DIY Compression Checking

D

dellenwood

Well-known member
We are starting to build our first HP engine and want to get it right. I'm looking for practical advice on calculating compression. I understand the formula for calculating compression, but I'm not sure how to guarantee that the measures used in the formula are accurate. For example, how do you determine the volume of the head chamber? It's so irregular, it seems like we would need to "cc" it in order to be perfectly accurate. But, that seems a bit beyond our capabilities. Is this something you typically take to a machine shop? What about the other measures? Some seem relatively easy, but there seems to be room for error . . .

I know this is a pretty wide open question, but I would appreciate any advice that you've got.

Thanks!
 
You could use the "generally accepted" cc for your heads, and use that in your calculations. That would be a close estimate, unless you have done some head work. The height of the valve seat, or a new valve in one cylinder, will affect your cc reading between the chambers. But, the most accurate is to cc the head as described below.

It is a simple home shop procedure. I use a flat piece of plastic or polycarb with a small hole in it. Have used larger calibrated syringes (some drug stores carry them) or burettes. A good machine shop can do it for you.


http://www.circletrack.com/techarticles/ctrp_0611_cylinder_head_volumes/viewall.html
 
I measured with a piece of plexiglass and syringe and came out within 1/2 a cc of what my machine shop measured.

There are several online spread sheets to plug in numbers to calculate CR.

http://www.csgnetwork.com/compcalc.html
 
Here is a simple kit that you can use. The plastic beaker is a little hard to pour out of but works. You can also use it to CC the dome/dish on a piston. You set the piston down into the cylinder a given height and CC it like the head. Since you know the bore and the height the piston is down in the cylinder for your test you can figure the total dome/dish on the piston.

http://www.speedwaymotors.com/Speedway-Economy-CC-Checker,3172.html
 
OK, I'm confident that we can cc the head accurately. We'll use the piston dish cc provided by the manufacturer. Measuring the deck should be easy. Measuring the cylinder diameter is easy. Two questions:

Do folks typically use the stroke published in the GCR since we all use stock rods?

How do you measure the "compressed" head gasket?

Am I missing something? Do folks typically leave some margin for error?

Thanks for the help!

DE
 
Felpro VW is about 064. But the bore is oversize about 83ish
The head size for 11/1, VW 1.8 is near 132mm . But do the work that you have going.
And please post your results. Thanks,MM
 
How many times your old head has been shaved for blown head gaskets, how many valve jobs, and any other head work will really change your factory nominal cc beyond belief. I bought the last two (according to Honda) blank heads for the EW engine because my old heads had been shaved down past the nibs and yet when cc'd were greater than the mfg claim. Looking at the new heads the valve seats were over a 1/4 inch pulled down into the head. I decided that was possibly a shed failure and bought the $1k heads. (each). When I cc'd the brand new heads with brand new Honda valves, the cc was 5 less than than the head that had been shaved .060 inch. The old head passed the whistler, passed the cc test, but when I thought about it for about 5 minutes figured it was outside the intent.

I checked the heads myself with the circle track kit and also let the machine shop do it. The plastic and syringe was spot on with the pros. I used the piston mfg declared cc and the math was about what the whistler returned. We all know that the whistler is optimistic.

YMMV

James
 
I have the glass 100cc burret on stand, because I do this often, but you can get 50cc feeding tube syringes form the drug store that will do the job for a couple of bucks, in fact that's what I use to transfer my liquid into glass burret. It's good idea to make a couple of holes in the plexiglass plate to chase air bubbles with. I use regular wheel bearing grease to seal the plexiglass plate to the cylinder head. Once I read my actual CC volumes, and know that number, I can use a compression ratio caculator to figure where I'm at (there are palnty of good/free ones online), then if I need to cut the head to increase the compression ratio, I simply lower the combustion cc volumes on the caculator until I get near my target, then put that amount of liquid back into the combustion chamber, and use a depth micrometor with a wide base to see how far down it takes to reach the liquid, double check myself, then I know exactly how much to cut off the head. I use that green rubbing alcohol as a liquid, it seems to work fine, and not have any static/magnetic qualites that some liquids do when doing this job. Hope this helps.
 
dellenwood said:
Do folks typically use the stroke published in the GCR since we all use stock rods?
Click to expand...


The rods don't determine the stroke, the crankshaft does. Since you're measuring everything already, you should measure the piston travel as well.
 
easier than pouring from a beaker or syringe:


http://www.amazon.com/ACRYLIC-BURET-PTFE-STOPCOCK-50/dp/B0018C4DRK/ref=sr_1_1?s=industrial&ie=UTF8&qid=1353559197&sr=1-1&keywords=BURET
 
Because I have a 948cc sprite engine the head gasket is egg shape, and I machine (carve) my own pistons domes I decided on a one step calculation. First I install a piston w/rings to tdc and seal the piston to wall area with grease. Next install the head w/valves using a used gasket. Rotate the assembly till the spark plug hole is at the top of the chamber. Fill with fluid from graduated burette to the bottom of the hole and calculate using stroke and bore numbers.
 
If SCCA still uses a whistler to verify CR in tech, your best bet is to locate one and actually verify the final build.

I have use it to baseline a new combination and then calculate modifications. After which I use fluid validation.

I have heard that they are tempermental, to date I find this is not true, especially if working in the shop with ambient temps.

J
 
Anyone whos had their compresion checked by the SCCA want to weigh in here? Was a Whisler used? Did you agree with it's results?
 
I've had mine tested by the SCCA and using the one we own at the shop. I've found that you really need to get the temperature of the engine correct. Ambient isn't good. Further my experience is that the unit will consistently read about a point lower than real.

James Rogerson
 
When a competitor at the Runoffs was found to be non-compliant a few years ago a whistler was used initially, but when the motor was found to be over the CR limit it was torn down and measured. I believe that they used liquids to verify volumes but do not know that for sure. My motor is right at the correct CR per the liquid tests that I have run and it whistled on the low side by almost half a point, so I would say that the SCCA whistler is conservative from those results.
 
I've been whistled several times, at both The Runoffs (FP) and the ARRC (ITA & ITB). I'd agree that usually, they ended up reading a little lower than I expected them to. Being an engine builder, and having used a Whistler many times myself, it really pays to know how to operate one while you're watching someone else check if your car is legal or not. Those things were originally designed for the roundy-round guys, and their V8's, so you have to enter displacement in full-integer cubic inches. Do the math on your car, convert your cc's to ci's, round it up and down to the nearest whole ci, and see what kind of a range that is. For example, here's my car:

engine cc's = 1860 (or something like that, after the legal overbore)
convert that to ci = 113.5
113 ci = 1851.7 cc
114 ci = 1868.1 cc

That's a pretty large range, and will definitely sway the compression ratio reading by quite a bit. It's already been said earlier, but the temperature that's inputed into a Whistler can also make a HUGE difference. Even the depth you have the thing installed into the cylinder head makes a difference, like with or without the valve cover installed on engines that have spark-plug tubes through the center of the head (like a Honda). All makes a difference. One year at the ARRC, I saw them try to bounce a competitor for who they were reading like 12.5:1 compression on an engine that was supposed to be 9.7. Once they were shown everything they were doing wrong, they finally got the number right.
 
cc'ing with burette;
When checking the engine parts with a fluid I use a mixture of water and anti-freeze @ about 30% with 6 drops per gallon of water wetter. It seems to work really well for me.

whistling:
As kruck mentioned, getting the whistle engaged all the way to the base of the spark plug adapter is very important but make sure that it doesn't protrude through the bottom. With the older whistlers the whistle itself is rather large in dameter and makes it very difficult to get the correct reading if there is any obstruction around the unit. Getting it to seat properly in a DOHC engine with the recessed spark plug location is problematic. If the air can't escape the whistle unobstructed it affects the reading. Also (and I haven't seen anyone try this in a while) don't use nitrogen, it affects the reading as well)
 
Thanks, everyone, for the comments. We'll be trying to do this ourselves in the next few weeks. We are interested to how all this "fits together" for us.
DE
 
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