I have the HKS 264/264 cams and I am running the stock injectors. The stock turbo will only flow about 38 lbs/min of airflow at the most. Assuming an 11.7:1 air fuel ratio at a maximum of 38 lbs/min, you need 3.25 lbs/min of fuel. Multiplying by 60 and converting to lbs/hr. = 195 lbs/hr. To convert lbs/hr to cc/min, you multiply by 10.5 and you get 2047.5 cc/min. You have 4 injectors to create this much fuel flow, so you divide by 4, you get 511.9 cc/min.

Therefore you need 512 cc/min injectors.

If the stock injectors are 540 cc/min, rated at 43 psi (which is standard). And for the stock turbo to create 38 lbs/min it needs to run 21 psi of boost pressure. Therefore the fuel pressure will rise in a 1:1 manner with boost due to the factory fuel pressure regulator. This will yield a 43+21 psi fuel pressure. Using the formulas found here: http://www.rceng.com/technical.htm . The 540 cc/min injectors at 64 psi fuel pressure will flow 540 * sqrt(64/43) = 658.8 cc/min .

Therefore the projected fuel injector duty cycle to maintain 38 lbs/min of air flow at a 11.7:1 a/f ratio at 21 psi of boost pressure, while using 540 cc/min injectors would be 82%.

This seems to suggest that the stock injectors are fine with all the air flow that the stock turbo can provide as long as the fuel pump can maintain the fuel pressure needed.

My $.02!!!


Brian

 

For those wanting a comparison of cam specs I put one together here: https://www.evolutionm.net/forums/sh...ad.php?t=86242

It covers duration, duration at 1mm, lift and the need for upgraded valve springs.

 

very nice write up Brian. to the person that said piper jun and tomei cams are just for bigger turbos, you are wrong. many people in the uk are running piper cams (ultimate road or rally) with huge sucsses. they work well on bigger turbos coz they offer more lift (thus sortening spool up times) but just as well on stock to!

erioshi, as already said mate, on very good idea. well done!

thanks Chris.

 

Careful! As a 1:1 FPR raises fuel pressure with boost, all it will do is keep the same delta_P across the injectors. You will get no more pressure differential than you did before and you will not turn 540 cc/min injectors into 658.8 cc/min ones! For more details, see: https://www.evolutionm.net/forums/sh...41#post1002309

 

Please see my response on that post.

Brian

 

okay guys, I drive an avarage of 200+ miles per day when I work so putting cash in the gas tank is now a priority to me especially here in Cali. The gas here is averaging 2.50+ per gal. Before I invest in getting cams or cams/cam gear combo, what are you guys averaging in mpg. thanks for your input.

 

What is the duration on the stock camshafts? Does anyone know this? I've looked several places and always come back with no answers.

 

Heh heh... I had a guy in a '98 993TT ask me the samething after we did a brief roll on the highway.

Oh yeah, I have the HKS 264/264 combo.


Oh yeah again, I have the stock injectors and a BR Stg 3. I'm seeing 100% injector duty cycle at wide open throttle at ~6300 RPMs and higher. I'm DynoFlashed with BR's maps.

 

so if i have a big turbo ,did i order a 264/272 or 272/272 ,because somebody said if i have a big turbo it will be 1000-1500rpm late of the turbo spool,so if i have a 264/272 cam is this will be help for the turbo to spool faster

 

S**T 100% duty! vmate you are in trouble. get some larger injectors quick, or dont run the car over 4.5K with WOT!

chris.

 

I can't log knock activity but I'm not seeing the ECU pull any timing from what I can tell in the logs. Timing advances at a steady rate to 18 to 19deg at 7500rpm with no pull back. Seems normal but I'm not sure. Also according to my WBO2, I max out at 11.6 AFR, which interestingly is almost exactly the "conservative" 11.5 AFR that BR quotes on their website for the flash tune.

Regardless, injectors and fuel pump are on the short list of mods to get done. I think I'm currently running on the ragged edge of the stock fuel pumps ability to safely deliver enough fuel. I just gotta decide how I want to do the injector scaling, either with a reflash or use an SAFCII-like device I have.

With 3" turbo back, 264/264 and running catless @ 19psi of boost, my car pulls like a freight train all the way to redline. I'd like to try the 272s but i can't imagine that it gets any better.

 

Hey guys I've been thinking about getting cams. Just wanna know about reliability between the 264 and 272. Does it in anyway affect the integrity of the engine? Also if I want to run 264 is it a must that I need bigger injectors than stock. I already have a walbro 255lph fuel pump, intake, testpipe, turboback exhuast,MBC and dynoflash. Also how much do Pipers go for?

 

Dang, you are brave. Running that much power on the stock pump and injectors. At least I have the upgraded fuel pump. Try and swap in the fuel pump and see what kind of duty cycles you get then. How are you reading the injector duty cycle?

Brian

 

Bear in mind that I've got a Buschur Racing Stage 3 set up with the "BuschurFlash". While not justification for running under such conditions, I would imagine my situation is not unique.

Okay, I was WRONG when I mis-quoted at what RPM I was hitting 100% duty cycle. Based on my logs my injector pulse width flattens out at 18.8ms. This would mean my injectors are out of steam at about 6383 rpms. This jives with where my logs say I'm hitting 99% duty cycle. At 7500 rpm my max injector pulse width should be 16ms.

<half baked thinking>
I'm thinking that by simply slapping in a fuel pump I'm not going to affect injector pulse width since the ECU is running in open loop mode at wide open throttle. It's using a fixed fuel map for a known injector size at what it thinks the fuel pressure should be. With the extra fuel pressure a bigger pump would give it will be running mucho rich (but safe) and still running at 100% duty cycle. I'll have to use some MAF output scaling to get the injector pulse width under control. Correct me if I'm way off base.
</half baked thinking>

Oh yeah, I'm using an ECU+ for logging, MAF scaling and timing control.

Sorry for the off topic ride into the weeds.

 

You are right. You are running in open loop when at WOT. I was thinking that you could modify the MAF signal to lean out some to maintain the same air fuel ratio that you have now. With the fuel pump upgrade, you would be able to keep the fuel pressure up and hopefully you would be able to achieve that air fuel ratio at a lower duty cycle. That was my thinking.

Brian