Model specific tech items are listed here:
Saab 900 Calssic—Short Shifter
Volvo V70 XC—Horn Relocation
Performance Calculator
I came across this great performance calculator online developed by Jeff
Chojnacki. For anyone who is running a rising-rate regulator with larger
injectors on your car it makes adjustments a breeze. If you are running larger
than stock injectors it is necessary to lower the static fuel pressure to
keep from running rich at idle. This, unfortunately, negates the effect of
the larger injectors under boost because the stock FPR works on a 1-1 ratio.
If you add a rising-rate regulator you can increase your fuel pressure
under boost at a higher ratio.
Downtime is running 33pph injectors at a static fuel pressure of 37, 20 lbs of boost, and a gain rate of 4.5:1. I arrived at these numbers with Jeff's calculator. Here's how:
I first input all the information for the stock setup for my car at 0lb.
boost. This will give me some baselines to hit with the revised numbers.
- The stock T1 injectors are 27pph.
- The injectors rated pressure (the stock fuel pressure) is 55lb.
- I set the Duty Cycle to 80%. This is the average for most fuel injected
vehicles.
- The number of injectors is 4
- The Static Fuel Pressure is 55 (this is stock)
- The boost pressure is 0lb.
- The rate of gain is 1:1 (stock FPR)
The important number here is the "Calculated Flow Rate". This is the number
you will need to match (or come close) with the new injectors in order
to keep the car from running rich at idle. To do this you will need to
lower the "Static
Fuel Pressure". You will need an adjustable fuel pressure regulator to do
this (see below).
- The new injectors are 33pph. (T2)
- The injectors rated pressure (the stock fuel pressure) is 55lb.
- I kept the Duty Cycle to 80%.
- The number of injectors is 4
- The Static Fuel Pressure is lowered to 37.
- The boost pressure is 0lb.
- The rate of gain is 1:1
As you can see the Calculated Flow Rate on both screens is almost the
same. Unfortunately, so is the Max HP. Now you need to calculate
the rate of gain for the RRFPR. Here, the important number to watch
is the "Fuel Pressure at Boost". Fuel injectors can accept only
about 70psi. at the tip. To calculate this you subtract the amount
of boost from the actual fuel pressure at boost. Play with the "FPR
Gain Rate" and "Boost Pressure" numbers until you get as close as
you can to 70lbs of pressure at the tip at max boost. For me the
magic numbers were 20lb. of boost and a rate of 4.5:1.
As you can see the Fuel Pressure at 20lb. boost is 90lb. If you subtract
the two numbers you get 70lb. at the tip of the injector. Now
take a look at the Max HP :-).
In order to set the RRFPR you'll need an acurate fuel pressure gauge,
boost gauge, and air/fuel ratio gauge that you can see in the
cockpit.
Download Jeff Chojnacki's calculator. Windows
only (134k).
Adjustable Fuel Pressure Regulator
Exploded view.
To save myself some cash on a part that would be useless once I do the TI to TII conversion, I made my own adjustable fuel pressure regulator. To my surprise it works! Here's how I did it.
I picked up a used TI FPR from a local bone yard for about five bucks and the rest of the miscellaneous parts I bought at the hardware store.
Here's what you'll need:
- A stock FPR
- A 1.5" fender washer with a 3/8" center hole
- A 1/8" NPT to 1/8" hose fitting
- A 1/2" copper pipe cap
- A 3/4" 10/24 set screw
- 8 small screws
- RTV silicone sealant
Here's what you do:
  1. Put the FPR in a vise like this and use a rotary tool to cut through the upper casing. The idea here is to cut the upper hose barb off and make a hole JUST BIG ENOUGH to get the spring out. The vise keeps the spring from popping out at you as you're cutting.
2. Once the spring is out, clean up the hole you cut in the casing so that there is 1/32" of space around the diameter of the spring.
3. Take the spring to the bench grinder and grind about 1/16" off of each end to shorten it. This is so you can lower the fuel pressure for larger injectors.
 4. The 1/2" copper pipe cap should fit inside the spring. We use this to compress the spring, so it will need to have a lip on it to overlap the spring at the top. I was able to begin the lip with a pair of pliers and finish up with a hammer on the anvil. You will also need to drill a 1/8" hole through the side of the cap so the vacuum line will work.
5. Now tap the center of the fender washer to accept the 1/8" NPT nipple. Put some RTV sealant on the threads and screw the nipple into the washer. It should be very snug and barely protrude from the back side of the washer.
6. Next tap the inside of the nipple to accept the 10/24 set screw.
7. Now drill eight holes around the perimeter of the washer. These holes will accept the small screws that will hold the washer to the FPR casing. The holes need to be large enough for the screws to just slide through. The holes should also be halfway between the outside of the casing and the large hole you made in the casing to get the spring out.
8. With the spring out, put the washer on the FPR casing and mark for the corresponding holes in the FPR casing. Drill and tap these holes for the small screws.
9. Now the tough part. With a drill bit smaller than the allen wrench fitting in the end of the set screw, drill a hole down the center of the screw without coming out the bottom. Then, cross-drill near the bottom of the screw to meet this hole. This is so the vacuum line can still work when the set screw is in place.
 NOTE: the best way to drill down the center of a screw is to drill a hole in a block of wood using a drill press, put the set screw snugly within this hole, then drill down the center of the screw.
10. To put it all together, put the spring inside the FPR casing, put the copper compressor inside the spring, place the washer in place and compress everything in the vise. Attach the washer to the casing with the small screws. Only screw them in about half way. Take the FPR out of the vise, squeeze RTV in the area between the washer and casing (don't overdo it). Then, tighten down the screws and wipe away the excess sealant.
11. Let the sealant dry overnight. Then, put the set screw in place inside the new vacuum hose fitting and turn it till it seats on the copper spring compressor. That's it!
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