injectors
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39's are too much. The stock 24# units would be better for a bolt-on car and wouldn't require a tune.
save the money , the stock 24s are fine for bolt ons , you would have to add a blower or one hell of a built engine to need anything over the stock 24s
Use the formula for determining what size fuel injectors you need. Remember that fuel injection has to support all the power an engine makes, including the power lost due to friction.
(Fuel flow in LBM X Number of cylinders) / 0.5
If you are forced induction you would use 0.6 instead. By this formula, 24# injectors only support 384 flywheel HP. That's only 326HP at the wheels.
Depending on the mods you have, or the HP goal you envision, 24# injectors might not be enough. Breaking 326 RWHP in a four valve is not too hard at all. You probably can't do it with bolt ons, but it is easily achievable with cams, exhaust and intake. Heck, pick two of them and you could do it.
(Fuel flow in LBM X Number of cylinders) / 0.5
If you are forced induction you would use 0.6 instead. By this formula, 24# injectors only support 384 flywheel HP. That's only 326HP at the wheels.
Depending on the mods you have, or the HP goal you envision, 24# injectors might not be enough. Breaking 326 RWHP in a four valve is not too hard at all. You probably can't do it with bolt ons, but it is easily achievable with cams, exhaust and intake. Heck, pick two of them and you could do it.
Crzyhrse
Active Member
^ I agree with this. If you've just got long tube headers, midpipe, catback, CAI and dynotuned then don't bother. You'll come out at about 315RWHP. If it was me I'd go with 30lb. injectors if I was to get cams or a new Intake manifold. 42's with a blower.
Maxpowers
Member
Save your money. Your stock injectors will be able to handle what I assume you have.
Keep in mind, to add a larger set of injectors you will need a tune.
Keep in mind, to add a larger set of injectors you will need a tune.
The formula is correct but the number it puts out is rwhp at 100% duty cycle, not flywheel hp (technically it could also be flywheel hp @ 85% duty cycle since our drivetrain losses tend to be ~15%). So 24lb injectors will support 384 rwhp at 100% duty cycle. For proof look at the dyno chart here on a trick flow headed NA car putting out 345rwhp on a mustang dyno (notoriously reads 3-5% lower than a dynojet), this with 24lb injectors (this example is running 90%, a little high but
):http://www.stangnet.com/mustang-forums/834521-those-doing-research-na-setup-trickflow-4-6-heads.html
For a street car 85% duty cycle is where you want to be so that is where the 326rwhp number comes from. That being said you are going to need much more than "some boltons" to even reach 326rwhp and even when you reach that level you are only truly at that hp for fractions of a second.
To the OP: if you have no plans for a dry nitrous kit, aggressive cams, or s/c in the future than get stock sized (24#) injectors. Yes you may be able to get some stock 39# terminator injectors for cheap but like maxpowers and I have both pointed out you are going to require a tune. Even if you have a SCT tuner and the free AM bama tuning this is a significant jump in injector size and I would be a little nervous w/ a mail order tune for injectors more than 60% bigger in size.
The formula was originally intended for 85% duty cycle and gross horsepower in naturally aspired conditions (cars used to be rated on gross horsepower way back in the day). When you get to talking about parasitic losses such as accessories or superchargers it gets a lot more complicated. Using for 100% duty with RWHP naturally aspired is mathematically pretty darn close....But there is an issue with 100% duty cycle.
A fuel injector running at 100% is always flowing fuel. The problem with running past 85% is that you will inject liquid fuel droplets straight into the cylinder. This seriously raises the risk of fuel detonation because you don't have a homogeneous mixture in the cylinder at the end of your compression stroke. Multi port fuel injection was designed to inject fuel onto the intake valve (hot metal) to assist in vaporizing the fuel while the remaining liquid would be uniformly swept into the chamber by incoming air and conventionally evaporated.
When you operate above 85%, even a competitive tune in naturally aspired form, to say nothing of forced induction, introduces the possibility of detonation.
When you get to a point where you are above 85% on your stock injectors, you will be needing a tune anyway. You might as well tune with bigger injectors to support future improvements. Also you will make more power through your proper fuel mixture by being able to run a more aggressive tune.
But like Dark04GT says, a mail order tune is not the way to do it. A major change in injector size necessitates a custom tune. Mail order could do it, but it would have to be real conservative to be considered reliable.
That's just my two cents.
A fuel injector running at 100% is always flowing fuel. The problem with running past 85% is that you will inject liquid fuel droplets straight into the cylinder. This seriously raises the risk of fuel detonation because you don't have a homogeneous mixture in the cylinder at the end of your compression stroke. Multi port fuel injection was designed to inject fuel onto the intake valve (hot metal) to assist in vaporizing the fuel while the remaining liquid would be uniformly swept into the chamber by incoming air and conventionally evaporated.
When you operate above 85%, even a competitive tune in naturally aspired form, to say nothing of forced induction, introduces the possibility of detonation.
When you get to a point where you are above 85% on your stock injectors, you will be needing a tune anyway. You might as well tune with bigger injectors to support future improvements. Also you will make more power through your proper fuel mixture by being able to run a more aggressive tune.
But like Dark04GT says, a mail order tune is not the way to do it. A major change in injector size necessitates a custom tune. Mail order could do it, but it would have to be real conservative to be considered reliable.
That's just my two cents.
It is a good two cents. I agree, above 85% can lead to suboptimal conditions. It is do-able but things start getting really tight.
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