Lower compression change timing?

[CrazyPrime]

Alright so I took my head/block to the machine shop and he decked both. It ended up being 0.10 thousands off both sides 0.03" on the block and 0.07" on the head. I'm planning on using a cometic MLS to make up the difference I have read that .069" is the stock HG thickness. What thickness should I use to return the CR to stock? Should I get a even thicker one and lower it even a little more? (I'm turboing is why I ask) and how would lowering the CR effect the timing? I would assume I could just use the 0.80" MLS and it would return my CR to stock correct? But if I buy a thicker HG would it allow for more boost and be safer then stock?

[digger]

why was so much taken off? i would just add what was taken off to the stock thickness if youre getting a MLS.

[jlevie]

Changing the position of the head relative to the block (as compared to stock), will affect cam timing. That can be fixed with an adjustable cam sprocket.

I'm not into forced induction, so I can't speak about using a thicker gasket to lower compression. From what I have read, whether or not to lower compression ratio is a function of the level of boost you are aiming for. For modest boost, it would seem that you'd be fine by using an MLS gasket that would make up for what was removed from the head & block. Which will restore stock cam timing.

When I build a race engine I'll deck the head the maximum amount to increase CR. That throws off cam timing enough to affect performance. I fix that by a mod to the cam sprocket that restores the original cam timing. These engines are built for NASA Spec E30 rules, where an adjustable cam sprocket isn't allowed.

[CrazyPrime]

The head was warped so alot had to be taken off before it was true. I'm just wondering if its worth it to lower the compression a bit and if I would have to do anything with my timing.

[digger]

Why would you want to lower the cr?

[TobyB]

so much

ten thou is a lot?

Whether you want to lower the compression is entirely a function of

How much boost you want to run
What fuel you're going to use
and
How good your engine management is going to be.

You're stuck with the 2v head, which is pretty primitive compared to modern forced induction engines,
but you can help a bit by cleaning up all the sharp edges.

If you've set an 8-10 psi limit, then no, a little over stock compression will probably be fine.
10-15, get the compression back to stock, or a little lower,
and
over that, you might have some trouble.

he says,

wild- assed guessing....

t

[digger]

ten thou is a lot?

Whether you want to lower the compression is entirely a function of

How much boost you want to run
What fuel you're going to use
and
How good your engine management is going to be.

You're stuck with the 2v head, which is pretty primitive compared to modern forced induction engines,
but you can help a bit by cleaning up all the sharp edges.

If you've set an 8-10 psi limit, then no, a little over stock compression will probably be fine.
10-15, get the compression back to stock, or a little lower,
and
over that, you might have some trouble.

he says,

wild- assed guessing....

t

unless i misread 0.07" (1.78mm) would take the dimples out of most heads, then some

[TobyB]

Oh, shit, you are right- I read thousandths, and he meant hundredths, as in .030" and .070", so about a tenth, .100" total.

Yeah, I take it back- you want a thicker head gasket. Or run N/A and check your clearances
and re- time the cam....

t

[CrazyPrime]

Ok I'm pretty terrible with measurements. 0.010 was how much was removed in total my head still has the dimples.

[AndrewBird]

.010" total, .003" off the block, .007" off the head.

.003" is about the thickness of a sheet of paper or a human hair.

[digger]

just use a stock bmw gasket 2.05mm (i.e. +0.3mm version) with good bolts and forget about it?

[TobyB]

0.010 was how much was removed in total

Then I retract my retraction- you're fine. We should all be so lucky...

t

[Som]

Rollercoaster of reactions on my part reading this thread with the different measurements. :)

Agree with the +0.3mm. For one, it adds about 0.012 back to the height, which is slightly more than the 0.010 you removed -- so the timing should be unaffected.

As for the CR, keep in mind that you removed 0.007" of volume from a "domed" container (in the head) and you're replacing it with 0.009" of cylindrical volume from the new head gasket.

So you have 2 factors that will decrease your CR -- the extra 0.002" from the HG and the cylinder vs. dome volume.

As far as how much the CR is affected, this thread has an M20 CR calculator (http://www.mye28.com/viewtopic.php?t=51939), that allows you to tweak head/HG thickness, but it does say it's assuming cylindrical volume and not the head's CCs.

According to that, you'd be running 8.76 ( vs. 8.8 ) to 1. Accounting for the dome vs. CC bit, I'm guessing you'd be running more like 8.72 (~ 1/3rd of .007).

Som