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

[Dennis K.]

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

Hi Guys,

Sorry I went silent for a while... business and family called. I finally got around to this project... and found my distributor was "A-OK." Dug a little deeper... and found my harmonic balancer had gone bad... at least 10 degrees off. This explained alot.. becasue nothing in my car had changed over last 10 years... distributor checked out, etc. Once I replaced the HB life was good again. Thx for all of your assistance. FYI... been VERY happy with my Mallory unit. It has been flawless. Can send anyone details if hey want to know the specific model number I run...

Anyone know where I can get a harmonic balancer repaired in Northern Califoirnia area?

-Dennis

[John H.]

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

Anyone know where I can get a harmonic balancer repaired in Northern Califoirnia area?

-Dennis

Dennis -

Google "Damper Doctor" and "Damper Dudes" - both companies are in Redding, California; those are the only outfits I'm aware of that reliably rebuild harmonic dampers.

[Michael M.]

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

Wow!
This sound exactly like my problem. Too much total advance so it runs ok with vacuum advance disconnected @ 36 degrees total advance but overheats. The other variation is a mild hydraulic cam in this L76, So would you use 250 or 300 HP timing specs? I know of a NCRS member with a Sum Machine so that sounds like my next step. Mike

[Duke W.]

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

I have no way to quantify a "mild hydraulic cam", but if you can state the idle speed, idle vacuum, initial timing, total idle timing, along with vacuum advance specs and the number stamped on the vacuum advance bracket I might be able to give you some direction.

Duke

[William C.]

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

Recurve kit without knowing the baseline is useless! First step is to either pull the distributor and find someone around you with a dist machine and find out what the current curve is, or do it yourself using a timing tape on the damper and having a friend run it up in about 500 rpm intervals while you track and write down the mechanical advance. I've seen way to many good distributors messed up by well meaning folks swapping parts without knowing what their baseline is not any idea of what the factory spec is. The correct oem spec for the SHP '65 engine is 2 degrees @900 rpm 24 degrees at 2350 rpm. These figures are crankshaft degrees and crankshaft rpm, most of the printed info is distributor degrees and dist RPM, which is a little harder to use with a timing lamp. AFTER you know where your distributor is relative to this spec, you will be in a position to make changes, but first I'd consider 93 octane fuel if available in your area. If you have overheating problems after getting to the original curve, and assuming you have toe correct vacum advance can (also a VERY overlooked part of the shp engine package, BTW the specs are 16 degrees (crankshaft) of vacuum advance at 8 inches of vacuum, Most replacement parts will not come anywhere close to meeting this spec, and that is the cause of many overheating problems with the '65 SHP engine. Also, vacuum can must be hooked up to a direct "full time" manifold vacuum source, not a "ported" advance source. I hope this helps, I've DRIVEN 365 hp and FI cars since the late 60's and using this as a baseline will at least give you a good known base to start from.

[Duke W.]

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

The trouble is, Bill, he says it has a "mild hyraulic cam", so the '65 SHP/FI spark advance map is probably way too aggressive. All the exhaust dilution from the 30-30 cam at low revs slows down the rate of combustion propagation, so it can tolerate very aggressive centrifugal and vacuum advance curves.

That's why I asked him about idle speed and vacuum. That's a hint about how much overlap his cam has. Also we know nothing about the installed VAC, so that info is needed to because if it's OE it's not suitable for a low overlap cam, but we have no idea what is installed.

Like you say, a baseline is needed. Is it OE L-76 or has it been modified, but the cam may require something different than OE.

Duke

[William C.]

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

Yes, I agree 100%, the factory curve is wicked unless used with the proper cam. It will also work well with a 350 hydraulic cam but if the cam is unknown then all bets are off, but starting with a Known curve is always a step ahead of shooting in the dark.

[Joe C.]

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

His camshaft choice alone, even if all of the cam specs are known, is of no value unless you also know the engine's static compression ratio. Without both of these parameters, arriving at a reasonable spark advance "map" is fruitless.

[Dennis K.]

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

Hi Duke (Jerry, Joe, Bill, and others),

Thx for the comments here. I pulled the distirbutor, put it on my distributor machine, and found that it was mapped perfectly - and ws running flawlessly. Next - I decided to investigate the harmonic balancer... because this detonation had to come from somewhere (e.g. "somtheing" had changed)... and low and behold... the harmonic balance had gone bad. The keyway was acutally right in-line with the timing indicator (and it should have been about 10 degrees off). Anyway - I fixed the harmonic balancer, reinstalled it, stuck the distributor back in, and set my initial advance at 12degrees at idle (with no vaccuum advance)...as per OE spec. My situation definitely improved - about 90% of the detonation went away... but I still had some faint detonation when under a light load (e.g. climbing up a slight hill @ 40mph and lightly depressing accelertor in order to maintain speed up the grade). As expected - I could accelerate the detontation away. So... I decided to back the initial off 2 degrees and set the distributor at 10 degrees. Re-ran the car... same situation. Light detontation. I decided to back it off 2 more degrees, so I set it at 8 degrees initial advance. Ran the car... same situation. Light detonation. Against my better judgemet - I decided to back it down 2 more degrees as a final resort (I am realy uncomfortable with it at 6 degrees initial, but I was curious to see what would happen). Ran the car. Still light detonation... except now the car was hesitant off the line and tended to stumble/backfire through the carb. Any thoughts? My next thoughts are: 1) to check the vaccuum off the side of carb that feeds the vaccuum advance can - and see what's going on, 2) run a compression test (maybe the intake manifold gasket is bad with and creating a vacuum leak?), or 3) maybe start tweaking the rich/lean on the carb, or 4) increase the size of the primary jets in the carb? Thoughts or advice?

Thx,
Dennis

[Timothy B.]

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

Dennis,

If this is a 1965 motor I believe the keyway in the damper is supposed to be in line with the timing mark on the outer ring. In (Ithink) 1969 it changed because of interference timing the engine with accessories like power steering.

You can also double check TDC mark using a piston stop in #1 cylinder but be careful NOT to crank the motor, just turn it by hand, info. in archives.

If I am correct, fix the damper and time the engine at 10-12*BTDC and check your vacuum advance control as it should be part # 236 and it's possible the rubber stop that's usually on these things is missing which would allow a few degrees extra advance. After the engine is timed correctly and warmed, go through the emulsion screw adjustment at the proper idle RPM.

[William C.]

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

Also, what are you using for fuel?

[Dennis K.]

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

Hi Tim,

Thx for feedback. Mine is the L76 SHP performance engine with a 30-30 cam. the harmonic balancer is different on this motor and the Fuleie... than on all other small blocks. The correct damper for my engine is 10degrees off of the keyway. I did use a piston stop to identify that my damper was bad... and I did replace it. As for the rubber stopper in the distibutor - mine is actually brass (or copper), but either way - I pulled the distibutor out of the car and ran it on a distirbutor machine. It is working flawlesly to the GM spec. Going to have to look elsewhere to solve my probloem...as it is not in the damper or distributor.

-Thx
Dennis

[Dennis K.]

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

I am running 91 POM (Shell). I am in Northern California. I have also aways added about 2.5 gallons of 110 leaded Sunoco Purple (leaded) to a tank as the car has always loved it and ran better (figured it was a samll price to pay for my car to run better... and since I don't drive it that much - I didn't worry about the aded cost). So - my car is detonating even with 2.5 gallons of race fuel added to 91 POM in the tank.

Cheers,
Dennis

[John H.]

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

Hi Tim,

Thx for feedback. Mine is the L76 SHP performance engine with a 30-30 cam. the harmonic balancer is different on this motor and the Fuleie... than on all other small blocks. The correct damper for my engine is 10degrees off of the keyway. I did use a piston stop to identify that my damper was bad... and I did replace it. As for the rubber stopper in the distibutor - mine is actually brass (or copper), but either way - I pulled the distibutor out of the car and ran it on a distirbutor machine. It is working flawlesly to the GM spec. Going to have to look elsewhere to solve my probloem...as it is not in the damper or distributor.

-Thx
Dennis

Dennis -

Your balancer is NOTdifferent from other small-blocks in terms of the location of the timing index line on the inertia weight; that index line is directly in line with the crank/balancer keyway and a pulley bolt hole. The index line location wasn't re-indexed 10* CCW until 1969, and that only occurred on passenger cars, not on Corvettes. See the note at the top of the balancer drawing below that specifies the index line orientation relative to the keyway.


BalancerDwg.jpg

[William C.]

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

Are you sure the distributor advance is as you have specified? I'd want to run it on a machine before leaping to that conclusion. Plus the variation from the actual advance point onto the spec on one of those really long curves is pretty wide based on a few I have run on my machine over the years. Those curves were really pretty strange even for the time....