What Would My Compression Ratio Be?

[PM-R33]

A standard RB25 runs a head gasket thickness of 1.2mm and runs a CR of 9.1:1

Now when my head was off it got resurfaced and headwork got done to it (porting, polishing, valve seats cut etc). When we put it back on we used a genuine Nissan head gasket of 1.2mm.

So from this, would the CR be that much higher than 9.1:1? Was it wrong to use a Nissan 1.2mm gasket due to the head resurfacing and a thicker one should have been used?

My tuner thinks it is running higher compression as a compression test also came back with over 170psi per cylinder (a compression test will be redone to check this).

Thanks

Edited July 9, 2010 by PM-R33

[zebra]

How much was taken off the head, It normally doesnt affect comp ratio drasticly unless you had .5mm taken off or somthing.

[tridentt150v]

Usually a head skim on a good head takes off stuff all and as Zebra said would not really affect your comp ratio in a big way.

[PM-R33]

I got told the head got minimum taken off it with turb stuff (what does that mean?) and wouldn't have been over 0.5mm.

So you don't think it would be higher compression than 9.1:1?

What about the compression test results of 170psi? I remember doing a compression test years ago and I thought I only had like 130-140psi. Like I said I will redo this compression test later on.

[tridentt150v]

You have a fresher motor and a new head with valves that seal great and if you openned up the head a bit then it will flow better all these things = more psi

Check and see if you have any knock, no knock is great, if you do have knock then it just gives you an excuse to run with WMI - a win win

[PM-R33]

Hmm that does make sense I suppose.

Well the knock part of it is the main reason this whole thing has come about. My engine simply won't take any ignition timing without the thing starting to ping.

Due to not being able to run ignition timing the power level is absolutely terrible at the moment and yet nothing has been touched since the head got done. My tuner keeps thinking that the comp ratio is a lot higher than 9.1:1 and hence on boost levels close to 20psi is takes next to nothing to knock.

I don't know. It really doesn't make sense to me as we made good power on this head, yet now it suddenly is crap with nothing changed and after checking every little thing nothing can be found to be wrong with it. It even sat at my mechanics for a week and they can't figure it out either.

Edited July 10, 2010 by PM-R33

[snozzle]

a light skim is usually 10 thou which is 0.25mm - will make minimal difference

0.50mm skim is actually quite alot i think

running a higher comp ratio on an oem gasket is probably not the smartest idea but it all depends on what boost your going to be running and how much was actually removed from the head, im also assuming the block hasnt been skimmed ?

i wouldnt say its wrong as such so dont stress, but i would have done it slightly differently

Edited July 10, 2010 by snozzle

[PM-R33]

Nah the block wasn't touched.

I am on the verge of pulling the head off again and putting a thicker gasket into it. But I would really hate to go through that pain again if that aint the problem.

[snozzle]

you should investigate the quench distance/area in your head, pretty sure its the distance between the piston and cyl head at tdc if this is incorrect it can lead to detonation all over the shop

i managed to find this info off another forum

"

From my experience RB's are notorius to knock based on their cylinder head design alone. Whats good for response, and fuel economy isnt always the best for high timing.

The quench pads in the cylinder head love to heat up when EGT's are high, and when timing is too far advanced or to retarded. They act almost like a glow plug and hold in heat more so on the intake side. When the fuel is squirted into the combustion chamber some of this air/fuel mixture can get trapped between the piston and quench pad. As this mixture is compressed when the piston is traveling upwards into the glowing hot quench pad it can be pre-ignited, this occurs right before the spark is actually fired. This small amount of pre-ignition isnt shown on an a/f meter since its getting expelled with the rest of the fuel burnt by the actual spark on the exhaust stroke. So the a/f meter might be reading pig rich when actually the engine is pre-detonating its head off.

The next issue is the fact that the quench pads take up most of the flack from this small amount of pre-det so it goes almost unseen on the spark plug readings. Therefore a good knock monitoring system should be used. I have in the past been against them, but after burning a couple of motors here due to this same pre-det issue caused by a CAS/ECU related error I have decided to go to one next time around.

If anyone cares to see what type of damage I'm talking about look for my past build thread, it shows plenty of it on the cylinder head, and pistons.

"

[rob82]

I got told the head got minimum taken off it with turb stuff (what does that mean?) and wouldn't have been over 0.5mm.

So you don't think it would be higher compression than 9.1:1?

What about the compression test results of 170psi? I remember doing a compression test years ago and I thought I only had like 130-140psi. Like I said I will redo this compression test later on.

That cranking compression is fine for a fairly well std RB25. Alot of the evo's are around 175-195psi with a static of 8.8:1. Cranking compression has more variables than just static compression ratio.

[PM-R33]

you should investigate the quench distance/area in your head, pretty sure its the distance between the piston and cyl head at tdc if this is incorrect it can lead to detonation all over the shop

i managed to find this info off another forum

"

From my experience RB's are notorius to knock based on their cylinder head design alone. Whats good for response, and fuel economy isnt always the best for high timing.

The quench pads in the cylinder head love to heat up when EGT's are high, and when timing is too far advanced or to retarded. They act almost like a glow plug and hold in heat more so on the intake side. When the fuel is squirted into the combustion chamber some of this air/fuel mixture can get trapped between the piston and quench pad. As this mixture is compressed when the piston is traveling upwards into the glowing hot quench pad it can be pre-ignited, this occurs right before the spark is actually fired. This small amount of pre-ignition isnt shown on an a/f meter since its getting expelled with the rest of the fuel burnt by the actual spark on the exhaust stroke. So the a/f meter might be reading pig rich when actually the engine is pre-detonating its head off.

The next issue is the fact that the quench pads take up most of the flack from this small amount of pre-det so it goes almost unseen on the spark plug readings. Therefore a good knock monitoring system should be used. I have in the past been against them, but after burning a couple of motors here due to this same pre-det issue caused by a CAS/ECU related error I have decided to go to one next time around.

If anyone cares to see what type of damage I'm talking about look for my past build thread, it shows plenty of it on the cylinder head, and pistons.

"

Ok that sounds very interesting.

So when you say the distance between the piston and cyl head at TDC, this would mean a high CR correct?

See when you say they take of 0.25mm when resurfacing the head, this means to get the same CR I should have used a 1.5mm (1.45 to be exact) head gasket? So in essence my CR would be higher tham 9.1:1 yeah?

An NA RB25 runs 9.5:1 correct?

Edited July 10, 2010 by PM-R33

[snozzle]

do u know if ur head got skimmed before you had it skimmed this time round? sounds like it has been skimmed more than 10-20thou - which you definently need to make up for with a metal head gasket

id also run arp head studs as oems arent too flash on high boost.

its a hard concept to understand but an incorrect quench distance results in pre-ignition NOT because of the consequent CR. unfortunately i know more about the sr20 quench pad operation as ive built one before but im quite new to the rb25/26 combustion chamber design, i have not built one before so i cant give you very specific advice, sorry

[rob82]

you should investigate the quench distance/area in your head, pretty sure its the distance between the piston and cyl head at tdc if this is incorrect it can lead to detonation all over the shop

i managed to find this info off another forum

"

From my experience RB's are notorius to knock based on their cylinder head design alone. Whats good for response, and fuel economy isnt always the best for high timing.

The quench pads in the cylinder head love to heat up when EGT's are high, and when timing is too far advanced or to retarded. They act almost like a glow plug and hold in heat more so on the intake side. When the fuel is squirted into the combustion chamber some of this air/fuel mixture can get trapped between the piston and quench pad. As this mixture is compressed when the piston is traveling upwards into the glowing hot quench pad it can be pre-ignited, this occurs right before the spark is actually fired. This small amount of pre-ignition isnt shown on an a/f meter since its getting expelled with the rest of the fuel burnt by the actual spark on the exhaust stroke. So the a/f meter might be reading pig rich when actually the engine is pre-detonating its head off.

The next issue is the fact that the quench pads take up most of the flack from this small amount of pre-det so it goes almost unseen on the spark plug readings. Therefore a good knock monitoring system should be used. I have in the past been against them, but after burning a couple of motors here due to this same pre-det issue caused by a CAS/ECU related error I have decided to go to one next time around.

If anyone cares to see what type of damage I'm talking about look for my past build thread, it shows plenty of it on the cylinder head, and pistons.

"

Thats post is a little ambiguous - first he assumes there is a problem with the combustion chamber design incorporating the quech pads and then he goes onto say that he's torched two motors due to bad CAS/ECU. The bad CAS/ECU are a known problem on all skylines and I too have had issues with high rpm ignition scatter. But I've never believed there is any issue with the quench pads/sqiush bands.

I can tell you that removing quench pads on high comp 4valve motorcylce heads will only cause more detonation. And from what I've seen you could correlate the results of bucket actuator 4valve motorcyle heads to that of the RB series 4valves heads.

If I were you I wouldn't be concerned about running the factory head gasket even up to 25psi if you have head studs. I would stay away from dropping the comp with a thincker head gasket as you are reducing the squish - which wont help at all!

Is there a reson your concerned about what has been done to the head thus far? Is there an issue with detonation or the engine not liking timing?

[The Dan]

A head skim is only going to take off 5 thou. 10 thou is considered heavy. None of this 0.25mm crap.

Just because you made more hp before the head was ported, doesn't mean the person you got to port it knew wtf they were doing. Most instances they have no frickin idea and do more harm than good....like opening up the ports too far....I'd be worrying less about CR and focus more on the actual cause of the detonation.

[PM-R33]

do u know if ur head got skimmed before you had it skimmed this time round? sounds like it has been skimmed more than 10-20thou - which you definently need to make up for with a metal head gasket

id also run arp head studs as oems arent too flash on high boost.

its a hard concept to understand but an incorrect quench distance results in pre-ignition NOT because of the consequent CR. unfortunately i know more about the sr20 quench pad operation as ive built one before but im quite new to the rb25/26 combustion chamber design, i have not built one before so i cant give you very specific advice, sorry

Yeah running ARP head studs and it was the first time the head had been removed, so first time it has been resurfaced.

I'm thinking that yeah maybe a bit of meat was taken off the head.

If I were you I wouldn't be concerned about running the factory head gasket even up to 25psi if you have head studs. I would stay away from dropping the comp with a thincker head gasket as you are reducing the squish - which wont help at all!

Is there a reson your concerned about what has been done to the head thus far? Is there an issue with detonation or the engine not liking timing?

It's not so much about the boost, it's the fact the car simply won't take any timing to make power.

The strange thing is when the head first got put back on it made more power throughout the rev range (as in i got a dyno chart with more power). However with that same tune it was detonating hard and really it didn't have much timing in it and had quite rich afr's.

It's not so much about me wanting to drop the CR, it's more about trying to get the CR back to what it was if it is indeed higher than what it should be.

A head skim is only going to take off 5 thou. 10 thou is considered heavy. None of this 0.25mm crap.

Just because you made more hp before the head was ported, doesn't mean the person you got to port it knew wtf they were doing. Most instances they have no frickin idea and do more harm than good....like opening up the ports too far....I'd be worrying less about CR and focus more on the actual cause of the detonation.

Is there a way I can tell how much has been taken off the head? What if it has been more, ie 10-20 thou?

It was a fairly reputable place that did the head so I'm not to worried about that. Like I said it is mainly the fact we can't run timing into it without it detonating. I mean NO timing what so ever. We than tried running my new nitrous setup (as that was the point of getting it redynoed) and the car absolutely hated it, kept detonating no matter what AFR or what timing was in it (the jets were even ran on the rich side but no luck).

To me everything is pointing to the CR being a lot higher than 9.1:1 the more I think about it. Would these symptoms be correct in thinking that though? As in lets assume there was a bit taken off the head and I have bumped up compression to something silly like 9.4:1, with something like that it would keep detonating on high boost with timing put into it wouldn't it?

Also since I havn't mentioned it, the camshaft timing has been checked a handful of times as that was the first thing I thought of and it really does all appear to be spot on so I don't think it is that.

Edited July 11, 2010 by PM-R33

[rob82]

No way the base timing on the CAS has moved?

[PM-R33]

Another thing we thought of and checked with a timig light. Even re did the base timing (locking it on the ECU and redoing it) just to check but it's not that either.

Checked the CAS aswell as I have had one die on me in the past, wasn't that.

The Mafia on these forums has said he will have a look at it when I take it down to him in Mackay or something. Just trying to rule out this CR thing as there wouldn't be much he could do if that was indeed the problem.

Unless the harmonic balancer is f**ked and actually aint showing the true timing location at the bottom of the belt. Suppose it wouldn't hurt to take it all off and check the actual timing belt location on the crank snout.

Edited July 11, 2010 by PM-R33

[The Dan]

What ECU?

Have you checked the trigger angle is correct?

[PM-R33]

Sorry man, what's the trigger angle?

It's using a Microtech LT12S.

[The Dan]

Sorry man, what's the trigger angle?

It's using a Microtech LT12S.

Trigger angle is the angle before/after the trigger at which the coil/injection fires. If the figures are wrong or the setup in the ECU was changed, this means your timing will be way off.

Try checking the timing while it's on power on the dyno.