2.5" v 3" exhaust on a 500HP 406sbc ?

[SBC-68-FIREBIRD]

Hi Guys,

I have a 406 sbc in a 68 Firebird. The car runs 11.4 @ 118mph & I would think is somewhere in the 500-520hp area. Currenty it runs a 1.75" 4 into 1 header with 3" collectors into a twin 2.5" system through an X pipe then 2.5" Hooker Aerochamber mufflers dumped @ the diff.

Would I benefit in stepping up to a 3" X pipe system with 3 " mufflers ?
Is there a rough rule of how much HP a certain size pipe will support ?
Just notsure if the 2.5" system is restricting it at all.
Thanks
Scotty

[Jeff Pancher]

I doubt you would see much difference. I have a single 3" exhaust on my camaro that runs 11.60's. I would say spend the money somewhere else.

[67RS502]

I doubt you would see much difference. I have a single 3" exhaust on my camaro that runs 11.60's. I would say spend the money somewhere else.

mid 11s with 1/2 of a exhaust system (only 1~ 3" pipe) and you dont thing it will pick up?
I dont understand where people come up with this stuff.
unless youve done some track testing how can you say that his wont pick up, bad advise.
Both of your combos would pick up.

I would say firebird is lookin at around 2 mph increase with a 3" system, with his current combo,
and more with his "new" engine.

[Jeff Pancher]

You think that at 500HP and with 3" collectors it would pick up a bunch? His may. I doubt mine would at only 450HP. Maybe I will try cut-outs someday and see.

[OldSStroker]

Scotty,

Mufflers can be the biggest culprit in limiting the power an exhaust system can support. No offense against your mufflers, but Magnaflw or Dynomax straight thru designs have been shown to be the class of the field for not limting power. Virtually all Engine Masters Challenge folks use them. Have you run the car back to back with the mufflers removed? That might be a good first step. If you pick up mph look at changing the mufflers.

Your 2.5" system might be suficient for 500 fwhp if:

1) All bends are mandrel bends. IOW no "crush" muffler shop bends which reduce the cross section of the tube in the bend area which is the most critical area.

2) Your transition from the 3" collector to the 2.5" head pipes is not abrupt. An 8" or longer transition would be my choice.

As the exhaust travels along the system it cools drastically which reduces the volume which reduces the required tube size to prevent losses. By the time the exhaust gets to your mufflers it has lost a lot of its energy. You might consider 3 inch pipes from the headers to a few feet from the mufflers, then the long transition to 2.5 inch Magnaflows or Dynomax 2.5s.

Good luck.


Jon

[bmftech]

My 69 camaro with 406 picked up 2.5 tenths and 3mph going from a 2.5 inch flowmaster to a 3 inch magnaflow. Went from 11.60@ 116 to 11.36 @ 119. it is a 10-1 406 afr 195's comp hydro roller, 1.750 headers, 3.55 gears, coan 3800, 27 tire, hp 750 holley.

[brket racer]

I use to have a 64 Nova that had a mild 350,glide,hooker 1 3/4 super comp hedders and a 3 inch exhaust through flowmaster mufflers.This car ran a best of 12.14 at 110.I decided,after years of using this exhaust system,to remove the flowjunkers,and installed a set of dynomax bullet mufflers.The following weekend I went to the track and first pass my Nova ran 11.85 at 114.3 tenths and four mph from ONLY changing the mufflers.I would change your entire exhaust to 3 inch and trash your current mufflers.

[greywolf]

Before spending a penny on a new system, why not try open headers at the track one weekend and compare that to your existing system. That should give you a good idea how restrictive your current system is.
I noticed an big improvement going from 2-1/2 dual to open headers. Then installed 3" dynamax ultraflows and they're pretty close to open headers.

[Nitrousknob]

You might try Schoenfeld Headers. They make a muffler that breaths real good. You can drop a coke can through it, if your not concerned about noise. They are also priced right. Moroso makes a sprial muffler that is a decent peice.

[cjperformance]

Forgetting about mufflers for now, for that hp you need 3" dual at least down to your X pipe, 2.5" from there will 'just' support this hp IF it is a well made system. The X pipe is generally (not always, there are good ones), the most restrictive part of the system, so it is VERY important that this is sized correctly, the smallest diameter in your X is going to need to be minimum of 3.5" if well made, 3.75" best or 4" if the confines dont allow an optimum design. The 'idea' behing the X is good but due to room availably, they usually fall short. (this is why I prefer the H pipe, still it must be done properly),
3" from the X back will allow the optimum in top end hp but will be a little droney at lower rpms on the street, so it depends on what you want.
I will almost bet right now that your X pipe is holding back more hp than any other part of the system. Most of the X's available off the shelf or fabbed up in average exhaust shops are terribly restrictive.
As has been said, drop the exhaust and do an open header run just so you get a bench mark of the actual difference the system makes. Then you at least have an idea of the 'exhaust on' performance level you are looking for.
Then all you need is good mufflers.

[BrazilianZ28Camaro]

Wouldn't the carb jetting be different with open headers to max power?

[zums]

have a 372 11.1 510hp running through 3" on the street- 11.10 et 122mph

[rfoll]

3" pipes essentially give you several feet of collector length. Last info I heard, it is always good to have plenty of collector length on muffled street systems. I squeezed times in the 12's on street rubber with a nearly stock 350 using a 3" full length system. It will be LOUD!

[F-BIRD'88]

My 11sec 406sbc runs thru a custom made 2.5" full dual exhaust system with hand made custom 2.5" in/out mufflers of my own design and full tail pipes.
The mufflers are a modified tri-flow design but with propriatory acoustic and flow management.
No voo-doo, just physics at work.
In back to back drag tests the car runs the same thu the closed exhaust as uncapped.
(Actually runs just a hair quicker ET, thru the exhaust).

It's street legal quiet, yet powerfull. Without annoying interior resonance.

A 3" system is going to be louder than your 2.5" system.

[SBC-68-FIREBIRD]

Hi Guys,

Thanks for all the replies. From what I am seeing hear I should see some improvement stepping up to a 3" system so have booked it in with my exhaust guy for the week after next. I am not concerned if the car is louder, I like loud as I drive a quiet car at work every day so it is nice having some noise.

I have a mate who has a 350sbc in his 67 Firebird. He has a twin 2" system on that so my 5000 mile old 2.5" system is going to go onto his car.

It is winter down here at the moment & too cold at the track of a night so will be a couple of months before I get to run it. When we do go I will post up how both cars have gone.

Thanks
Scotty