1st Gen Exhaust

R

Rx7Carl

New member
I've tried many homemade setups, but it seems that any over the axle system drops power. Right now I'm using a long primary system with a pacesetter(dont roast me ok, and stop laughing!) header, 4 heavy duty cherry bomb type presilencers merging into one 2 1/4 pipe right before the axle, then a turndown towards the ground. It's Reasonably quiet. Is the RB system really worth the money? It seems that the muffler would be too restricting. Comments? Advice? Bueller?

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Carl
#33 CSP/soonITA/IT7
[email protected]
 
Carl, I use a RB header with 2 " heavy wall pipes to beyond (and underneath) the rear axle. Then I built a 2-2" into 1-3" collector and then thru a dynomax straight thru muffler. I added a 5" extension to the backside of the muffler to keep from burning my rear bumper. This system is hand bent (no muffler shop bends....crimping) with a rose bud torch end and a lot of patience. With the muffler I never go over 97 dba. Without the muffler (with a 3" pipe the same length as the muffler and a turn down) 101 dba. My thought process was this: the header has 2 - 2" pipes. A 2" pipe has an inside area of 3.1428 sq. inches. 2 - 2" pipes have an inside area of 6.2856 sq. inches. 1 - 3" pipe has the inside area of 7.0713 sq. inches. Thus no restriction in the exhaust system. A 2 1/4" pipe has the inside area of 3.9423 sq. inches. This is a restriction in the exhaust system and inhibits the exhaust flow and reduces power. Hope this helps you in your design of exhaust system. By the way, the car sounds awesome thru the tri-oval at Daytona with no muffler but my ears are still ringing a week later, but I'm not gripping about it. I love the sound of a Mazda at 8,300 rpms in 5th gear with a 4.88 gear.
 
Mike, thanks for the info. What you describe is exactly what I envisioned runnig, with one exception. Why should the collector be 3"? The pulses should never meet the collector at the same time (assuming equal length pipes), so wouldn't the big collector drop velocity and reduce scavenging? My minscule knoweldge of exhaust theory tells me that the 2 1/4 might actually be better. BTW what Dynomax are u running? the "cherry bomb" type with the high temp packing? I have used one of those and it held up well.

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Carl
#33 CSP/soonITA/IT7
[email protected]
 
Carl,
Based on the radius, the pipes are not the same length. I'll measure it tonight and let you know. We are running the dynomax with the hi temp packing. One other thing I found out, make sure you leave the system loose. I built in a bracket to allow the system to grow as it heated up. I blew a hole in a RB header because I had a rigid system. Now my hanger at the muffler allows for growth and haven't had any more problems. Make sure you have the rear pipe flush to the rear bumper or you'll burn your bumper off the car on downshifts. My car throws a 3' flame out on downshifts.
 
How restrictive is the muffler you are using? I am aware of the extreme heat that the RX-7's put out that no other racecar does, and have seen a burnt rear bumper without installing the turn-out. You seem to have the "exhaust" knowledge. How did you go about choosing the muffler?

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Corey L. Clough
ITS RX7 Soon-To-Be
[email protected]
 
Another reason for the 3" merge into the muffler is the exhaust pulses from each rotor will expand after exiting the housing. Therefore, even if both 2" pipes were exactly the same length, there would be overlap of the pulses. By having the 3" into the muffler, it allows for both pulses to be there at the same time.

But...I don't claim to have a huge amount of knowledge in this area...If I am mistakin', please let me know.
 
Okay, related question. When you run the exhaust under the rear axle, are you doing anything to keep the axle from dropping on the exhaust when jacking the car up, or do you just let it hit it? I've seen guys put a steel cable around the axle that's attached up by upper spring perchs...

Mike, I'm surprised you can even get down to 101db without the muffler on that set-up! I tried a similar approach (RB Race header, 120 2"pipe back to the RB collector and a straight pipe out) and it made my ear drums bleed at idle!

I was running two pipes back to collect just before the axle, then one pipe OVER the axle to a 24" long "turbotube" muffler. Good high -end power....maybe a bit light on low end torque. Since I bent that set-up in half backing off the tire wall last year, I thought I'd try a new approach. Anyone see an ITA RX-7 that exits the exhaust in front of the rear wheels?
 
We ran a stock manifold(PRO-7) with a 3" straight out dumping just before the rear axle pointed down. I thought it was heating up the rear axle assy, but never ran a probe there to test. Ran @ Laguna and had to add dual mufflers to quiet 'em down just to pass. With so much heat out of the exhaust I don't think you would want it to exit in front of the rear tire. How could you protect that tire from the heat? Just a thought. Is water injection further back in the exhaust pipe too unreal? Might as well squirt oil on the track to keep them off your tail(Some things should not be said at all).

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Corey L. Clough
ITS RX7 Soon-To-Be
[email protected]
 
I use a turn down on the straight pipe setup and aim it away from sound control. At Daytona sound control is on driver's left in the infield so the sound goes to the right, however in the banking and the trioval next to the wall........
what a sound...........music.
Hey it may not have been any faster than with the muffler on but it sure did sound faster..lol
 
I've tried several combinations; here's what worked the best for me: (No dyno sheets to substantiate) just lap times

RB header, two 2" pipes under axle (very slight bend to allow suspension droop w/o contact). The GC panhard setup will limit axle droop on right side of car.

RB collector with megaphone outlet, Megaphone cut at 2 1/2" diameter and mated to a 3" inlet 2 chamber flowmaster which had the inlet heated and formed around collector. Nowhere near the 91dB limit that laguna seca required. So I borrowed a Mr2 (which I eventually purchased) when going there.
 
Originally posted by rlekun:
Okay, related question. When you run the exhaust under the rear axle, are you doing anything to keep the axle from dropping on the exhaust when jacking the car up, or do you just let it hit it? I've seen guys put a steel cable around the axle that's attached up by upper spring perchs...
Click to expand...

Whoops, thats not technically legal. Not being a nerd here,just helpful. you can limit it legally, just run your limiting cable from the top of the shock to the eye at the bottom, therby invoking the "any" shock rule
wink.gif


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Carl
#33 CSP/soonITA/IT7
[email protected]
 
Ok, heres what Im gonna do. Twin 2" pipes running under the axle collected in front of the muffler with a flowmaster 2/1/4 in 3" out wye connector (it looks like a good free flowing design)http://racerpartswholesale.com/flomstr1e.htm

Then I'm going to get a Dynomax ultra flow muffler, cut it open, gut it, pack it with lava rock (I got this idea from a guy on mazspeed.com) and re weld it shut. Much cheaper than Mazdacomps $400 muffler. And lastly, make a tailpipe that goes past the bumper cover. Sound ok?
http://www.a-1performance.com/race_magnum/...um_mufflers.htm

I'm trying to decide between one of the ovals or rounds, I'm thinking the biggest I can fit, guess Ill have to measure once I get the other pipes in place.

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Carl
#33 CSP/soonITA/IT7
[email protected]

[This message has been edited by Rx7Carl (edited May 20, 2002).]
 
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