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Old 11-17-2007, 12:50 PM
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Lower compession = lower octane?

Right now my blown 4.6 DOHC is stock 9.8 to 1 compression and requires 93 octane. If I rebuild it and drop it to around 8.5 to 1 could I get away with 87? Right now it runs good and never pings even under boost but I retard timing as boost goes up. I tried 87 once and it pinged even under moderate load (non boost) conditions.

Thanks
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Old 11-17-2007, 12:55 PM
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You would probably have to do a comprehensive computer reprogramming. The reason you can get away with 93 octane on 9.8:1 plus boost is due to a sophisticated package of programming that modifies fuel, spark, timing, EGR, etc.

If you switched to lower compression you might be able to get away with lower octane if you clued the computer in to what you were doing Find a good programmer who has done it before so you can capitalize on your new tune.

Edit: I see that its installed in a rod... what is your engine management?
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Old 11-17-2007, 04:46 PM
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Quote:
Originally Posted by curtis73
The reason you can get away with 93 octane on 9.8:1 plus boost is due to a sophisticated package of programming that modifies fuel, spark, timing, EGR, etc.
Well thanks for the compliment!

It's a Megasquirt EFI system that I built and programmed. It runs the fuel and spark systems, so I have total control. I probably could have made it run on 87 just the way it is, but I never messed with it after I got it running good. I could add knock sensing to the Megasquirt, the 4.6 already has the sensors for it.
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Old 11-17-2007, 05:56 PM
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I didn't think megasquirts could deal with knock sensing. being your motor is so expensive, I think with the crap they pass today as gasoline, spend the extra .30 as cheap inschurance
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Old 11-17-2007, 06:33 PM
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by lowering the compression ratio you can run more advance thus making more power, you can effectivelly stuff more air in the motor and burn it accordingly with the rigth timing not partially with retarded timing, all boosted motors must run low C.R.'s to take advantage of the boost, I'd go with 8:1

about octane, lowering the C.R. will not let you run lower octane, you could do under low boost levels, but why, you'll losse the power the compressor gives, run high octane and as much boost as the blower can push and enjoy your motor.

you will not need to reprogram your ECM, it will take control automatically, if it does not sense pinging it will hold the timing at the rigth point.

I asume it can handle fuel enrichment under boost, correct?

Augusto.
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Old 11-17-2007, 06:58 PM
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Since you have total control over the system add the knock sensors, and adjust it to give more fuel pulse width. If there are larger injectors available you might be able to use the compression ratio you have allready. Those motors run great stock. Keep me posted as to what you try and the results. I was thinking of using one of those in my 73 Grand Torino Sport with a super charger.
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Old 11-17-2007, 06:58 PM
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Quote:
Originally Posted by Augusto
by lowering the compression ratio you can run more advance thus making more power, you can effectivelly stuff more air in the motor and burn it accordingly with the rigth timing not partially with retarded timing, all boosted motors must run low C.R.'s to take advantage of the boost, I'd go with 8:1

about octane, lowering the C.R. will not let you run lower octane, you could do under low boost levels, but why, you'll losse the power the compressor gives, run high octane and as much boost as the blower can push and enjoy your motor.

you will not need to reprogram your ECM, it will take control automatically, if it does not sense pinging it will hold the timing at the rigth point.

I asume it can handle fuel enrichment under boost, correct?



Augusto.
megasquirt runs fixed values over 70% throttle, so. sometimes it would have fuel enrichment, using the o2 sensor but not often under boost.
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Old 11-17-2007, 08:38 PM
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I'd do some research into either retarding your cams, or swap the cams for something "larger".
In both cases you what you will be doing is dropping the dynamic compression ratio. You may loose a bit of lowend torque, but I doubt you're short in that department with the blower. It'll also be cheaper than new pistons.
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Old 11-18-2007, 05:45 AM
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Actually, with the free flowing 4 valve, it is a little lackluster in the low RPM range, its really a high RPM design. The engine was originally designed to use a two stage plenum setup with butterflys that are on 8 of the intake runners. These stay closed until about 3500 RPM, then open up so you have 16 intake runners open. I removed those valve's so they are open all the time and I think that might have hurt my low RPM TQ. The blower is great, but it doesn't help until it runs up to boost. Which doesn't happen under normal driving. I'm thinking it might be wise to reinstall those butterflies and reprogram the fuel table.

The motor isn't as expensive as it looks, it just proves you can shine a turd.

The MS is very versatile and you can program it to do anything, which is good and bad. It doesn't keep you from inputting parameters that can blow it up. Under cruise conditions, the MS runs off a fuel table and makes minor corrections with the O2 sensor (if you have one and have a Air/Fuel ratio table filled out). When there is any enrichment going on (acceleration, decceleration, warmup, etc.) the O2 reading is ignored. I can adjust the fuel table to dump as much fuel as I want at anypoint, so your only limited by the size of your injectors.

Yea, it might sound like I'm cheap, and I am. I like to drive and usually put 5,000 miles on a season, but not so much this year and the cost of gas was part of it. I like to take long trips too, so I'm just thinking ahead for my next project for what I can do to make it more efficient. I'm not so worried about HP. I can make that blower put out all the boost I want, it's so big. As it is now, there is a restrictor plate under the bugcatcher and I'm running 20% underdriven.

Last edited by Arrowhead; 11-18-2007 at 05:55 AM.
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