Duke - advance curve plot

[Timothy B.]

I tried out my new dial advance timing light today and obtained the results in the chart attached. I read several earlier posts on how to gather data for both vacuum advance and centrifugal advance. The dark blue diamonds are the first readings I obtained, and as you can see the centrifugal advance topped out at only 2000 rpm!! I was quite surprised, since I had recently replaced the advance springs with new ones from LIC that say they are right for my dist and engine, which is a 327/300HP car with MT. The kit from LIC was called the "Spring and weight kit", and included one light spring set and one heavy set, and new weights. The package labels the weaker one being applicable for pretty much every hydraulic lifter engine, and the heavier spring for solid lifter engines. As you can see, the result is no where near the "spec" values that are plotted with yellow triangles. Specs are from the service manual and are only quoted for four engine speeds.

I tried the heavier springs, and those results are plotted with the violet squares in the chart. They are much closer to spec, and maybe even too heavy. Duke and others, should I try to find some other slightly lighter springs or just stick with the ones installed now? If you assume the in-between values would be consistent, you can basically connect the dots in the chart.

By the way, my vacuum can appears to be close to spec. Spec from the service manual is 0 deg advance at 6"Hg, and 15 deg advance at 12"Hg. My results are about 0-1 deg at 6"Hg and about 18 deg at 12"Hg. So, the advance is a few deg high, but probably not worth replacing the can?

[Timothy B.]

Re: Duke - advance curve plot

Oh, and by the way, I haven't driven the car yet since this was done. Will provide an update. I really hadn't had noticeable problems with the old weights, so it will be interesting to see if I can tell the difference.

[Timothy B.]

Re: Duke - advance curve plot

Oh, and by the way, I haven't driven the car yet since this was done. Will provide an update. I really hadn't had noticeable problems with the old weights, so it will be interesting to see if I can tell the difference.

[Dave Suesz]

You could also

mix the springs, if what you have is a bit stiff, swap one for a slightly lighter one in the leftover pile. Dunno about GM, but this is common practice on Ford distributors.

[Dave Suesz]

You could also

mix the springs, if what you have is a bit stiff, swap one for a slightly lighter one in the leftover pile. Dunno about GM, but this is common practice on Ford distributors.

[Timothy B.]

Re: Duke - advance curve plot

Also, my engine runs about 18"Hg vacuum and idle.

[Timothy B.]

Re: Duke - advance curve plot

Also, my engine runs about 18"Hg vacuum and idle.

[William C.]

Re: Duke - advance curve plot

I believe the printing on the spring instructions is backwards from GM actual practice. Typically lighter springs and faster curves are used on the SHP packages and the heavier ones with longer curves on the base engines.

[William C.]

Re: Duke - advance curve plot

I believe the printing on the spring instructions is backwards from GM actual practice. Typically lighter springs and faster curves are used on the SHP packages and the heavier ones with longer curves on the base engines.

[Duke W.]

Re: Duke - advance curve plot

As Bill said the instructions are "backwards". The mechanical lifter engines have a more aggressive centrifugal curve because their late closing inlet valve lower dynamic CR and allow more advance at low revs. You should call the supplier and point this out to them so others don't get caught in this trap. A check of the '65 300 HP engine versus the SHP/FI engine advance specs in any reliable source will back you up.

It sounds like you vacuum can is reasonably in spec and the idle vacuum is good.

As far as the centrifugal is concerned, the basic rule is to get it in as quick as possible without detonation. I doubt if the most aggressive curve could be used without detonation, but you have to experiment. Also keep in mind that the engine will probably accept a more aggressive curve in cold weather than hot weather since higher inlet air temperatures will increase the tendency to detonate.

I assume that you set the initial timing at zero to check the centrifugal curves, so with about 30 centrifugal that would imply 6-8 initial to yield 36-38 maximum WOT timing, and 18" vacuum would yield 24-26 total idle timing, which is right in the ballpark.

You didn't state the model year, but IIRC the 300 HP engine from '65 up had about 30 @ 5100 total centrifugal and a 12" vacuum can. The earlier 300 HP engines are 24 @ 4600.

You can use a postage scale to weigh the original and new weights. If they are different, you can put together more combinations for more curves. You can also use one light and one heavy spring with each set of weights. Since you have the most aggressive combination installed now, test it for detonation and install the next least aggressive set if you get any low rev or mid range detonation. The way to test for this is to give the engine about 3/4 throttle at 1000 RPM going to WOT at about 3000 and letting it rev to about 4000. High load and low revs is the acid test for detonation.

Once you've found a curve that allows detonation free operation or nothing more than slight transient detonation you are set, but watch out for detonation as temperatures warm up this summer and be prepared to slow it a little more if necessary.

Duke

[Duke W.]

Re: Duke - advance curve plot

As Bill said the instructions are "backwards". The mechanical lifter engines have a more aggressive centrifugal curve because their late closing inlet valve lower dynamic CR and allow more advance at low revs. You should call the supplier and point this out to them so others don't get caught in this trap. A check of the '65 300 HP engine versus the SHP/FI engine advance specs in any reliable source will back you up.

It sounds like you vacuum can is reasonably in spec and the idle vacuum is good.

As far as the centrifugal is concerned, the basic rule is to get it in as quick as possible without detonation. I doubt if the most aggressive curve could be used without detonation, but you have to experiment. Also keep in mind that the engine will probably accept a more aggressive curve in cold weather than hot weather since higher inlet air temperatures will increase the tendency to detonate.

I assume that you set the initial timing at zero to check the centrifugal curves, so with about 30 centrifugal that would imply 6-8 initial to yield 36-38 maximum WOT timing, and 18" vacuum would yield 24-26 total idle timing, which is right in the ballpark.

You didn't state the model year, but IIRC the 300 HP engine from '65 up had about 30 @ 5100 total centrifugal and a 12" vacuum can. The earlier 300 HP engines are 24 @ 4600.

You can use a postage scale to weigh the original and new weights. If they are different, you can put together more combinations for more curves. You can also use one light and one heavy spring with each set of weights. Since you have the most aggressive combination installed now, test it for detonation and install the next least aggressive set if you get any low rev or mid range detonation. The way to test for this is to give the engine about 3/4 throttle at 1000 RPM going to WOT at about 3000 and letting it rev to about 4000. High load and low revs is the acid test for detonation.

Once you've found a curve that allows detonation free operation or nothing more than slight transient detonation you are set, but watch out for detonation as temperatures warm up this summer and be prepared to slow it a little more if necessary.

Duke

[G B.]

Zoom parts give-away

I've removed a bunch of those Mr. Gasket type advance weights from Corvette distributors. I was going to throw them at cats. However, I will mail these pieces of #@$?&* to anyone for the price of postage. You will be paying too much, regardless of the postage cost.

[G B.]

Zoom parts give-away

I've removed a bunch of those Mr. Gasket type advance weights from Corvette distributors. I was going to throw them at cats. However, I will mail these pieces of #@$?&* to anyone for the price of postage. You will be paying too much, regardless of the postage cost.

[William C.]

I Agree! No Good even in the 60's! *NM*

[William C.]

I Agree! No Good even in the 60's! *NM*