Distributor Timing Map/Curve for 65 SHP (365hp L76)?

[Dennis K.]

Hi Guys,

Does anyone have a good ignition curve/map for running the stock 1111069 distitrbutor on a 1965 SHP (365hp L76) engine running 91 octane pump gas? Need to recurve my distributor... Engine is stock (with 30-30 cam and 11:1 compression).

Also - anyone know who might stock a recurve kit?

thx,
L76

[Duke W.]

Re: Distributor Timing Map/Curve for 65 SHP (365hp L76)?

The OE spark advance map is perfect! All you should do is verify that it meets OE specs in the Shop Manual. The specs are also in the vehicle information kit, which is a free download from the GM Heritage site.

Use a dial back timing light and Mityvac to verify that both the centrifugal system and VAC meets OE spec. If they meet OE spec, set the total WOT timing (VAC line disconnected and plugged) at 38-40 degrees at 2500 or more. If it will tolerate this without detonation, it's optimized. With 91 PON fuel you may have to back it down a few degrees, but maybe not. It's something that has to be determined experimentally for each individual engine and driving environment.

If it detonates with only 34 degrees total WOT advance there are other solutions.

It's also a good idea to bring revs up to about 6000 to verify that it's not overadvancing at high revs.

Duke

[Jerry G.]

Re: Distributor Timing Map/Curve for 65 SHP (365hp L76)?

Is all the mechanical advance in by 2500 RPM? I thought there was about 6 degrees more advance that comes in on a stock advance curve after 3000RPM.

[Duke W.]

Re: Distributor Timing Map/Curve for 65 SHP (365hp L76)?

Nope. Check the specs - and I believe the "curve" is linear.

Some curves are non-linear. For example, the '66-up 327/300 curve maxes out at 30 @ 5000, but most of it is in by 3500. Chevrolet was still learning how to optimize the spark advance map.

As a rule of thumb, it's helpful to add about 1 degree per thousand revs beyond the torque peak because mixture density is fallling off, which means combustion time is increasing slightly; and that's what the the '66-up 327/300 curve seems to do.

Another rule of thumb: power decreases about one percent if spark advance is 3 deg. less than optimum, so adding a degree per thousand beyond the torque peak can yield about one percent more power in the peak power range.

Duke

[Joe C.]

Re: Distributor Timing Map/Curve for 65 SHP (365hp L76)?

Hi Guys,

Does anyone have a good ignition curve/map for running the stock 1111069 distitrbutor on a 1965 SHP (365hp L76) engine running 91 octane pump gas? Need to recurve my distributor... Engine is stock (with 30-30 cam and 11:1 compression).

Also - anyone know who might stock a recurve kit?

thx,
L76

I saw your post on the Forum, and the responses there give you some good advice. If you have the OEM cam plate, autocam (main shaft), springs and weights, then getting your advance program "dialed in" should involve nothing more than setting the initial timing, which is 12 degrees BTDC, and VERIFYING THAT YOUR SPARK ADVANCE LIMIT SLEEVE IS INTACT AND IN PLACE ON THE PIN ON THE UNDERSIDE OF THE AUTOCAM. If this sleeve is missing, which is often the case, then expect the distributor to overadvance by about 4-6 degrees! If any of your original components have been changed, then a good amount of trial and error will be needed, unless you have access to a Sun distributor machine. I don't use a dial back light, and prefer referring to the degree tape that I have laminated onto my engine's harmonic balancer. Accel sells a spring and weight kit in which the most valuable part is a BRASS advance limit pin SLEEVE. Three sets of springs of varying tension are handy as well. It is NOT necessary to install both springs having the same tension; mixing and matching springs is perfectly acceptable.

The stock specs work well, although the 30-30 will tolerate more initial advance, up to 18 degrees. If this is done, then the advance slot in the cam plate must be shortened (brazed) to limit WOT spark advance to no more than 36-38 degrees.

Here is the distributor setup from the Shop Manual:

[Jerry G.]

Re: Distributor Timing Map/Curve for 65 SHP (365hp L76)?

Sorry to belabor this, but I need to get this correct. I have a 914 distributor ( no vacuum advance). The advance at 1200 RPM is speced at 0-2 , at 1500 RPM 1.5-3.5 advance and at 3900 RPM 6-8 advance and finally at 6000 RPM 10-12. So if I'm trying to get 38 degrees total advance I could rev it to 6000 RPM and set it at 38( since all the advance is in) or I could go to 4000 RPM and set advance at 34 degrees( 38 less [12@6000 minus 8@4000] = 34@ 4000 RPM ) Is this correct? Another way to think of this is that with a distributor that has 12 degrees of total mechanical advance and no vacuum advance I could set the advance at 26 degrees at approx 1000 RPM and end up with 38 degrees total advance at 6000RPM ,Correct??

[John H.]

Re: Distributor Timing Map/Curve for 65 SHP (365hp L76)?

Sorry to belabor this, but I need to get this correct. I have a 914 distributor ( no vacuum advance). The advance at 1200 RPM is speced at 0-2 , at 1500 RPM 1.5-3.5 advance and at 3900 RPM 6-8 advance and finally at 6000 RPM 10-12. So if I'm trying to get 38 degrees total advance I could rev it to 6000 RPM and set it at 38( since all the advance is in) or I could go to 4000 RPM and set advance at 34 degrees( 38 less [12@6000 minus 8@4000] = 34@ 4000 RPM ) Is this correct? Another way to think of this is that with a distributor that has 12 degrees of total mechanical advance and no vacuum advance I could set the advance at 26 degrees at approx 1000 RPM and end up with 38 degrees total advance at 6000RPM ,Correct??

Jerry -

Note that the 914 distributor specs in the chart in the ST-12 are shown in DISTRIBUTOR degrees and rpm, not CRANKSHAFT degrees and rpm; you have to double both to get crank degrees and rpm. Doing so gets you:

0-4* @ 1200 rpm
3*-16* @ 3900 rpm
20*-24* @ 6000 rpm

If your distributor meets the above specs and stops advancing at 6000 rpm, if you're looking for 38* total advance, you'd get that by setting your initial (base) timing at 14* (14* + 24* = 38*).

[William C.]

Re: Distributor Timing Map/Curve for 65 SHP (365hp L76)?

That assumes the mechanical advance is fully and correctly functioning. But if it is, just adjust to the lowest idle it will run at, set 12 degrees and you should be right on with much less danger. A spin on an old-fashoned distributor machine should set you straight as to where you are---need to be.

[Jerry G.]

Re: Distributor Timing Map/Curve for 65 SHP (365hp L76)?

AH HA ! Thank you. A little knowledge is dangerous. I forgot about distributor degrees versus crank degrees. Thank you thank you.

[Duke W.]

Re: Distributor Timing Map/Curve for 65 SHP (365hp L76)?

That 914 curve is WAY too slow for your vintage racing engine. The centrifugal should be all in by 3000-3500 or the lowest revs you see on the slowest corner. Set up the dist. with 24 and set the total advance at 38-40 at above the speed where the centrifugal is all in.

It would be very difficult, with light springs, to set initial timing on your engine running because it probably won't even run below the speed where the centrifugal starts.

The other alternative is to static time it using an ohmmeter, which usually gets the initial target to within one degree.

Duke

[Joe C.]

Re: Distributor Timing Map/Curve for 65 SHP (365hp L76)?

Here's a chart, and it recommends 18 degrees BTDC initial spark advance, at the crankshaft.

http://www.carnut.com/specs/gen/vette60.html

In light of the fact that the recommended curve might be deemed too slow, plus the very important fact that 2-4 degrees additional advance is recommended above the torque peak, then the best course of action should be to:

1. Time it with 20 degrees static timing.
2. Shorten the advance slot in the cam plate enough, by brazing, to limit centrifugal advance to 20 degrees (instead of 24).

This would accomplish 2 things, first, it would "quicken" the advance by giving you more initial and low RPM advance, and second, it would maintain the 16 degrees centrifugal at 3900 (near the torque peak) while still providing an additional 4 degrees (instead of 8 degrees) above the torque peak and out to 6000.

You'd end up with spark timing of 36 degrees near the torque peak (around 3900 RPM) with an additional 4 degrees slowly coming on by 6000 RPM. Detonation potential is always at its highest at the torque peak, where cylinder pressure is at its highest. As revs increase beyond the torque peak, it's necessary to add spark advance because of the increased velocity of the piston.

This is an advantage that you have over the OE specs for the later model SHP engines which did not allow for any additional spark advance above the torque peak.

[Dennis K.]

Re: Distributor Timing Map/Curve for 65 SHP (365hp L76)?

Thx Guys.

Joe - I am a little hesitant to go this route as I am getting detonation in my car after 9 years of flawless performance. Based on when I witness pinging (and my timing light reads) I think I am getting too much mechanical advance (due to bad bushings enabling the fly weights to advance too far). I will pull the distributor this weekend and put it on the distributor machine to see what it tests at. Think I might start with the stock curve and go from there ... Unless you can convince me otherwise (I am always game for a little improvement!)!!

Cheers,
Dennis

[Joe C.]

Re: Distributor Timing Map/Curve for 65 SHP (365hp L76)?

Thx Guys.

Joe - I am a little hesitant to go this route as I am getting detonation in my car after 9 years of flawless performance. Based on when I witness pinging (and my timing light reads) I think I am getting too much mechanical advance (due to bad bushings enabling the fly weights to advance too far). I will pull the distributor this weekend and put it on the distributor machine to see what it tests at. Think I might start with the stock curve and go from there ... Unless you can convince me otherwise (I am always game for a little improvement!)!!

Cheers,
Dennis

Dennis,

I'll assume that you're responding to post #5 and not #11, which was meant for Jerry Gollnick.

A very important question, which has not been asked, is: When do you hear detonation? During full throttle, part throttle, or both?

Something has changed that is causing your engine to detonate now, after 9 years. By far and away, the most likely reason for this is because the sleeve has either disintegrated or become dislodged from the pin on the bottom of the autocam. This will cause an additional (and unwanted) 2-3 distributor degrees (4-6 crankshaft degrees) of advance. If this is the case, then if you use what I wrote in post #5, especially by using a brass sleeve, which you should cement onto the pin with a very small dab of epoxy, then you should be very pleased with the results.

You have never discussed your vacuum advance. The most common operating zone for detonation to occur is during transient throttle application to maintain speed while cruising. Reason being, that the engine is near its torque peak at this time, considering that many vintage Corvettes (especially L76 which mostly were equipped with 4.11 axles) generally cruise at 3000 RPM. In the case of your distributor, your centrifugal advance should be "all-in" (@ 2350 RPM) far before cruise speed is reached. If the distributor were set properly, then you'd have 36 + 17 = 53 degrees spark advance. If the sleeve is missing, then you're getting almost 60 degrees.

[Dennis K.]

Re: Distributor Timing Map/Curve for 65 SHP (365hp L76)?

Hi Joe,

Actually I was erroneously responding to post #11. My fault for reading and replying to the post on my iPhone (a little tough to follow the thread splitting to cover two ignition systems on such a small device). Sorry. Looking at it on my computer now... I will actualy proceed with your suggestions in post #5. This seems inline with my thinking. My detonation occurs during transient throttle application. I can "almost" accelerate it away if I go to WOT. I say almost becuase I can;t get it to go away entirely, but it does "fade" significantly under WOT. I run 3.70s in rear-end so I am almonst always above 2350 rpm (where GM indicates the ignition curve/advance is "all-in").

Just pulled the distributor tonight and will have it on the Sun machine tomorrow. Haven;t looked to see what the autocam situation looks like (I converted from stock points to Mallory conversion). Oddly... I ran the timing light one more time before I pulled the distributor tonight - and I was getting 26 degrees spark advance at 800 rpm and 53 degrees spark advance at 3,000 rpm. Will be interesting to see what's gpoing on here. Want my car running well again! I'm gonna try to drive it to San Diego for the national convention in July.

Cheers,
Dennis

[Timothy B.]

Re: Distributor Timing Map/Curve for 65 SHP (365hp L76)?

Dennis,

Not sure if I missed this or not but when you report 25* inital timing at 800rpm is that with the vacuum advance disconnected?.

The only thing that's changed is the Mallory electronic switch. I have a friend that installed a Petronics III and we could never set the timing. I am not sure if he had voltage problems because of resistance or if he ever got it set with a timing light. I have another friend that installed the same Petronics on a Olds distributor and same problem with timing light.

The Breakerless SE is what is in my 67 and it's performance is exactly like stock points with no problems setting timing. You may want to switch back to points while you check the advance curve on the sun machine and if it's in spec try the points just to see if the car runs OK again. Some of these electronic switches I am not so sure about..