Fuel injector sizing

[Chubbs]

What size injectors are people running for 350hp? I have a k27/8 turbo and I don't think the stock ones are up to feeding it. Also, what injectors physicaly fit? I've seen people who run generic aftermarket injectors, but are their certian things I need to look for in an injector? Do injectors off of any other make or model fit a 944 turbo?

[wild man]

I'm running about 335-340hp on a K26/8, and the stockers seem to be OK for that (A/F reading is around 800mV). But I will be going K27/8 soon (the big one), so we will probably see exactly where they will be topping out at that time. If you use different injectors, you need to make sure that the impedance matches the stockers (6-7ohms, if I recall correctly).

[Lawrence Coppari]

Been running mine with stock injectors and 3 bars fuel pressure for 14 years. I run 16 psi boost with a K27 and MAF. I am sure mine puts out over 350 hp. Car has about 100 DE events on it so if it were too lean, it would have welded itself together by now.

[74goldtarga]

To be honest this is a bit surprising to me, I assumed if you went to maf and larger turbo you had to run larger injectors. The stock injectors are around 35lb aren't they. You can run a MAF and larger turbo with those? - I assume to be sure you've got to monitor your AFR after the upgrades and make sure they're ok but I think you could find out the hard way. Lawrence, what does your car have for fuel control, is your turbo K27/8?

[Lawrence Coppari]

Originally Posted by 74goldtarga

To be honest this is a bit surprising to me, I assumed if you went to maf and larger turbo you had to run larger injectors. The stock injectors are around 35lb aren't they. You can run a MAF and larger turbo with those? - I assume to be sure you've got to monitor your AFR after the upgrades and make sure they're ok but I think you could find out the hard way. Lawrence, what does your car have for fuel control, is your turbo K27/8?

My power upgrade was a complete kit from PowerHaus, purchased in the winter of 1996. I don't monitor anything other than looking at the spark plugs once a year when I change them. They look fine to me.

All this was installed when the car had about 93K miles on it. It now has 179K. The only things that are not stock on my engine are:

MAF, chips to operate it
K27DR turbocharger
Extrude honed intake
Triple ground head matched to intake (both were a kit)
Larger boost tubes
Reamed throttle body
Several carburetor jets to change boost. I use one that puts out 1.1 bars boost.
3 bar fuel pressure
Two oil coolers

There is a threshold that requires larger injectors. Mine is below that. Some of the other guys on this list would know.

If something were wrong with mine, it would have grenaded by now. I do not run the engine above 6K.

[Lawrence Coppari]

Here is a link to an episode at Road Atlanta at Octoberfest. My friend's 911, who caught me (in the maroon 951), weighs 500 pounds less than I do so he eats me under braking. Notice what happens on the straight. He spins at the end and I go by.

http://www.youtube.com/results?search_query=phasterphaster&search_type=&a q=f

[nize]

at 350whp the stock injectors are running at around 99% duty cycle. i would recommend going to at least 55lb injectors for safety.

[Lawrence Coppari]

Powerhaus sold the setup. If they thought I needed larger injectors they would have included them in the kit and charged more for it. It would be to their benefit to do so. This was 14 years and about 100 track events ago. If there were a problem, it would have surfaced by now. The stock injectors were sent off and cleaned/balanced about 6 years ago.

There seems to be two camps of thought on this topic.

[74goldtarga]

Lawrence, in defense of Nize, he probably isn't criticizing Powerhaus, they know the needs and output of their system. To be fair, we don't know exactly how much power your car is putting down at the wheels, I looked at Powerhaus' web site and they are not claiming 350 wheel hp, they claim 289 wheel hp or 339 at the flywheel. You can calculate the duty cycle of an injector of known capacity for a 4 cylinder 4 stroke engine with a certain power output - at a certain point the injectors are running wide open and you can get lean, it sounds like your car is safely below that level based on the excellent reliability of your system. I think the safe thing to say for someone trying to upgrade their car is that you can go a good ways with the car with the stock injectors as your car proves but that as you approach 350whp you'd want to be watching AFRs and the duty-cycle on the injectors to be safe and that to go beyond this level you probably need bigger injectors.

As an aside Lawrence, your car looks fast at Road Atlanta. It looks like you are passing other 944s but I can't tell if they are NA or Turbos. I am tempted to call Powerhaus myself about the system on their website. My car is pushing 130,000 miles and I'd like to bump the power without blowing it up.

[74goldtarga]

This information is borrowed (mostly copied) from "How to Tune and Modify Engine Management Systems" by Jeff Hartman, Motorbooks Workshop. You should all go buy this book. The price printed on the back of my copy is $29.95

Flow Rate (pounds/hr) = (HP x BSFC) / (#Injectors x Maxcycle)

HP = flywheel horsepower
BSFC = brake-specific fuel consumption in pounds/horsepower-hour (Assume 0.55 for turbocharged engines, 0.45 for NA)

#Injectors = 4
Maxcycle = the maximum duty cycle of the injector. (Above 80 percent, some injectors could begin to fibrillate or overheat, which could eventually cause them to lose consistency - or even fail completely!)

So now to calculate for 951

Flow Rate = (350 x 0.55) / (4 x 0.8) = 60lb/hr

This is at the flywheel mind you, not the wheels, so for 350 wheel hp the recommendation might be slightly higher.

If you change maxcycle to 100% duty-cycle then the calculation works out to Flow Rate = 48lb/hr so I'm not quite sure where the calculation breaks down.

[Lawrence Coppari]

Originally Posted by 74goldtarga

As an aside Lawrence, your car looks fast at Road Atlanta. It looks like you are passing other 944s but I can't tell if they are NA or Turbos. I am tempted to call Powerhaus myself about the system on their website. My car is pushing 130,000 miles and I'd like to bump the power without blowing it up.

The 944's were the turbo variety. The red one is a stock club racer. I also have the extrude honed intake and O-ringed head, item 6, from their list of go fast parts. Also have the oil pan and pickup tube modifications.

So according to their sales pitch on their website, I'm supposed to have a little extra umph - the extra 37 hp which is probably at the crank. I have saved the last set of spark plugs that were in the engine for two years. I might have them read.

I arrive at my hp figures based on car weight and what cars it keeps up with on the long straight at Road Atlanta which is about 3/4 of a mile.

The reason why I don't want to change fuel injectors is because the car gets really good mileage on the highway if you putt putt around. I wonder how larger injectors would affect the highway fuel mileage. I also get a charge out of going as fast as a lot of others do while spending far less money. Injectors are not inexpensive items.

I appreciate all the comments you guys are providing. I could be wrong but it seems the engine would have grenaded by now if the injectors were not providing sufficient fuel. Given the longevity of my setup, race ipsa loquitor.

[wild man]

Let's not for get that the stockers put out 35 lb's per hour @36 psi. When boost goes up, so does the flow rate of the injectors. But other mods that increase air flow, without increasing the pressure, DO NOT increase the flow rate of the injectors. That's what you have to watch out for, if trying to max out the cars hp on stock injectors. And increasing boost does NOT produce an equal increase in fuel flow, either. A 20% increase in boost will only net you about a 10% increase in fuel flow. So increasing duty cycle IS required to maintain a safe ratio.

So where is this leading? To the 100% level of duty cycle, obviously. Beyond that, A/F levels will start going leaner. Then the question becomes, how lean are you willing to go? I hope you have a knock counter to answer that one. Remember that the higher your boost is when you get knocks, the more energy each knock has in it...

Let me add that this is EXACTLY where a rising-rate, adjustable FPR would come in VERY handy. And that is assuming that the stock FP can handle that much extra pressure. But I have yet to find the kind of FPR that I mentioned, that is "bolt-on" for our cars. Hello AFTERMARKET people...................................

[Lawrence Coppari]

[QUOTE=wild man;5059084]Let's not for get that the stockers put out 35 lb's per hour @36 psi. When boost goes up, so does the flow rate of the injectors. But other mods that increase air flow, without increasing the pressure, DO NOT increase the flow rate of the injectors. That's what you have to watch out for, if trying to max out the cars hp on stock injectors. And increasing boost does NOT produce an equal increase in fuel flow, either. A 20% increase in boost will only net you about a 10% increase in fuel flow. So increasing duty cycle IS required to maintain a safe ratio.
QUOTE]

The kit included a 3 bar FPR. So instead of 36 psi I have 43.5 psi fuel pressure. That's 20% more pressure that provides a little more fuel. Evidently it's enough to keep me out of trouble.

[Lawrence Coppari]

Would some spark plug pictures help the discussion? I recently changed the plugs and saved the old ones that had been in service 2 years and probably a dozen track events. Here are pictures of one of them. In addition the the DE's, the plugs had about 7500 miles of normal driving. There does seem to be a coating on it. All plugs had same coating.

[Rogue_Ant]

Originally Posted by wild man

Let's not for get that the stockers put out 35 lb's per hour @36 psi. When boost goes up, so does the flow rate of the injectors.

This is incorrect.
The pressure delta across the injectors remains static at whatever initial base FP is set at, regardless of boost pressure (manifold pressure).
At 0 psi of boost the fuel pressure is, say, 2.5bar. If boost is 1bar, then fuel pressure rises an equal 1bar. Accordingly, the pressure change across the injector (rail to manifold) is still the same 2.5bar.
This is done to keep injector flow fairly constant regardless of manifold pressure.


-Rogue

[Rogue_Ant]

Originally Posted by Lawrence Coppari

The kit included a 3 bar FPR. So instead of 36 psi I have 43.5 psi fuel pressure. That's 20% more pressure that provides a little more fuel. Evidently it's enough to keep me out of trouble.

Going from a 2.5bar to a 3bar is about 10% more fuel:

Sqrt ( 3 / 2.5 ) = 1.09545


-Rogue

[Lawrence Coppari]

I posted the plugs on another list and got comments from too lean to too rich. So I guess without a knock counter or something that gives me the fuel to air ratio I am flying blindly. The over 350 hp I mentioned in my first post was crank power, not rear wheel power. I figure 315 at the wheels. Does this change anything?

What would larger injectors do for my fuel mileage in normal driving? I assume they would open for shorter intervals based on what the oxygen sensor tells the ECU?

Where does one buy larger injectors?

[Lawrence Coppari]

OK, guys......I'm leaning towards getting larger 55# injectors.

That's what I like about Pelican. You get good information.

[wild man]

I guess that's correct about flow remaining constant as boost/fuel pressure increases. It looks like my next step, after installing the K27, will be to go 3 bar on base fuel pressure. We'll see how far that can take me. If another step needs to be taken on top of that, it will be in the form of an auxiliary (5th) injector that only kicks in under full boost.

[wild man]

No, scratch that. I'm already running too rich ar 2.5 bar, with autothority MAF. I'll either go with the extra injector, or, if a chip is available for the 55# injectors used in conjunction with autothority MAF, I wiil go with that. Does such an animal exist?