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oldtuckunder

Help MJ Decision Time, MAP or TPS

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OK about to byte the bullet and buy the MegaJolt Kit, but have to make decision about using either MAP or TPS as the driver.

I have read the odd guides about using MAP or TPS but am still hovering, this is slightly complicated (or perhaps overcomplicated) by the fact that I already have a TPS and MAP sensor installed which I log to my data logger so I have a fairly clear picture of what my engine does, in relation to MAP, TPS and RPM.

So first off I'm not really worried about idle or cruise (happy for some compromise in this area) the areas I am concerned about is 3K-6.5K range and what happens when I transition from WOT to SHUT and back again.  I can see from my data logs that RPM can still be rising whilst TPS is dropping by up to 25%, and through gear changes RPM can be rising whilst both MAP and TPS fall to the floor and then jump back to max. Now with mechanical advance in a distributor this is all invisible, the advance is going to be stable based on engine rpm, which as I see it is safe.

So lets say normally I have max advance in the low 30's mechanically, at somewhere in the low 3K band.  At launch say engine at 2K, not max advance, fairly low TPS and high MAP vacuum, I drop the clutch, revs drop a bit (and hence may be mech advance) just as I hit WOT and TPS goes to the roof, MAP goes to zero, but my mechanical advance follows the engines RPM. What the Fu.k does MJ think is happening, and what's it doing with advance. If MJ is ignoring MAP and TPS in these situations and using rpm, then why even bother having MAP or TPS as an input?

NB from the Logs I can also see that the movement of MAP and TPS, whilst at the extremes of WOT and SHUT mirror each other, I can also see that in between one can be  rising as the other is falling, so which is most important?

Or is TPS or MAP a bit like having vacuum advance/retard circuits on a mechanical distributor (which I don't use) so that MJ can pull advance up/down for essentially emission purposes. I can see why with something like Megasquirt that is controlling fueling that these would be important inputs, but for ignition timing only?

Am I creating my own dragons here?  does megajolt care about rpm? should I care?  Why have I got hold of the wrong end of the stick?

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Hello Alan

                   Will you still have mechanical advance?

I thought that is why you are thinking of Megajolt! Maximum advance on lighter loads and retarded a bit when you think or hear pinking!

I use a map sensor as it can adjust ignition advance/retard to suit vacuum.

I am not sure how TPS works on carbs

Roger

ps order one with Map sensor and you have options I think? plus 2 ignition maps to play with(normalish advance or just plain silly or 1 in between!)

Edited by rogerguzzi

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9 minutes ago, rogerguzzi said:

Will you still have mechanical advance?

I am not sure how TPS works on carbs

Roger

ps order one with Map sensor and you have options I think? 

No mechanical advance, those comments were to try and make some correlation between things.

TPS on carbs, works same as throttle body, it just measures the angle of the throttles shaft 0-5v (well within that range)

With MJ I seem to have to either order a TPS version or a MAP version (hence my conundrum)

Alan

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We'll be moving in parallel, then, OTU!

I bought the kit a while ago now, various probs meant I've never fitted it, but this winter is the time!     Got the trigger wheel mounted and the coil pack and ECu on the bulkhead.  Now got to make sense of the wiring.   It's a vacuum sensitive version.

JOhn

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2 hours ago, oldtuckunder said:

No mechanical advance, those comments were to try and make some correlation between things.

TPS on carbs, works same as throttle body, it just measures the angle of the throttles shaft 0-5v (well within that range)

With MJ I seem to have to either order a TPS version or a MAP version (hence my conundrum)

Alan

Hello Alan

                  I still think order Megajolt with Map sensor because you still have the TPS port if you want to try it(I think)

This from Triggerwheels uk

In the vast majority of cases you can use either the Manifold Air Pressure or Throttle Position Sensor setup.  The easiest installation is with the MAP sensor version as the sensor is built into the ECU and simply needs a vacuum take-off from the manifold connecting up.  The MAP system also can run forced-induction if that is something you need. 

The TPS setup requires you to fit and wire up a throttle sensor if you do not already have one fitted so is slightly more work than the MAP system.  If you are running extreme race-spec camshaft profiles then the resultant pulses in the inlet can confuse the MAP sensor readings unless you are careful so in that case you would typically go the throttle sensor route.

I do not think you can use both as in Megasquirt(Nick will know)

I tapped the inlet manifold for the MAP sensor and put a tee piece in then ran it through a plastic fuel filter as the plenum chamber plus it can catch the fumes/fuel which they say would damage the TPS sensor!(plus mount it lower than TPS inlet)

Roger

 

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You can use both in Megasquirt (most versions of anyway).  We did have this discussion before and I suggested using an MS ECU rather than MJ, partly so you would be "injection ready", but also because MS makes a pretty good data logger in it's own right.

As regards what the ECU does with the MAP or TPS signal.....  It's simply reading from your timings table.  The box where the actual rpm and load intersect at any given moment (it samples 1000s of times a second) will be the timing figure used (and it interpolates).

Vitesse PH3 20102016.JPG

RPM is the X axis (horizontal).  Idle on left, max rpm on the right.  You can configure the range.

Engine load is the Y axis and could be be using MAP or TPS.  In this table it's MAP; 100 KPa is WOT and the lower number is chosen to encompass the full range or where the engine actually goes.  Lowest figures with be on the over-run and maybe cruise at very light throttle.  Idle will be somewhere in the bottom 3rd dependent mainly on cam timing.   Note that this is an MS table so WOT is at the TOP.  MJ have their tables the other way up with WOT at the bottom..... it does my head in!

My preference is for MAP as this is measuring what's actually happening as opposed to what you'd like to be happening.  Possible exception is if you have a really wild cam or weird induction system which gives a very poor vacuum signal in which case TPS might have the advantage.  With your cam and twin carbs, MAP should be fine.

You can think of the X-axis as being the same as mechanical advance and Y axis being the same as vacuum - except it's much more versatile.

Hope this helps a little.

Nick

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26 minutes ago, Nick Jones said:

You can use both in Megasquirt (most versions of anyway).  We did have this discussion before and I suggested using an MS ECU rather than MJ, partly so you would be "injection ready", but also because MS makes a pretty good data logger in it's own right.

You can think of the X-axis as being the same as mechanical advance and Y axis being the same as vacuum - except it's much more versatile.

 

Hi Nick 

Yes helpful!  so in essence if I plotted all the cells vertically to the same value and then across adjusting the advance to RPM then you would have a MJ that behaved exactly like mechanical advance. Not saying I want to do that but just getting my head around it.

Did look at using the MS option, but to do timing and logging only, the price was getting silly, and as this car can't ever be injected (well not electronically) as long as I compete in it, and I already have data logging, I decided that it was MJ.

Sorry you won't have the fun of watching the threads as I attempted FI :biggrin:

Alan

 

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4 hours ago, oldtuckunder said:

I have emailed to ask If MAP version will support using TPS instead.

Alan

Ok great response from Trigger Wheels in the evening with a few back and forth questions.

The answer is the MAP version of MJ will only do MAP it has no TPS input. The TPS version can do MAP if you have an external MAP sensor (I do) and hook the 0-5V output from the MAP sensor to the TPS input of the MJ.  The MAP sensor version of MJ has an inbuild MAP sensor that generates 0-5V which MJ uses.

So for me the best option is the TPS version as I have both TPS and MAP sensors intalled, so can see which is best.

Alan

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