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Do I need bigger injectors?

 
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buzzinhalfdozen



Joined: 30 Mar 2006
Posts: 93

PostPosted: Thu Jun 14, 2012 8:29 am    Post subject: Do I need bigger injectors? Reply with quote

Have a 276 ci. V6, 74# injectors engine redline is 10,200 doing the math on PW looks like I can go just above 10ms before I'm over 85% duty cycle at max RPM. Anyone know how to figure if this will be enough fuel for this App. or do I need bigger injectors and decrease the PW? Engine specs, 4.125 bore 3.45 stroke. I realize theres probably not alot of combos running this type of RPM range, but I'm sure the Nascar engines are near this. Thanks for any suggestions. Joe
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RootesRacer



Joined: 04 Apr 2005
Posts: 485
Location: Arvada, Colorado

PostPosted: Thu Jun 14, 2012 8:44 am    Post subject: Re: Do I need bigger injectors? Reply with quote

buzzinhalfdozen wrote:
Have a 276 ci. V6, 74# injectors engine redline is 10,200 doing the math on PW looks like I can go just above 10ms before I'm over 85% duty cycle at max RPM. Anyone know how to figure if this will be enough fuel for this App. or do I need bigger injectors and decrease the PW? Engine specs, 4.125 bore 3.45 stroke. I realize theres probably not alot of combos running this type of RPM range, but I'm sure the Nascar engines are near this. Thanks for any suggestions. Joe


More than anything, you need to know the target horsepower and at what RPM that will occur.
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buzzinhalfdozen



Joined: 30 Mar 2006
Posts: 93

PostPosted: Thu Jun 14, 2012 9:39 am    Post subject: Reply with quote

Made 650 HP peak @ 9500. This was with a carb.
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RootesRacer



Joined: 04 Apr 2005
Posts: 485
Location: Arvada, Colorado

PostPosted: Thu Jun 14, 2012 11:33 am    Post subject: Reply with quote

buzzinhalfdozen wrote:
Made 650 HP peak @ 9500. This was with a carb.


Lets figure on 700hp with 80% max duty and GASOLINE.

1/.80*700/2=437.5 lbs/hr total
You would therefore need 73lbs per hour with 6 injectors.

If you have 74lb injectors that sounds about right.

Be sure that the opening/deadtime is not too slow for those injectors because
at high RPMs (10,000 for instance) one engine cycle is 12ms so having 1ms dead time would eat 8.3% off your 20% duty cycle margin.
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RootesRacer



Joined: 04 Apr 2005
Posts: 485
Location: Arvada, Colorado

PostPosted: Thu Jun 14, 2012 11:38 am    Post subject: Reply with quote

I should also mention that the above figures assumes and AFR of around 13.0. If you are running richer then that at those high RPMs, you may need more than 20% duty cycle margin to meet your goals.
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buzzinhalfdozen



Joined: 30 Mar 2006
Posts: 93

PostPosted: Thu Jun 14, 2012 11:53 am    Post subject: Reply with quote

Looks a bit close to being limited out on injector size...agreed? I'd much rather buy larger injectors and reduce the overall PW, thus my question. Would you think the smart move would to increase injector size now instead of finding out they're basically at their limit? As you stated the time gets very short at high rpms hence less time to inject enough fuel. I'd rather spend money now than have an issue, if in fact I buy injectors what ones to buy are there better ones for my application? Thanks Joe
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Pantera EFI



Joined: 12 Feb 2005
Posts: 1718
Location: So. California

PostPosted: Fri Jun 15, 2012 9:28 am    Post subject: Flow=Fuel Pressure Reply with quote

The "dead time" can be reduced, with some (most) injectors, by an increase in fuel pressure.
The fuel flow would increase with an increase in fuel pressure.
The fuel spray pattern will, most often, become better with an increase in fuel pressure.

Thus, I would adjust your regulator to give a rail pressure (static) of 60psi.

There may be a "new style" injector with a better (smaller) droplet size though for drag racing you would have little gain.

Lance
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