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Old 03-22-2020, 03:22 PM   #1
jamie2370
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Default injector size

Stocker, 350 tpi motor, Fast efi. What would be a recommended injector size? Am I being crazy by installing 60lb injectors?
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Old 03-22-2020, 09:25 PM   #2
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Default Re: injector size

Nope, depends on the EFI system you are using. A newer system, like Holley, and there are others, will let you time when the injectors open, but to do so they need to be much larger than "typical". If you were to use a 30 pounder or so, you wouldn't be able to change when the injector opens in relationship to the intake valve, you wouldn't be able to get enough fuel in the chamber at higher rpms. By over-sizing it, you have the flexibility to try different things. Again, only if your particular EFI system will let you adjust the actual "timing" of the injection events.
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Old 03-23-2020, 12:19 PM   #3
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Default Re: injector size

Thanks for the PM's guys. You know who you are. I appreciate the advise.
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Old 03-23-2020, 12:44 PM   #4
Robin Lawrence
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Default Re: injector size

Boy this question can have a ton of correct answers. Minimum would calculate to about 40 PPH.
Based on an estimated 600 HP and a rough .45 BSFC that is around 33 PPH.
Add another 20% margin since we don't like to run injectors above 80% duty cycle and you get near 40PPH.

As said above you can run a larger set that can be timed to cam intake valve events giving some advantages depending on camshaft/intake design. I have seen some where it was not worth any gains. The feature can really help the Idle in come cases.

If you want to run sequential injector strategy you will need a cam sensor. It will allow the engine to run bit cleaner at low RPM and when you go up on the converter. It's usually not an issue with the smaller injectors to run batch or bank to bank strategy. Many older combinations run batch fire at close to 100% duty cycle. Some of those guys still use fuel pressure adjustments to fine tune their EFI rather than adjusting the base fuel map.

Hope this helps

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Old 03-23-2020, 03:20 PM   #5
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Default Re: injector size

Robin , Do you see a gain by running the injectors at 50 or 55 psi instead of oem 42 psi ?
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Old 03-23-2020, 04:30 PM   #6
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Default Re: injector size

When the injectors are timed to valve events does that lesson the energy in the reversion wave. Will it generate a "sweet spot" with the reversion wave to pack the cylinder with A/F mixture? With the larger injectors, will droplet size suffer by being larger or will the droplets be close to the same size.

Trying to learn!

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Old 03-24-2020, 07:54 AM   #7
Robin Lawrence
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Default Re: injector size

Quote:
Originally Posted by ss3011 View Post
Robin , Do you see a gain by running the injectors at 50 or 55 psi instead of oem 42 psi ?
As in may scenarios my answer is "It Depends". Only testing at the different pressures will tell you if there is any value.
Typically I try to run injectors at their design fuel pressure. As you get away from the design pressure some injectors characteristics can change. An injectors opening and closing can be effected by a couple of things, fuel pressure and voltage. changes in these areas effect the way the injector reacts and can alter flow.
Most modern ECU have voltage compensations tables. Getting the correct "dead times" for a specific injector offset any voltage fluctuations and deliver what you have programmed in your base table.
As for pressure changes only flow testing at the various pressures can tell you what that injector will to a setting different than what it was designed for.
And only track testing can tell you if there is a benefit. (notice I didn't say dyno)

As drag racers we affected less by these variables. A race car that is driven at part throttle or through a wide rpm range will see a benefit. A street car will have better drivability with good injector setup data.

In my opinion you will see the most benefit in a drag race scenario by tuning the individual cylinders at wide open throttle through the RPM band of your combination.

I would used the injector timing tables to help mitigate the effects of a huge cam at idle. If an auto car this can also help when loading the converter.

Sorry if I got long winded

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Old 03-24-2020, 09:19 AM   #8
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Default Re: injector size

.

Sorry if I got long winded

Robin[/QUOTE]

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Old 03-24-2020, 12:02 PM   #9
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Default Re: injector size

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Originally Posted by jamie2370 View Post
Stocker, 350 tpi motor, Fast efi. What would be a recommended injector size? Am I being crazy by installing 60lb injectors?
30lb would be plenty. I have made more power with my LT1 SS engine using 36lb injectors, at 60 psi. At your power level, I would use no more than 30lb. Myself, I would try 24lbs at a higher pressure. I have had best luck with Lucus injectors in my cars.
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Old 03-25-2020, 06:21 AM   #10
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Default Re: injector size

I have been running 30 lb Ford Racing injectors in my LT1 for a while. I have an injector flow bench that I will test a few boxes to try to set match them. They seem to work pretty well for me. Generally have my fuel pressure between 52 lbs and 58 lbs depending on the air.

Also have some 60 lb injectors that I want to experiment with, but I have my hands full at the moment with other stuff.
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