pistons and compression
Thread Starter
Registered User
Joined: Oct 2012
Posts: 8
From: Gouda Netherlands
pistons and compression
hallo everyone,
i own a 1968 olds delta 88 coupe. it have the standard 455 with the 2 barrel rochester.
i have reviseted the engine. not me but a company who do that stuf. now the engine was finished and i couldn't get the timing right. engine knock. now is the thing that the old pistons had a dish in it of about 0.276 and the new pistons have a dish of 0,138 in it. can that be the trouble? everything is new on the engine. the static compresson is 11 bar. is that to high? please responce.
Raymond.
i own a 1968 olds delta 88 coupe. it have the standard 455 with the 2 barrel rochester.
i have reviseted the engine. not me but a company who do that stuf. now the engine was finished and i couldn't get the timing right. engine knock. now is the thing that the old pistons had a dish in it of about 0.276 and the new pistons have a dish of 0,138 in it. can that be the trouble? everything is new on the engine. the static compresson is 11 bar. is that to high? please responce.
Raymond.
Connoisseur d'Junque
Joined: Sep 2010
Posts: 21,183
From: The Hudson Valley
11 bar = 160psi, so, no, that is a normal cranking pressure for a fresh low-compression (9:1) engine with a factory cam.
I am not sure what you mean by 0.276 and 0.138. Do you mean inches of depth?
If so, what were the diameters of the respective dishes?
Also, note that the 2bbl 455 could be ordered with either a 10.25:1 CR or a 9:1 CR in 1968, so the other question is, what was the original compression ratio of your engine?
- Eric
I am not sure what you mean by 0.276 and 0.138. Do you mean inches of depth?
If so, what were the diameters of the respective dishes?
Also, note that the 2bbl 455 could be ordered with either a 10.25:1 CR or a 9:1 CR in 1968, so the other question is, what was the original compression ratio of your engine?
- Eric
Registered User
Joined: Feb 2013
Posts: 15,370
From: Phoenix, AZ
The dish in the pistons is how Olds made the compression ratio different. Larger dish = lower compression ratio, smaller dish = higher compression ratio. You would need to also know the diameter of the dish in order to calculate the dish volume and then calculate the compression ratio.
You say the "static compression" is 11 bar, but that can't be - 11 bar would be the pressure within the cylinder while the engine is cranking, which translates to about 160 psi. I think that pressure is typical of a low compression engine or a medium compression engine with a longer duration camshaft; either way I don't think it is high enough to cause the problem you are experiencing.
You say the "static compression" is 11 bar, but that can't be - 11 bar would be the pressure within the cylinder while the engine is cranking, which translates to about 160 psi. I think that pressure is typical of a low compression engine or a medium compression engine with a longer duration camshaft; either way I don't think it is high enough to cause the problem you are experiencing.
Thread Starter
Registered User
Joined: Oct 2012
Posts: 8
From: Gouda Netherlands
hallo eric. its the depth. what do mean with 10.25:1 CR or a 9:1 CR ? sorry i am dutch and not a expert. i try to understand it all. we don't have a great forum like this in the netherlands. i am just a european guy who loves his olds.
Connoisseur d'Junque
Joined: Sep 2010
Posts: 21,183
From: The Hudson Valley
The compression ratio of engines from about 1964 to 1970 was either 9:1 or 10.25:1. After that, it was 8.5:1.
These are nominal specifications, and the actual compression ratios are about half a point to a point lower, depending on manufacturing tolerances.
The depths of the dishes are not very helpful without also knowing the diameters, and also the letters of the heads (probably C), and whether they have been modified in any way.
Ideally, we would want to know the manufacturer and model of the pistons and camshaft as well.
- Eric
These are nominal specifications, and the actual compression ratios are about half a point to a point lower, depending on manufacturing tolerances.
The depths of the dishes are not very helpful without also knowing the diameters, and also the letters of the heads (probably C), and whether they have been modified in any way.
Ideally, we would want to know the manufacturer and model of the pistons and camshaft as well.
- Eric
Registered User
Joined: Nov 2008
Posts: 8,472
From: Central Fl
The .138 dish piston is most likely the Federal Mogul cast dished piston, like this;
http://www.summitracing.com/parts/sl...ake/oldsmobile
It has a short comp distance, 1.725 but a small dish. Normally it equates to about a 9.75:1 depending on the head, deck etc.
http://www.summitracing.com/parts/sl...ake/oldsmobile
It has a short comp distance, 1.725 but a small dish. Normally it equates to about a 9.75:1 depending on the head, deck etc.
Last edited by cutlassefi; Sep 16, 2015 at 04:08 PM.
Thread Starter
Registered User
Joined: Oct 2012
Posts: 8
From: Gouda Netherlands
this is my question in a diffrent form. what are the consequences of putting high comp pistons in instead of the low comp standard pistons whithout adjusting anything. what can happen? to much compression? different air/ fuel mix? do i need to adjust the jets of my standard 2 barrel rochester?
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Joined: Apr 2015
Posts: 47
Hounddog88: To high of a compression ratio can cause pre-detonation, or the gasoline exploding before the spark plug fires. This is an issue and can damage your engine.
Generally, higher compression engines need higher octane gasoline. I do not know how your gas is there, here street gas is sold in 3 grades, ranging from 85 to 96 octane. Low octane is "regular gas" and is what most standard engines use. A low compression Olds engine can run regular gas just fine. Performance cars, (Corvette, Porsches, BMWs, ext) with high compression engines have to run high octane gas, "premium", to avoid the issues I mentioned above. If you ran regular gas before, you likely will need to run premium now with high compression pistons.
However, more factors than just your compression ratio effect this. Dwell time/stroke, cam timing, and ignition timing all are factors. The physical size and shape of the combustion chamber as well.
This is why everyone is asking for the camshaft information, head information, ext. Sometimes you can adjust the timing to make the engine run well, but with to much compression for your gas, you reduce the power of the engine. Ask the person who did your engine what gasoline he recommends you use. As a test, auto parts stores may sell a octane booster, which you might be able to use, however, long term use of this can damage your engine.
Do you have new spark plugs, wires, and distributor? These are commonly replaced in a rebuild but can cause an issue if they are not.
I hope this is helpful, and I hope my English is simple enough. Let me know if there is anything that confuses you.
Generally, higher compression engines need higher octane gasoline. I do not know how your gas is there, here street gas is sold in 3 grades, ranging from 85 to 96 octane. Low octane is "regular gas" and is what most standard engines use. A low compression Olds engine can run regular gas just fine. Performance cars, (Corvette, Porsches, BMWs, ext) with high compression engines have to run high octane gas, "premium", to avoid the issues I mentioned above. If you ran regular gas before, you likely will need to run premium now with high compression pistons.
However, more factors than just your compression ratio effect this. Dwell time/stroke, cam timing, and ignition timing all are factors. The physical size and shape of the combustion chamber as well.
This is why everyone is asking for the camshaft information, head information, ext. Sometimes you can adjust the timing to make the engine run well, but with to much compression for your gas, you reduce the power of the engine. Ask the person who did your engine what gasoline he recommends you use. As a test, auto parts stores may sell a octane booster, which you might be able to use, however, long term use of this can damage your engine.
Do you have new spark plugs, wires, and distributor? These are commonly replaced in a rebuild but can cause an issue if they are not.
I hope this is helpful, and I hope my English is simple enough. Let me know if there is anything that confuses you.
Registered User
Joined: Feb 2011
Posts: 4
From: Gouda, Netherlands
Hounddog88: To high of a compression ratio can cause pre-detonation, or the gasoline exploding before the spark plug fires. This is an issue and can damage your engine.
Generally, higher compression engines need higher octane gasoline. I do not know how your gas is there, here street gas is sold in 3 grades, ranging from 85 to 96 octane. Low octane is "regular gas" and is what most standard engines use. A low compression Olds engine can run regular gas just fine. Performance cars, (Corvette, Porsches, BMWs, ext) with high compression engines have to run high octane gas, "premium", to avoid the issues I mentioned above. If you ran regular gas before, you likely will need to run premium now with high compression pistons.
However, more factors than just your compression ratio effect this. Dwell time/stroke, cam timing, and ignition timing all are factors. The physical size and shape of the combustion chamber as well.
This is why everyone is asking for the camshaft information, head information, ext. Sometimes you can adjust the timing to make the engine run well, but with to much compression for your gas, you reduce the power of the engine. Ask the person who did your engine what gasoline he recommends you use. As a test, auto parts stores may sell a octane booster, which you might be able to use, however, long term use of this can damage your engine.
Do you have new spark plugs, wires, and distributor? These are commonly replaced in a rebuild but can cause an issue if they are not.
I hope this is helpful, and I hope my English is simple enough. Let me know if there is anything that confuses you.
Generally, higher compression engines need higher octane gasoline. I do not know how your gas is there, here street gas is sold in 3 grades, ranging from 85 to 96 octane. Low octane is "regular gas" and is what most standard engines use. A low compression Olds engine can run regular gas just fine. Performance cars, (Corvette, Porsches, BMWs, ext) with high compression engines have to run high octane gas, "premium", to avoid the issues I mentioned above. If you ran regular gas before, you likely will need to run premium now with high compression pistons.
However, more factors than just your compression ratio effect this. Dwell time/stroke, cam timing, and ignition timing all are factors. The physical size and shape of the combustion chamber as well.
This is why everyone is asking for the camshaft information, head information, ext. Sometimes you can adjust the timing to make the engine run well, but with to much compression for your gas, you reduce the power of the engine. Ask the person who did your engine what gasoline he recommends you use. As a test, auto parts stores may sell a octane booster, which you might be able to use, however, long term use of this can damage your engine.
Do you have new spark plugs, wires, and distributor? These are commonly replaced in a rebuild but can cause an issue if they are not.
I hope this is helpful, and I hope my English is simple enough. Let me know if there is anything that confuses you.
in holland we have the normal 95 or the super 98 gas. i always pick the 98 octane.
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