differences in ET/MPH full exhaust vs open exhaust - Chevelle Tech

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post #1 of 12 (permalink) Old May 23rd, 10, 8:22 PM Thread Starter
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differences in ET/MPH full exhaust vs open exhaust

i have a feeling my exhaust is holding me back. i have a 2.5 inch system with an x pipe, ultraflows with full tails out the back. it is hand bent. i'm looking for a close to optimal exhaust setup and i am thinking of going to 3 inch with an x pipe dumped right before the rear axle. i havent run with open headers because after unbolting the exhaust there is no where for the exhaust to go as it rides up against my doughnut style gasket. if i push it down it scrapes up the gasket and the gasket is stuck to the collectors. i'd rather not break them.

i am trying to get some real world information on your cars with full exhaust vs open headers or exhaust that is real short or dumped. please give me your before and after ET/MPH.


where can i get a 3 inch system with an x pipe that dumps right before the rear axle as a kit?
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post #2 of 12 (permalink) Old May 23rd, 10, 8:32 PM
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Re: differences in ET/MPH full exhaust vs open exhaust

I just ran my car yesterday pypes 2.5in. x pipe race pros with tailpipes .11.75@113.With open headers 11.85@112.once.Mostly 11.90 -12.0s with the same system with street pros.

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post #3 of 12 (permalink) Old May 23rd, 10, 11:01 PM
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Re: differences in ET/MPH full exhaust vs open exhaust

Quote:
Originally Posted by curley8788 View Post
i have a feeling my exhaust is holding me back. i have a 2.5 inch system with an x pipe, ultraflows with full tails out the back. it is hand bent. i'm looking for a close to optimal exhaust setup and i am thinking of going to 3 inch with an x pipe dumped right before the rear axle. i havent run with open headers because after unbolting the exhaust there is no where for the exhaust to go as it rides up against my doughnut style gasket. if i push it down it scrapes up the gasket and the gasket is stuck to the collectors. i'd rather not break them.

i am trying to get some real world information on your cars with full exhaust vs open headers or exhaust that is real short or dumped. please give me your before and after ET/MPH.


where can i get a 3 inch system with an x pipe that dumps right before the rear axle as a kit?
Mike,

Back in the the late 90's on my old 383 SBC combo, I picked up 4 MPH in the 1/8th going from a 2.5" compression bent exhaust with 3-chamber Flowmasters to 3" home made mandrel bent exhaust (mitered and welded 90 deg. mandrel bends etc.) with 2-chamber Flowmasters. The car was running mid 11's at ~116 MPH at the time.

I've run 111.3 MPH in the 1/8th with 3.5" exhaust and 3" straight through design mufflers.

Jason Gore
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post #4 of 12 (permalink) Old May 23rd, 10, 11:28 PM Thread Starter
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Re: differences in ET/MPH full exhaust vs open exhaust

great info. i was thinking of reusing my 2.5 inch dynomax bullets on the end of the 3 inch system (with reducers of course) to cut cost. am i correct in assuming this will not affect performance?
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post #5 of 12 (permalink) Old May 24th, 10, 1:51 PM Thread Starter
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Re: differences in ET/MPH full exhaust vs open exhaust

i called pypes and they said their x pipe kit puts the mufflers too close to the driveshaft so i cant use my bullet mufflers id need some offset mufflers. does anyone know of a kit that spreads the exit pipes out far enough to clear a large driveshaft
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post #6 of 12 (permalink) Old May 24th, 10, 1:58 PM
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Re: differences in ET/MPH full exhaust vs open exhaust

the pypes kit with THEIR race mufflers
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post #7 of 12 (permalink) Old May 24th, 10, 2:20 PM Thread Starter
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Re: differences in ET/MPH full exhaust vs open exhaust

id rather not buy new mufflers when all i need is the pipes to be spread apart a couple inches.
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post #8 of 12 (permalink) Old May 24th, 10, 2:56 PM
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Re: differences in ET/MPH full exhaust vs open exhaust

summit and jegs both sell universal X pipe head pipes. Did you ever get your vibration fixed?
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post #9 of 12 (permalink) Old May 24th, 10, 3:09 PM
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Re: differences in ET/MPH full exhaust vs open exhaust

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Originally Posted by curley8788 View Post
id rather not buy new mufflers when all i need is the pipes to be spread apart a couple inches.
More than likely your mufflers are the restriction. I have a friend who has a '70 Chevelle with a 450 h.p. - "502" and he did the small mufflers thing. It has always been a dog. Low 12s is the best it has ever run, he even changed to a bigger hydraulic roller trying to get it to run better, no luck. Still ran low 12s no matter what he did. It's a 4000 lb. slug. At about 110 mph it's done. Mustangs are blowing dirt in his face with regularity.
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post #10 of 12 (permalink) Old May 24th, 10, 3:15 PM
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Re: differences in ET/MPH full exhaust vs open exhaust

I like to use http://www.torquetechexh.com/. They should be able to set you up with whatever you need.

When I went with their 3" x-pipe (over the axle and out the back), I had a 400-450hp 383 and the old system was a 2.5" muffler shop cheap job. At the time, all I changed was the exhaust and if I remember right, it was worth 0.3-0.5sec, but my old system had some nasty bends. I tuned the carb with open cutouts, but at the track it ran the same open or closed. On the highway, it got much better fuel economy with the cutouts open (2-3 mpg better).

I kept the exhaust for my current engine, which is in the 550-600hp range. Now there's a couple tenths and couple mph lost when I close the cutouts and the plugs seem to foul out faster. This could be somewhat due to the flowmasters, which probably weren't the best choice for performance.
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post #11 of 12 (permalink) Old May 24th, 10, 3:34 PM Thread Starter
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Re: differences in ET/MPH full exhaust vs open exhaust

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summit and jegs both sell universal X pipe head pipes. Did you ever get your vibration fixed?
i havent balanced the tires yet. i want to do this exhaust and that and go to the track before i restall this converter.
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post #12 of 12 (permalink) Old May 24th, 10, 3:57 PM
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Re: differences in ET/MPH full exhaust vs open exhaust

I have a full 3" mandrel bent system, H pipe right after trans tailshaft and Dynomax mufflers. I have cutouts right off the headers. I have run it back to back open vs closed and it runs the exact same ET/MPH both ways. This is a 525 tq/475 hp SBC.
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