Reassurance question:Cylinder Heads

[Roman S]

Re: Reassurance question:Cylinder Heads

Sorry for the double. The tech at Edlbrock replied and said that my compression at the present time is low with the 76cc chamber. The e-tecs would add one full point of compression. I am guessing I am somewhere in the 8:5 range, maybe less, and I would start pushing 9:? with the vortecs. I still need to know 64 or 72. And, will I still be able to use 87 or 89 Octane.
Thanks Roman

 

[FreeBeeTony]

Re: Reassurance question:Cylinder Heads

Just curious..........why not go with the GM 062 or 906 heads?
You can probably get a pair of 062's from GM for ~$500............

 

[John_S]

Re: Reassurance question:Cylinder Heads

You need to pull the heads before making any final decissions. You need to 100% confirm the pistons, make sure the engine has not been already bored, or decked. Once you measure this, you can now check the static compression in the calculator.

Here is specs for the GM 350 HO crate engine. "http://www.gmperformanceparts.com/EngineShowcase/tech_specs.jsp?engId=350HO&engine=350%20HO&sku=12499711&engCat=sb" It is close to a vortec marine spec. They advertise 9.1:1 compression ratio, and recommended fuel is 92 octane. Here is the standard 350 gm crate: "http://www.gmperformanceparts.com/EngineShowcase/tech_specs.jsp?engId=350290&engine=350/290%20HP&sku=12499529" It is relatively close to your current engine. (8.5:1) Cams and timing are different from marine, so not 100% comparable.

IMHO, to stay with 87 octane, you will need to be in the 8.8 - 9:1 "true" static compression ratio range for an iron vortec style head. For alum vortec style, you can bump that up 2/10ths.

More reading:
"http://www.chevyhiperformance.com/howto/47075/index.html"

 

[Roman S]

Re: Reassurance question:Cylinder Heads

John, Thanks. I looked at the specs of the engine. The Stock compression ratio is 9.0:1. On the value cover the octane called for with this engine was 88, meaning 89.
Roman

 

[John_S]

Re: Reassurance question:Cylinder Heads

I have seen compression ratio specs, all over the page. Bottomline is you can't trust the spec. If you have flat-top pistons and 76cc heads, the only way you get to 9.0:1 is with steel (0.017) shim head gaskets. I'd bet you have ss composite gaskets, which are thicker.

 

[Mkos1980]

Re: Reassurance question:Cylinder Heads

Why does the 90 Mercruiser 350's state 9:3:1 compression and rec. 87 octane. I've always ran 87 octane and even upgraded the TB module to a 24 unit with more timing. Gained 2 mph's and no problems even with more timing.

 

[John_S]

Re: Reassurance question:Cylinder Heads

"Why does the 90 Mercruiser 350's state 9:3:1 compression"

Because it is overstated. I have seen mine listed as 9.3, 9.1, 9.0, : 1 and actual is 8.8.

 

[Mkos1980]

Re: Reassurance question:Cylinder Heads

Maybe its because merc used both 76cc and 64cc heads?