(Remember the later 16V Euros have same hot wire MAF and Computers as early 32V cars)

 

A MAF system can usually correct for changes in inlet or exhaust flow - within the system design.

Speed/density systems need to be re-cal'ed for WOT operation when you change anything that impacts the VE. Part-throttle closed-loop, S/D systems normally can adjust unless there are big VE changes.

FYI- Increased fuel flow/pressure does not increase power unless the optimum AFR is unobtainable with the existing system/calibration. In other words, in an NA engine a richer AFR than LBT, does not produce more power.

 

Share????

 

Similar but not 100% the same.

I've interchanged them before for testing purposes.
There are enough small differences you wouldn't want to permanently run an 85/86 US on an 84/85 EuroS brain, or vice versa.

 

Probst' book pointed out a little of this, and he couldnt be more wrong in that one small area. (its only error in my estimation) The speed density , or AFM can very easily adjust for changes in VE, as well as altitude changes, without altitude sensors, that were installed to further increase their already accurate capabilities up to 10,000feet as their design description points out.

Proof is in my 1984 AFM system that went from near 170rwhp to over 295rwhp, at WOT, using the same AFM, keeping the air fuel ratios near the same along the way, while changing anything you can imagine in the intake and exhaust.

 

Thanks guys, that was some good feedback. Mark, I presume this is the book you referred to and is a good book on the subject? http://www.amazon.com/Bosch-Fuel-Inj.../dp/0837603005 I think I'll add this to my reading list.

Also, anyone looking to understand how the LH functions should check out the concise description written by Jim Corenman in the Sharkplotter manual found on page 21: http://www.sharkplotter.com/

I’m still trying to understand how the MAF works. Externally I now what it does… measures air mass and reports it to the LH as a voltage. I believe this voltage is 0 for nothing and 6 volts for max. I’ve read that it does this by heating up a wire and the in rush air cools it. More air mass, results in more cooling, results in higher voltage. For tuning purposes, it’s not necessary to know what going on inside. I’m just curious; does anyone know how to explain what's going on inside in layman’s terms?

On the timing front, I’d like to explore the basics of ‘base’ timing. Intuitively, it would seem like at a slight moment just past TCD when the piston starts heading down is when the ‘ideal’ spark should occur. Again, I have no idea, but in addition to all the details about how much to retard (which from what I understand is all the EZK does… no advancing), I thought it would be best to understand the ‘base’ timing and how that relates to crank/piston position.

I’m thinking back to the basics of suck, squeeze, bang, blow analogy that an engineer once used to describe how an engine works. So in slow motion:

1) intake valve opens
2) piston moving down draws in air & fuel mixture
3) intake valve closes
4) piston moves up to compressing air & fuel
5) spark set off explosion
6) piston forced down by combustion
7) a few mores step to clear and start over

So the question is, on the ‘base’ (the furthest most advance setting) where does step 5 occur in relation to TDC?

 

5) can be as much as 35 dgrees before TDC on the compression stroke. Around 10deg before TDC at idle

You description of MAF operation is fine. On a 928 the MAF will give about 2.7v at idle. Up to 5.5v at WOT max power.

 

The "base" spark is relative to engine speed due to combustion time and pressure rise vs. (real time) piston speed. You need to start the spark before TDC to have the peak cylinder pressure rise at ~15-20 degrees after TDC for optimum performance. At low engine speeds you need less ignition "advance" so as John noted 5-10 degrees Before Top Dead Center (BTDC) is typical at idle and as much as 35 deg. BTDC is typical at higher rpms around 6,000.

 

Now that is interesting… so the spark that initiates the combustion can or does occur before the compression stroke is complete. I believe you, but at 10,000 foot level, that seems counter intuitive.

Why is that, does it take a little while for the ‘fire ball’ to ‘get going’? Does it need more time to completely burn? Is there a delay between spark and bang?


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Edit... I see my question has already been answered. I get it... it's not instantaneous; combustion takes time and has varying rates of expansion or pressure increase through the cycle. The object is to get the max power (peak cyclinder pressure rise) lined up in the 'sweet spot' of the down stroke.

So the piston is accelerating between TDC and half way down. That’s when we want max energy transfer to occur… right? After halfway down it’s decelerating to a stop at the bottom, but during the entire power stroke energy is being transferred to the crank. Is that about it?

 

^^^^ Yup you've got the idea now.

FWIW the ex. valve opens a bit before Bottom Dead Center, (BDC), to release the cyl. pressure for better exh. scavenging and less pumping loss when the piston is headed upward on the ex. stroke.

 

I see your question. Yes, ignition occurs, but requires some time to propogate. Its a science in itself to design combustion chambers to do this in the best posible way. obviosly, if its too early, you compress this expanding flame front, along with the unburned fuel and air and it ignites and you have an explosion (detonation). So the trick is to never have an explosion. Thats why shapes of piston edges, squish areas on the flat part of the piston, and other combustion chamber shapes are so important. I think it becomes much more intuitive when you see one of those high speed videos of the entire 4 stroke cycle. Still, much of it is voodo science in my mind too!

 

Wow, I agree… I’m sure there is some over the top science involved in combustion technology and piston design, etc. I think it’s safe to say there are a couple ideas one should be able to understand.

One, combustion take time and there is a delay between when the spark fires and when peak cylinder pressure increase occurs. Two, in order to keep the pressure rise in the ‘sweet spot’ it becomes necessary to adjust the timing relative to the RPMs.

So, in general in order to give the flame front enough time to ‘get up to speed’ it becomes necessary to advance the timing as the RPMs rise. However, the amount needed varies because of other conditions like load and who know what else. Also, if we aren’t careful we get bang (max pressure rise) on the up stroke… and we all understand that’s bad.

So ignition tuning is the art & science of turning the ***** (adjusting maps) to accommodate a wide range of varying condition in order to optimize what is going on at particular time. That’s fair enough.

 

I believe what you're asking though is : If you increased the pressure would the LH and stock chip compensate for it. I believe the maps are tuned to an expected pressure and injector size.

Ken (Porken) found that with his modded chips, the stock injectors were approaching 100% duty cycle at WOT so he increased the pressure (with the '87 FPR) to 55psi and was able to dial back the maps so the injector duty cycle was lower. That implies (to me) that the chips need to be dialed in for the FPR and injector size (or flow) in order to work.

 

That is correct.

The LH/EZK combo has no way of knowing what it is connected to the engine as far as fuel injectors are concerned. So except for the not always used closed loop mode where the O2 sensors feedback helps fine tune the A/F mixture, the combo is determining what to use base on what it’s been programmed to think is connect (a.k.a open loop mode).

Bottom line, change what’s connected… need to change programming or more accurately the maps.

 

I believe when reducing back pressure, this allows more air mass to flow through the engine so the MAF will see this difference and automatically compensate for it.

As already mentioned, that would not be true if you changed the fuel pressure or injector size.

The LH doesn’t meter the fuel per se, it thinks it knows how much it is giving base on open time and pre programmed amounts. Change the fueling = change the maps.