SteelBallz

I am a couple of weeks away from putting on a set of Patriot Performance Stage II LS6 heads and then getting it dyno-tuned. I was wondering if it is going to be necessary to invest in new injectors. I know that you could look at the logged data from when it gets tuned, but I was wanting to know in advance so I could set aside the extra funds and possibly do everything at once. I will be driving quite far to get my car tuned, so if the new injectors are necessary for a good tune then I would like to know.

My mods are in my signature and my car made 397hp before I got the throttle body ported and I have no idea what to expect after the head swap. I tried to search on the topic, but after 4 pages I gave up. I also explored the SAE forumla on the sticky, but I would like some personal advice...

SRPerformance

It depends on what size injectors you have?

On 28lb injectors thay maxed out at 360rwhp, but we still could push the same injectors to over 400rwhp. As we are trying to improve our 1/4 mile time we are going to 36lb injectors which are rated at 3bar (43.5lb) and had to scale them to 4bar (58lb) using redhardsupra spreadsheet (injector.xls).

We are fitting the injectors this week, will post results.

SteelBallz

I have the stock injectors, and I am just wanting to know if I would need to replace them in-order for everything to work out okay after I do my head swap.

I would like to know if it is something I should do before I go to get the car tuned after the head swap....

SRPerformance

If you change the injectors you must change the Injector flow rate table in the PCM, full stop. So if you change the injectors than you must tune it straight away!

SRPerformance

It was a hot day, 102F with 15% humidty, the car would only pull 382rwhp, swapped in the 36lb injectors and recalibrated IFR with HP tune. It started the same, scanned and logged AFR the car was running a little lean, adjusted the VE table. Did a power up the AFR was running in the 14s adusted PE table and finaly made 392rwhp. The power curve was the same but went to a higher peak. The interesting point was that: after max HP the torque curve did not drop as fast as before, around 10NM more all the way. This happened after 5000rpm all the way to the redline, 6700rpm.

Overall, the smaller injectors were limiting max power and torque afterwoods! The duty cycle is now down from 100% to 78%!

bshell

iTrader: (24)

Congrats, its always nice to see a real world example validate the math! Did you compare the two afr curves on the dyno as well? I'm thinking the pull with the injectors maxed would show a much more unstable afr curve than the one with the new injectors.

bshell

iTrader: (24)

To answer the original question: You will need new injectors after the head swap. What size you will need is based on what kind of hp you expect from the heads. You also will need to estimate drivetrain loss and bsfc for your engine. You can use 15% loss to the wheels and .5 for the bsfc to ensure you error to the conservative side of things. Using the equation from the sticky, it looks like you will need at least 40# injectors at 58psi. There are several injectors that cover that fuel mass:
SVO 30, 42
Lucas / Delphi 37, 38, 42
The SVO 30 and Delphi 37 injectors will cover 450rwhp pretty well, but not much beyond that. If it were me, I would go up a step in size (they will idle the same) to SVO 42s or Delphi 42s to cover any future mods.

How far will you be driving to get the car tuned? Avoid high rpm at all costs until you are able to get the new injectors installed.