respectable

[tonydenton]

what is a respectable [ healthy ] CYLINDER compression pressure for an EA81 engine that is putting out decent power. I've been checking mine sometimes and as usual don't know whats normal ??? and for the smarter ones out there how does one come up with a compression ratio that often gets thrown into conversations.

[WEST OZ FLYER]

Tony my best guess would be 200-230psi on a compression test. Alot depends on cam, compression ratio etc. From memory my standand EA81 in my Brumby was around the 170psi. It was a long time ago now.

Compression ratio is best chosen after you know your cam specs. You can pluck whatever figure you like out of the air for conversational purposes but reality dictates otherwise. The EA 81 has the wrong shape combustion chambers for unleaded petrol thus requiring a lower compression ratio than a modern engine. I would stick to a maximum of 10:1 but you need a suitable cam for this. Detonation in an EA81 will destroy bearings in no time.

Mad Max is the best person to ask cylinder compression as he had them written on his rocker covers. As usual there is a story behind it.

[tonydenton]

thanks for that. yes I figured that max would be a wealth of info. I am happy with the performance. I just started thinking of what was normal. thanks Matthew.

[Tim]

G'day Tony,
Re Compression ratio. Stock unmodified EA 81 is 1781cc = 445cc swept volume per cylinder. Total volume with 55cc combustion chamber = 500cc, (445 + 55). Therefore Compression Ratio = 500/55, = 9.1 : 1
Working out the volume of the combustion chamber is a little more complicated but it can still be readily done. It must include piston deck height (N/A with a stock EA81) and the volume of the compressed head gasket.
As for compression pressure, it can vary considerably with even minor modifications. At cranking speed I would be looking for ~ 170psi, or less with a turbo engine.
Regards Tim

[tonydenton]

ok thanks Tim, I thought it would be something like that , but you have explained it perfectly. mine is on the 150PSI mark. still happy with the performance thou. . of course it would be better if I lost some weight but that's not going to happen. thank you TIM.

[Tim]

With the variation in cylinder heads, test cranking speeds and testing equipment, it would probably be more important that all cylinders were within cooee of the same reading. I.e. I would be concerned if 3 were ~170 and one was 110.
Tim

[tonydenton]

2 are the same tim with the other 1 @ 10 lower and the last @20 lower. but luckily they are on opposite sides. I do know what the problem is and will strip it down and correct it soon.

[tonydenton]

if you wanted some thing to do I could always bring it down and leave it with you. .

[Tim]

G'day again Tony,
Before you strip it down do a simple check. Stick a hose in your ear and the other end in 1: the exhaust pipe, 2: down the carby, 3: inside the oil filler cap, and 4; check the water in the radiator.
Rock the prop back and forth on the compression stroke (ign off) of the low cylinder. You should hear a slight hiss from the leak, or see bubbles in the radiator.
1: = leaking exhaust valve, 2: = leaking inlet valve, 3: = leaking piston rings and 4: = leaking head gasket.
The problem with this system is that it does not clearly identify a leak in the head gasket between the cylinders, but at least it allows you to identify the correct side so that you may only have to remove one head.
Hope this helps
Tim.

[tonydenton]

ok thanks again TIM, that all makes sense when reading it for the first time in my life. [ not having any engine building skills ever . I had been guessing it is the actual ring gap. I discussed with the engine shop what the gap should be considering the constant engine RPM. but after paying and taking home for me to assemble I found the ring gap to be a shambles. so I was always thinking that it would be that happening.
if you don't mind TIM, what have you done in the past. do you still stay with standard gap on new rings and bore as per the suby manual. thanks in advance.

[Justin]

If it works dont stuff with it

[WEST OZ FLYER]

I am with Justin. They are close to a 10% difference, which is acceptable in my eyes. My only concern would be if there was a leakage to the cooling system. Do Tim's test to see where the problem area is and keep a watch on it. Just a query, did you have WOT when cranking and all of the sparkplugs out?

[tonydenton]

all spark plugs out but not WOT. I never read that any where. i'll try it again on the week end. I was not going to stop flying it as it has good power it was just my tinkering little voice on my shoulder saying to keep an eye on it. thanks MATTHEW.

[WEST OZ FLYER]

Tony, I have always done them at WOT. An elderly gentleman told me that doing it at WOT there is the least amount of resistance for the incoming air. It can make a small difference. Did you do the test hot and/or cold? Have you done a wet compression test? Have you checked/adjusted valve clearances? There are a few things that can change test results but make sure your basics are covered.

[tonydenton]

WOT makes sense, no panic , I was just doing a check and needed to ask the question for comparasion. I had just landed so it was hot but im keen to do another with WOT. .
seeing the forum is on fire again what about distrubitors. mine has never had the vacume advance fitted , been blocked off. but the GYRO engine builder that did that and all the things I have found out some things are not correct , so question is do you have the vacume fitted to a dizzy that flies ? if the answer is NO , then just to enrich my knowledge of all things smart , WHY NOT ?