compression ?

FS5 · 30-04-2006, 04:52 PM

we did a compression test on our i6 the other week and it had 180 psi in all the pots ,bar number 1 it had 175 .the test was done while the engine was cold and the head has had 12 thou shaved off.
dose anyone know how to work out the comp ratio going off the psi numbers ,also what sort of compression should a healthy i6 make ?
thanks,jake

xr8ute · 30-04-2006, 05:02 PM

Its not really possible to calculate the comp ratio from cylinder pressure.

e.g. two motors might have the same static compression ratio, but one has a bigger cam with more overlap than the other. The cylinder pressure at cranking speed will be higher on the motor with the smaller cam...

FS5 · 30-04-2006, 05:09 PM

thanks for the info mate,would the cam size alter the cold cranking psi?

xr8ute · 30-04-2006, 06:12 PM

Yep, more overlap would mean more cylinder pressure escaping through the exhaust.

FS5 · 30-04-2006, 06:19 PM

thanks mate

jonbays · 30-04-2006, 06:52 PM

Cold cranking psi doesn't really say much i am afraid this side of a total disaster

FS5 · 30-04-2006, 09:49 PM

jonbays the part about ,this side of total disaster has got me worried .it deosn't have any nasty bottomn end rattles or piston bearings ticking.
well i'l give you the full run down on why i ask.as most know the old girl made 159rwk with the jmm streetfighter cam and cai and ran a personal best of 14.59 @ 94.9.
i had an au head ported and it flowed 387 hp @ 600 thou lift and i think from memory 368 @ 500 thou ,exhaust flowed 75% of intake .
after fitting the head and gettinjg tuned it made 156 rwk . it ran a best time of 15.1 @91.8 in the middle of summer,wich wasn't to bad considering what times other cars were running.
then i fitted a gnd cam and it made 158 rwk and ran a best of 15.1 @92.3 .i played around advancing the cam and got it down to a 14.9 @ 92.8 ,although the air was a lot cooler and had a lower RA.
some how i had lost a 1/2 second and a couple of kw's and 2 mph . so i thought i'd get a second dyno done and went up to RDP and got em to do a dyno run . it made a hole 130 rwk ,this is with the cam advanced 1and a 1/2 degree's.
so now i am in the process of going through and ticking each thing off the list as to whats going on .
i'm going to wilowbank for one final crack to see how it stacks up .
next on the list is getting the cat checked
air fuel ratios ,and seeing if the unichip has crapped it .
one other thing i have gone from running a rpm hd clutch back to a stock au one .
it's a long list and it all cost money.
hopefuly it'l work outin the end

xr8ute · 30-04-2006, 10:01 PM

You are really looking for consistency between each cylinder when doing a pressure test. Your motor sounds like its pretty healthy with all cylinders between 180 and 175psi.

The last time i did a pressure test on my ute, the cylinders where between 120 and 135psi (not including the one that was 0psi :P )

Going backwards is frustrating, I hope your able to find the issue.

**Stav** · 30-04-2006, 11:40 PM

the car must be hot when you do a compression test mate.Ifthe same happens then,squirt some oil into the spark plug hole.If it gives a rise in compression its your rings.

jonbays · 01-05-2006, 12:10 PM

What i meant was a cold crank test doesnt say much about the engine unless its like 50psi different.

You want it hot normal running and then crank for three compression strokes on the cyl under test and watch the 1-2-3 results. The psi sounds good and high I doubt you have a problem there

SIKSIX · 01-05-2006, 05:33 PM

Quote:

Originally Posted by FS5

jonbays the part about ,this side of total disaster has got me worried .it deosn't have any nasty bottomn end rattles or piston bearings ticking.
well i'l give you the full run down on why i ask.as most know the old girl made 159rwk with the jmm streetfighter cam and cai and ran a personal best of 14.59 @ 94.9.
i had an au head ported and it flowed 387 hp @ 600 thou lift and i think from memory 368 @ 500 thou ,exhaust flowed 75% of intake .
after fitting the head and gettinjg tuned it made 156 rwk . it ran a best time of 15.1 @91.8 in the middle of summer,wich wasn't to bad considering what times other cars were running.
then i fitted a gnd cam and it made 158 rwk and ran a best of 15.1 @92.3 .i played around advancing the cam and got it down to a 14.9 @ 92.8 ,although the air was a lot cooler and had a lower RA.
some how i had lost a 1/2 second and a couple of kw's and 2 mph . so i thought i'd get a second dyno done and went up to RDP and got em to do a dyno run . it made a hole 130 rwk ,this is with the cam advanced 1and a 1/2 degree's.
so now i am in the process of going through and ticking each thing off the list as to whats going on .
i'm going to wilowbank for one final crack to see how it stacks up .
next on the list is getting the cat checked
air fuel ratios ,and seeing if the unichip has crapped it .
one other thing i have gone from running a rpm hd clutch back to a stock au one .
it's a long list and it all cost money.
hopefuly it'l work outin the end

could it be the head over flowed???

xr8ute · 01-05-2006, 05:49 PM

Quote:

Originally Posted by SIKSIX

could it be the head over flowed???

overflowed?

XR6-VCT-2000 · 01-05-2006, 08:40 PM

hmmm , yeah bugger having to hunt backwards trying to find ya problem , that was a fair drop to 130 rkw ! , i very strongly doubt imo if going from a heavy duty clutch back to a standard clutch again would loose you that much rkw. Sounds like a restriction of some sort somewhere.

maybes..-->
.buggered muffler or something blocked on the exhaust end ??..
.bad spark , maybe a coil problem ??..
.If you have been messin with the timing constantlly you might have confused the ecu , have u tried reseting it ??..
.fuel/air ratio - check map sensors ( free them of oil if you run oiled air filters ) ??.

gee the list goes on , could be any of many a things unfortunatelly. Maybe go back to the normal timing and reset the ecu and see if you can achive the same results you was getting before without the advanced timing , if you can't then at least you know it has nothing to do with the advanced timing or the ecu learning the new timing, it's somewhere to start thats free ! . good luck

jonbays · 01-05-2006, 09:44 PM

Quote:

Originally Posted by FS5

jonbays the part about ,this side of total disaster has got me worried .it deosn't have any nasty bottomn end rattles or piston bearings ticking.
well i'l give you the full run down on why i ask.as most know the old girl made 159rwk with the jmm streetfighter cam and cai and ran a personal best of 14.59 @ 94.9.
i had an au head ported and it flowed 387 hp @ 600 thou lift and i think from memory 368 @ 500 thou ,exhaust flowed 75% of intake .
after fitting the head and gettinjg tuned it made 156 rwk . it ran a best time of 15.1 @91.8 in the middle of summer,wich wasn't to bad considering what times other cars were running.
then i fitted a gnd cam and it made 158 rwk and ran a best of 15.1 @92.3 .i played around advancing the cam and got it down to a 14.9 @ 92.8 ,although the air was a lot cooler and had a lower RA.
some how i had lost a 1/2 second and a couple of kw's and 2 mph . so i thought i'd get a second dyno done and went up to RDP and got em to do a dyno run . it made a hole 130 rwk ,this is with the cam advanced 1and a 1/2 degree's.
so now i am in the process of going through and ticking each thing off the list as to whats going on .
i'm going to wilowbank for one final crack to see how it stacks up .
next on the list is getting the cat checked
air fuel ratios ,and seeing if the unichip has crapped it .
one other thing i have gone from running a rpm hd clutch back to a stock au one .
it's a long list and it all cost money.
hopefuly it'l work outin the end

Honest answer here is the GND cam doesnt have the power of a dev4 with or without porting the head.

159rwkw and 14.6 @94.9 are all good honest numbers that line up well with each other.

After the GND cam 158rwkw doesnt sound likely as this would give better than the 15.1@92.8 which is close to what useless and I are almost getting with heavier autos and 140rwkw honest kw. This sounds like you've only got 130rwkw odd to me in a manual to run 14.9-15.1 like a good XR6 basicly.

I am not sure how good the headwork is but the cam loooks to me like your problem.

The head well flow at .6 lift isn't very useful as you won't get that much without hitting the pistons. Flow of 368 at 0.5 inch is good but again claims can be a bit wooly sometimes ie made up.

With heads the big port EF XR6 are the best stock and the small port AUII not very good stock and need lots of port straightening and openening up a bit.
early AU and late EL heads seem good stock too.

know too there is a big difference between most peoples PB on say a cold still winters night say 15.0 and a warm humid day say 15.4.

I will bet your biggest problem is the cam. Get a 1521a or a dev5 regrind is my tip. Both will need the unichip retuned.

FS5 · 02-05-2006, 08:46 PM

Quote:

Originally Posted by SIKSIX

could it be the head over flowed???

thats what i was thinking to,i know the i6 dosen't like it when the ehaust port is opened up to much.
one thing i notice is that when i'm up revving 5000 to 5500 rpm its blows a bit of smoke sort of mustard colour.it dosen't do it all the time only when i havent give it some for a while.
i am getting the original head flow tested by the same shop,hopefully i will have the results thusday arvo.then i will be able to see what difference is between the two.

jonbays · 02-05-2006, 11:00 PM

Can't see the head as the problem myself. If it was flow bench tested honestly it would be hard to f it up that bad that you would lose 20rwkw odd. If the ex ports were opened up too much you would have a hole into the water jacket for starters and the loss of velocity at low engine rpm would be made up by the gain in flow at high rpm. Worst case it would be peakier than a well done head but not lose that much in peak power. could lose a bit in average power but with a manual you could drive around that. Look for something else that has changed when the head was changed.