I wanna put a true dual 3 inch exhaust off a set of headers straight into a set of 40 series flowmaster mufflers is 3 inch to big?

 

I am sure someone will chime in with the whole lack of "back pressure" angle, but IMHO there is no such thing.

As for 3", I think it's bigger than you could ever need with a 351ci and 2 1/2 is plenty but it's your preference and $$$. So I say if it's what you want, go for it!

 

what size motor? I'd suggest a 2.5" H-pipe and then 3inch to your mufflers and 3" out to the tailpipe. Hell get a 4" tip.
but 2.5" off the headers to take advantage of exhaust scavenging.

 

So 2 1/2 would be better ? I had a flowmaster 2 1/2 catback on my 94 f150 it had a 302 and it screamed so I thought half inch bigger for a bigger motor would do the same effect

 

exhaust scavenging is what your referring to and trust millions of engine builders around the world. It does exist.

 

From my understanding of scavenging is the process of pushing exhausted air out of the cylinder chamber and drawing in fresh air for the next cycle.

The only way exhaust size would possibly effect this is by being to small not allowing enough exhausted air out fast enough. My point is, you can run 5" exhaust and it will not effect engine performance whereas, some claim to big of an exhaust is a bad thing or claim not enough back pressure. Under sizing exhaust is the only "bad thing" that I know of as exhaust goes.

 

goes both ways... too big doesn't allow it to be pulled out just like too little doesn't allow it to be pushed out, thats what I have learned.

3" works better for turbo applications as the turbos need the exhaust volume to spin the turbine and then the boost takes over the push and pull.

3" from the headers is more for a large displacement motor like the big blocks or realyl high revving motors.

 

Ok so 2 1/2 it is haha thanks

 

As with what Preston said no worries about back pressure or lack there-of BUT, where do you use the power of your truck the most...the bigger the piping the more the power curve will shift upwards... if you want torque off the line down lower than go 2.25 duals...2.5 duals do good mid-range and 3in for top end...

although back pressure isnt a good thing but some swear by it...but the factory has specs on what is too much...

on a stock 302 or 351 I would go 2.25in duals and mufflers but if you spend more time above 3000rpm then go 2.5in


you can have too much scavenging and that can hinder lower end performance...



Close but not quite....

Scavenging means to Scavenge...means to Pull...

It happens more in headers and is strongest on Long tube header... right where the tube come into 1 at the collector there is pulses from the cylinder exhausting the pulse creates a Vacuum and creates a slight suction in the other tube and Pulls or scavenges the exhaust out of the cylinder...

the hardest part is getting the parts to work nicely... pipe size DOES play an important role in this effect... too small and you under scavenge but create low end torque at the sacrifice of top end... too big and you over scavenge and kill any low end you hope to have...

same with pipe sizing for exhaust...scavenging isnt much of a factor on stock manifolds but piping play a HUGE role in how the cylinder filling/Emptying happens...

in stock vehicles like our truck that scavenging is built into the camshaft artificially...its cheaper to do it in the engine like that and scrimp on the exhaust to save money per vehicle...this is why you get the most from header with a cam swap...you get better cylinder filling and the exhaust can actually do its job

 

Well I like to drive fast and I don't pull so I don't need a lot of torque