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astro awd 1997 rear box (muffler) change

PostPosted: May 17th 2020, 9:45am
by GSouthee
Hi this sunny UK am.

Now I am not upto speed on modern exhausts etc and was wondering if I can change the rear box for something a bit sportier. I have in my garage a 2.5 inch Hedman turbo muffler that i had spare from an old project.

However when I take look under the van it appears that the pipe leading to the existing muffler (that is as big as a dustbin) is a lot larger then the pipe from the existing muffler It appears to be about 3.5 inches. Why and should I use a reducer to connect the hedman or do I need a larger bore muffler.

Will a turbo muffler aid performance or just be louder?

Does the astro engine need the back pressure of the existing muffler?

Normally on old school motors the more free flowing the better is it true for these as well?

Cheers and hope your all staying safe over there.


Re: astro awd 1997 rear box (muffler) change

PostPosted: May 17th 2020, 1:17pm
by petema
Gary wrote: "Cheers and hope your all staying safe over there."

Over WHERE? :D
Have You checked with RockAuto? Inlet is 2,75 or 3 inches, outlet is 2,5. There is much about exhaust pipes on the forum. In my opinion, if you can make it work with a reducer and appropriate hangers it should work. P

Re: astro awd 1997 rear box (muffler) change

PostPosted: May 17th 2020, 2:12pm
by Mmusicman
GSouthee wrote:Does the astro engine need the back pressure of the existing muffler?

Back-pressure is often a "misused and misunderstood" term.
Generally.. back-pressure is bad.

But there is a BIG science to proper pipe-size (specifically before the muffler) for proper velocity and scavenging (look those terms up). Engine exhaust travels in pulses, and behind each pulse is a small vacuum that essentially siphons exhaust behind it (very important).. thus creating scavenging. This is done to siphon pressurized and remaining exhaust from each cylinder, dramatically improving performance in every way. A vacuum in the cylinder will allow a much better and more efficient charge from the intake too. Smaller pipes have higher velocities, larger pipes have lower velocities. This becomes important in relation to timing and the speed of the pulses at different rpm ranges. This is generally just as important with headers too.. but another similar discussion. There are reverberated back-pulses too (where pulse timing is critical).. but that's for advanced discussion and engineering.

Pipe too large, or too small BOTH can work against you... and they are generally "tuned" to a desired rpm range where their velocity speeds are most efficient.

But it's also important to know this "pipe size science" mostly applies BEFORE the muffler. The muffler itself is an expansion chamber where all kinds of things happen with those exhaust pulses both physically and sonically... and scavenging/velocity science pretty much ends here. Generally what come out of the muffler is a much smoother exhaust flow... again both physically and sonically.

The muffler must have as little back-pressure as possible... as it no longer siphons exhaust, but receives it. Often it is designed as a compromise between flow and sound output... where quiet muffler have some back-pressure and loud mufflers tend to have less back-pressure.

Pipe size AFTER the muffler may have less importance in regards to velocity and scavenging.. but it must not be restrictive either. If it is too small, it may choke the output and create additional undesirable back-pressure at the muffler. This back-pressure will increase at higher rpms. A tail-pipe also has ability to lower or change sound output.

So to recap.. proper pipe size leading from the engine to the muffler is somewhat critical for maximum efficiency at specific rpm ranges, affecting torque range, horsepower, and fuel economy. Back pressure at the muffler however will damage this flow output, but often it's a choice of balance of sound output vs flow. The tail pipe mostly just routes the final flow from the muffler.

The tail-pipe "should" match the output size of the muffler. Reducing it's size would be undesirable. But mufflers can also be purchased with almost ANY input/output size, so this comment doesn't stand much on it's own. None the less.. reducers should be avoided, unless the muffler has unusually large ports.

But it is possible the smaller tail-pipe size (depending on how much) might have minimal affect overall (especially at lower cruising rpms) where it might soften the loud mufflers output. In racing applications where you wind out at top rpm full throttle.. this would certainly be detrimental. But in a daily cruiser, this may be fine.

GM engineers have designed our exhaust systems with maximum efficiency while remaining virtually quiet. This would be the pipe size to emulate. Yes everything can be improved on with more compromise and/or cost. The V6 is not a torque/hp monster. The output at the tailpipe (much like the air-filter input) can likely be measured in CFM. If the pipe size is adequate to pass the desired CFM with minimal back-pressure, then you'd be fine. But that also applies to intended RPM range too.. where you may occasionally need additional power. But will you feel the difference of 10hp (for example)... not likely. Will it affect you mileage (at cruise rpm range).. not likely.

You want more.. it might matter. 2 1/2" is generally a good number for each side of the engine. Better for performance in V8's.. less important in a V6. Two into one pipe size (input side) is usually a little larger as well.

That's my technical rant.

A reducer at the muffler can be used with minimal negative affect on a "stock performance" engine. The restriction it adds may not be much better than the over-sized stock muffler... that's undetermined. It may only become slightly restrictive at higher rpms.. also undetermined.

GSouthee wrote:Will a turbo muffler aid performance or just be louder?

Will a turbo muffler improve performance?
Maybe a little.. but enough to notice or matter? Uncertain.
How often do you floor it and wind the engine out to top rpm?
There's not enough "performance" in a stock V6 to matter, in my opinion.
It certainly won't hurt performance.

The stock muffler is very large to aid in reducing sound while attempting to reduce back-pressure.
The aftermarket turbo muffler is much smaller, usually louder.

Will it "sound" better? That's for you to decide.

Those are my thoughts...

Re: astro awd 1997 rear box (muffler) change

PostPosted: May 17th 2020, 2:37pm
by markmitch
the pipe coming into the muffler should be 3 inches so you should stay at 3 inches into the muffler then stay at what the tail pipe size is after the muffler. Mine is 3 inches in then 2.5 inches out. A better muffler definitely will not hurt. good flowing muffler good price.

Re: astro awd 1997 rear box (muffler) change

PostPosted: May 17th 2020, 3:45pm
by GSouthee
Blimey, a thorough reply re the muffler etc if a little to technical for me.

Cheers all for replies. Maybe I will try o source a muffler with the correct size inlet.

And stay safe every where.

Cheers G

Re: astro awd 1997 rear box (muffler) change

PostPosted: May 17th 2020, 4:37pm
by Big_kid
Just be sure to keep the tailpipe exiting on the side of the van. I had mine exit under the rear, and it filled the cabin with exhaust fumes. The tailpipe didn't stay there way very long.