HEI Ignition question

[harshard]

Can anyone explain the advantages and disadvantages of an HEI with vacume advance versus mechanical advance on a slightly modified small block 350? Thanks, Shawn

[crakarjax]

Should have both! Mechanical advance depends purely on RPM while vacuum depends on load, I think. Both are reqd for a proper timing curve. There are plenty of guides out there on how to adjust curves, but it boils down to swapping mechanical weights and adjusting the vacuum canister. You can probably get away with a stock setup.

[harshard]

Well right now I have a stock HEI in the truck but it has no vacume canister. I can see where it use to be on the ignition but can not figure out why it was removed. I just purchased a new better than stock HEI from Eastwood and it has a vacume canister, just wondering what the performance diff. is. Shawn

[68gmsee]

When you are at idle or real low rpms, the engine needs to advance the spark to properly ignite and burn the fuel. This is where the vacuum advance comes into play.

As the rpm's increase the vacuum pressure in the intake diminishes to the point where there is "0" vacuum pressure at wide open throttle. That's because the butterflies are open allowing air into the intake, therefore, equlizing intake with outside pressure. This is where the mechanical advance in the distributor comes into play because the engine still needs to advance the spark for proper combustion.

A lot of drag racing cars don't have a vacuum advance since they are mostly run at wide open throttle.

[Autometer]

People would lock down the mechanical to say max advance at 3500 or 4500 so they could set a total advance. Since the travel of the mechanical is limited, you could have more timing at idle and not need a vacuum to pull timing at idle.

[harshard]

So I guess when I install the new HEI with a vacume advance I should get better performance off idle or crusing around town?

[chevy_mike]

The HEI without a vacuum advance could be a later model version that was computer controlled timing. My '92 Camaro had one. Most aftermarket or earlier GM ones had the vacuum advance.

Your mileage should go up with the vacuum advance, assuming you get everything set correctly.

[crakarjax]

Originally Posted by chevy_mike

The HEI without a vacuum advance could be a later model version that was computer controlled timing. My '92 Camaro had one. Most aftermarket or earlier GM ones had the vacuum advance.

Your mileage should go up with the vacuum advance, assuming you get everything set correctly.

Hmmm, computer controlled timing? Sounds like something to look into. What what performance like on that? Did it have mechanical or purely electronic?

[harshard]

No, the HEI that is in the truck right now at one time had a vacume canister on it. It has been removed for on reason or another, it is a very old HEI that needs to be replaced any way I just can not figure out why the vacume can was removed, thought maybe someone could shed some light on this reason. Thanks, Shawn

[chevy_mike]

Opps, missed that part. Why it was removed, who knows, PO's can do some very strange and stupid things over the years. Maybe the vacuum canister started leaking and it was cheaper to remove it than to replace it.

As to crakarjax, it had a mechanical advance which would be controlled by engine RPM's but instead of having a vacuum advance for idle/cruise, it was handled by the computer, since it has more info then just vacuum. If you don't have a computer setup to control the timing, this distributor will not work outside of the mechanical part.

[mr48chev]

I can see no advantage to removing the vacuum advance from an HEI on a street driven rig. But as someone said, previous owners often do strange things to vehicles in the name of performance or saving a buck.
I'm not sure that a computer controlled unit will operate without being hooked to the computer but it most likely would cause the the truck to run poorly if it did operate at all.

[OhOneWS6]

That HEI may have been used in a marine application. You do not want vacuum advance on a jet boat. It causes it to bog.