Boost Timing Master and Vac Advance?

[70Blwnova]

Anybody use the MSD 8962 Boost Master Timing with Vacuum Advance? It doesnt appear on there website anymore but I see it at Summit and Jegs.

Since there is only one vac connection to the MSD box I assume its manifold vacuum and therefore present at idle?

Are any of you running blowers with dizzy vac advance?

Is there any difference to using this or just using the vac advance canister on the dizzy and connecting to manifold vacuum?

Thanks

[HILROD]

The BTM has a vacuum hose so it knows boost. It pulls timing out only under boost. I also run vacuum advance.

[Blown353]

I've used one. Worked OK, but it's somewhat limited in function like the standard 6BTM box or BTM add-on. I've also used a 6BTM in conjunction with a standard vacuum advance canister.

However, there is a much better option available now: MSD Digital 6AL2 and pick up a 2 bar GM MAP sensor to go with it. You lock the distributor and use the software to setup a fully programmable timing map and set the centrifugal advance, vacuum advance, and boost retard plus rev limiter. Much more precise and versatile than the BTM, and I have a lot more faith in the reliability and accuracy of an external GM MAP sensor than I do the sensors built into the BTM boxes.

http://www.summitracing.com/parts/MSD-6530/

I've set up a couple street cars now with both the 6AL2 and a Digital 7; same software and it's simply a superior setup compared to the old analog BTM boxes, either the integrated ones or add-ons.

[70Blwnova]

Thanks Guys - all this blown motor timing thing is new to me and half the problem is that I don't have a ported vac and I dont want detonation. Also I have an Accel dizzy that is not co-operating hooked up to an Accel 300+CD that I got for a good price. Looks like I may have to go MSD in the end. It seems to be that the adjustment of the mech advance in an average dizzy is very limited and that the MSD pro-billet is the only one that gives enough adjustability. Electronic seems like the way to go if you need something real specific and not generic. I'll look into the 6AL2 - thanks for the hook up.

[70Blwnova]

Dang you - Blown353 - I've looked at the programmable 6AL-2 and now I want one! Seems sooo much easier than screwing with weights and springs. Might need to wait until next year! Thanks for the hook up.

[HILROD]

Most people you'll find lock out the mechanical timing when running boost. Mine is locked. The object mainly is to keep all the timing advance down low. At higher RPM as boost rises vacuum advance goes away and the BTM pulls timing out.

[Blown353]

Dang you - Blown353 - I've looked at the programmable 6AL-2 and now I want one! Seems sooo much easier than screwing with weights and springs. Might need to wait until next year! Thanks for the hook up.

LOL... I can have that effect on people.

Trust me... after screwing around with mechanical advance, locked out mechanical advance, standard vacuum advance, and the analog boost retard boxes and various combinations of the above for the last 7 years or so the Digital-7 or the less expensive but still very capable 6AL2 plus a GM MAP sensor is the way to go. Much more precise to tailor your ignition advance any way you need it plus built in programmable rev limiters and start retard.

Lots more reliable IMO too... the GM MAP is time tested and proven and you don't have to worry about mechanical weights wearing and hanging up, you don't have to worry about a vacuum advance canister sticking, etc. You end up with a locked out distributor and a MAP sensor which is a lot less to fail, drift around, or move.

Does the old method work? Sure, I ran that way for a while. But the new digital & MAP sensor based systems are leaps ahead in terms of precision, functionality, and ease of setup. Why go back when the 6AL2 plus a MAP sensor costs just a little more than either a 6BTM or a 6AL plus a BTM add on box?

The MSD software can be a head scratcher at first as it works "backwards" for most people's line of thinking. It works in degrees of retard per RPM and by manifold pressure. So you retard more at lower RPM and less at higher RPM to simulate a mechanical advance curve and you retard less at high manifold vacuum tapering down to a nominal amount at 0 inches of manifold vacuum to simulate vacuum advance, then retard more than that under increasing manifold pressure to serve as boost retard.

I typically lock the distributor at around 45 degrees with all the software retards at 0; this will allow up to 45 degrees full advance for cruising conditions and still allow you plenty of timing retard under boost. If you don't need too much boost retard then I like to set them at 50 degrees so you can run up to 50 degrees advance at light load cruise.

This scheme is very similar to a standard distributor trigger for an aftermarket EFI box where you put lots of advance in the distributor and then the EFI box time delays the ignition box trigger output to produce the desired timing. This scheme does however create an out of phase relationship between the rotor and the cap plug terminals; if the distributor is set at 45 degrees for pickup purposes and you're at WOT and have the box only firing at 25 degrees the rotor will be out of phase with the cap terminals by 20 crank degrees. This can be compensated for by re phasing the reluctor wheel on the distributor shaft such that it sends the signal to the box at 45 degrees BTDC but the rotor will be pointing directly at the cap terminals at 25 crank degrees BTDC. Ideally you want the distributor to be phased such that the reluctor signals the box at your setup maximum advance (for example 45 degrees) while the rotor will be centered on a cap terminal at your peak torque timing advance (for example 25 degrees) because peak torque is where cylinder pressures and spark current demand are highest and thus there is the greatest chance of a misfire. Rephasing the reluctor to rotor relationship takes care of that.

Is it needed? I've never had a spark or misfire problem in either an EFI or MSD Digital install I attributed to an unmodified reluctor to cap relationship. I always rephase my own stuff or really high power applications just to rule out the possibility though.

Keep in mind a phase difference of 20 degrees at the crank between distributor mechanical position for triggering and peak torque timing is only 10 degrees of movement at the distributor rotor so the phasing error really isn't that great.

[70Blwnova]

....its gonna take me a while to absorb that - thanks for the great info - I'll be in touch when I save enough pennies for the box. Thanks again!

[camcojb]

....its gonna take me a while to absorb that - thanks for the great info - I'll be in touch when I save enough pennies for the box. Thanks again!
Troy has that affect on people......................... :)

Jody