Bringing this back because I never answered TTA89 lol. Jeez I've been MIA for 7 years. :eek!:
Your assumption was right on the money!!!! Now for the reason for this huge difference in boost. One word......FLOWMASTER.
Capped up I was running through a 3" single muffled by a Flowmaster muffler and seeing 16 psi.
Uncapped, the boost went to 27psi in 1st & 2nd gears and 30psi in 3rd gear. No changes to the ATR external wastegate mind you.
14 PSI of back pressure through the Blowmaster.
I was running a RA race chip and a tight Pat's 2800 converter. This caused the car to be a dog to the 1/8th mile (97 mph). When I hit 3rd the car woke up and even though she lean popped 4 times through the intake (340 pump but not an M) she was able to run 126 mph in the 1/4. The non 'M' 340 pump can't handle 70+ psi of fuel pressure, the M does. I was running about 41-42 psi FP with the hose off at idle.
Had a grudge race with a buddy's 80 Z28 (468 BB) so I capped the exhaust back up and cranked the waste gate to 25 psi. Ran an 11.5 at 121 or so and beat him by a couple of cars. After the race I was seeing bubbles in the overflow tank so I knew I popped a gasket. Hmm let's see...25 psi on top of 14 psi of Blowmaster back pressure..........39 psi. :cheers:
Shortblock was stock 109, .020 JE pistons, stock rods with ARPs, stock crank, 2 steel caps.
Top end was ported 1.77 heads, ported intake, stock TB, ESP stage 4 cam.
Induction was a 9" K&N, 3" MAF pipe, stock MAF with screens (RA says to take them out with the race chip), Limit T63e, KB Big Boy (lol).
Exhaust was an ATR 3" DP with external wastegate, stock manifolds and 3" single.
Drivetrain was a slipping and stock 2004r out of a 307 powered car, Pat's 2800, stock rear. Tranny wouldn't shift higher than 4000 RPM when left in D so I had to manually shift off the stock tach. lol
Suspension was all stock including having the front sway bar in.
Wheels/tires were stock fronts, Welds and 27x10 M&H slicks in the rear.
With a 340 M pump, real tranny and a 9'' non lock up she would have easily ran 10's at 130.
I believe you are correct. I know Walbro used to make a 255 L pump in both low and high pressures. I know it was a 255 L pump but it wasn't the high pressure one and fuel flow would significantly diminish as the pressure went up. I should have just just put a RA XP pump with the stronger bypass spring in there from the get go.