HEI use on the 1968 Olds 350 motor

[Redog]

1 quick questions:

My mech wants to use the HEI on the '68 motor instead of the points

I need to know if the plugs that are in the 1985 307, which are new (10 miles on them) with fit the 1968 block.

I know the gap will have to be .045 instead of the .032 for the points, but are the R45S standard plugs I bought for the '68 are the same size as the 41-630 plugs from the '85 307?

[Jokers69]

I run an Olds HEI in my 69 350. I'm using ACDelco R46SZ, I believe they're spec'd out for a mid 70's 350 with HEI. Hope that helps...

[88 coupe]

Assuming a stock (or close to it) engine:

Originally Posted by Redog

........ My mech wants to use the HEI on the '68 ........

He is probably not comfortable dealing with points.

........ if the plugs that are in the 1985 ........ will fit the 1968 block ........

They will not fit the block.

If you want to know if they will fit the heads, you can try to install them, visually compare them, or have your mechanic check his application guide.

........ I know the gap will have to be .045 instead of the .032 for the points ........

It will not. HEI was part of the '73 emissions package along with (among other things) lower compression, leaner mixtures, and wider gaps to fire those excessively lean mixtures.

Use the same plugs, and gap, that were engineered for your engine. More gap = unnecessary wear on the ignition system.

You will have to use more initial to get enough total advance and you may, or may not, have issues with the advance curve in the HEI.

Norm

[joe_padavano]

Originally Posted by 88 coupe

HEI was part of the '73 emissions package along with (among other things) lower compression, leaner mixtures, and wider gaps to fire those excessively lean mixtures.

Actually, the main reason GM (and other automakers) switched to electronic ignitions with higher voltage coils was not directly related to tigher emissions levels but to a federal requirement to maintain these emissions levels for 50,000 miles without a tuneup (now 100,000 miles, by the way - which is why new cars have platinum plugs). Electronic ignition by itself has nothing to do with the emissions, but as you correctly point out, the higher voltage WILL fire lean mixtures and fouled or worn plugs more reliably. Point ignition would have satisfied the emissions requirements with regular tuneups, but people don't maintain their cars so the feds have to do it for us. Yes, the advance curve in the distributor will be tailored to meet emissions requirements, but this is the same for points or electronic.

Use the same plugs, and gap, that were engineered for your engine. More gap = unnecessary wear on the ignition system.

There IS an advantage to a larger plug gap if the distributor can fire it. A larger gap will ignite a wider variety of mixtures in the cylinder as you pointed out previously. Again, this is one reason why the HEI uses a much higher voltage coil than a point style distributor. Every resource I've seen reccommends using the larger gap on the plugs when retrofitting HEI to an older engine, and since that distributor was designed to fire the larger gap, there should be no issue on wear. Will the car work with the smaller gap? Sure, but why bother doing the conversion if you don't plan to take advantage of it?

You will have to use more initial to get enough total advance and you may, or may not, have issues with the advance curve in the HEI.

Agreed. You will almost certainly want to change the mechanical advance curve and will probably want to use an adjustable vac advance can. This brings up another important point. You MUST use a non-computer HEI. The HEIs from the 307 are computer controlled and will not operate as a standalone distributor. You'll notice that these units have neither mechanical or vacuum advance provisions. Be sure to get a distributor with a vac advance can.

[88 coupe]

Thanks for the clarification.

Norm

[Eightupman]

Dont forget to have him advance the timing an additonal 10 deg or so to compensate for the different timing curve in the HEI. Should come out around 18deg or a little more. I made that mistake when I swapped mine to HEI and could not figure out why she ran like a wet dog...

John

[joe_padavano]

Originally Posted by Eightupman

Dont forget to have him advance the timing an additonal 10 deg or so to compensate for the different timing curve in the HEI. Should come out around 18deg or a little more. I made that mistake when I swapped mine to HEI and could not figure out why she ran like a wet dog...

John

Well, the right thing to do is to recurve the HEI to match the requirements of your particular engine. All HEIs come off of smog motors, so it's a given that the curve will be wrong for a performance motor.

[Eightupman]

Originally Posted by joe_padavano

Well, the right thing to do is to recurve the HEI to match the requirements of your particular engine. All HEIs come off of smog motors, so it's a given that the curve will be wrong for a performance motor.

understood... I am in the middle of a high performance build and will address that properly once I get the final specs down. I threw it out there to give him a ballpark to start in.

John