# Compression



## Kinnakeet (9 mo ago)

Is it possible to get 8:1 compression from a 1968 YZ 400 with #16 heads with thicker head gaskets or would it need dished pistons.
My Father had his 1968 GTO motor rebuilt and he told me the builder put 8:1 pistons in it but he could not tell me if the builder used dished piston or maybe thicker head gaskets which I do not think that will get it down that much the problem he is having is it overheats after he drives it thru the neighborhood and then parks it he then told me he was using 89 octane. I would hate for this motor to expire as the car is a numbers matching car i have read that to low octane burns to fast and can cause overheating any help would be great.


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## PontiacJim (Dec 29, 2012)

Kinnakeet said:


> Is it possible to get 8:1 compression from a 1968 YZ 400 with #16 heads with thicker head gaskets or would it need dished pistons.
> My Father had his 1968 GTO motor rebuilt and he told me the builder put 8:1 pistons in it but he could not tell me if the builder used dished piston or maybe thicker head gaskets which I do not think that will get it down that much the problem he is having is it overheats after he drives it thru the neighborhood and then parks it he then told me he was using 89 octane. I would hate for this motor to expire as the car is a numbers matching car i have read that to low octane burns to fast and can cause overheating any help would be great.


Thicker head gaskets will not do it, nor the way to go as it can create more problems.

Dished pistons are the way to go UNLESS you want to swap heads with larger combustion chambers. So 2 ways to drop compression.


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## Sick467 (Oct 29, 2019)

Even though overheating can be a symptom of running too low of an octane fuel...there are other symptoms that would show us as well. 

The most noticeable would be "pinging". This is most obvious when at highway speeds just after going into high gear with a light throttle, going up a slight grade. Like you are just trying to maintain speed. You will hear a "ping-ping-ping" that could go away with the change of the throttle.

Another one is when your car is well up to running temp and you shut it off, most evident on a hotter day, and it continues to run, run like doo-doo, but still runs-on...also known as dieseling.

If these symptoms are not showing up, I'd say you have a low compression build. To make sure, I'd put some 87 octane in it and see if anything shows up. 8:1 compression should run just fine on 87 octane.

Plus one on what @PontiacJim says about the pistons being the correct way to drop CR and that the gasket alone would not drop it very much.

Back to the overheating issue. How hot is it getting? Have you verified it with a second thermometer (like a temp gun)? OR is it just puking antifreeze after a short drive? If the radiator is filled up much past the top of the core, it will puke some antifreeze. These old cars did not have a coolant overflow and just expelled the excess.


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## Baaad65 (Aug 29, 2019)

You could get a camera like this that's inexpensive and look into the cylinder to see if you have dished pistons.


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## PontiacJim (Dec 29, 2012)

Sick467 said:


> Even though overheating can be a symptom of running too low of an octane fuel...there are other symptoms that would show us as well.
> 
> The most noticeable would be "pinging". This is most obvious when at highway speeds just after going into high gear with a light throttle, going up a slight grade. Like you are just trying to maintain speed. You will hear a "ping-ping-ping" that could go away with the change of the throttle.
> 
> ...


If I am reading his post correctly, he said it was heating up *after *he parked it. If he means when he shuts it down, then that is normal as the cooling system is no longer cooling and the heat built up in the engine will cause the temps to go up - normal happening. 

Maybe that is it?


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## lust4speed (Jul 5, 2019)

Holding the normal parameters constant for a 400 engine with 16 heads, it would take a 35cc dish to arrive at a true 8:1 compression. Doable since I've ordered out custom 33cc pistons from Ross, but I doubt the mechanic went to that much trouble or expense. Is the mechanic still around for questioning? It would be interesting to discover how he determines his 8:1 compression. A few years ago I built out a '67 .020" over 400 engine with 24cc pistons ending up with 8.6:1 compression that ran fine (and cool) on 87 octane. Even a 9:1 engine running on 87 octane driven mildly around the neighborhood wouldn't overheat.

There's so many areas that can cause overheating that it's hard to blame low octane fuel and detonation. Why not start by removing the current gas and refilling the tank with some 91 or 93 octane and do a run and see if things improve? I'm betting that the heat remains about the same and the problem is a substandard cooling system. Wonder if the mechanic installed the plate(s) in the water pump? Surprising how many Chevy oriented mechanics screw this up. Need to check the fan clutch, condition of radiator, and all the other things that might be wrong with the system. As Jim said, it is normal to gain an additional 10° or so on hot soak, but if the engine is right at or above overheating when it is shut off, then the hot soak is going to spew coolant out of the overflow.


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## Kinnakeet (9 mo ago)

Baaad65 said:


> You could get a camera like this that's inexpensive and look into the cylinder to see if you have dished pistons.
> View attachment 159462


i HAVE 1 AND WILL CHECK THE NEXT TIME i GO THERE


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## Kinnakeet (9 mo ago)

lust4speed said:


> Holding the normal parameters constant for a 400 engine with 16 heads, it would take a 35cc dish to arrive at a true 8:1 compression. Doable since I've ordered out custom 33cc pistons from Ross, but I doubt the mechanic went to that much trouble or expense. Is the mechanic still around for questioning? It would be interesting to discover how he determines his 8:1 compression. A few years ago I built out a '67 .020" over 400 engine with 24cc pistons ending up with 8.6:1 compression that ran fine (and cool) on 87 octane. Even a 9:1 engine running on 87 octane driven mildly around the neighborhood wouldn't overheat.
> 
> There's so many areas that can cause overheating that it's hard to blame low octane fuel and detonation. Why not start by removing the current gas and refilling the tank with some 91 or 93 octane and do a run and see if things improve? I'm betting that the heat remains about the same and the problem is a substandard cooling system. Wonder if the mechanic installed the plate(s) in the water pump? Surprising how many Chevy oriented mechanics screw this up. Need to check the fan clutch, condition of radiator, and all the other things that might be wrong with the system. As Jim said, it is normal to gain an additional 10° or so on hot soak, but if the engine is right at or above overheating when it is shut off, then the hot soak is going to spew coolant out of the overflow.


The plates are in that is one thing I checked the distance is good
The radiator is kinda dirty and useing cleaner/flush did not help to much
It never pings
At idle it gets up to 190-200 and 240 after it is parked and turned off, temp reading from gauge in the intake before the stat
Another question is the hose after the T-stat supposed to be filled with water after the stat opens because it does not feel like that if it shoud be
The radiator is not the original but it is a 4 core
After I check the piston with the camera Ill et you guys know


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## chrisn (2 mo ago)

with stock pistons that engine was only rated at 8.6 to 1 i dont know if they used a flat top the heads have 72cc chambers and 2.11/1.77 valves so if he put pistons for a 8 to 1 ratio you should be ok because 8 to 1 is lower than the 8.6 from stock and with thicker gaskets your just lowering it


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## Kinnakeet (9 mo ago)

chrisn said:


> with stock pistons that engine was only rated at 8.6 to 1 i dont know if they used a flat top the heads have 72cc chambers and 2.11/1.77 valves so if he put pistons for a 8 to 1 ratio you should be ok because 8 to 1 is lower than the 8.6 from stock and with thicker gaskets your just lowering it


YZ 400 10.75 compression 360 HP from factory


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## BearGFR (Aug 25, 2008)

Kinnakeet said:


> YZ 400 10.75 compression 360 HP from factory


Except it wasn't actually 10.75:1. Those were the factory blueprint specs from the engineering drawings, but production line engines weren't built to those specs. It was a game that most OEM's played. Back in the day, that's what "blueprinting" an engine meant: re-machining all the tolerances and clearances to match those blueprint specs. That included milling the heads to reduce combustion chamber volume down to the "blueprint" spec in order to achieve "blueprint" compression, thus making the engine more competitive yet still NHRA Stock class legal.

There's an Excel spreadsheet available here in the FAQ section that you can use to calculate actual static compression ratio, provided you have all the correct measurements.

Taking a stock spec 400 with #16 heads (usually around 72cc's in the chambers) and all other measurements being factory nominal, the actual compression ratio from on a production-line engine would have been closer to 9.9-10.0:1.










(You can also use this tool to "figure out" how much piston dish volume you need to arrive at a desired compression ratio. Remember though, GIGO. Garbage In Garbage Out. To get accurate results, you have to provide actual and accurate measurements. Don't guess at anything.)

Now, on to your overheating issue. If that engine is stock spec, ain't no way it's going to be happy on 89 octane. Run the highest you can find, 93 if available. Even with that,
there's a good chance it's still going to rattle/detonate. One of the "common bandaids" for that is to retard ignition timing. Notice I said bandaid. You MIGHT be able to stop the rattle by doing that but it's going to cause other problems. One, it's going to murder performance. Two, and less obvious, it's going to make it tend to overheat. Why? because with the timing retarded, the fuel charge is being ignited "later" which will tend to cause it to still be burning in the cylinder when the exhaust valve opens. That's going to put a ton more heat into the edges, back, and stem of the valve and also into the exhaust port in the head -- and you get an overheating problem. That's the also the second reason that factory ignition systems have that vacuum advance mechanism. Under part throttle conditions, when manifold vacuum is high, fuel mixtures are also more lean due to the flow restriction imposed by the throttle. Lean mixtures burn more slowly and they burn hotter. Adding additional ignition advance under those conditions helps to make sure all the burning is done before the exhaust valve opens.
So, make sure your vacuum advance mechanism is functional and working properly.

PJ mentioned a couple good points (as always). All engines will get a little hotter right after shutdown because the coolant is no longer circulating - that's normal. When you said it's overheating after shutdown, what does that look like? Is it puking coolant out of the overflow tank? That COULD be normal if it was overfilled to begin with. It's normal for a warm engine to push a little coolant into the tank, that's one of the reasons it's there. Is the temp gauge in the car climbing? How high and for how long?

Thick head gaskets. Never a good idea because they increase the distance between the cylinder head and the piston crown at TDC. This distance, "quench" or "squish" distance is important because it induces turbulence in the cylinder which causes the fuel mixture to tend to stay mixed and circulating prior to lighting the fire. Not enough "quench" reduces turbulence and can make it easier for edges in the chamber to cause hot spots which can actually make a detonation problem _worse. _I know it's tempting because it seems "easy", but don't use fat gaskets to drop compression unless you truly have no other options, and even then get ready to deal with other problems that include taking a hit on performance. 

Bear


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## Kinnakeet (9 mo ago)

BearGFR said:


> Except it wasn't actually 10.75:1. Those were the factory blueprint specs from the engineering drawings, but production line engines weren't built to those specs. It was a game that most OEM's played. Back in the day, that's what "blueprinting" an engine meant: re-machining all the tolerances and clearances to match those blueprint specs. That included milling the heads to reduce combustion chamber volume down to the "blueprint" spec in order to achieve "blueprint" compression, thus making the engine more competitive yet still NHRA Stock class legal.
> 
> There's an Excel spreadsheet available here in the FAQ section that you can use to calculate actual static compression ratio, provided you have all the correct measurements.
> 
> ...


Thanks for the info


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