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MAF re-scaling on a car with VVT

Practical Reflash Tuning

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Hi Andre,

I have a question when it comes to flashing a car with VVT such as the EVO X which has it on both intake and exhaust cam shafts.

How does this fit in to the 6 step process? For instance, when I re-scale the MAF, do I optimize my fuel next and then optimize the VVT? Will the MAF need to be re-scaled again after I have changed the cam values in the VVT map?

Also, in terms of exhaust valve timing.....this is something you have never addressed in detail in the webinars....how can I go about optimizing this? Do I dial the intake in first and then dial the exhaust in? Or is it an iterative process where after tweaking the exhaust you will then need to re-address the intake side? An lastly, in terms of cam advance and retard with respect to engine speed......is it just completely opposite to the intake cam tuning? ie retard the cam at low rpms and advance it at high rpms? Is this a table that sees a noticeable benefit from tweaking on an evo x?

Chris.

Chris,

Generally I will get the fueling in the ball park '(and ignition timing) of what I want to target (no point getting to fussy as the fueling will change with cam control)

I will start with the intake cam as this will make the largest change in power, torque and AFR. Get this about right and tweak fueling as you go.

Then start work on the exhaust cam, this will end up being a bit disappointing as the gains are tiny compared to the intake cam. Generally in factory form the exhaust cam control is more emissions over power.

You will find the exhaust cam will stay pretty consistent throughout the rev range and not vary a huge amount unlike an intake cam which will generally move around 35-40 degrees. The difference between say -20 exhaust angle and 0 could only be a couple/few kw in the top end.

As you tweak the exhaust cam and find a good setup I will then go back and move the intake around a few degrees (generally you won't see much change) The go back and finish off your fuel tuning and ignition timing.

If the MAF scaling is done correctly then it shouldn't need to be touched again. Since it is measuring actual air mass it will be able to alter the fueling for the change in air mass from cam control tweaks.

Hi Chris,

Thanks again for the response. I understand what you've said about the MAF scaling, so when you speak about re-visiting the fuel after making changes to the cam angle, I assume you are speaking about making small 1-2% changes since the accurate MAF calibration should have already corrected the larger change in fueling due to changing the cam ange on its own?

Also, can you explain some more the sign convention of -20 degrees when it comes to tuning the exhaust cam? I am accustomed seeing values from 0 to say 30-40 degrees on the intake cam. Can you explain what a value of -20 actually means with respect to if the exhaust cam is retarded or advanced as well as which direction you want to go in at low rpms vs high rpm to optimize the exhaust cam timing under these conditons.

I have one more question on the topic of re-flashing particularly on an evo x with regard to ignition timing in the over run areas of the map, I'll ask after you reply on the above.

Thanks,

Chris

Chris,

If the MAF is scaled correctly and your AFR is targeting your desired AFR then when changing the cam angle the AFR should be perfect. If not then it may need small amount of tweaking.

Same as the intake cam the exhaust cam is measured in degrees of crankshaft rotation. The intake cam with advance from its base position and the exhaust will retard (hence the - numbers)

As I said earlier don't expect a huge gain from the exhaust cam, in stock form it is used more for emission reason.

I have attached a screen shot of a stock and tuned exhaust cam map. As you can see in the stock map most of the retard is around light load low rpm cruise areas. This is to closed the exhaust valve later and promote scavenging to help reduce NOx. Kind of like an EGR system.

The tuned map has a small amount of retard which does increase to -10 in the top end. From memory the difference between the stock and tuned map was around 3-5kw so not huge.

Attached Files

Hi Chris,

So does this mean that the exhaust valve will remain open while the intake cam is also open as the piston goes from tdc to become?

If so then this may be why the maximum retard is only -10 because more than that and it'd start being counter productive?

My last question is on evos and their ignition timing in the high rpm lower load areas.....why does Mitsubishi have so many negative values in the stock map? Do you make any changes to this area when you flash an evo? What are the implications if any of using values of ignition timing after tdc in these areas? Can it be detrimental to the engine in any way?

Thanks for all your help so far

Depending what your current exhaust valve timing is, you will find at certain rpm ranges that having the exhaust valve close later and with more overlap will help with evacuating the cylinder of exhaust gases from having boost pressure force it out.

You will find most stock cams will close the exhaust valve around tdc or just before (like 5-10 degrees) so retarding the valve by 10 will only equate to the valve closing at tdc or maybe 10 after when the piston has barely started to move back down the cylinder.

It all comes down to what each engine wants and that's our job as a tuner to be able to read what the motor wants and unlock its potential. If you tell an engine what it wants that's when things can go wrong fast.

I believe the areas you mean are low rpm high load? These areas generally can't actually be accessed. like 22psi boost at 2500rpm etc and from memory the areas at peak torque (say around 4000rpm) where the engine will be running through the map are still positive numbers (usually quite low though as this is where the engine is most likely to knock).

I will tweak all areas which can be accessed by the engine and then tidy the map so all the dead zones (like low rpm high load) look nice and smooth etc

Having the ignition timing firing ATDC isn't exactly dangerous to an engine although it won't be doing ti any favors either. Your peak cylinder pressure will happen way to late in the cycle and not have a huge impact of force on the top of the piston. If you'r at the point where the engine was knocking so bad to be firing ATDC I would look at reducing boost pressure etc to be able to get more advance in and reduce heat.

Other thing firing the timing ATDC or quite retarded is that it will generate a lot of heat. This is transferred into the exhaust valves, manifold, turbo etc which can cause issue with premature failure, over-heating and over boosting.

Hi Chris,

Yes sorry I did mean high load, low rpm. I've attached the stock high octane timing map of a buddy's evo x and you can see what I'm talking about, there's quite a lot of negative values in there in these areas so I found it a bit odd hence why I asked. I'd imagine with a reflashing job like this, you won't be able to do steady state tuning to see what the engine really wants at part throttle so it'd just be a case of leaving this as Mitsubishi has set it?

Another thing is, the car sometimes spits some flames at high revs when you let off the gas, is this something that you would consider a no no and needs to be addressed in the tuning or is this something quite normal and acceptable?

Chris.

Attached Files

Chris,

You will find the engine won't operate in those areas. Most of those negative numbers are at around 22psi and 2000rpm which the engine generally won't ever be able to achieve.

"Spitting flames" generally isn't a bad thing, Is quite typical from higher rpm and load due to heat and unburnt fuel in the exhaust.

Only time I started to get concerned is when it comes with a back fire which could damage mufflers or push back on the exhaust valve and pop off rockers etc on certain engines.

Hi Chris,

Thanks again for the reply.

With regard to the backfire, is there anything in the tuning that would specifically make this more likely to occur?

Also, what about a car running rally style anti lag, would those issues of push back on the exhaust valve, pop off rockers etc still be an issue and if so how is this dealt with in these applications?

Hi Chris,

Thanks again for the reply.

With regard to the backfire, is there anything in the tuning that would specifically make this more likely to occur?

Also, what about a car running rally style anti lag, would those issues of push back on the exhaust valve, pop off rockers etc still be an issue and if so how is this dealt with in these applications?

Sorry for the late reply Chris, Have been overseas the last couple of weeks.

That sort of back firing only really occurs when you're using an ignition cut and ignition retard when using launch control etc.

As for anti-lag, the cut isn't as aggressive (when managed correctly) so the back firing isn't really an issue other than the heat which is created. But that's another story

Hi Chris,

Thanks for the reply and understood.

This same car I am tuning, i'm seeing total fuel trims in the vicinity of -12% at idle and cruise. The intake setup in the car is 100% stock with the exception of a ralliart air filter inside the stock airbox. I find it a bit strange to see such a large negative trim.....would this raise any flags for you or should I just go ahead and rescale the MAF to correct this?

That is quite a large trim. Is this a short term fuel trim or long term?

Reason I ask is sometimes you may need to reset the LTFT as these will be stored from before the upgrades.

Do you recall what the trims were before the upgrades or were they checked?

Chris,

The short is sitting around -4% and the long term at -8%.

No I didn't get to checking the trims before the upgrades. The catalytic converter on the car has also been removed and its fitted with a HKS cat back system.

I can't say I have ever seen trims this far out on an EVO X (I have on Holden's or Ford) however it wouldn't surprise me to be honest.

Is it just at idle or all over the place?

If everything is working correctly and there are no boost/vacuum leaks or exhaust leaks then I would just tune the fuel and MAF tables to get them back within a more acceptable range

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