so, what are the propor readings for a "safe" tune? - Miata Turbo Forum - Boost cars, acquire cats.

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Old 02-04-2007, 09:45 PM   #1
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Default so, what are the propor readings for a "safe" tune?

12.4 afr, <10psi on 9:1 compression, and no knock (15*btdc)?

I'm just curious. I hear that lots of people tear stuff up on too aggressive of a tune, but I don't see how if its stoichiometric, and no knock.

Sorry, I'm a newb.
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Old 02-04-2007, 09:53 PM   #2
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At high rpm a car makes max torque at 13:1 and max hp at 12.5:1. I read a sae paper on it one time but I dont have a link to give you. Another reason for a non stoich mixture in boost is because the extra gas actually helps cool the cylinders to help prevent knock. Also, the extra gas helps delay the explosion so that a hot edge or other deposit doesnt pre-ignite the mixture, much like the previous reason.
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Old 02-04-2007, 09:54 PM   #3
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Quote:
Originally Posted by neogenesis2004 View Post
At high rpm a car makes max torque at 13:1 and max hp at 12.5:1. I read a sae paper on it one time but I dont have a link to give you. Another reason for a non stoich mixture in boost is because the extra gas actually helps cool the cylinders to help prevent knock. Also, the extra gas helps delay the explosion so that a hot edge or other deposit doesnt pre-ignite the mixture, much like the previous reason.
so I should tune the thing to like 12:1 under boost to be on the safe side...or is that too fat?

I also read in the megamanual to tune to like 17:1 under cruise. Is that right?


what kinda afr's do I want under load for both natural aspiration and fi?

Thanks for the help on this.
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Old 02-04-2007, 10:02 PM   #4
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also, how do I tune ms for decel and cruise if we don't use a TPS? How will the MAP show this? Do all map's show vaccum?
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Old 02-04-2007, 10:05 PM   #5
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17:1 is safe for a cruising, but I wouldnt go over that. I got 32mpg at 15:1 on my MS. Anything at atmospheric or below you would be safe at 14:1 under like 5krpm. I personally would tune to 13:1 above 4.5k below atmo. Above atmo, below 4.5K I have it tune around 13.5:1 and then above I tuned to 12.5:1 above 4.5k. 12.5:1 is optimal but to be safe 11.5:1 is what you should tune for really. I had water cooling and that is why I felt safe to tune that lean in boost. What ended up killing one of my cylinders was my timing adv.
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Old 02-04-2007, 10:07 PM   #6
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17:1 is safe for a cruising, but I wouldnt go over that. I got 32mpg at 15:1 on my MS. Anything at atmospheric or below you would be safe at 14:1 under like 5krpm. I personally would tune to 13:1 above 4.5k below atmo. Above atmo, below 4.5K I have it tune around 13.5:1 and then above I tuned to 12.5:1 above 4.5k. 12.5:1 is optimal but to be safe 11.5:1 is what you should tune for really. I had water cooling and that is why I felt safe to tune that lean in boost. What ended up killing one of my cylinders was my timing adv.
thanks man. Can you give me a quick run down of what your time averages at say cruise, decel, and boost? So 13:1 in boost until 4500rpm then to 11.5:1 at say 10 psi is a safe afr if my ignition timing is in check? What do you run for mechanical time...+8?
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Old 02-04-2007, 10:07 PM   #7
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additionally, places on the map like the right bottom corner you can run pretty lean because you will only e in that part of the map when you are going downhill and under next to no load at any speed.
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Old 02-04-2007, 10:09 PM   #8
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cruise is easily like 70% of the time. You'll hit boost when you want to and going up hills. With the small turbos we have youll easily be under enough load to hit atmo at only like 30% throttle. So boost I'd say is even with decel. But that depends on where you live. I go to school at VT which is in the middle of the appalachin mountains so i go up and down hills all day long when I drive.
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Old 02-04-2007, 10:14 PM   #9
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My car made its best power at ~13:1 on the dyno
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Old 02-04-2007, 10:16 PM   #10
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ok, so i will set up pulse width to correlate with boost pressure, but how do I get my afr correct since boost and rpm are independent variables? What does "boost is even with decel" mean?

thanks so much for the help, I can't seem to find this info anywhere else.
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Old 02-04-2007, 10:39 PM   #11
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Quote:
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ok, so i will set up pulse width to correlate with boost pressure, but how do I get my afr correct since boost and rpm are independent variables? What does "boost is even with decel" mean?

thanks so much for the help, I can't seem to find this info anywhere else.
im not sure what any of the terms you just mentioned are.....the VE tables are absolute pressure (kpa) vs rpm (rpm...:P). The resulting afr is dependent upon the cell the MS is reading at the time. So you just make a datalog using one of the base maps on the settings forum and then you can tune those cells to the appropriate AFR. With megalogviewer you can view your datalog, and ve and ign tables all at the same time. At any time t in your datalog it will show you what cells it is reading from in the corresponding table so that you can tune that exact part of your map.
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Old 02-05-2007, 04:39 AM   #12
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I wouldny try 17:1 on a Miata motor. 17:1 might be ok for an oldy low compression Vette engine with big-bore cylinders, not so for ours. Be on the safe side, stick to 15:1 at cruise.
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Old 02-05-2007, 02:15 PM   #13
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Quote:
Originally Posted by neogenesis2004 View Post
At high rpm a car makes max torque at 13:1 and max hp at 12.5:1.
That's very dependent on the engine and head design. Also, power is a function of torque and engine speed... so max torque across the board should also give you max HP.
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Old 02-05-2007, 03:21 PM   #14
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The add to that the variables of what different gauges/senors read. This is why egts are often seen as a valuable tuning tool- just one more thing that can tell you what your changes are doing for combustion. My process is to datalog, then sit down at the computer and compare the log for desired results, make adjustments and then repeat. The key is to make small adjustments to meet your goal.
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Old 02-05-2007, 05:53 PM   #15
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Thanks guys. The more I talk about it and get input from here, the more feasible it seems.
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Old 02-06-2007, 04:12 PM   #16
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Quote:
Originally Posted by hustler View Post
12.4 afr, <10psi on 9:1 compression, and no knock (15*btdc)?

I'm just curious. I hear that lots of people tear stuff up on too aggressive of a tune, but I don't see how if its stoichiometric, and no knock.

Sorry, I'm a newb.
I'd probably shoot for 11.8-12 but with a few more degrees of timing, I usually go static at 15* when I get lazy and then only when I'm in the 15+ psi zone.

Buy a jewelers loupe so you can examine your plugs for specks of piston start the timing low and work up until you get scared or you see little ***** of metal stuck to the plug and then back it down a bit. Pull them after each run while you let the car cool. But you should also lower timing a few degrees around peak torque.

You guys with experience tuning Miatas feel free to shoot that down as the closest I've come has been a turbo MX3, what I've said above is just general crap I've gleaned screwing around with other cars over the years.
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