[monzter]

Can anyone here tell me what the compression ratio is in my engine, it is 350 SBC from 1974 Chevy Blazer/Pickup truck.
It Has 76cc heads (3998-993) With 1.94"/1.50" Valves, The Pistons are MOST likely stock dish one´s and it the engine is at std deck height.

Best Regards.

 

[techinspector1]

Early to mid-70's motors were the worst.....I'd guess 8 1/4 to 8 1/2.

 

[monzter]

Still that much, i was thinking of 7.5:1 hehe but......
So it would maybe go to around 8.75:1 / 9:1 If i would use steel shim head gasket like fel pro 1094 ?

Best Regards

 

[ericnova72]

Nope, it had a steel shim gasket from the factory...like all '55-95 iron head SBC's

 

[techinspector1]

Still that much, i was thinking of 7.5:1

You may be closer to correct than I am.

If we assume a motor at 8.27 with a 0.040" gasket, replacing it with a 0.015" gasket would make the cr 8.65.

 

[curtis73]

I *think* the factory advertised them at 7.8:1, so it was probably in the mid-7's. I'll have to get my standard catalog out tomorrow and double check that.

 

[cobalt327]

The '74 Chevrolet passenger car and light duty truck Overhaul Manual has the 145, 160 and 175 HP 350's all having 8.5:1 CR. The 245 HP 350 was rated at 9:1 CR.

 

[monzter]

Yes, 8.5:1 is what I red out of the suffix code, if I remember correctly

 

[curtis73]

Ok... those guys cleared it up before I could get to the book. Maybe I'm thinking of 76?

 

[cobalt327]

Ok... those guys cleared it up before I could get to the book. Maybe I'm thinking of 76?

Possibly there were some changes that didn't make it into the advanced info manuals.

I'm showing '73 as being the same as '74 except for the slightly different HP ratings.

My '76 Chevrolet advanced info shop manual shows the 110 HP 262 cid, 140 HP 305 and all 350's as having 8.5:1 CR, except for the 210 HP 350 which had 9:1 CR. The 400 SBC and 454 BBC are also shown as having 8.5:1 CR.

My 1977 Chevrolet advanced info shop manual shows all V8's (inc. 454) to have 8.5:1 CR.

 

[ericnova72]

The manuals are correct as far as blueprint "paper" ratio's given by GM, but actual measured and calculated ratio's at production machining tolerances and casting tolerances tend to have the engines up to 1/2 a point lower as assembled.