Need thoughts on new dizzy, vacuum advance or not?

[whytry]

Well it looks like some of my problems came from my dizzy. I currently have the Unilite 9000 Mallory, mech advance locked out @ 40 per Kevin @ CSU taking out 2 degrees per lb boost, ending @ 28.... The module went out, got great use out of it, 12 years.... Now I have read both +/-'s of vacuum advance on Forced induction with locked out dizzy. For those of you running boosted applications, are you running mech advance or vacuum? Car will be street driven mostly, some track time, but mainly street driving....

I am looking for rec's on a new dizzy, what and why? Vacuum advance or not?

THX..

[HILROD]

How are you pulling the timing? MSD? I am running MSD dist. on 2 boosted cars now. I tried to run vacuum advance. It helped a lot down low, but I couldn't keep the advance steady at part throttle. It runs way smoother with vacuum and mechanical advance locked. Why pay to get stuff you don't need.

[whytry]

Right now I am pulling timing via the MSD BTM.... My current setup is mechanical advance only. I am trying to decide if I should get a new dizzy that has vacuum advance or not.

[whytry]

BTW Mechanical will be locked as it is now.

[Fuelie Fan]

Is it street driven? Do you care about mileage?

[Blown353]

I'd recommend you go with either a digital programmable ignition or a distributor with vacuum advance.

When I was still running my engine blowthrough carb I had my distributors mechanical advance locked out at 36 but I still had vacuum advance (up to 46 degrees of timing with vacuum advance) and I used an MSD 6BTM to pull timing out with boost.

Without something (i.e. either a vacuum advance or a programmable ignition) to advance the timing during conditions of higher vacuum such as light load cruise your cruise mileage will suffer and to a lesser extent it won't keep the plugs as clean. For a street driven car a vacuum advance is a must IMO if you even want to think about any sort of light load/cruise combustion efficiency.

For example, right now I have my engine idling at about 30 degrees and cruising down the freeway at light load it runs about 15.5:1 AFR with 44-50 degrees of timing depending on the RPM (about 44 degrees at 1800rpm/60mph up to 50 degrees at 85 mph/2500 rpm.)

You will probably have to dink with the vacuum canister itself to match it to your engine-- the vacuum rating of the diaphragm & spring and total allowed vacuum advance will need to be tuned to your application. Or if you go with say a MSD Digitial ignition you can simply program in your complete timing curve.

[buns]

Could someone enlighten me on the term " locked out distributor"?. Maybe I'm just not understanding the terminology, but is it just a matter of changing the stops on the advance weights? When some posters talk about it, it almost sounds like the advance is " all in", all the time.

Maybe it's just a geographical language problem as I live on an island in the middle of the Pacific Ocean.



https://static1.pt-content.com/images/pt/2007/12/funnynewcar-1.jpg

[69Bbird]

No advance vacuum or mechanical it's locked out

[Blown353]

No advance vacuum or mechanical it's locked out

You can also lock out just the centrifugal (mechanical) advance and keep the vacuum advance. That's what my blowthrough setup liked/performed/got the best mileage with.

[whytry]

You would basically take the springs/weights out of the mechanical advance and put stop bushing there instead with the advance "all-in" all the time as you mentioned. Blowthrough setups don't like advancing dizzy's when making boost..

So I am leaning toward the MSD 8361. That should help with mileage becuase its vacuum and mech. I will probably lock it out @ 36 and go from there for starters, and see where to go from there. If I am going down the wrong path let me know.... To shut the car off I will just be dragging the clutch in gear, nothing wrong with that right?

[Blown353]

So I am leaning toward the MSD 8361. That should help with mileage becuase its vacuum and mech. I will probably lock it out @ 36 and go from there for starters, and see where to go from there. If I am going down the wrong path let me know.... To shut the car off I will just be dragging the clutch in gear, nothing wrong with that right?

That's what I would do. Start by locking out the mechanical at around 36 degrees but leave the vacuum advance on to help idle, light load, and cruise conditions, and use your BTM to pull out timing under boost.

As far as shutting it off, no problem doing that. Back when my car was carb'd I shut it off in-gear when I had the automatic, or I'd very lightly drag the clutch once I put the TKO in.

Just remember to watch out for MSD's distributor gear depending on your cam...

[whytry]

I was planning on pulling the gear off of my Unilite, Bronze one, and putting it on the MSD unit. The gear is about 2 years old but probably 500 miles on it... Hyd roller cam...

[Blown353]

Whose hydraulic roller cam and on what kind of core? You might be able to run a honed out GM Melonized gear which will wear practically forever, unlike the bronze gears...

[whytry]

It's from Comp Cams, they recommended to change to a bronze gear when I changed cams... I read somewhere on here about that GM gear. Might be a good time to change it anyways.

How do I know if my current one is wearing out? I have not pulled to dizzy out yet, will once I get the replacement. I do know that when its put in place in it still "rocks" a little bit even when the dist bolt is tight...

[Blown353]

A little bit of backlash is normal.

Depending on what HR cam blank you have from Comp, you could have an all-cast cam, all-billet cam, or billet core with pinned & pressed iron gear. Post up the grind # on your cam card and I can tell, a -8 core is cast, a -9 is billet, and if it's a billet cam with the iron distributor gear under "Spc Instr" on the cam card it will say "cast dist gear"

If the cam is all-cast or billet/iron gear you can run the GM Melonized gear. If it's billet with a billet gear, then you're stock with the bronze (or poly) gear.

[whytry]

I don't have the card with me but I remember it is a -8, so I guess its a cast core. Can I get the Melonized gear @ the dealership? Would Tognottis carry it?

[camcojb]

I don't have the card with me but I remember it is a -8, so I guess its a cast core. Can I get the Melonized gear @ the dealership? Would Tognottis carry it?

first off, I owe you an apology. I got your pm, but was in a hurry and didn't respond. Then I forgot............... :hand:

The melonized gear comes from GM, Togs won't have it. Also, with the MSD distributor the gear has to be honed to fit, as the MSD has a slightly larger shaft size than the gm distributor.

I have a new melonized GM gear, honed to fit here at the shop. I bought it from Lewis Racing and never used it. I can let you have it and you can order me a replacement from Mike. He's the only one I know who stocks them honed for the MSD's. Or if you have time you can order one from him, but if you need it this weekend you'll need mine.

Jody

[Blown353]

The GM Melonized gear on a -8 cast core is fine and definitely the best & longest wearing (practically forever) option. Just slather it up with moly assembly lube since it's a brand new gear.

What are your valvespring pressures? The cast core rollers (even the HRs) can wipe the lobes off with too much spring pressure.

[whytry]

Hey Jody, don't worry about it, I know this time of year is usually pretty crazy... I might be headed over to watch some friends on Saturday at the track if its open. I would be more than happy to get you a new gear if I can get yours. I am supposed to pick up the distributor tomorrow @ the track. One of my friends has one that he put in his motor for 1 start, then pulled it out for a ProMag setup.... So I might be somewhat in the area. I could call you tomorrow if it works out. If not I will probably order one.

I will have to check valvespring pressure. I really don't know off hand...

[whytry]

Did not meet up with my buddy, so I tried to do some testing with my current setup. Went to total timing from 40, to 36, to 32, to 28. Problem was still there not as bad, but still there, and there was definitely a loss in power in acceleration as timing went down. I also tried it @ 28 with no BTM hooked up to make sure it was not moving from 28 and the power was definitely not all there, and there was a miss still at cruise. :(

[Y-TRY]

Does your blow-through carb still have the ported vac. port in the primary metering block? I wanted to run vac. advance but don't have anywhere to hook it up. Out of boost, it runs like any regular car and could probably benefit from more advance while cruising.

I opted for the MSD Digital E-curve distributor and love it. No vac. canister or anything, no weights. Really cool for non-efi cars like us.

I think I used an intial 28* for start-up with a very steep advance to 38 with 2*/lb boost retard. I'd also like another 4* possible advance through the vac. (for a total cruise advance of about 42*). The problem is that I have no port left for the vac. advance from the carb. MSD says that boost won't hurt the sensor in the distributor

[Blown353]

You'll probably have better luck with manifold vacuum to the distributor. It'll pull more timing in at idle and clean it up.

[Adam's 55 Chevy]

Thanks good thread guys. I bought an 8361 for the same reason. Racing and street use. I will be locking out the mechanical at 36/38 and pulling it out with a digital 7531 box. The vac advance will be referenced to the intake plenum for street driving, cruising and driving in the pits. If I get it right it seems like the best of both worlds.