leeperryracing

I have a bullet racing custom hydraulic roller in my 355ci motor that specs out at .573" lift in/exh. 234/242 @.050, advertised dur 289/297 on a 110lsa and a 106 intake center line. How much initial advance will make this cam happy? I've been told to go 18*+ initial and limit the mechanical curve in the dist. so total advance comes out to around 34-36 max. I also run a 125 and ocassionally a 150shot of nitrous and 18*+ initial sounds scary or am i over worrying? so lets say 20* initial and 14* mech.=34* total, if i pull 6* for the nitrous wouldn't that still be too much initial timing for N20? I don't run any vacuum advance either.....hows does vacuum advance tie in with nitrous use? car is NOT a daily driver but street driven regularly.

Apeiron

For that much cam, I'd try locking the advance out and setting it at 34-36, maybe pulling some out when you spray, with however much vacuum advance it likes on top of that.

Vacuum advance doesn't matter with nitrous. Your manifold vacuum is essentially zero at WOT when you're spraying.

305sbc

iTrader: (3)

That wasn't bad advice but you might want to go even higher on the initial to about 22* or more, and take more out of mechanical. As long as your total advance isn't too high (that's why you remove some mechanical advance ) you won't have any issues using N2O.

The only issue with running lots of initial advance is starting the engine - especially hot starts. Your engine will not spin as fast and it could break your starter and/or flexplate gears.
The cure for this is an iginition kill switch. Kill the ignition (spark) before starting, then turn the key to spin the engine, then turn on the spark while the engine is spinning. It will make for easy starts and much longer life for your starter.

Apeiron

A start retard works too, if the convenience is worth the money to you.

leeperryracing

didn't think that cam was such a "radical" piece....I had the comp NX288HR in there before and that cam was a .520"/.540" lift, 236/248@.050 with 288/315 advertised dur. on a 113lsa/108ICL and it ran like a champ with 18* initial(on motor, pulled 4* for a 125shot) and 18* mech. for a 36* total....even with slightly longer duration than the bullet cam i'm running now.

rjmcgee

I have a lot smaller cam (XE274) and it has the best idle with 24* base timing. I limited the mech advance to 12*. First thing I did before changing the advance was buy a good gear reduction starter.

Then just hook your vacume advance to manifold vacume for what light throttle cruising you do. By doing this I have seen 18 - 20 mpg on the freeway with 4.10 gears.

leeperryracing

so say i set the base timing to 2X* and limit the mech. advance accordingly, all i have to do to have the vacuum advance work "right" is to hook it up to a ported vacuum source right? i'm running an air-gap intake and a holley 750dp(#4779C)....between both of those, which or what source should i hook the vaccum advance to? in other words, where on the carb or the intake i must tap into for the best source?

305sbc

iTrader: (3)

Straight manifold vacuum.
On a performance application it's best to have some adjustability in the vacuum advance mechanism. With a lot of initial advance you also need to limit the vacuum advance as well so it doesn't add as much as for a factory application. Between 10 and 15* is all you'll need from the vacuum advance.
If your vacuum advance isn't adjustable then you could just install a limiter screw to limit the travel of the advance arm.

I've had to graft vacuum advances onto other style distributors and modify them for a limited amount of advance. Here's an example of a Chevy advance mounted on a Toyota distributor installed in a Dodge Omni. The V8 part added too much advance for the 4 cylinder. Here (pic link)you can see what I'm talking about with the limiter screw. Having an adjustable style advance unit saves the trouble though.
link:
http://i165.photobucket.com/albums/u...stributor1.jpg

rjmcgee

On my Demon there is a manifold vacume port, that is where I have my advance plugged in. You'll end up with about 50* timing cruising down the highway, but as soon as you floor it the manifold vacume drops to zero and you'll have your base + mechanical at 34*-36* total. This also gives you a better idle with a bigger cam since the vacume advance will be seeing manifold vacume at idle. I think I have about 38* at idle with the vacume advance hooked up.

leeperryracing

thanks a bunch for the insight guys! so how do i go about getting 10-15* of vacuum advance and keep it around there and no more?

rjmcgee

You probably need to get an adjustable vacume can. There is an allen screw inside where the vacume hose hooks on, it only changes how fast or slow the can works. There is a little eccentric that goes on a screw on the base of the distributor that you adjust to limit the travel of the arm off the vacume canistor.

The eccentric looks like this.

leeperryracing

ok, i'll pop the dist. out and examine it properly to see where I need to go from there based on the info you gave so far....thanks a lot