Hypothetical speed density questions

hoopty5.0

hoopty5.0

mechanicus terribilis
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Dec 14, 2010
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#1
Let's say I have a 351W in a "mustang" that is controlled by a speed density computer. If I swap out the E7s and stock truck intake manifold for a GT-40 intake and heads:
- Do I need 24# injectors?
- Will that mess with the computer?
- Adjustable fuel pressure regulator needed in place of OR in conjunction with the injectors?

Thanks!

*I'd ask these questions in the truck forums, but those guys seem to all wear velcro shoes.
 

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#3
24lb injectors are not needed for a gt40 intake and heads so I think you'll be fine with the speed density. Unless your going to swap a cam in as well because Speed density depends on your MAP sensor. A cam swap depending on the cam would alter your vacuum and would be a problem unless you do your research and get a speed density friendly spec cam. Or toss your parts on it and see how it goes if it needs a maf conversion then tackle that then.
 
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#5
Boostedpimp said:
24lb injectors are not needed for a gt40 intake and heads so I think you'll be fine with the speed density. Unless your going to swap a cam in as well because Speed density depends on your MAP sensor. A cam swap depending on the cam would alter your vacuum and would be a problem unless you do your research and get a speed density friendly spec cam. Or toss your parts on it and see how it goes if it needs a maf conversion then tackle that then.
Click to expand...
No cam for the moment. Just putting the P heads and intake on my truck to get a little extra grunt.
 
#6
- The injector size would depend on an airflow calculation (though it definitely wouldn't hurt to upsize a little in this specific scenario).

- The speed density tune IS affected by parts and injector changes since airFLOW is changed throughout the range of MAP and RPM.

- I don't think an adjustable regulator is necessary in this scenario.

These are the reasons all the SD mustang guys (1987-1988) do mass air conversions. As long as the maf isn't pegging, the ECU will see the correct amount of airflow and can accurately compensate with fuel. In an SD tune, the computer doesn't see the added airflow at a given MAPxRPM condition and won't know to add fuel. Closed loop correction would see the AFR out of range and try to compensate but it never works out well, considering the narrowband sensing accuracy window. It'd probably overshoot in both directions and surge your life away.
 
#7
5.0Thunder said:
- The injector size would depend on an airflow calculation (though it definitely wouldn't hurt to upsize a little in this specific scenario).

- The speed density tune IS affected by parts and injector changes since airFLOW is changed throughout the range of MAP and RPM.

- I don't think an adjustable regulator is necessary in this scenario.

These are the reasons all the SD mustang guys (1987-1988) do mass air conversions. As long as the maf isn't pegging, the ECU will see the correct amount of airflow and can accurately compensate with fuel. In an SD tune, the computer doesn't see the added airflow at a given MAPxRPM condition and won't know to add fuel. Closed loop correction would see the AFR out of range and try to compensate but it never works out well, considering the narrowband sensing accuracy window. It'd probably overshoot in both directions and surge your life away.
Click to expand...

So assuming that a MAF swap is off the table (for now, at least) What would you do in this scenario? Leave the injectors alone, or ?
 
#8
probably leave the whole engine alone. Lol!

But for real, anything you do to change airflow in/out of the engine more than a few percent is going to obsolete the factory tune. I'd for sure leave the injectors alone if you're bent on modifying the thing. Maybe do the heads and intake only and see how that goes. Typically the SD mustang guys got away with that much, though the cars ran rich from overcompensations to lean conditions.

I did a b303 and exploder intake on my 87 back in high school and the ECU flipped me the bird. lol might have been better without the cam but that's a mild cam.

So why is MAF off the table? and why would microsquirt/mega be off the table? cost maybe? MS tuning on stockish engines is a breeze.
 
#9
5.0Thunder said:
probably leave the whole engine alone. Lol!

But for real, anything you do to change airflow in/out of the engine more than a few percent is going to obsolete the factory tune. I'd for sure leave the injectors alone if you're bent on modifying the thing. Maybe do the heads and intake only and see how that goes. Typically the SD mustang guys got away with that much, though the cars ran rich from overcompensations to lean conditions.

I did a b303 and exploder intake on my 87 back in high school and the ECU flipped me the bird. lol might have been better without the cam but that's a mild cam.

So why is MAF off the table? and why would microsquirt/mega be off the table? cost maybe? MS tuning on stockish engines is a breeze.
Click to expand...
MAF is off the table because if I was going to go to the trouble, I'd do MS. But then, I'd have to buy a trans controller. And since I'd have a good tuning platform, I'd have to buy better engine parts then. And then the transmission would sht itself... again.

SO -- I'm purposely limiting myself until I can afford a coyote swap. That way, it's in, its done, and I can stop dicking with it and only have a foxbody that is a continual source of issues :D
 
#10
hoopty5.0 said:
MAF is off the table because if I was going to go to the trouble, I'd do MS. But then, I'd have to buy a trans controller. And since I'd have a good tuning platform, I'd have to buy better engine parts then. And then the transmission would sht itself... again.

SO -- I'm purposely limiting myself until I can afford a coyote swap. That way, it's in, its done, and I can stop dicking with it and only have a foxbody that is a continual source of issues :D
Click to expand...

Does it need heads and intake before the coyote? lol
 
#12
The displacement change is the only real element that concerns me.

If I were going MegaSquirt, I'd be [more] likely to run SD. TwEECer is out of the picture too then. You want to know if it'll run well in stock form over the top of that motor and, I think you said it's an EEC from a 351?

If you're staying with the stock cam, a 351 SD EEC, and swapping only heads and intake, it should all work just fine. Keep the 19 lb injectors. I doubt you will need more but you can always upgrade them later if you see that fuel is a bit short. I would install an adjustable regulator even if I didn't use it. If you find that you're running just a hair rich or lean, you can adjust that through fuel pressure. Make sure your pump is up to the task. An OEM 88L/hr would not be my first choice.

Did I miss anything?
 
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#13
Noobz347 said:
The displacement change is the only real element that concerns me.

If I were going MegaSquirt, I'd be [more] likely to run SD. TwEECer is out of the picture too then. You want to know if it'll run well in stock form over the top of that motor and, I think you said it's an EEC from a 351?

If you're staying with the stock cam, a 351 SD EEC, and swapping only heads and intake, it should all work just fine. Keep the 19 lb injectors. I doubt you will need more but you can always upgrade them later if you see that fuel is a bit short. I would install an adjustable regulator even if I didn't use it. If you find that you're running just a hair rich or lean, you can adjust that through fuel pressure. Make sure your pump is up to the task. An OEM 88L/hr would not be my first choice.

Did I miss anything?
Click to expand...
Uhh no, nothing other than your usual unintelligible rantings that typically follow your moments of clarity. :p But it's 100% factory right now, it was a 351 vehicle with a 351 SD computer. The only things changing are the P heads and GT40 intake. All else remains the same.
 
#15
hoopty5.0 said:
Uhh no, nothing other than your usual unintelligible rantings that typically follow your moments of clarity. :p But it's 100% factory right now, it was a 351 vehicle with a 351 SD computer. The only things changing are the P heads and GT40 intake. All else remains the same.
Click to expand...

This will run fine. I did heads(TFS), intake, TB and exhaust, on my 86 initially. Even went 306 a bit later, all on SD.

That's one of the big reasons I"m such a fan of the 86 cam profile. Ok, the heads were horrible but....
 
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#16
Ummmm, I feel like I'm in the Twilight Zone... Let's say for arguments sake I know this guy, we will call him "Collin" in this story. In this hypothetical he has a "Mustang" and he put a "351" in it. Now here's the thing... he's had that "351" in it for a while running "speed density" using a "Megasquirt" computer.

Now here's where it gets weird for me. You seem awfully similar to the story in my head except you're asking about a stock 351 computer... One of us is drunk. I don't think it's me.
 
#17
bird_dog0347 said:
Ummmm, I feel like I'm in the Twilight Zone... Let's say for arguments sake I know this guy, we will call him "Collin" in this story. In this hypothetical he has a "Mustang" and he put a "351" in it. Now here's the thing... he's had that "351" in it for a while running "speed density" using a "Megasquirt" computer.

Now here's where it gets weird for me. You seem awfully similar to the story in my head except you're asking about a stock 351 computer... One of us is drunk. I don't think it's me.
Click to expand...
Collin might have another vehicle with a 351 in it, that may or may not have a factory speed density system
jump.jpg
 
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#20
We had a racing buddy with a speed density mustang. He put Brodix heads on it and a systemax intake, but left the stock cam. He ran that car to the bottom 12's and never had an issue.

Joe
 
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