For my 65 350 HP engine: What should be the total vacuum advance? We set the timing at 8 degrees. The distributor was professionally rebuilt using a B28 can from NAPA. The distributor was ran on a SUN machine with a spec sheet of the results. We had to move the coil 1/4" towards firewall to allow us to achieve the initial timing, since the vac can would run into coil. According to my builders timing light total advance is about 54 degrees. Way too high. I am considering changing the can to lower this total advance. I have not figured out which vac can to use yet. The camshaft is not an original 350 HP cam but a COMP brand cam similar but not the same. Any suggestions. Thanks DG
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Re: Total vacuum advance
Sounds about right for the parts you have.
Static advance is 8 degrees.
Vacuum can is 16 degrees.
Add 30 degrees for the distributor and you get 54 degrees total.
The B28 can is at full advance with about 8 mmHg vacuum - do you really have that poor vac at idle? If not, you want to get a vac can that is about 4 dagrees lower than your idle vacuum - like a 12 or 15 mmHg can.
Does the engine knock? With 11:1 compression, 38 degrees may be a little too much depending at what RPM the curve is all in.
Mark -
Re: Total vacuum advance
Thanks for the reply. The engine has no knock. The engine runs great, but won't it run hot at such a high total vacuum advance? If I remember correctly, my builder told me compression is about 9.5. I have not checked my vacuum yet on this car. At this writing, I just completed the first run of this engine and am waiting for the engine to cool down. I will be changing oil and filter and topping off cooling system. Thanks DGComment
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Re: Total vacuum advance
Dick, You need the Duke for this question. One thing you said that I didn't like at all was: "We had to move the coil 1/4" toward the firewall" etc,
Could you possibly be a tooth off? Others that know more will chirp in but I know from talking to guys on the phone that this may be your problem.
Here is who we were told many years ago how Flint installed the distributors.
Starting from the bottom gear the installer looked for the depression or dimple on the gear. Then lined it up with the seam on the distr case(facing the radiator). Then he knew the rotor was pointing at number 1. The cap was on us well as the spark plug wires dangling. Now when he pushed it in the block before it was completely installed he took a rubber hammer or mallet and smacked the passenger side of the advance to insure he was getting the assembly installed correctly. To insure he was not one tooth out. That's why you see a ton of original VA cans with the dent on the side. Good luck, John. The B28 is what we use for the 30-30. Course we like the 236 16's real ones for sure.Comment
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Re: Total vacuum advance
Dick -
Vacuum advance is not a factor in "total advance", as it varies based on engine load, not rpm. "Total timing" is initial plus max centrifugal, normally 34-36*, and is checked with the vacuum advance disconnected and the source plugged. Your vacuum advance should add 15*-16* to your initial at idle, assuming it's connected to a full manifold vacuum source - that's "total idle timing".Comment
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Re: Total vacuum advance
Your distributor is either installed a tooth off of the gear is on 180 degrees out.
This has been discussed a zillion times. Search the archives.
You distributor map sounds okay. The 54 degrees is total CRUISE timing if cruise revs are equal to or more than the point of maximum centrifugal and manifold vacuum is less than 8" with a B28 VAC.
There are three total timing values.
1. Total WOT timing
2. Total IDLE timing
3. Total CRUISE timlng
Again, the above have all been thoroughly explained many times before.
DukeComment
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Re: Total vacuum advance
HI Duke, I can't believe I gave the right answer. Or a possible right answer. Seriously one tooth off on a Chevy is very common. They run like crap that way.Fuel cars hate that. I had a buddy though that said he was going to leave the screw up alone and just change the spark plug wires. I said to him I give up. JohnComment
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Re: Total vacuum advance
The dimple location is correct on the gear. We went back and forth probably 8 times one gear tooth then back the other way. We were either too advanced or retarded. Now, when I say too, I mean there was not quite enough room to set initial timing. Either the intake or the coil bracket restricted our movement. The distributor can swing only about 1-1/2"(guess). Again the timing is now set at 8 degrees acheived by moving the bracket towards the firewall about a 1/4". Yes, this has been discussed a zillion times. This will not be the last either. Sorry. Thanks DGComment
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Re: Total vacuum advance
As I've stated before there is one and only one way a C2 SB distributor can be installed and achieve the correct initial timing spec, BUT this is contingent upon the gear being installed in the correct orientation - dimple pointed the same direction as the rotor. Everthing is usually correct if the cap window is close to normal to engine centerline with the initial timing at the correct value.
On the L-79 the maximum centrifugal advance of 30 degrees is not achieved until about 5100 RPM, so it is unlikely that the 54 degrees sum of initial, full centrifugal, and full vacuum adance will ever be achieved in normal operation assuming the centrifugal curve has not been quickened.
One the other hand, the '64-'65 L-76/84 achieve their full 24 degrees total centrifugal at a mere 2350, so with 10-14 initial, typical highway cruise speed will yield 50-54 degrees spark advance, which those engines like because of the exhaust gas dilution from the high overlap of the 30-30 cam. And because of the late closing inlet valve, they can tolerate a lot of advance at low revs without detonation.
Low mixture density(part throttle, light load) and exhaust gas dilution slow flame propagation speed, which is why these conditions require lots of spark advance, and the combination of initial, OE vacuum advance, and centrifugal advance do a pretty good job of providing a correct range of spark advance for all operating conditions on most, but not all Corvette engines of that era.
DukeComment
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Re: Total vacuum advance
There have been some prior discussions where this was the case. And in some cases achieving correct timing without VAC interference could only be accomplished by installing the gear 180 out from OE indexing.
One possible explanation is that some aftermarket cams may have incorrect distributor drive gear indexing.
DukeComment
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Re: Total vacuum advance
As stated earlier, the cam in this car is a COMP brand cam. Not an original GM cam. Your possible explanation of an incorrectly indexed cam gear makes sense. Thanks Dick GutmanComment
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Re: Total vacuum advance
Dick -
I've found in almost every case where correct distributor orientation couldn't be achieved with specified timing setting, that the car had an aftermarket non-GM cam; those cam manufacturers either don't understand the drive gear indexing notes on the GM cam drawing, or just ignore it, as they don't know why it's necessary. It only matters on Corvettes.Comment
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