engine timing for cruising and very low load acceleration

[wrxtacy2003]

Hello everyone

I have a question about how to determine how much USEFUL timing can be run while cruising (highway situations), and under low load acceleration, mainly in regards to when using e85. The car specifically in question is a 2005 Subaru legacy gt, the engine is a turbocharged 2.5L, very similar to the sti engine and almost identical to the 2.5L wrx models. I will be referring to tuning using e85 from here on out. When I tune my car for WOT, I use road dyno software to tune for MBT. Normally though, I have just used the factory timing values for cruising and loads under 5psi of boost. Well, my new motor i'm building is going to be 9.8:1 CR, compared to the factory 8.2:1. The problem with using e85 is that when tuning, on both subys and evos atleast, our knock sensors are QUIET!!! when using e85. So, if you do not tune for MBT using road dyno software, you can continue adding timing and the knock sensor wont show anything, and you'll go right past MBT and just be causing excessive cylinder pressures(obviously not good). (If you don't believe this, go read threads among threads and pages among pages on both evolutionm.net and nasioc.com, this discussion is not about this). So my problem is, I don't want to be running excessive timing values at cruise or low load when I put my higher compression motor in, since the factory values are for an 8.2:1 engine. I would also like to do the opposite as well, meaning maybe the factory left some timing on the table for safety purposes, and because the values they have in these areas are for pump gas and not e85. Finding timing values is not an issue at high loads/WOT because I can just use the road dyno software to find MBT. So my plan was to attempt to use the road dyno software at VERY slight accelerations, say engine vac is around -10mmhg, -5mmhg, and 0 vac, and then into 2psi, 5psi, etc. (at all these values my car will actually accelerate, and not just be "cruising") so I figure the road dyno software should be able to record HP/TQ and thus I can attempt to find MBT for these low load conditions. I have never used any of the road dyno softwares for anything other than WOT pulls. So, i'm just preparing for it to maybe give funky/inconsistent readings that will not allow me to find MBT for these areas. So, I'm looking for other ways of how to go about doing this,in preparation for epic failure of the road dyno software? Also, I am just interested in the subject as well. How does the factory find these values? Using and engine dyno?

Thanks

Dustin

 

[Lou Scannon]

An alternative pretty accurate method you could try is BSFC or BSAC.
Both of these will be minimum at MBT (I'm assuming steady state closed loop operation). You'll probably need a decent stretch of flat road, and the ability to log fuel and/or air flow. I would test at constant speed (need light traffic for this). I would first do a series of back and forth runs with everything held constant, and evaluate the repeatability of the data. That will indicate whether the method is viable, and allow you do determine how many repeat runs are needed to get a sample that is statistically valid.
You can map specific speed-load points by varying the gear and road speed. For higher loads at a given engine speed (that aren't on the road load curve), you could hold the accelerator pedal constant and hold vehicle speed constant by modulating the brake pedal. I would allow time for the brakes to cool between these runs.

OEMs will perform the base steady state mapping on an engine dyno, where there are the fewest uncontrolled variables.

 

[wrxtacy2003]

Thanks hemi, I'll try that if the software doesn't pan out.

Dustin

 

[patprimmer]

The timing that will maintain the set speed on the same road and conditions at the highest manifold vacuum and lowest injector duty cycle will be pretty close.

Regards
Pat
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[wrxtacy2003]

Thanks Pat

Dustin

 

[LionelHutz]

I agree with what Pat posted. Usually easy to monitor on a fuel injected vehicle too.

 

[Lou Scannon]

Agreed. Highest manifold vacuum = minimum BSAC; lowest injector duty cycle = minimum BSFC, engine speed & load being held constant.

 

[patprimmer]

Hemi

I know. I rewrote it sans acronym for the layman.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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