Throttle Bodies

[Injuneer]

At 500 flywheel HP, the 36's will be close to 80% duty cycle, which is where you want to be. 42# is as large as you an go on a stock PCM, because that is the largest high impedance injector you can get. To run larger, low impedance injector on the stock PCM, you need a "black box" converter.

An alternative to larger injectors is higher fuel pressure. The pintle style injectors are comfortable at 58psi. That gets you 15% extra flow out of an injector that has been flow rated at 43.5 psi.

The larger the injector, the harder it is to get it to flow the low fuel rates required for idle. Its hard to turn an injector down below a 1 milli-second pulse width, it that's what it takes to idle. That's why you need to go to a low impedance injector when the size goes over 42#. The low impedance injectors are much faster, particularly when combined with a high resolution, high quality driver in the ECU.

[Z Ya Later]

42# is as large as you an go on a stock PCM, because that is the largest high impedance injector you can get.

Actually, RaceTronix carries some high-impedance injectors that flow higher than that (50# and above). Not that they should be used for this application, but they would be compatible, right?

Here's the link if you would like to check them out.

[Kevin - Blown 95 TA]

Jerry, If you put a 58 mm TB on an A4 car, you had better know how to reprogram your transmission because a lot of torque will be made at a smaller blade angle (opening) and the trans pressures will not be set up for it, so they need to be altered. A shift kit might also be a big help in that case, but I am not sure if it is all you would need.

[fastTa]

Here is the Acceleronics Versa Fueler (Black Box) that Fred mentioned:



55 lb/hr is the limit for high impedance(z) injectors.

Your MAF will be fine.

I used SVO 30 lb/hr injectors on my n/a 488 RWHP LT4 and they were at about 90%-95% duty cycle, so I would suspect that 30-36 lb injector will be more than sufficient for only 500 at the crank.

AS&M makes very nice TB's. A 58mm would suit you just fine and would leave you a little room to grow. And, like Kevin pointed out, it will require alittle additional tranny setup within the PCM. I would also suggest adding at least Corvette if not Billet servo's depending on whether or not this car is your dailiy driver.

[jerrysta]

The car is my summer daily driver, and the same guy who is doing the engine is rebuilding the trans, too. And he's doing it with heavier duty parts to take the power.

I know that I will need to get the CPU done again, without question. There is going to be so many changes when the motor comes back so I have already planned for that. I am going to have Ion set up a baseline tune for me and then I am going to have it dyno tuned.

Fred, you are saying I can keep the stock injectors and just increase fuel pressure? Or were you saying I should get 36# injectors and raise fuel pressure? It's problably not a bad idea to replace them at this point anyway. If I need to increase fuel pressure is that just done with a new fuel pump or is that another CPU controlled adjustment?

Thanks, guys.

[fastTa]

The car is my summer daily driver, and the same guy who is doing the engine is rebuilding the trans, too. And he's doing it with heavier duty parts to take the power.

I know that I will need to get the CPU done again, without question. There is going to be so many changes when the motor comes back so I have already planned for that. I am going to have Ion set up a baseline tune for me and then I am going to have it dyno tuned.

Fred, you are saying I can keep the stock injectors and just increase fuel pressure? Or were you saying I should get 36# injectors and raise fuel pressure? It's problably not a bad idea to replace them at this point anyway. If I need to increase fuel pressure is that just done with a new fuel pump or is that another CPU controlled adjustment?

Thanks, guys.

You will need larger injectors. The stockers will not support your goal of 500 HP at the crank.

[jerrysta]

Ok, got it. Should it be the 36# injectors or the 42# ones?

As far as the pump, Racetronix has a replacement for the LT1 pump that is rated at

255L/hr@43.5 psi with 13.5V

Will this pump be sufficient?

[Injuneer]

I think the 42# limit may be inherent to the LT1 PCM, not a limit on injectors in general.

You can increase flow capacity by raising pressure, but there are limits. The stockers (RP ball and seat) don't like high pressures, so 58psi might be the limit. That only gets you 28.75#/HR (using the 24.92 #/HR used in the stock programming). To add 50% to the injector capacity, you would have to run them at almost 100psi. That's not a practical approach.

A lot of people use the Walbro 255 LPH pump, that Racetronix sells. That will be more than adequate for 500fwHP (I run a single Bosch 205 LPH pump at that level). I'm not sure about the quality of the Walbro pumps at this point. With so many people using them, it isn't unusual to see a post on one of the bulletin boards claiming a premature failure of the pump. Some of these failures have been traced to incorrect installation, but I wonder about the reliability of that pump - I'm not saying they are "bad".... only that there seems to be some problems, if you believe what you read on the bulletin boards.

I use a pair of Bosch fuel pumps, and one of them has been going strong for 10 years, the other for more than 5 years.

[jerrysta]

Do I have to use an in tank pump or can I install an in-line pump? The stock pump was replaced about 20k miles ago and it is working fine. Would I be able to use an in-line pump with my existing pump? The idea of replacing the pump does not sound fun right now.

Also, do I need a certain pump specific to an LT1?

Where did you get your Bosch pump?

[jerrysta]

Thunder Racing has a Holley 255/lph pump for only 125.00. Does that sound like a good idea?

I'd like to buy from Thunder just because of the rough time they are going through with the Hurricane. Keep that commmerce going in LA!