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89 mercruiser compression ratio / octane question

7

70wellcraft

Member
Hello,
I am in the middle of a rebuild of my 89 350 mercruiser. I am putting on vortec 64 cc heads, new intake, same stock cam, new speed-pro pistons with 6.9 cc total volume for valve indents, .039" (compressed) thick head gasket. Compression ratio calculator says 9.66:1. Can I run 87 octane or would this be asking for detonation issues?
Thanks a million.
 
70wellcraft said:
Hello,
I am in the middle of a rebuild of my 89 350 mercruiser. I am putting on vortec 64 cc heads, new intake, same stock cam, new speed-pro pistons with 6.9 cc total volume for valve indents, .039" (compressed) thick head gasket.
Don't forget that the piston deck height must be added to this.
For example: .039" compressed gasket thickness + .016" in-the-hole-clearance = a .055" quench dimension.
.055" might be OK for HP auto use, but you will want to stay near .038".

Compression ratio calculator says 9.66:1. Can I run 87 octane or would this be asking for detonation issues?
Unfortunately there is not a simple answer to that question.
The Q dimension, piston profile, Aluminum -vs- Cast Iron heads, E-10 -vs- Non-Ethanol fuel, Ignition advance, etc, all play a role in this!

Thanks a million.
Click to expand...

May we assume that the Speed-Pro pistons with only 6.9 cc total volume are Flat Tops?
If so, and while using .016" deck height clearance and .022" compressed head gasket thickness (quench dimension of .038"), I come up with just over 10.22:1 on the S C/R.


You may want to run the numbers again.

http://www.csgnetwork.com/compcalc.html

https://speedmaster79.com/tools/engine-compression-ratio/


For the SBC w/ 4.030" bore and a 3.480" stroke, the vortec cylinder head quench surface would perform better with a LCQ style piston profile.
Perhaps with a dish volume in the 16 cc to 18 cc range.

I would run the numbers again using several on-line calculators.


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o2batsea said:
Oh gosh incoming Rick quench-effect piston diatribe
Click to expand...


NOUN;
  • a forceful and bitter verbal attack against someone or something

First off, I do not see where I have attacked anyone or anything in the past regarding this topic.
These guys post their questions in hope of finding suggestions and answers. In this case, it would appear that the OP is questioning his build.

Secondly, if you would care to share your knowledge re; how to properly build a Marine version SBC...... please do. :D If you can show us a better build technique, that would be great!





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Last edited:
Thanks, the pistons are flat with the 6.9 cc valve indents, Hypereutectic Pistons, deck ht is .025, vortec heads are cast iron, carb tuned with a little richer primary and secondary metering rods. I ran the numbers though a few different online calculators and got the same 9.66:1. The original engine was spec'd at 9.3:1. I will stay with 89 Octane to be safe.
 
70wellcraft said:
Thanks, the pistons are flat with the 6.9 cc valve indents, Hypereutectic Pistons,

deck ht is .025,
Add the .025" to your compressed gasket thickness of .039" and you have a .064" quench dimension...... of which is too large in my opinion.


vortec heads are cast iron, carb tuned with a little richer primary and secondary metering rods.

I ran the numbers though a few different online calculators and got the same 9.66:1.
Keep a tight quench, and that should be OK.

The original engine was spec'd at 9.3:1. I will stay with 89 Octane to be safe.
The ignition advance plays an important role as well.
Click to expand...


Typically the F/T pistons can be used with the older 76 cc chamber cylinder heads, giving us a S C/R that suits the SBC Marine build.
With the 64 cc Vortec cylinder head quench surface, you will do better if you were to use a LCQ style piston with a dish volume that brings the S C/R into a more desirable range.
I would target a Q dimension of approx .038".



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Rick, you're as hilarious as ever! I don't need to know how to make my own shoes in order to go for a run!
 
o2batsea said:
Rick, you're as hilarious as ever! I don't need to know how to make my own shoes in order to go for a run!
Click to expand...

Perhaps not.
However, do you want to run in a regular pair of shoes, or would you prefer an actual running shoe?
I like an actual running shoe! :D

Let's help the OP, instead of trying to derail his thread...... fair enough?



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