Overheating SB C2 w/AC when not moving

[Elliott P.]

[Thomas B.]

Re: Overheating SB C2 w/AC when not moving

Does anyone have the design specifications on how much temperature drop is expected across the radiator (inlet to outlet)? Seems that would be a good way to determine if the engine were generating too much heat (timing, mixture, etc.) or if cooling system were not doing its job (radiator, fan, fan clutch shroud, etc.)

[James W.]

Re: Overheating SB C2 w/AC when not moving

Here is the B26 vacuum canister showing the part number VC1765 I purchased from NAPA. I installed this in my '65 L79 and a/c equipped Corvette. Runs great and does not overheat.

James

[Elliott P.]

Re: Overheating SB C2 w/AC when not moving

I am still peeling the onion here. I further tested the Niehoff DR-305 vac-adv I purchased, which I believe should be equivalent to the original 236__16 for my application. However, I noticed that there is very limited diaphragm pull-rod movement for advance. The diaphragm is pre-loaded by the internal spring against the inner diaphragm housing, thus fixing the “zero” vac-adv position. When vacuum is applied, the diaphragm pull-rod is pulled into diaphragm only 0.2”, being limited by the slot in bracket arm. I crudely plotted a vac-adv curve that looked like this below, giving a total of 0.2” movement between 5”hg and 8.5”hg. Using rough geometry calculations, that 0.2” of movement should be equivalent to only about 9 degrees of advance, which does not seem like much. It would seem that the spec should be 16 degrees.

At this point, I do have a vac-adv that is “operational”, so I will install it and see how it does; but, this ole engineer is not happy if it just sorta-works (my Mama raised me better than that). Where does one find access to the original vac-adv specs for these various vac-adv parts?

BTW, I have attached picture of Niehoff DR-305 (B28) I purchased. (On the back of box was a sticker with V329 handwritten on it, which may suggest interchangeable with Borg-Warner V329 and BWD V329 from previous listing.)

Best regards, Elliott
DR-305 Vac-Adv Plot.jpg IMG_1129.jpg

[William F.]

Re: Overheating SB C2 w/AC when not moving

Duke,
When doing the test4 to see which vac can will meet the 2" rule, do you have the vac advance plugged-inoperative/
Thanks

[Duke W.]

Re: Overheating SB C2 w/AC when not moving

I don't understand your question.

Duke

[William F.]

Re: Overheating SB C2 w/AC when not moving

Duke,
What I'm asking is: In order to determine how much vacuum engine is pulling at idle so as to determine which vac can to use to meet 2" rule, do you plug the existing vac advance line?
Thanks

[Duke W.]

Re: Overheating SB C2 w/AC when not moving

No! Plugging the VAC signal line changes the engine configuration. Normal idle vacuum is with the VAC connected at normal idle speed. For example a L-79 with an OE cam is 14-15" @ 750.

If the VAC is dead or or you think the engine has a different than OE cam, tie up the centrifugal with a rubber band. Then increase initial timing to about 25 degrees and use measured vacuum at whatever your normal idle speed is and apply the Two-Inch Rule.

In the case of a L-79 when you remove and plug the VAC signal line revs will drop. In fact the engine might stall. Even if you manually raise idle speed back to 750 measured vacuum will be lower than the normal configuration with the VAC connected because of the loss of advance provided by the VAC.

Duke

[William F.]

Re: Overheating SB C2 w/AC when not moving

Thanks, Duke.

[Elliott P.]

Re: Overheating SB C2 w/AC when not moving

Mea culpaParce mihi placet, and best regards, Elliott