Reading The “motors” Auto Repair Manual

[chromebumpers]

For 1968 they list 2 different Pontiac 400 cu in motors. With both having 2 barrel carbs, one is 8.6 and 265 hp, the other is 10.5 compression with 290 hp. How can I tell which one is in my car?

 

[Indycars]

Pull a spark plug and use a borescope to look at the piston top.

https://www.summitracing.com/search...-6l-400/engine-family/pontiac-v8?fr=part-type

 

[chromebumpers]

Can I assume the piston on the right is the 10.5 CR? If I find a flat top I have the lower CR? The Summit ad doesn’t state which is which,

 

[Grumpy]

you can't assume anything without a few documented facts to base the assumption on, to start

do you have the original engine?
do you have the vin number ?
and engine casting number ?
and cylinder head casting numbers?
so we can try to look the info up?
have you done a compression test?
what were the readings?

1967-1969
Beginning in 1967 casting nnumbers moved to the rear of the block just behind cylinder #8



Year CU IN Casting #
1967
326

9786339
1967 400 9786133
1967 428 9786135
1968 350 9790079
1968 400 9790071
1968 400 RA 9792506
1968 428 9792968
1969 350 9790079
1969 400 9790071
1969 400 RA IV 9792506
1969 428 9792968
1969 303 546313
1969 400 RA V 9792968
1969 428 RA V 545887

 

[Thunderbolt]

The piston on the right is low compression.

 

[chromebumpers]

I haven’t had a chance yet for the other things to check but here is the VIN 252678X103756