Calculating compression ratio of an IDI

[ocnorb]

Just like the title says. How does one go about getting an accurate number?

I am guessing that the pre-injection cup volume + the volume of the chambers + the volume of the swirl chambers on the piston are all necessary to determine static compression. (in addition to bore, stroke and piston deck clearance.)

Anybody run the numbers on a stock IDI? I keep coming up with a total combustion chamber volume of about 35cc on a stock motor to get 22.38:1 compression.

Not sure I am doing the calcs correctly. Any help would be great. My motor is at the machine shop. Looking for a clear path forward after they tell me how much has to come off my heads and block deck.

 

[FordGuy100]

Sounds about right!

Remember the pistons stick out if the block so the only clearance between the pistons and the head is just head gasket thickness minus how much the pistons protrude out. I would say for calculation purposes you could say the piston is essentially flat.

 

[ocnorb]

Remember the pistons stick out if the block so the only clearance between the pistons and the head is just head gasket thickness minus how much the pistons protrude out. I would say for calculation purposes you could say the piston is essentially flat.

I actually measured mine; it crowns pretty close to 0.010 over the deck surface.



My plan is to lower the compression a bit by shaving the pistons. Just need to figure out how much. I will know more after I talk to the machine shop guru; he wasn't there when I dropped my stuff off unfortunatley.

 

[itsacrazyasian]

kinda off topic... but guess what showed back up in my mailbox yesterday?

 

[ocnorb]

kinda off topic... but guess what showed back up in my mailbox yesterday?

??

A compressor wheel???

 

[itsacrazyasian]

??

A compressor wheel???

Yup. Stamped Aug 27th, 2011...

Undeliverable as Addressed.

***?!?

I came home saturday to the box and i was wondering what it was. thought it might have been my pissed off neighbor trying to bomb me or something.

 

[racer30]

When I first started trying to work out the compression I did some tests on the engine. This is what I found on my 94 turbo engine.. precup vollume 20cc with a BB injector and glow plug installed. cylinder head intake and exhaust valve closed vollume 6cc intake 3cc exhaust. piston top depression 8cc. With these numbers on a 7.3 engine with a head gasket with a .065 compressed hight the piston needs to come up .014 above deck to get 21.45 to 1 compression.

 

[ocnorb]

I wondered what had happened, but I didn't want to be presumptuous or pushy about it.

Thanks for trying!

I actually still have that turbo laying around, as I ended up having the local guru fit my housings to a BW center and wheels. It works pretty good- one of these days I need to measure it up and see how big the cold side is.

 

[ocnorb]

When I first started trying to work out the compression I did some tests on the engine. This is what I found on my 94 turbo engine.. precup vollume 20cc with a BB injector and glow plug installed. cylinder head intake and exhaust valve closed vollume 6cc intake 3cc exhaust. piston top depression 8cc. With these numbers on a 7.3 engine with a head gasket with a .065 compressed hight the piston needs to come up .014 above deck to get 21.45 to 1 compression.

Thank you! I was hoping you would see this having read most of your posts on this type of stuff.

I measured my piston height at .010 above deck. Machine shop called me today and said that they took .011 off my heads. Hoping thats not too much.

 

[racer30]

If your engine is all the way apart I would cut the pistons down and coat them. because you have a turbo, you dont need more static compression. If not, you can run them as is with no problems as long as you keep an eye on the pyro. lots of people have had there heads cut and have no major problems. If you think about it to much, You will convince yourself you have a problem and then you will lose confidence in your build.........

 

[icanfixall]

Well 11 thousands off the heads is twice as much as they can be milled so those heads are out of spec.. Sorry to say that too. Now lets discuss why they can or can't be used. Milling anything off the heads changes only the thickness of the head and where the stems are located. The cr does not change. Your probably wondering how the cr stays the same. The heads are flat. The valves have a depth they need to be set into the heads. All your doing with a milled head is raising the valves up into the seats and head. So that will stay the same. As much research I have done on milling the heads I still can't find a reason for the thickness demension. The springs need a certain amount of pressure on the seats when they are closed but shimming them will fix that on a milled head. Now that the heads are milled you may have some issues with the intake manifold to head gasket match up. But the intake can be milled to fit.. Its a tuff demension to find what needs to be cut off too. The actual mill setup is easy as is the milling of aluminum.

 

[racer30]

The compression will change a little if he dosent set the valves deeper in the head. the valve relief in the head has 9 total cc of space (in my turbo heads) cutting .011 off the head will reduce this space. adding 1cc of space in the cumbustion area lowers static compression by .47 taking away space will increase compression. Some people say the piston clearance to the valves is close on the exhaust stroke when the piston is chasing the valve closed at the end of the exhaust stroke. My research indicated over .090 of clearance with a stock cam and its verry slowly closing valve action. I know there are specs for building this engine and staying inside of them is a good idea. But in this case I see no reason to be alarmed. I would say check the valve setback in the head, maby cc the pocket to see what it is and the make a call on the situation. shimming the valve spring is easy to do to keep the closed seat preasure. The intake manifold is pretty forgiving at most might need to drill the bolt holes a little oblong to get them to thread up, a test fit will help with that, unless you are doing the heads in the truck. Ouch my back hurts just thinking about it. check them out and see where you at before Freeking out.

 

[icanfixall]

racer30.... HMMM. Milling 11 off the heads and not setting the valves the proper depth in the heads will change things like cc area. Soooo. Setting the valves the proper depth in the heads be they milled or not will keep all the cc issues they same . Thats what I feel you missed in your post above. I feel we are saying the same thing but its just not getting on paper that way. I understand specifications on how machine work is to be done. What I nor anyone else has found out is why the heads have this spec for thickness if we have no combustion chamber like a gasser. Milling 11 off a sbc will change the cr because its cut that much off the head chamber. We have no head chamber. Our pistons come up out of the top of the cylinders as much as 31 thousands. Thats within the factory specs. I astually ran my first engine ar 54 out but.. Every piston had a valve imprint on them. You could only see it but not measure or feel it. It looked like a painted or stained mark and nothing more. Would I tell anyone that 54 out is ok and do it ...Nope... Thats too much but I did get away with it for a while anyway... Then I lost a free plug and melted the engine pulling the highest grade out of LA.... Its called the Grapevine and its a long pull with water stops along the trip. Just a note on what happened to me. When you loose all the coolant suddenly you also loose the ability for the coolant touching the temp sender. So you have no idea whats going on till the engine slows down because the pistons are melting to the cylinder walls. By that time is all over but the crying..... Been there and have the pistons to show for it.....

 

[ocnorb]

Just picked up my heads>>>

I found these specs in my FSM:

Valve height:
Intake .042-.054
Exhaust .051-.063

Minimum head thickness is 4.7950

I took the printout with me to the shop so we could measure and talk about it. Tough call to lay out the cash for heads that may be paperweights...

We measured the head thickness at 4.780
The intake valve was .040
The exhaust was .053

Dave told me that he thinks that my intake clearance will not be an issue IF I shave the pistons down. (makes sense.)

He was also of the opinion that the factory specs are designed to sell new parts and not always true "reject" limits.

He has machined far more off of IDI chevy motors with no issues.

Then he asked me if I could get a thicker head gasket... and I tried to explain the current state of IDI head gaskets to him.... not gonna happen in the next few weeks.

So to sum it up... I paid the man for his labor and now its decision time. Run' em or don't run 'em?

 

[wmoguy]

If he did all the work to these heads how much $$ would you have into them?