Fuel rail questions
Steve Lovelock - 12/3/07 at 08:27 PM
Hello,
I have connected my GSXR TBs to my modified inlet manifold after what seems like five years (every thing I do takes hours) and can't remember
what pipe is the feed and what one is the return. Also I can't remember what to do with the vacuum outlet on top of the fuel rail. Any ideas
would be welcome. Just to help I have a Vauxhall C20XE and will be running Megasquirt and Spark (hopefully)
Cheers, Steve.JPG)
stevebubs - 12/3/07 at 08:44 PM
A is feed
B is return
EFi regulator goes after the Injectors
stevebubs - 12/3/07 at 08:46 PM
Blank off the vacuum pipe. If you're using a megasquirt of such you won't need it.
stevebubs - 12/3/07 at 08:49 PM
Should qualify that last remark before I get shot down.
vacuum pipe is normally connected to the vacuum pipes on the Throttle Bodies and allows the fuel pressure in the rail to be altered by the intake
vacuum (e.g. instant WOT = more pressure = more fuel)
There Megasquirt or whatever you're using can use a mixure of TPS & MAP to simulate this mechanical tweak.
Exception to this is when your injectors are on the limits of capacity. In this case, the extra pressure allows you to squeeze every last ounce of
fuel into the engine..
Steve Lovelock - 12/3/07 at 09:07 PM
Thanks for the reply. I have actually got a spare vacuum tube connector on the TBs which can be seen on the first body in from the left. However,
the tube on the fuel rail is very close to the bonnet so I would be pleased to get rid of it. So with a standard Vauxhall fuel pump and injectors do
you think I could get by on the TPS / MAP sensors and the megasquirt alone?
stevebubs - 12/3/07 at 09:11 PM
I would suspect so but can't be 100% sure as I don't know the flow rate of the injectors or expect hp of your engine...
Steve Lovelock - 12/3/07 at 09:17 PM
Well, it is a standard engine and injectors and I am not expecting anything super special on the BHP front. What I am most worried about is the car
driving well and the engine being good to use. Anything above 165-170 would be great. Thanks for your assistance.
Steve
martyn_16v - 12/3/07 at 09:19 PM
quote:
Exception to this is when your injectors are on the limits of capacity. In this case, the extra pressure allows you to squeeze every last ounce
of fuel into the engine..
It's the other way around actually, having the vac reference decreases the fuel delivered at idle, it won't add any more at WOT unless
it's a 'charged engine.
I'd connect the FPR vac personally. At idle the increased pressure difference between the inlet vac and the fuel rail pressure will effectively
increase the flow rate of your injectors, so you'll have to run shorter pulsewidths to compensate, thus losing some resolution and possibly
giving yourself a hard time getting a nice stable idle. They are there for a reason, and it's not exactly a struggle to connect it...
miegru - 12/3/07 at 09:40 PM
A programmable ecu definetly doesn't need the vacuum fitted. But don't close the hole!
If you find that your injecters are running above 90% at full load; get some bigger injectors. Don't play with pressure to accomplish the higher
fueling loads. The injectors aren't designed to handle this higher pressure and will wear quicker.
TangoMan - 13/3/07 at 01:33 AM
quote:
Originally posted by martyn_16v
They are there for a reason, and it's not exactly a struggle to connect it...
What he said 
I have connected mine as the lowered fuel pressure will help to tune idle.
If it is not needed, why do manufacturers spend he money to put one there. I am sure they spend thousands on mapping the injection system.
martyn_16v - 13/3/07 at 09:58 AM
quote:
Originally posted by TangoMan
I am sure they spend thousands on mapping the injection system.
And then some, they often spend thousands of hours on it 
MikeRJ - 13/3/07 at 01:19 PM
quote:
Originally posted by stevebubs
Exception to this is when your injectors are on the limits of capacity. In this case, the extra pressure allows you to squeeze every last ounce of
fuel into the engine..
Blanking off the vacuum port won't get you any extra pressure, you would need to gently squash the regulator in a vice to achieve this!
The purpose of the vacuum control is to keep the pressure accross the injectors constant, i.e. as the manifold vacuum reduces, the fuel pressure is
reduced accordingly which means you get a consistant flow rate from the injectors. Without the vacuum connection, the injectors will flow more under
part throttle than they will under full throttle.
This can be compensated for by the ECU, but it has the downside of requiring even smaller pulse widths at light engine loads which can be a problem
with high flowing injectors.
MikeRJ - 13/3/07 at 01:22 PM
quote:
Originally posted by martyn_16vThey are there for a reason, and it's not exactly a struggle to connect it...
But connecting it to just one inlet when running individual throttle bodies is likely to cause problems with the pulsing vacuum, just as connecting a
vacuum advance in the same way does. For ITB's I wouldn't bother connecting it.
If you used the injector bosses in the throttle bodies themselves rather than the manifold you don't have this problem at all as the injector is
then before the throttle butterfly, so sees a more or less constant pressure.
[Edited on 13/3/07 by MikeRJ]
martyn_16v - 13/3/07 at 05:52 PM
quote:
Originally posted by MikeRJ
If you used the injector bosses in the throttle bodies themselves rather than the manifold you don't have this problem at all as the injector is
then before the throttle butterfly, so sees a more or less constant pressure.
Umm, no it's not. I'd hate to be near an engine like that when it's idling (if it even managed to), fuel spraying at the outside of a
closed butterfly? Keep the fire extinguisher handy...
To be fair there are setups where you do have injectors outboard of the throttles (F1, newer GSXRs), but they either also have a set in the
'normal' place to cope with the part throttle running, or in the case of F1 engines just don't idle 
The outboard injectors on road
systems don't spray when the throttles aren't open.
I didn't think vacuum pulsing would affect the rail pressure that much though, didn't think an FPR was quite that sensitive (or fast). If
you've got all of the original setup from the bike the four throttles do actually have all their vac ports tee'd together, just take your
feed off that if you're worried.
martyn_16v - 13/3/07 at 05:53 PM
quote:
Originally posted by MikeRJ
Blanking off the vacuum port won't get you any extra pressure, you would need to gently squash the regulator in a vice to achieve this!
The purpose of the vacuum control is to keep the pressure accross the injectors constant, i.e. as the manifold vacuum reduces, the fuel pressure is
reduced accordingly which means you get a consistant flow rate from the injectors. Without the vacuum connection, the injectors will flow more under
part throttle than they will under full throttle.
This can be compensated for by the ECU, but it has the downside of requiring even smaller pulse widths at light engine loads which can be a problem
with high flowing injectors.
I'm sure I said that 
MikeRJ - 13/3/07 at 06:49 PM
quote:
Originally posted by martyn_16v
Umm, no it's not. I'd hate to be near an engine like that when it's idling (if it even managed to), fuel spraying at the outside of a
closed butterfly? Keep the fire extinguisher handy...
Apologies, I assumed these were the same as TB's I've seen that have the injector before the throttle butterfly.
However your fears regarding the risks of such a setup causing fires are unfounded. When the throttle is closed the injector would only be spraying a
tiny amount of fuel anyway, and it should be directed towards the small opening at the edges of the butterfly where gas speed is high and the fuel
stays in suspension.
To make the best power the injector needs to be placed far back from the port, you might have seen race engine where the injector is actualy suspended
over the end of the intake trumpets. Placing the injector very close to the inlet valves is done for emissions and economy reasons, e.g. less area
for the fuel to condense on when engine is cold. This wouldn't work well at idle to to fuel drop out, but an injector mounted just behind the
throttle should be fine.
Have a look at http://www.usrallyteam.com/content/tech/injector_staging_article.pdf to see the considerable power advantages that mounting
the inectors before the throttles gives (this car used staged injectors to get best of both worlds).
quote:
Originally posted by martyn_16v
I'm sure I said that
Yes you did, I was replying in reponse to another poster that suggested injector flow could be increased.