318 and my compression ratio

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7duster4

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I was talking to a buddy of mine about my 318. I had it rebuilt a few years ago. All the machine work was done by a very well known engine builder who spends more time building mopar engines than anything else. Its just what he prefers and I have regarded him as a very knowledgable guy when it comes to this stuff. He said he raised my compression to 9:1 since low compression was always the biggest drawback on these.

My buddy dosn't think I have 9:1. Basically becuase my engine dosn't ping. Anytime I get gas I usually just use the mid 89 octane. If I take it down the track I put 93 in it. I have never heard pinging. So my question is would a 9:1 318 ping with 89 octane gas? I have about 10,000 miles on this engine I would really hate the fact that my engine isn't what builder said it would be. That sure would be a total dissapointment.

Any thoughts or helpful info is appreciated, thanks.
 
Well if you could find a buddy with a tool to check your compression, that'd be the route to go. I know they aren't super common to come by, but I mean I have a buddy who has one. Just saying, that's the only way to know for sure.
 
ask the expert engine builder..what is the deck height on your pistons and what pistons he used....cc on the heads...and thickness of the head gasket...

then you will be able to cal your compression ratio...
 
As Coryduran suggests, I used to think one could determine the compression ratio by a compression measurement. Indeed, I wondered why it is hard to find stated values for what is a good measurement, usually they just say "varies less that 20 psi between cylinders". The reason is that the camshaft makes a big difference. Some "wild" cams even overlap the intake and exhaust. They may work great at high rpm, but not make a good compressor at idle, or during a cranking test.

If a cylinder measures <100 psi, that indicates a sealing problem. If all cylinders are >200 psi and stabilize by the 3rd stroke, the rings and valves are probably fine. Otherwise, you need other tests (leak-down), listen for hissing in the manifolds or crankcase.

Re your question, short of pulling the heads and making dimensional measurements, I don't know how you can determine the compression ratio. Maybe there is a way to take the cam out of the picture, like if you have adjustable rocker arms and can back them off to make it act as a smaller cam. Your friend's suggestion of using pinging as a diagostic has some basis, but isn't definitive. Besides cylinder pressure, pinging is also strongly affected by temperature, heat sources in the cylinder (carbon, spark plug), and of course gas octane. Maybe the "89 octane" you buy is actually 91. They only claim it at least 89. Also, pinging is not yes or no, but a varying level. I recall an SAE paper ~1979 that said a little pinging (aka auto-ignition) is good since it improves mixing. But by the time you hear it in the passenger compartment it is severe enough to damage pistons.
 
The only way to find out for sure is to cc the heads, take some measurements, and calculate it.

Bigger cams will lower your cylinder pressure and will be less likely to ping.

I would not expect an iron head small block with 9:1 compression to ping, assuming it doesn't run too hot or too lean, and has a hotter-than-stock cam.
 
A 318, bored .030, with flat tops, and open chamber 318 heads, shouldn't ping at 12* at 1000, 24* at 1800, and 34* by 2500, with a fairly mild cam. (Thanks partially to the small bore some don't like).

I have had them cc'd at 9.5, with a 270adv cam, and they didn't ping on 90 oct, til you got over 38*
 
Thanks everyone, I thought about asking the guy who built it but I haven't talked to him in such a long time. May not even remember lol but if he did I can see him saying exactly what he did and it being 9:1. He's a stand up guy. I was just curious. My buddy has a stock 318 Dart I believe its a 1970 block. He swapped the slant six. Everything is stock except headers, intake and a Holley. Stock cam and engine has never been apart. He told me he hears some pinging with 89 when getting on it hard. I thought for a basically stock 318 thats was pretty extreme.

thanks again everyone
 
stock 318 w/ or w/o any bolt ones can ping from carbon build up which can raise the compression as well as glow red hot and pre ignite.

you can get away with 175 psi cranking and pump gas

it would be nice to know what cranking comp the motor has, knowing that along with the cam specs does give a realm
 
1wild&crazyguy said:
stock 318 w/ or w/o any bolt ones can ping from carbon build up which can raise the compression as well as glow red hot and pre ignite.

you can get away with 175 psi cranking and pump gas

it would be nice to know what cranking comp the motor has, knowing that along with the cam specs does give a realm
Click to expand...

The motor in my buddys Dart if I'm not mistaken has over 100,000 miles on it and has never been apart. He could have a lot of carbon causing some pinging on full throttle runs where my engine has around 10,000 miles on it. I know for a fact he is running a stock spec cam and I have a 340 spec. But I'm not sure if it's from mopar or somewhere else. I still beleive mine has the compression as stated becuase quite frankly the car just feels much better now than before the engine was rebuilt.

Before it came apart the odometer showed 26,000 miles, I am assuming 126,000. Even then I used 89 octane and I don't remember hearing any pinging but its been a while.

He is just thinking that about my engine because his Dart is almost a full second faster than my car in the 1/4 mile. The two biggest advantages he has over me is a well tuned Holley and hedders.
 
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