3 inch exhaust
 

My question is that I have a Pontiac 428 making around 500-550 horsepower is it worth changing my exhaust system from the current 2-1/2 system to 3inch what are the pro and cons for doing this .

Is your current system mandrel bent? If not just going to one that is would be worth more than going up one size. If racing through closed exhaust you would see some difference but maybe not enough to justify the expense. On the street you probably won't notice the difference between 2 1/2 and 3 inch mandrel bent.

Check out this video: https://youtu.be/_PVXvHkr-Vs

If you're chasing a timeslip then yes it's worth it. If your just cruising on the street then probably not. I picked up a couple tenths going from 2.5 full mandrel bent to 3" from manifolds to mufflers w 2.5 tails. If I had it to do over I would go all 3 but was concerned with fitment around the 9" rear.

Agreed. Another example on minimal impact of smaller pipes down stream... I have full 3" mandrel pipes with Ultraflow mufflers. For improved fitment, the 3" tail pipes are necked down to 2.25" chrome tips. I ran back to back 1/4 mile runs with and without the tips and zero difference on ET or MPH.

Of course the exhuast gasses have cooled considerably by the time they reach the tips, and the tips are only 10" long.

I have since installed cutouts after the H pipe when I race, which did provide an extra 1/10th or so compared to full exhuast.

As mentioned, if mostly street i would use a 2.5" mandrel bent system and reduce hassels of the larger pipes.

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you will notice a sound difference. Love the 3 inch sound.
i did full all the way back 3 inch on a 68 FB conv and a 70 GTO. Both needed a lot of cut, fit and re weld even with kits. used American Thunder on Bird and Pypes on Goat. Bird had to trim the quarters since little room between leaf and lower quarter panel. I agree that it may not give a whole lot of power at that hp level.

"Bird had to trim the quarters since little room between leaf and lower quarter panel"

Yep, this is why I necked mine down to a 2.25" chrome tip. A person could also zip tie a small piece of 3/8th fuel line cut length wise to the leaf adjacent to exhaust pipe if still making some contact.

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That's how I feel about the matter, too. All of the dyno testing out there which shows the power difference between 2.5" and 3.0" exhaust is done under WOT conditions, which are most applicable to racing conditions rather than street driving. That's not to say the performance advantage of a 3.0" exhaust isn't real, but it's going to be small potatoes on the street.

Here is my dyno testing on a WAY milder motor.

First exhaust system (blue lines) consisted of 2.5" pipe to 3" FlowMasters, into 2.25" tailpipes. All pipe was compression bent.

2nd exhaust (red lines) was all 3". Compression bent from headers to mufflers, and with a 2.5" "H" pipe. Tailpipes are mandrel bent 3" (a gift from Steve C.). Mufflers are 3" Borla ProXS.

Impossible to say if the pipe size or the mufflers were responsible for the gains, or what % is attributable to each. Note that the gains increased as RPM increased - upper rpm gains were more substantial than those at "peak".

I had to install much thinner metering rods with the new exhaust, to correct the AFR.

7.8:1 compression 455 with unported pressed-stud heads, smallish cam, HEI, Q-jet.

So I guess this comes down to money spent to see a justifiable gain. The saying goes is the juice worth the squeeze. As mentioned if your not racing then a smaller mandrel system is fine. Good previous posts . Fitting a 3 “ system can be a war. I went to a bigger system ( make more power) but I’m racing a little and thought build bigger for the future . In your case that’s the only consideration u will need to decide .

I picked up at least 2 tenths by changing to 3 inch exhaust than 2 1/2 from the mufflers than to the tail pipes which were 2 1/2 then I switched out the tail pipes to 3 inch and picked up another 1 tenth.

Your mufflers are 3 inch in and out then???? Or 2 1/2 in and out with necked down pipes
I run 3 inch to the mufflers necked down to 2 1/2. With 2 1/2 full tails…..
I have mandrel bent new tails 2 1/2 to install

Use 2-1/2" pipes with mandrill bends. No X or H pipe. Use the Dynomax Super Turbo, I like the 17749 the best.. Use Walker stainless band clamps and put exhaust pipe sealer on every joint.

Once the exhaust is finished, put one zip screw in each connection. Make sure it gores through the clamp and both pipes. This will keep the exhaust from moving.

Before installing the mufflers put a couple pints of transmission fluid inside the mufflers, let the fiberglass mat soak it up. When you first start it make sure its outside there will be some smoke for a few miles..

Once the whole system is finished, let it sit for 24 hours to let the sealer set up.. This is so hard because you want to hear how it will sound..

The straight dedicated dual exhaust is the best set up. Those X pipes get super hot and there usually right under the transmission.

Yes 3 inch in and out , originally just reduced after out to 2 1/2 .

Than went Back and replaced to 3 inch Tailpipes.

It’s very tight and needs to be adjusted some times as it rattles, Thinking of going back to 2 1/2 inch tail pipes so I can run stock reproduction chrome tips

That is nearly the identical engine I run in my Chevelle with very similar output.

I run a full 3" mandrel bent system with Torque Tech tailpipes and Dr. Gas "X" with Dynomax Ultraflow mufflers.

The 3" system is very easy to fit on an A-body with plenty of room. Been street driving this setup for 20 years.


For me, making the choice on exhaust size would come down to the car itself. F-bodies are pretty fussy to fit a 3" system on, and I'm not a fan of dual mufflers hanging under the floor pans either. I prefer the transverse deal on these cars and almost always use 2 1/2". For 500-ish HP that works just fine for me. If it were an A-body, I'd probably step it up.

Since Torque Tech went out of business I stopped looking at 3" stuff for F-bodies. They made some of the best fitting stuff in the industry.

Just put an adapter between the tips and tailpipes.

It can be done with the existing 3" if you want to take the time to squeeze in some adapters.

I did something similar on our 69 Z. I wanted the factory tailpipes on the car as they are a bit unique and stand alone amongst anything seen in the aftermarket exhaust industry. The issue is they are only 2" in diameter.

I run a Pypes 2 1/2" transverse system on the car, so I grafted in a pair of mild reducers to step it down. Took some time but now looking at the car from the outside it appears it has an OEM exhaust system on it.

The tail pipes mainly rub against the springs so I adjust them every once in a while

I'm talking about mounting the smaller tips on the 3" tailpipes.

I understood what you were saying but that wouldn’t solve the problem of the pipes rubbing on the springs and not hitting the lower fenders.

The plan is right where they stop dropping down to reduce it to 2 1/2 inch for the rest of the tail pipes and put the correct tips on