ideal compression on an m20 stroker with custom pistons

[tschultz]

I know this is not an e30 site, but my brother thought you guys could throw me some knowledge on my choice, so I am using his profile to post and thus my problem and my possible solutions.

I am rebuilding an m20 right now, it came with Ross Racing forged pistons and rods after the previos owner snapped the timing belt. Ultimtely I would like to turbo this engine and thus am stuck deciding on a compression ratio and the necessary parts to make that happen.

The pistons I have are meant for an m20b30 by using the s52 crank (89.6mm) however I pulled it apart due to the low compression numbers I was seeing and found out it still has the stock 76mm crank in it. With this setup the compression ratio would be ~9.75:1 and 2979 cc. (the numbers I am seeing are about 6:1 with the crankshaft I have in it!!) With a turbo down the road I know this is not ideal with the likelihood of detonation under boost.

I also looked at the s50 crank (86mm) and am considering this as an option. This would resultin 2860cc displacement and a compression ratio of ~8.1:1. The head is stock as of now and I know the 745i turbo had a 7 or 8:1 compression ratio and this could be a good range for it. The interesting twist is that I am a mile high above sea level. I have been looking at low boost applications and am tentatively planning the rrfpr and chip method-- please do not turn this into a which fuel management method is better argument.

I compared a low boost stock m20b25 setup with 8 psi at sea level to both of the setups i am looking at right now. The results surprised me and got me to thinking, here is the link to the stock setup:


http://www.zealautowerks.com/index.html ... 0,6500,0,8


As you can see the stock compression ratio of 8.8:1 yields an effective compression ratio of 13.6:1. It is safe to say this is the limit on the stock motronic fuel system with a chip, many builds have used this method with a rrfpr. Remember this is a 2.5L engine.

I then punched in the ross racing setup with the same 8 psi but at an altitude of 5000 feet (Denver is 5280', I am around 6500' in Colorado Springs). This would be a 3.0L engine.


http://www.zealautowerks.com/index.html ... 500,5000,8


These results surprised me because the 9.75:1 Ross Racing Setup yielded an effective compression ratio of 8.78:1 at 5000' and an effective compression ratio of 13.56:1 under 8 psi of boost at 5000'. This means I would be seeing the same power at altitude as a stock m20 would be seeing at sea level.

The other option with the s50 crank would yield lower results.


http://www.zealautowerks.com/index.html ... 500,5000,8


The compression ratio would be lowered to a lesss than impressive ~8.1:1 in a 2.86L engine. Remember the 745i turbo had 7:1 or 8:1 compression out of a 3.5L engine. One 8 psi this engine would effectively be 11.06:1. This would leave room to up the boost but fuel management would need to changed to accomodate.

So what do you think? I am seriously considering the 89.6mm crank as it should be ideal and would maximize performance without including additional management costs. Of course this setup would need some sort of sea level valve that is opened below 5000' to keep detonation from occuring and have maybe 1 psi of boost.

What are your thoughts on the shorter 86mm crank and resulting compression ratio? We could get into porting of the head and cams as well but for now assume the head is stock and the same for all applications, the difference is solely the crank and resulting compression ratio.

[turbodan]

Boost pressure doesn't affect the compression ratio. The compression ratio is limited by how much heat will be added to the fuel and air during compression. Turbocharged engines tend to have retain more residual heat in the combustion chamber and higher intake air temperatures depending on how well intercooled a system is. Thats why you can run 20+ psi on pump gas and still have a margin of safety before detonation occurs. Doubling the pressure of the intake air does not double the compression ratio. The CR has a multiplicative affect on combustion chamber temperatures while increasing air density has an additive affect. Think of it like changing the numerator VS the denominator. It works like that. Thats what limits NA motors to ~13:1 when you can more than double the air mass with forced induction at an 8.x:1 CR on pump gas with ease.

Hopefully that makes sense. I don't know if I'm 100% correct with the terminology though.

If you're going to run under 10 psi you would be fine with 9:1. 8.5:1 would allow well over 20 psi. Low 8's gives a larger margin of safety but you're beginning to trade away to much off boost efficiency at that point. 8:1 is a pretty good lower limit IMO. With sufficient intercooling, a good tune and everything else worked out you shouldn't need to go lower than that.

[tschultz]

Thanks Dan, this is the answer I was looking for. Do you have any calculations to help determine where the line is between different boost and compression ratio combinations?

I understand what you are saying and that is why rally cars can run insane amounts of boost (I think the Ford RS200 had 23 psi on a 1.8L) as it is not a linear relationship between the compression ratio. (the Ford has something like 7:1).

My main goal was to have a reliable setup with a rrfpr and chip pushing low amounts of boost. It was pointed out that the CH on my calculator may be wrong so I double checked the paperwork and it turns out CH = 1.026 in = 26.06 mm.

Plugging this in to the calculator we get a 9.11:1 CR.

http://www.zealautowerks.com/index.html ... 500,5000,8

They used 8.11" (206 mm as opposed to 206.7mm) as the block height and I think this will effectively lower the compression ratio to work in my favor.

I was mainly asking the limit of the upper end of compression ratio on boost and you answered the lower end, Dan. I think the s52 crank is ideal for my setup and am leaning toward that.

What do you think of the barometric boost valve for sea level driving?

[winfred]

my build may or may not be useful or amusing http://www.e30tech.com/forum/showthread.php?t=44729 short version, 84mm bore/stroke stock i pistons (around 9.2 compression with longer stroke) m52b28 crank, E rods, BAS 280 cam, extrudehoned intake and bored throttle body, paxton sn-92 blower with upgraded impeller pushing 8# of boost through a water to air charge cooler, run by MSII

[turbodan]

Calculations would be extremely complex and probably useless. Way over my head.

9.1:1 would be okay for 10 psi. I would feel better about 9:1 because I like round numbers and comfortable margins of safety.

I'm not sure what kind of valve you are referring to. Would the purpose be to reduce boost slightly below a certain altitude? Shouldn't be necessary really.

[tschultz]

my build may or may not be useful or amusing http://www.e30tech.com/forum/showthread.php?t=44729 short version, 84mm bore/stroke stock i pistons (around 9.2 compression with longer stroke) m52b28 crank, E rods, BAS 280 cam, extrudehoned intake and bored throttle body, paxton sn-92 blower with upgraded impeller pushing 8# of boost through a water to air charge cooler, run by MSII

I will be reading through it. How much boost were you looking for and any reason you didnt go with a chip a rrfpr? I realize tuning is more difficult but I figure its worth a try to I can always upgrade later if it doesnt work out. I am starting with a motronic 1.0 system (35 pin) and dont feel like upgrading yet as it involves the cps, ecu, harness, and a few other sensors and connections on the tranny and fluids.

Calculations would be extremely complex and probably useless. Way over my head.

9.1:1 would be okay for 10 psi. I would feel better about 9:1 because I like round numbers and comfortable margins of safety.

I'm not sure what kind of valve you are referring to. Would the purpose be to reduce boost slightly below a certain altitude? Shouldn't be necessary really.

Thanks Dan I figured, but was more looking at the upper limits of the sohc turbo'd car on motronic if you have any experience with this. 9.75:1 is probably fine with a good tune and standalone fuel management but what about the motronic setup?

The idea would be that without a full standalone unit the system would be more susceptible to detonation as it is not a perfect tune. With a high CR the detonation is more likely at the same level of boost. The value I was talking about is a comparison between the higher CR and boost and where the limit is for that depending on the end result (amount of boost, CR, fuel management, etc).

[winfred]

8-10#. i didn't want band aided old tech, not saying it doesn't work just not for me

I will be reading through it. How much boost were you looking for and any reason you didnt go with a chip a rrfpr? I realize tuning is more difficult but I figure its worth a try to I can always upgrade later if it doesnt work out. I am starting with a motronic 1.0 system (35 pin) and dont feel like upgrading yet as it involves the cps, ecu, harness, and a few other sensors and connections on the tranny and fluids.
.

[altus22]

I think what Dan is referring to is dynamic compression ratio. This takes into account static compression ratio and boost pressure, in addition to when the intake valve closes, but I won't touch on that.

For a simple example, let's say your static compression ratio is 8:1 and you want to run 1 bar (14.7 psig) of boost. Essentially, you're looking at DCR = SCR * (Boost in psia)/14.7

For the example above, this works out to DCR = 8 * 29.4/14.7 = 16.

If you want to run the highest DCR possible and not break something, look at similar builds that run well, calculate the DCR and choose a combination that fits your goals without exceeding the DCR of a known good build. This number gives a simple way of comparing FI builds of different compression ratios. I think that it goes without saying that the higher your DCR, the more likely you are to knock/detonate, everything else being equal.

Note: psia (psi absolute) is psig (psi gauge) + atmospheric pressure, usually 14.7psi at sea level .

[tschultz]

I think what Dan is referring to is dynamic compression ratio. This takes into account static compression ratio and boost pressure, in addition to when the intake valve closes, but I won't touch on that.

For a simple example, let's say your static compression ratio is 8:1 and you want to run 1 bar (14.7 psig) of boost. Essentially, you're looking at DCR = SCR * (Boost in psia)/14.7

For the example above, this works out to DCR = 8 * 29.4/14.7 = 16.

If you want to run the highest DCR possible and not break something, look at similar builds that run well, calculate the DCR and choose a combination that fits your goals without exceeding the DCR of a known good build. This number gives a simple way of comparing FI builds of different compression ratios. I think that it goes without saying that the higher your DCR, the more likely you are to knock/detonate, everything else being equal.

Note: psia (psi absolute) is psig (psi gauge) + atmospheric pressure, usually 14.7psi at sea level .

Thanks altus, yes I am referring to the dynamic compression ratio. I am aware that this changes depending on cam profiles and tried to calculate the potential dynamic compression ratios, but I like your idea and didn't realize the forumla was so simple.

I had found this calculator but was unsure of the angle in which the inlet valve closes after ABDC. I found a value for the m20b23 but that did not help much as I think the cams were different.

http://www.wallaceracing.com/dynamic-cr.php

Is the calculation you used above appropriate to use for a stock head/cam and does it change with differences in cam? Or is that just a simplified example for a rough guideline of dynamic compression ratios?

And winfred I didn't notice you went with a supercharger on your build, that somewhat changes things vs a turbocharged project.

Thanks

[winfred]

8-10# of boost is 8-10# of boost, the pistons/rods/crank don't really know the difference with decent charge cooling and fuel/ignition management. i have 27k miles on the build with about 10 of those boosted with several autocrosses and lotsa hard driving

And winfred I didn't notice you went with a supercharger on your build, that somewhat changes things vs a turbocharged project.

Thanks