jif

static compression down the LHS, boost across the top row, and dynamic compression as the product in the table.

eg you get roughly the same dynamic compression at (9:1 + 15psi) and (11:1 +8psi) ...

[edit] to actually answer your q, I'm not 100% but your guess looks right :
http://en.wikipedia.org/wiki/Compres...pression_Ratio

rmcelwee

iTrader: (1)

Here are some notes I had at home. It is a quote off a thread or web page I saw years ago. I have no idea where I got it. The 9:1 pistons mentioned in the last line are the pistons in Beast (from an automatic Miata):

Higher static compression creates more power throughout the rpm band, but it'll lower your maximum allowed boost before the onset of detonation. Boost is worth way more power than compression, because boost raises your compression and your total air flow at the same time. With the down side of, when you're not on the boost, you have slightly less power.
Effective Compression Ratio = static compression ratio x (1 + boost/14.7)^1/2

For a car running 8.5:1 pistons and 18psi(~max on pump gas)
8.5 x (1 + 18/14.7)^1/2 = 12.67 ECR

If you run 9.0:1 pistons and want to maintain the same 12.67 ECR (~max on pump gas), you'll have to lower your boost to: 14.4psi
[(12.67/ 9.0)^2 - 1] x 14.7 = 14.4 psi

So you have to run 3.6 psi less boost to maybe pick up a tinny bit of bottom end. Or to take it even further for 9.5:1 you can only run 11.4psi. I'll tell you right now that the difference between 11.4psi and 18psi is huge. And at some point (12.67:1 in this case) you can run no boost and be maxed out on ECR for pump gas. And how fast is a N/A car with 12.67:1 compression, ask the Honda boys running 15's.


Therefore:
Stock Miata 9.8:1 pistons = 9.87 psi
My LC 9:1 pistons = 14.4 psi

Joe Perez

iTrader: (8)

Nice find- that's a very cool chart. Which engine is that for, exactly?

The closest data I've seen from FM is the chart in the documentation for the non-piggyback Link, which gives ignition advance for RPM vs. KPa. I don't know where they got that data from however- it might have been the result of dyno sessions rather than observing the stock ECU. And I doubt that Bell has anything like this available since the XCEDE is just a piggyback (although in fairness I've never actually asked).

I believe that someone once was on a project to reverse-engineer the ROM section of a stock 1.6 ECU, however I don't recall seeing any actual data come out of that effort.

The biggest point of confusion for me has always been that the stock ignition curve is based off a "load" parameter, which is a pretty arbitrary concept given that AFM voltage varies with RPM for a given manifold pressure.

If someone had a lot of time on their hands, it would actually not be too difficult to slap an EMU onto a stock engine and datalog the whole thing. That's basically what I did to reverse-engineer the AFM voltage vs. MAP/RPM table when I did the writeup on AFM removal. It took about two weeks' worth of "average" driving, plus another couple of evenings to crunch all the numbers and build the table.

Unfortunately I do not have my CAS wires connected to the EMU so I've never been able to chart actual ignition advance. Someday I may go back into the dash and put those wires in, but the way I've got my EMU mounted it's a real trial to get to.

MiazdaBLUE

hmmm...so the forged pistons i'm getting are 8.5:1 does this mean i will be able to run more boost at the same amount of retard as someone with higher compression pistons?

this wont necesarrly make the car fast will it since it is compensating for the lower compresssion.
???

Chamuko

Was the head shaved and prepped before he blew the HG or after? If its after, its worthless, you will have to take it in to the machine shop again.

Also, with a shaved head not only does it affect compression, but on some cars the timing might be a bit off, im not sure if this is the case in the miata.

And to answer the last guy, yes, you will be able to run more "boost" to make up for the compression loss.

High comp low boost engines are fun for faster spool, more "oomph" while out of boost and overall just a fun car to drive when fast response is needed.

Low compression and high boost has a bit more "lag" but tends to win out in the power output category. This "sudden" rush of power feeling is increased even more if you have a bigger turbine.

personal preference really.

Slidin'Miata916

iTrader: (-1)

After he blew the hg. Why would it need to be taking to a machine shop again>?

Braineack

iTrader: (62)

head is aluminum, block isn't, the head will warp when the gasket blows.

Slidin'Miata916

iTrader: (-1)

I just emailed him asking if he had it checked or not. He said he blew it comming up in his driveway. But we will see what he says.

lazzer408

iTrader: (1)

-1

Braineack

iTrader: (62)

let me elaborate....if overheated the head will warp...

Slidin'Miata916

iTrader: (-1)

He emailed me back saying it was all checked and resurfaced.