Yep 2 1/2 I.D.

 

We'll put an H pipe crossover in, and see how that works. Thanks for the ideas gents.

 

Doing all the involved math to figure out flow for the engine the calculations say 2" is more than enough. I don't use 2.5" in any of my cars as they will never see speeds high enough to utilize all that extra cross section. That includes the modified 302 in my Cougar. This is one area where people over do it just like overdoing a carbs CFM.

Next I don't think the H pipe will make any difference. Mopars of that era never used an H pipe which I didn't know at the time of switching my Polara from single to dual. I put in an H pipe and then had a terrible time with drone between 35-45 mph. Annoying as hell to drive around town. The problem was the mufflers. They were Dynomax turbos but they were the 13" instead of the 20". Pulled them out after three weeks for the 20" and all the drone disappeared. So I'm going with the mufflers.

Wish I had seen this some years ago since it does the math and it turns out I was pretty close to what it says.

 

Well maybe you're right, but the consensus seems to be (from spending a while searching the Internet forums) that an H pipe crossover can help with drone and maybe quiets things a bit while deepening the tone. Should be a little better scavenging and a smidge more power.

Had thought about having a crossover installed right from the git go but had read some on that too, some prefer "true duals" so figured could always add one later without too much hassle. The exhaust was real quiet at first, in fact I was even a little bit disappointed. Turns out though exhaust or mufflers seem to need a little bit of break in for some reason.

 

Your call but remember drone is caused by sound frequencies between 133-138hz in cars. Typical in rpm range from 1800-2200 depending on final drive ratio. I have seen enough reports that an H pipe makes almost no difference. Once those sound waves start to resonated inside the muffler, making it act like a drum, you are going to get drone. Chambered mufflers have much less of a problem than straight through mufflers.

 

I must agree with Mike. As he stated and I stated above, the drone sound is coming from your mufflers. Add to that, your trunk bed acts like a speaker cone, amplifying that sound.
H-pipe, crossover, or what ever you wish to call it, will most likely improve the exhaust flow = better performance. ALTHOUGH you may not feel the difference. Yes, in some cases it may stop the drone frequency, but

 

If you go with a H pipe be sure they put back far enough that you can drop the trans if needed without having to remove the exhaust system. I've worked on several cars that I had to do that.

 

Damn good point, been meaning to look at the routing and see what's what, will keep this in mind.

 

Everything I've read says to put the crossover as close to the engine as possible. That means it's in the way of stuff.

One way to make it easier to drop the exhaust is to use header flanges or something similar, to make the center portion of the exhaust system removable or simply make the cross pipe removable.

 

There are as many opinions on exhaust systems as there are those sharing those opinions. TBM3FANs chart shows what the maximum horsepower can be handled by a certain size pipe. It does not say anything about if you only have say 300HP you should not go bigger than say 2 inch pipe. With that said you and I have pretty much the same thing 292 V-8 in a 64. I have headers you have rams horns. You have straight thru glass packs I have straight thru magna flows. Both systems go out to the rear bumper. All I can tell you for sure is I had true duals and convinced myself that adding an H pipe would eliminate the crackling of the exhaust under acceleration and deceleration. And would also eliminate the drone I was getting at certain constant rpm. I put in the H pipe and everything I had issues about were gone. I now enjoy driving my truck even more. There was one thing I noticed after putting in the H pipe that kind of bothers me. Not as many people turn their heads to see what is coming as before the h pipe. So I do not get as many thumbs up as I once did.

 

Heh, it does sound great. I was kind of thinking about Charlie's idea of a removable crossover, a flapper valve or blocking plate might be useful.

 

Here is a link to a page of a thread about exhaust testing. On this particular page, 2 1/4", 2 1/2", and 3" pipe and open header is tested on the same engine, with the same headers. For each size bigger, the engine picked up some power everywhere or didn't really lose any over the previous diameter. The scoring system used is the same as Engine Masters, which averages tq and hp and gives a much better picture of what is happening with the engine than a simple peak does.

Exhaust test to end all tests?

This thread contains lots of useful info, even crossover testing. All testing is done on a Y-Block but that really shouldn't matter as far as the actual data goes. It was done back to back, on the same engine, on the same dyno, by the same dyno operator, verifying performance at the very end with the baseline setup to ensure that the engine was performing the same at the end as it did at the beginning, so it is about as direct a comparison as you will find for different manifold and header styles, pipe sizes, etc.

What I get out of all of that testing is that the header is of the most importance. After that, the pipe doesn't matter a whole lot as long as it is not too small for the output of the engine.

This engine was producing around 300hp so according to the chart posted by tbm3fan, 2 1/4" should be adequate, and it pretty much was, but by going to 3", a little bit more was gained - very useful info for the guy trying to squeeze as much as possible from his engine.

 

All the info about headers, pipes and exhaust pipe size is great if you are worried about getting 5 hp more at 5000 rpm on some high performance engine but that isn't the issue here. He is having a problem with the drone sound on a basically stock street engine from what I have read.

 

Yep, true enough. Completely stock. It's still good info though! One of the best things about this forum and others like it is being able to use the search function and scour the archives. Once I get a hankerin' on a particular subject it's easy to spend a lot of time reading up on it.

 

Yet then that is your case. My case was an exhaust that already had an H pipe and the muffler was the problem until it was changed out. The biggest variable here is the actual muffler and how that particular muffler is constructed and how it flows exhaust gases.