compression ratio, whats too much?

[2drxploder]

working on my 5.0 swap, was thinking cam, valve springs etc,... but the block has some nice scarring in hole #1. thinking of buying a 347 short block from carolina machine engines, running stock heads n cam. heads are at machine shop getting a rebuild which will include a slight decking to ensure a good mating surface. my rsearch shows gt40p heads @ 58-61cc, according to CME website, that will put me around 10.5 to 1. i think this may be a little high for a street truck. cant afford aluminumheads, cam, and block right now. thoughts? suggestions?

 

[adsm08]

10.5:1 is OK, you might need to run higher octane fuel though.

20 or 30:1, that might be a bit much.

 

[Dirtman]

Run 94 octane and a can of octane boost and you'll be fine.

 

[adsm08]

Run 94 octane and a can of octane boost and you'll be fine.

I'm not sure adding boost to an engine that is already running 10.5:1 is a good idea.

 

[Dirtman]

I'm not sure adding boost to an engine that is already running 10.5:1 is a good idea.

Go stand in the corner...

 

[adsm08]

Go stand in the corner...

Make me.

 

[2drxploder]

94 + octane boost?
only have 93 locally, and would prefer not to pay for it every fill up

 

[Dirtman]

Might run on 87 if you had the timing set perfect?

 

[2drxploder]

id be ok with 93, octane boost necessary?

 

[gaz]

2drxploder,

@ 10.5:1, you will be safe with whatever octane premium fuel is available (92-94), so stick with premium and enjoy the extra power from the higher compression.

 

[Junglejoe]

Daily driver?

 

[rusty ol ranger]

I'm not sure adding boost to an engine that is already running 10.5:1 is a good idea.

My cummins ISX has like 20 or 22:1 compression and pulls 40psi.....

Just sayin.

 

[adsm08]

My cummins ISX has like 20 or 22:1 compression and pulls 40psi.....

Just sayin.

Slightly different beast.

 

[2drxploder]

Yes it will be daily driven

 

[RonD]

An EGR system helps you run lower octane, just FYI, it cools combustion chambers so prevents heat ignition of lower octane fuels

Regular gas, 87 octane, will ping/knock with compression above 9.5:1
You might be able to use 89 octane, give it a try, if you get pinging on acceleration then go to 91 octane

I wouldn't think you would need 93 octane unless you were racing it and running engine hot

Octane is of course a heat rating nothing more, a gallon of 87 octane has the same energy as a gallon of 93 octane
Well if straight gasoline, 87 and 89 octane usually has ethanol added to keep stable octane rating so slightly less energy than 91 or 93 octane, but not much less, you wouldn't feel it, but could chart it on a dyno, loss of a few HP

Car makers add knock sensors on higher compression engines so they can run lower octane, and even on some lower compression engines, who's cylinders get hot spots
Engine Computer uses Knock sensor to adjust spark timing and fuel mix when knock/ping is detected
If your engine barely pings/knocks on 89 octane you can probably tune that out by adjusting spark and fuel mix, without too much power loss