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12:1 compression 75mm pistons http://www.teamswift.net/viewtopic.php?f=7&t=17540 
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Author:  racinghart GTi [ Tue Sep 13, 2005 8:15 pm ] 
Post subject:  12:1 compression 75mm pistons 
Yes Guys I'm currently building a mk2 with 12:1 75mm pistons. My car have a standard head on now have 198psi cyl 1 200 psi cyl 2 206 psi cyl 3 and 201 cyl 4 what will these 12:1 put me at? 
Author:  jones [ Wed Sep 14, 2005 12:10 am ] 
Post subject:  
I believe compression readings should increase in proportion with the increase in compression ratio. I don't know the stock compression ratio but I'll guess it's 9.5:1 judging by your current numbers. This means you should see around 260 psi if I'm correct. 
Author:  Murr [ Wed Sep 14, 2005 11:33 am ] 
Post subject:  compression 
sorry for being of topic Compression Ratio is measure by taking the volume of 1 chamber at BDC, then taking the volume at TDC and dividing. Volume @ BDC / Volume @ TDC = Compression ratio IE Piston is at BDC < bottom dead center > the measured volume is 10 cubic units. When piston is at TDC < top dead center > the measured volume is 1 cubic unit. This yeilds 10/1 = 10 therefor 10:1 compression. Your engine is taking an area 10 times larger and forcing it into an area of 1. Compression < measures how good of shape an engine is in. It tells you if you rings are sealing properly, maybe whether there is a hole in a piston etc. When engines wear, rings become less effective, pistons wear and become oval and loose, head gaskets might be leaking. A compression test is usually used to check the life in an engine and how hard it has been run. You can also diagonse simple problems with a compression test. If you do regular compression tests and find your compression dropping, you could try to add some oil into the chamber and retest. If your compression goes up, then it is a sign your rings are worn and maybe need to be replaced. You can not measure the compression ratio with a compression tester, nor can you measure the compression by a known compression ratio. 
Author:  Casey [ Wed Sep 14, 2005 12:11 pm ] 
Post subject:  Re: compression 
Murr wrote: Wow 12:1 that will get ya a one way trip to the race gas supplier. Them pistons will need 104 octane or better.
12:1 pistons 75mm, are they from a honda? You should look into how compression is calculated. Do some CCing pick up a calulater and calculate the compression your self. In no way does compression readings tell you anything about the "static" compression of the engine. How do people think this? 12:1 w/o any head work is quite a bit. I was going to do a 12:1 compression total, after pistons and head. Murr, he won't need 104 octane, although he will need some fine tuning on the cam gears to not ping, probably retard a bit on them. Talk to Geozuki about high Comp pistons, he is running 12.5:1 total compression in his, overbored and loves the power it produces. He gets no pinging on 94 octane. 
Author:  Guest [ Wed Sep 14, 2005 12:17 pm ] 
Post subject:  
Can 12:1 pistons even fit without head work?? i thought they'd smack into the valves! i could be wrong though. I had suzuki sport 11.5:1 compression pistons and from my understanding these were the highest compression i was able to fit without head work! Were's our piston specialist on the board! 
Author:  Casey [ Wed Sep 14, 2005 12:23 pm ] 
Post subject:  
gypsydigi wrote: Can 12:1 pistons even fit without head work?? i thought they'd smack into the valves! i could be wrong though.
I had suzuki sport 11.5:1 compression pistons and from my understanding these were the highest compression i was able to fit without head work! Were's our piston specialist on the board! i don't see why they WOULDN't fit because the head work would actually be SHAVING the head bringing them closer to the top right? lol jus trying to think of simple basics. There definately will need to be block boring done though in the sleeves thats for sure, they won't jus drop in I hope he realizes that haha. 
Author:  jones [ Wed Sep 14, 2005 2:22 pm ] 
Post subject:  
Quote: originally posted by Murr
In no way does compression readings tell you anything about the "static" compression of the engine. How do people think this? I'm curious how you came to this conclusion. I based my statement on personal experience. My talon comes with 8.5:1 compression stock and the compression reading should be 185 psi. I've swicthed to 9.5:1 pistons and get higher compression readings now. The math works perfectly for the readings I now get. 9.58.5=1 1/8.5=0.1176 185 X 0.1176=21 185+21=206 I get 205 psi everytime I do a compression test and there is no explanation for the increase besides the change in compression ratio resulting from the new pistons. If you think of the logic, you're squeezing the same amount of air into a smaller air space so it is going to be under more pressure. 
Author:  Murr [ Wed Sep 14, 2005 2:41 pm ] 
Post subject:  
sorry for being off topic 
Author:  Murr [ Wed Sep 14, 2005 2:43 pm ] 
Post subject:  Re: compression 
sorry for being off topic 
Author:  suprf1y [ Wed Sep 14, 2005 3:26 pm ] 
Post subject:  
jones wrote: Quote: originally posted by Murr In no way does compression readings tell you anything about the "static" compression of the engine. How do people think this? I'm curious how you came to this conclusion. I based my statement on personal experience. My talon comes with 8.5:1 compression stock and the compression reading should be 185 psi. I've swicthed to 9.5:1 pistons and get higher compression readings now. The math works perfectly for the readings I now get. 9.58.5=1 1/8.5=0.1176 185 X 0.1176=21 185+21=206 I get 205 psi everytime I do a compression test and there is no explanation for the increase besides the change in compression ratio resulting from the new pistons. If you think of the logic, you're squeezing the same amount of air into a smaller air space so it is going to be under more pressure. Comparing static compression of different motors is even more ludicrous. Static compression ratio, and dynamic compression are two, totally different things. 
Author:  jones [ Wed Sep 14, 2005 7:38 pm ] 
Post subject:  
Quote: originally posted my Murr Oh my, sorry that does require a response, please go back to 10th grade math. Please go back to grade 10 math? How about if you wait until you get to grade 10 math before you started tellin' people to go back. An 11.76% increase in compression ratio increased my compression readings by almost exactly 11.76%. Check the math. I've got 20 years of school under my belt and I'm an accountant. I do nothing but math everyday. I've taken math classes much more advanced than what you will see as you make your way through high school. If you're going to say I'm wrong, you could at least back it up with some facts or references instead of telling me to go back to high school. I used real world experience and have actually had this discussion with some knowledgable people on another bulletin board. What do you have? Quote: originally posted by suprf1y
Comparing static compression of different motors is even more ludicrous. Static compression ratio, and dynamic compression are two, totally different things Never did I compare between engines, but I did use another engine as an example. It's a 4cylinder and operates on the same principles. It just happens to be turbo and pushing over 300 horses to all four wheels. Obviously static and dynamic compression ratios are different things and do you know which one we are discussing here? You guys should back up your thoughts with some facts and this could turn into a really good thread. Oh and don't start hating on me just because I think a little differently than you. I am totally open to learn more and if I am wrong I'd love to know so I don't go around giving out false information. I was originally just trying to help someone. 
Author:  Jardamuth [ Wed Sep 14, 2005 9:58 pm ] 
Post subject:  
suprf1y wrote: Comparing static compression of different motors is even more ludicrous. Static compression ratio, and dynamic compression are two, totally different things. Don't worry Mike, we all know they are two, totally different things. The point is that they relate to heach other just like your kids relate to you and your wife. What this guy is saying is not bullshit. Of course you have the leak percentage to take into acount but still it's minimal on a good condition engine. 
Author:  Murr [ Thu Sep 15, 2005 1:56 am ] 
Post subject:   
sorry for being off topic 
Author:  fordem [ Thu Sep 15, 2005 7:50 am ] 
Post subject:  
I've got this question for our mathematician  atmospheric pressure (at sea level) is accepted as 15 psi  if I take a given volume of air and squeeze it into 1/10 the volume (static CR of 10:1), the pressue should go up by a factor of 10, so we're looking at 150 psi, right? That means, that mathematically, there's no way your 8.5:1 Talon could generate 185 psi 
Author:  jones [ Thu Sep 15, 2005 10:56 am ] 
Post subject:  
Quote: originnally posted by Jardamuth What this guy is saying is not bullshit. Of course you have the leak percentage to take into acount but still it's minimal on a good condition engine. Thank you very much. Someone agrees with me (here), I am not alone. Quote: originally posted by Murr Static Compression is calculated or measured by Finding the volume in 1 chamber, when piston is at BDC, then find the volume at TDC. You can either calculate the volume or mechanically measure it. Vol @ BDC / Vol @ TDC = compression ratio Period !!! That is excellent. It's too bad that no one has questioned how compression ratio is calculated. I believe the question is how an increase in compression ratio affects compression readings. You said there is no effect and I asked you to back it up. Please enlighten me. With all your experience you would think you would have had some time around high compression engines. I guess you just never bothered to do a compression test on them. It's too bad because I think if you had, you just might agree with me. Is there any chance you currently have access to a high compression engine? It would be awesome if you could do a compression test on a high compression engine and compare the readings to the same stock compression engine. And then of course post the results here. I just happen to have the day off so I can go back and forth with you all day if you choose. With all your posts I assume you'll be around all day too. I will not shutup until you prove to me that increased compression ratios do not effect compression readings. The reason is because I believe you are spreading misinformation. Every bulletin board I visit is strictly for technical reasons and I would love to learn something here. Quote: originally posted be fordem
I've got this question for our mathematician  atmospheric pressure (at sea level) is accepted as 15 psi  if I take a given volume of air and squeeze it into 1/10 the volume (static CR of 10:1), the pressue should go up by a factor of 10, so we're looking at 150 psi, right? That means, that mathematically, there's no way your 8.5:1 Talon could generate 185 psi I'm not a mathematician. I'm an accountant. I was trying to say my math skills exceed the grade 10 level. Atmospheric pressure is actually 14.7. I like to calculate pressure ratios and stare at compressor maps in my spare time. Unfortunatley I do not know how to use atmospheric pressure and compression ratio to calculate compression readings. If you do know how then I would greatly appreciate if you enlightened me. Fords are my first love. Suzukis are a close second though. It is only within the past few years I've been getting into turbo 4cylinders. The talon just happened to be the one I chose to mess around with. I would especially love it if a Ford guy could explain the concept abovve to me. My guess is that there must be some constant value that is factored into the formula. Let me know if you know the answer. Thanks for the great discussion guys. I'm sure people are learning some important concepts from this thread. 
Author:  tuffcarguy [ Thu Sep 15, 2005 12:14 pm ] 
Post subject:  Re: 12:1 compression 75mm pistons 
racinghart GTi wrote: Yes Guys I'm currently building a mk2 with 12:1 75mm pistons. My car have a standard head on now have 198psi cyl 1 200 psi cyl 2 206 psi cyl 3 and 201 cyl 4 what will these 12:1 put me at?
i think it will be hard to calculate your new PSI mathmaticly. is 260 going to be close? maybe. but there are other factors involved to say a math formula with tell you exactly what it WILL be. how many miles on the motor when the compression test was performed? was it warmed up? oil in the cylinders? battery full on charge? pull all the plugs and hold the throttle open? also on the new motor the type of rings will have an effect on compression. zero gap, high or low tension. and if you are building an all new engine i am sure you will be putting a new cam in. what profile of camshaft you choose will have a huge effect on what cranking compression will be. either way i cant wait to see a right up of how the build up and finally the test drive goes!!! 
Author:  jones [ Thu Sep 15, 2005 12:18 pm ] 
Post subject:  
Quote: originally posted by tuffcarguy
what profile of camshaft you choose will have a huge effect on what cranking compression will be. Are you sure about that? A camshaft will affect dynamic compression but has no effect on static compression. Both valves are closed at top dead center and that is what produces the compression reading on the gauge. I agree with the rings comment though. I hadn't considered that. 
Author:  tuffcarguy [ Thu Sep 15, 2005 2:21 pm ] 
Post subject:  
positive 
Author:  jones [ Thu Sep 15, 2005 2:24 pm ] 
Post subject:  
The only way I can see cams affecting the readings is if you advance or retard the cam timing. My logic tells me that under normal circumstances with stock cam timing that a change in the camshaft(s) should not make any difference in compression readings. Don't you agree? 
Author:  tuffcarguy [ Thu Sep 15, 2005 2:40 pm ] 
Post subject:  
static compression is your compression ratio,thrugh simple math. http://www.rbracingrsr.com/compstaticcalc.html dynamic compression is basicly what you will read on a guage. http://www.rbracingrsr.com/comprAdvHD.htm " Of the variables, the most important is cam timing which has a dramatic effect on your "dynamic" as opposed to your static compression ratio." so, your kinda right,but kinda wrong 
Author:  fordem [ Thu Sep 15, 2005 3:52 pm ] 
Post subject:  
jones wrote: A camshaft will affect dynamic compression but has no effect on static compression. Both valves are closed at top dead center and that is what produces the compression reading on the gauge.
Static compression is a mathematical calculation. As soon as you get into the real world, you find the inlet valve is typically open with the piston at BDC on the compression stroke, and closes as the piston rises  because of this, some of the volume in the cylinder escapes through the open valve instead of being compressed and this is the primary cause of the difference between dynamic (or actual) compression and static (theoretical/ideal) compression. 
Author:  jones [ Thu Sep 15, 2005 4:03 pm ] 
Post subject:  
Another factor that can change your dynamic compression ratio is the addition of a turbo. This is why I am pretty sure you cannot measure dynamic compression unless your engine is operating. So your regular compression test is in fact not a measure of your dynamic compression ratio. When there is a turbo the dynamic compression ratio of an engine will change as the level of boost rises and more air is stuffed into those cylinders. This also implies that the dynamic compression ratio will change with RPMs. The efficiency of a particular engine will also effect dynamic compresison. In short I don't think you or I will ever have the resources available to measure dynamic compression ratio. That is if it is even possible. The compression readings we see are an indication of the engine's static compression ratio and therefore they will change with changes in static compression. This is just as I implied in my original post. The one that started this mess. 
Author:  tuffcarguy [ Thu Sep 15, 2005 5:18 pm ] 
Post subject:  
you seem to have some real mental block on this!!! oh well your good with numbers on paper. i recomend getting out into the garage and actually putting some of these to use. racinghart only wanted to know what his cranking compression would be with a fresh engine build. you should check the stickys on the proper way to do a compression check and then PM me to tell me how you will have boost while checking compression with no sparkplugs at about 300 rpm and so on. i am done here. 
Author:  racinghart GTi [ Thu Apr 06, 2006 8:46 pm ] 
Post subject:  
Ok People the car is finally finish the compression was 290psi cylinder 1 288 psi cylinser 2 284 cylinder 3 and 288 cylinder 4. The car feel superb up stairs and down stairs. By the way had no pinging problems on 91% octane gas. 
Author:  jones [ Thu Apr 06, 2006 8:55 pm ] 
Post subject:  
Congratulations on getting it together and thank you for reporting back with the results. 
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