What's the max CR safe for a P90 head?

[Zmanco]

I've begun porting the P90 head and need to decide how much to shave it for my NA L28. I know most of the write ups suggest taking off .080", but LEngine says that for me (.030 over bore with flat top pistons) that will give me a CR of around 10.3. That's what I currently have with an N42 head and reground cam (advertised as 284/284 .480" lift) and I have to retard timing to no more than 24 degrees to avoid detonation. This is in Colorado where we're always at least 5k elevation. Again, I'm NA, no turbo.

 

I know the P90's combustion chamber prevents detonation better than the N42, but is it possible to go as high as 10.3 on pump gas (91 is the best we have) and not get detonation? If I only shave 1.6 mm (.063") that yields a CR just under 10.0.

 

I'd rather trade a little CR for ignition advance, but don't want to give up any CR if it isn't necessary.

[z-ya]

I don't think you can use LEngine to compute the CR when shaving a head. How are you estimating the head CCs? You really need to shave, then measure the combustion chamber volume. Then you can plug that number into the "head CCs" box in LEngine. Maybe someone has already done this at 0.020, 0.040, 0.060, 0.080? I have a junk P79 that I could donate to the cause.

[Daeron]

my understanding is that an untouched P90 combustion chamber is roughly 42ccs when the head has been shaved .080", but that is JUST a rule of thumb... compression ratio is dropped by increasing the volume of the CC on the head, so any grinding done on the chamber itself will have the effect of lowering it a hair.

[Zmanco]

Good point Z-ya. I've measured the chambers (after unshrouding valves but before polishing) and they are all 54.0 or 54.5 cc.

 

To determine how the volume changes with shaving I traced the outline of the chamber on graph paper and added up all the squares. The surface area is 49 sq cm. That means a 2 mm shave removes 9.8 cc and a 1.6 mm shave removes 7.8 cc.

 

Daeron, in my case, a 2 mm shave would result in 44.2 - 44.7 cc chambers.

 

The CRs I mentioned in the original post were all calculated using my measurements. See screen shots attached.

[Zmanco]

Maybe another way to ask my question is to ask those who have done this (shaved P90 head on a NA L28) what CR are you running and can you run full advance of 35* without detonation on pump gas?

[1 fast z]

My head is shaved .110" plus I have a 3.1 liter with flat top pistons, compression is 12-1 and I run on 91 octane, 36 degrees full timing. My elavation is about 1000 feet, but have run at sea level with no problems.

[Zmanco]

Great! Then I should have no issues with my 2.8l at .80".

 

But this has me wondering: what is the trade off for a closed chamber head design such as a P90? Did Nissan not understand these benefits when the original heads (E31, E88, N42) were designed? Otherwise, why wouldn't they have gone this route from the beginning? Perhaps emissions?

[Tony D]

But this has me wondering: what is the trade off for a closed chamber head design such as a P90? Did Nissan not understand these benefits when the original heads (E31, E88, N42) were designed? Otherwise, why wouldn't they have gone this route from the beginning? Perhaps emissions?

 

Well, we got the P90... Lots of NON-North American Markets got the N42 till the day they stopped production.

 

When the engine was designed, the gas was much better.

 

In 1975, the USA mandated Unleaded Fuels. And had very strict emissions compared with everyplace else on the planet, save for Japan (whose requirements were stricter 75-83...)

 

Everything with engine design is a compromise. Paid for tooling accounts for a lot of decisions in the OEM world. It took Chevrolet over 50 years to reverse their venerated Small Block, even after they did the testing and showed the advantages in the early 60's! What was that, almost 50 years to institute that change?