rhkcommander
Full Access Member
instead of shaving the pistons, can the rods be machined to have the piston sit lower?
Calculating compression ratio of an IDI
- Thread starter ocnorb
- Start date
ocnorb
IDIT
I am into them $265 for cleaning, magnafluxing, shaving and vacuum testing.
They passed the vac test at 6 bars... Dave says thats almost as good as a fresh valve job.
They passed the vac test at 6 bars... Dave says thats almost as good as a fresh valve job.
racer30
Full Access Member
My coment on the compression ratio change from milling the heads is a fact. Some machinest dont reset the valves deeper in the head to offset the milling of the cumbustion face. those in the know also will only cut the head on a IDI as little as posible to corect the surface. His machinest must have felt .011 was the minimun needed or he cut to much. We will never Know. I still feel that with the pistons above deck .010 he will be fine. The fact that a engine ran with pistons over .050 above deck without complete piston contact shows he could be as much as .030 on the safe side. Better yet, Put the crank in the block, one rod and piston time it to the cam and use light temporary valve springs ( I get springs from the hardware store) so the lifters won"t colapse,or(I have used a wooden solid lifter that I made) temporary cardbord gasket and clay balls on the piston tops, roll that over and start calculating the clearance. It cant hurt to check it for clearance before you panic about needing new heads. I have used this trick on Highly modified briggs engines, a pontiac 400 v8 and a few sb chevy's with pop up pistons. works every time. you will know if you are to tight real quick. just takes some time to find out for sure.
I'm no help in answering your question. But I wanna spend some of your money and tell you to get some of those 30% over heads from that company in New Jersey. I'll live vicariously thru you on it then.
ocnorb
IDIT
I know you can on some motors. Going to the machine shop today to ask a few questions and decide which direction my build is going to take.
FordGuy100
Registered User
Can we have a set of rods made that are shorter? Probably $$$$
ocnorb
IDIT
Machine shop says an offset pin can be done, but they do not have the equipment to do it.
icanfixall
Official GMM hand model
Milling a .006 offset in the small end pin bushing has been done. That dropped the piston .006 in the cylinder right there. Our heads do not have a combustion chamber so the comment about some shops not sinking the valves can be said to be true... But not on our heads because there is no combustion chamber so mill down said amount.
Now the if word... If x amount is milled off the head and the valves are sunk the "factory required amount".. The compression ratio can't change. Nothing has changed demensionally to cause a change. Now if the valves were not set into the seats on the head the proper amount there is now telling what will happen to a possible piston to valve contact or cr.... Only .004 can be taken off the block deck and another .005 can be taken off the heads and stil be in factory specs. But again, I need to ask why do we have this spec in the first place. Nobody has come up with an answer yet. There still is a few puzzles with these idi engines. I can post those puzzle questions in a new question later.... I understand that milling the block puts the heads closer to the center line and the intake manifold gets closer to not matching up to the heads properly. With milling the heads or block some exhaust crossover pipes will be a struggle to fit too. That struggle was talked about in a recient past post...
Now the if word... If x amount is milled off the head and the valves are sunk the "factory required amount".. The compression ratio can't change. Nothing has changed demensionally to cause a change. Now if the valves were not set into the seats on the head the proper amount there is now telling what will happen to a possible piston to valve contact or cr.... Only .004 can be taken off the block deck and another .005 can be taken off the heads and stil be in factory specs. But again, I need to ask why do we have this spec in the first place. Nobody has come up with an answer yet. There still is a few puzzles with these idi engines. I can post those puzzle questions in a new question later.... I understand that milling the block puts the heads closer to the center line and the intake manifold gets closer to not matching up to the heads properly. With milling the heads or block some exhaust crossover pipes will be a struggle to fit too. That struggle was talked about in a recient past post...
ocnorb
IDIT
Squish!!!!!!!
OK. I've been mulling this over so much my co-workers are sick to death of hearing about my motor... maybe some of you are too!
My factory manual says piston spec is .010-.031 over the deck.
Now if the head gasket compressed is .035 and the head is flat that leaves only .004 piston to head clearance!!! This is what worries me. If the clearance is .004 cold I guarantee its going to hit when hot! Isn't it??
My block needs to be decked. I don't want to end up with a $350 paper weight. What should our squish be set at?? (Ideally)
OK. I've been mulling this over so much my co-workers are sick to death of hearing about my motor... maybe some of you are too!
My factory manual says piston spec is .010-.031 over the deck.
Now if the head gasket compressed is .035 and the head is flat that leaves only .004 piston to head clearance!!! This is what worries me. If the clearance is .004 cold I guarantee its going to hit when hot! Isn't it??
My block needs to be decked. I don't want to end up with a $350 paper weight. What should our squish be set at?? (Ideally)
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racer30
Full Access Member
Where did you get .035 compressed gasket? 
...I was under the impression that the gasket was thicker than that. I am used to GM stuff and I had several different gasket's to choose from. It sure makes it easier when you have more options to work with. Well it seems like cutting the pistons is looking like the only way to get clearance in the engine at ths point. I had a engine that was so tight that after running a complete race season, we opened her up and you could see the cylinder head milling imprinted on the piston tops, just a bit to tight at 8200rpm, Things sling to there max at that rpm, needless to say we used .010 thicker copper head gasket when it went back together. Keep up the good work and you will overcome the problems one at a time. I am thinking I will cut .030 to .035 off my pistons, but I will be only cutting the top inside .25" from the edge to keep the ring lands from running hot. Just my .02 on cutting. I am shooting for 18.5 to 19.0 to 1. I will know more when I test fit my parts. I have a few spare pistons from junkers that I will use to cc the cut for calculations. Then I will get them coated. I have lots of time and no money, so I have to wait for summer to get more done on this project.
...I was under the impression that the gasket was thicker than that. I am used to GM stuff and I had several different gasket's to choose from. It sure makes it easier when you have more options to work with. Well it seems like cutting the pistons is looking like the only way to get clearance in the engine at ths point. I had a engine that was so tight that after running a complete race season, we opened her up and you could see the cylinder head milling imprinted on the piston tops, just a bit to tight at 8200rpm, Things sling to there max at that rpm, needless to say we used .010 thicker copper head gasket when it went back together. Keep up the good work and you will overcome the problems one at a time. I am thinking I will cut .030 to .035 off my pistons, but I will be only cutting the top inside .25" from the edge to keep the ring lands from running hot. Just my .02 on cutting. I am shooting for 18.5 to 19.0 to 1. I will know more when I test fit my parts. I have a few spare pistons from junkers that I will use to cc the cut for calculations. Then I will get them coated. I have lots of time and no money, so I have to wait for summer to get more done on this project.
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ocnorb
IDIT
Where did you get .035 compressed gasket?
My old gasket measures .065 at the ring and .085 around the edges. (It may have swollen.)???The new HS3789 VR gasket set has no documentation, but it measures .055 around the edges and at the ring its about .080- but it has not been compressed yet. I am assuming that with studs the .055 will drop when installed.
I can't seem to find my source on the compressed thickness of the VR gaskets right now. Will try calling NAPA when they open.
racer30
Full Access Member
well .065 is what my gaskets rings on both my turbo engines measured used. So I feel that is a good number. I have seen on this site other's measurements from both felpro and VR gaskets, But I have never checked one myself in new condition. I cant say for sure if it smashes anymore than .065 when installed but this number is what I based all my calculations on when I ran this engine on the computer engine simulater. once I had all the info in the computer I was able to run several tests on compression and the changes I could make by changing the cumbustion area space. I found that cutting the piston .010 (the complete piston surface) lowers the static compression 1.2 to 1 Adding 1cc of space in the cylinder lowers the compression .47 to 1 So this can be calculated by several ways. remember Dynamic compression (the running compression) and the Boost compression (the compression under boost) is all different and higher than static compression. I used "wallace Racing" engine calculators web site to do alot of my research. My engine compression work has been mostly on race engines where we had rule's about flat top pistons and valve angle's so we needed to build the engine as tight as possible without breaking the rule's. I was a race official also so it was important not to break the rule's or I had more than my championship points to loose. I feel that cutting the pistons gets you every thing you need so thats what I woud do. Researching your option's is the right thing to do and you are doing that so you can feel good about your choice of action.
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Don't mean to bust your balls Gary, but if you machine the heads to an IDI properly (including sinking the valves) you will change the compression slightly, because of the precup when you machine it the bottom of the precup gets thinner therefore the volume of it (the throat from the prechamber to the cylinder) is decreased ever so slightly.
Now this change may be negligible but it is still there.
Sorry but I like to be as exact as possible, just like you can compress water. (it takes 68,000psi to compress a given volume of water bu 10% of it's original volume)
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