IS THIS NORMAL?????????

mustangfan1990 · Feb 16, 2008

Ok I was driving down the road and was looking at my fuel pressure. When I am cruising in 5th gear or in gear slowing down my pressure drops to 30 psi and as soon as I hit the gas ( 1/2 down to WOT) it jumps right back up to 40 psi. Right now there is no SC on it and it has 24s on it...just wondering

795.0pacecar · Feb 16, 2008

Yes, when you let of the gas and you are coasting your vacuum is high, so your fuel pressure gets lower because the FPR is controlled by engine vacuum.

mustangfan1990 · Feb 16, 2008

oh ok thanks alot bud sharp ride too BTW I use to have one

Foxfan88 · Feb 16, 2008

when are you getting your blower in?

mustangfan1990 · Feb 16, 2008

Foxfan88 said:
when are you getting your blower in?

it will be here Monday 2-18...cant wait I am so impatient

85_SS_302_Coupe · Feb 16, 2008

795.0pacecar said:
Yes, when you let of the gas and you are coasting your vacuum is high, so your fuel pressure gets lower because the FPR is controlled by engine vacuum.

Yeah...that's one of the crappiest down sides to running a carbed engine. If you don't have the "tricks" done to keep this from happening, nailing the brakes can stall a carbed engine quick. Luckily there's lot of things to do that'll prevent it. Unfortunately i haven't done a single one yet

Fast63 · Feb 16, 2008

85_SS_302_Coupe said:
Yeah...that's one of the crappiest down sides to running a carbed engine. If you don't have the "tricks" done to keep this from happening, nailing the brakes can stall a carbed engine quick. Luckily there's lot of things to do that'll prevent it. Unfortunately i haven't done a single one yet

I don't get what you mean. Are you saying that carbed engines have vacuum controlled fuel pressure regulators? If your carb and the rest of your system is set up "normal" you shouldn't have any stalling at all, in corners or on hard braking, it doesn't take any "tricks" as far as I've had to deal with.

85_SS_302_Coupe · Feb 16, 2008

No i guess that didn't quite make sense....

Vacuum is the basis of how a carb works...when you stomp the gas you drastically change the vacuum in the engine and that can stall it out (does on mine anyways). Part of it is fuel starvation in the bowl because all the fuel sloshes to the front away from the jets. It's the opposite of what happens to the secondaries when you're drag racing and all the fuel sloshes to the back of the rear bowl away from the rear jets. Jet extensions can prevent a lot of this.

I didn't mean to relate it directly to the EFI vacuum situation..i guess i left out a bit of what i was trying to say.

HISSIN50 · Feb 16, 2008

What RPM are you at when you do this?

mustangfan1990 · Feb 16, 2008

HISSIN50 said:
What RPM are you at when you do this?

at about 1500-2000

HISSIN50 · Feb 16, 2008

mustangfan1990 said:
at about 1500-2000

At such low RPM, you wont see tremendously-high excessive vacuum (from engine braking). Alternatively, upon decelerating from 3500 RPM or so +, you can easily ramp over 20" hg.

30 PSI is just within absolute spec. I'd keep an eye on it. Also, if you happen to have a vac gauge lying around, you could hook it up to see how your vacuum gauge relates to what the fuel gauge is doing.

mustangfan1990 · Feb 16, 2008

HISSIN50 said:
At such low RPM, you wont see tremendously-high excessive vacuum (from engine braking). Alternatively, upon decelerating from 3500 RPM or so +, you can easily ramp over 20" hg.

30 PSI is just within absolute spec. I'd keep an eye on it. Also, if you happen to have a vac gauge lying around, you could hook it up to see how your vacuum gauge relates to what the fuel gauge is doing.

well I do have a boost gauge all wired up and installed just waiting on the SC to get here to FINALLY get started on hooking it up. and when I am timing my motor do I set my timing at 10 degrees as base and then run the car to see how it does. If so and there is no det. then would I bump my timing up?

jrichker · Feb 16, 2008

A sudden decrease of vacuum affects how much air is drawn through the carb. This will cause the engine to stall.

Fast63 · Feb 16, 2008

jrichker said:
A sudden decrease of vacuum affects how much air is drawn through the carb. This will cause the engine to stall.

Not necessarily. Haven't you ever pump the brake pedal at an idle and noticed the RPM's increase? I had problems with cornering and braking causing my engine to stall and it was because my floats were not adjusted properly. If you have this kind of problem, something is wrong, it's not "just how it is" so to speak.

HISSIN50 · Feb 17, 2008

mustangfan1990 said:
and when I am timing my motor do I set my timing at 10 degrees as base and then run the car to see how it does. If so and there is no det. then would I bump my timing up?

You can do that. If you're just dinking around for a little while before the blower goes on, I'd just put it at 10* and leave it. You'll be hosing with your timing for the blower anyhow.

Just remember that we're talking about setting it at 10* with the SPOUT connector removed. Otherwise your timing will be really retarded and the car will barely idle.

mustangfan1990 · Feb 17, 2008

HISSIN50 said:
You can do that. If you're just dinking around for a little while before the blower goes on, I'd just put it at 10* and leave it. You'll be hosing with your timing for the blower anyhow.

Just remember that we're talking about setting it at 10* with the SPOUT connector removed. Otherwise your timing will be really retarded and the car will barely idle.

Sorry I didn't specify but I meant start it at 10 degrees with the blower. Right now I am running 16 degrees of timing I know I will have to set it back down to 10 but cant I keep bumping it up (w/ the SC) until det.

Foxfan88 · Feb 17, 2008

i would set it at 10 with the blower and leave it. i ran more timing and it sounded alright but i ended up blowing a HG and most likely it was detonation related. i was also running the base kit so i didnt have any control of timing retard like the bigger kit does.

the stock eec tune usually would have too much advance in the higher rpms for a blown car. it works fine the a N/A engine but with a blower you have to be careful. the ideal way to adjust timing is through the EEC, taking out more timing than usual in the higher ropms when you are under boost.

some people actually retard the timing even from the factory 10* to keep the advance out at higher rpms when untuned.

the s trim kit will come with an ignition/boost control box. it hooks to vacuum and you can adjust how timing will come out per pound of boost. it should work well for you. to keep things simple i would set it at 10 and use the boost ignition box vortech provides. the blower is going to add so much power the small amount of timing you lose will mean nothing. you will gain it all back ten fold.

mustangfan1990 · Feb 17, 2008

cool deal I will set it at 10 then. I just know at 16 it runds GREAT, but when I put it at 10 bammmm flat on its face....lol I know it's different with a S/C. How do you accually adjust with the BTM box. I know you said it adjusts how much comes out per pound of boost. How exactly do I know what to set it at?

Foxfan88 · Feb 17, 2008

you will get a knob to adjust in the car, you can retard the timing up to 4 degrees per pound. i'd start there and work your way up a bit. dont get too close to pinging but you will have some sort of range to work with. with this method you have control over it. with my kit i didnt have control and no wideband or anything sort of instrumentation or any tune and i was pretty much in the dark, pretty silly of me. oh well i learned my lesson.

mustangfan1990 · Feb 17, 2008

Foxfan88 said:
you will get a knob to adjust in the car, you can retard the timing up to 4 degrees per pound. i'd start there and work your way up a bit. dont get too close to pinging but you will have some sort of range to work with. with this method you have control over it. with my kit i didnt have control and no wideband or anything sort of instrumentation or any tune and i was pretty much in the dark, pretty silly of me. oh well i learned my lesson.

Oh I see I can start at 4 and down the line 3,2,1 and if at 1 and there is still no pinging then what? or better yet want to travel over here to NC and hook it up for me lol also how do it know how much boost I am boostin. I have a boost gauge but lets say I put it on 4 how does the box know how much boost I ma boosting?

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