New full exhaust system, no additional power???

[JC6715]

Here's the situation: '67 289 C4, Stealth intake, Holley 600 Vac Secs, Ignitor electronic ignition, blaster coil, 100 amp alt, ram air hood scoop. Just installed hooker comps to 2.5" H-pipe to Hooker aerochambers dumped before the axle, 2.5" for all tubing... Ran good before the upgrade

I don't really feel any power difference in the seat of my pants... I thought for sure I would, sounds good as hell, but doesn't feel any faster...

Any ideas? Did I go over-board with the 2.5" system (Dual system before was 2")??? Please help, I am very disappointed right now...

Thanks everyone!

 

[Dark Knight]

For a pretty much stock 289, it's probably to big. The stealth is probably over kill too.. a mild cam and some steeper gears would help it use that bigger exhaust.

 

[D.Hearne]

You've got a 250 hp engine feeding an exhaust for a 400+ hp engine. I don't agree though about the Stealth. High rise dual plane intakes aren't rpm sensitive. HAve you changed the secondary spring in the Holley to let it open sooner?

 

[JC6715]

I am planning to do new heads, cam, and gears down the road... Am I still going to be over-exhausted then? I could have sworn somewhere that I read 2.5 was good for 250 to 400, not 400+... Great... What a buzz kill... Is there anything I can do to help it out until "down the road"?

 

[JC6715]

Yes, D. Hearne, I have monkied with the secondary spring... Speaking of rejetting, from my motorcycle experience I know you have to rejet after an exhaust upgrade... Same thing go for cars?

 

[D.Hearne]

You could try it, but I doubt it'll make a difference you can feel. You've unfortunately bought into the "bigger is better" exhaust marketing. I've got a dual 2.25" system that was bent locally (no fancy mandrel bends here) on my Ranger with Shelby tri-Y's and a home made equalizer pipe before the mufflers (this is basically a 4" x 12" long section of exhaust pipe that the dual 2.25" dumps into then exits back to duals) And this system was plenty good enough for the 400+ hp 331 stroker motor I had in it. Yea, a bigger system eliminates back pressure, but that's not always a good thing and not always something that you can feel after it's installed. And I love the guys who think that just because it's louder, must also mean it's faster. Run your car for a week or so, then pull the plugs to see what they're telling you as to if it needs more fuel. Then go up if needed in 4 size increments on the jets. Your 600 should have 66's in the primary side go up to 70's and run it a week then read the plugs again.

 

[gsxrken]

I wouldn't say you are over-exhausted and now losing power b/c of that, but it does sound like your original exhaust was not limiting your output... and so a freer flowing one would not make a difference.
Kinda like putting a 5" tall air cleaner woudn't pick up any power, either, if your 3" one already allowed more air than the engine needed.

 

[bad68coupe]

You've got a 250 hp engine feeding an exhaust for a 400+ hp engine. I don't agree though about the Stealth. High rise dual plane intakes aren't rpm sensitive. HAve you changed the secondary spring in the Holley to let it open sooner?

What do you mean when you say high rise dual plane intakes aren't dual plane sensitive? is an edelbrock performer rpm a high rise?

 

[ga289stocker]

I went from 2" to 2.5 inch on my 65 289 and I could tell a difference. I also at the same time went from stock exhaust manifolds and h-pipe to tri-y's w/ x-pipe w/ magnaflows and turn downs at rear valance. My car is very similar build has X-celerator intake, holley 600 dp, it's a 4sp. My old system had some very tight bends as well. The new is nice mandrel bends. With this exhaust plus, electric fan, dialing in my rear suspension, pertronix and making sure I got WOT i went from 15.80's to 14.40's...

 

[2nd Mustang]

I have a similar setup, but with a 302, single plane manifold, mild cam, 3.25 limited slip rear, etc. The previous owner installed a 2.50 inch exhaust from the header collectors to the turbo mufflers, but used the skinny original pipes from the mufflers to the tailpipe GT tips. I dumped the skinny pipes and tips and had the muffler shop weld on 2.25 inch pipes to the rear valance. That woke up the performance noticeably. When it comes time to change pipes, I think I'll go with a 2.25 inch exhaust all the way to the rear.

I read somewhere that some pro racers tend to keep the exhaust on the small side for their motors to force the motors to work harder.

 

[D.Hearne]

What do you mean when you say high rise dual plane intakes aren't dual plane sensitive? is an edelbrock performer rpm a high rise?

High rise dual planes simply do not lack anything on the bottom end, or as they say "off idle" You've got the each side feeding 1 (at a time) cylinders, vs a single plane with a huge open plenum feeding off 4 bbls that all cylinders feed off of, or in other words each cylinder is drawing the fuel/air mix from a large open reservior, and that slows down the mixture velocity at the bottom end and you get a sluggish engine till a certain point in the rpm range. High rise dual plane intakes like those I listed are all high rise intakes (taller than the stock 2 & 4 bl intake with larger runners) and give you almost the same performance on the top end as a single plane (notice I said almost) but don't suffer from a "too slow" velocity at the bottom end. I've used the Performer RPM and the Ford A321 on 302's and neither suffered from dead spots off idle. I've also used the same type intakes on 390's and 427's (428PI, and 428CJ) and had the same results, good bottom end, no different from shorter "low rise" 4 bbl intakes but a bit more top end power. The manifold makers need to revise their advertising when it comes to these intakes, it's really misleading. They have a somewhat definate top end limit for power production, but no limit on bottom. Sure when you use one on a mild engine build you may not use all the capacity at the top, but you'll leave nothing on the table at the bottom, plus if and when you decide to upgrade to bigger cams, heads, etc, you won't need to run out and buy another intake. As an illustration, I used the RPM intake and a Holley 600 on a completely stock 88 Crown Vic 5.0 with it's crappy E6SE heads and base roller cam and it worked fine off idle. The only mod I did with this engine was add 1.7 Cobra roller rockers and remove the Thermactor bumps from the exhaust ports. This is a 302 rated at 150 hp. The intake, and carb might have gave it some more power over the EFI, I can't tell though for certain, and the two mods plus dual exhaust might have given it 20 hp more. You could hardly describe it as even a mild build , but it worked fine with the RPM intake.

 

[65notch]

i agree that the 2.5 may be a little big for ur current setup. 2.25 would have been the way to go. when going to big you run the chance of loosing the velocity through your exhaust system which can hurt your low end tq.
are u have full tailpipes or are u dumped right after the muffler?

 

[JC6715]

65Notch- I am dumped right after the mufflers... Would running 2.25" full tail pipes help at all?

 

[Max Power]

Performance exhaust will only help you if the previous exhaust was a restriction. Looking at your combo, I would say your heads and cam are the restriction.

 

[rebel65]

yes, go all the way back to the bumper, you will notice a difference.

 

[bnickel]

think of it like this. since your engine is basically just a big air pump the bigger exhaust isn't necessarily hurting but the engine isn't taking in any more air than before and the old exhaust was adequate for the setup so the new exhaust isn't putting out any more than the old system. change the cam and port the heads or install different heads and the engine will take in more air and make better use of the new exhaust.

re-jetting the carb is likely to make a couple more hp as the engine is better able to get rid of the spent gases and it's a good idea to maximize the combo anyway but it probably won't much of a difference you can feel.

adding tailpipes from the muffs back will help build a little backpressue and may produce a little torque so you might notice a small difference from that.

 

[xoxbxfx]

I think your expectations were to high... I have never been in a car where a full exhaust system gave you any power you can feel in the seat. Maybe a few HP...but not enough to really make a difference. You will see it on a dyno, but not in the car.

 

[65notch]

65Notch- I am dumped right after the mufflers... Would running 2.25" full tail pipes help at all?

yes go all the way back. i believe the hooker aero chambers are designed to be a mid car muffler, meaning it should have a tail pipe so that atmospheric air cant go in and cause some restriction...versus a muffler that can be used on a car like a modern camaro where the mufflers at the rear and the exhaust dumps after it...these mufflers actually have a champer at the rear or the muffler to help trap that thats trying to back up the exhaust.
another thing i thought about was with the too big of exhaust pipes, you lose that velocity and then reversion happens which will cause restriction also. cause no power gain or loss in power...
so yes put 2.25 the rest of the way.