Rail Pressure Gauge

[DLeno]

Doug, are you saying that one could see high rail pressures with just injectors?



High enough that when there is a sudden lift of the foot it could cause enough pressure to pop a relief valve?



JRG

I don't know about actually openging the valve, but definately there will be a short spike. under the conditions you mention, the CP3 is delivering VERY high volumes of fuel to the rail, and it cannot react instantaneously to a sudden lift of the foot. So where does all this pressure go? I have a hunch that if we understood the rail better, and the real mechnical dynamics that happen (it is a very complex pressure behavior), we would probably find that the relief valve opens more often than we think.

 

[DLeno]

I maxed out my SPA gauge with just injectors and TST. The high number recall was 27,000 psi. I don't remember if this was at WOT momentarily or if it occurred when I lifted off the throttle quickly.



Before the TST, I popped my relief valve late last summer when I was running the 03 Edge Juice (timing/duration/pressure) with injectors.

you are not maxing out the SPA you are maxing out the on-board factory (Bosch) rail pressure sender.



there are two problems with a 27,000 psi figure coming from a digital gauge. First, you don't know if you are reading a short, momentary spike or a sustained pressure. In particular, the peak detect is probably showing you the highest detected peak, not the highest sustained average. So it really isn't meaningful. There are very short spikes going on all the time in the rail, but those are not the ones you care about.



Second, 27,000 psi isn't really meaningful -- this figure is at or just a bit above the saturation point of the factory (bosch) sending unit from which the SPA receives its signal. In my tests I cannot read over 26,500, but that could be because I read the rail pressure signal with respect to signal ground at the sensor and not with respect to chassis ground in the cab. I found that the vehicle chassis is noisy and that I got slightly higher readings when using chassis ground. So that probably explains why the SPA reads a bit higher than I have been able to measure. There will also be some differences from truck to truck. my sender might saturate at a lower pressure than yours.



The Pricol gauge that I worked on has an input circuitry that snubs the short spikes to you see sustained pressures. it also brings in two wires from the engine compartment to avoid the noise inherant in the chassis ground.



so the bottom line is that when you read 27,000 with the peak recall, you don't know if actual pressure is higher than that (the sender is blind to pressures above approximately 26,500), and you dont' know if that pressure is sustained or just a digital spike. and if it is a spike you dont' know how short or long it is, or whether or not you should care.

 

[DLeno]

Thanks for the info DLeno I did some playing tonight with the drbIII an found that it is within 1000 psi of gauge usually on the high side. I think your right about the gauge not reacting as quickly I did not see those spikes on scanner making a pass it would hold about 18000-19000 an jump to about 22000 between shifts an came down to 16000 on a long run in 6th gear. This is with box plugged in but on 0 an injectors no programmer. Checked pressure at cp3 pump holds 14psi at idle an drops to 9 on a pass. With the programmer on 70hp tow or the box on anthing over level 5 the pressure wouldnt get above 16000psi anything over lvel 5 an box foooled drb an gauge on box would barely hold 15000 at best. Does this sound like my pumps going bad or is a little over 100hp all a stock pump can supply?

The stock lift pump is good for only about 100HP. the CP3 itself is good for way more -- guys are getting over 500 HP to the rear wheels with the stock CP3.



There are just too many variables to know how to interpret the rail pressures you mention. they don't look scary, but again I'm so used to my analog gauge I find it hard to interpret digital readings. I don't know how your digital gauge interprets the factory sender, thats all I'm saying. keep in mind that DRB will alwasy read lower than a properly installed and calibrated rail pressure gauge if you have a pressure box installed.