When do I need to upgrade injectors?

XR7 Dave

Registered User
This is not a question, but rather an answer. It is often debated/pondered how much HP the stock injectors will support and when you need to upgrade.

Well, this particular car ran out of injectors at about 245rwhp. The car is a 91 AOD and the relevent mods are as follows:

Full catless exhaust system
Ported exhaust manifolds and cleaned up exhaust ports.
S-Port blower with 10% OD
76MM MA meter
Double IC
CAI w/ cone filter.

For reference, the car hits 14psi immediately and will peg the boost gauge by 3500rpm. The 245rwhp figure comes from a Gtech so take that for what it's worth, but based on my experience and how the car runs I'd say it is pretty accurate. Keep in mind this is a very strong running and well tuned 91 SC. Your results will vary.

Now the interesting thing is how the injectors responded. At 4000rpm the car pulls a commanded 11.8 AF ratio but by 4500 it has climbed to 12.5 - still reasonably safe, then by 5000rpm it has climbed to about 13.5 AF which is NOT safe. Also, since we can see that the injectors are running 100% duty cycle at 4000rpm, operation at this rpm or higher for an extended period of time would be very bad for the injectors.

Just thought some people might find this information useful.

:)
 
Thanks for that, im sure it will come in handy for someone. Not me at the moment but soon. :)

jOe
 
....I have a 77mm ProM mafs...will I have to have it reprogrammed if I change my 30's to 36's? I've heard yes and no.....opinions?
z
 
Dave,

Was the fuel pump able to maintain pressure at max boost/max rpms ?

For SCs I've always gone by the following rule of thumb.

30# will support 300 fwhp
36# ................360 fwhp
38# ................380 fwhp
42# ................420 fwhp
50# ................500 fwhp

If you assume 20% drivetrain loss, the car in question was making 306 fwhp, so it looks like the rule of thumb is pretty close.

David
 
Good thread, was going to PM you this question in the next couple days, GET OUT OF MY MIND! :p
 
hmm..

XR7 Dave said:
This is not a question, but rather an answer. It is often debated/pondered how much HP the stock injectors will support and when you need to upgrade.

Well, this particular car ran out of injectors at about 245rwhp. The car is a 91 AOD and the relevent mods are as follows:

Full catless exhaust system
Ported exhaust manifolds and cleaned up exhaust ports.
S-Port blower with 10% OD
76MM MA meter
Double IC
CAI w/ cone filter.

For reference, the car hits 14psi immediately and will peg the boost gauge by 3500rpm. The 245rwhp figure comes from a Gtech so take that for what it's worth, but based on my experience and how the car runs I'd say it is pretty accurate. Keep in mind this is a very strong running and well tuned 91 SC. Your results will vary.

Now the interesting thing is how the injectors responded. At 4000rpm the car pulls a commanded 11.8 AF ratio but by 4500 it has climbed to 12.5 - still reasonably safe, then by 5000rpm it has climbed to about 13.5 AF which is NOT safe. Also, since we can see that the injectors are running 100% duty cycle at 4000rpm, operation at this rpm or higher for an extended period of time would be very bad for the injectors.

Just thought some people might find this information useful.

:)


Dave is there any way we can set up some kind of tuning session within the next I'd say month for around the monroe/detroitish area because I will need mine tuned as soon as I bring it out. I know I talked to you before and theres 1 other person in this area, but didn't know if you had any other inquiries about or if I should try to set something up. If not, its not a big deal, I'll just drive her down to you. Thanks.
 
Too many people.......

I have heard too many people say that "well, Mustangs run 24lb injectors and there fine". SC's are NOT and I repeat NOT Mustangs. We're talking displacement and HP here.

I had 36 lb injectors on my SC that's pretty well modded and ran out of injector also. Had to go to 42's. People fail to realize that fuel on a supercharged motor is the most important thing. Timing being second.

Thanks Dave for making the community aware of a large problem we have here which is head gasket failure due to lean mixtures and knocking.


Before modding these cars, I always advise them to search the boards and talk to a few KNOWLEDGABE people about what they are doing. On another note, that's EXACTLY why the NJTACC started!!!!!!!



Don
 
lube70 said:
I have heard too many people say that "well, Mustangs run 24lb injectors and there fine". SC's are NOT and I repeat NOT Mustangs. We're talking displacement and HP here.

I had 36 lb injectors on my SC that's pretty well modded and ran out of injector also. Had to go to 42's. People fail to realize that fuel on a supercharged motor is the most important thing. Timing being second.

Thanks Dave for making the community aware of a large problem we have here which is head gasket failure due to lean mixtures and knocking.


Before modding these cars, I always advise them to search the boards and talk to a few KNOWLEDGABE people about what they are doing. On another note, that's EXACTLY why the NJTACC started!!!!!!!



Don


That's the Fearless Leader of NJTACC, Always looking out for the Fellow SCers ...

Hey Don great point on our cars and we definitely are not Mustangs :eek:
 
Dave,

Are you using a scan tool or something else to monitor the injector time?

I have access to a $3k Snap-On scan tool but haven't tried it to see what options are available in the 91 EEC. I know when I used it on my g/fs 2000 Concorde it showed almost everything imaginable.

Later,
Steve
 
Dyno Dave Tells It Like It Is...

Thanks for the Great Info, Dave. I have a few observations and questions.
Does this car still have the stock 60mm Throttle Body? I'm assuming that the MAF was changed out, but the TB and Injectors are stock. Would you attribute this Injector fall-off to the 10% SC Overdrive, since the injectors run out of fuel over 4000 RPM?
What is your solution? In this case could one simply upgrade to 94-95 parts? If one where to change to the late-model 38 lb stock Injectors,late-model 70mm TB, and late-model fuel pump, would this remedy the problem?
Or would you go with aftermarket TB,Fuel pump and Injectors (#lb?), so that the fuel system is upgraded to a higher level and thus improve the fuel system for addtl. mods down the road?
As always, $$= HP!!, just curious if going the low-budget route would be an alternate choice if the car remains at this HP level, or slightly above
(up to 3-325 HP).
The numbers you refer to are the Air/Fuel Ratio, correct or no?
If so, what is the ideal A/F Ratio across the RPM Range?
I must be mistaken, as I thought that it was around 14 to 1, or is this only for normally aspirated engines? (Incl. all fuel-injected engines-)
Thanks for the great tips, and your willingness to share with us your vast knowledge, Dave!
 
Steve,

no I did not use a scan tool but you can on the 91. That will be one of my next investments. ;) They don't measure duty cycle, just ms so you have to convert/calculate.

Dave,

14.64:1 is stoich for running in vacuum conditions and would destroy a motor under boost. 11.8 is desired for most boost conditions.

The stock 94 injectors are only 34lb so they would only give a small margin of increase over the 30's. I universally recommend 42's because IMO, upgrade once and be done with it. Of course 42's should also include a tune but they don't absolutely have to.

In this case we could have turned down the boost to about 13psi and been ok (ditch the OD pulleys), but the torque was just too much fun. :D

All SC TB's are the same size, and yes this car had a stock TB. This just goes to show you that bolting on a 70MM TB on your stock SC is a waste of money. I'm sure this car would have benefited from a 70MM at this Hp level, but this person invested his money where he would get the most benefit for the $$.

Really, this power level is all you should consider if you are on a strict budget. Once you get above this level you start to need some expensive stuff and you better be prepared for some unexpected maintenance from time to time. If you are pinching pennies (nothing wrong with that!), then you should stop at about this level. This car is capable of a 13 second time slip which is about all you should target for a daily driven budget minded SC.

;)
 
Wow, Dave. I was worried before, now I'm really worried. Good thing our local tune is on friday/saturday.

Would a workable solution be to (during the course of the tune) set the rev limiter still at a safe a/f ratio to prevent any engine damage until a time when those of us who fall into the category of needing injectors can get them? I know it's not the best solution, but I know that I'll need a workable one until I can afford injectors. And I don't trust myself to be on the honor system to keep the rpms below a certain level.

You've got some good info here and it should be added to the FAQ section just for reference. I've got a few more mods and I was mildly concerned that I was starving for fuel. Now I'm genuinely worried that I'm going to hurt my SC. I guess no driving until Friday....
 
David Neibert said:
Dave,

Was the fuel pump able to maintain pressure at max boost/max rpms ?

For SCs I've always gone by the following rule of thumb.

30# will support 300 fwhp
36# ................360 fwhp
38# ................380 fwhp
42# ................420 fwhp
50# ................500 fwhp

If you assume 20% drivetrain loss, the car in question was making 306 fwhp, so it looks like the rule of thumb is pretty close.

David



I have planned to do these mods:

Better exhaust

D I/C with 800cfm fun

Raised blower top

73mm MAFS

70mm T-body

MP air intake system

180 degrees t-stat

255 lph fuel pump

5% overdrive pulley

3.27 rear end ratio


I thought I don't need bigger injectors. With these mods I will not gain so much extra hp. So please tell me do I need bigger injectors than those stock 30# ones!? I was also thinking that 30# will support about 300rwhp.

And also it would be very nice if you could tell your opinions about my mod plan! Anything!

Thanks!

PS. Dave, have you changed your mind about that "injectors size thing"? :D
 
As I said above, your results will vary. David is right about the 300hp level for the stock injectors and this car has demonstrated just how accurate that is. For this vehicle we were very strict about only doing mods which would provide maximum results. Unless you have all the mods listed and are still hitting 15psi I don't think you have anything to worry about.
 
XR7 Dave said:
As I said above, your results will vary. David is right about the 300hp level for the stock injectors and this car has demonstrated just how accurate that is. For this vehicle we were very strict about only doing mods which would provide maximum results. Unless you have all the mods listed and are still hitting 15psi I don't think you have anything to worry about.

Ok, I just wanted to be sure... I will change them later but not yet! Thanks! :)
 
say i have light mods would i benefit from injectors (overdrive pulley, under drive pulley 2.5-3-2.5 dual exhaust, 70mm tb, 70mm mafs,kooks headers midlength) ive been thinking abouta copule of options and injecotrs are cheaper then some and more expensive then others
 
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