dawalkster

iTrader: (1)

So one question that's been on my mind recently is what exactly compression is, and how you tune it. I understand that boosted cars need a lower compression so that it can handle high amounts of boost; and high revving all motor cars usually have high compression. What else should i know?

Also, say i wanted to lower or raise compression - how would one go about doing that? I realize its not a simple thing to do. And once you do lower/raise compression how do you test to see what your compression is?


help a noob out, or atleast give me links to places where i can learn the answers

Nealoc187

iTrader: (23)

you calculate what your static compression is. it's just a mechanical calculation, combustion chamber volume at BDC vs TDC. if the volume is 10 times bigger at BDC than TDC, you have 10:1 compression. If the volume is 8x bigger at BDC than TDC, you have 8:1 compression.

You can change it (static compression) by changing the thickness of your head gasket, milling the head or block, pistons of different combustion area design, etc. You can't "tune" it, you have to actually change parts to change it.

JClaw

iTrader: (17)

Increasing displacement with the same volume combustion chamber is a real winner because it also yields higher compression.

nismology

iTrader: (8)

This statement is rather vague.

eng92

iTrader: (3)

Just a roundabout way of saying that you can get more power by "stroking" an engine

nismology

iTrader: (8)

Understood but i didn't quite understand how it was germane to what the OP was asking about.

f550maranello2

iTrader: (2)

changing pistons to deep dish also changes the compression... same with connecting rods...

DAVE Sz

iTrader: (2)

I just used Seafoam.

YellowNinja

seafoam wont do anything but restore the original compression in the engine. so it wont really be changing it.

nismology

iTrader: (8)

It won't even do that.

He was being sarcastic, BTW.

GoalieKeg

iTrader: (4)

I put in octane booster and injector cleaner at the same time and it gave me .5 seconds in the 1/4 mile, it also polished my heads for me.

MaximaSE96

iTrader: (8)

if u want to lower u can also use 3.5 heads on a 3.0...but there are many ways....to raise...milling heads....different pistons made for Higher or Lower Comp.....thicker head gasket.....stroker kits for the 3.5 guys....

BEJAY1

iTrader: (19)

Are there any aftermarket head gasket's avail for the VQ30DE? I was just reading about a CRX that used a .025" thinner gasket for about 1pt compression increase.

YellowNinja

haha d*mn it, now i feel like a dumb a$$.

Luquire

So if i was going to use forced induction like nitrous on the max i should use the 3.5 head on my 3.0.

SPiG

iTrader: (7)

To go through the trouble and expense of changing the heads you might as well use the entire VQ35 block. The VQ can hold a lot of boost if tuned correctly. The only reliable way on a VQ to change compression is to use aftermarket pistons.

Different thickness head gaskets aren't the best idea as they will affect cam timing and possibly fitment of the lower IM and the timing chain covers. If we had inline engines with adjustible cam gears like RBs, SRs and hondas then we could do this.

nismology

iTrader: (8)

I personally wouldn't use heads with a larger bore than the block. The name of the game is to sqeeze the a/f mixture toward the center where the spark plug is. That's what the squish area is for. With that hybrid setup, some of the mixture would sneak out to the periphery. It could work, but it just wouldn't be optimal IMO.

nismology

iTrader: (8)

It has more do with with the fact that they use timing belts than anything else.

DrTySOUFMaX23

iTrader: (6)

im trying to understand this too but you lost me at "combustion chamber volume at BDC vs TDC"

nismology

iTrader: (8)

It's the ratio between the volume of the cylinder at the bottom of it's stroke versus the volume at the top of the stroke. For example, if a cylinder's volume is 400cc's at bottom dead center and 40cc's at top dead center, it has a compression ratio of 10:1.

MaximaSE96

iTrader: (8)

word......like the vg guys could get away with adjustable cam gears...i dont remember id anyone makes them maybe for the 300zx vgs

datsun40146

You really can "tune" compression, and the best way to increase it is to shave the heads down.

nismology

iTrader: (8)

I would disagree. That's the worst option because of the reasons posted in the second paragraph of post # 16. For these engines, aftermarket pistons are THE option.

speed racer

iTrader: (10)

So if guys wanted to stay all motor with the 3.0 block and raise compression would that mean that one would have to get custom piston and rods or does a manufacture sell them already?

SPiG

iTrader: (7)

OK let me rephrase, If you change the CR through the head or head gasket many parts will not line up correctly since we have a V engine (could be fix through some creative dremeling and machining), and our cam timing will be off because we have a timing chain and can't use adjustible cam gears (not as easy thing to fix).

If you are going to go throught the trouble of building an engine you should probably just get a VQ35. Increasing the CR will not give you that much power for it to be worth tearing the motor apart. Hell for that trouble you should get the 100mm Darton sleeves and pistons with a VQ40 crank or some other stroker kit and get a 3.8-4.3L VQ and have 350-400whp NA with some good cams.The VQ30 has a good amount of potential NA, but the VQ35 does have more and has off the shelf internals that are becoming cheaper and cheaper. Building a VQ30, especially a NA VQ30, just doesn't seem practical.

However you really don't need to think about building any VQ anytime soon. A well modded, EMU tuned, VQ30 with a DE-k manifold could easily have more Hp than most VQ35s and will be plenty fast. Very few Maxima people have built their engine with turbo, and no one has done it for NA AFAIK.

nismology

iTrader: (8)

I understand all this. All i meant was that it's not easy to dial the cam timing back to stock because we don't use a timing belt setup.
I plan on being one of the first. The plan eventually is headwork, 11.5:1's, and cams in a 3.5.