What is the best comp ratio for 280-300rwkw?

[BOOSTD]

What is the best comp ratio for 280-300rwkw?

[Bl4cK32]

-Joel- was looking at this as well. I think it worked out like if you had 8.5:1 it would be "safer" for the engine. But it would take more boost to get it going. If you had 9:1 comp it would have better off boost performance, less boost to get it going, BUT it would have to be tuned good to prevent knocking....(like any engine i suppose)....

[T0nyGTSt]

Is this an interesting question.

The GT-R is 8.5:1

You can make what you will of engine response compared to the RB25DET.

An extreme case would be the F40 (7.8:1) or the VL Turbo (7.5:1).

You would get laggy with the same turbo, gears, cams.

Cost/benefit as always.

T.

[Steve]

Matt, I have just had a HKS 1.6mm gasket done, CR 8.6:1 - which is similar to RB26 CR. Off boost response is not noticeably different.

I did it purely so that I could turn the boost up to 1.5 bar + and still have a safety margin. It works on the RB26.

[BOOSTD]

Matt, I have just had a HKS 1.6mm gasket done, CR 8.6:1 - which is similar to RB26 CR.  Off boost response is not noticeably different.  

I did it purely so that I could turn the boost up to 1.5 bar + and still have a safety margin.  It works on the RB26.

Ive decided to go with CR 8.5:1

[JimX]

Steve, did you O-ring it as well? Or do you think it's not necessary with the thicker gasket and lower compression?

[BOOSTD]

Steve, did you O-ring it as well? Or do you think it's not necessary with the thicker gasket and lower compression?

I was told o ringing the block was a waste of time if you had a metal head gasket. Has anyone seen a RB25 blow a head gasket? SR is suited to o rings ,but, apparently wasted on an rb25.

[Roy]

Has anyone read into the ceramic (HPC) coating the combustion chamber and valve faces?

Theory goes that less heat is transferred into the head and subsequent water jacket, improves flame travel and combustion effeciency, and provides a denser air/fuel charge by preventing overheating of the exhaust valve and transferring heat from the intake valve to incoming cool air and fuel. (Claims from HPC)

So looking at alloy head development and turbo cars, amongst reduction in hot spots and the many design quirks for making an effecient combustion chamber/head, the temperature of combustion has always been a critical factor.

So with the ceramic coating dropping the combustion temp by dropping head temp etc, it stands to reason that you may be able to creep a bit more boost pressure into an engine running say 9.0:1 compression ratio.

I wish i had the coin to test all these little theories.

[Sydneykid]

Hi guys, we always try to run as high a compression ratio as we can. Around 9 to 1 seems to be the best compromise at the moment, it used to be 8.5 to 1 when you couldn't buy 98 octane fuel. Now you can, so no need to go down that low anymore. That's one of the reasons why VL Turbos were 7.5 to 1 'cause there was only 92 octane then.

Using 9 to 1 compared to 8.5 to 1 is a potential 6% increase in exhaust gas velocity. So we are not just talking about off boost performance, we are also affecting turbine speed and acceleration (to maximum boost).

Everybody has an opinion on this, 9 to 1 happens to be mine, and my personal RB31DET is 9 to 1. I see no reason to change for a road or circuit car up to 1.8 bar. A drag car running over 2 bar, then there is a case for lower compression.

As for o'ringing being "a waste of time", that is unbelievably stupid. There are far more competition engines in the world with o'rings than there are without. There is only one reason I can think off telling anyone not to o'ring an engine. That is so you could sell him an expensive head gasket instead or 'cause you don't know how to o'ring or don't have the equipment.

What did you pay for your metal head gasket? Bet it was more than a standard head gasket and a set of o'rings. Answer is not "waste of time" the real truth is "I make more money".

Hope that adds to this interesting thread.

Yes Steve, I know that you knew I was going to put the case for 9 to 1 didn't you?

[Sydneykid]

Almost forgot Roy, ceramic coating the combustion chamber and the piston crown does not lower the combsution temperature. Ceramic coating the crowns prevents the temp getting into the pistons and therefore the oil. Ceramic coating the conmbustion chamber prevents the temp getting into the head and therefore the water (and a bit of oil as well).

So the engine runs cooler (oil and water) but you actually have higher combustion chamber temperature. The exhaust gas is hotter (the extra temp has to go somewhere) and has more velocity. You know what that means.

Hope that helps

[Steve]

Yes Sydneykid, I was waiting for it:) I suppose I am a bit detonation paranoid, if I can get a large margin for error, without costing me too much power, I will gladly take it.

Jimx, I didnt o-ring, as I believe the HKS gasket is more than adequate without o-ringing. If I was dropping CR by different means I would look at getting o-ringed with stock gasket, but I didnt want to spend a fortune on head work or rebuild the engine, ie, took the quick fix. But if it works, why not?

[Roy]

So the engine runs cooler (oil and water) but you actually have higher combustion chamber temperature.

OK i follow, how about this then? If an engines head has poor cooling then the thing will typically run hotter and ping more readily when be it ignition or boost is increased. Correct?

So the actual exhasut gas temperature remains higher as more heat (energy) is being contained within the combustion chamber, and once the exhaust valve opens, the hotter exhaust gasses run thru the exhaust helping to increase the torque on the turbine.

This is as opposed to a combution chamber that doesnt have any sort of thermal barrier/ceramics/HPC coating which some of the heat is transferred thru the aluminium head and into the water galleries, and perhaps oil. So HPC helps build boost/torque/increase effeciency of the combustion process.

So how does this correlate to the onset of pinging, there would seem to be some evidence that says it increases the onset of pinging, other evidence that says it may reduce the onset of pinging.

Call T1- the inlet temp, T2- the compression temp, T3- the ignition temp and T4- the exhaust temp.

So this is my guess...anyone else help with these assumptions?

Comparing HPC vs std head, approximations of temps could be...

HPC

T1 - 40 deg C

T2 - 80 deg C ( i cant be bothered doing the calc, but it is increased due to the compression, anyone know by how much given a rough compression ratio of 9.0:1)

T3 - 950 deg C

T4 - 910 deg C

Std Head

T1 - 45 deg C (head runs hotter, heat transfer into inlet air)

T2 - 80 deg C (increased slightly as heat is transferred from the the head/water galleries)

T3 - 950(decreases slightly as heat is transferred into the head/water galleries)

T4 -860(decreases slightly as heat is transferred into the head/water galleries)

Is this making sense? Im sure its only a small % difference, but any process improvement comes back to measurable gains, even small gains.

[Steve]

OK, lower temp in the pistons, but what about the thermal characteristics of the surface of the ceramic coating? would it heat up more than a standard piston? same with valves?

If the coating prevents the transfer of heat into the pistons/head, wouldnt it also therefore prevent some cooling? Increased heat after the exhaust valve shuts could be nasty, even if it does mean greater exhaust gas velocity you will have a higher charge temp on induction - yes?

I dont know the answers, but I think the questions should be asked - it is very rarely that a mod gives without taking somewhere:)

And BOOSTD, sorry for the postwhoring, but you have finished with your question yes?

[Roy]

Im only guessing but...

OK, lower temp in the pistons, but what about the thermal characteristics of the surface of the ceramic coating? would it heat up more than a standard piston? same with valves?

Im not considreing ceramic coating the pistons, only the combustion chamber and all valve faces, and perhaps the exhaust valves' stems. But the following should hold true for either

The coating has negligible mass, which means it cannot absorb large amounts of heat, and subsequently transfer it to the incoming air. So it is a barrier that stops heat from transferring into the head which has the mass to allow it to act as a heat sink.

Im not sure, but if such a coating allows you to reduce the inclination for an engine to ping whislt running higher compression, then it could be a good thing. Against the agro of removing head, cutting valve seats etc.

[Sydneykid]

Hi Roy, I think I need to explain something about heat barriers and combustion processes.

On an engine with ceramic coating, at the time of combustion, the temperature in the combustion chamber is the same as an engine without ceramic coating. After the explosion, the temperatures stay hotter longer. Due to piston action, the spent gases are exhausted and leave the cylinder, but they don't loose as much heat on the way as in a non ceramic coated combustion chamber.

Obviously camshaft over lap helps here with the inflow of new fresh air and fuel rushing in and helping to push the exhaust gases out.

The smooth finish of a ceramic coated combustion chamber (over a standard cast one) certainly reduces the engine inclination to pre ignition as there are no sharp, pointy, hot spots.

Ohh and Steve, that is one of the secrets to running 9 to 1 compression ratios, the polished combustion chamber actually makes the engine less likely to pre ignite than an 8.5 to 1 combustion chamber with dags casting marks, sharp edges and the minor bumps and dips you get in a standard head.

Back to Roy, there is another thing to consider, a ceramic coated engine runs cooler in both water and oil, so the incoming charge, inlet manifold, inlet ports are all cooler. This helps offset any minor increase in temperature just prior to combustion.

Finally, the exhaust is hotter and therefore it exits the combustion chamber faster. This improves the scavenging and further lowers the temperature as there is less exhaust left in the combustion chamber and more (cooler) inlet charge.

Hope that adds some more

[Roy]

Thanks, ill try and tidy up my rambling :Oops:

[JimX]

Ok, so someone wanna summarise this? Come rebuild time, you polish the inside of your head and coat it and valves in ceramic. Actually, wanna go one further and do the pistons/rods as well? Shotpeen the rods and put both rods and pistons through that cryogenic freezing process?

Anything else?