I put a GM crate 454ho in my wife's 73. I installed an hei distributor from an aftermarket source. The initial timing is set at 10 degrees (as per GM recomendation). I have had problems with the car heating at low speeds/idling. Tonight out of curiosity I checked the advance and found no change when disconecting the vacuum hose. The mechanical advance turns out to 32 total/ with 10 initial. My question is which can do I need and would the lack of advance cause the heating problems. I read all the posts a while back about the cans and have'nt quite figured it out yet. The car runs like stink with no pinging on 91 octane but sometimes idles poorly and the heating problem. I am hoping one of you sharp engine guys can save me some time and heart ache. Sorry for the book.
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Re: distributor advance
You probably have the vacuum can signal line connected to a "ported" source, which doesn't provide vacuum with the throttle blade in the idle position.
You need to find a full manifold vacuum source to plug it into. Once you have found one check the total timing and manifold vacuum at idle speed. The correct vacuum can spec is a function of the engine's vacuum characteristics, which are primarly a function of valve overlap.
Not sure what the proper total timing should be for your big block configuration, but most are around 40 degrees - total initial plus centrifugal, so you might want to modify the dist. for more centrifugal or bump up the initial timing up.
Duke -
Re: distributor advance
Sounds like you have the vacuum advance hooked up to ported vacuum source, meaning that at idle no vacuum. With ported vacuum the vacuum is created when the throttle is opened. Try another vacuum port that has engine vacuum at idle.Comment
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Re: distributor advance
sorry guys, forgot to mention the hose has vacuum, but the timing does not change when hooking or unhooking the hose. What sort of vacuum advance numbers should I be seeing with the hose hooked up and the engine at idle? Thanks JohnComment
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Re: distributor advance
Check the vacuum advance unit with a hand activated vacuum pump and see if it holds vacuum. The diaphram could have ruptured.Comment
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Re: distributor advance
There are a couple of dozen different spec vacuum cans for the HEI, so I have no idea what the specs are for your particular unit. If you have vacuum
at the hose, there is either insufficient vacuum to pull the vacuum can
plunger, or there is something wrong with the vacuum can or distributor.
You haven't given us enough information to be able to narrow down the possibilities.
Info such as distributor manufacturer and number, vacuum can ID, engine idle speed and vacuum would be useful. You should also contact the distributor manufacturer and obtain the specifications for you particular distributor.
DukeComment
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Re: distributor advance
OK, after reading the responses from the first entry I went back out to the shop for more looking. The diaphram in the can must be rupured( no resistance when vacuum applied). The unit is manufactured by D.U.I. and is a mechanical tach drive H.E.I. GM remunufactured. I have 10 degrees initial and 32 total at about 3000 rpm. I think the total should be in the neigborhood of 36 total at 3600 rpm. The engine at idle makes about 12 inches of vacuum. I am wondering how much change the can should make when working properly ( ie degrees advance) and would this make the engine heat at idle. If I go any more than 10/12 degrees initial the engine pings when run on 91 octane. I am assuming this is because the can is not working, yes/no! The mechanical advance is 22 degrees so 14 initial should yield 36 total. I can get the cop ratio/cam numbers/etc. if I need that info for more accurate tuneup specs.
Thanks again for your time, I am pretty mechanically inclined, just haven't spent the time around dyno's/ racetracks etc.Comment
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Re: distributor advance
You want the vacuum can to provide full advance of about 16 degrees at no more than 2" less than idle vacuum. You can determine this by increasing your idle timing to about 25-30 degrees. This will increase engine speed, so adjust the idle speed to whatever you have selected as the best idle speed and see how much more vacuum it will pull. Use this vacuum number and the document I e-mailed you to select a vacuum can, and select one that has close to 16 degrees total.
Also verify that the signal port you are using does, indeed, show full manifold vacuum at idle. "Ported" vacuum signal sources can show some vacuum at idle, but less that manifold vacuum.
Keep the initial at 10 but modify the centrifugal mechanism to achieve about
28 to 30 degrees, which will give you 38-40 total WOT advance at around 4000. Advancing the initial increases the low end WOT advance which is why it is pinging, but it will probably take more advance at higher revs.
DukeComment
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Re: distributor advance
Thank You Duke, I know that some of you guys have way more historical experiance than I, for more insight. I probably would figure this out eventually but at this point in life less time and less head scraching is better.
JohnComment
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What Duke and Jim said was my problem.
I was in a similar situation a couple months back. Duke told me the same thing about the ported vacuum source. Turned out he was right (imagine that!). I should mention that I could feel vacuum from the hose, so I was mistakenly thinking that wasn't my problem. But when I hooked up a vacuum guage, I saw the the vacuum there was low, even though it seemed fine with the "thumb over the end of the hose" test.
I say this because you mentioned you have vacuum. I just want to make sure it's enough.Comment
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Re: What Duke and Jim said was my problem.
Thanks Mike, I did not put my vacuum gauge on the hose ( I will) but I did draw on the can and NO resistance. I remember Duke and Clem talking about this situation awhile back and needed more in depth info and lo and behold you folks provided it. I gleen a LOT of good tech info from this board and it shure makes my life easier.
JohnComment
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