5.0pntydrpr

Hello all. I just recently installed a comp cams XE270HR-14 and unported gt40 heads into/on to my car which is a 1966 fastback with an 89 5.0, long tube headers, 2.5 inch exhaust with magnaflow mufflers, pertronix ignition, and a weiand stealth dual plane intake.

I also installed a home rebuilt holley 650 dp which ended up running pig rich at idle even after tuning it correctly. So rich in fact that it fouled some of my plugs. I fixed that and got the car to idle a little leaner (about 10.5:1 a/f or so). With the carb like this, I could hear the cam loping (sp?) pretty well. I took it for a test drive and the car just felt like she was begging to go faster and it just didn't feel right.

Long story short, I put on an older carb of mine which is a holley 600 vacuum secondary p/n 80457. Now it still runs a little rich, but is SOOO much smoother. So smooth in fact that you can't even tell I have an aftermarket cam in the engine.

So my question is: Does carburetor sizing/tuning play a direct role in how car sounds, especially with a cam in it? I don't want an excessively rough idle, but I don't want it to sound stock, either.

Any and all information and past experiences are welcome. Thanks in advance.

Starfury

Tuning? Yes. 10.5:1 is extremely rich. It would definitely cause a lopey sound at idle, which is actually a misfire. A better tune would improve the mix ratio and fix the misfire problem.

Your cam shouldn't lope much, if at all. Do not tune for sound. No good comes of that. Tune for highest vacuum at idle and go from there.

5.0pntydrpr

Thanks for your input, Starfury.

That's what I was thinking. I just wanted to get a second opinion so to speak.

Would i be better to stay with this vacuum secondary carb or would a double pumper compliment the cam?

Starfury

There are more variables than the cam when choosing a carb. Do you have an auto or a manual transmission? What gear ratio are you running?

Double pumpers like low gear ratios and manual transmissions. They offer good throttle response if tuned properly, but are slightly more difficult to tune than vacuum secondary carbs. They are not known for their fuel economy.

Vacuum secondary carbs are better for cars with automatics and/or higher gear ratios, as well as daily drivers that see more city driving.

I run a 600CFM Edelbrock (air-valve secondary) on my aggressive 331 and it works fine in most scenarios. A double-pumper would probably give me a little more tuning power, but I like the ease of tuning of the Edelbrock.

In your situation, I'd probably stick with the 600CFM Holley you have. Tune it properly and I doubt you'll have any regrets.

5.0pntydrpr

That sounds good to me - less money I have to spend. Thank you for your insight.

boogerschnot

Try tuning the 600 holley with a vac gauge. Then check your accelerator pump shot. It should squirt as soon as you touch the accelerator but not bottom out with full throttle.