Fuel Pressure Gauge
02-26-05, 11:48 PM
#2
The gauge I *am* using and the gauge I *want* to use are two different things.
I *want* to be using the gauge that Paul Yaw sells. Why? Well he's got an entire write-up somewhere talking about the inaccuracies of automotive Fuel Pressure Gauges. Here's a summary:
Gauges are rated for accuracy by percentages, for example my gauge has an accuracy of 3%-2%-3% . What this means is that in the middle third of the gauge, it can be off by plus or minus 2%, and in the upper and lower thirds of the gauge, it is accurate to plus or minus 3%.
Now this doens't sound like a lot, until you realize that the 2% is 2% of the FULL SCALE of the gauge. It's nearly impossible to find an automotive fuel pressure gauge for any smaller than 0-15PSI. With our carburetors, we're aiming for a fuel pressure around 2.5-3.5 psi, which puts us in the lower third of the guage.
That means on my 0-15psi gauge, my fuel pressure actually can be off by almost half a psi! When you're aiming for 3.5psi, you could be getting 3psi, or you could be getting 4psi.
What you want is a gauge that puts 3.5 somewhere in the middle third of the gauge. A gauge going from 1-8psi would be perfect, but alas, it doens't exist.
The gauges that Paul Yaw has are originally intended for Industrial use, and they're extremely accurate. They have a small enough range, and a high enough accuracy that you won't be second guessing your fuel pressure when you need it. This comes at a price though, as his gauges cost almost fifty dollars.
The last thing that you need to know is that since the fuel pressure regulators are mechanical devices, they don't kick in *right away* when the car turns on. This means that there is a momentary spike to 6psi in my case, which would damage a sensetive instrument like Paul Yaw's gauges, which are only rated for (I believe) 4psi. He installs what he calls a "snubber" to deal with this. This gauge should also not stay mounted in the engine bay because the vibrations will cause it to become innacurate over time.
ALL of this having been said, I run an el-cheapo Mr Gasket gauge that you can find at almost any store that sells ricer accessories and car parts in one store. Get the gauge with the smallest range you can find. It will get you in the right ballpark. You only really need that extra accuracy when you're going for the last 10% of horsepower that you get from proper tuning, jetting and dyno testing. For street, as long as you're sitting between 2 and 4 psi, you *should* be good.
Jon
I *want* to be using the gauge that Paul Yaw sells. Why? Well he's got an entire write-up somewhere talking about the inaccuracies of automotive Fuel Pressure Gauges. Here's a summary:
Gauges are rated for accuracy by percentages, for example my gauge has an accuracy of 3%-2%-3% . What this means is that in the middle third of the gauge, it can be off by plus or minus 2%, and in the upper and lower thirds of the gauge, it is accurate to plus or minus 3%.
Now this doens't sound like a lot, until you realize that the 2% is 2% of the FULL SCALE of the gauge. It's nearly impossible to find an automotive fuel pressure gauge for any smaller than 0-15PSI. With our carburetors, we're aiming for a fuel pressure around 2.5-3.5 psi, which puts us in the lower third of the guage.
That means on my 0-15psi gauge, my fuel pressure actually can be off by almost half a psi! When you're aiming for 3.5psi, you could be getting 3psi, or you could be getting 4psi.
What you want is a gauge that puts 3.5 somewhere in the middle third of the gauge. A gauge going from 1-8psi would be perfect, but alas, it doens't exist.
The gauges that Paul Yaw has are originally intended for Industrial use, and they're extremely accurate. They have a small enough range, and a high enough accuracy that you won't be second guessing your fuel pressure when you need it. This comes at a price though, as his gauges cost almost fifty dollars.
The last thing that you need to know is that since the fuel pressure regulators are mechanical devices, they don't kick in *right away* when the car turns on. This means that there is a momentary spike to 6psi in my case, which would damage a sensetive instrument like Paul Yaw's gauges, which are only rated for (I believe) 4psi. He installs what he calls a "snubber" to deal with this. This gauge should also not stay mounted in the engine bay because the vibrations will cause it to become innacurate over time.
ALL of this having been said, I run an el-cheapo Mr Gasket gauge that you can find at almost any store that sells ricer accessories and car parts in one store. Get the gauge with the smallest range you can find. It will get you in the right ballpark. You only really need that extra accuracy when you're going for the last 10% of horsepower that you get from proper tuning, jetting and dyno testing. For street, as long as you're sitting between 2 and 4 psi, you *should* be good.
Jon
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