Engine guru's advice please.. Re: Upping the fuel pressure
Discussion
I know a fair bit about engines, however I know there are folk around here on PH that know tons more than I do, so I wanted some wisdom and advice on the matter of fuel pressure on injected cars..
On a modern injected car, forced induction or NA.. Are there are gains to be had by way of simply upping the fuel pressure? I tend to think not, assuming everything else including mapping is left alone.
Common sense tells me that either the car will run rich, which doesn't = more power. Or the ecu via its myriad of sensors is going to sense the rich mixture, and simply dial the additional fuel back out anyway. Again, no power gain.
However, some swear blind they feel an improvement in their cars performance after upping the fuel pressure via a higher pressure fuel regulator or whatever..
Some cars for example have a 3.8bar regulator in, so owners are swapping them out for a 4bar one. Or the car already has a 4bar regulator, so folk install a 4.5bar one etc.
What gives? Discuss please..
Thanks in advance, P
On a modern injected car, forced induction or NA.. Are there are gains to be had by way of simply upping the fuel pressure? I tend to think not, assuming everything else including mapping is left alone.
Common sense tells me that either the car will run rich, which doesn't = more power. Or the ecu via its myriad of sensors is going to sense the rich mixture, and simply dial the additional fuel back out anyway. Again, no power gain.
However, some swear blind they feel an improvement in their cars performance after upping the fuel pressure via a higher pressure fuel regulator or whatever..
Some cars for example have a 3.8bar regulator in, so owners are swapping them out for a 4bar one. Or the car already has a 4bar regulator, so folk install a 4.5bar one etc.
What gives? Discuss please..
Thanks in advance, P
PJR said:
Common sense tells me that either the car will run rich, which doesn't = more power. Or the ecu via its myriad of sensors is going to sense the rich mixture, and simply dial the additional fuel back out anyway. Again, no power gain.
I'd agree with that, why up the fuel pressure unless it actually needs upping....
To understand why people up the fuel pressure you need to understand the fundamentals on what is going on.
In essence the engine controller is instructing the fuel system to drop x ccs of fuel into the engine at a certain speed and load point. The amount of fuel is determined by the size of the injector nozzle, the time opened and the fuel pressure.
If you put bigger injectors on - say those rated at 40#/hr vs 36#/hr, you get roughly 10% fuel in at all times. HOWEVER, as the car will have a lambda sensor, it will realise the engine is running rich when the engine management is in closed loop operation and just reduce the fuel needed (it thinks) by 10%, getting you back to where you were.
Alternatively the fuel pressure could be upped and you would get the same effect.
The problem is that when the engine is in open loop control (acting like a carburettor would) the engine would be running more richly. This open loop control is normally at idle or during transients (acceleration). Typically those selling the kit have wrked out what increase will make the car accelerate fastest.
For a track car that wouldn't be that big a deal but if you were commuting it would start to be a pain in traffic, and if you want the car to pass an emissions test - it might be marginal on a good day if the engine can't run in closed loop at warm idle.
In essence you have to combine the effects together - chip, pressure, and injectors. The other complication with chips is that they tend to have a lot of the safety protection against knock removed, so you can hole a piston if the wrong combination of events occur.
With cars like the Cosworths and Imprezas where tuning companies have a huge market and the time to put the effort in to getting the combinations right the cars will be driveable and deliver the power with adequately legal emissions (for the UK). Crude tuning will fix one aspect to the detriment of another, and that may not make the car legal or driveable.
I hope this isn't too much of a disappointment.
In essence the engine controller is instructing the fuel system to drop x ccs of fuel into the engine at a certain speed and load point. The amount of fuel is determined by the size of the injector nozzle, the time opened and the fuel pressure.
If you put bigger injectors on - say those rated at 40#/hr vs 36#/hr, you get roughly 10% fuel in at all times. HOWEVER, as the car will have a lambda sensor, it will realise the engine is running rich when the engine management is in closed loop operation and just reduce the fuel needed (it thinks) by 10%, getting you back to where you were.
Alternatively the fuel pressure could be upped and you would get the same effect.
The problem is that when the engine is in open loop control (acting like a carburettor would) the engine would be running more richly. This open loop control is normally at idle or during transients (acceleration). Typically those selling the kit have wrked out what increase will make the car accelerate fastest.
For a track car that wouldn't be that big a deal but if you were commuting it would start to be a pain in traffic, and if you want the car to pass an emissions test - it might be marginal on a good day if the engine can't run in closed loop at warm idle.
In essence you have to combine the effects together - chip, pressure, and injectors. The other complication with chips is that they tend to have a lot of the safety protection against knock removed, so you can hole a piston if the wrong combination of events occur.
With cars like the Cosworths and Imprezas where tuning companies have a huge market and the time to put the effort in to getting the combinations right the cars will be driveable and deliver the power with adequately legal emissions (for the UK). Crude tuning will fix one aspect to the detriment of another, and that may not make the car legal or driveable.
I hope this isn't too much of a disappointment.
PJR said:
On a modern injected car, forced induction or NA.. Are there are gains to be had by way of simply upping the fuel pressure? I tend to think not, assuming everything else including mapping is left alone.
Increasing the fuel pressure is likely to increase the amount of fuel going in to the engine.
What you want is just the right amount of fuel. If the engine is standard and in good condition with a modern engine management system then it is probably getting the right amount of fuel as standard.
If there's something wrong with it, or it had been modified and getting more air in and the engine management system hasn't already compensated by adding more fuel, then it might be running lean. If so, raising the fuel pressure would probably improve things. It's a very crude way to do it though, better to sort the map out properly to get the fuelling accurate everywhere.
If the engine isn't running lean then adding more fuel is just likely to reduce power, increase engine wear and throw money down the drain.
If the engine actually needs more fuel, then it can have benefits. If it doesnt, then you wont lol.
Pretty simple really.
Until you know how its fuelling at the minute, and can make a proper decision as to whether it needs more fuel, then guessing, and just doing it, is a bit stupid.
Pretty simple really.
Until you know how its fuelling at the minute, and can make a proper decision as to whether it needs more fuel, then guessing, and just doing it, is a bit stupid.
Cheers chaps.. You all confirmed what I had suspected already.
It wasn't something I was particularly wanting to do myself by the way. But on 1 or 2 other forums I use, there is a bit of a fad going on where people are fitting higher pressure fuel regulators as a 'mod' and claiming performance improvements. Which to me didn't make sense. I could only think it was a placebo effect.. It is an easy part to fit after all, and to some folk "more fuel" must mean "more power"..
I did try to explain that this is not especially the case, but they were not having any of it..
Anyway, thanks again for clarifying, but if any other folk have anything of interest to add, by all means do
Cheers, P
It wasn't something I was particularly wanting to do myself by the way. But on 1 or 2 other forums I use, there is a bit of a fad going on where people are fitting higher pressure fuel regulators as a 'mod' and claiming performance improvements. Which to me didn't make sense. I could only think it was a placebo effect.. It is an easy part to fit after all, and to some folk "more fuel" must mean "more power"..
I did try to explain that this is not especially the case, but they were not having any of it..
Anyway, thanks again for clarifying, but if any other folk have anything of interest to add, by all means do
Cheers, P
I have a Caterham 21 powered by an early VVC (1996 EU2 engine) which use to have a 48mm tb on it.
When I swapped for a 52mm tb the car was running very rough below about 2500, which was a pain while cruising and when accelerating it had some hesitation as well.
I slightly increased the pressure and it is much better than before, but it still does it sometime. I don't know by how much I have increased it.
The engine has a bespoke exhaust system.
Why after I have increased the fuel pressure the engine is less rough and hesitate a lot less?
Could it have been running lean before?
Jack
PS: the engine is decated and a the last MOT, it passed the emission test with flying colours!
When I swapped for a 52mm tb the car was running very rough below about 2500, which was a pain while cruising and when accelerating it had some hesitation as well.
I slightly increased the pressure and it is much better than before, but it still does it sometime. I don't know by how much I have increased it.
The engine has a bespoke exhaust system.
Why after I have increased the fuel pressure the engine is less rough and hesitate a lot less?
Could it have been running lean before?
Jack
PS: the engine is decated and a the last MOT, it passed the emission test with flying colours!
[quote=jack&mle]
Why after I have increased the fuel pressure the engine is less rough and hesitate a lot less?
Could it have been running lean before?
[/quote]
All those changes could have affected the fuelling requirements substantially, it's remarkable that you managed to pass an emission test without correcting the fuelling.
Why after I have increased the fuel pressure the engine is less rough and hesitate a lot less?
Could it have been running lean before?
[/quote]
All those changes could have affected the fuelling requirements substantially, it's remarkable that you managed to pass an emission test without correcting the fuelling.
[quote=jack&mle]I have a Caterham 21 powered by an early VVC (1996 EU2 engine) which use to have a 48mm tb on it.
When I swapped for a 52mm tb the car was running very rough below about 2500, which was a pain while cruising and when accelerating it had some hesitation as well.
[/quote]
I would suggest that something was disturbed when the larger throttle body was fitted, or something was not replaced correctly such as a beather pipe. A slightly larger throttle body should make no difference to the fueling requirements at part throttle.
When I swapped for a 52mm tb the car was running very rough below about 2500, which was a pain while cruising and when accelerating it had some hesitation as well.
[/quote]
I would suggest that something was disturbed when the larger throttle body was fitted, or something was not replaced correctly such as a beather pipe. A slightly larger throttle body should make no difference to the fueling requirements at part throttle.
One question related to this - might you not get slightly improved atomisation running a higher pressure at the injectors? A minor effect maybe, but a smaller droplet size in the cylinder at ignition could yield a slightly faster flame front expansion away from the spark plug and a marginal power increase.
you have to remember that ecu's have fixed reference points and variables, the fixed points are things like injector nozzles where the ecu will use this figure to calculate the opening duration in comparisson to fuel pressure giving the calculated amount of fuel required , fixed variable is when the throttle pot detects load the air mass meter calculates air flow the ecu will assume that fuel rail pressure will have been increased by the pre-programmed amount as the vaccuum operated fuel pressure regulator will restrict fuel returning to the tank by means of vaccuum pipe fron the inlet manifold , if you replace the regulator with one which increases the pressure then the lambda sensor will know if there is rich running situation thus the ecu will decrease the injection pulse time in accordance to maintain lambda
nel said:
One question related to this - might you not get slightly improved atomisation running a higher pressure at the injectors? A minor effect maybe, but a smaller droplet size in the cylinder at ignition could yield a slightly faster flame front expansion away from the spark plug and a marginal power increase.
Niels, Better atomisation and better mixing on a Spark ignition engine as an end to itself doesn't reap the rewards as is often fabled in my experience. Theoretically it could give a slightly faster burn- but if you compared two engines one with a slower burn and one with a faster burn and everything else equal (very unlikely in the real world) there wouldn't be much torque output benefit.
Manufacturers measure burn duration usually quoting 10% burn to 90% burn duration. Typical values for a good pent roof engine at full load could be 24-28 degrees. Higher injection pressure would effect that figure minimaly (unless we're talking GDI pressures).
There are other much more effective ways to get a faster burn, through charge motion (usually at the expense of flow).
Better atomisation could be good for Hydro carbon emissions and may even allow you to run leaner at low loads (if such things were allowed!)
However Faster burn can give a better knock limit, thus allowing more ignition or allow you to specify a higher Compression ratio (as Porsche did when they went to twin plug)- the net result is more power.It can also allow higher EGR tolerance at part load for better fuel economy, and allow you to retard the ignition more without the combustion becoming unstable- beneficial during the catalyst warm up phase for emissions...
With diesel engines it's a totally different story with a different regime of combustion so better mixing via higher rail pressures COULD indeed yield an appreciable increase in power.
Edited by Marquis_Rex on Saturday 28th October 11:37
Edited by Marquis_Rex on Saturday 28th October 11:38
Edited by Marquis_Rex on Sunday 29th October 16:46
If its currently running lean, then you may gain a little.
if its currently running rich....you wont. Its as simple as that.
Speculating about pressure and a lot of other crap is totally irrelevant, when its the all important AFR's that matter.
Check to see what sort of AFR's it is running, and they seem off, go to a rolling road, and alter your fuel pressure to attain the AFR's you want to see, and that acheive the best power/torque.
if its currently running rich....you wont. Its as simple as that.
Speculating about pressure and a lot of other crap is totally irrelevant, when its the all important AFR's that matter.
Check to see what sort of AFR's it is running, and they seem off, go to a rolling road, and alter your fuel pressure to attain the AFR's you want to see, and that acheive the best power/torque.
In times gone by, we used to up the fuel pressure by a couple of psi by "squeezing" the factory fuel pressure regulator on pinto and cologne EFI Fords, The idea came from a friend who spent a lot of money(for lot read some!!) on an FSE Boost Valve, which was a replacement pressure regulator that simply had a stiffer spring in it to raise the fuel pressure a couple of PSI, (bear with me). The blurb with the FSE thingy was that it gave improved throttle response, coming off idle and when coming off then back on the gas(gear changes). I believe the extra fuel pressure squirted in a bit of extra fuel at these times, combined with better fuel atomisation. It certainly seemed to work but we never put the cars on the rollers to check. Sqeezing the factory regulator with a large pair of water pump pliers whilst checking the fuel pressure on a guage connected to the fuel rail worked for me. It may have been a placebo though!!
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