flyrod

I rebuilt an engine recently and was curious about compression numbers so I did some calculations. This is a TECH web site and I thought others might be interested, so here they are:

Stock US H22’s have 10:1, so to calculate the combustion chamber volume (v):

(bore area)(stroke) = (8.7/2)(8.7/2)(Pi)(9.07) = 539.18 cc per cylinder

(539.18 + v) / v = 10 v = 59.91cc

This is the total combustion chamber volume, with head and piston cc’s combined. Shaving the cylinder head or decking the block basically reduces this volume. So suppose you shave the head by 0.030”, or 0.0762cm. The volume is reduced by roughly this amount:

(bore area)(stroke) = (8.7/2)(8.7/2)(Pi)(0.0762) = 4.53 cc

Note, this would be different for a head with quench areas flush with the surface of the head, like on a B18C1 head. A H22 head has recessed quench areas, so milling the head doesn’t really remove metal from the combustion chamber. So we subtract 4.53 to get the new combustion chamber volume and calculate our new compression ratio:

(539.18 + (59.91 – 4.53)) / (59.91 – 4.53) = 10.74

The same equations can be used for JDM engines because you know the starting compression ratio. A thinner head gasket is equivalent to shaving the head. Here is another example using Type S pistons (11:1 before shaving head) and shaving the head 0.015”:

(539.18 + v) / v = 11 v = 53.92cc

Shaving 0.015” translates to 2.26cc, or a 51.66cc combustion chamber
(539.18 + (51.66)) / 51.66 = 11.44

Shaving 0.030” translates to 4.53cc, or a 49.39cc combustion chamber
(539.18 + (49.39)) / 49.39 = 11.92

With custom bores, pistons, and cylinder heads, you have to “CC” the heads to calculate the CR. You might also be able to estimate CR from compression testing. I had the opportunity to borrow a friend’s compression tester. Stock US H22’s tested at 150psi, and my rebuilt H22 with ~11.4:1 tested at 170psi. This was at ~6000ft altitude, so testing at sea level would give different results. Cam profiles can affect compression tests. Static CR is a function of displacement and combustion volume, while compression testing is more closely related to the dynamic compression ratio. For example, compression tests on an engine with a high overlap cam would give lower numbers than for a low-overlap cam.

RHD_BB4T

Yes, thank you so much for the enlightening info!

Just the other day, I was having a discussion with a friend of mine about the differences in static and dynamic CR and whether or not compression test results and CR values would or would not change depending on different cam profiles. He didn't understand where I was going with the whole thing; this will help him out a bit, I hope!!!

Thanks for the info!

Dan P.

flyrod

Just wondering, does anyone have compression test numbers for JDM engines, or at sea level? Do the engine importers check compression?

civicsitek GanGsTa

when I had 8.8:1 wiseco pistons at 1800 ft I tested at 175 across the board, I think there will be more difference in between compression testers than anything.

satan_srv

you left out head gasket and any deck clearance. I don't think there's any deck clearance but you defintely left out the head gasket.

93ludeSi

nice info,

That is static compression right??

Dynamic would have to include cam duration?!?

satan_srv

most other sources put the H22a cc at 53.3-.8 cc for reference.

rjr162

The programs I wrote in VB calculated out the static compression based off of various info, and i also wrote one for Dynamic (based on RPM, Air flow, etc)... but the thread got locked because ppl freaked and were like "I bet it's a virus!!!" Sad, isn't it... even if it was, oooo, you'd have to re-install. Yet the blaintently incorrect threads that could seriously damage someone's engine costing $1500+ in repairs don't get locked.

flyrod

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by satan_srv &raquo;</TD></TR><TR><TD CLASS="quote">you left out head gasket and any deck clearance. </TD></TR></TABLE>

Those numbers were just for total combustion volume (everything combined). If you know the Head CC (~53.8cc) and gasket thickness (~0.027"), then you can break down the combustion volume into components and do calculations with different parts. The generic formula for CR:

(Original Volume)/(Combustion Volume) = Compression Ratio

Broken into parts it becomes:

(Vc - Vp + Vg + Vy)/(Vc - Vp + Vg) = CR

Where:
Vc = volume of the combustion chamber (just the head)
Vp = volume of the piston (relative to the top of the deck)
Vg = volume of the gasket
Vy = volume of the cylinder
CR = compression ratio

Vc and Vp are usually measured, looked up, or calculated based on other knowns
Vg is just the volume of the hole cut in the gasket, so it depends on the cylinder bore and thickness of the gasket. In cc's:

Vg = Pi()*(thickness inches)*(.00254)*(bore cm / 2) ^2

Vy is the same formula, except the thickness is the stroke:

Vy = Pi()*(stroke cm)*(bore cm / 2) ^2


Now you can plug all this into excel or a VB script etc. and mix and match parts:
If the H22 combustion chamber is 53.8cc, you can calculate the piston cc's:

US 10:1 pistons: -2.0cc
JDM 10.6:1 pistons: 1.7cc
Type S 11:1 pistons: 3.9cc

H22 head, H23 block, type S pistons CR = 11.5
H22 head milled .020, H23 block, type S pistons CR = 12.1
H22 head, H22 pistons, H23 block CR = 10.4
H22 head, 90mm bore H22 block with 2cc pistons = 11.3
etc.

Flat bottom valves change Vc. Also as mentioned above, milling or decking can be treated as changing the thickness of the headgasket. So this is all there is to it guys. The hard part is finding accurate cc measurements of different parts.

93H22ACX

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by flyrod &raquo;</TD></TR><TR><TD CLASS="quote">

Now you can plug all this into excel or a VB script etc. and mix and match parts:
If the H22 combustion chamber is 53.8cc, you can calculate the piston cc's:

US 10:1 pistons: -2.0cc
JDM 10.6:1 pistons: 1.7cc
Type S 11:1 pistons: 3.9cc

H22 head, H23 block, type S pistons CR = 11.5
H22 head milled .020, H23 block, type S pistons CR = 12.1
H22 head, H22 pistons, H23 block CR = 10.4
H22 head, 90mm bore H22 block with 2cc pistons = 11.3

</TD></TR></TABLE>

so that means that the US H22 pistons are "out of the hole" when you have a
-2.0cc? What are you using for piston to deck height and the pistons dome cc?

prelittlelude

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by PirateMcFred &raquo;</TD></TR><TR><TD CLASS="quote">

No -2cc is a Negative volume i.e. the valve reliefs on the OEM 10:1CR have a larger volume than the piston dome resulting in a negitive volume (a dished piston). Look at it this way: if you had a flat-topped piston flush with the engine deck, effectively the cumbustion chamber would be 55.8cc's instead of 53.8cc. For 11:1CR it would be 49.9cc's etc. Positive piston volume is why you see all motor engines with the pistons sticking out of the engine deck.

Flyrod's equations do not account for piston-to-deck height, that volume is included in the piston cc. For OEM H22 applications the Piston to deck height is essentially 0 unlike many B series OEM apps. This of course will be different for aftermarket pistons which usually give you the CR and piston cc's anyway. you've got the equation so just plug the numbers in and figure it out.

the Pirate</TD></TR></TABLE>

also the head gasket takes up some space so the head is lifted alittle off of the block, plus there is alittle pistont o deck height difference like pirate said, so the usdm piston is not actually -2cc

93H22ACX

So you're saying that the piston to deck height for the H22 is stock 0.000 meaning piston and block deck is flushed?

so what kind of compression will this be:

Dome=6cc
Removed middle layer of head gasket=0.01933333(Per C-Speed racing)
AND the piston to deck height is 0.000 like the above posts.

This means that the compression ratio would be 11.63?

Also what if I did it this way too...

Dome=6cc
Removed middle layer of head gasket=0.01933333(Per C-Speed racing)
AND the piston to deck height is 0.000 like the above posts.

MILLED 0.020

This means the compression ratio would be 12.3?

Is this correct?

93H22ACX

thanks! let me know if you did measure it.

dfoxengr

what is the thickness of a stock h22 OEM headgasket?

4norzn

pardon my ignorance for bumping this but i have a few questions about the calculations.

"Stock US H22’s have 10:1, so to calculate the combustion chamber volume (v):

(bore area)(stroke) = (8.7/2)(8.7/2)(Pi)(9.07) = 539.18 cc per cylinder

(539.18 + v) / v = 10 v = 59.91cc"

*i understand that the bore area , 87mm (8.7cm) is multiplied by itself (8.7cm) multiplied by pi ( 3.14159265) multiplied by 9.07cm (97mm) stroke.

what is (v)? and is it multiplied or squared to (v)?
------------------------------------------------------------
then this formula:

"Vg = Pi()*(thickness inches)*(.00254)*(bore cm / 2) ^2

Vy = Pi()*(stroke cm)*(bore cm / 2) ^2"

*is pi divided or multiplied? to what? and is "bore cm" divided or squared? and what does ^2 mean?

4norzn

bump?

4norzn

this... yes.

i have a target cr and i dont know what exactly the dome cc i need to get there.
so i just need to play with different spec. dome cc to get to my taget stat. cr.

thanks you again pirate