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Discussion Starter #1
I am asking for some help on a questions. I have a 94 Stock BPU+ Supra with stock ECU. I am thinking of running 87 Octane with a Methonal injection kit. Bring the Meth on line at 8 psi. My question is can I run 14 psi of boost on a daily bases with out any problems?

Thank you.


F16Supra
 

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Depends on if you have tuning to go with it. If you can tune your timing and your fuel, you could definately optimize a setup like that, the problem is that 87 is so completely different than 93 that you'd be taking a huge hit in power and response at the end of the day just to run that low of octane.
 

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Discussion Starter #3
Thank you. I understand what you mean. My boost controller ( how fast it spools up) and where the psi the meth come on is the only two things I can control. I can not tune the timing or fuel. Is a hit in power my only problem?

Thank you very much

F16Supra
 

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I dunno, I would think that using that low of octane the car would knock a lot in the lowend, off boost, light throttle conditions. The bottom half of a factory timing map is really where the factory spends the most time tuning, and driveability is one of their top concerns, hence why timing and fueling are so important there and consequently why you can ramp up the topend in your car so much with mods....one is conservative and rich, the other finely dialed in. Losing that octane in the lowend is going to hurt because your cruising timing and fueling is going to make the car knock most likely.
 

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Discussion Starter #5
Timdog1650 said:
I dunno, I would think that using that low of octane the car would knock a lot in the lowend, off boost, light throttle conditions. The bottom half of a factory timing map is really where the factory spends the most time tuning, and driveability is one of their top concerns, hence why timing and fueling are so important there and consequently why you can ramp up the topend in your car so much with mods....one is conservative and rich, the other finely dialed in. Losing that octane in the lowend is going to hurt because your cruising timing and fueling is going to make the car knock most likely.
Thank you very much for your info and time.

Final conclusion. NOOOOOOOOOOOOOOOOOOOOO! I like my F16 too much.

Thomas AKA F16Supra
 

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Timdog1650 said:
I dunno, I would think that using that low of octane the car would knock a lot in the lowend, off boost, light throttle conditions. The bottom half of a factory timing map is really where the factory spends the most time tuning, and driveability is one of their top concerns, hence why timing and fueling are so important there and consequently why you can ramp up the topend in your car so much with mods....one is conservative and rich, the other finely dialed in. Losing that octane in the lowend is going to hurt because your cruising timing and fueling is going to make the car knock most likely.
Are you sure about that? knocking while cruising? :faint:
 

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I know it sounds counterintuitive, but the majority of knock comes from lugging the engine at low speeds/high load. In a car like a supra, it's logical to think that boosting the crap out of it and adding timing would cause knock....it does, but when you take a car like that and run 87 in it, you're introducing a whole new level of crappiness to your fuel system. There is a big difference between "29 psi, 25*, 11.9:1 afr's and your car knocked"-tuner knock and "we cut 6 octane points out of your fueling setup and even the car's base timing below 2500 RPM is not allowing it to accelerate without detonation." The first is the one we all hear about because we drive the cars to the limit, the second is rarely ever considered because pretty much noone uses low test gas.

Now, let me qualify all this by saying that if you setup your meth to spray based on RPM and throttle position, so like....above 20% throttle and between 1000rpm and 4000rpm, you could safely run 87 as long as your boost was at a rock bottom low wastegate pressure(assuming stock timing).

Professor Jim Cowart at the Naval Academy my "IC Engines Theory" teacher and former powertrain production engineer/ecu tuner for Ford gave us a set of simple rules to follow when thinking about knock and octane based on data collected using government standardized testing platforms:

1 octane =~ 1* spark
5 oct. =~1 compression ratio
1 oct. =~7*c air temp
1 oct. =~10*c Coolant
2 oct. =~1afr
3.5 oct. =~3 inHg or 1.5psi (Manifold Absolute Pressure)

So using these rules, you can add 1* spark timing accross your map if you have 1 more octane, assuming you're tuned to the very edge of knocking accross the baord. You can add 1 compression ratio point if you add 5 octane before you see knock, assuming you're tuned to the threshold, so on and so on...

So logically, assuming stock timing and fueling requirements under boost, if you lower your boost 1.5psi you can cut 3.5 octane. If you lower it 3-4psi, you can cut 7 octane. That means you can run safely WOT at 4psi less than stock on 87 octane if you roughly equate this test data from a research engine to the 2jzGTE. Now, when you think of all this you have to take into account that the engineers did not design these cars to the very ragged edge, hence why a boost controller and exhaust is such a good mod, this test data assumes absolute knock threshold is reached, therefore you can probably get away with more in the real world than you could in this data. You still have to think about spark though, and if you ditch 6 octane points by switching to 87, in a vacuum/cruising/light load situation you would need to retard your timing 6* to make it safe on that crap gas. That's a whole ton of a lot, assuming 1* timing = .5% torque. If your engine is making 45 to 50 ft-lbs of torque at a low speed/high load cruise condition, cutting those 6* of timing would only reduce your torque by about 1.5 ft-lbs, but would make the car feel unresponsive and sluggish in throttle transitions.

I dunno if all of this makes sense as I type it, it sounded better as it was explained to me in class, however, I think it's safe to say that without something like a standalone (AEM) or specially designed internals to optimize lower octane gas and slower burn rates, you'd be causing more driveability and knock problems than it'd be worth. Best way to cut your gas bill is to drive your car less or never step on the brake ever.

~Tim
 

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FWIW you can run straight water without meth....

-M
 
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