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vernw 03-26-2002 11:25 AM

Exhaust sizing help needed for 454HO with EFI
I'm confused over how and why to size an exhaust system. My Beast is an 86 Suburban 1/2 ton 4WD. Power is from a GMPP 454HO (425HP@5250, 500FT-LB@3250, hyd roller cam with Lift: .510" I, 540" E. and Duration @ .050: 211 I, 230 E., has rectangular port heads and 2.19" intake valves and 1.88" exhaust valves with 118cc open combust. chambers and 8.75 compression). I've also added Hooker super comp headers and custom 2.5" dual exhaust with H-pipe. I also added a Holley Commander 950 multi point fuel injection kit w/30LB injectors.

When I initially set up the exhaust system, I bought some Dynomax headers for it, but as soon as I opened the box I realized they were smaller diameter than the exhaust ports on my rect. port heads so I sent them back. I learned the Hooker super comp headers were 1 7/8" diameter so I went with them since that closely matched the exhaust port size. At that point I had a local shop bend up the rest of the dual system using 2.5" pipes, Magnaflow mufflers and two cats. I also had a H-pipe put in ahead of the cats.

But I keep reading that smaller pipes yield more torque down low (and sometimes mileage?) at the expense of HP at upper end. OK, but does that mean you should use something smaller than the exhaust port size? I'd always heard you wanted to reduce back pressure, not introduce it. So what gives?

Also, the system I've got is NOT mandrel bent. Would replacing it with a freer flowing mandrel bent system hurt the lower RPM performance? Or would the difference be so small I probably wouldn't even be able to tell a difference?

4 Jaw Chuck 03-26-2002 04:36 PM

With your cam and setup your 2 1/2" exhaust should do just fine, a three inch system might give you a little more up top and boost the bottom end too. Corking up the exhaust to get backpressure does one thing, reduce emissions. Not something I am figuring you or I much care about. Finding mufflers that are pipe size all the way through is hard because the MFG's like to make one muffler and then expand the ends to fit a different size pipe, the cheap ba_tards.

The good thing is this restriction is far enough back of the exhaust port that gains are small if you did fit a proper sized muffler. You could try a diesel engine muffler to get a big muffler, you probably have the room under your truck. Probably wouldn't gain much HP although it would last a lot longer as they are heavy duty. I have always liked the late sixties early seventies big block Chrysler mufflers, very quiet and have 2 1/4" through hole size I believe. Big and heavy mind you.

vernw 03-28-2002 08:25 AM

So, we meet again, eh Chuck? Thanks, once again, for taking the time to try and educate my dumb <you know what>.

You're absolutely right on the emissions concerns, but my guess is the EFI ought to keep it a lot cleaner that a carb would have. So my 2.5" duals sounds okay, the Magnaflow mufflers shouldn't be introducing much back pressure.

Does mandrel bent pipe vs. regular crushed type bends give much noticeable benefit?

Again, THANKS!

4 Jaw Chuck 03-28-2002 04:31 PM

I have never spent the extra dollars to get mandrel bent pipes because the benders around here couldn't be bothered. I honestly don't know if it is worth the money. I would go up to 3" regular bend to get more flow. If they were the same price 2 1/2" as compared to 3" non mandrel I think I would pick the 3" regular only because I think it looks nicer and nobody seems to care what bend the pipes are anyway. At any rate gains will all take place at 5000+ rpm so I doubt you would notice a few extra HP at this rpm level. I don't know if it is worth the money, I kind of doubt it.

vernw 03-29-2002 10:59 AM

Thanks for the reply, Chuck. If I'm not going to see any benefit below 5000RPM, there's not much point in messing with this right now (or ever?)

Think I'll just concentrate on the EFI tuning for awhile (a perpetual task it seems).


--- Vern

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