Was wonderering why I never hear anything about this? Couldn't I just Install a standard cheapo ebay vac/FPR on the return line of my 85GS 12a? Then I could adjust base fuel pressure and with the vac hooked up I'd get a leaner cruise fuel ratio?

Not looking for performance here just beter MPG... Anyone doing this? Would you suggest teh vac supply to the FPR be constant or just at cruise?

Thanks...

Like so...

http://cgi.ebay.com/ebaymotors/Unive...Q5fCarQ5fParts

 

Search for posts by member peejay. He has lowered his fuel pressure and changed timing to obtain better fuel economy.

 

I found his postings. But all the links to pictures are down. Do you know if they were putting the regulators on the feed line or the return? If there is no restriction in the return from the factory I don't see how you'd decrease pressure?

Also there isn't much info on those posts. Claims of 1psi on the guage but he's guessing more like 2.5? No wideband reading etc. I just don't want to lean out at WOT if I turn the base pressure down to far?

What is the factory setting on pressure?

I've got one of the cheap universal electric "v8" pumps. rated to 220hp or so the guy at the parts store claimed.... I'm sure I need to turn the pressure down.

12-volt Electric Fuel Pump, 7 to 9 psi, 45 GPH Delivery Capacity

 

Can you elaborate on how lower fuel pressure would affect mileage? If I remember how carbs work, there's a float in the fuel bowl that lowers when the level drops to allow more fuel to enter. When the fuel bowl is full, no more fuel is allowed to enter. It's like your toilet bowl. I don't see how altering the inlet pressure would affect your mileage -- unless the inlet pressure is so high or so low that it doesn't allow the valve to operate properly. If that happens, you have other problems.

 

if you're carbureted the regulator goes before the carb in the feed line. put it in the return line after the fuel rail only if you're fuel injected.
elwood you are correct. in carbed apps if lowering fuel pressure gives you better miileage its because you have other problems.

 

Other problems like an aftermarket pump pumping 3 times the psi It should be pumping? I would think that as you accelerate and the float bowls drop, once open the pressure going into the carb would be effecting your a/f mixture? I'll look into it a little more.

Thanks for the input all... but if what your saying is true then I could run 30 psi worth of fuel pressure and it wouldn't effect how the carb operates? Something isn't kosher about that....

I also don't see any dead head systems that have the ability to lower FP through VAC assist. So I'm sure your right there.

 

Your fuel reservoir isn't pressurized -- it's vented to atmosphere. So no matter how much pressure the carb sees, it won't affect your AFR, UNTIL you overpower the inlet valve. Once that happens, all bets are off, and you'll begin dumping fuel into your engine at an uncontrolled rate. The carb will cease to be a carb -- you'll have some sort of unmetered constant flow injection.

The fuel inlet valve has pressure limits -- upper and lower. Keep the fuel pressure between those limits under all operating conditions (including under WOT), and the carb will work like it's supposed to.

 

Thanks... makes more sense to me now. Next question would be what pressure is the inlet valve rated to?

The car starts idles and drives great. so I doubt I've hit that point.

But without a doubt my MPG has gone down since I've installed this new pump in march of this year. If I do install a regulator the temps are in the teens now. I can't get any good data on MPG increases until it warms up.



Also peejay claims his car ran better after installign a FPR and lowering pressure?

https://www.rx7club.com/showthread.p...=fuel+pressure

Without a wideband on the car I'll always be guessing anyway. I just have a hard time forking over the 200 dollars for my mostly original daily driven rx....

 

i installed a carter pump and a holly FPR. I turned the fuel pressure to 2.0 PSI, IIRC, and got the best results.

 

So I was talking this over with some "old-timers".


They are saying the lower the pressure, the lower float level, the carbs. main system is delayed (pullover) which results in the leaner cond
raising or lowering float level can make the equl. of a jet change?

Make sense? I suppose if my float levels were set at 2psi and I raised FP to 7psi. Woudln't my float levels change?