Piston Choices and Compression PSI

[dvbrien]

So I was just wondering... anyone know what type of compression numbers you could probably expect in a cylinder based on what ratio pistons you choose.

Senario... Currently I have a Spankin new "Built" motor with Ross 8.5 : 1 @ .20 bore. In 3 & 4 I'm seeing a respectable 150 Psi on my Guage. #2 is showing a Dismal 132 (testing several times "Warm") and #1 is about 145ish... So based on the Pistons (with the Mitsu 4 layer SlowBoy HeadGasket) Where should I assume my psi range should be. Also if timing were off (which it doesn't appear to be) and a valve is partially open not letting enough compression build... Wouldn't adjusting the cam (to "fix timimg") cause the compression to drop in a different Cylinder? Btw Idle Vac is -5 ?? Just some additional info 272 /264 setup... Idles like stock... @ 750-800 RPM / Me... Dupped!

THX

 

[Burnett03]

sounds like your timing is off.. -5 vaccum is really low. also you should have higher compression than that on a 8:5:1 motor. Also you dont want the numbers to be that far off from each other, they should all be about the same.. check your timing

 

[underradar92]

Cam timing would affect all cylinders evenly. You are losing compression somewhere else on the weaker cylinders, valve seat, rings, like that.

 

[paper_clip_boy]

Check your Base timing with a timing light. Read the VFAQ on how to do it.

 

[T9S1i]

yeah something is off. How many miles on the "fresh" motor? Rings might not have fully seated yet, but id be more skeptical of timing with the low vacuum.

I am running weisco 8.3's with a MLS HG and a head that has been milled a few times (3 maybe). At like 250 miles on my rebuild i had 180 Compression across the board. Im at like 4700 miles now and i havent checked to see if its gone up but id think 185 or 190 at the very most, IF it has gone up at all. Id still be happy with 180 though. I also get about 16-18in of vacuum with 264/272 combo.

 

[dvbrien]

Timing base is good, knew how from my 67 Camaro Build... Miles on the new motor, I'd figure run time better maybe 2 hours at most. Actual load miles 6 total. I have the marks lined up exactly. Hard to say though AEM cam gears in silver aren't the easiest to make out. I'll post a pic later to see what yall think. Thanks for the responses.

 

[steel_3d]

Hmm there was a thread on here a few months ago where a guy had 0 compression in only one cylinder due to shitty cams.

I have FFWD DKS2 272's, 2.3 with 8.8:1 wiseco pistons.

With the adjustable cam gears at 0, I get low 170's compression and a shitty idle, vacuum about 15 inhg.

With the intake retarded 8 degrees an dthe exhaust advanced 8 degrees I get mid 150's-low 160's and a smooth idle, vacuum about 18-19inhg. Maybe your grind is even beyond this.

I just did another compression check today at +-6 degrees and I got 162 156 162 162. This was pretty much the same as the previous setting of -6 on the intake and 0 on the exhaust. I found that strange, but it seems that adjusting the intake cam has a bigger effect on compression.

Honestly I don't even know if my findings are normal The numbers seem a bit low for 8.8:1 compression, but hey...

 

[T9S1i]

id pull the plugs out to check if the piston tops are still wet. Your compression #'s will come up as the rings seat. They dont always seat right away. If it were me id not worry about compression #'s until about 200 miles on the new motor. Just remember to vary rpms and do a lot of engine breaking and decelleration while in gear.

Im also running dks cams. The 264/272, i must have gotten a good grind because they do what they are supposed to when installed straight up! 16-18in vacuum and pull like a mofo on my 16g!!!

 

[dvbrien]

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here is a pic of the Gears... what do yall think about the angle...

 

[Maglin]

Looks good on the timing. Also you don't degree your cams for best compression. You defeat the purpose of having a higher duration cam by doing so. The reason is that intake will be open a tad bit into the compression stroke for more high RPM air flow (which effects idle a lot). And the same with the exhaust cam. but it has more to do with overlap from the exhaust and intake valves which again help with high end flow but actually hurt low end flow because of reversion.

I personally install all cams straight up and degree them so that they are in time with the crank. Milling and decking effects this relation ship. Then only change it on a dyno. If their is enough time to even bother with fudging with cams.

 

[steel_3d]

Maglin, I find my own results strange, because I also thought that more overlap (and shittier idle) should come with less compression, but I'm seeing the opposite. Unless I'm trippin, when I'm advancing the exhaust and retarding the intake I'm reducing overlap. When I do that my idle definitely gets smoother, *almost* passed emissions even, but my compression goes lower instead of higher. Can anyone shed any light on this?

 

[Maglin]

It's probably from your intake cam staying open well into the compression stroke. Your idle should be sounding different but you are eliminating the valve overlap and loosing power from the intake being open to long and probably to far into the compression stroke. And only the intake cam will effect your compression numbers. I would put your motor on TDC and then sent your cams so they are straight up. Hello lopy idle. But you will get a lot more power and definitly more top end.

If you want to make it idle better and are willing to sacrafice top end power for it advance the exhaust in 2* incraments until you have the idle you are happy with. But I wouldn't do anything with the intake cam. Just leave it straight up.

 

[steel_3d]

Hmm. I'll give that a try, I haven't tried that combination yet! Thanks.

If I was dvbrien, I'd try advancing the intake cam in 2-4 degree increments, and check compression, idle quality and idle vacuum each time to see if you're moving in the right direction. I keep the plugs out and turn the crank over by hand to check for valve contact after every adjustment. That definitely doesn't guarantee safety, but I figure as long as the idle is gettig better I'm probably out of the danger zone. Not much else you can do without a proper degreeing setup.

You definitely need to fix your low idle vacuum, and the compression numbers will probably get better along with it. It's really weird that you have a smooth idle with 5inhg vacuum The compression numbers might even out after a proper break-in. Unless they're caused by a shitty cam grind. You've done at least some 80% throttle pulls already, right? What break-in procedure did you use?

[edit] You showed us that the cam gears are aligned, but is the crank aligned as well?

 

[dvbrien]

Ya see, I thought I had all this figured out... as far as setting the motor at TDC and setting the cams "straight up" I'm not sure what you mean... Other than that you said is my crank alligned as well. I assume you mean when I did the timing in which case yes it was. The only other thing I can figure is if you mean whether or not I am sitting a 5* TDC on the lower cover. The vacume I have found may be the result of a clogged P valve on the throttle body. I have change my guage tap location but haven't started it yet. I am currently checking all vacume leaks and everything before the TB was good but, have found a large leaky gasket on the elbow side of the TB.

LMK

 

[steel_3d]

I believe the timing mark on the crank pulley should be lined up with the 0 mark on the cover at TDC.

 

[dsm-onster]

id pull the plugs out to check if the piston tops are still wet. Your compression #'s will come up as the rings seat. They dont always seat right away. If it were me id not worry about compression #'s until about 200 miles on the new motor. Just remember to vary rpms and do a lot of engine breaking and decelleration while in gear.

Im also running dks cams. The 264/272, i must have gotten a good grind because they do what they are supposed to when installed straight up! 16-18in vacuum and pull like a mofo on my 16g!!!

I have to agree. wait for the rings to seat. So what? one has started to seat earlier than the others.

Asking questions about what compression tester nubers are correct should be rephrased to: what perssure does your guage give you when doing a compression check? then aske yourself does my compression tester have the same length line? same width? same volume in the "register chamber" at the tester head? An uneven result is what's important AFTER a thorough break in.

Best results are when you do a test w/ YOU personal tester before the rebuild then use the same tester AFTER the rebuild AND break in.
---------------------------------------------------

I would put your motor on TDC and then sent your cams so they are straight up. Hello lopy idle. But you will get a lot more power and definitly more top end.

I'm not so sure about that. We're talking about 272s right? . . . The common consensus based on research is that the 272s idle like stock. . . My FP2Xs w/ a little more duration idle like stock. They sound different next to a stock running engine, BUT, they can idle down to 700 rpms just fine w/ no missing or inconsistancy at all.

 

[Maglin]

loopy idle is opinionated. But next to a stock car it is loopy. It isn't a V-8 with like 114* of overlap loopy. I biggest thing that effects idle that I've seen are large injectors. Pretty much 1000cc+ injectors will effect idle the most out of just about everything out their in terms of quality and RPM.

And as far as length of hose for compression gauge goes. It really doesn't make any difference. Maybe 2-3 PSI but yes the number isn't totally important, but it is. You definitly have an issue with your car with those 2 low cylinders. I would put a few TSb's of oil in the spark plug holes and then do another compression test. If they all rise to where you thing they should be or at least with-in 10% of each other then you either have rings not seated in yet or just bad rings or possibly a ring set that is gapped way differnet then the rest giving you more blow by. Is their a lot of air coming from your vent's on the vealve covers with the PCV unhooked? Their shouldn't be really any at idle. Hell my stock bottom end with over 135K on it barely has any blow by.

 

[groundPork]

The numbers do sound low, and it is definitely weird to have that big of a spread in pressure numbers. Squirt some oil into the cylinders and test after another dry test, and see if they seal up... that would mean rings are not sealing. remember to avoid free idling as much as possible. it doesn't help your break in.

 

[dvbrien]

Vac prob. blocked P port on Throttle body... Compression low (?) rings not seated yet... only 6 miles.

Thx for the responses, thought I'd state the solutions since so few do on here and no-one ever finds out the resolution to many issues they have too...