My engine has been bored 20 over, greatest bore that Nissan makes stock pistons for. After experiencing some blowby, I checked compression on all cylinders with the following result: 150 lbs on all but cylinder #4 which was 145 lbs. Compression is within normal limits and normal differences between cylinders (128-173 limits and 12 lbs difference if I remember correctly).

My question is this, if the engine is bored then would that not decrease my compression given no other changes to the cylinders/rods/etc? I'm trying to figure out if given the bore, if 150 lbs is the max compression achievable or if I'm really mid way within tolerance of a turbo engine. If I am below peak norm on compression, would this translate into ring issues? Clear as mud?

 

when I did compression check on my old vq they always came up to about 180 to 185 but the spec's you give are correct though

 

it would decrease compression, i dont think that the rod and crank will be any issue, this is die hard stuff downthere so i heard, the compression depends if you have the stock dished turbo pistons in there or the flat top na ones

 

Is there any way to calculate what my maximum compression should be given a 20 over bore?
Then, how do I calculate what my compression ratio is? So, if I'm running 150 psi, it is equal to.....what? 178psi = 9.0:1? Thanks for all the help!

 

what stock pistons are you running?

 

I probably should not have used the word "stock." I am utilizing Nissan oversized dished pistons (and rings) specific for VG30ET engines that have been bored 20 over. So, the only thing that has changed is the bore of the cylinder walls and subsequently the increased piston size which are manufactured by Nissan. Hope that helps so my questions can be answered!

I think they also sell pistons for 10 or 15 over bore as well. Although, I should say that Nissan USED to sell oversized pistons for the VG30E (and ET). I was told they have discontinued these pistons due to lack of sales. That's why it took me almost 6 months to get a total of 6 pistons from Nissan. They had to do a country wide search for anyone who had even a couple in stock. They finally rounded enough up for my 6.

 

Lets get one thing straight, given no other changes, an overbore increases compression. This is because you are compressing more air into the same space.

As for calculating your compression ratio, there are several sites that will allow you to plug in all your specifics and can give you a very accurate answer.
Just google "calculate compression"

Since you had your engine overbored, I hope you told the shop the type of piston being used so they could calculate the proper piston-to-wall clearance. If they assumed you were using forged 4032 or 2618 aluminum pistons, your piston-to-wall clearance will be too wide, resulting in lots of blow-by and low compression numbers.

I noted you sited the same compression pressure for the Maxima VG as the Z31 VG. The VG30ET had 7.8:1 and a 8.3:1 compression ratio. I dont know for a fact that the VG30ET has less compression pressure than the Maximas 9:1 compression ratio, but I would assume so

 

You're right about the Max/Min compression I listed. I knew they are close between turbo and non-turbo (3-4 lbs difference) so I listed the limits I could remember. The actual TURBO compression limits are: 125-169 lbs. Non-turbo limits 128-173 lbs. Either way, I'm still within "normal limits" but below maximum compression for some reason. Although, I have a thought as to possibly why......

I installed an aggressive cam. The slight lope shouldn't affect compression because this occurs during the exhaust duration of the stroke. However, the intake valve stays open for a slightly longer duration which, during a compression check, I would expect to push some air back out the intake valve, therefore reducing the compression. Where on a milder cam, there would be more air in the cylinder due to the valve closing sooner which would increase compression. Of course this cam on a turbo, allows more time to force air into the cylinder under normal operating conditions to better utilize the turbo. I think this is why I am getting a slightly lower compression whether a bore increases or decreases compression. I will have to read up further on that. Your point is well taken Maxpower. It looks like I will also need to further investigate whether I am right on the aspect of the cam contributing to my compression numbers.

As far as the type of piston, everything was coordinted between myself, Nissan, and the shop. Nissan sent 6 oversized pistons and ring kit for the specific bore on the engine. Any blow-by I'm getting has been tracked down to the PCV system and too small of lines venting the crank case previously.

Update: As far as searching on how to calculate compression ratio, there is alot of information that I cannot locate in my FSM for the Maxima or the Nissan z31. All the FSM's will tell me is 1. Bore: 3.43 (stock) and 2. Stroke: 3.27. I assume a 20 over bore equates to 3.45?

I still need the following information.
3. Connect Rod Length 4. Cylinder Head volume (cc)
5. Deck Height 6. Head Gasket Bore
7. Head Gasket thickness 7. Piston Dome Volume (cc) - if dished (it is)
8. Piston to Cyliner Wall Clearance.

Of course some of these I could do without and still get a CR. But, this is the info that is asked by nearly every site as well as the demo program I downloaded. Can anyone help plug in some numbers?