Originally Posted by Bill Hoffmann
I'm thinking about upgrading the exhaust in my '75 2.0E Manta. I had a nice mandrel bent 2" system but it rusted out (after almost 20 years - I can't complain). I plan to cut the head pipe at the 'Y' and go to 2" diameter. Bob's recommendation is to use at least a 2.25" over-the-axle pipe. Two questions:
1. Where is the best place to make the transition? I would think before the muffler, since there's more clearance there. The muffler outlet is crowded enough as it is, needing to bend immediately up. My concern is whether that will have an adverse effect on the muffler's designed function (high flow with sound reduction).
2. Should the rear muffler be 2.25" diameter also, or would it be better to reduce it back to 2"? The same concerns apply.
O.K., one more question: Any recommendations as to baffled or straight-through mufflers? I imagine the rear will be straight-through, but will it be too noisy with both that way? I have a straight-through Dynomax on my autocross Manta, and with just a pipe out the back, it's just too loud for a street driven car.
All opinions are welcome.
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Bill
On my 75 Manta with +.030 flat tops, Roger Wilson big valve head and CamTech cam, I'm running a DynoMax Super Turbo muffler with a Hooker dual resonator tip. The sound is pretty close to the compromise I had hoped for. I think the back pressure loss is less than a regular muffler, but may be a little more than a straight thru type.
The sound at idle is deep and throaty but not too loud. When you pull redline++ in 1st and 2nd, everyone can hear it. Cruising down the highway at 60+ you really don't even hear it, so you don't have to turn the radio up. When you are cruisn' its hardly noticeable, but when you get on it you can hear it nicely. Sounds great going under the overpasses. It sounds so good, I find myself driving around without even turning on the stereo just to hear the exhaust note!!!
The only one thing I'm not over joyed with is the drone in the cabin between 2200 and 2500 rpms. But you're gonna get that with anything other than the stock set-up. I really don't even notice it unless there is someone else in the car....
I chose the 2-1/4" muffler since the 2" pipe from the header slips just inside of the inlet to the muffler. Exiting the muffler we used 2-1/4 mandrel bent tubing over the axle and into the inlet of the dual tips. Looking back, if I had to do it again, I might try using just a 2" muffler and 2" mandrel bent tubing. I know RallyBob has always said to use 2-1/2" for the over the axle due the necking down of the pipe from the bending process. So I kinda tried to split the middle. But, after lowering my car, I found there was not much room for the 2-1/4" pipe so we have a few rattles on hard bumps. And who knows for sure, but the larger 2-1/4" vs 2" mandrel bent stuff may have cost me a couple of lb-ft of torque down low. However, when I run it up to 7,000 RPM I sure like the bigger stuff....
Here some links to follow...
Dynomax :: Mufflers :: Super Turbo
Hooker Headers 21417HKR - Hooker Exhaust Tips - summitracing.com
You can see pictures in the thread called Finally It Lives, in the Manta forum
For my daughter's GT with a stock, Low comp engine, I used the Walker Thrush Turbo muffler since it was cheaper. It still has a very good sound. Relatively quiet at idle and gets progressively louder as it revs up. In her car there is no interior drone. Still it sounds great driving with the windows down or reving the engine at idle. I used 2 single resonator tips for the classic look.
I cut her head pipe at just the right spot where a 2" pipe blended in nicely. Also, I took extra time to do a little clean up in the interior where the two down pipes merge as there is a big hunk of tube sticking out causing lots of flow restriction....
Sorry for the long and somewhat off topic post in the middle of Bob's great thread....