Grand Prix Injectors


Registered User
i may not be a long time supercoupe guy but here`s my take on the whole injector sizing issue since i have been building motors both n/a and boosted for many years now

on a n/a engine the general optimal a/f is around 14.7 to make good power ( very general)
on a boosted engine its closer to 12.2
so sayu have 2 engines both of same size and combo other than one is boosted and one is n/a the boosted engine is by default going to require a a larger injector just because of all the extra air being forced into the engine to even get the same a/f at max tq it is going to take more fuel to get the same a/f ratio in the boosted engine as it would the n/a engine due to having more air being forced inside it not to mention the fact that a boosted engine needs a fatter a/f ratio to run safely without detonation
so it makes complete sense that a 42 lb injector will be too small on a engine that only makes 320 hp v6 where a 42 lb on a 5.0 will go a lot further since the v6 has to work those 6 injectors much harder to make the same hp than it would have to on the v8


Registered User
All engines require 14.7 to 1 a/f ratio to run at optimal emmissions and fuel mileage. They DO NOT make the most power there. N/a you look for around 13 to 1 boosted 12 to 1. This is why you have to look at BSFC numbers. They will be higher boosted because of the "saftey factor" you are allowing for by running boost but is also effected by many other factors, like combustion chamber design. A boosted engine is nothing more than a. N/A engine that is having more air forced into it than can be done via normal atmoshperic pressure. Thus leading to the same amount of power being made at less rpm. At low rpm the cylinder can only handle so much air. Which leads to the higher cylinder pressures, need for reduced timing, and the ability top pop head gaskets.


Registered User
I think there's too many folks getting confused in here by my simple general rule of thumb math.

As I stated previously, this is MY forumla that I developed MYSELF from MY OWN EXPERIENCES its not perfect just to give you an idea.

Again, It's not perfect, as fraser stated, different engines are going to have different efficiency's so brake specific fuel consumption will be on a per engine basis just as jdsgallops stated.

jdsgallops said:
I find it hard to believe a mild bolt on car will be maxing out the factory injectors.

as hard as it may be for you to believe it that is fact, simply bolting an s-port on an early model supercoupe will max out the injectors

the numbers i posted are RWHP since thats what my primary dyno measures and thats how i've come up with this, I don't typically tune many engines on our engine dyno as its not feasible most of the time.

Hell even with your math a v8 should be putting out 154 more HP than a supercharged v6 just because it has 2 more cylinders

you calculated my formula wrong

19 x 1.6 x 8 = 243rwhp on a v8 with 19lb injectors, x .8 = 195rwhp boosted

14.64 is stoichiometric for gasoline, its not usually where maximum horsepower is attained, although I have tuned a few naturally aspirated engines that made max power at that afr, its not typical, from memory those few exceptions were very efficient engines that were either yates or chi headed.

13:1 afr is a typical afr for max power on an n/a engine, 12.2-12.8 is where most boosted engines seem to make max power. For safety concerns you'd typically richen it up a tad unless your running very high octane fuel where detonation isn't a concern. This excludes straight alcohol as alch tends to make more power the more fuel you dump in, contrary to gasoline.

hope this helps some one


Registered User
When I asked the best SC engine builder to give me 300ish hp he sent 60# injectors.

Good enough for me.




Registered User
Yep sure is clear as mudd. The 331in my 95 cobra put down 320rwhp on a land and sea dyno. This is a load bearing dyno like a Mustang dyno. On a dynojet that would be about 350rwhp. Before the engine was together I did the math and the stock 24lb injectors would have been rright on the 80% limit. I had that math checked by my cam grinder who also said I could use 24's. I have not paid attention to duty cycle on any dyno results or data logging I have done because the injector was sized appropriately so I can not verify exactly what it is. I just have a hard time fathoming Ford would put a relatively large injector on this engine and leave so little wiggle room. It is not characteristic of them. A 5.0 Mustang can safely run a 19lb injector up to about 300fwhp a full 75hp over stock. Yet you are telling me a SC will run out of injector at 230rwhp, only 40 to 50hp over stock? I don't like your math. Hell even with your math a v8 should be putting out 154 more HP than a supercharged v6 just because it has 2 more cylinders! At the rpm the SC supercharged 3.8 makes peak power I find it hard to believe a mild bolt on car will be maxing out the factory injectors.
I pretty sure the cams play a key role in determining this along with volume of air. Lots of underdrive on supercharger and larger cold air intake require larger than 30# factory injectors. For proper fuel to air ratio. My opinion.