davidmartin148

Can someone tell me what the timing should be set at on a 1970 Olds F85 rocket 350 with an HEI distributor and a mild cam?


Thanks.

442Harv

As much as you can, before it pings, or hard to start.

joe_padavano

Unfortunately, you have a non-stock cam, a non-original distributor of unknown mechanical and vacuum advance specs, and you didn't tell us if this is a high or low compression engine or what octane gas you use. Bottom line, as Harv posted, is to keep advancing the initial timing a couple of degrees at a time and drive the car until it starts pinging under acceleration. Then back off a couple of degrees.

So long as you don't hold the car in the pinging for any length of time, you aren't going to really hurt anything doing this trial and error method.

davidmartin148

Thanks for the advice, Unfortunately I know nothing about what's been done to this Rallye besides it is a Rallye 350 and not a clone, It has been restored in 1991 and has been garaged since and the engine has ben rebuilt with a mild cam. I bought it this way I don't think the tranny is the original because the speed o meter is way off. could you tell me what drive and driven gear set I need for this same car? it has a 37 driven gear in it now, Here is the drive train, rocket 350 engine, Rochester Quadrajet Carb, TH350 Tranny, 3:23 gear with P235/60R14 tires. and at 75 MPH by my GPS I am turning 3.000 RPM,s (verified with a secondary tachometer) which doesn't seem right either. Shouldn't I be running in the lower 2.000 RPM range? But far a small block this car will really run, I am wondering if they might have stroked it when they rebuilt it cause that would explain the higher RPM,s I only run the high grade fuel and I will pull a compression test this week end and let you know the results. I am picking your brains because y'all know a great deal about these cars and I really appreciate all of your help and advise.

oldcutlass

On your timing, your going to have to figure what the car likes. Since you don't know whats in the engine, you'll need to experiment.


Whether it was a T350 or T400 neither will change the speedometer as they have the same final gear ratio. The only thing that will change the speedometer is a rear end gear change and major swing in tire size.


With 3.23 gears your Rpm should be 3250 @ around 75 with your gear ratio, tires, and trans.


Heres a calculator you can play with.


http://www.angelfire.com/fl/procrastination/rear.html

davidmartin148

I think this tranny came out of a truck with larger tires. Thanks for the advise.

joe_padavano

Well, first, your tires are about an inch smaller in diameter than the stock ones, so that alone will increase RPM at speed. I don't know why you think the RPMs should be 2000 at 75 MPH. That's about right for a newer car with an overdrive trans, but your TH350 has a 1:1 top gear. As noted, sounds like your RPM is about what should be expected.

Uh, no.

The RPM in at a given speed is a purely mechanical calculation that's unrelated to the engine. It is only related to the effective final drive ratio (the rear axle ratio times the gear ratio in the trans, or in your case 3.23 x 1.0) and the number of revolutions per mile from your tires, which is a function of the tire diameter (actually, it's a function of the loaded diameter, as modern radial tires deflect more than bias ply tires due to the more compliant sidewalls, so the effective radius is smaller than theoretical and the revs per mile are higher).

joe_padavano

Well, what does the speedo say when the GPS says 75 MPH?

Also, most truck TH350s would be a long tail and the A-body cars use a short tail.

oldcutlass

Since Buick, Olds, nor Pontiac manufactured a truck I find it very unlikely.

davidmartin148

By GPS at 75 my speedometer 90 and the tranny is a short tail housing.

davidmartin148

Yep your right about the truck I just had a brain fart on that one.

joe_padavano

Yeah, there's that...

Now, while there are no BOP TH350 transmissions from trucks, there ARE BOP TH400s that came out of Chevy and GMC trucks, namely those with Olds diesels. Yes, this is off-topic, sorry...

joe_padavano

All that means is that the speedo gears in that trans are for a 2.73 rear axle, not a 3.23.

oldcutlass

Good point, but like you stated earlier, probably long tail.

davidmartin148

just trying to get it right, when I first bought the car it didn't run the way it should and I found that the timing advance didn't even have a vacuum line ran to it. and now when I first get on it I have a little backfire through the carb but it will run pretty well after that.

oldcutlass

The reason the vacuum advance was disconnected is probably because they were running very hi initial timing. With that vacuum advance connected, it was probably causing to much advance while cruising. I would disconnect it until you get your timing straightened out.


Do some reading on how to tune an HEI distributor to an engine.

davidmartin148

I will try that