Curious as to what compression numbers you guys are running with clone heads on predators. I'm talking non owing using gaskets. And not looking for compression ratio but actual lbs of compression. Group of Karter talking about there's got to wondering what would be kind of an ideal number to shoot for but not have blown head gasket problems. The group I was listening to had anywhere from 180 to 260lbs. What are your thoughts, thanks.
compression question for open motors
- Thread starter dirttrax
- Start date
jsstump#70
Member
it seems very very few actually check static compression on these 4-cycle engines that have a compression release on the cam. While it can be done by backing off the exhaust rocker to the point the compression release is disabled, many feel that a true compression can not be measured unless the valves are working as designed to allow the air in and out as it should be. If the compression release is not disabled, any number gathered is of absolutely no value. Most simply use a leak down tester, if anything, so that they can determine the source of the compression demise, whether it be rings, gasket or valves. One could use the numbers from a compression check to determine when the rings "seal" and how that engine is holding up throughout its life. Comparing one engine to another through compression numbers is not going to be a good gauge of anything I'm afraid. Lets say you build a super tight smallish cc engine that has really high static (super low rpm) compression numbers. Someone else has a slightly larger engine that the rings are not as tight at non firing rpms because they are gas ported and it has a cam grind with tons of overlap. It shows WAY lower static (super low rpm) compression numbers than the other engine BUT it makes WAY more power. Runs rings around it in fact. Static compression numbers, if used, may be used to keep a check on either engine individually but is a poor tool to compare one to the other, I'm afraid. Now the pressures generated at 7000rpm in those engines would be quite telling and "dynamic" I bet.
alvin l nunley
Site Supporter
As I am often heard saying, "compression is the Holy Grail". When your engine is putting out the maximum compression pressure it can, and in spite of that you're running slow, rings and valves can pretty much be eliminated as a cause. And a compression check is so easy.
You check the compression after break-in. The first thing you check, when you get back to the shop after a race, is the compression. If it hasn't changed since the last time you checked it, or has maybe gone up, it would tell you that the rings and valves are still in good shape. If it has gone down, since the last time you checked it, that would indicate a problem. This is where a leak down test could come in handy. It would tell you if it's the rings or valves are causing the drop in compression pressure.
With my two cycles, if our times were slow, the first thing we would check would be the compression pressure. It's not the end-all for tuning, but it definitely has its place.
If you find your lap times getting worse, a compression check is a fast, easy and convenient way to eliminate one possible cause.
As far as the compression release is concerned, it doesn't eliminate compression, it only lowers it. The fact that a modified engine may or may not show more or less compression pressure means nothing! If your engine makes a certain amount of compression pressure, whatever that amount is, that is your base number to compare subsequent readings to.
You check the compression after break-in. The first thing you check, when you get back to the shop after a race, is the compression. If it hasn't changed since the last time you checked it, or has maybe gone up, it would tell you that the rings and valves are still in good shape. If it has gone down, since the last time you checked it, that would indicate a problem. This is where a leak down test could come in handy. It would tell you if it's the rings or valves are causing the drop in compression pressure.
With my two cycles, if our times were slow, the first thing we would check would be the compression pressure. It's not the end-all for tuning, but it definitely has its place.
If you find your lap times getting worse, a compression check is a fast, easy and convenient way to eliminate one possible cause.
As far as the compression release is concerned, it doesn't eliminate compression, it only lowers it. The fact that a modified engine may or may not show more or less compression pressure means nothing! If your engine makes a certain amount of compression pressure, whatever that amount is, that is your base number to compare subsequent readings to.
Yes I was meaning non compression release motors. I check mine weekly just to see if I've lost any. My question was just what everyone sees on their built motors. Not that I was gonna build mine to another's numbers etc.
My animal has 180lbs
My hemi has 165 lbs
Got a friend that has 2 predators that have 245 and 250
So I was just trying to figure out if my build is just low compression built or his is extremely high
My animal has 180lbs
My hemi has 165 lbs
Got a friend that has 2 predators that have 245 and 250
So I was just trying to figure out if my build is just low compression built or his is extremely high
alvin l nunley
Site Supporter
It would be interesting to see how much smaller his combustion chamber is than yours. 28% more compression pressure is a lot. Do you run in the same class?
With that much compression, what fuel is he running? How about you?
alvin l nunley
Site Supporter
If his runs good on 110 octane, then yours should run better on less.
Last edited:
youngengines
Member
Cam overlap plays a major role in cranking compression too.
jsstump#70
Member
That was my point I was trying to make above. Comparing two completely different builds by using cranking pressure will mean very little. Cam overlap being the major reason why. In the above engines, the smaller "combustion chamber" very well may be the lower compression engine checked. Using cranking engine pressure to determine what fuel may be OK to run is also not something that should be attempted. Cranking pressure and actual running pressure and mathematically calculated compression ratio are very very different things and may or may not have a direct correlation to the other.
alvin l nunley
Site Supporter
It's hard for me to see how the overlap could have any effect on compression pressure? Could somebody explain that? With both valves open, I find it hard to believe that the exhaust stroke could add anything to the compression pressure. I know there is some pressure on the exhaust stroke because you are pushing the burnt gases out the exhaust valve, it takes some pressure to do that, but the idea that that could add to the much higher compression stroke pressure escapes me.
flattop1
Dawg 89
The over lap does not add any thing to the compression cycle.
At slow cranking speeds it allows compression out the valve.
as the cycle is not fast enough to capture it, as it would be at say 5000 rpm.
Even though it is a 4 cycle, the exhaust still has a pressure wave that returns back through the exhaust pipe.
At slow cranking speeds it allows compression out the valve.
as the cycle is not fast enough to capture it, as it would be at say 5000 rpm.
Even though it is a 4 cycle, the exhaust still has a pressure wave that returns back through the exhaust pipe.
jsstump#70
Member
As it was explained to me at Holman Moody, while running parts, particularly camshafts, to and from several race shops, any volume that comes through the intake that is used to expel the exhaust and NOT compressed for combustion, will reduce the available charge for combustion, thus lowering the dynamic compression. They are also the source of the evidently faulty (sarcasm) information that says vastly different ring designs will also greatly influence the pressure differences from "cranking" pressure vs running pressure. The OP implied he was wanting to compare combustion chamber pressures at cranking rpms as a way to compare one engine to another. I contended and still contend that several factors, cam overlap being a major one, column inertia being even more so relevant, will definitely make doing so of no value. As flattop1 just clearly stated "(cam overlap)at slow cranking speeds it allows compression out the valve". (I would have added "potential" prior to the word compression) If it can not be understood by some that by NOT capturing ALL of the incoming available volume to compress, then no amount of explaining by me or anyone else will change that. I also contend that by using some of this incoming volume to aid in expelling the exhaust, while potentially reducing compression pressures, although slightly, will also increase overall engine efficiency making more "power'. Keep in mind we are talking about measuring the compression at very very low rpms. I am not trying to reinvent anything that has been established many years ago. I am not offering anything new or insightful. No new "theories" being presented by me. Just letting the OP know that you can NOT compare two totally different engines by using the pressure measured that they produce in the combustion chamber at cranking rpms. Cams have much varying grinds. Some have HUGE overlap. Some don't. Even two engines with the same cam grind may vary widely due to how its oriented to the cranks position. There is just no way to use compression pressure at low rpms as a means to compare two different engines. Sorry.
As a side note, be sure to check compression on any engine with the throttle (if carb is in place) at wide open position. You can NOT compress what you cant "suck" in.
As a side note, be sure to check compression on any engine with the throttle (if carb is in place) at wide open position. You can NOT compress what you cant "suck" in.
Well thanks for all the great reading info, but to be clear I wasn't wanting to compare my engine to his based off cranking compression. It was just a general question just to hear what guys are running. Beings I didn't know if 180lbs in mine was good, average,low etc. I just figured there was a baseline where one could say yeah 180 good on gas but 200 plus is better on alky 225 plus and head gaskets are an issue etc. But after reading you great explanation on overlap I can see why know one would really check there's other than just to see if they lost any. Thanks