Measuring Head Volume / CC's / Compression Ratio

[Gordo2]

Tonight my goal was to measure my head volume so that I could accurately calculate my engine's compression ratio (CR).


Background

I'm rebuilding a 3.2L engine with intent to increase daily driving / street performance.

I've had some machine shop work done to the heads, which included re-surfacing the head-to-cylinder mating surfaces (which appears most people refer to as "fly cut" heads).

From what I understand, re-surfacing the head-to-cylinder surfaces can affect your compression ratio to some extent (more head material removed = higher CR). As such, I thought I should measure the head volume to make sure I'm achieving the CR that I'm shooting for.

My engine rebuild includes new JE 9.5:1 compression ratio pistons. For performance purposes, I would like my CR to come our pretty close to 9.5:1 as I understand this is the maximum recommended CR for readily available high octane gasoline (92-93 octane rating in my area), on a single plug configuration.

I plan to make minor CR adjustments with cylinder spacers/gaskets (0.25mm or 0.50mm) if my deck height allows (shooting for 1.0 mm deck height).


Tools & Preparing to Measure the Cylinder Head Volume

Lets start with the tools....

I bought a cheap, plastic burette from Amazon for ~ $35 shipped (ACRYLIC BURET, PTFE STOPCOCK, 100 ML, 1/PK: Amazon.com: Industrial & Scientific). This is the primary tool needed to measure head volume.

I also bought a 6"x6" piece of 1/2" thick acrylic plexiglass (from E-bay ~ $10 shipped) to cover the heads and allow for accurate volume measurements. I cut the plexiglass piece to 112mm circular diameter and drilled a countersunk hole in it (to allow me to fill the head with fluid).

I would have bought a 1/4" thick piece of plexiglass, but I couldn't find any local places that sold small sections of plexiglass or had scraps.

All good so far - here's the stuff I had bought / prepared to take my head volume measurements:

(Syringe was used to top off the burette to a precise level)


For fluid, I used isopropyl alcohol colored with green food coloring (not sure why many web threads recommend alcohol over other fluid options - I had some readily available so I went with it).


Taking the Cylinder Head Volume Measurements

I greased the head-to-cylinder surface with white lithium grease to seal the plexiglass disk against the head.

Once the plexiglass was sealed, I attempted to fill the head with green dyed alcohol.

Attempted is the key word here- Initially, the fill hole would overflow and wouldn't allow any more fluid to enter the head chamber even though there was a huge air bubble under the plexiglass. I think its essential to drill at least 2 breather holes, otherwise you end up with a bubble that you chase and try to fill like some kind of tortuous circus game .

After a few modifications to the plexiglass (enlarging the fill hole and drilling/enlarging 2 breather holes to ~ 1/8"), I was able to get what seemed like an accurate measurement:



My volume for this head turned out to be a surprising 92.4ml/cc - quite a bit larger than the 90ml or smaller volume that I expected (shaving the heads should reduce the total head volume...).

According to this calculator: Engine Compression Ratio (CR) Calculator, this resulted in a calculated CR of 9.2:1 (assuming I can get 1.0mm deck height, and accuracy of JE's piston advertised dome volume of 35.1cc's)

I'm going to measure & remeasure each head tomorrow to see what I come up with.

More to follow.

Gordo

[haycait911]

sub'd.

[brighton911]

Hi Don. Thinking back a long way, the issue of an irregularly shaped piston can make it very difficult to get the true piston volume and thus skew your result. In my case, to get the true volume, I assembled the head to the cylinder, had the piston at TDC and positioned the engine so the spark plug hole was level. Rather than alcohol in the buret, I believe I used a very light oil as alcohol could get past the rings. I know it's a bit awkward but you will end up with a volume that is a true value. Another way to do it is to make a mold of the piston at TDC position using plasticine or similar then pull it off and fill the mold with buret fluid to obtain the volume. I haven't tried that method. My bet is your actual CR is higher.

[jhg41977]

My guess on the alcohol is; it will not entrap air like oil, it will flow easily into all voids and it will not leave any residue behind.

[Gordo2]

Originally Posted by brighton911

... irregularly shaped piston can make it very difficult to get the true piston volume and thus skew your result...

Thanks,

I'm using the piston dome volume right off of JE's specs - I could measure, but think it would come out pretty close...

I re-measured the same combustion chamber this AM (coffee as a helper vice a few beers...) - came in at 92 ml / cc's.

Researching, but it appears the 3.2L engines may have a slightly larger combustion chamber volume than the 3.0L. The 3.0L is generally recognized as having a combustion chamber volume of 90 ml. I've found other posts where folks were measuring 91.3 ml from their 3.2L heads (Chris Bennet).

Still looking at it.


Gordo

[haycait911]

when I check C/R here's what I do.

with the engine on a stand assemble one cylinder/piston/head assembly with a little axle grease to seal the base gasket and the cylinder to head. turn engine so plug hole is vertical. pour in a tablespoon or so of Lucas Oil Treatment or similar, very sticky/gooey stuff. turn engine over a couple of times, now the rings are sealed. turn engine to put piston at bottom dead center. fill cylinder with oil, while measuring how much goes in. do it in a timely fashion but no need to rush. I use trans fluid because it's easy to see and flows well. turn engine over and allow oil to drain. turn engine so cylinder is back to vertical and move piston to top dead center. fill with oil again. drain oil. divide A by B and you have your C/R.


not real scientific but it seems to work.

[brighton911]

Yeah, I suspect JE has a good idea of what their dome volume is.

[cgarr]

Here is what I made for checking total volume assembled on the stand. Screws into the plug hole then you can roll the engine slightly to get any air out. I like the mm oil or fuel oil for measuring.

[Gordo2]

Thanks for the input on the other methods to measure CR. Before staring into measuring, I considered both (assembled measurement vs. component i.e. combustion chamber volume + piston dome etc.) and decided to go with component measurements.

-----------------------------------------

JE Pistons Nominal Compression Ratio Calculation

If anyone is curious how JE comes up with their nominal compression ratio - the following example should provide a good feel for it.

JE Piston specs from their website (PORSCHE - PORSCHE 911 (Sport Compact Section) - JE Pistons)



Using the info off of this chart - you can plug the #'s into a compression ratio calculator such as this one: Engine Compression Ratio (CR) Calculator

JE assumes a standard 3.2L engine with the following:

- 95mm bore
- 74.4mm stroke length
- 90cc combustion chamber volume
- .040in / 1.016mm deck height (deck clearance)
- 0mm base gasket (really this is part of the deck height)
- 35.1cc piston dome volume (JE's spec for this piston)

If you plug the above #'s into the calculator - you come up with 9.5 to 1 compression ratio.

Obviously if any of your engine's measurements vary (deck height, combustion chamber volume etc.) - then you will come up with a different compression ratio.

Just tossing this out as additional info & resources for the community - not really new but possibly explained another way.

Gordo

[Gordo2]

I went back tonight and remeasured each combustion chamber:

Placing my homemade plexiglass puck on the head:



I drilled the fill-hole to a size that allowed the the nozzle end of the burette to fit snug in the hole, without extending the nozzle into the chamber:



With each consecutive head measurement, I got more proficient at filling and dissipating the bubbles that would form under the plexiglass:



I found you could quickly lean the head to consolidate bubbles - then lean it to get the bubble below one of the breather holes. From there you could force the air out by allowing more fluid to enter the chamber.

If the bubble isn't under the breather hole, the fluid backs up the breather hole and flows out of the combustion chamber area - skewing your measurements:

As shown here - fluid on top of the plexiglass next to the right breather hole:



My 3.2L head volume / combustion chamber volume measurements:

- Head #1: 92.4cc
- Head #2: 92.4cc
- Head #3: 92.0cc
- Head #4: 92.2cc
- Head #5: 92.8cc
- Head #6: 92.6cc

Average combustion chamber volume measured out to 92.4cc's.

Since this is higher than expected - I'm going to bring my burette to a machinist who can use theirs to confirm accuracy of mine.

Enjoyed the job & learning new stuff as I go.

Gordo

[Mark Henry]

Another way to do this is to seal just the top ring with grease and place the piston in the cylinder, then bolt or clamp the head on (grease again at the head sealing surface) then you have to rotate the head so that the plug hole points straight up. Measure the volume (and do the maths) and you're killing two birds with one stone.

I use the block on an engine stand to do this, no need for crank and rod assembly.
You can reset the piston at several deck heights and recheck. Write down your results and then as you build you can figure out your CR by just measuring the deck height.

Also if you do my way at zero deck, then measure the head your way, you have the piston dome volume.

Doh...Don beat me to it..
Guess I shouldd have read the whole thread

Originally Posted by cgarr

Here is what I made for checking total volume assembled on the stand. Screws into the plug hole then you can roll the engine slightly to get any air out. I like the mm oil or fuel oil for measuring.

I like!

[justinp71]

Hi Gordo, I like your component method of checking the cr, I'm currently in the same boat. I checked with components using the compression calculator and then did it with a head assembled and put oil in the cylinder with it at TDC,

I got similar measurements for both (9.6:1 cr)

btw, its a 3.2 with 9.5:1 JE's, 3.0 large port heads and a deck height of .9mm.

[justinp71]

Gordo-

Fyi, make sure you check the pistons specifications paperwork that comes with your pistons, my actual net piston volume ended up being 36.2cc, not the 35.1cc on the website. That would give you closer to 9.5:1. I would check your total combustion chamber volume with an assembled engine to be sure.

[Gordo2]

Originally Posted by justinp71

Gordo-
Fyi, make sure you check the pistons specifications paperwork that comes with your pistons, my actual net piston volume ended up being 36.2cc, not the 35.1cc on the website. That would give you closer to 9.5:1. I would check your total combustion chamber volume with an assembled engine to be sure.

Great call - finally got around to checking the paperwork that came with my pistons. Turns out they measure 36.2cc as opposed to the online listing as 35.1cc.

As such my CR calculates at 9.3:1.

Thanks for the point out.

Gordo

[HawgRyder]

I am used to the pit inspection type of checking.
In Formula Ford....the inspectors just plug a pipette into the plug hole...back off the valve adjusters...and rotate the crank though 360 deg and watch the high and low levels in the tube.
With the car tilted to get the plug hole at the top of the assembly...it seems to verify what we already knew when we built the engine.
With Porshe engines...single or double plug...it would be very easy to do it on the engine stand and rotate the beast as needed.
This would give you over-all ratio...but would not give the differences in either piston or head numbers to see which ones to change.
Bob

[Eagledriver]

Originally Posted by HawgRyder

I am used to the pit inspection type of checking.
In Formula Ford....the inspectors just plug a pipette into the plug hole...back off the valve adjusters...and rotate the crank though 360 deg and watch the high and low levels in the tube.
With the car tilted to get the plug hole at the top of the assembly...it seems to verify what we already knew when we built the engine.
With Porshe engines...single or double plug...it would be very easy to do it on the engine stand and rotate the beast as needed.
This would give you over-all ratio...but would not give the differences in either piston or head numbers to see which ones to change.
Bob

While this check will give you the displacement, I don't see how it will give you the compression ratio or the volume of the cylinder at TDC.

-Andy

[cgarr]

There are calculators online you just plug in the bore, stroke and combustion chamber cc and it figures it out. You skip the deck height, piston dome, head cc etc, makes it pretty accurate. A 3.2L with a combustion chamber measuring 62cc would give you 9.5/1 cr

http://www.csgnetwork.com/compcalc.html

[Walt Fricke]

Bob H

On a 911, backing off the adjusters is not enough to allow this kind of displacement checking. You have to remove the pair of rockers for one head.

You don't have to use fluid and spark plug holes vertical to do this. You can use air and a large syringe-like plastic tube with a cylinder in it. Get the engine to BDC and the measuring piston down to its bottom, and turn to TDC. If you have compensated for temperature, this gives a pretty accurate reading of cylinder displacement. Works great for pushrod V8s, because it is so easy to pull the rockers. Not so our 911s.

[HawgRyder]

Andy...the ratio difference between top and bottom of stroke is the compression ratio.
Eg....if it's 100cc at the bottom of the stroke...and 10cc at the top...the comp ratio is 10:1.
I know it never works out this simple but you get the drift.
In pit inspections the guys are mostly looking at displacement...but sometimes we got questioned on comp ratio (we played around with it something wierd...LOL).
After learning about Bill Elliot's engine in NASCAR running 17:1 on gas...we tried his secret and it worked...gave lots of power...and wore out the engine in 1 weekend of racing!
So...depending on your wallet...
Bob

[cabmandone]

For nearly 8 years this bothered my poor brother... so he made me test my heads prior to putting my engine back together. I used a 60cc syringe and was hitting 90cc checking just two heads and rechecking to make sure.