Mixed pipe diameter
 

I restored my 72 Lemans vert T41 with a stock 2 1/4" exhaust system. I did this whole frame off myself in my garage. I was going to drive it to town to have an exhaust put on, then decided I could do that too. But, I wanted zero problems with fit. So, hence the stock replacement. Now, with the 400 dyno at 310 HP and 400 ft. lbs. torque, I feel I've cheated myself on the performance end with the exhaust. I'd like to up to a 2 1/2" system and different flowing mufflers. I had to make my own rear tail pipe hangers going out the sides, so I have to make sure those work. My thought now is, am I cheating my self if I just change my down pipes from stock manifolds and new mufflers, but keep my 2 1/4" tail pipes?

I would think the 2 1/4 pipes would still be a restriction. I would go 2 1/4 down pipe to 2 1/2 system/mufflers

My understanding (and someone correct me if I'm off a bit) is that the exhaust gas cools down significantly after the mufflers. As such the tail pipes do not need to be a large as the head pipes.
With your build I think the 2 1/4s will be fine.

If your present exhaust is mandrel bent you will not see any difference in performance on the street with going to a 1/4" larger exhaust. If you do not have mandrel bent pipes that will be your biggest restriction. Yes if you go with bigger engine pipes you will not need bigger tail pipes. The only time larger pipes would make any possible difference is at wide open throttle and you would not feel it in the seat of your pants.

Going 2-1/2” up to and including new 2-1/2” mufflers will make a noticeable difference. At your power level the 2-1/4” tailpipes will not be any real restriction.

The exhaust gasses have cooled and condensed at that point so you’re good with the smaller pipes after the mufflers. The car will be a little quieter with the smaller tailpipes too.

I'm doing essentially the same thing as you are on my 350HO Custom S. I will be replacing the kink-bent 2 1/4" headpipes (aka downpipes) with the Pypes 2 1/2" mandrel downpipes (DGA10S 2.5" mandrel bent downpipes) with a reducer to the 2 1/4" pipes to the mufflers. I am going to replace the standard style regular reverse flow mufflers (restrictive) with 2 1/4" DynoMax UltraFlo's or the MagnaFlow equivalent. This will allow the regular performance 400, like yours, to breath better. If it was making 375 hp (flywheel) or more, I would definitely go 2 1/2" all the way back. I'm not sure if you 310 hp/400 tq was measured on a chassis dyno, if it was, your proposed system is better than what you currently have and will help preserve more power than your stock system. We have found the most hp is preserved with 2 1/2" mandrel downpipes for at least the first 3 ft. and substituting hi-flow mufflers, even with sections of 2 1/4" pipe here and there further back in the system. As mentioned before the exhaust gases are cooling as they move down the pipe and are spreading out as well, so slightly smaller pipe isn't as big a restriction at lower rpm's and lower hp engines.

Dennis

After I put a 455 in my '72 LeMans Sport vert I added RA manifolds and custom 2.5" exhaust with Dynomax Super Turbo mufflers and it is perfect.

This is all very encouraging and a refitment I can do in my garage (I hate working on my back). I hate to waste good, new parts (1500 miles on this), but I want to make it as good as possible now. I'll be looking at the 2 1/2" mandrel bent (have crush bent now) and 2 1/2" in, 2 1/4" out mufflers.
Thanks to all who enlightened me positively.

After thinking about this, does it make any sense to even switch out the dual mufflers? Just stick with the 2 1/4" inlet/outlet? I guess I could try it and if it doesn't seem to do much, then switch.

Just run 2-1/2” to 2-1/4” reducers right before your current 2-1/4” mufflers and you’ll realize the performance boost from the new 2-1/2” mandrel bent head pipes. The gasses have cooled enough by that point that the 2-1/4” mufflers will handle the gas flow.

Reducers with a smooth transition in the shape of a cone are what you’re looking for.

You will realize more of a performance increase from the mandrel bent pipes than anything else. If you check your old pipes where they bend I'll bet there is no more than an inch and three quarters inside.

Thanks. Yes, the crush bent style has a lot to be appreciated. I guess I can get the new bigger mandrel head pipes and adapt pretty easily with what I have.

For giggles, here is a comparison of two exhaust systems as tested on my '73 T/A clone. At the time of this testing, the motor had "66" Dport heads (stock other than springs) on a 455, giving 7.8:1 cr. HEI, Qjet, Performer, mild cam, and a set of Tri-Y headers.

First test was with the exhaust that came on the car. 2.5" pipe to 3" FlowMasters (not sure of the model), into 2.25" tailpipes. Rather ugly junctions where the sizes were mated, all crush bent. Carb tuned to give ~12.8:1 AFR at WOT.

Planning on future upgrades, I went a bit overkill (for the motor at that time, at least) on the replacement exhaust.

The 2nd exhaust was 3" right off the headers, H-pipe, into 3" Borla ProXS mufflers, then into 3" mandrel tailpipes (thanks to Steve Coombes!). The pipes from headers to mufflers were crush-bent, but the guy has 40+ experience and does a pretty decent job (see photo). I was shocked that the AFR leaned out noticeably, yet the power was still better than with the old system. After a couple of metering rod changes, I got the results shown on the graph.

If only looking at peaks, the new system gained 14hp/9tq at the wheels. But notice the old system nosed over at 4,400rpm, while the new system extended the powerband. By 4700rpm, the new system was 21hp/25tq higher.