S4S boost gauge.....are they reliable

[pureguava]

Hello

i have just replaced a boost gauge (original lotus part from SJ)

with engine idling it reads well below 0 and only ever gets to 0.5 at boost.....

the car seems to drive well and the boost kicks in pretty hard but not having anything to compare it to I don’t know if everything is working as it should. 

I will plug in freescan and go for a blat and see what MAP I get.....

long question short : are the boost gauges unreliable? Is my gauge needle possibly not set correctly ? ( reads -ve when boost off)

al. 

[Barrykearley]

Get freescan plugged in.

car needs to be over 72 degrees for full boost to be allowed but the ecu. And it’s absolutely critical the TPS and throttle is set to 100% when you press the peddle to the floor - this is where the magic happens! Two of my esprits have had the pedal not set correctly. From the factory they were something like 85% and meant to be adjusted at first service - dunno if that’s an urban myth - but I’ve read and heard that many times

[jonwat]

17 hours ago, pureguava said:

long question short : are the boost gauges unreliable? Is my gauge needle possibly not set correctly ? ( reads -ve when boost off) 

The gauge on my old '95 S4s always seemed pretty accurate showing about 1.1 bar max & not going over 0.5 bar when the CEL bulb had blown or when the thermostat stuck open. It certainly shouldn't go negative, sounds like you've got a duffun.

Give SJ a call & ask what they think, I always found Steve pretty helpful. 

Edited November 25, 2018 by jonwat

[Chillidoggy]

Hi Alastair,

My boost gauge always read the same as John’s description above, and still does following it’s recent overhaul by Speedy Cables (as part of my refurbish job I had all the gauges done). And if the car is indeed only boosting to 0.5 bar, that would be immediately noticeable as a big drop in power. 

I’m m curious. Why did you replace your original gauge?

[pureguava]

Hi thanks for input

i replaced it as the old one was broken. No movement at all.  

I will ask SJs what they reckon.  I will try to put up a video.   It’s as if the needle has been put on the shaft 30 degrees out!!  

I am tempted to pry off the front and push it a bit to the right....but I think that will make it look aweful when I crimp it back on

Al 

[jonwat]

1 hour ago, pureguava said:

I am tempted to pry off the front and push it a bit to the right....but I think that will make it look aweful when I crimp it back on 

I wouldn't do that, the body of the gauge is made of plastic & breaks easily, speak to SJ first. 

[davevr6car]

Buy a mechanical Stack boost gauge and plumb it into the boost circuit from the inlet manifold ...this will give you a real boost figure which you can compare to your electrical dash gauge.

[snowrx]

Zero boost would be atmospheric pressure, engine idle would be vacuum (below atmospheric) so if it's a true boost gauge that sounds right to me.

I've run the VDO boost gauges in other cars and they seem reliable and relatively accurate.

[fjmuurling]

On ‎24‎/‎11‎/‎2018 at 19:33, pureguava said:

Hello

i have just replaced a boost gauge (original lotus part from SJ)

with engine idling it reads well below 0 and only ever gets to 0.5 at boost.....

the car seems to drive well and the boost kicks in pretty hard but not having anything to compare it to I don’t know if everything is working as it should. 

I will plug in freescan and go for a blat and see what MAP I get.....

long question short : are the boost gauges unreliable? Is my gauge needle possibly not set correctly ? ( reads -ve when boost off)

al. 

Remember that the MAP calculations in Freescan are not correct.

Freescan uses the following conversion for calculating/displaying the MAP pressure in bar: MAP (bar) = (MAPADC - 130)/100 + 1

The right conversion is: MAP (kPa) =  (MAPADC / 255 + 0.04) / 0.005

1 bar = 100 kPa

MAPADC = MAP sensor AD counts

[jonwat]

10 hours ago, fjmuurling said:

Remember that the MAP calculations in Freescan are not correct.

Freescan uses the following conversion for calculating/displaying the MAP pressure in bar: MAP (bar) = (MAPADC - 130)/100 + 1

The right conversion is: MAP (kPa) =  (MAPADC / 255 + 0.04) / 0.005

1 bar = 100 kPa

MAPADC = MAP sensor AD counts

That's just what I was going to say. 

[pureguava]

Hello again

 

here is the MAP from freescan....I almost get to 2.0 so I guess thats about right? Interestingly my gauge still reads negative until the boost gets to about 0.5 on freescan....

 

Thanks again

 

Al

Edited December 3, 2018 by pureguava
removed an extra duplicate graph