[carbking]

This subject is outside my area of expertise, so the following is NOT a recommendation, only an observation:

My personal 1974 GTO is running a Pontiac 350 with an 0.040 overbore, 350HO heads and distributor, and an RAIV camshaft. MEASURED compression was 10.38:1 Timing is set at 8 BTDC. Using an original RAIV intake with no crossover, and an 850 Carter TQ.

It runs with NO pinging or detonation on 93 octane pump gas. And runs pretty good in a car that now weighs less than 3000 pounds.

Jon.

[Jay S]

Originally Posted by Jay S

Staring out it this 389 is likely 10:1 and has a 68-70 cc head. Probably a 17, 47 or an 11 from a 68-70 350s would work well, low to mid 80s cc’s. Compression should be 9 ish. 68-70 small valve 400 could be about the same too. The next step below that would be the 70 455 heads. The 71 and later 350 and 400 heads are 90-100 cc’s, getting on the soft side for compression.

??? My reply earlier in #27. Maybe I was wasn’t clear enough. 80cc to about 90 cc would be what to look for. IMO the best pump gas friendly with no cam change is a 68 to 70 350 head, or a small valve 1968 to 69 400 head. Next best is the 1970 small valve 455 heads. There are 90-100cc heads used in 400s and 350s from 71 up, but the compression would be lower to mid 8s, but you could have them milled. All those heads would work on a 389 with a mild cam, the 71 and later heads would likely work better with a different intake than the 64 vintage. Look up head casting numbers on Wallace’s site it you want numbers. I think the 68-71 head choices would match up to your old 64 intake the best because of the exh cross over is bigger on the 72 and later heads.

[Goatracer1]

Thanks 80-90 cc’s is the answer I was looking for. I have a good pair of #62 heads that have had the chambers enlarged. I’ll have those measured. I thank you for your suggestions but you have to remember that a Bonneville with AC is a lot heavier than an “A” or “X” body and might not be happy with an 068 cam and stock gearing.

[Sirrotica]

I think the 068 would be fine in a B body with 2 series gears, and A/C. The mid 60s B bodies aren't that much heavier than a second Gen F body. There is a lot of weight in the mid 70s F body cars. The 068 still has a very smooth idle, so I can't see it being a problem loaded with A/C at idle. I've used the 068 in 350s, and 400s with low compression, and it still picks up performance over factory stock cams installed straight up, same as the factory installed them.

I do know that your fuel mileage will suffer with the 068, but performance will increase as every car I have taken a 066, or a 067 out that got good fuel mileage, the 068 did drop the mileage, but it was worth the increase in performance.

To be perfectly truthful, I have never tried the 068 cam to drop dynamic compression, so I have zero knowledge if it's going to work out as planned.

For the ease of installing a cam and lifters, and a relatively low cost, if it were my car I'd probably try it, before swapping heads. It may be an experiment that works out better than expected. If it doesn't suit you, you can still swap heads without being buried in time, and money in the experiment.

Edited post to add one example that I had forgotten about:

I just remembered one vehicle that I put a 068 cam in a 65 389 Bonneville premium fuel engine that I swapped into a Jeep Wagoneer (not a lightweight vehicle by any stretch). Automatic T 400 with 3.31 axle no A/C. I did run regular fuel in it, because in the late 70s you couldn't buy anything other than 89 regular fuel.

Lots of problems back then trying to still run the premium fuels engines that were still around on low octane regular fuel. Water injection was huge during that time period for suppressing fuel/spark knock.

I don't remember it detonating with that combo, so perhaps the 068 cam prevented it. I surely didn't put that combo together to drop dynamic compression, but it may have inadvertently. I used the Wagoneer to haul cars with all the time, so it never lacked any low end, and idled just as smooth as any stock camshaft did. It was a 4X4, so with that 389 it was a beast, even on snow it would throw 4 rooster tails up about 10 feet into the air when you stomped it.....

I used to run it against snowmobiles a few times during really bad snowstorms when guys took to the streets with their snowmobiles. They never expected a Jeep Wagoneer to be right with them on a snow covered road..........

I also used the same cam in a (low compression) 72 455 powered T 400 equipped Jeep J4000 3/4 ton wrecker. I used it to plow snow, and haul cars, so no lack of low end with an 068 cam.
See below:



It's your car, so ultimately your choice.

[padgett]

Personally like the 068 in an automatic car and 744 in a 400 with manual trans.

[Goatracer1]

The Bonneville already gets terrible gas mileage. My 421 GP and my blown 434 GTO both get better highway mileage.

[Jay S]

I have never had any luck with fuel economy when ignition timing must be taken out to nurse high compression. To get more fuel economy it will likely take quite a bit higher octane and the full timing put in.


The 62 heads would have potential, after a valve job they often run 77-78 cc, i have a set of untouched that measure 75 cc. It they have had the chambers opened up some they could be in the lower 80’s cc.

[Sirrotica]

Had a customer with a 77 T/A 6.6, 4 speed Bandit car, in 1977.

Took the catalyst off, put on true dual exhaust, with headers, Holley Street Dominator, re-jetted OJ, recurved distributor, it picked the mileage, and power up a bunch.

He decides it's still not fast enough so we put an 068 cam in it, the power picked up, but the mileage dropped. It wasn't 5 MPG drop, but it did drop maybe 1-2 MPG of of the nearly 20 MPG he was getting before the cam change.

I just wanted you to be aware that you might lose some mileage, at least in my experience.

I didn't think it was of utmost importance to lose a little fuel mileage, as you describe the car as used infrequently.

[Goatracer1]

The base timing is at 6 degrees BTDC which I believe is factory. Distributor is not modified. Both other cars get about 15 mpg on the highway but the Bonneville gets maybe 10 highway mpg.. I hate to lose any more.

[Sirrotica]

I'd say you may possibly have some other type of problem.

My own car, 68 Bonneville, 400 high compression engine, bone stock, got about 17 MPG after changing out a flat camshaft. Mileage with the half worn away lobes wasn't great. Matter of fact, the cam/lifters that came out of a 77 6.6 T/A customers car that I put a 068 cam into, that I already posted about, went into my Bonneville......

Maybe you have lobes going away too? that would shorten the duration of the cam which could raise the dynamic compression. That maybe far fetched as a theory, but 10 is pretty poor, unless it's mostly city driving, then I'd say it's close to normal.

[geeteeohguy]

Originally Posted by Goatracer1

Assuming the engine was rebuilt to close to stock how big a chamber would I need on a factory later model head to lower the compress ratio to something usable?

I am running 1970 455 heads on my '67 400 in my '67 GTO. Casting number 15, small valves. They are 87cc and I converted them to screw-in studs. Been running them about 12-15 years now and CR is about 9.3 to one....and it will detonate on a 105 degree day pulling a long grade under load (Pacheco Pass) on our crappy 91 octane fuel. For a 389 with a stock-ish cam, you can bolt on any later head with 85-100cc chambers and lower your CR to pump fuel levels.