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I recently upgraded to a MSD street fire ignition. After install I needed to adjust the timing with the distributor a significant amount. I have a 454 with HEI and a throttle body. Now I got it running smooth untill I connect the vacuum advance. After 1500 RPM's it really misses. I disconnect the vacuum advance and plug the port on the throttle body and it runs great. Why does it cause the engine to miss. and do I really need it?
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where do you have the timing set at?
 

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Did you adjust the vacuum canister with an allen wrench. I dropped the same distributor in my nova yesterday and had to turn it almost all the way in to bring the amount down where I wanted it.
 

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Vacuum advance needs to be tailored to your engine just like the mechanical advance.
 

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You need to get the specs for YOUR 454. Some 454's only run initial and mechanical. NO VACUUM. There was another guy on here with a 454 with the same issue. He found that the 454 he had DID NOT use vacuum advance.
 

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You need to get the specs for YOUR 454. Some 454's only run initial and mechanical. NO VACUUM. There was another guy on here with a 454 with the same issue. He found that the 454 he had DID NOT use vacuum advance.
The only way this would happen would be a later style setup where the HEI had a computer controlled advance, which is not what you want. All of the older 454s had vacuum advance.
 

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Discussion Starter #8
The engine is a 2006 GM PP HO 454. I have it set at 30 degrees without the vac connected. It runs well and does not knock. My sense is it does not use vacuum advance.
BTW, the MSD Street fire runs smooth and has noticeable more throttle response.
 

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Bob Is that 30degrees of initial? My streetfire distbutor has about 18 of mechanical. I set mine at 18degrees initial and with mechanical makes it 36. If you have 30 initial with another 18 mechanical you have 48 then hooking up vac.advance will add even more. Try checking the timing at 3000 rpm or so and see how much timing you have got.
 

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This is a later 454. Computer controls the advance.
 

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The engine is a 2006 GM PP HO 454. I have it set at 30 degrees without the vac connected. It runs well and does not knock. My sense is it does not use vacuum advance.
BTW, the MSD Street fire runs smooth and has noticeable more throttle response.
30 initital? this means that you are running close to 50 degrees total.
that sounds like about 10 degrees too much. this sure sounds like a recipe for disaster when the detonation monster shows up.
back the initial off to 18 or whatever it takes to get the total mechanical advance in the high 30's somewhere and hook the vacuum advance to full manifold vacuum to bring the initial back up to 40 or so at idle.
this should be a decent starting point- the rest is all tuning to see what your particular motor wants.
 

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This is a later 454. Computer controls the advance.
we're not talking about a stock 454 as installed in a stock vehicle here-it is a GM aftermarket crate motor installed in an old car with a carb and regular distributor.
 

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Oh I got ya. It's just funny how it ONLY misses when the VA is connected. To me, that says it doesn't need the VA, especially if he is only running 30* total. It can't be over advanced.
 

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Unless he has the mechanical advanced locked out, he has around 50 degrees total like derrik stated.

Charlie,just at idle he has 30degrees and i bet quite a bit of manifold vacuum. When he hooks up the vac advance, it adds to the initial. The hei he has adds 22 degrees of advance with 14inches of vacuum and that is with it at the lowest setting. While i dont know how much vacuum he is pulling or how many turns out he has the advance unit adjusted, when hooked up the motor is overadvanced. Just reving the motor to 1500 will increase the vacuum advance even more. When i set mine up right out of the box I set it at 36 total and 18 initial, but when I hooked up the vac advance my initial jumped up to 40 before I adjusted the advance canister down to 30 with the advance hooked up to it.

Hey Bob stick a 3/32 allen wrench in your advance canister nipple,turn it counterclockwise till it wont go in any more. Set your timing to whatever u want the total(no vac advance) to be at with motor around 3000rpm. Check your initial timing to see where it is and aid in changes, then hook up the vacuum advance. I'm betting you'll still end up around 30 with the advance hooked up. Lets us know how its working.
 

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I recently upgraded to a MSD street fire ignition. After install I needed to adjust the timing with the distributor a significant amount. I have a 454 with HEI and a throttle body. Now I got it running smooth untill I connect the vacuum advance. After 1500 RPM's it really misses. I disconnect the vacuum advance and plug the port on the throttle body and it runs great. Why does it cause the engine to miss. and do I really need it?
Thanks
Car is running a throttle body (retroteck) from what I see in the sig. I think he needs to see what the intial is when his electronics to the dist are turned off (spout connector pulled if it has one) I know nothing about the retrotech set up but most throttle bodies you have to pull a connector to see what you intial is then go from there. I'll bet it does not use vacume in this set up since most electronic systems don't. He should check his retrotech install intructions and see what they say.
 
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