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compression in a 400 ci


Guest ralsy

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Guest ralsy

Can anybody tell me why somebody would reduce the compression by increasing the gasket thickness on an Olds 400 big block?

There might be a logical reason, but not sure.

Cheers.

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Guest ralsy

I don,t understand what difference it makes. If you use valve saver additive it should not be a problem. I dont know what pump gas you guys have in the US, but here in Australia i would use Premium unleaded which is 98 ron.:confused:

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In my area fuel with a high octane like 98 has gotten tough to come by. 93 is about it at a normal station. On 93 the 400 with the higher compression will not be happy. Ping and knocking will result none which are good for the motor. We have a slew of different additives on the shelf. None of them seem to perform that well to me.

By the way I own a 69 442 50,000 mile un-touched original car. It has the 400. When the weather gets hot and the car likes to run hot I leave it home. I call it my great spring and fall car

Don't forget in 69 premium fuel was a good 101-102 octane with lead here in this area.

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Guest ralsy

Well it will be interesting to see how my OLDS handles the Aussie hot weather. I have had a few early 60s car before and have run them on 98 octane fuel with the Valve saver additive thats available here without any problems (pinging). Thats not to say that i wont have any probs with the olds 400.

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I'm not sure how BOLD text helps, but there are a couple of reasons. First, as noted, octane rating of gasoline in the US is pretty pitiful. I also don't know whether the octane rating system in Australia is the same as in the US. I do know that the European scale is different and cannot be directly compared. Increased gasket thickness to reduce compression ratio was common here starting in the mid-1970s when high octane gasoline pretty much went away with the removal of lead. Today there are some fairly high octane unleaded brands available, but they are priced accordingly.

The other reason may be just due to gasket availability. Olds used a steel shim head gasket from the factory, with a compressed thickness of about 0.017". The most common head gaskets available today are the FelPro blue gaskets, with a compressed thickness of about 0.040". These do seal better, however. Steel shim gaskets are available, but not from your corner parts house.

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Guest ralsy

Sorry if I offended anybody with the bold txt. I didn’t know that there are subtleties in writing on forums. I just wanted it to stand out as the main part of the posting. Wont do it again.:(

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Sorry if I offended anybody with the bold txt. I didn’t know that there are subtleties in writing on forums. I just wanted it to stand out as the main part of the posting. Wont do it again.:(

As with all-caps, it usually connotes shouting. One unfortunate "feature" of electronic correspondence is the lack of facial expressions to help communicate intent. ;)

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Guest JeffreyAlman

re the bold text if your intention was to have it stand out then I think it was appropriate, IMHO

re the gasket thickness altering compression, can someone please explain that to me

I am assuming head gasket? but maybe intake manifold? I just can't imagine how that small an amount would change it, or maybe I don't get how it's all intertwined

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Guest docfrahm

A thicker head gasket increases the total volume of the combustion chamber, thus decreasing the C.R. It doesn't seem that a 0.025 deeper (larger) combustion chamber would be that significant but it usually decreases the C.R. by approximately 1 point.

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Guest Julian

On 93 the 400 with the higher compression will not be happy. Ping and knocking will result none which are good for the motor.

I have an all original 67 442 with 58k 400 engine.. It does not like 93 octane. I have to "mix" 100+ with 93 and use a lead subsitute to get it to run like it should. A few years back, I was able to get some "real" nascar racing gas that really woke up the ole gal. I have it timed to factory specs. When it is running good, I get a solid 10-12 mpg :)

Edited by Julian (see edit history)
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