Building a simple 383w/ hot cam/ported heads/pacesetter long tubes.Just
installed new 24lb. svo injectors less than 1000k ago.Just wondering if they
are going to be large enough?

 

I've got 30lb. injectors with my set up. Stock cube engine. I want to get a bigger cam. Probably a custom grind in the 232 duration, .570s in lift, 113 LSA, and I've heard "might wanna go to 36lb. injectors." 24lb. might be a tab small for a 383. JMHO though. You're putting in the time, effort, and $$ for a performance engine, why have small injectors that might choke off the engine. Stock LT1s have 24 lb. injectors.

 

At the stock fuel pressure, my stock 24lb injectors were maxed out at 330 hp. Data loging showed that they were at 103% duty cycle. I swaped in 30lb injectors and the duty cycle was at 80%. This was with a large stall unlocked, so a M6 could probably put down 350 or so with stock injectors.

 

Quoting "rear wheel HP" with reference to injectors is pointless...... they are sized based on flywheel HP. Injector size also depends on the "efficiency" of the tune.... a factory tune might be closer to 0.50 #/HR/HP, and a really good dyno tune, on an engine with efficient heads might be closer to 0.45 #/HR/HP.

Assuming an average tune (0.48), the 24# SVO's will max out (100% DC) at about 420 flywheelHP, at the stock 43.5psi LT1 fuel pressure.

To get in the ballpark on injector size, multiply estimated flywheel HP by 0.07....

example.... 500 flywheelHP x 0.07 = 35 #/HR

Remember to adjust the injector's "rated" flow to the actual pressure you are running your fuel system at.

 

injuneer, what do you mean by set the injectors rated flow to your fuel system pressure. are you talking about using an adjustable fuel pressure regulator? Like 43.5 psi?

 

You have to tell the PCM the injector's "flow constant". And that number has to be adjusted to reflect the actual fuel system operating pressure, if the fuel system pressure is not the same pressure the injector was flow rated at.

A 24#/HR SVO injector is flow rated at 2.7bar (39.15psi). If you put that injector in an LT1 with a STOCK fuel system pressure of 3.0bar (43.5psi), the injector will flow about 5% higher than it is rated.

(43.5 / 39.15) ^0.5 = (1.111) ^0.5 = 1.0541

The 24# SVO needs an injector constant of 24 X 1.0541 = 25.298#/HR

Or, if you are using LS1 injectors (flow rated at 4bar = 58.0psi) in an LT1, you will find that a 28# LS1 injector is actually about the same flow rating.....

(43.5 / 58.0) ^0.50 = (0.75) ^0.50 = 0.866

The 28# LS1 flow constant needs to be set at 28 x 0.866 = 24.289#/HR

 

So at what point does a stock LT1 injector become too small.
By that math the stock injectors are barely big enough to feed 350 flywheel HP. Depending whether you use .07 or .05.

Also, 84zed's mild 383 would need at least 32# or even 36#.

 

The "0.07" factor is a guideline that gets you the "right" size injector, not one that is "barely big enough". It represents a BSFC close to 0.48 #/HR/HP, and a duty cycle of 85%. That is the smart way to size an injector, insuring it has adequate headroom.

I explained when the SVO (25.3#/HR) - and by inference the stock injectors (programmed as 24.9#/HR) - would be the limiting factor.... about 415-420 flywheelHP. You would be running the injectors at 100% DC.... you really don't want to do that, because the injectors will see a shortened life, and its a crap shoot as to whether the spray pattern will remain stable and the flow rating can be held for an extended period of time.
84zed's 383 might be mild enough to "barely" get by with an injector that flows 25.3#/HR. But lets say it can make 450 flywheelHP at SAE conditions. He would be smart to get a set of at least 32# injectors. That leaves him at an 85% DC. Let's say he makes a pass an a really cool, dry day. He should be able to pump out 495 flywheelHP based only on a 50degF inlet air temp reduction... the DC on his 32's would now be increased from 84% to 92%. The 36's are now looking like they might be a better choice.

 

K, don't mean to hijack the thread but since it seems his question has been answered and you are obviously the right person to ask.

Right now, by the math, my car should be somewhere in the 330-340 rwhp range or close to 400 flywheel. Stock injectors, I'll check duty cycle in spring. I plan on spraying a 125 shot using the NOS setup that bumps fuel pressure to richen the mixture with the nitrous.

Should I upgrade the injectors or should the stockers work with this setup.
Theoretically of course, if you said it should and it didn't I'm not stupid enough to blame you.

And, if I do upgrade the injectors now for future power levels, how bad is it to run say 36# on this setup. Is there a minimum duty cycle suggested?

Sorry for so many questions, but I just need info from an experienced and knowledgeable racer such as yourself.
(I couldn't find the *** kissing one)

 

The stock injectors are going to be near the edge if its pushing 400 flywheel HP on motor (maybe 95+/- % DC - depends on how efficient the tune is), but if they are in good condition, and if you are sure the stock fuel system is holding pressure at the required flow, they should be OK.

When you spray, with the NOS dry system, you could be in big trouble with the stock injectors. The NOS system boosts fuel pressure to 80-85psi. The stock RP ball and seat injectors do not like to operate at pressures that high. If they lock closed under pressure, you will have a major problem. The general recommendation when using the NOS 5176 kit is to use a pintle type injector, that will work correctly at the elevated pressures. The FMS, SVO, Bosch, Denso, etc. injectors are pintle types. I ran the NOS 5176 kit for about 4 years. I had problems with the stock injectors, and switched to Densos (FMS).

Running a 36#/HR injector in a 400 HP engine will simply mean your duty cycle will be lowered to the 65-70% range. That will not cause a problem. The only issue with "too large" injectors is the ability to turn the injector down to the very low pulse width required for a smooth idle. But people are running 42# injectors and not having a problem. I run the equivalent of a 78# injector, but they are low impedance, and I'm using an ECU with extremely accurate, high resolutiuon drivers, so the engine has no problem idling smoothly, while still holding 70% DC at 800HP, including a 300-dry shot.

 

Would the Delphi disc type work as well?

Thanks Fred. The check is in the mail.