The only way you would really build any sort of substantial vacuum at the top end would likely be if your induction system was too small for the motor.
chevy small block timing
- Thread starter ERIC87442
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the info i found for adjusting a chevy V8 for performance is:
initial + mechanical = max 36 @ 2500-2800
max with vac 52
http://www.corvette-restoration.com/res ... v_Spec.pdf
initial + mechanical = max 36 @ 2500-2800
max with vac 52
http://www.corvette-restoration.com/res ... v_Spec.pdf
im talking about ventury vacuum, not manifold vacuum. i am running a vic jr and a blowthrough 750.so i highly doubt there is a restriction in my induction system. you are supposed to hook up your vac adv to a ventury vac, not a manifold vac source.Doober said:
😳 Understood 🙂 Mine is hooked up to manifold vacuum and it's much more responsive down low, I have no ping to speak of.
torjan said:
so according to this? mine is initial (20) + mechanical (30) = 50? in order for this to work id have to retard my timing alot? i really really feel im missing something here
It still holds true for the most part, I'm pretty sure HEI has more advance added from vacuum. Basically what that pdf says is that 52º is while you're cruising down the road (and yours may not really see that much at cruise speed), not while the car is sitting and idling. The reason it has so much timing while cruising is the engine is under cruise mode, where there is vacuum, which in turn is causing the vacuum canister to add timing. At the same time the engine is turning somewhere between 1800 and 2600rpm (all dependent on vehicle speed, gearing, tires, etc.), so there is also mechanical advance... in which case it could be nearly all the mechanical advance, which could give that high advance. The only way you will really find the optimum spark curve for your own is to set it and tinker with it, every motor is a little different from the next. 10-15º with the vacuum line disconnected is a decent starting point. After the car warms up, shut it off and see if it cranks over ok. If it starts fine, give it a little nudge in a +timing direction, repeat. When it starts getting hard to turn over, back it off a little until it easily starts successively, and drive it. If it's responsive and doesn't knock, it's likely you've found your happy medium. If it knocks, take out a little more timing until it stops.
SLICK79WGN said:
sry but not seeing a difference between HEI and other distrubutors in adjusting. 😳
I didnt know if there was a difference so I was just pointing that out, and allot of the points type didnt have vac advance so it makes a big difference in tuning :?
all i know is when i ran my n/a motor i was running total( all mechanical advance in) 36 degrees. that was at 3500 rpm and i was running a 3800 rpm stall and some drag radials. it never knocked and it ran slower at 38, and slower at 34. 36 was its happy place. the engine was a 13.5:1 406 with a solid roller cam and good fuel. i never ran vac advance.
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