that setup and love it. Very nice sound, no drone, and with a high flow design (no baffles or chambers) they should support a decent amount of power.

[Lee]

I appreciate the suggestion, but the plan for the Firebird is for a MUCH bigger engine in the near future. I'm torn between using 3" and 3.5" pipes/mufflers on it :-) If I were keeping the 350, though, I would seriously consider your suggestion!

Thanks :-)

[Formulajones]

My father uses a larger system on his 571. 3 1/2" coming off the header collectors into a Dr. Gas X that transitions out of the X to 3", (X is 3 1/2" inlet/3" outlet) into 3" race mufflers (forget the brand) and then into 3" Torque Tech tailpipes.
This same system was on a friends Buick GS that made 700hp and ran bottom 10's through the tailpipes. So what you are thinking should work nicely.