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Cranking Compression vs. Octane

bigburban383

bigburban383

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Apparently my engine knocks a lot, even though I don't hear it, I know this because the sensor tells me. I checked the cranking compression on a few cylinders and I am getting 150 psi. Would 93 octane be good enough for pressures like that, I have no clue of the compression ratio. I have dished pistons and unshrouded 305 heads.
 
how much of a dish? Unshrouded 305 heads are probably like 59/60ccs. With flattops that would be about guessing at 10.5:1 compression. You are probably in the mid or upper 9s, I'd say try a little bit of 89, then try a little bit of 93, whichever gives better results(power,knocking,etc.) go with that. If it runs good on 89 there is no reason to run 93.
 
How dished are the pistons? Stock 83 305 heads on a 305 produced 9.2:1 compression, hence the need for ESC. Bigger piston diameter on your engine will lower the compression a bit. /forums/images/graemlins/k5.gif
 
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How dished are the pistons? Stock 83 305 heads on a 305 produced 9.2:1 compression, hence the need for ESC. Bigger piston diameter on your engine will lower the compression a bit. /forums/images/graemlins/k5.gif

[/ QUOTE ] actualy a bigger piston diameter will raise your compression due to the greater volume being compressed /forums/images/graemlins/deal.gif /forums/images/graemlins/waytogo.gif
 
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actualy a bigger piston diameter will raise your compression due to the greater volume being compressed

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Then the compression ratios of all those smog 350's in the 70's with 76cc heads would of been really high, right?
 
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actualy a bigger piston diameter will raise your compression due to the greater volume being compressed

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Then the compression ratios of all those smog 350's in the 70's with 76cc heads would of been really high, right?

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No. Larger chamber heads will reduce compression ratio. 76 cc heads are huge.
 
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actualy a bigger piston diameter will raise your compression due to the greater volume being compressed

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Then the compression ratios of all those smog 350's in the 70's with 76cc heads would of been really high, right?

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No. Larger chamber heads will reduce compression ratio. 76 cc heads are huge.

[/ QUOTE ] /forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gifExactly /forums/images/graemlins/peace.gif
 
After a re-read of that article it makes sense to me but its funny. Overboring an engine RAISES compression by ADDING volume, however using larger chamber in the heads ADDS volume and LOWERS compression. That is the part I forgot about. /forums/images/graemlins/k5.gif
 
I try not to question, a 400 can have the same compression with 76cc heads that a 350 has with 64s, I just think of that and well thats that.
 
/forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gif you got it ! Sounds wierd though at first, I'll give you that one! /forums/images/graemlins/wink.gif /forums/images/graemlins/thumb.gif
 
you are pushing probably 9.5:1+, like I said 89 might work but I'd say much more towards 93. any pictures of your chambers? I remember them a while back but forget now.
 
Actually he's closer to 10:1 or right at it. I would be willing to say that with 305 heads, 10:1 CR, and any kind of torquey cam, that engine is going to knock like a Diesel under load no matter what pump gas you choose.
 
I'm asking about his heads cause, if he has a nice polished chamber it would be a help. I said a little less than 10:1 cause he elarged the chamber by unshrouding the valves. You are right though, it will be tough to get around.
 
Since your engine and my engine are the same CID, I opened up the compression ratio calculator that I wrote in Excel and checked a few things.

With a 4.06 bore, 3.75 stroke, 12cc dish piston, 72cc head etc etc my comp ratio is 9.463:1. I changed nothing but the head cc...which I changed to 58cc to reflect the 305 head......and the comp ratio jumped to 10.9:1! /forums/images/graemlins/eek.gif /forums/images/graemlins/doah.gif /forums/images/graemlins/yikes.gif

You probably did not square deck the block for good quench which will lower the comp ratio, but make the engine more prone to detonation.

I would guess you are between 10-10.5:1 which is WAY too much for a street motor in a heavy Burb. Loose the 305 heads and find a good set of 350 heads with either 64 or 76cc's. You'll be much happier with the engine. /forums/images/graemlins/waytogo.gif
 
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you are pushing probably 9.5:1+, like I said 89 might work but I'd say much more towards 93. any pictures of your chambers? I remember them a while back but forget now.

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16468heads1.JPG
 
Those pistons must have a big dish in them (like 20+cc's) to get the compression #'s you posted in a 383!

Tearing into the motor might not be something you want to do...but putting in a larger cam will help bleed off some pressure and you might be able to get away with the cheeper gas. Cam's don't cost a hole lot so you shouldn't have that long to break even...on the plus side you will have some more HP which is always fun /forums/images/graemlins/waytogo.gif
 
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Since your engine and my engine are the same CID, I opened up the compression ratio calculator that I wrote in Excel and checked a few things.

With a 4.06 bore, 3.75 stroke, 12cc dish piston, 72cc head etc etc my comp ratio is 9.463:1. I changed nothing but the head cc...which I changed to 58cc to reflect the 305 head......and the comp ratio jumped to 10.9:1! /forums/images/graemlins/eek.gif /forums/images/graemlins/doah.gif /forums/images/graemlins/yikes.gif

You probably did not square deck the block for good quench which will lower the comp ratio, but make the engine more prone to detonation.

I would guess you are between 10-10.5:1 which is WAY too much for a street motor in a heavy Burb. Loose the 305 heads and find a good set of 350 heads with either 64 or 76cc's. You'll be much happier with the engine. /forums/images/graemlins/waytogo.gif

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Oh My Got, 10.9:1, I suppose that is the problem. I'm an idiot! Thank you Chevy for ESC and Knock Sensors /forums/images/graemlins/bow.gif
 
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You probably did not square deck the block for good quench which will lower the comp ratio, but make the engine more prone to detonation.

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Decking the block into a good quench zone does the exact opposite. It will INCREASE compression ratio, and make the engine LESS prone to detonation.

Even with perfect quench distance in a 383 with a torquey cam and 305 heads and 10:1 static CR, it's going to be VERY knock prone.
 
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