drwet

I am about to pull the distributor (MSD Pro-Billet HEI) out of the '79 for a rebuild. Last time I had it out there was too much end play and I didn't have the right shims to correct it. I now have the parts, and I'm going to take the time to set it up properly. I have been setting up distributors for more years than I choose to remember, so I don't need advice on how to do it. What I would like however, is some opinions on what timing specs to use. My last engine was a 383 with a moderate cam (224/224), Trick Flow heads, and 9.5:1 compression. It seemed to like 14 deg initial, 34 deg total all in by 3000, and another 16 degrees of vacuum advance. My current engine is a 427 small block, AFR 220 heads, 10.25:1 compression, and a hydraulic roller cam with 233/241 duration. My inclination is to add a little more initial because of the longer duration camshaft, reduce the mechanical to stay with about 34 degrees total, and keep the 16 degrees of vacuum advance. This sound about right?

gkull

I would buy an adjustable vacuum advance and turn it down to 8-10 additional. 16-20 initial is a good starting point

Gunfighter13

10-12 initial, 24-26 mechanical all in by 3000 (total 34-38). The 16 vacuum is OK. All on 93 or better fuel.
Just my recommendation.

cv67

You know youll have to experiment and give the engine what it wants but most heads with a nice chamber dont need a bunch of total. Bet 33-4 gets you real close .

lars

16 - 20 initial. 34 total mechanical at about 3000 rpm. About 10 degrees of vacuum advance on manifold vacuum to produce actual timing at idle of about 30 and total combined maximum timing of about 44.

Lars

drwet

Thanks everyone. This is when it would be nice to have access to a dyno. The numbers are all pretty close to what I had in mind. I was thinking the 16-18 initial, as well as the 34 by 3000. However, I have always shot for about 50-52 total with the vacuum advance. Why would you limit the total to around 44? (I should have added its a stick.)

gkull

I had a high dcr motor in my vette it would rattle ping going down the highway at light cruise throttle

the fix was the adjustable vacuum advance can turned down to the 8 10 range

drwet

Thanks. I will keep that in mind. I didn't have a problem with 52 degrees total in the 383 and I've been running the same curve on the 427. What do you consider a high dynamic CR?

gkull

I'd have to run all the numbers again, but it was up over 8.4. it was a 10.8 static with a 232/238 6 inch rod 3.750 stroke. It was always on the edge of detonation and the ultimate fix was to increase the cam duration dropping the DCR

lars

In addition to the dangers of part-throttle detonation when running more than 12 degrees-or-so vacuum advance (with total mechanical in the mid- 30's), modern fuels have a quicker burn rate than the fuels that these cars were originally designed for. Henry Olsen has done some excellent testing with the oil companies using a 5-gas analyzer to demonstrate that you do not want to run total combined timing in excess of 48 max. We see excellent results when staying within these limits.

Lars

Jebbysan

drwet

Thanks. That makes sense. The fuels we have today bear little resemblance to those we had 30, 40, and 50 years ago. I don't suppose you have a link to Olsen's test results?

drwet

Heads are ca 2017.

v2racing

I'm at 34 degrees all in at 2600, 20 initial, with an adjustable vacuum advance limited to 8 degrees, but I'm running ported vacuum. I run the ported because my vacuum at idle is 9 inches in neutral and 7 in gear. When you put it in gear the vacuum drop lowers the advance which makes the engine idle speed drop which makes the vacuum drop which..... There is a great deal of difference in off idle response, and light throttle cruise as well as gas mileage with the ported vacuum hooked up.

I run 3/4 93 octane mixed with 1/4 100 street legal pump gas with my 11.8 to 1 compression. For straight 93 I still ran 34 total all in at 3000 but only 10 initial and vacuum advance limited to 6 degrees. It ran good on 93, but the mix allows me to kick the timing in quicker and really makes this thing come alive.

Mike

bjankuski

very good start position with the afr heads.

My go to is 20 degrees from zero to 2000 rpm and then ramp in from 2000 to 3000 rpm the additional timing to get to 32 degrees with afr hesds. Add vacuum on top of that, I usually limit total mechanical and vacuum to 45 degrees.

gkull

Drwet, skip white racing makes turn key crate motors and go to their website and dig down to engine and cam specs. It will be an eye opener to most people of how big of h roller cam it takes to make something dyno tested over 600 hp from a 427 or 434 ci sbc

drwet

I had a look. Those are some big cams. The only change I would make to my engine were I to do it over might be to go with a slightly smaller cam. Don't get me wrong, it works great, but the idle can get a little annoying in traffic. I can't imagine what it would be like with 253/261 !

cardo0

Without a dyno you might want to consider old school testing on a long slow inclined slope using seat of your pants for a performance gauge. Now to make it more scientific add a stop watch and a note pad to record your accelerations between 2 locations at the same starting speed. Next step up is a dashboard mounted accelerometer like the G-Tech (BTW made by Tesla) or there maybe copy cats dashboard accelerometers by now (G-Tech has been on the market over 20 years).

Something that lars made me realize years ago is to stop adding advance when power stops (or in this case accelerating) stops improving. Because below 3000 RPM you can add a whole lot of advance w/o any knock but it's just not good for your engine at all to run with excessive advance.

Hope this can help and good luck.

drwet

In case anyone is interested I pulled the distributor today, shimmed it to .030" and set up the curve. I took a couple of degrees out of the mechanical advance, and set it to 16 deg. at idle. That gives me 34 deg. by 3000 rpm. Vacuum advance is 14 for a maximum of 48 degrees. Now I just have to wait around for 4 or 5 months to try it. Bring on the global warming!!