[Slick Poncho]

Wondering if my exhaust X pipe is a restriction. Long story short I wheeled into test and tune Friday night a little late and didn't have much time to prepare for my first pass and ended up running through the full exhaust where as I would normally open my electric cut outs located on the back of my pypes 2.5" X pipe. The car went 12.00 through the exhaust, the next pass no other changes other than opening cut outs went 11.86. This got me wondering if my X pipe is possibly a restriction. The car is a 72 lemans sport 428+.030, forged rods/Pistons, comp xr288hr (236-242 110lsa,)using endurex solid roller lifters,kre d ports with mild port work, worked t2,1"open spacer, 950 Holley, hooker super comp 1 7/8 to 3" collector reduced right down to a complete 2.5" pypes stainless exhaust. Anyone else see any gains going to a 3" X pipe even with cutouts on the 2.5"??

[Mikes455Wagon]

I have always heard to maintain collector diameter thru the X. My collectors are 3.5" and the x pipe is also 3.5". I have long tapered reducers after the x to 3" mufflers and saw no real loss of performance compared to open exhaust.

[455TA]

What was your 1/8 mile times? I have the same cam, 305 cfm round port e heads, performer rpm intake with no spacer, 1 7/8 inch dougs headers with 3.5 inch collector into 3 inch x pipe. I run 7.40's in the 1/8th at 94 mph with a 3.42 gear and 2600 stall. I think you have a restriction for sure.

[Lee]

2.5" pipes are, IMHO, small for your combination. If you cutouts are PAST the X, then you really didn't test whether the X is a restriction or not. I would not run less than 3" on motor such as yours.

Either the rest of your exhaust system is a restriction, or your carb is tuned rich (opening the cutouts leaned it out some, improving the AFR).

[Slick Poncho]

1/8 mile was 7.48@90.5 with the cutouts opened up. I'm aware I didn't test if the X pipe is a restriction, I was wondering if it's worth my time to disconnect the pipes at the header, sounds like it is. I feel like this combo is capable of mid 11's or so. I don't get out as often as I'd like to so my tuning process is slow. My intentions are to mess with carb and timing this week if I make it. I'm running a 200-4R and 3.90 gear,continental 10", 28" tires. 60ft times are typically 1.60-1.62

[ta man]

I'd step up to 3 inch..On my car with 3 inch exhaust..I had 2.5 inch mandrel tailpipes..many thought they weren't a restriction..but I took them off and picked up a full tenth...

[Slick Poncho]

Ran Friday night with X pipe removed and just a turn down bolted to header. Car actually slowed up to 11.91. Only other change was I ran my 28-10.50 slicks instead of my 295-55-15 mt et SS since I hauled it in anyway figured may as well run slicks. The plugs do look a tad lean, I will bump up a jet or two and see what's up this weekend. This car is making me beat my head against the wall. Installed a fiberglass front bumper and hood and first outing on a cool September night it barely runs any better than at Norwalk in 90 degree humid air.

[BruceWilkie]

Couple things to look at...

You can generally support 500 hp with a dual 2.5" exhaust before it becomes a restriction to making more. 3" will get you to @700 hp before it can start getting difficult to find gains.

Any pipe beyond your collector flange(rest of ex system) changes the collectors effective range. Some lengths will actually kill power because the reflected waves will arrive back at the ex port at the worst time.

Losing restriction often leans mixture but be sure you are reading your plugs correctly.

You can easily find bottle necks in your ex system by taking pressure readings at various points. harder to make the test points but overall not too challenging.

You can do a systems approach and put expansion chambers between tuned open headers and a quieting system behind the expansion capable of adequate flow thru it. The expansion chamber volume needs to be double or more ci per side. Make expansion entry as large as practical. The sudden large increase in area and volume "fools" the engine into "thinking" its open atmosphere. A 500 ci that expansion area could be a pair of boxes 4x9x14 or larger or oval/cone shape at same or greater volume.
Obviously under chassis real estate determines what you can do.

[PAUL K]

If the car went slower with open headers, why would the exhaust system be a problem?

[BruceWilkie]

Originally Posted by PAUL K

If the car went slower with open headers, why would the exhaust system be a problem?

C'mon Paul there are plenty of reasons why... most often its both collector length and the overall effect on tuneup. He was running exhaust with dumps on his x pipe with better results. Then slows down with only open turn down off collector. hes changed effective collector length which may be worse length than before and hasnt verified the tune to see what may or may not need adjusting.

[Slick Poncho]

I had a local engine builder tell me they use a collector and strait piece of pipe equal in diameter with a painted stripe down the side of it. They make a pass and the point the paint is burnt off to Is where they cut the pipe off at. I figured this would br a cheap try anyway, any thoughts? I like the idea of the box, having room may be a issue, an exhaust will have to back on at some point for street driving whether it's the 2.5" or a 3" is still the real question.

[slowbird]

I see one big change that no one seems to be pointing out. He removed the radials and went to bias-ply. That is a loss of et and mph. Who knows what the exhaust change did vs the tire change?? It was a mistake taking the radials off

[PAUL K]

Originally Posted by BruceWilkie

C'mon Paul there are plenty of reasons why... most often its both collector length and the overall effect on tuneup. He was running exhaust with dumps on his x pipe with better results. Then slows down with only open turn down off collector. hes changed effective collector length which may be worse length than before and hasnt verified the tune to see what may or may not need adjusting.

Bruce the OP feels his engine is off by half of a second. He's ran it with a decent exhaust and with open headers and found little difference. Do you really think he will find a half second gain in the exhaust?

[BruceWilkie]

Originally Posted by slowbird

I see one big change that no one seems to be pointing out. He removed the radials and went to bias-ply. That is a loss of et and mph. Who knows what the exhaust change did vs the tire change?? It was a mistake taking the radials off

Good catch! Focus on one thing at a time. Just know sometimes one thing alone may net you a loss untill you try another thing to compliment it. Holds true with suspension, drivetrain as well.

[slowbird]

Fyi a good exhaust should show very little change in et/mph on or off. I ran 9.0@147 with and with exhaust.

[Slick Poncho]

My mistake, I had no clue there is that much difference between the radials and slicks. I intended going back to radials this weekend anyway so that's what I will do. I'm not sure if I'm off by half second of et, finding guys with pontiacs at the track to talk to is hard to come by let alone someone with a 428 combo. Lots of 460+ci combos similar that seem to be in the low 11 territory.

[slowbird]

My guess is you'll run slower with the x-pipe off but you need to test. Also try to test it all in the same day to limit weather difference as a variable.

[Cliff R]

From the testing I've done here these engines LOVE big head pipes, X, H, or nothing it doesn't matter. I would use nothing before the mufflers less than 3" for any reason.

On the same topic it doesn't seem to matter much what you use after the mufflers, I've seen NOTHING (in terms of ET or MPH) with the same exact headers/3" "H" pipes and straight thru 3" mufflers using 3" turn downs, crappy 2.5" full length tail pipes, and really nice mandrel bent 2.5" tail pipes.

I would also say here that there is a HUGE difference in interior/exterior noise, and inside the car resonance with different tail pipes, but that's usually not a big deal over hear in the "Race" section.....FWIW.....Cliff

[Steve C.]

Something to consider, a Dr. Gas X-type crossover that is 3-1/2" from the collectors to the mufflers is VERY heavy !


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[Skip Fix]

FWIW my car ran almost identical times when I had the 455 roller motor and loose convertor in it with open exhaust and no mufflers or tailpipes even on as it did with a 3" Dr. Gas X into 3" Ultraflows and 3" Torque Tech tailpipes even with that additional weight. 6.90s @ 99 in the 1/8