3.08 to...3.55

[William C.]

Re: 3.08 to...3.55

I'd also have the distributor curve checked before spending the time and $$ for a rear gear change. The distributor is a majorly overlooked area to gain low end punch.

[Bob S.]

Re: 3.08 to...3.55

I'd also have the distributor curve checked before spending the time and $$ for a rear gear change. The distributor is a majorly overlooked area to gain low end punch.

You know, Bill...when I first got the car, I think the distributor was curved and it was smokin' fast..even for a 3.08. Then, I had the car tuned up and it doesn't seem to have the same low end now. The mechanic I use isn't in to curving. Do you know anyone out my way (Camarillo, Ca) that could do this for me?

By the way, the engine is a stock 427/390

Thank you,

Bob DeSimone
(9944)

[Gene M.]

Re: 3.08 to...3.55

If you do change the ring and pinion, the replacements are generally a finer pitch gear than the originals. That is not a bad thing in that you will have more teeth in mesh than the older bigger pitch. The tooth count on both the pinion and ring gears will be more but the finish ratio will be same as GM built.

[William C.]

Re: 3.08 to...3.55

I'd check with the chapter guys out in your area if you need a local contact. I have a machine here in Ohio, but it's not really local to California, although UPS and USPS have both worked in the past. Also a good idea to check the actual engine TDC against the damper timing mark just to make sure your timing efforts aren't being wasted by a damper that has moved on the mounting ring.

[Bob S.]

Re: 3.08 to...3.55

I'd check with the chapter guys out in your area if you need a local contact. I have a machine here in Ohio, but it's not really local to California, although UPS and USPS have both worked in the past. Also a good idea to check the actual engine TDC against the damper timing mark just to make sure your timing efforts aren't being wasted by a damper that has moved on the mounting ring.

Thanks, Bill...

This was sent to me by the previous owner of the car. Like I thought, I was right. The distributor was curved. My mechanic then put a new Rochester Carb on, and retimed the engine. Accelerates, but nothing like before. Here is what the previous owner said:
Hi Bob,


"Yes, basically, it goes like this: Vacuum advance in inches will determine how much you should advance the dizzy. If you have a vacuum can that's pulling 12 inches of vacuum, you will need to advance the timing to 38- 40 degrees at about 2750 RPM. They say you can advance the timing between 2500 and 3000 RPM, so just split the difference at 2750. The number you are trying to reach is a combination of Vacuum advance and total timing to reach 50 - 52, Do not worry about the timing at idle, it's irrelevant. I hope that makes sense.


It is suggested that this be done with a dial back timing light so the mechanic can toggle back between the timing and the RPM's. After the timing is set, then the carb air/fuel mixture will need to be adjusted to match the new timing settings. Lars' mantra was: "90% of carburetor problems are timing problems' Lars is a wise man!"

If this makes sense to you, or you have a better idea, feel free to let me know.

Appreciate your time,

Bob DeSimone
(9944)

[Bob S.]

Re: 3.08 to...3.55

If you do change the ring and pinion, the replacements are generally a finer pitch gear than the originals. That is not a bad thing in that you will have more teeth in mesh than the older bigger pitch. The tooth count on both the pinion and ring gears will be more but the finish ratio will be same as GM built.

Thanks, Gene...

I'm also looking into curving the distributor...might be what I need instead of the Final Drive being lowered

Regards,

Bob DeSimone
(9944)

[William C.]

Re: 3.08 to...3.55

Lars is a wise man, but centrifugal advance and vacuum advance are not married together as you have indicated. essentially the centrifugal advance matches spark advance to best performance of a given engine under full load at points across the rpm curve. Vacuum advance is applied to optimize response and fuel economy under light loads, where due to the lean fuel mixtures flame propagation is slower and more advance is tolerated for optimum economy. Although the two systems work in parallel, they have different functions, and it would take a real magician to optimize that interaction with a dial-back timing light.

[Bob S.]

Re: 3.08 to...3.55

Lars is a wise man, but centrifugal advance and vacuum advance are not married together as you have indicated. essentially the centrifugal advance matches spark advance to best performance of a given engine under full load at points across the rpm curve. Vacuum advance is applied to optimize response and fuel economy under light loads, where due to the lean fuel mixtures flame propagation is slower and more advance is tolerated for optimum economy. Although the two systems work in parallel, they have different functions, and it would take a real magician to optimize that interaction with a dial-back timing light.

OK..thanks...Is there a Blueprint available for Curving?

Thanks,

Bob DeSimone
(9944)

[William C.]

Re: 3.08 to...3.55

Not that I'm aware of, the factory curves are available in service manuals and other references, and problem # 1 is that most older distributors don't (maybe never did) replicate the original curves very well.

[Duke W.]

Re: 3.08 to...3.55

The PO's explanation of how to set timing is one of the most screwball procedures I've ever seen. He clearly has no understanding of the spark advance map requirement and how to optimize it.

The original spark advance map is in the CSM and AMA specs that is a free download from the GM Heritage Web site. Everything else you need to know to optimize the spark advance map is in my San Diego presentation.

http://www.metroli.org/pdf/2012 Nationals tuningseminar.pdf

Duke

[Bob S.]

Re: 3.08 to...3.55

The PO's explanation of how to set timing is one of the most screwball procedures I've ever seen. He clearly has no understanding of the spark advance map requirement and how to optimize it.

The original spark advance map is in the CSM and AMA specs that is a free download from the GM Heritage Web site. Everything else you need to know to optimize the spark advance map is in my San Diego presentation.

http://www.metroli.org/pdf/2012 Nationals tuningseminar.pdf

Duke

Thanks, Duke....

Man..nothing is easy, I guess....what's that they say about Opinions?

Your presentation is what I hear the most about...Again, THANKS!!!

Bob DeSimone
(9944)