I have questions about how the ignition timing is controlled on the supercharged x308s.

I have a Snow stage II water/meth kit and I am going to get an ECU tune after the installation of an intake tube, bored TB, and a "few" other ported parts.

I'm going to end up running 16-18 psi on my overdriven Heaten M112.

It is my understanding that the gains from the water/meth injection are maximized when/because it allows running more boost and more total ignition timing.

My question is: Does the computer control allow the ignition timing to advance until it gets feedback from the knock sensor, and then back the timing off, OR does it advance to only a certain limited degree, even if there is no knock/detonation and therefore leaves room for gain by advancing the timing when more octane/resistance to detonation is present, like with the addition of super high octane fuel, or, in my case water/meth injection?

I'm just trying to wrap my uneducated brain around how the timing control works in our cars so I can better understand what would be available through a tune.

 

You can easily find out yourself, but these are actually questions you need to ask the one that is going to tune you ECU imho. He needs to take everying into account to provide you a custom tune, where this question is just the easiest one. Who are you asking for the tune?

 

In general, engines have a point at which additional ignition advance will not result in any increased power and in fact will start to reduce the output irrespective of whether there is detonation or not and octane level.

A good example is the classic Chev small block- it reaches peak power at 34-36* BTDC. Rasing octane levels to permit more advance (avoiding detonation) returns no additional power. Advancing the ignition to 40* or beyond gradually reduces the output as the piston is still travelling upwards when peak cylinder pressure is achieved.

I'm quite sure Jaguar has established the equivalent sweet spot for our engines and has set the stock timing curve accordingly.

 

See this article: http://www.snowperformance.net/produ...le-pdf-116.pdf This is a good reference because it is dealing with supercharged engines.

 

I'm not sure yet, just starting my research. I will keep you posted.

I found this article off the Snow website which thought was an interesting read http://www.snowperformance.net/produ...le-pdf-116.pdf

 

Look for the two long threads by XJREngineer in the X300 section detailing his two methods of increasing spark advance on the sixes.

The so-called sweet spot you refer to is affected by Jaguar policy which dictates that it be less than what the engine actually wants. This of course is related to concerns about tolerance to conditions out in the field and resultant customer care costs.

The timing modification for the sixes has been purchased by a number of JF members with universally good results as to power and fuel economy. The most notable JF luminary who has installed it and is pleased with the results is Brutal. Another one is long time member Aholbro. There are others in the thread who can be recognised as frequent and credible posters on JF. No one has reported a bad result.

Therefore, there may be a factory designed limit, but that limit is likely to be sub-optimal for any given enthusiast application given that it must take into account all types of customers, conditions and tolerance stacking.

Certainly, in the case at hand, there are substantial differences from the engine as it left the factory and its current state.

++

 

Absolutely- but the physics still remain that additional advance cannot be dialed in simply because the engine is not detonating with the expectation of gaining additional power.

 

I agree that additional timing beyond a point would not be productive, but I don't think we are at or near that point, and more importantly,I not think that is the limiting factor in this situation.

I don't think you are taking into consideration that running 18 psi is a completely different situation than the stock 11.6 psi parameters. More heat and more pressure will absolutely move the operating conditions towards detonation, as these conditions (higher heat and pressure) make combustion happen faster. These conditions make the molecules not only move faster with more energy, which means that more of the collisions will result in a reaction, but also they will be bumping into each other more often, increasing the frequency of collisions, both of which make the explosion/combustion happen faster. It is my understanding that the detonation is the result of combustion happening too fast.

SOOO if we can once again slow the rate of combustion with the addition of water and meth, we are left with the ability to add timing again/Now that we have slowed down the burn again to make it more push than pop, more propellant than explosive, we can start the burning process sooner to make more power. By using the meth/water we are increasing the point at which the detonation would happen, and this situation would not have been foreseen by the programmers of the stock tune.

Please explain more if I am misunderstanding your point.

 

Yes, you're misunderstanding me or more probably I'm not explaining it well. To be clear, it is solely the additional ignition advance that provides the boost in power and not the octane rating of the fuel.

More compression results in more heat which increases the probability of detonation subsequent to normal ignition at a given advance setting. Higher octane fuel reduces the probability.

Conversely, using a fuel that is resistant to detonation offers the possibility of advancing the ignition to an optimum setting as measured in degrees of advance before TDC.

The key is 'optimum ignition setting'. Given that the flame speed is a known constant for a given compression ratio (and unrelated to octane rating as per common myth), it would be desirable to have the cylinder reach peak pressure at a given piston position/crank throw angle with respect to TDC. Typically the desired peak pressure should be achieved at about 10-12* ATDC. Advancing the timing will correspondingly reduce this number and in an exaggerated case will cause the piston to be almost 'pushed backwards'.

Using the example of the Chev SBC again, the optimum ignition advance setting to achieve maximum cylinder pressure at 10-12*ATDC is approx. 34-36*BTDC at speeds of 2500-3000 RPM or higher. Again, advancing the ignition beyond this setting will cause the peak pressure to occur at a less than optimum crank position and loss of power.

Given that flame front speed is related to cylinder pressure and that 'higher pressure burns faster', feeding in more timing with higher boost may be counterproductive, especially if masked by detonation-free combustion through the use of higher octane fuel.

Hope this helps.

 

I think a big issue here is what does the stock program limit the most advanced timing to..and is that at or even near the max theoretical max power arena. Until that question is answered, this discussion is not terribly practical (though it is interesting)


but to your points:
"Given that the flame speed is a known constant for a given compression ratio (and unrelated to octane rating as per common myth)"

I think I disagree with the above. The flame speed, as I understand it, is ALSO regulated by the fuel type used, as not all fuels burn at the same rate. With the addition of meth and water, we are changing the chemical makeup and therefore the burn rate.

Do you agree with this?

I totally agree with you that the absence of detonation allows the possibility of more timing, and that it is only when you add more timing that you can make more power.

I am not well versed enough to know what the optimal timing is, but if the combustion is not completely instantaneous (which it isn't given we try to make the fuel a propellant and not an explosive), then top dead center is not the complete limit, nor do I know what the stock program sets the most advanced timing to be, but this IS beyond my current knowledge of ignition timing and combustion. Someone with more understanding of this subject will have to chime in to educate me. My speculation could only take me this far...

 

Yes- agreed. Different fuels may have different flame speeds. Ironically, the much maligned and hated ethanol fuels feature a faster burn speed than gasoline.

My own experience with water/meth injection is on gas turbine engines but the theory and benefits are similar. Given the limited volume of water/meth being injected, I believe any reduction in burn speed of the blended mix would be relatively minor and does not fully explain the advanced ignition timing described in your link.

 

Disadvantage to tune your ECU based on Methanol, is that you always need it to work.

As said before, you need to ask these questions to the one tuning your ECU, as there you trust this all too. If they don’t have the experience (so from the Jag ECU and combination of mods you want), you might want to go your own route. Have you asked xjr-0220, he said before he already tuned his ecu, so maybe he can help.

If your tuner isn’t able, you need to go your own way to find it for your setup:

Get yourself first the jaguar IDS, this way you can monitor all professionally, and learn your car. Next get yourself a separate knock sensor system, so you can monitor that as well for just in case, as you need to find the boundaries so you can take some steps away from it.

Then get yourself a piggyback ECU, so you can control the ignition dynamically, but first learn your stock setup, before starting to adjust. Advantage of the piggy back is that you can change the tune dynamicaly for your specific needs, fi if your water/meth tank is empty).

Then find a dyno that has an eddy current brake system, and one that has a good cooling setup (so not the office type of fans you sometimes see, more like 8 or 10 kw strong fans).

Now you are able to find the optimum timing, taking your full setup into consideration.

 

Man, you always get the prize for the most complete transmission of understanding of the real issues in your posts

While I definitely agree with all in your post to completely maximize the timing available, I'm wondering if I could get 80% of that gain by simply relying on the computer to NOT back off / retard the timing because it won't be knocking with the meth and just leave it at that, without all the fuss of maximizing the tune...will the computer do this, or does every tune automatically override the existing computer function of retarding only when it senses knocking happening?

I will ask XJR-0220 about what his tune guy can do, he seems to have cultivated lots of really high quality connections with very skilled auto people.

 

All I mentioned can be had relatively cheap, and can always be sold once you’re done, key is good monitoring imho, and then it’s easy to adjust. In the above posts you didn't add the richness of a mixture as the richer the slower the flame front again, so I see no other way in determining the best setup without actually measuring the results.

Your ECU has a max ignition advance, iirc around 21 degrees top rpm (it starts lower), it will not advance any further. Then based on temps the ecu will retard, and of course when a knock is detected in a cylinder it will be retarded for that one but will come back when no further knocks are detected.

 

the above also tells me that I will likely still see a decent gain from reducing the temps and lessening the tendency to knock and therefore the automatic reduction in timing by the computer. So it sounds to me, that even without a "tune" the meth/water injection will still give a good increase when paired with the 4lbs of extra boost.

Clearly if I can find some tuner to do as you mentioned, that would be by far the best. I would like to explore the benefits of a proper tune, without risking melting stuff

 

check out Turbo Buick.com and go to alcohol injection system,

these guys started water.meth many yrs ago, exception being german 50,50.

 

FYI,, we give credit to GM, they had the 1st production car that used water/meth. auxilary injection on ther 1962 OLDS F85 TURBO ,all aluminum V8.

i had the rare chance to drive one,back then, kool little car.