I see a lot of questions about this, and have asked some myself.


Well the experiment is finally done and I don't have to worry about it anymore.

It turns out that the stock injectors on the 1998 C5 really don't do anything past 100% duty cycle. The DC's and PW's reported are accurate, and the actual AFR does slowly start going lean after 100% DC as the RPM continues to climb & DC increases further past 100%.

After seeing it happen I can see how easy it is to be fooled by the wideband data. It would've been plainly obvious in an M6 equipped car because the RPM would be in a high range for most of a test run.

The highway geared A4 that I tested is just the opposite. The RPM stays low for most of the run, and in fact in 3rd gear stays between 4000 and 4500 RPM for the entire last 1/8 mile of a run. On the bottle the high RPM's flash by rather quickly with a 200hp shot so there is little time to register a slight lean condition that is only momentary. It does show on the wideband but looked like a hiccup and was so small as to be ignored. Most of the run on the wideband was well down in the 10:1 AFR so there seemed little to worry about.

In actuality as the RPM climbs slowly (NA) at the end of second gear, there is time to clearly see an upward trend with the AFR after 100% reported DC. With the injectors static & working like a carburetor, there is still no evidence of a massive lean conditon. As RPM increases the need for fuel increases, yet the need for fuel per cycle gets smaller to an extent too cancelling part of the fuel deficit. The result is a smooth rise in AFR.

Here's the data:
RPM----DC-----PW-----AFR
5150--99%--20.850---10.89
5332-101%--21.232---11.16
5476-103%--21.582---11.30
5639-106%--21.963---11.42
6109-108%--23.013---11.63

I forced a commanded 10.75 - 10.61 AFR for the testing to see how much was left in the injectors.
At the lower RPM there is quite a bit left to easily drop down in the 9:1 AFR range or a little lower. In fact 90% of my test run averages about 10.97 AFR and only leaning out in very small areas where the RPM is the highest.
It's much more dependant on RPM than anything, which is why this C5 was able to digest a 200hp shot of N2O without a problem and put up 11.0 @127 MPH with stock fuel system/injectors and still have room left.

Conclusion: A highway geared A4 car that keeps the RPM down can get away with murder with stock injectors. 500 - 550hp is a piece of cake as long as you don't need to spin high RPM to make it... and the bottle don't need to do that.

If you have a M6 car that makes its power higher in the RPM range, then you're going to be sorry for not upgrading injectors before you put the bottle to it.

I said I would do the experiment and I did it. It all makes sense now, and there isn't any real trickery or mystery involved at all... happy ending.

 

Here is a quick question for you. I have an M6 with the 26.6lbs injectors (2000 C5), it also has 3:90 gears. I would like to add 150hp shot without upgrading my fuel system.

Can I get away with 150hp shot without upgrading my fuel system?\

If I can't, what (as a minimum) do I have to change?

And if 150hp shot is too much, could I just spray like 100hp shot or maybe 125hp shot without upgrading the fuel system?

The reason I ask is because I don't want have to replace the fuel pump. However, If I had to replace the injectors...........I think that would be easier and more importantly cheaper (due to the labor of having to pull my fuel tank)

Your input is greatly appreciated.

 

this is on a dry or wet set up?

 

Im assuming this was on a dry shot? Thanks for the info and doing the research into it. Even at 108% duty cycle its still not a bad AFR

 

https://ls1tech.com/forums/fueling-injection/472708-max-rwhp-28lb-injectors.html

i found where he said the vette was a 200 dry shot.

 

Based on your signature line I'm going to say no, not without an injector upgrade and preferrably a fuel pump too.

If you were all stock, then yes a 100hp DRY shot should be no problem for your M6.

Oh yes I was talking DRY system. (sorry I forget about the wet guys)

As long as you've got the pump upgrade, then you could add your 150hp as a WET system without a problem. I'm pretty brave and I wouldn't do it without the pump.

 

GOOD STUFF WHITE!!!!!!

So it appears under certain conditions you could survive for a time. But under other conditions it could be a recipe for disaster.

So for everyone out there...even though under certain curcumstances you may be able to get away with alot more han previously thought....injector upgrades by the old rule of thumb are still a safe bet.

PS...I still dont worry about the 80% rule. but I do worry when getting close to 100% for a longer period.

Good stuff man.

 

Damn...Nice work white, but Im going to have to upgrade my SVO 30's for when I spray my 1st stage dry shot. I already get to almost 90% on the SVO's NA at 6800 rpm.... Wonder if the 30's would be enough for a 50 shot dry first stage???

 

Would I be able to get away with just changing out my injectors?

 

so white on my 98 z a4 with the 28.6# injectors, stock fuel pump and the stock 2:73 with a couple bolt ons you think i can run a 150 shot with no need to touch my fuel setup. with proper timeing of course

 

Probaby, to both questions.
I'd start out with a just a 100hp shot at first and log the AFR with a wideband, and datalog the runs so you can get a good idea what your duty cycles are and how much room you have left.
That's really the best way, instead of guessing.

You will need a way to tune the PCM in order to manage a big dry shot anyway, and scanning some test runs just makes sense.

 

White2001s10, good info for sure. I also have had this scenereo when I went 490rwhp on 28lb'ers. I also say that I was starting to go lean in the upper rpms, above 6000 to redline (at that time)6500rpm. Your data has proved what i have been trying to say for some time, injector duty cycle of 80% is a sustained cycle limit, or a cycle limit you would see often and only stated for longevity of injectors. Now the short time of the 1/4 run is not going to wear your injectors out, and we in fact have much more room than the 80% rec for wear/tear. I too was suprised when I ran a 200 dry on my stock injectors, i don't remember the exact numbers but they were close to what you found, however, at that time I was using NB for a/f.

On the vette fuel pump, the stock Walbro is good to 550 and some take it above 600hp. The bad thing is having the return at the tank, this causes a slight pressure loss, so converting to regulator and return line at the rails is the ansewr and you will maintain a rock steady 58/60psi to redline. this pumps are actually very good. I am doing the mod above and adding a Kenny Bell Bap to be activated when spraying only. Jon, remember I said you could have my 28lb injectors, and if I remember correctly you were at 380hp, so a KB Bap would be a good idea as it can boost the stock pump upto 30% which will cover you for sure. you can set the pressure you want for spraying and this can overcome smaller or close to the limit injectors. how ya doing over there? see ya soon.
Robert

 

Robert do you think you could paste the results up into the FAQ section?
I have seen these type of questions come up a lot here.

 

Me no can do anymore, gave up my modship cause of the cry-babys.
Robert

 

I'm doing good, but they moved me from Germany to Turkey. Oh well, it could be worse! Thanks for asking. How you been doing? I miss

My car is trapping 119.50mph in the 1/4, so I would say my car has well over 400rwhp. I'm guessing around 420rwhp......What do you think with that mph?

Does the C5 Coupe have the walbro fuel pump stock? And I have some 28lb injectors from that LS6 intake I bought. When I get that intake on it should also add some more hp. Then with the 150hp shot, that would put me at around 550rwhp and way more torque.

So you think that the 28lber's will be alright combined with the KB Bap?(what is a KB Bap?)

Or should I buy bigger injectors? When I get home, I'm really gonna need you help a whole lot..........with istallation and picking out parts. I want to try and stay around $1500 dollars or so! We can talk more later.....

Thanks for your help Robert and I'm eagerly awaiting your 10 second run. Should be pretty soon.....

 

I think KB BAP is Kenny Bell boost-a-pump

BTW, I spent a year in Pirinclic Turkey, but that base is closed now.
durka durka abbi

 

Jon, yea, you'll need some bigger injectors to make it easy, or we can tune it to make the 28lb'ers work. this would be a little harder, but doable. we will compare the $$ to do it each way. The Kenny Bell BAP is a great product, and a tuners best friend, here is a link with info,
http://www.kennebell.net/accessories...boostapump.htm
and here are a couple threads for insight,
http://forums.corvetteforum.com/showthread.php?t=739059
http://photos.strasser.us/gallery2/m...g2_itemId=7882
here is the math for upsizing injectors by increasing fuel pressure,
http://<a href="http://photos.strass...temId=7882</a>
and I have many more if ya want to read up.
Robert

 

so you're sayin i should be fine with a 100 shot on my 99 Z A4 with 28lb injectors?

 

Well we can talk about this more when I get home. I really appreciate your help. I also love the idea of boosting the output of my fuel pump without having to replace the fuel pump altogether. That sounds great!

So if I buy some bigger injectors ($200..?) and this boot a pump, I should have no issues spraying 150rwhp dryshot of nitrous? If I can get this all done safely and effectively I should be able to run 10's! I know you have a vast knowledge of nitrous and the safety equipment to run it, but I really want to run it as safely as possible.........even if it means spending a little extra money! IMO, it would be cheaper to purchase and install all the necessary safety equipment now, rather than trying to save a few bucks and blowing my engine down the road as a result! Like mentioned earlier, we can work out these issues when I get home! I'm getting pretty excited about all this.

 

You should absolutely be okay with a 100 shot..........but remember that different systems are rated differently. So your 100 shot set up may only be 80rwhp setup!