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Discussion Starter #1
I can't find where anyone else has posted on this, so before I start diving down the rabbit hole trying to figure this out. Driving to work this morning, I realized I had my COBB plugged in and displaying the boost gauge. I was sitting at a light and happened to notice that the COBB gauge had a different value than the OEM gauge displayed in my digital instrument cluster. I then checked against the PP gauge and same thing. The 2 OEM gauges agreed with each other and the COBB was exactly 1/2 the value. Once the light changed, I noticed under varying load that at all times the OEM gauges agree with each other and the COBB gauge was exactly 1/2 the value of OEM. This applied to vacuum only. Once boost was being measured, all 3 gauges agreed with each other. Has anyone else noticed this behavior either previously or currently? I'm going to remove my custom tune and put on the COBB OTS Stage 0 when I get home from work tonight, to see if it is a product of a specific tune or the COBB device itself. Another related question on this would be are the OEM gauges and the COBB using the same source for vacuum. I would assume that since the OEM gauges are CAN and the COBB is plugged into OBD, that it would be the same, but maybe someone knows different. I really would like to get a definitive answer on this to know if it is normal and if not, what I should be looking at as a fix, COBB, dealer warranty, something simple.
 

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An aftermarket device that communicates directly with the PCM will always show a more accurate value over the stock gauges. Most gauges on any car have a safe zone that they will stay in so people don't freak out when they don't know what they are looking at.

For example the water temp gauge will stay in the middle even though the water temp will constantly be fluctuating between 175-200 degrees on a properly running engine, based on if your moving sitting still etc. If the stock gauges fluctuated that much, your average joe would be freaking out.

The accessport also displays a wider range for all the parameters it monitors, another example are the AFR readings. The center stack gauge will go up to 20.0 with the throttle closed, where my accessport displays 40.0 - 44.0 with the throttle closed.

The discrepancy you are seeing is the fact that the accessport is a more accurate device than the stock gauges.
 

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Discussion Starter #3
I hear what you are saying, but I can't believe that a 100% deviation can be chalked up to accuracy. I've dealt with that issue a multitude of times over the years with different builds and the deviation at its worst was never more than 10%. I have a mechanical vacuum gauge somewhere that if I can find it I will plug into reference vacuum and check. I'm betting it is going to read fairly close to the OEM gauges. A 2x difference is hard to swallow as a variance in accuracy or a function of input value smoothing. I'll post my results once I change tune tonight and if I can find my vacuum gauge I'll post that also.
 

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I hear what you are saying, but I can't believe that a 100% deviation can be chalked up to accuracy. I've dealt with that issue a multitude of times over the years with different builds and the deviation at its worst was never more than 10%. I have a mechanical vacuum gauge somewhere that if I can find it I will plug into reference vacuum and check. I'm betting it is going to read fairly close to the OEM gauges. A 2x difference is hard to swallow as a variance in accuracy or a function of input value smoothing. I'll post my results once I change tune tonight and if I can find my vacuum gauge I'll post that also.

Mine does the same thing with vacuum vs boost, just saying. I think you're fine.
 

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Discussion Starter #5
Mine does the same thing with vacuum vs boost, just saying. I think you're fine.
Thats good to hear, but I still want to prove out which one is reading closer to actual. Once I plug in a mechanical vacuum gauge to reference vacuum that will tell me. If the OEM gauges are reading close to actual my next step is to contact COBB to find out wtf. If the COBB is close to actual, then I might pursue it a bit further with regards to how OEM is obtaining the values it displays, but that is a different ball of yarn.
 

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Thats good to hear, but I still want to prove out which one is reading closer to actual. Once I plug in a mechanical vacuum gauge to reference vacuum that will tell me. If the OEM gauges are reading close to actual my next step is to contact COBB to find out wtf. If the COBB is close to actual, then I might pursue it a bit further with regards to how OEM is obtaining the values it displays, but that is a different ball of yarn.
I'm interested to see what you find!
 

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Discussion Starter #7
Another angle just occurred to me. Can anyone on this forum with an SCT or any other device for that matter verify what deviation (if any) they see between OEM gauge and their device for vacuum? I would really be interested to know. A reading at idle where it is steady would be sufficient to compare the values.
 

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I think the issue is with the units. My Accessport displays vacuum in PSI, but the factory gauge displays vacuum as in.HG. PSI is roughly half of in.HG for the same vacuum reading. So everything is correct and in sync, the units of measurement are just different.
 

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Discussion Starter #9
I think the issue is with the units. My Accessport displays vacuum in PSI, but the factory gauge displays vacuum as in.HG. PSI is roughly half of in.HG for the same vacuum reading. So everything is correct and in sync, the units of measurement are just different.
I believe that is the answer! I verified that changing the tune did not change the variance. A mechanical gauge hooked to reference vacuum matched OEM gauges within a couple decimals. Doing the math to covert COBB reading to PSI matches the OEM gauges within a couple decimals.
 

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Have you asked over at Cobb yet? You are more likely to get a good answer straight from the source.
 

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I think the issue is with the units. My Accessport displays vacuum in PSI, but the factory gauge displays vacuum as in.HG. PSI is roughly half of in.HG for the same vacuum reading. So everything is correct and in sync, the units of measurement are just different.
You're absolutely right, I completely forgot that that cobb only measures it in PSI where the center stack/ PP gauges go from PSI to HG
 
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