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?