C4 Tech/Performance L98 Corvette and LT1 Corvette Technical Info, Internal Engine, External Engine

Foe people who have converted to F-body ignition

 
Old 05-17-2013, 09:52 AM
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bjankuski
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Default Foe people who have converted to F-body ignition

I converted my 406 over to an F-body ignition (remote coil) years ago due to clearance issues with my intake. The ignition system is all stock other then a high output MSD coil. It has always appeared to work fine but on my new 406 I am seeing a big drop in power (On the chassis dyno) after 6000 RPM and I do not believe it is related to valve train issues. The current combination of engine parts should rev to 6500 RPM peak power and hold strong to 6800 RPM. Instead the power climbs straight up to 5900 RPM and then goes over a cliff and drops hard and the engine appears miss slightly. (AFR is fine goes a little rich)

I am wondering if the stock distributer and ignition module are not up to the task of running the engine at 6000 RPM and above? I am looking for people opinion on this subject. Just replacing parts is not an option.

Thanks
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Old 05-17-2013, 10:08 AM
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C409
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..... I'm using the MSD small cap HEI distributor with external coil for the same reason ... clearance ... As I recall reading somewhere , the TBI trucks used the same distributor as the F-bodies but had a different , possibly governed , control module ... I don't remember the details ... but I believe it was a rev limiting issue ... perhaps you can check the part nos on your ICM to rule out that possibility .........
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Old 05-17-2013, 10:10 AM
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cgantner5150
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If I had to guess (I'm running the same setup on my 385 HSR) I think possibly spark scatter inside the cap? The small diameter of the cap and the amount of advance would lead me to believe that it is trying to fire the plugs when the rotor is between lugs. Maybe it jumps the gap to the previous lug instead of the intended one? At high RPM that rotor is really moving. Coupled with the high power coil and accompanying spark. The large cap HEI spreads the lugs a bit farther apart - maybe it helps?

If you really sit and think about it, it's a wonder it works at all. I may be way off in my thoughts but it makes sense to me.
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Old 05-17-2013, 10:14 AM
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383vett
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I have run a stock distributor and now an Accel replacement distributor on my 406; both with a remote coil. I've always run an Accel 7 pin ignition module and an Accel Supercoil and have not had problems running to 6800.


Last edited by 383vett; 05-17-2013 at 10:17 AM.
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Old 05-17-2013, 02:40 PM
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Rohn
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I shift at 6000. stock GM dist Petronix cap and rotor and Delco module.

I had same issue come on last summer. I replaced cap rotor with above and module(28 years old). Cap rotor was visably worn.

Car ran like new. OBTW it threw a code for EST in datalog but no CE light.
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Old 05-18-2013, 01:59 AM
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Cliff Harris
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Originally Posted by bjankuski View Post
Instead the power climbs straight up to 5900 RPM and then goes over a cliff and drops hard and the engine appears miss slightly. (AFR is fine goes a little rich)
Sounds like it's going into knock retard. You need to datalog to see if that's what is happening.
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Old 05-18-2013, 07:48 AM
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..... The OP is an experienced tuner ... you might want to take another look at the valve spring rates ( measured vs advertised ) at YOUR installed heights and clearances ... hyd roller lifters are heavy little lumps and can be tough to control ... improperly set up springs can exhibit all sorts of dance moves if harmonics are uncontrolled ... springs should be installed to within .050-.100 ( no more ) of coil bind at max lift with as much retainer to stem seal clearance as possible ... keep in mind that advertised coil bind heights don't include the inner spring step ( .100" ) on the retainer ... put the spring in a vise with the retainer on the spring and measure actual cb and double check push rod length and thus , rocker geometry ... hyd roller cams and lifters limit spring rates to around 150# on the seat and 450# over the nose at max lift .......
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Old 05-18-2013, 09:24 AM
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bjankuski
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Thanks for the replies. I am suspect of the ignition and valve train. The rocker arm geometry is fine i set it up myself and have double checked it. The valve spring set-up has me concerned. I was shooting for 150 the on the seat at 1.8 installed height. I had my local machine shop check the pressure at 1.8 and it was 150 according to him, but when i check the pressure with my arm bar pressure tester i get 180. I need to varify the pressures
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