[Zooguy]

I guess I'm a collector now because I just bought my second 1966 Pontiac this one is a Bonneville and after looking closely turns out it does not have a 389 under the hood still it has a 1970 400 and some 15 heads. Did anybody happen to know what kind of compression ratio that would give me?

And assuming the stock camshaft what type of power it may make?

[steve25]

The YB motor was a 2 bbl out of a Grand Prix and was not the base motor, but special order.

It was rated at 265 Hp and was said to have a 8.8 compression, likely in reality the motors compression was 8.5..

The 2 bbl Cam was dinky, with 269 / 277 duration and the typical .406” lift.

I had a 1965 GP for quite a while, and even with its high comp 4 bbl 389 it was not a power house, but it was ok.

I was lucky that it did not have AC so it had a decent street gear for its barge like weight!

[johnta1]

1970 400 and some 15 heads

Can you check what the block casting is on the back of the block, passenger side?

Also possibly the date code on the heads?

[Zooguy]

*Update* casting number behind the passenger cylinder head: 9799914

[johnta1]

Good, that makes it 1970 400 for sure.
The 15 heads could be for a 1970 455 (very low compression on a 400) or the 68 heads. (what Steve said about them)

The 400 is a good block to have so a cam change could make a big difference.

[Zooguy]

Hard for me to tell
What do y'all think 🤔

[steve25]

The air injection boss running length wise below the valve cover rail ( above the exh ports ) is the tip off to these castings being 1968.

These heads where cast in late 67, and by the end of 68 during the start of the 69 production run in August these air passages where never to be seen again until the mid 70s.

I will post up later what the chamber size is on these heads.

[steve25]

Well things are looking better for your compression ratio.
In 1968 your # 15 castings where rated at a 10.5 to 1 compression, so in real life with the way Pontiac did things let’s call it a true 9.5 compression.

[Zooguy]

So assuming the camshaft has not been changed but kind of approximate power/ performance could I expect

[Stan Weiss]

Originally Posted by Zooguy

So assuming the camshaft has not been changed but kind of approximate power/ performance could I expect

How about doing a cranking compression test on the engine and posting those numbers?

Stan

[Zooguy]

Originally Posted by Stan Weiss

How about doing a cranking compression test on the engine and posting those numbers?

Stan

Yes that is on the to-do list. Pull out the spark plugs get a fresh set and before installing them do a quick compression test across all eight. Also temporarily hook up an oil pressure gauge to make sure it is building oil pressure adequately.

[steve25]

That same basic. Combo was used over a 3 year time span albeit with different casting number heads.

With a 2 bbl Cam the factory called it 265 hp, some of the 2 bbl motors also got the 066 Cam and where rated at 290 hp.

All the 2 bbl motors however only got the 40 psi oil pump with its not so good thin flexible bottom cover and also they where made down graded rod bearings .

If this where my motor and without peaking inside to look at the condition of the bearings and to put a standard 60 psi pump in, I would not rev it up passed 4500.

You have a good block there so don't chance blowing it up!

One other thing I would do even before doing what could be a misleading compression test would be to pull all the plugs out, pop the Dizzy cap off and bar the crank back & forth to check for timing chain slop.

If the Dizzy rotor does not follow the Crank movement well then doing a compression test will only tell you how even the cylinders are, but this is still worth taking the time to do.

[Zooguy]

Ill check that too.

[imposter]

i'm all in with the 66 bonneville rescue as well