Menu toggle

944 DME Project

Yes I have heard the same about the AFM and WOT. I suspect it is because WOT is triggered by the TPS - but the TPS advise WOT at around 60-70% throttle opening IIRC. This means the AFM must be used to determine the correct amount of fuel for the amount of air the engine ingests. I would love to know where the code is in the EPROM that calculates this is. Presumably it proportions the fuel in the WOT map based on the AFM calculated air flow and temperature.
 
I presumed that at WOT as the load becomes a known constant the fuelling would be effectively derived from engine speed and the intake air temperature. I think the TPS triggers WOT at 60-70 as a butterfly valve is not a linear control valve, as in the last 20-30% of movement has little effect on the manifold pressure. This is valve movement not pedal movement as this can be linearised via a cam.

GPF is there a WOT fuelling table in the DME at all? I can see your logic that if the fuelling stays correct at WOT despite a change in manifold pressure the DME must be compensating using the AFM signal.
 
Isn't the AFM fully open at around 4,500 rpm? how can it provide much feedback above that point if this correct? I am suprised the throttle position is ignored.
Tony
 
There is no consensus on wether the AFM is maxed out at 4500 rpm. Some say yes some say no, I have not yet tested this. I know the turbo AFM is different to the lux AFM and by my calculations the lux AFM can measure ~200hp. You could easily datalog the AFM output.

The throttle position isn't known absolutely by the DME just closed or wide open signal. It has no knowledge in between. The throttle position switch is used to enable an idle mode and too switch to a separate set of wide open ignition maps.
 
I can wait to see how much difference this system make I think I will go for TPS(Potentiometer)/MAP based setup rather than using a MAF sensor. Might just use TPS on its own for the Lux. A decent MAF sensor is too expensive, you can get a MAP and TPS sensor for ~60 quid. Can't go wrong really and there isn't any major benefit to MAF that I can see.
 
I'm wondering about monitoring engine oil pressure as a safety feature. Would cutting the engine out when oil pressure is below a set threshold be worthwhile? Is it already too late?
 

ORIGINAL: barks944

I presumed that at WOT as the load becomes a known constant the fuelling would be effectively derived from engine speed and the intake air temperature. I think the TPS triggers WOT at 60-70 as a butterfly valve is not a linear control valve, as in the last 20-30% of movement has little effect on the manifold pressure. This is valve movement not pedal movement as this can be linearised via a cam.

GPF is there a WOT fuelling table in the DME at all? I can see your logic that if the fuelling stays correct at WOT despite a change in manifold pressure the DME must be compensating using the AFM signal.
Click to expand...

Yes, the WOT fuel table is a 1x16 table with values for 16 RPM points from 1,000revs to 6,240revs (as standard). There is one in each of the three map sets, together with matching WOT ignition advance tables.
 
I take it it is just an AFR demand then and the DME uses the AFM to work out the mass air flow and injects accordingly.
 

ORIGINAL: Eric_Oz_S2

Yes I have heard the same about the AFM and WOT. I suspect it is because WOT is triggered by the TPS - but the TPS advise WOT at around 60-70% throttle opening IIRC. This means the AFM must be used to determine the correct amount of fuel for the amount of air the engine ingests. I would love to know where the code is in the EPROM that calculates this is. Presumably it proportions the fuel in the WOT map based on the AFM calculated air flow and temperature.
Click to expand...

Yep, I'd like to know that too - or at least how the calculation works!

I know that the AFM signal has less significance than the values in the WOT fuel table, but it doesn't seem to be by a constant proportion.[:mad:] Possibly because of the AFM transfer function tables, but I *think* there is also something else going on.
 
It might happen like this.

1. DME uses AFM and temperatures sensor to calculate mass flow of air into the engine.
2. DME reads required Air Fuel Ratio from WOT Fuel Table using engine speed as a reference.
3. DME calculates required mass flow of fuel into the engine by dividing mass air flow of air by the air fuel ratio.
4. DME calculates required injector pulse width.
5. DME drives the injectors to calculated pulse width.
 
Tom,
I reckon that would work, its a bit crude but was state of the art in 1985 when most manufacturers had vacuum advance and carb's.
Ref your earlier Oil pressure question, I think its partly a good idea, in 99% of cases its not too late to save the engine (unless you are at 160mph when it happens) but I dont like the idea that anything can stop the engine at any time. If you are half way past a queue of trucks in 3rd gear, I'd kinda like to know that even if the oil pressure failed, I wont be left there with no power.
The big red light on the dash is OK for my ECU when you get around to it :)
keep up the good work
George
944T
 

ORIGINAL: George Elliott

..... If you are half way past a queue of trucks in 3rd gear, I'd kinda like to know that even if the oil pressure failed, I wont be left there with no power.
....
Click to expand...
I think if that happened you would be left without power even if the ECU didn't cut the engine, it'd seize/throw a rod in fairly short order?
 
ORIGINAL: George Elliott

Tom,
I reckon that would work, its a bit crude but was state of the art in 1985 when most manufacturers had vacuum advance and carb's.
Ref your earlier Oil pressure question, I think its partly a good idea, in 99% of cases its not too late to save the engine (unless you are at 160mph when it happens) but I dont like the idea that anything can stop the engine at any time. If you are half way past a queue of trucks in 3rd gear, I'd kinda like to know that even if the oil pressure failed, I wont be left there with no power.
The big red light on the dash is OK for my ECU when you get around to it :)
keep up the good work
George
944T
Click to expand...

Its been so long since I drove my 951 I've forgotten if what lights there are on the dash.... Could certainly drive the oil warning light using this kind of signal. Is there a general engine warning light?
 
Darren,

you are right, it wont last long at full tilt, and the damage will be catastrophic, but the thought of a total cut in power is not nice. for example, you are in 5th on the m/way, 3rd lane, busy traffic, raining, 3 artic's in convoy in the middle lane to your left, transit 2 feet behind, and the oil pressure flickers (could be a dodgy sender unit) but the ECU deceides to shut the engine, so no servo brakes, no power steering, wipers stop as you try to restart the engine, if its dark the headlights drop out too.........and if the pressure sender is u/s, the car wont re-start......

I prefer to slip into neutral, leave engine on tickover (it wont throw a rod at tickover inside 1 minute) slow the transit behind, and steer through to the hard shoulder with wipers and lights.

Tom,
I think the only oil warning light is at the bottom of the scale on the pressure guage, but the general warning in the centre of the instruments may light too?

George
 
George, are you saying the oil light already comes on at low pressure? I though it only came on for low level but I dunno.
 
Tom, I believe you are right in your thinking. Perhaps your new ECU will make low pressure (less than 1 bar??) flag a main warning light?
George
944T
 
The hard part is to get the oil pressure signal into the ECU. I think it would need an additional oil pressure sender. This isn't too hard though as I could just fit a T-piece where the standard sensor fits and add an additional sensor.
 
I've been following this thread avidly (reminds me of my A-level electronics) and would happily be a guinea pig when you want the system tested in the real world.
 
Thanks Eldavo I appreciate the offer of support. Once I have proven the system on my own vehicle it would be great to have some other people testing out the design.

I am building a test rig atm to test out the circuits for the crank and reference sensors. This is the last bit of circuitry I want to test before I will commit to making some boards. Hopefully I can get this done next weekend and then I will get some boards made up.
 
You must log in or register to reply here.

Posts made and opinions expressed are those of the individual forum members

Use of the Forum is subject to the Terms and Conditions

Disclaimer

The opinions expressed on this site are not necessarily those of the Club, who shall have no liability in respect of them or the accuracy of the content. The Club assumes no responsibility for any effects arising from errors or omissions.

Porsche Club Great Britain gives no warranties, guarantees or assurances and makes no representations or recommendations regarding any goods or services advertised on this site. It is the responsibility of visitors to satisfy themselves that goods and/or services supplied by any advertiser are bona fide and in no instance can the Porsche Club Great Britain be held responsible.

When responding to advertisements please ensure that you satisfy yourself of any applicable call charges on numbers not prefixed by usual "landline" STD Codes. Information can be obtained from the operator or the white pages. Before giving out ANY information regarding cars, or any other items for sale, please satisfy yourself that any potential purchaser is bona fide.

Directors of the Board of Porsche Club GB, Club Office Staff, Register Secretaries and Regional Organisers are often requested by Club members to provide information on matters connected with their cars and other matters referred to in the Club Rules. Such information, advice and assistance provided by such persons is given in good faith and is based on the personal experience and knowledge of the individual concerned.

Neither Porsche Club GB, nor any of the aforementioned, shall be under any liability in respect of any such information, advice or assistance given to members. Members are advised to consult qualified specialists for information, advice and assistance on matters connected with their cars at all times.

Porsche
Lockton
Pirelli
Officially approved
Oracle Finance
WEX Esso
Collecting Cars
Archant Dialogue
Back
Top