Stephen Max

iTrader: (59)

I'm thinking about flycutting some cylinder heads by about .025" in order to raise compression to about 11:1 on a NA VQ35 engine I am putting together for a guy who is going to drop it into a 4th gen.

I know it will retard cam timing a bit, but only by less than a degree, so that shouldn't do much.

Also, the car has a NA JWT ecu, so ignition timing is more advanced than stock.

Anybody have any strong opinions one way or another about doing this? What is a practical CR limit to shoot for, keeping in mind that the car will be running on 93 octane fuel?

98SEBlackMax

iTrader: (9)

Thats a great idea to raise compression ratio and not increase the mass of the pistons. 11:1 is a mild bump in compression and I think it would be ok. They sell 11~11.5 compression pistons for the VQ35, though I have not heard of anyone using them. Heck Cosworth now sells a high compression piston for the VQ35, I think it's rated at 11:1. I've bumped up timing quite a bit and with almost no detonation occuring with +8 ~ +10 timing on 93 octane. I would think a little bump in compression shouldn't hurt.

But raising or lowering the height of the heads changes the cam timing. Doesn't it make one bank retard while the other one advances? IIRC when they were talking about making thicker custom head gaskets to lower the compression on a VQ30 and this problem came up.

Stephen Max

iTrader: (59)

It will retard the cam timing for both heads, but because the heads are at a 60 degree angle to each other, moving them down towards the crank also causes them to get closer to each other by the same amount. So the cam timing on the left bank ends up being retarded twice as much as the right bank. But I can take account of that when I drill the intake cams for the dowel pins.

96sleeper

iTrader: (4)

11:1 I would think would be fine, but I was always told it was a bad idea to cut heads on a OHC V-engine. I inquired about it when I was doing my engine. Besides changing the timing, is there anything else? With higher lift cams, is there clearance for the valves?

I think I have seen custom thickness head gaskets for the VQ35, it might be a better option is there are no other repercussions.

nismology

iTrader: (8)

It's doesn't matter if you cut the block, cut the heads, or put in a thinner HG. The same issues are going to arise no matter how you do it.

Kevlo911

iTrader: (36)

But replacing a head gasket is much easier then cutting the heads.

nismology

iTrader: (8)

I was referring to the first part of his post when he said it was a bad idea to cut OHC heads.

96sleeper

iTrader: (4)

I know, thats why I said the HG would be easiest if there were no other repercussions (meaning if timing was the only thing affected and you could deal with it, HG is easier than cutting)

You always come across like everyone but you is an idiot. You might mean it, you might not.

nismology

iTrader: (8)

I thought you were saying that the only reason you'd wanna put in a thinner HG was because cutting the heads was "bad". My mistake...
I don't mean it. Sorry for coming across that way.

Stephen Max

iTrader: (59)

The problem with the head gasket idea (for raising compression, at least) is that the VQ35 head gasket is already only .036" thick. So if I want to move the heads in by .025", that would require a head gasket that is only .011" thick. That would be like shim stock. Is it even possible to get a head gasket that thin?

96sleeper

iTrader: (4)

Cometic has a .030" head gasket that is multi-layered (MLS), wonder if you could peel off a layer or two to get what you need? lol.

They also make a .040" and a .054" gasket. I haven't seen a smaller one yet.


edit: JUN's stroker kit says it comes with a 0.7mm head gasket, which is .0285".

JClaw

iTrader: (17)

Increase the displacement instead

Stephen Max

iTrader: (59)

Sounds expensive.

pawnstar12

crowers 4.15 stroker its a little less than $4k but thats without the needed sleeving but its way cheeper than that aebs kit

sonicrunch

You are gonna have to mill the lower IM as well.
Well, maybe not have to, but the might be an issue.

Why not have some welding done to add material to the combustion chamber?

Stephen Max

iTrader: (59)

Oooh, very good point, I'm glad you mentioned that. That may just put the kibosh on the whole idea.

nismology

iTrader: (8)

Why would you have to touch the lower intake manifold?
Has this ever been done successfully?

Stephen Max

iTrader: (59)

Moving the heads down also causes them to get closer to each other by the same amount (for a 60 deg V configuration). So there may be fitment issues with the LIM. Maybe not enough to worry about, now that I've had time to think about it. I'll do a test fit tonight with the heads on but no head gasket. That would simulate flycutting the heads by .036".

nismology

iTrader: (8)

Ah ok. That makes sense. Thanks...

Kevlo911

iTrader: (36)

I think it will still work just because of the SMALL amount. But you never know. Glad someone even brought it up, I didnt even think about it.

sonicrunch

I do not know. It's one of those things I thought about just before I fell asleep.

What I do know is cracked aluminum heads are successfully repaired by welding and grinding. This includes the combustion chamber.

So, why not add material and even reshape the chamber?

UK-nissan

If you milled the combustion face, then you would mill the intake face pro-rata (2:1??) and the IM would still mount properly, you would need to elongate the chaincover holes though.

Mike

pawnstar12

for that cost just get custom pistons made

Matt93SE

iTrader: (44)

call up JE and buy custom pistons. it'll cost about $1100 but you can get them in any CR you want.. an they'll be much stronger than the stockers and able to take the abuse you're planning to give them.

sonicrunch

True, but you will get more power from a smaller combustion chamber than a domed piston

pawnstar12

really? even if the cr was something like 14:1?

Kevlo911

iTrader: (36)

I am 99% sure you can't use that for pump gas...

Hoooper

although that would most likely work for changing the cr, i would think that it would make a drastic change in the flow characteristics and take away any power that might be added due to the increased cr.

UK-nissan

Some people have coaxed 12.4:1 to run on pump gas with a standalone, but over 11:1 it gets quite hard to do without losing power because you cant put enough advance on.

Mike

nismology

iTrader: (8)

UK-nissan

Well this thread has drifted into discussing how much the compression can be raised for N/A use. Each engine responds a little bit different, but that is a ballpark figure of how high you might get away with, if everything else is done right. What bit do you want me to explain further. Engine efficiency and power increases with compression ratio, until you get too much detonation, then you have to knock back the timing, hence you lose the power you might have gained at that higher compression ratio.

Mike

Mike

sonicrunch

agreed

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