Ok..so I have been reading all the posts on vacuum advance and the whole ported vs manifold vacuum debates.Interesting reading so far.Question is why is it that I cannot run vacuum advance on my z without it detonating at cruise???I had a Summit Hi Performance HEI in it that I swore was breaking up/bad and chased the problem for over a year..replacing other parts that wasn't needed.I finally broke down and installed an MSD pro billet (swearing the HEI was bad..and I wanted it anyway) only to have no difference..car still detonated at cruise..which I originally mistook for a miss.Even with the MSD billet the car still missed/detonated at cruise.Brainstormed for awhile and unhooked vacuum can and woila..car ran excellent without it hooked up.So here I am with no vaccum can and a good running car..I have it set right now at 18 degrees at idle with 36 total timing (black bushing in pro billet giving 18 degrees mechanical advance) in before 2500rpm.I want to run the vacuum can for the added benefits when street cruising (which is often in summer months)..but it detonates right where I cruise.Car has a Demon 750dp..MSD 6AL fired 10:1 355 with AFR 195's..Weiand Stealth..Comp XE274 flat tappet hydraulic..4 speed..3.73's.How or what do I need to change to be able to use the vacuum can??Less initial timing also develops a flat spot on initial take off on street driving.Carb has the blue pump cam on #2 hole with a #31 squirter...80/86 jets..5.5PV.Anyone have an idea??

 

Adjust the vacuum advance for less of it. Very typical when no EGR is used.

 

The MSD Pro Billet can isn't adjustable as far as I know..it is a 15 degree can IIRC...and the Crane one for HEI's doesn't fit?!?!

 

It is adjustable. Stick a 3/32" allen wrench into the vacuum port. Counter-clockwise will decrease the total amount of vacuum advance it gives you. Keep backing it out a few turns until it goes away.

 

What Pro Billet are you using?

 

I wanna say P/N 8361? The one that requires an MSD box.

 

You have the one with vaccum advance?Then yes it's an 8361.Thanks for that info..I never knew it was adjustable..there is no mention of it being so in any of the paper work that came with it from MSD..damn..duh

 

The 3\32 allen adjustment in the vacuum can adjusts the " rate " of when the vacuum pulls in...not the total amount. Although you can limit the amount of vacuum dialed in at cruise with this method, a better way is to limit the travel with a bushing.

Limit vacuum total advance to 15 degrees. Don't know if the MSD distributor has limiter bushings for the vacuum can but it might. If no adjustemnt you can always shorten the travel slot by welding or brazing.

Check amount of vacuum advance with engine running. Use a mighty vac pump.

Your timing curve is also too agressive.

quote:
I have it set right now at 18 degrees at idle with 36 total timing (black bushing in pro billet giving 18 degrees mechanical advance) in before 2500rpm

That's coming in way too fast, even for mechanical advance with no vacuum advance. Try 14 degrees static, 22 degrees mechanical advance in the distributor for a total of 36 degrees total mechanical. Total mechanical should be in NO sooner than 3,200 rpm. Try to get it all in by 3,500 RPM. BTW...what is your cruise RPM

That should eliminate the over advance at cruise. BTW....hook up vacuum advance to manifold vacuum...that helps with tip in stumble.

Deal with the flat spot on light acceleration ( if it still remains )with carb tuning.

Couple of other things. What gear ratio do you have and what type of tranny...Auto or stick? Auto's and tall gears may require a slightly bogger shooter.

Fuel quality????. What are you running for fuel? You have a fairly high C.R. and fuel quality in some States frankly sucks. Shell is one of the worst. Chevron not as good as it used to be. With that CR you should be running a good 94 octane. Try to run a fuel that runs an Ethonol blend,,,,10% or so. Fairly common these days. I believe that Union 76 and Sunoco have alcohol blends. Car will run much better and detonation should be a thing of the past.

My car is a 355 with 10.4 to 1 CR, Edelbrock Performer RPM Heads, heavily modded 780CFM VS and a Performer RPM manifold. Cam is Hydraulic Roller 218\228 at .050" , 112LCA , .500"\.520" lift.

I switched to Mohawk fuel ( Ethonal blend ) several years ago. Difference in running of car was amazing. Car used to diesel in hot weather running Chevron 94 Octane. Switched to Mohawk 94 Ethanol blend and I can easily run 2 to 4 degrees more static with no pinging or run on. Typically run 14 to 16 static, giving 34 to 36 total with Crane adjustable vacuum can setup with 15 degrees total and full vaccuum in by 6" HG.

 

Could be a vacuum leak on one or more cylinders.
A lean mixture at cruise on one cylinder can cause it to ping. a vacuum leak can also be the source of a pesky
of idle flat spot or spudder as one or two cylinders misfire while other seem to be getting plenty of fuel.
Propane is very usefull in tracking down a manifold leak.
I agree advance curve is too agressive.
All in by 3200 is better.

 

[/B][/QUOTE]Limit vacuum total advance to 15 degrees. Don't know if the MSD distributor has limiter bushings for the vacuum can but it might. If no adjustemnt you can always shorten the travel slot by welding or brazing.

Check amount of vacuum advance with engine running. Use a mighty vac pump.

Your timing curve is also too agressive.

That's coming in way too fast, even for mechanical advance with no vacuum advance. Try 14 degrees static, 22 degrees mechanical advance in the distributor for a total of 36 degrees total mechanical. Total mechanical should be in NO sooner than 3,200 rpm. Try to get it all in by 3,500 RPM. BTW...what is your cruise RPM

Couple of other things. What gear ratio do you have and what type of tranny...Auto or stick? Auto's and tall gears may require a slightly bogger shooter.

Fuel quality????. What are you running for fuel? You have a fairly high C.R. and fuel quality in some States frankly sucks. Shell is one of the worst. Chevron not as good as it used to be. With that CR you should be running a good 94 octane. Try to run a fuel that runs an Ethonol blend,,,,10% or so. Fairly common these days. I believe that Union 76 and Sunoco have alcohol blends. Car will run much better and detonation should be a thing of the past.

My car is a 355 with 10.4 to 1 CR, Edelbrock Performer RPM Heads, heavily modded 780CFM VS and a Performer RPM manifold. Cam is Hydraulic Roller 218\228 at .050" , 112LCA , .500"\.520" lift.

I switched to Mohawk fuel ( Ethonal blend ) several years ago. Difference in running of car was amazing. Car used to diesel in hot weather running Chevron 94 Octane. Switched to Mohawk 94 Ethanol blend and I can easily run 2 to 4 degrees more static with no pinging or run on. Typically run 14 to 16 static, giving 34 to 36 total with Crane adjustable vacuum can setup with 15 degrees total and full vaccuum in by 6" HG. [/B][/QUOTE]

I run 3.73's with a T10 4 speed.AFAIK MSD supplies no limiters for the vacuum can with bushings..I will have to look into limiting the total amount of vacuum on the MSD can with the spot weld.I can slow down the timing curve with springs..right now I have one light and one medium in the dist.I'll set it up with two medium springs next weekend.Cruise RPM is around 2700-3200rpm with the 3.73's.Usually try to run Sunoco 94 as it is the highest octane (supposedly) around my area.I can set it up for 15 degrees initial with 21 degrees mechanical advance with the MSD supplied bushings..but when I had it set up like that I had a wicked dead spot off idle..but then again that was before I got the carb set up as close as I do now..so I will try it again.Willing to try anything..thats the fun of tuning..1 step forward..2 back..at least for me this past year.

 

i have a pic of a way to adjust vac adv for virtually free. i just need someone that can post it here.

i recieved this pic from a guy on another board.


he basically uses a small plate with a slot and (2) 6/32 SCREWS to hold in place. He did tell me i have to grind the screws down on the back side so they clear the rod.

the pic explains it all. This will work on ANY vac adv.

send me an e-mail address to someone that can post the pic here.

 

Here it is..many thanks for that Mike.

 

thanks for posting that. Again the guy said instead of the 2 holes to make a SLOT, then it's adjustable.

 

I can set it up for 15 degrees initial with 21 degrees mechanical advance with the MSD supplied bushings..but when I had it set up like that I had a wicked dead spot off idle..



you shouldn't have that off idle stumble if you hook to MANIFOLD VAC. because with 15 initial and 15 vac adv( only obtainable at idle with manifold vac) you will have 30 deg of timing at idle.

 

i also run a pro billet MSD dist with the vacuume advance. when i hit higher RPMs, the car kinds studders and bogs a bit like it cant hit it right. cant figure it out...

 

retard your timing

 

yeah the whole car has to be timed and tuned anyhow. going to take it to someplace since im terrible at all that stuff. i just like putting it togeather