I hope it's not low class to quote myself, but I posted the below in a thread in another forum. Any input to add to my understanding of timing would be welcome.

A number of factors play into optimum timing. What you are trying to achieve is maximum cylinder pressure at ~14-18 degrees after TDC. Sometimes, you can't add enough timing to do this without causing detonation. With a higher compression ratio, the motor will tolerate less timing before detonation occurs. Ditto for high intake air temps. A combustion chamber which is smaller, and more efficient combustion chamber (the GMPP "Fast Burn" is a good example) needs less timing to achieve max pressure at the correct time as does a smaller bore. Fuels which burn faster need less timing. The higher the rpm, the more timing is needed. A leaner mixture needs less timing (it burns faster), and so on.

In any case, no matter what, optimum timing has to be determined empirically and can't simply be calculated in advance. I doubt even the OEM's have a sufficiently detailed model of each combo to simulate it. Contrary to what you often see on the board, optimum timing cannot be determined by simply continuing to advance the timing until you experience knock and then backing off. More timing, even without detonation, is not good if it advances max cylinder pressure too far towards (I used the misleading word "beyond" in the other thread) TDC.

This is one of the many reasons why tuning is best done on a dyno. You can tell if you are making more hp or less with each change and you also can instrument so you know what the conditions are (IAT's, etc.).

One other point just FYI. The less timing needed to achieve max cylinder pressure at the optimum point the better. That's one reason why those GMPP "Fast Burn" heads work well, they need very little timing to make hp. The earlier the spark, the earlier the cylinder pressure starts to rise. Keeping in mind that this begins before the piston reaches TDC, you can see how power is lost as the piston is forced up against the already igniting fuel/air mixture. In an ideal world, instanteous complete combustion would occur with the piston at ~14-18 degress ATDC. Of course, this is impossible. But the closer you can get, the better.

Rich Krause

 

Hey Rich,

I'm doing as much digging on info before I make my first dynotune. I'm gald you posted this. After I set the baseline A:F, how should I go about dialing in timing? Should I be very concerned with adjusting A:F as I dial in the timing?

(This probably requires a long reply - skip down if you don't wanna type a bunch! )

Optimum timing = most power?

IOW, the right timing to use would be the timing were the ___ (Rwhp? Rwtq? Both?) is greatest at that MAP/RPM?

I don't think its low class .

Ryan

 

Nice post Rich.

For my non technical response to the last reply:

best timing = best HP on the dyno or highest MPH at the track.

 

Simplified tuning guide:

Warm engine enough to bring oil temp up before WOT runs, try to have water temp about the same at the start of each run. Water temp should be in the 165-170 degree range. The point isn't to show a big number but to make the runs reproducible. Begin with 28 degrees of timing. Make a run and check the AF ratio. Adjust AF to ~13:1. Make AF ratio adjustments after to each run to try to bring it back to ~13:1. Add two degrees of timing on each subsequent run. Once you see the hp begin to drop, or stop increasing you are almost done. At that point, lean it out a little (to 13.2-13.3:1) and take out a degree or two of timing. You can then try adding the little bit of timing you just took out back.

Keep in mind the effect of density altitiude. If you are tuning with bad air, the motor will be leaner and tolerate less timing on a day with good air. So err on the rich side if the air is bad when you tune and subtract a degree or two of timing.

Rich Krause

 

Rich:

I'm assuming that you keep a close eye on all parts of the graph (hp). After a few pulls you can begin to decide which individual rpm points make more power with a little higher (leaner) than 13:1 and which points make more hp with less than 13:1 (richer). Am I on the right track?

Thanks Rich
Ryan

 

Timing is really only dialed in well on a dyno like Rich said. I can't agree more. Track times or MPH are really not a accurate way to do it, but sometimes the only option. I would tune the car for the max coolant and oil temps you plan on running it at. The heat from them will make the motor detonate on a timing curve that was at the ragged edge on a lower temp motor.

Engines that are sensative to timing will make optimum power at the right timing, 1 deg +/- optimum will drop power and 2 +/- will kill it. Some motors you can run at 36 degs, and then drop to 32 and nothing happens.

Fast burning fuels are really not a option for us. Street gas is going to burn at about the same rate, and things like Alcohol and Nitro are completely different. Nitro doesn't really burn but "crakel" and it gets like 60degs of timing just to try to get there. The burn rate is really related to the chamber and dome/dish shape along with the quench areas.

Winston Cup guys run as little as 25 degs of total timing. From what I have heard that only happens in Fords at events like the Winston.

People forget that it's not a explosion but a burn and burns take time. One reason why I can't emphisize paying close attention to the quench area.

"IOW, the right timing to use would be the timing were the ___ (Rwhp? Rwtq? Both?) is greatest at that MAP/RPM?"

If you adjust the whole curve, or the area where the timing is maxed (max total timing) a degree or two you will see that part of the curve drop or raise. If it drops, meaning probably RWTQ and RWHP are down then you need to go back to where you were. It's mainly trial and error.

One thing that makes me laugh is guys who tune by ear. That's just crap. If you listen for knock you are doing the wrong thing unless you have too much compression. Then you should get better gas because the lower timing hurts you more than the higher compression helps. I've tuned motors to have slight detnonation in the low end to bring up the power, but it's only to get good numbers on the dyno. On a street car you should never tune it to have any detionation.

Bret

 

You will basically find that the motor will like to be leaner (make more power) untill it burns a piston. Leaner or richer at any RPM is not really optimum for performance and engine life. A blower motor might be different because it might like the cooling properties of the fuel, so more fuel will make more power at a point.

Shooting for a flat A/F table is a good idea. A carb doesn't really get that option unless you are really good. With EFI you should shoot for a flat curve.

Bret

 

Ok..makes sense - thank's for explaning it it .

How do guys know that they are ok to disable the knock sensor? If my A:F is dialed in at 13:1 across the board and my timing is dialed in (power curve is looking good), would it be perfectly safe to turn the knock sensor off when I'm at the track?

I think I understand the idea of leaner is meaner. Am I correct to assume 13:1 is the point of diminishing returns for the A:F? IOW, any leaner and your risk of damage goes way up, while your power increase goes up very little?

You guys are helping me out big time!
Ryan

 

Lean is mean.

13:1 is safe and will make power. High 13's is riskier and will make more power.

If you could get the motor to live at 16:1 and it made more power there you would run it there.

One of the reasons that NASCAR boys get such awesome fuel mileage is that they work on making the motor burn every ounce of fuel. Or optimizing the BSFC which is the lbs per HP per Hour of running. the lower the BSFC the better it is burning what it has and usually the lower the A/F ratio.

I know there were some sick readings in the Engine Masters last year on A/F ratio, like 17:1 at the low end. That's in the highest detonation area too, that's a bit iffy!

Bret

 

I'm guessing there's only way one way to know for sure what A:F the motor can take and last. Since I'm not anxious to find out myself I'll shoot for 13:1...lol

Thanks Bret.
Ryan

 

Yeah a mid 13's will work. As long as you don't run stupid compression.

Holing a piston is the way to find if it's TOO lean.

Bret

 

Good points raised. You got the "simplified tuning guide" 'cause there is obviously a lot more to it if you care to get into it.

For the kind of basic tuning I was talking about, leave the KS on. It won't help you find the optimum timing for the way your car is usually driven to turn it off. You will end up with "optimum" timing, but as soon as you turn the KS back on, your timing will probably change and negate your efforts.

Doing a run at high-normal operating temp is not a bad idea, to verify a safe operating mode. But OEM LT1 and LS1 programming already adds a lot of fuel and takes away timing at high operating temps (as well as high IAT's), so it almost certainly going to be safe. It will probably be too rich and have too much timing, but that's a nice safety margin. And I am assuming that someone with a high performance car will operate it with some common sense. Just like you wouldn't do a WOT run on a cold motor, you wouldn't put it in high gear at 2,000rpm and crack the throttle when it's 95 degrees and the water temp is already at 210 degrees. Of course, people do this kind of stuff, that's why the factory tune for these types of conditions are so conservative.

There is more power in certain combos by going leaner, and as Bret said, optimum AF ratio for forced induction is much richer. We can get into this at some point, but just one thing. It gets more complicated, risky, and time consuming (expensive) to start pushing the limits. If you want to experiment with lean AF ratios, you need to monitor exhaust gas temps for each cylinder and have the capability to tune fuel for each cylinder. You are flirting with major engine damge as the EGT's start to approach 1650 degees. What is optimum? It's like with timing, it takes experimentation. Mainly, the role of EGT is to help determine "how lean is too lean?" Keeping in mind that leaner - hotter. If you are making more power as you lean it out, and the EGT's are getting to the 1500-1550 range, I'd stop leaning it out at that point. But if you still have one or more cylinders that are cooler, there may be more hp to be found by continuing to lean out just those cylinders.

A nice thing about using a dyno for tuning is the ability to tune for different fuels. If you use one kind of gas at the track, and another on the street as I do, you need to different tunes to get optimum performance. Think about the density of the fuel for a minute. A 5% difference between fuels is common. If you already have your motor on the ragged edge of lean, if you put in a fuel that's 5% less dense, your AF ratio may well be too lean. And then ther's octane, burn rates, O2 content, etc. So don't just assume that everything will be fine if you put in a tank of race gas at the track. It should be safe, but you may not gain anything without a tune for that fuel. And, of course, for your base tune you must tune with the same fuel you are going to be using on an everyday basis.

Rich Krause

 

Take care with the idea that "lean is mean". If you are a little to the rich side your car will probably not slow down much. If you run too lean you will kill it. Remember that a dyno does not take things like drag into account.
I have spent quite a bit of time dynoing different combos and then testing them at the drag strip. I have found my best results by running leaner at the start line and richer towards the traps. On the timing I run about two degrees more on the bottom end than the top.
I will add that LT1's seem to love a bunch of timing.

Best Regards,

Daren

 

Great post guys!!! I love tinkering on the dyno (friend owns one) with timing and air/fuel. Rich are you sayin that you need to dial in the a/f ratio first and then the timing? Should the a/f change at all when changing timing? Whats the first indication of too much timing? (less torque?, less horsepower?, both?) Is more timing needed with bigger duration cams? Just trying to learn!!! Thanks again guys.

 

Thanks for the information, it's very helpful!

One question/comment: isn't tuning timing on the dyno a bad idea, if you plan on drag racing, since there is less load present on the dyno? Then when you get the car on the real road, the higher load will tend to cause detonation earlier with the same amount of timing, correct?