Okay, from what I have read, creating a hybrid VQ33 using VQ35 crank and rods in a VQ30 results in a deck height of .6 mm (i.e. the top of the pistons rise above the deck by .6 mm).

Can this be adjusted back to zero by using a thicker head gasket? I'm thinking yes, but is this advisable in a boosted engine?

Would it be better to mill .6 mm off the top of the pistons? (I'd rather not buy aftermarket pistons just yet.)

 

I REALLY don't think you want the piston to be sticking out of the block bore! Other than the obvious reasons, you might have valve clearance problems. I don't know if milling 0.6 mm+ makes the pistons too weak or not.

You would be much better served by finding shorter oem rods or oem pistons with a higher wrist pin placement. Or finding aftermarket of one of the two.

 

If you raise the cylinder head with a thicker head gasket, then you won't have any valve interference. I'm worried about possible hot spots that could result in preignition, though, as well reduced gasket strength.

Milling the pistons reduces ring land area, too, something you don't really want to do in a boosted car. But it's only .6 mm, and as long as you keep detonation under control ...

 

But the piston would still be sticking out of the block bore. I can't imagine that would be a desirable thing to do.

 

I'm not sure what the ramifications of that would be.

 

Well one would be the risk of the piston ring coming out. I've never seen an engine with the piston higher than the block deck. Actually all engines I've seen are lower than the deck.

Let's say it sticks out 0.6mm. How much more at temp + 7200 rpm would that equate to?

 

Rod stretch due to rpm and dT can't be more than the thickness of the head gasket. I don't know what the head gasket thickness is, but I bet it's not a whole lot more than about .050".

Judging from this pic (thanks to ejj), it looks like there is plenty of distance before the rings come up out of the block:

 

Domed pistons rise up above the deck (That thang got a hemi? Swaaaayyt.) Of course, that is a bit different than what we're talking about.

 

what it the legnth difference between VQ30 and VQ35 ronds?

 

I know the VQ30DE rod is 147.6 (center distance). Not sure on VQ35. Matt knows the head gasket thicknesses I think steve, you can check with him on the gaskets. I can't find the posts where he had them, thought I had it saved

 

http://forums.maxima.org/showthread.php?t=363610

 

Thanks. If this were a pushrod engine life would be easier. I just can't see where this would be a good setup for a FI engine. Now for a NA setup, it might be a different story. Does the VQ have a 0 deck height stock or is it a -.X? If it is 0 and you go to +.6mm with the VQ35 rods and add a thicker head gasket, or double head gasket, you would get a lower compression, which on the surface seems good for FI, however I would be worried about the same gasket integrity issues that you have raised. I say custom forged pistons.

 

I think .6mm is approx .024 inch, pretty small amount. There are two ways to get to 0 deck height. One way is by resizing the connecting rod big end. This is done by shaving material off the rod and then bolting the cap back on and machining the big end round again (stock size). This has the effect of making the rod shorter. The other way is by offset grinding the crankshaft. This works by grinding the rod journel size .020" or .030" undersize is such a way as to actually reduce the stroke a small amount.

Any competent automotive machine shop will know about both methods - not rocket science by any means.

Paul w

 

If the VQ had independant timing chains for each bank I would say a headgasket change would be worth the effort. When you change the HG thickness you're also changing the cam timing. A thicker gasket will advance the cam timing and a thinner one will retard it. Since a VQ has one chain that drives both banks you'll get an uneven timing change between banks. If bank 2 (front) gets a advance of X then bank1 (rear) will get an advance of X+Y. X being the added differance from the crank centerline to the cam centerline and Y being the added difference between the bank2 and bank1 cam centerlines.

I also can't fathom why anyone would do this complete waste of time hybrid. Of all the hybrid combinations IMO this is by far the worst one to do.

 

That's very interesting about the cam timing. I wonder how much it would really affect power? I'll have to measure the timing gear and calculate how much of an angle change we're talking about.

Thank you for your opinion, which is well received. All I can say is there are reasons why this route is attractive to me.

 

Well then explain them to me in detail (PM) and give me the chance to convince you otherwise.

 

What ever happend too the old thread that went over all this? What comp ratio would a 3.5 block/pistons with 3.0 rods and crank have with 3.5 heads? Im wondering cause i have a VQ35 with a bad rod journal.

 

just a suggestion. you could have the rod modified. have the cap on the rod milled just over 0.6. then have it bolted to the rod and then line bored back to the right journal size. that should put the piston back in the block for you.

later.

 

i'm very interested in working with this on my car for a na setup any more useful info or sites would be breatly appreciated.

 

also, the stock head gasket is less then 0.035.

you could leave it out of the block with a thicker gasket as long as it is the same bore as the cylinder.

the other thing the muscle car guys do is, they shim the gasket. you get a machine shop to make a steel plate to a certain thickness and put it between the block and gasket. you may also need to make reciever groves in the block to line up with the shims.

it will also lower the compression ratio with either way you go