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post #1 of (permalink) Old 12-06-2000, 05:56 PM Thread Starter
 
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HEI in 258 - port or manifold vacuum?

I've been experimenting with my HEI, having run it off manifold vacuum w. a Crane adjustable vacuum advance for quite a while now in my 84 CJ7 (258). I recently tried using ported vacuum off the MC2100 carb, and it seems to improve the bottom end power, possibly at the expense of the top revs. For those of you using an HEI unit, are you using ported or manifold vacuum for the advance? Stock vacuum advance pot? (Is that 20 or 10 degree pot)? Faster or slower mechanical advance?

Tim

84 CJ7, 258, HEI, M/C 2100 carb, 5 inch lift, RS9000's, 33x12.5 BFG M/T's, Lockright, full cage & belts, Xenon flares, AMC 20 1pc axles, GM dual diaphragm brake booster
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post #2 of (permalink) Old 12-06-2000, 09:46 PM
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Re: HEI in 258 - port or manifold vacuum?

TIM,

MY CHOICE IS PORTED. IT LET'S THE IDLE BE MORE FLEXIBLE AS FAR AS CARB ADJUSTMENT ETC. DOES'T GIVE THE OFF IDLE DROOP WHEN THE VACUUM DROPS ON ACCELERATION EITHER.
IT WILL MAKE MORE POWER IF THE TIMING ADVANCES AT A LOWER SPEED(MECHANICAL). ONLY PLAYING WITH YOUR PARTICULAR COMBO WILL NET THE MOST GAIN. PUMP THE TIMING UP AS FAR AS THE ENGINE WILL STAND WITHOUT SPARK KNOCK. YOU CAN FINE TUNE THE MECHANICAL WITH THE CURVE KIT AVAILABLE IN MOST PARTS STORES. BACK OFF A COUPLE OF DEGREES MORE TO BE ON THE SAFE SIDE. YOU MIGHT ALSO TRY ONE OF THE VACUUM RESTRICTORS THAT WERE USED ON ALMOST EVERYTHING IN THE LATE 70'S AND EARLY 80'S. THEY WILL DELAY THE ADVANCE FOR A FEW SECONDS SO IT ONLY ON WHILE CRUISING. YOU HAVE A LOT OF ADJUSTMENT AVAILABLE TO TWEAK IT TO YOUR DRIVING STYLE.

GOOD LUCK,
JACKIE

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post #3 of (permalink) Old 12-07-2000, 08:11 AM
 
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Re: HEI in 258 - port or manifold vacuum?

GM designed the HEI distributor to use Ported Vacuum.
I recommend Ported vacuum with an adjustable vacuum advance so you can play with advance rates.

"I Have The Body Of A God... Buddha"
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post #4 of (permalink) Old 12-08-2000, 10:12 AM Thread Starter
 
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Re: HEI in 258 - port or manifold vacuum?

Switched to ported vacuum. Big difference. I can now drop the pedal all the way at about 1400 rpm and have the engine just roar to life. I am finding that less accellerator pump gives me more throttle response. This is odd. I have the pump linkage on my MC2100 set at the 2nd lowest setting. The lowest setting I can't get the rod retainer to hook up, so I can't try it. Perhaps this carb is jetted too rich, or the Power Valve is set to kick in too early. I am very impressed with the setup and performance of the MC2100 and HEI. I am interested in fine tuning to get the best out of it that I can. The 258 has all the performance I could ask for, and I sense that I can tweak even more out of it.

Thanks for your suggestion to use ported vacuum. I will experiment with the adjustable vacuum advacne some more, as well as using lighter spring on the mechanical advance.

Tim

84 CJ7, 258, HEI, M/C 2100 carb, 5 inch lift, RS9000's, 33x12.5 BFG M/T's, Lockright, full cage & belts, Xenon flares, AMC 20 1pc axles, GM dual diaphragm brake booster
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