Subarutex Posted August 3, 2004 Share Posted August 3, 2004 Is it possible to test the compression with the engine on the stand? Also, what have other people seen compression wise running the delta cams? Seems like it should be the same as stock, yeah? Link to comment Share on other sites More sharing options...
offroadsubaruguy Posted August 3, 2004 Share Posted August 3, 2004 Is it possible to test the compression with the engine on the stand? Also, what have other people seen compression wise running the delta cams? Seems like it should be the same as stock, yeah? i would say you could test it on a stand, you might have to turn the motor over by hand, but you should still be able to do it.... as for the delta cams, they wont change compression, just valve timing and duration, things like that..... Link to comment Share on other sites More sharing options...
thealleyboy Posted August 3, 2004 Share Posted August 3, 2004 I don't think it will be possible to spin the motor fast enough to get an accurate compression reading. However, I do believe the heads can be pressure tested off the block, if necessary. If you have done a complete teardown, and replaced the cam with an aftermarket, your compression could very well change, but I wouldn't expect it to be significant. Unless other internals were replaced/modified, you will not benefit greatly from running a "hot" cam. good luck, John Link to comment Share on other sites More sharing options...
Guest taprackready Posted August 3, 2004 Share Posted August 3, 2004 IF you set the engine up like on a dyno meaning you can hook up a starter to it then ya sure you can do a compression test on it. Bill Link to comment Share on other sites More sharing options...
SubSandRail Posted August 3, 2004 Share Posted August 3, 2004 Most of the guys I go to the sand with use VW engines and transaxles, with the starter on the tranny just like a soob. They took a blown tranny and used a sawzall to remove the front of it with the starter. That way you could drill holes so you can bolt in on an engine stand. Link to comment Share on other sites More sharing options...
naru Posted August 3, 2004 Share Posted August 3, 2004 Is it possible to test the compression with the engine on the stand? Also, what have other people seen compression wise running the delta cams? Seems like it should be the same as stock, yeah? Sure you can test compression on a stand as long as you use the starter. If you have installed increased duration camshafts w/no other mods,you will have a lower gauge reading because of the decreased dynamic compression ratio. Significantly increased duration requires increasing the static compression ratio to maintain a reasonable dynamic ratio and compression gauge reading for good low RPM operation. Link to comment Share on other sites More sharing options...
WJM Posted August 3, 2004 Share Posted August 3, 2004 Steven (Pleiades) and I tried to compression test an engine on a stand...ironically it also featured delta cams. No luck really....couldnt get over 30 psi out of any cylinder.... :-\ Link to comment Share on other sites More sharing options...
naru Posted August 4, 2004 Share Posted August 4, 2004 Steven (Pleiades) and I tried to compression test an engine on a stand...ironically it also featured delta cams. No luck really....couldnt get over 30 psi out of any cylinder.... :-\ 30 psi? YIKES! How did it run? You guys should read this Pontaic based discussion of cam selection/installation and compression gauge readings: http://www.thepontiactransampage.com/engineblueprint.html Link to comment Share on other sites More sharing options...
gravelRX Posted August 4, 2004 Share Posted August 4, 2004 I checked the compression ratios on all four cylinders on my EA82T with the 260 grind cam. All were about 85lbs. Naru could you explain the terms in a more simplistic way? I'm not proud. Jay Link to comment Share on other sites More sharing options...
Subarutex Posted August 4, 2004 Author Share Posted August 4, 2004 Thanks for the reply WJM. I couldn't get any cylinder over 30 either. OH well, time to take it apart. Got the engine out in about an hour and 15 minutes. Link to comment Share on other sites More sharing options...
naru Posted August 4, 2004 Share Posted August 4, 2004 I checked the compression ratios on all four cylinders on my EA82T with the 260 grind cam. All were about 85lbs. Naru could you explain the terms in a more simplistic way? I'm not proud. Jay Sure, but they say it better: http://e30m3performance.com/myths/more_myths1/comp_ratio/comp_ratio2.htm http://www.kennedysdynotune.com/Dynamic%20Compression%20Tech.htm http://webpages.charter.net/darrell1/dynamiccr.htm Link to comment Share on other sites More sharing options...
gravelRX Posted August 4, 2004 Share Posted August 4, 2004 Did you have the valves reseated? I was pondering mine and the low compression. How about this. If the valves were resurfaced in the seat, couldn't the valves be taller because they are seated deeper into the head? Would that make then open earlier and close later resulting in the lower compression? When I checked my heads for problems the first go round, I took them off and remounted the cam towers with the cams and everything in place. I filled the intake and exhaust chambers independantly with alcohol and found no leaks until I turned the cams over duplicating normal operation. Because I didn't have a baseline, they looked good to me and I put it all back together. My RX is pretty quick over 2700, but anything before that would be considered wanting.(low compression?) Pulls hard up to redline and seems to still have more. factory boost. Jay Tex, how did you come to the conclusion you needed to pull the engine again? Idle issues? Link to comment Share on other sites More sharing options...
WJM Posted August 4, 2004 Share Posted August 4, 2004 the 30 psi was with the engien out and turning by hand....engine in and turning by starter yeilded 120+ on all four....but #4 was lower than the rest and misses on taht one at idle. Link to comment Share on other sites More sharing options...
Subarutex Posted August 4, 2004 Author Share Posted August 4, 2004 Compression tested the pass. bank and found cylinder 3 at 30psi, and 1 at 70psi. That, and it wouldn't start anymore. So i pulled it. New thread about to begin... Link to comment Share on other sites More sharing options...
thealleyboy Posted August 4, 2004 Share Posted August 4, 2004 Sorry, but I have to disagree about getting an accurate compression reading on the bench. In practice, you'll need the engine running at normal operating temp to assess its true ability to build and "hold" pressure. This is the true test of whether the motor is solid in terms of compression. Sure it's possible to pressurize a motor on a bench (or in the car for that matter) but this is not a very reliable method. The readings will not be very useful, unless you want to establish a "no pressure" condition. Even something like a weak battery can throw your results off in a major way. You can try it, but don't draw any firm conlusions from your readings. I will say that consistantly high readings would probably be a good sign, though low compression is not necessarily bad. good luck, John Link to comment Share on other sites More sharing options...
naru Posted August 4, 2004 Share Posted August 4, 2004 Sorry, but I have to disagree about getting an accurate compression reading on the bench. In practice, you'll need the engine running at normal operating temp to assess its true ability to build and "hold" pressure. This is the true test of whether the motor is solid in terms of compression. Sure it's possible to pressurize a motor on a bench (or in the car for that matter) but this is not a very reliable method. The readings will not be very useful, unless you want to establish a "no pressure" condition. Even something like a weak battery can throw your results off in a major way. You can try it, but don't draw any firm conlusions from your readings. I will say that consistantly high readings would probably be a good sign, though low compression is not necessarily bad. good luck, John In theory yes,in practice no.Have you ever compression tested a subaru engine hot and cold? VERY little difference. The question was whether different cams could affect compression gauge readings. The answer is yes. To see how much, a cold test should be compared to a cold one and hot to hot w/the cams as the only variable. "no pressure condition" Huh? Whazat? A weak battery will affect results hot or cold, in or out of the car.As long as you have consistent cranking rpm you`re OK.FSM suggests 250-350 rpm w/corresponding pressure variations. Link to comment Share on other sites More sharing options...
Qman Posted August 4, 2004 Share Posted August 4, 2004 High lift cams will to a degree affect compression. Not much mind bit they can. What effects compression is decking heads, changing pistons, even the type of rings used can effect it. What may have happened is a burnt valve. Increasing boost without increasing fuel pressure will result in a lean condition. To much pressure could also result in broken or burnt rings. I feel it still should have started with low compression in only one cylinder. Where are you going to take the parts for inspection? Make sure to tell them all the mods you have done so they can make understand the conditions and causes better. Link to comment Share on other sites More sharing options...
Subarutex Posted August 4, 2004 Author Share Posted August 4, 2004 Took the parts to Bow Wow and Action Machine today... see other thread for results. Link to comment Share on other sites More sharing options...
thealleyboy Posted August 4, 2004 Share Posted August 4, 2004 In theory yes,in practice no.Have you ever compression tested a subaru engine hot and cold? VERY little difference. Yep, owned quite a few of them, and done it many times. As I said, consistantly high readings would indicate an exceptionally strong motor. Inconsistant/low readings would suggest a motor with wear - maybe significant wear. In practice, temp does make a difference if you want accurate readings . If you don't believe me, check your FSM. Checking compression after warming your engine is standard practice. "no pressure condition" Huh? Whazat? How about a blown head gasket? Ever hear of one of those? Or perhaps a cracked head. The guage would read "0" PSI. This means no pressure on that cylinder. A "no pressure condition". Get it?? A weak battery will affect results hot or cold, in or out of the car.As long as you have consistent cranking rpm you`re OK.FSM suggests 250-350 rpm w/corresponding pressure variations. Yeah, exactly. Problem is {as explained above} cranking for a few seconds, even with a fully charged batt, is not equivalent to normal operating conditions. Not trying to start an argument here. If you feel comfortable with your method, good luck. I'm just sharing my own experiences, and recommending only what I would do on my own car. John Link to comment Share on other sites More sharing options...
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