Andial951

Interesting post Jimbo. This is the first I have ever heard of having to match your MAF to your injectors. Since were on the subject what are the pros/cons of 55# vs 75# injectors?

tedesco

read very carefully. Still the question remains:

Why would you need to tune fuel if you what to change boost? Assuming that MAF and fuel capacity is sufficient for the flow requirements at new boost levels.

I understand that a certain MAF and injector size needs to be matched but once that is done why would you need to adjust anything?
With the piggyback (that alters the MAF voltage) I mentioned you should be also more flexible with the selection of injectors. Certainly it will be hardly possible just to use a MAF change injectors and expect anything to work properly.

special tool

Wow. What happened, Luis??

mark944turbo

"read very carefully. Still the question remains:"

Stock chips go to full throttle maps after a certain airflow per rpm is reached. The airflow/rpm value is still used, but the chips are set up to depend on a final full throttle correction value, which stays the same no matter what boost you run. So if you run more boost, you need to increase the full throttle values in the chips to keep it rich enough.

A way to avoid this is to set the chips up to not use any full throttle value, just have the calculation for fuel based on airflow only. Perhaps this is what vitesse chips do.

fast951

I'm not sure I follow what you are saying. No the Vitesse code does not function as you describe it. We use the proper "Transfer function" which translates a voltage to mass air (same as the original code does). Each MAF sensor has a unique transfer function. We can change MAF sensors, change the transfer function and not touch anything else and get the exact timing and exact AFR as with the prior MAF or AFM for that matter.

Chris White

Chips are best with beer.....

Chris White

mark944turbo

John, what I was saying has nothing to do with maf vs afm or transfer functions.

The question was, why is it necessary to retune when you run more boost, since the airflow meter should sense this and tell the ecu to add fuel.

I answered by saying that the stock chips depend at least partially on "full throttle map" which enriches the mixture at full throttle, the same % no matter what boost level you are at. So when boost is added to a stock car, it adds some fuel, but not enough due to the nature of the full throttle enrichment.

Since your chips are known to handle many boost levels without retuning, I was supposing that you might no longer use full throttle enrichment, but instead base the fuel values 100% on airflow (not including the accel enrichment, temp correction, all that of course).

Changing the transfer function should not allow you to run any boost you want without tuning. It should still go lean at full throttle under higher than tuned for boost, because the full throttle enrichment values will not be high enough.

TonyG

mark944turbo

>>>The question was, why is it necessary to retune when you run more boost, since the airflow meter should sense this and tell the ecu to add fuel.<<<

You should add into that questions, "... and since the boost referenced fuel pressure regulator also adds fuel when the boost is raised".

TonyG

jimbo1111

Chris White will ansewer this one.

TT

Hell, I'll answer this one. A 1:1 rate FPR does not add more fuel as manifold pressure increases. Its job is to maintain a constant pressure at the fuel injector/intake manifold interface. As an example, a 3bar FPR will attempt to maintain 3 bar of fuel pressure relative to the base pressure in the intake manifold. Whatever the pressure (vacuum included) in the manifold, the fuel pressure will be 3 bar above that value. So injectors flow rated at 350cc/min at 3bar fuel pressure will flow at that specified value using a 3bar 1:1 rising rate FPR regardless of the manifold pressure.

Now, rising rate FPRs with a ratio other than 1:1 will flow more fuel under boost due to the multiplied increase in fuel pressure relative to manifold pressure. Those setups are the ones prone to retuning because of boost changes.

TonyG

>>>A 1:1 rate FPR does not add more fuel as manifold pressure increases.<<<

Incorrect.

>>>Whatever the pressure (vacuum included) in the manifold, the fuel pressure will be 3 bar above that value. <<<

Correct. However:

At 1 bar absolute (no boost), a 1:1, 3 bar fuel pressure regulator is putting out 3 bar of fuel pressure.

At 2 bar absolute (1 bar of boost), what's the net fuel pressure to the fuel injector using the same 3 bar, 1:1 fuel pressure regulator with 1 additional bar of boost?

And conversely, what happens at idle and low-load vacuum situations to the fuel pressure? Is it still at 3 bar? Or does it go down?


Don't confuse maintaining a relationship between fuel pressure and manifold pressure for the purposes of maintaining a certian a/f ratio, with the net pressure appied to the fuel injector. That relationship exists to add additional fuel. More boost.... more fuel pressure, more fuel.

If apply more pressure to a fuel injector, using the same pulse-width, the injector will flow more fuel.

Rising Rate fuel pressure regulators simply add fuel pressure in a non-linear rate, doing the same thing as the 1:1, just more of it...




TonyG

fast951

The WOT table is simply an array of desired AFR values, indexed by RPM. It doesn't matter if you are running 1bar or 2bar boost, K26 or Vitesse S5. More boost means more air flow and the MAF will generate the appropriate signal (assuming MAF is calibrated to support such flow). When determining fuel to be injected, the DME uses the AFM/MAF signal (one of the many variables in the equation) as well as the desired AFR in the WOT.
As long as the DME knows the actual air mass, it figures out the fuel to get the desired AFR (static in WOT table).

The important thing is for the DME to know the "actual" air mass. This is done in the transfer function. So changing a MAF sensor and using its unique transfer function allows you to run without having to retune (as long as you are not maxing out the MAF sensor! Of course the fuel system must be able to keep up). So boost is irrelevant when using a properly sized MAF.

This is why people using the VR MAF & software can alter boost and be fine (using proper octane of course). The AFR does not change.
Using a signal massager to alter the MAF signal to look like AFM signal forces you to retune when the boost (air flow) changes.

mark944turbo

This theoretically makes sense, but the neither the stock full throttle table nor any aftermarket version I have seen look like an index of desired air fuel ratios at full throttle. I will post it when I am on my other computer.

What it looks more like is a place for a tuner (Porsche at one time) to alter the final air fuel ratio at full throttle, accounting for any leftover variables that math so far have failed to handle properly. If vitesse chips are using a totally different code, then there is probably less need for this final correction table, and it would be ok to consider it a table of desired air fuel values.

But in the stock case, and the case of all chips but Vitesse, I argue that this is incorrect. Proof would be to take a stock car, and set all the full throttle table values to a constant. According to your theory, ft afr would be almost constant at all engine speeds, assuming low enough boost was run to keep the AFM in range. I am pretty sure this would not happen.

So the whole point of me saying this is to show why you can not just increase boost without retuning on most cars (vitesse not so much included), even if the MAF or whatever is in range.

Chris White

As much as it pains me (because Tony is usually right)….buzzzz – wrong (sort of)

All we really care about is the ‘relative’ fuel pressure – that is the relative pressure at the tip of the injector. The fuel delivery rating of an injector is based on the stated pressure drop across the tip of the injector. If you didn’t have a linear rising regulator you would be getting less and less fuel as the manifold pressure increased. In fact if you had a 2 bar regulator and you cranked the boost **** up and ran 2 bar of boost you would get no fuel out of the injector.

All injectors are is a hole that is opened and closed to allow fuel to pass. The quantity of fuel that is injected is equally dependant on the length of time the injector is open and the pressure differential between the upstream and down stream sides of the injector.

So the first statement that Tony said was incorrect is actually correct – in theory. If the amount of time an injector stays open is a constant then increasing the fuel pressure linearly to the manifold pressure will no add any additional fuel. I say in theory because in realty the time element will not stay constant since the engine will require more fuel to match the increase in boost.

As for low pressure situations that same happens in reverse – the key is keeping the pressure differential the same at the tip of the injector. So as the vacuum increases the fuel pressure drops – keeping the system linear.

The non linear FPRs were the result of some of the first attempts to run higher boost than the system was mapped for. Not great results – but at the time nobody knew what a chip was! Jets in the banjo bolts and nonlinear FPRs….

Chris White

special tool

AAARRRGH - you just beat me.
If the regulator was able to flow more fuel despite an equal pressure on both sides of the pintle - the universe would instantly collapse from the chaos. This is physically impossible.