Hello all.

I have a question that I hope someone could answer.

If I am doing a compression test on a fully built b18b engine with pistons that are at 9:6:1 ratio. What should the compression test numbers be?

 

I am going to answer my own question. I figured I would share it with the group in case others wanted to know the answer.

Found this info from the following website.

Evans Tuning's Tuning Checklist

per the evans tuning checklist:
8:1-8.5:1 compression: 150-170 psi per cylinder
8.5:1~9.5:1 compression: 170-210 psi per cylinder
9.5:1~11:1 compression: 210-275 psi per cylinder
11:1+ compression: 250+ per cylinder (highly depends on cams being used)

 

whats your elevation.

my pistion are pose to be 9:0.1 je guys. last two times ive checked it, gauage said 195 cold. hot engine was 210psi

 

looks like your numbers are perfect.

 

Easy:
Take atmospheric air pressure (14.7psi) and multiply it by your compression ratio (9.6): 14.7x9.6=141psi
A lenient statement yes, but You would think a engine should be either 100% or nothing when it comes to compression but they do have quite the range of readings, I'd say staying within 20psi of the 141psi is reasonable.

Basically the piston at BDC is in the midst if you will, of a crank and rod combination that potentially puts it at TDC... yielding a cylinder volume that is 9.6 times greater than it's BDC volume.

You could have even got away without knowing really, what matters is that there is compression, just use reason to gauge if your readings are reasonable, obviously theres no hurt in asking... one quick way is to look for equality among the four readings. Compression readings can only be low or on spot, you just don't see high readings (unless the head got milled down, wrong pistons installed etc.), so thats out of the question. Oh and make sure you pull out all the spark plugs! Causes for low compression are:
-Cracked cylinder head or cylinder wall
-Cracked pistons
-Worn piston rings
-Bent valves
-Burnt valve seats
-Damaged head gasket

Heres what I looked into trying to figure out the original question of how to determine compression readings haha, freakin math, I swear im a totally different person for at least an hour after trying to figure stuff like this out:
-Figure out your compression ratio which you said was 9.6:1
-Find the volume of a single cylinder (1.8liters/4=0.45liters)
-Find the density of air= 1.225 kg/m3 @ Sea Level (For every cubic meter, air weighs 1.225kg), this status directly applies to the air in our cylinder since it is a ratio or relationship you may apply this to different formulas to change units. (ex. Air density is also 0.001 kg/liter)
- BDC cylinder volume (0.45 liters)
-The weight of 0.45 liters of air: 0.001/0.45=0.002kg

 

That is quite the first post!. Thank you.

 

i think the evans numbers are a bit high for the upper end of the compression for each range but you get the point. for 9.6:1 i'd expect between 180-200. ive seen more than my fair share of stock vtec motors test 200-215 and they are in the low 10's.

my b20z tests 188-195psi and those are 9.6:1

 

Hmm. Just did a test.

I should also mention the block is sleeved. Has crower head package. (I did not do a leak down test though)

When I first took the pics I forgot to tell my friend to have WOT. So the numbers in the pics do not reflect WOT.

When we did WOT it added 20 psi to the end result. But I forgot to take pics of those results.

These numbers are also on a cold engine. Done dry not wet.

So do you guys think these numbers are good for closed throttle body and 9.6:1 compression?

 

By no means am I an expert but I can tell you that those number definitely aren't bad...

 

Damn sorry. More info I forgot to add.

It's bored .20 over and has 84mm je pistons and cosmetic head gasket.

 

Engine is supposed to be at operating temp and the throttle body has to be wide open or the test results are invalid. I'm also assuming the plugs were out of each cylinder as well?

You need to redo the test and do it properly to know what your numbers are actually supposed to be.

 

Plugs were out of each cylinder.

 

Well I wouldn't say invalid. But the numbers will be lower if the engine is cold.

 

yeah my f150 was all over the place when it was cold some cyls as low as 120 psi. when warm they were all 170-180. id say that you wont be in much trouble though the numbers seem to be off on the right foot

sleeved makes no difference. the cams will depending on overlap. if you are worried at all, do a leakdown test right after the compression check.

 

Well the whole point of the test is to find out the compression figures of the engine at operating temp. As others will tell you (and have told you) when the engine is cold the ring seal is hit or miss which can severely skew the numbers. At proper operating temp the piston has expanded in the bore and properly sealed the rings. Compression figures on a cold motor are basically worthless, granted they will still tell you if a cylinder is totally dead or severely low but other than that the results are garbage.

Also the other thing is that the engine has to have almost no resistance or pumping losses for the numbers to be correct. This means all plugs out of the engine and the throttle body is wide open. A closed throttle body increases the pumping losses and limits the amount of of air the cylinder ingests on each stroke, thus making the numbers lower.

So yet again if you want truly valid results that will generate useful data then redo the test with the motor at operating temps (both coolant and oil temps) and hold the throttle body wide open.

Doing it any other way results in numbers that are a crapshoot at best.

 

Redid the tests with warm/hot engine and WOT.

Numbers went up a bit. I DID NOT put oil in my cylinders.

This engine "should" have je pistons 9.6:1 but I did not take my head off to confirm.

Thoughts?

 

Still puts you in the range of what you had posted from evans tunning.

Also the same piston will have different compression from engine to engine. depending on other variables. Try the compression calculator with the specifications for your pistons to get a better idea
Honda B-Series Compression Calculator by ZealAutowerks

 

Compression test meh. As long as the numbers are really low and they're all close to one another your good. When you have variation between cylinder in big numbers it's a problem. Really should do a leak down test. That'll give you health of the engine. Compression number don't mean squat IMO