Shaved Heads for Higher Compression.
 

Im about to get a head thats has a valve job done on it and resurfacded and shaved for a little mor ecompression. How much will this affect the turbo? Will I still be fine Boosting? (If I go that route.)

You may have to pull a few more degrees of timing. I have around .025 shaved off mine...works out to nearly 10:1 compression. I'm finding I have to take out about 14-15 degrees timing for 10lbs of boost.....until I got water/meth injection.

Right now My bipes is tuned for six degree full retard. so do you mean more retard or up more?

With shaved head, you will make more power with the same ammount of boost. You also need to retard your timing more

Ok. I thought that Six degrees was the most you can before it starts running like crap?

Oh, I guess i can tune the bipes for as low as four degrees

are you getting a head to fix your short block?

Naw. I found a Different shortblock that the guy had blown his head gasket, and the block is still good and he had his head resurfaced and a valve job done. So ill be getting that off him and in it will go in my Car
A head to fix bad rings? Im not that stupid : / lol

Wrong direction. The Bipes can adjust the timing under boost anywhere from -4 degrees (least retard) to -14 degrees (most retard).

So let's say hypothetically that your engine is set up the same was as MiataNuTca's. Assuming you're using a 2bar MAP sensor you'd need to select 1.4 degrees per PSI on switches 2,3,4 (equivilant to 14 degrees maximum retard in AFM mode) and hope that your turbo never spikes above 10PSI, which is the point at which the ACU reaches "maximum" retard regardless of switch settings.

That is the case with static retard, where you don't have an active timing controller and are just turning the CAS back to retard the base timing. The ACU only retards when in boost, so you will not suffer any ill effects at idle or cruise.

When you said "never spikes to 10" why is that?

Because the ACU is maxed out at 10PSI. Beyond that it will not be able to pull any more timing.

Are you using a MAP sensor or are you in AFM mode?

Afm.
When you say "Beyond that it will not be able to pull any more timing"
Do you mean it will stop retarding? Lets say it retards six, then four, and by four your at 10psi, and then it just stays at four?

Ok, well then it's not quite as precise, but same concept...
Yes, but read further.
I think you're approaching this from the wrong standpoint.

We tend to think of the ignition timing in terms of the "base" timing, which is 10 degrees BTDC. But even in a bone-stock vehicle, the timing does not remain at 10 BTDC all the time- it advances based upon RPM and retards based upon load. 10 degrees BTDC is just where it happens to be at no-load idle, which is the point where we calibrate CAS.

So hypothetically if you rev the engine up to 5000 RPM with no load the timing might now be 25 degrees BTDC, and if you then start lugging up a hill in fifth gear it might retard back down to 20 degrees BTDC, or maybe 15 degrees or even less. (I'm making all these numbers up since I don't have the actual data in front of me, but it's the concept I'm trying to convey here, not the precise figures.)

When I read your message saying "Lets say it retards six, then four..." I get the impression that you mean "The actual ignition timing goes from 10 degrees BTDC, down to 6 degrees BTDC, down to 4 degrees BTDC..." but this is fallacious because the timing wasn't at 10 degrees to start with- the stock ECU has already advanced it somewhat because it's no longer at idle.
Since the ignition curve of the stock ECU is shrouded in some degree of mystery, we typically do not deal with absolute ignition timing when working with piggyback controllers. Instead we deal with a certain amount of retard relative to whatever the ECU ignition angle happens to be.

I realise you're using AFM, but for the sake of simplicity let's say hypothetically that you are running MAP, and that you have the ACU set to 1.4 degrees per PSI of retard.

At 1 PSI, the ACU will delay the ignition by 1.4 degrees. At 2 PSI it will delay the ignition by 2.8 degrees. At 5 PSI it will delay by 7 degrees, and at 10 PSI it will delay by 14 degrees.

At this point we don't know what the actual ignition timing is, because we don't know exactly what the stock ECU is doing. All we care about is that we are delaying the ignition by a certain amount based on pressure, relative to whatever the stock ECU thinks it should be, because we are accounting for a variable that the stock ECU does not understand- boost. The ignition tables in the stock ECU were written based upon the assumption that the pressure outside the throttle plate is always atmospheric. Since we are changing that assumed constant, we must provide our own compensation for it.

14 degrees is the maximum amount of delay that the ACU will provide, so whether you are using MAP or AFM, by the time you get to 14 degrees of retard, there will be no further retard applied for any additional increase in pressure. Thus, it is important that your engine never run in a condition that requires more then 14 degrees of retard, because we cannot provide it. Based upon MiataNuTca's observations for his engine, this point was reached at 10PSI. Bear in mind also that he has an intercooler. I didn't see one in your list of goodies.

Wow.....I dont know if I want this head now.....Seems too complicated. And no, no intercooler. Everytime I start saving up for a stripes piping kit, Something comes up. I want this head because its all prepped and valve job done and what not, but I dont even want to think about being in the same boat i am now if I do spike higher then ten.

You shouldn't be spiking to 10psi if you are just running off the wastegate. And before you start boosting more you need to get an I/C or WI (just for you Magna) which will allow you to run more advanced timing. Don't add an MBC to up the boost until you have it more sorted out, otherwise you'll be doing this all over again.

I have a formula at home (I'm at work now and can't get to it) that shows how compression affects boost. It will give you the theoretical numbers of what you can actually run. Something like you can run 10 psi with 9:1 compression at 10 degrees and that is the same level of knock as running 8 psi with 10:1 compression at 10 degrees. I'll try to remember to post it in the morning.

You can always use a thicker head gasket to drop the compression back down and maintain the benifits of this head your after.

Joe, does anyone here know someone at Bell or FM that might have that information? I think it was Corky (Bell), Justin (FM), or Henry from AEM that told me the Eclipse curve was very close to the Miata curve fwiw. If anyone is interested, here's AEM's ignition map from the basecal file they provide with the EMS.

damn, that was hard to find... it's been years since I last looked at it :D

http://members.aol.com/solomiata2/BoostCompRatio.gif

What do the numbers in the chart represent? na equivalent compression ratio?

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

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

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.

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.
???

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.

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

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

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.

-1 :nono:

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

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