megasquirt and VSS - Tuning Forum

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megasquirt and VSS
Friday, June 09, 2006 2:39 PM
I was on the highway crusing home I was doing about 85, I didnt even realise it I never go that fast there was just no one around for prospective. I was all alone on the highway so I figured I would test out one of my therorys

I figured that since the govener is basiclly a fuel cut I should be able to pass the 108 limit. (I planned on hitting and baking down right away) I climed my way to 108 and sure enough the car sputterd and the govener kicked in.

now the only thing I can think of is that somehow it used the fuel pump to stop the car, less pressure mabye? which really woulndt make sense because my ms would pick up on it and compesate. or was it a timing issue? mabye just droped it back.....alot

I really have no desire to spend any time over 90 miles an hour I was just curious to see if the limiter would kick in, and this is in no way a question on how to disable it.


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Re: megasquirt and VSS
Friday, June 09, 2006 2:52 PM
In my car (which is MPFI, not SFI so yours is probably a bit different) The Fuel pump clicks on when the ECM powers the fuel injectors. Perhaps when it goes to fuel cut like that it physically turns off power to the injectors (stock PCM) rather than grounding the injector that it needs. Maybe you need to wire that relay over so that it powers the pump relay from the Injection on the MS, rather than the stock injection harness.





Re: megasquirt and VSS
Friday, June 09, 2006 2:59 PM
alot of folks put resistors on the stock injection harness and run the injectors off the megasquirt if you do that you shouldnt have an issue because the pcm is no longer controlling whether they shut off or not



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Re: megasquirt and VSS
Friday, June 09, 2006 3:50 PM
what about running the injector power wire from an ignition wire (such as the one to your DIS) ?



Re: megasquirt and VSS
Friday, June 09, 2006 4:58 PM
SHOoff wrote:

In my car (which is MPFI, not SFI so yours is probably a bit different) The Fuel pump clicks on when the ECM powers the fuel injectors. Perhaps when it goes to fuel cut like that it physically turns off power to the injectors (stock PCM) rather than grounding the injector that it needs. Maybe you need to wire that relay over so that it powers the pump relay from the Injection on the MS, rather than the stock injection harness.


the MS uses any constant 12V and the MS ground to control the injectors so it takes the stock ecm out of the loop.


Rodimus Prime wrote:

alot of folks put resistors on the stock injection harness and run the injectors off the megasquirt if you do that you shouldnt have an issue because the pcm is no longer controlling whether they shut off or not


the ms controlls the injecotrs on every setup.

stevefire wrote:

what about running the injector power wire from an ignition wire (such as the one to your DIS) ?


you know I think my problem might be the fact that im using the fuel pump relay as a power source. and mabye my ms is actualy getting turned off???


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Re: megasquirt and VSS
Friday, June 09, 2006 5:00 PM
Maybe if you rewired the pump to a different source so the ECU doesn't control it?



Re: megasquirt and VSS
Friday, June 09, 2006 5:07 PM
well the MS can control the pump, but I severly doubt that the pump is my problem. to cut off flow to slow down a car is just stupid and I would hope thats not what happens.

but I appreciate your help


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Re: megasquirt and VSS
Friday, June 09, 2006 5:11 PM
as far as I know the ECU doesn't cut the fuel pump out, it only kills the injectors.

try running it off your power wire to your DIS, this should not get killed when you hit the speed governor.

I had trouble with my FP signal on my haltech, so I jerry rigged the relay to my 12v ignition wire just to get the car started intially.



Re: megasquirt and VSS
Friday, June 09, 2006 5:44 PM
There is also a spark ramp-out in addition to the fuel cut. Do you have the MS controlling spark as well?





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Re: megasquirt and VSS
Friday, June 09, 2006 5:48 PM
Jcavi wrote:

well the MS can control the pump, but I severly doubt that the pump is my problem. to cut off flow to slow down a car is just stupid and I would hope thats not what happens.

but I appreciate your help


Well, my other thought was that the ECU cuts spark, but I figured someone would say that's a stupid answer and flame my ass for it. But looks like Shifted brought it up so maybe I'm on the right track.



Re: megasquirt and VSS
Friday, June 09, 2006 5:52 PM
Jcavi wrote:

SHOoff wrote:

In my car (which is MPFI, not SFI so yours is probably a bit different) The Fuel pump clicks on when the ECM powers the fuel injectors. Perhaps when it goes to fuel cut like that it physically turns off power to the injectors (stock PCM) rather than grounding the injector that it needs. Maybe you need to wire that relay over so that it powers the pump relay from the Injection on the MS, rather than the stock injection harness.


the MS uses any constant 12V and the MS ground to control the injectors so it takes the stock ecm out of the loop.


Right, as it should. But, I was pointing out that his fuel pump relay is probably not recieving power from the MS, but rather the stock ECM. It gives a constant +12v down the line, then grounds out the injectors to complete the circuit. If it cuts the +12v then the fuel pump would stop as well as the injectors. I was taking a guess at the possibility of that's how the stock ECM runs the governer. Maybe it's dumb or whatever, but it would be effective.






Re: megasquirt and VSS
Friday, June 09, 2006 11:17 PM
Shifted wrote:

There is also a spark ramp-out in addition to the fuel cut. Do you have the MS controlling spark as well?


how or where does the ECU cut spark? Does it cut it through the ignition wire, or through the DIS itself?



Re: megasquirt and VSS
Saturday, June 10, 2006 7:25 AM
It doesn't cut spark, it ramps out the advance to reduce power near the fuel cut, so its not as big of a shock on the system going from WOT to nothing.





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Re: megasquirt and VSS
Saturday, June 10, 2006 7:30 AM
ah ok, thats good news I was concerned it actually cut power to DIS.



Re: megasquirt and VSS
Saturday, June 10, 2006 7:31 AM
Shifted wrote:

It doesn't cut spark, it ramps out the advance to reduce power near the fuel cut, so its not as big of a shock on the system going from WOT to nothing.


I'm not sure I completely understand. Is it just pulling timing or is it decreasing the spark energy, or what am I missing here.



Re: megasquirt and VSS
Saturday, June 10, 2006 7:53 AM
Its pulling timing.

I wrote:


it ramps out the advance to reduce power near the fuel cut


Ramps out advance = gradually pulls timing





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Re: megasquirt and VSS
Saturday, June 10, 2006 9:09 AM
Got it. Sorry, been a long week. Worked until 2am and I was right back in the office this morning at 8am.



Re: megasquirt and VSS
Saturday, June 10, 2006 12:27 PM
Shifted wrote:

Its pulling timing.

I wrote:


it ramps out the advance to reduce power near the fuel cut


Ramps out advance = gradually pulls timing


yeah I figured thats what it was, the question ( and this is all hypothectiacl because it will nevre get tested ) is how far does it get pulled, and what would happen if I kept my foot on the gas?


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Re: megasquirt and VSS
Saturday, June 10, 2006 5:49 PM
Actually, from experience it can fully retard the timing almost instantly. It only advances it so fast. For full retard you're looking at something like 30-35* I do believe. Pulling that much timing it would probably be possible to drive at 60mph, but not 110.

If you kept your foot on the gas it should start putting the timing back once you get down below 105mph (as thats where the gov is programmed return fueling) It would probably feel like its misfiring for a few seconds as the timing goes back.





Re: megasquirt and VSS
Monday, June 12, 2006 1:34 AM
The stock pcm has a constant in which the pcm compairs current vehicle speed vs rpm, and then sets timing accordingly. In our factory pcm's the curve drops off at the value set as the vehicle top limit, and vehicle top rpm not in park.

If your megasquirt is running your fuel, then any map within it would control the fuel cut, if there is no fuel cut then theoretically your fuel can go as far as your engine will before it fails.

But your stock pcm still does pull timing when you reach the top mph.

Why are you not running your megasquirt for timing and fuel. Which MS are you running. Our shop locally is learning the MS in order to tune cars, as we also do GM ODB1 in some models and some hp tuners. But what I was told was that the megasquirt can control all aspects of engine operation and the stock pcm is only needed to interface with the spedo/tach cluster options.

I guess what im asking is, doesnt your MS control your timing as well as fuel.
Re: megasquirt and VSS
Monday, June 12, 2006 4:56 AM
MS only controls fuel.

The MS-II will control timing as well.






Re: megasquirt and VSS
Monday, June 12, 2006 11:21 AM
ms 2 controls timing as whell


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Re: megasquirt and VSS
Monday, June 12, 2006 5:45 PM
MS will control both with the right software, MS2 does both natively.

SHOoff wrote:

MS only controls fuel.

The MS-II will control timing as well.

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