Got a fresh motor with 45 miles on it, unfortunately my head has been milled 3 times and the block has no deck left to mill anymore... on top of that my pistons are .016 out of the deck. My builder told me to get a stock size head gasket but with the above variables and .5mm oversize valves I decided to run a .065 cometic instead of the stock size to be safe. When I asked him to calculate my compression ratio he wouldnt do it so I just said screw him and took my motor back.

Can someone give me a ballpark guesstimate of what my Comp Ratio is based on my compression test??? Results are:

180 - 176 - 175 - 174 I think these are pretty good numbers for a fresh low compression motor but who knows, I was shooting for a 9 to 1, but will be satisfied with a higer CR due to the fact that I am going to be spooling a Stg5 60-1 with a .83 AR on an 81.5mm.

81.5mm pistons
head/block has been milled 3 times making everything closer together if I am not mistaken.
Combustion chambers have been welded and grinded down when my old pistons disentagrated and chewed up the CC's so no really significant change there.

The car feels quite strong when driving it non-turbo, making me believe my comp ratio is not below 9 to 1 like I have been told it probably is


Modified by 96 GSR-T at 1:02 PM 5/13/2006

 

I am not a genious at this by any means but I don't think you can calculate compression ratio by compression test results. They are two different things.

However, my B16 has 9:1 pistons and Eagle rods and the head has been cut .010 to true it and my compression test results are pretty much 205 accross the board with an OEM head gasket.

 

No you are around 9.5 to 9.8:1 compression..

Great compression numbers!

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Jay_Sensing &raquo;</TD></TR><TR><TD CLASS="quote">I am not a genious at this by any means but I don't think you can calculate compression ratio by compression test results. They are two different things.
</TD></TR></TABLE>

sure you can, but cam profile/etc will have an effect.. you can guess a good range, however..

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Jay_Sensing &raquo;</TD></TR><TR><TD CLASS="quote">I am not a genious at this by any means but I don't think you can calculate compression ratio by compression test results. They are two different things.

However, my B16 has 9:1 pistons and Eagle rods and the head has been cut .010 to true it and my compression test results are pretty much 205 accross the board with an OEM head gasket.</TD></TR></TABLE>

I am aware of that, but I know you can ballpark a motor by its compression test, the higher the comression ratio, obviously the higher the test will show..... mind that these numbers are on a fresh motor. Running GSR Cams if that helps. Also with that 205 across the board, how many miles did it have on it when you tested it?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by GaRn &raquo;</TD></TR><TR><TD CLASS="quote">No you are around 9.5 to 9.8:1 compression..

Great compression numbers!

sure you can, but cam profile/etc will have an effect.. you can guess a good range, however..</TD></TR></TABLE>

How could he be 9.5-9.8 when a 9.0 GSr motor put out 205 like the previous poster stated? My LS with 9.5 compression get 205 across the board too

 

I think once the rings seat properly, I will be in the 190-195 range...... hopefully

That previous car was a B16, not a GSR..... but irrelevant

 

The built motor had about 3k on it when I did that compression test.

Can anyone enlighten me to the formula used to convert static to dynamic compression then? There are a lot of other factors involved that just the pistons so I would like to know this one.

 

Bump for more info

 

#'s sound okay to me !

 

My stock b18a (9.2:1) averaged 175-180 psi

My ls/vtec (around 9.8-10:1) is around 225 psi

just for some reference.

 

Using http://www.C-SpeedRacing.com 's method of calculating the CR, It told me I am at....

8.57 : 1 using a .065" Gasket w/ pistons .016 out of bore and the dome of the pistons at -12. I would have to say that if that is a true figure, my rings are sealing pretty damn good, I just cant believe that my car has such a low CR when it moves like it does and with such a low amount of miles that the rings seal that good.

One thing that I noticed this method does not calculate ( or does it not make a difference? ) is the amount of material removed from the head during surfacing.... I does factor in the amount removed from the block, but not the head. If my head has been surfaced several times wouldnt that brong my CR up a good deal? I re-did the figures taking away .010 for every time the head was surfaced and that brought me up to a 9.1 : 1 Comp Ratio which seems more true to the compression test


Modified by 96 GSR-T at 7:27 PM 5/13/2006

 

Bump for my last post....make any sense?

 

Anybody else's compression test and CR's??

 

i have an 84 mm gsr, as well as a gsr head that has been milled quite a few times ( had to change the timing change out with cam gears). I used a stock thinkness headgasket and 9:1 pistons which put my compression test 172, 180, 175, 182. Im not sure how much opening the bore up to 84mm changes the test numbers tho..?

 

It possibly could, go to that website I posted and put your figures in....

I just took the amount of Mtl removed from the head and took it out of the headgasket thickness.

 

Up for more Comp. Test results and CR's

 

I have a stock D16Z6 with 172,000 miles on it. I got 175 across the board.
That's not too shabby concidering the miles on it.

 

Stock B16A2 with 65K and 15K of hard boost... 225-230psi all 4 cylinders.

 

I am not up on civic motors, is that the SI motor or the JDM R?