Decking Block

[Sean]

Ok, I know I've mentioned this before - but now that my motor is out and at the shop getting the work done (I tore it down - all on my own! :P), I need to figure this out.

I'd like to raise compression to around 9.0 to 9.2 from the 8.5 I'll have with the SuperEta pistons.

Questions with this - how much CAN I shave off without having to deal with timing issues? if no one here knows, is there someone I should call?

Secondly, how much would I need to shave to hit between 9.0 and 9.2?

And, what are the effects of decking the block and shaving the head? Sure, it'll raise compression - but no matter what the timing will be off a bit - how will I make up for this? I can shave a little, and the timing won't be too far off to cause damage - but won't it still be "off" ?

I think that's it for now! :)

[e30 gangsta]

I would like to know also. Seeing as i just pulled the motor and trans out for the 2.7 stroker rebuild.

[EtaSport]

10 thousandths is usually resurface limit. Anything over that will alter timing a bit. I believe 25 thousandths will change timing 2 degrees but will give you close to 9.0:1. 2 degrees isnt bad if your running a higher duration cam, but stock timing is still best. You could get an adjustable cam gear, or modify yours.

[Sean]

10 thousandths is usually resurface limit. Anything over that will alter timing a bit. I believe 25 thousandths will change timing 2 degrees but will give you close to 9.0:1. 2 degrees isnt bad if your running a higher duration cam, but stock timing is still best. You could get an adjustable cam gear, or modify yours.

Since I'm not going to be shaving anymore off my head, only decking my block - I could deck that much off my block and hit close to 9.0 then?

An adjustable cam gear will run close to $200 won't it?

How would I go about modifying mine? Couldn't I just turn the cam a little up top when I remount the belt - or is that a horrible idea? lol

I'd like to get a hotter cam in the future, just cannot afford it right now. When i do that, I'll also rebuild the top end completely - right now it's been cleaned and sealed (valve seals that is).

Should I even bother doing this? Will i see any benefit at all? Maybe 3 hp?

[MrK]

I've decked the block enough to need a camgear (as I think you're aware), and I've had to get a cam gear AND dial in a bit of spark retard to avert the consequent pinging all over the place. Not worth it IMHO.

Sure I got more compression which = more power, but at what cost?

cam gear: $275
smt7 (for timing): $650 + customs

JUST to get it running smoothly again.
But then again I'm running 10.7 CR lol

Hope that at least served as a reference, if not direct help.

[Sean]

I've decked the block enough to need a camgear (as I think you're aware), and I've had to get a cam gear AND dial in a bit of spark retard to avert the consequent pinging all over the place. Not worth it IMHO.

Sure I got more compression which = more power, but at what cost?

cam gear: $275
smt7 (for timing): $650 + customs

JUST to get it running smoothly again.
But then again I'm running 10.7 CR lol

Hope that at least served as a reference, if not direct help.

10.7? eh? What pistons you running? I'm gonna be running the following setup:

I Throttle Body
I Intake Manifold
M30 AFM
S50 Injectors
Resurfaced head - no shaving
SuperEta Rods (Eta)
SuperEta Crank (Eta)
SuperEta Pistons
New SuperEta Piston Rings
Stock headgasket
I ECU with Stock chip (may get Alpina B27 Chip soon)

I was hoping to bump compression up to around 9.0 - 9.1 just to get it up a bit, or at least closer to IS compression. That way I could leave room for a future turbo down the road.

If 25 thousandths hits close to 9.0 10 thousandths would get close to 8.8 wouldn't it?

I don't think I could hit 10.7 very easily, not with SuperEta pistons. Thats what I wanted to know more details about your engine. :)

How much power you putting down?

[MrK]

10.7? eh? What pistons you running? I'm gonna be running the following setup:

I Throttle Body
I Intake Manifold
M30 AFM
S50 Injectors
Resurfaced head - no shaving
SuperEta Rods (Eta)
SuperEta Crank (Eta)
SuperEta Pistons
New SuperEta Piston Rings
Stock headgasket
I ECU with Stock chip (may get Alpina B27 Chip soon)

I was hoping to bump compression up to around 9.0 - 9.1 just to get it up a bit, or at least closer to IS compression. That way I could leave room for a future turbo down the road.

If 25 thousandths hits close to 9.0 10 thousandths would get close to 8.8 wouldn't it?

I don't think I could hit 10.7 very easily, not with SuperEta pistons. Thats what I wanted to know more details about your engine. :)

How much power you putting down?

That's it. I PROMISE that I'll get an appointment with a dyno tomorrow morning! And to answer your question, I'm running high CR domed and valve-reliefed pistons 8)

Until dyno day comes up, my butt-dyno is registering about 40 whp over stock. So I'm expecting about 190 at the dyno. (Hope I'm not dreaming lol)

[Sean]

Wow, I hope you get at least that much! I'm hoping to hit around 170 to the crank, but we'll see. :D

It was informative nonetheless. Any suggestions on wehre to buy an adjustable cam gear?

[nando]

I was told that usually you only need to do 4 thousands to get an M20 block flat. that's not enough to worry about timing issues.

[EtaSport]

Sean, moving one tooth on the cam gear is supposed to alter about 9 degrees. I was talking to a guy on e30 tech who rotated his stock gear 182 degrees after decking .025" and drilling new holes. He claims "after 4 hours of dyno tuning at +,- 2,4,6,8 degrees, stock is the best setting."

[Sean]

Nando - what do you mean by flat?

EtaSport - so, decking .025" could still use stock timing? What did he benefit from it? Is the HP really that noticeable of an increase?

[nando]

inline six blocks are pretty long, they will tend to warp a bit over time. by flat I mean flat - you don't want an uneven or warped surface for mating the head to, or it won't hold a headgasket for very long. I was worried about it but after talking to metric mechanic it's not a big deal to have it shaved enough to make it flat. I also decided to go with their 86mm pistons rather than JE. :p

[Sean]

86mm pistons?? DAMN! That sounds sweet. Hmm...I'd love to get different pistons. Would really love that. You're buying MM pistons, then? Having the block decked about .010" and installing MM 86mm high CR pistons?

How much are they?

Makes sense now. I'll maybe have the shop deck the block close to .020" - would I still be able to use stock timing for that, and it would probably raise compression close to 8.8 right?

[EtaSport]

The altered timing may cancel itself out with the higher compression. But you can probably get away with it just fine.

[Sean]

The altered timing may cancel itself out with the higher compression. But you can probably get away with it just fine.

To have the block decked .020"?