Intake manifold for 75 vette

[Bill H.]

What would be a good street intake manifold that would fit under the hood of a 75 Vette?

[Mark E.]

Re: Intake manifold for 75 vette

Are you the same person who asked about headers a few days ago?

I recommend the stock cast iron manifold if your engine is stock, especially since it's a 1975 (165 or 205HP with a single restrictive converter with lots of back pressure). A benefit of the original cast iron is better thermal stability, although it does weigh a bit more. A benefit of the original cast iron exhaust manifolds is much less heat radiating to the interior and less noise.

Years ago I purchased a 1973 L82 with M20 and 3.55 gears. The PO installed long tube headers, an Edelbrock Performer intake, a Holley 650 CFM spread bore carburetor, and aftermarket open element air cleaner (the original cowl induction set up would not fit because of the added height of the aftermarket intake and carb). Luckily the car came with its original parts in a box. I reinstalled the original exhaust and intake manifolds, Quadrajet and air cleaner. Guess what? I think it actually ran a bit better. I don't know about 0-60 times but throttle response was better. Cold air from the cowl induction probably helped.

For your '75, the biggest performance bang is removing the single pellet-type catalytic converter and installing dual exhaust. Exhaust back pressure is the biggest reason why engine power declined 45HP from the non-catalyst '74 L82. The second biggest bang is recurving the ignition timing. My educated guess is these two things will gain 40-50 HP (for an L82). In contrast, the benefits of changing manifolds is negligible (maybe negative with the loss of the cold air intake). As a bonus, you'll save $, the interior stays cooler, and the exhaust won't leak.

Duke may chime in and I look forward to his input.

[Duke W.]

Re: Intake manifold for 75 vette

I agree with what you said, Mark. Dave McClellan mentioned in his book that the single catalyst exhaust added "half an atmosphere" backpressure. That's HUGE, 7.5 psi. The full dual system with a base engine is probably only 3 psi, max.

That's why I've always said that the Special 300 HP engine configuration (massaged heads, McCagh Special camshaft) that makes base engine low end torque/power and SHP type top end torque/power and revs won't work on the single catalyst exhaust engine because the extra inlet flow will just be eaten up with higher exhaust backpressure pumping loss, for little net gain in useable torque/power.

Duke

[Bill H.]

Re: Intake manifold for 75 vette

Thanks I already have converted the exhaust to dual pipes; was just wondering if anything else might help. Change cams?

Any suggestions on getting the A/C working again? Clean the system and change Freon ?

[Patrick H.]

Re: Intake manifold for 75 vette

No point in changing cams with the stock untouched heads.
Better off massaging the heads and starting there. Or, as we said before, tweak the timing and see how much that helps.

Old AC systems usually leak. I would have vacuum pulled on it, and check for leaks. If none noted, have them install R-12 and leak-detecting dye.
You may need a new drier first, because if the system has been open then the drier is in need of rehab or replacement.

[Joe L.]

Re: Intake manifold for 75 vette

Thanks I already have converted the exhaust to dual pipes; was just wondering if anything else might help. Change cams?

Any suggestions on getting the A/C working again? Clean the system and change Freon ?

Bill------


If by "change freon" you mean to remove what's in the system and refill it with fresh, that never needs to be done. However, freon may need to be added if some has been lost through leakage. And, if there is leakage, that needs to be fixed or you'll soon have the same problem.

If by "change freon" you mean change the refrigerant from the original R-12 to R-134A or something else, I don't recommend it. The system will not perform as well with anything but R-12.