I have used the MSD conversion module in my HEI for over 2 years now. It replaces ALL the stock stuff including the condenser. I added the MSD coil also. Not the cheapest combination but it works really well and is completely compatable and maint free. mgg
So your saying that the NAPA module is AS good as the MSD, or that stock modules don’t start to drop off at about 4500? What’s the price of the NAPA module (generally)? I might stop by a NAPA store tomorrow. I also work at high performance parts warehouse, so it’s equally easy either way. One other question for you IgnitionMan, what about re-curving an HEI? Does it help... etc
Thanks for the detailed post. Thank you also Manny Gomez.
Manny, when I build an HEI, I run it intact when I am finished with the rebuild, to check the output levels. I have not found any significant difference in spark output or rpm limits in any of the MSD, the GM 990 or NAPA TP45 modules.
What I have seen is a definate series of problems with the HEI that will lower the spark voltage output and rpm levels. Too much plug gap, excessive compression from either mechanical/chemical supercharging, the stock coils, weak pickup assemblies, low input run voltages, weak or bad plugs and plug wires, and modules that have been overworked and partially damaged from layer shorted high tension coils.
Moving the coil away from the cap in an HEI is the best thing to do for it. Not onlty do you get a much better coil than can be built for in the cap use. This also lowers the in cap temps as much as 45/50 deg/F.
Accel modules are no more than stock GM off-shore (NOT 990) modules, and have low output and are rpm limited from their internal circuitry. Next time you see an Accel module, flip it over, chances are you'll fing the words Made in Hong Kong or Made in Singapore on them, same as the stock, low performance GM modules.
Re-curving the HEI is mandatory for any serious performance work, both the mechanical and vacuum curves. Reasons why are the stock HEImis designed for lower output, emissions engines, and allow way too much mechanical and vacuum advance for use with modified engines intital timing requirements. Most HEIs were set from 0 to 6 degrees initial timing, and when advanced to the proper initial timing for the modified or early engines, causes a large detonation problem.
Example: 327 Chevy engine, mild cam, 10.00:1 c/r, headers, nice dual plane manifold and 600 carb wants about 12 degrees BTDC. Now has an HEI designed for 0 degrees BTDC installed.
Engine wants 36 degrees total mechanical timing (initial and mech curve), HEI has a 36 degree mech curve, and added up the engine now has 48 mechanical degrees total, way, way too much for non-detonation operation. Add a vacuum advance that gives 18 to 24 crank degrees, and the engine is in the twilight zone of timing.
Cures: curve the HEI to have 24 crankshaft degrees, coming in at the proper time for the cam, c/r, etc. Use about 6 to 8 vacuum advance degrees, on full manifold vacuum to help pull against the converter, not the massive amount of added timing from a stock vacuum can, and the engine will be happy again and live on junk pump gas.
I have found the over the counter HEI advance kit with the springs and weights/"limiter' plate are just about useless for the plate/weights. You can kill a lot of problems by getting the Crane adjustable vacuum advance kit, with the three sets of springs, adjustible vacuum can, and enclosed spiral degree limiter plate. Use the stock weights and plate. I don't remember the exact number for the Crane kit for stock HEI, but I seem to remember 99609-1, do check the number, please.
If the engine you are running has over 11.00:1 c/r, or is chemically/mechanically supercharged, then you might think about the MSD instead of the HEI. With the higher c/r useage, tighten the plug gaps down with any of the different ignition systems, and this includes the MSD, and other C/D systems.
I recently had a friend running a 150 horse nitrous shot in his 383 s/b, with an MSD, car went well until he hit the bottle, then laid down. He tried just about everything anybody would try, no help, still laid down and popped. He asked me, I asked what plug gaps, he sain .050 as MSD recammends, I said to go down to either .035 ot .032. He didn't believe me, said the MSD would take care of it. I said he should just humor me, he ended up doing that a couple of weeks later out of sheer desperation, set the plugs at .032. He immidiately dropped his et by .46 second, and raised the speed by 9 mph, with no more laying down or popping. He has since been able to continue tuning the carb and nitrous systems for another .15 sec and 3 more mph.
Compression ratios do affect spark system performance, and some higher c/rs won't let an HEI work as well as lower c/r engine packages.
Sorry this is so long, but the full book version is still at the publisher's.
Hey Ignition Man,
Good info! Thanks for taking the time to post this. It helps to hear from someone that has real hands on experience with this stuff. I've always wondered about the difference between performance ignition components and the stock stuff. Kind of hard to tell which coil puts out more heat just by using the seat of the pants. mgg
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