det suppression/ tuning tips

[slostar]

hey, heres what ive got: 1500cc 16v 4cyl engine with gt25 turbo. has a simple single stage wi setup.
at the moment its running just under 16psi with around 9 degrees advance on top a 10 degree base at that point. but has pretty much come to the limits of the fuel(95ron). as rasing the boost or timing gives light detonation. (listening through a detset) have yet to try retarting the timing some more and up the boost
was running about 180cc/m of straight distilled water. have uped the water to around 300cc/m but had little if any effect on the det. has 1800cc of injectors hitting a max of 75% at this stage. afr sits between 11.8:1 to 12:1 under full boost
is there anything im missing? or time to buy some race gas any ideas would be great.

injection point is just past the bend by the battery.

[hotrod]

Do you know what your ideal timing is for that engine?

There is really only one ideal timing for a given engine and fuel burn speed. Once you find it you should be very slow to crank in additional timing.

You might back off the boost to the point you cannot create any detonation with any reasonable timing and then run some tests to find your best mechanical timing ( ie puts the cylinder pressure peak at the ideal time after TDC)

If you have a safe environment ( ie enough octane or low enough boost) find the timing where you basically find no improvement in power with added timing, then back off the timing until you can just detect a hint of power loss. (about 1% power drop) that is your minimum best torque timing, and is the safest place to begin tuning from. Leave your timing fixed at that value and start running up the boost and playing with fuel air mix and injection rate. Once you have a good combo sorted out, then "tweak" the timing a bit to see if the minimum best torque timing has moved some ( burn speed changes with boost -- faster as boost goes up, and slows down with more water injection). There is a zone near 12.2 :1 AFR that the engine is most prone to detonation. You may be able to both pickup power and reduce knock by leaning it out some now that you have WI.

Have you looked at your EGT's for you engine ?
The engine is usually most detonation prone just slightly rich of max EGT. You might want to back off boost a bit and then tweak the fuel mixture a bit and see how close you are to that critical fuel air mixture that makes the engine most detonation sensitive. In general aircraft they do a lot of tuning based on where they are compared to maximum EGT. The engine will have peak cylinder head temps (on an air cooled engine) when the AFR is slightly richer than max EGT so that EGT is 30 or so deg F cooler than max. Max power is usually achieved rich of peak EGT, at about 80 degrees F cooler than max egt. Max tendency to detonate is about 50 degrees F cooler than max EGT on the rich side.

In short you are generally most prone to detonation just ever so slightly leaner than max power fuel air mix. If you start out well rich of max power mixture and then start leaning it out to make big power on WI you will pass through this "island" of detonation prone mixtures before you get lean enough to really make good use of your WI.

Just some thoughts on things you might want to experiment with.

Larry

[slostar]

thanks for the advice, just what i was after. helps alot.
havent got a EGT prob, but think i will look into getting one now. but will turn down the boost and have a play with the afr and timing for the moment, will let you know how i get on.

kris

[JohnA]

Another issue you might want to investigate is water mist distribution.
Do you know which cylinders are most prone to det? If it's always the same one, then perhaps all the water goes to the other ones.

Remember that water is much heavier than air, so it doesn't follow tight bends very well (more inertia).
So without a direct port injection your cyl1 might be getting totally different water quantities than cyl4 :wink:

[slostar]

good point. im guessing i'll know if the wi doesnt help the knock at all. then i'll consider going to a direct port setup. intake temp isnt really a problem anyway.

did some more tuning tonight. turned the boost right back to 8psi leaned the afr to around 12.5:1. then starting from about 10 degrees (+base) timming, slowly advanced it to a max of 20 degree where light det started. kind of hard to know when it stops making more power on the road (nearset dyno is 3 hours away..) but to me it felt as tho it was still gaining power, right the way till detonation. backed it off to 18degrees. did the same thing again but on 13psi. got to 14degrees and started to detonate. backed timming to 13 degrees. car does go a whole lot better now tho. seems to go just as well if not better than it did on 15 -16psi

no water injection was used, seemed to loose power when tuned on at those boost levels. will try going back to the smaller jet and see what happens from there..

[hotrod]

Best timing is frequently found about 2-3 degrees retarded from where you see detonation.

One low tech way to get relative power indications is what I call my "hill dyno"

I found a steep hill that has a good straight road with a good legal speed limit. I have one that max speed is 55 mph and another where it is 65 mph.

Then I found a stretch were I could pick out some reference points. In my case a couple road signs. I pass the first road sign at a given rpm in a specific gear, and then accelerate at WOT until I reach the second sign. By setting it up so the rpm range and gear put good load on the engine and force it to pull through the rpms you use most on the road, you get a good reasonably safe, and even legal if you set it up right road dyno. Price is right too.
As you gain power your elapsed time and max rpm at the second reference point will both change, time dropping and max rpm going up. Its reasonably repeatable, and sensitive enough to help you find that point where additional ignition advance does not make any significant improvement.

A few days ago I got to watch a friend do a dyno tune and by optimizing ignition timing, and AFR he made more power on 2 less psi boost without detonation. Boost does not always equal more power.

Larry

[slostar]

yeh it seems i have to take a good 1 degree advance out for every 1psi boost. i'm not sure at what point, rasing the boost and retarding the timing has negative effects. guess i will have to keep testing to find the best compromise. turbo defintly has alot left in it. witch is slightly annoying since its hardly in its efficiency range.

car is a bit ruthless to do the "hill dyno" at a legal speed. as it starts too wheelspin unless a higher gear is used. or its on a lower boost setting. (uses staged boost for traction, but thats no help when tuning high boost) so have to use the brakes to load it up a bit. hopfully i can get hold of a laptop this week so i can datalog a few things. should make it easyer


w.i seemed to help a little. but not much. maybe my jet is in the wrong position, its about 400-500mm from the throttle body on the bottom of the intercooler pipe. going closer help? or distribution problem as john says.. inlet runners sit flat as you can see in the pic. is it possible some of the water is falling out suspence in the plenum?
need to try a few more things with it first tho, bad weather has only let me do a quick test with wi

[slostar]

did a couple of runs with and without water injecton. interesting how it leans the afr. they may not be line up quite right being done on the road. then overlayed

first one, both runs were done on same flat road in 3rd gear at 3000rpm and finishing at rev limiter(7400) . solid line is with wi. dotted without. water and full boost come on at around the 4sec mark


same thing but on a hill