Acceleration flat spot / PI Swapped Confusion

[ramprat1983]

Alright guys, new here but I have an ongoing concern with my car. First off heres the specs/info all out of the way 1998 mustang gt autotragic 2800 stall 3.73 rear gears w/ carbon fiber clutch packs C&L upper intake manifold Pro Products 75mm TB Home made JLT style intake w/ K&N cone PI Intake PI Heads gasket matched ASP Underdrive pulleys (steeda style) MSD coils / MSD wires fms shorty headers hacked BBK 2 1/2" offroad H pipe custom side exhaust 24lb injectors w/ calibrated mass air stock springs all the way around w/ upr lower control arms, all new urethane bushings on everything etc and a Delta Force tuner w/ commando software (formerly Sniper) this is something that's gone on even before the PI Swap, tried tuning all around it - even had the actual Delta Force guys look at it (the ones who actually made the thing, as well as Diablosport etc) around 70-80 or so it gets this flat spot in the acceleration it feels like, and then it starts pull again. you can hear it in the exhaust note it just flattens out.. any way to get around this? what is it? also, before the PI head swap, i had just the PI Intake manifold on and i was running a 14.3 @ like 96-97.. after i put the PI Heads on i've only bested a 14.01 @ 99 mph - i'd of figured i'd run faster than that? the car laid down over 260 to the wheels after being tuned on the dyno netting me steady 12.2-12.5 A/F most of my runs ahve been on street tires due to having slower mph's before with my drag radials, on the street tires i cut 2.2 60's cus the car just lights em up. anyway if anyone can shed some kind of light on any of this please reply and let me know! thanks!

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[Hangwire]

When you say "calibrated mass air." What do you mean? Is it still your stocker MAF, just "calibrated" through your tuner?

[ramprat1983]

yeah, it's a stock cobra housing with the 98 gt unit on it calibrated to the 24'ers and the car.

[cliffyk]

I do not believe that what you have is a "calibrated" MAF*. Unless the stock '98 sensor's circuitry was modified by someone with the needed knowledge and skills, then it's just a messed up MAF with an unknown transfer function.

This is because the MAF sensor does not measure all of the air flowing through the housing, but only small a sample of the total air flow. It then produces an output signal (voltage on our cars) for the PCM to interpret.

The PCM does this by looking up that voltage in the MAF transfer function table, so as to find out how much air flow that voltage level represents.

But when a given sensor is placed in a larger diameter housing the sample size becomes proportionally smaller, as compared to the actual amount of air flow--for any output voltage there will be more air flowing. I.e. the transfer function changes.

The change factor for the transfer function can be approximated--here's a link to a post I made on the DeltaForce forum explaining this is some detail.

You need to get a wide band O2 sensor so that you can observe and/or log AFR--particularly at the time you experience the bogging. I would also suggest going back to the stock intake and MAF (which will easily support 350+ HP without pegging), as the MAF assembly you are now running has an unknown transfer function. Make sure the transfer function in the tune is returned to stock too.

-----------------------------------------
* -
So-called "calibrated" MAFs have had their transfer functions mucked up so as lie to a specific stock tune as to how much air is entering the engine.

Throughout most of the operating range it tells the PCM that less air than it really being drawn in is flowing, so that the stock tune will calculate a shorter injector pulse, so that the larger injectors do not inject more fuel than is needed. At or near the top of the flow range it lies less or does not lie and the stock tune uses what it thinks would be an appropriate pulse width for 19lb/h injectors--resulting in more fuel being injected.

I.e. "calibrated" MAFs are a kludge designed for a time when it was difficult or impossible to reprogram the tune's transfer function.

[ramprat1983]

well, the difference is, i have a wideband and i CAN monitor it. theres no change in A/F when the "bogging" occurs. it's been strapped to the dyno through this also, it's just a tonal change with a slight slowdown of acceleration, the rest of it , the car is screaming - the mass air has had 2 different tunes rewritten for one that we cut the sample tube off and vane, and one that we left stock. the transfer function, voltage, the injector pulse everything has all been changed. I'd assume that Sniper/Delta Force know what they're doing since they actually made the tuning program, as well as the old Diablosport system and superchips. I don't know if some of you know what the sniper/deltaforce software is but it's not like a regular little hand held, there is WAYYYYYYYY more you can do.


can't be the MAF because i've ran one of their bazooka maf's and while it performs a little better - there is still the exact same sagging of acceleration during that 70-80mph and flatter on up through the 1/4.

it's just something i'm trying to figure out so i don't have to tune around it, i can actually correct the problem.

funny thing is it's done the same exact thing when the car was bone stock? could it just be the internal gearing of the trans just sucks?

[cliffyk]

I am quite aware of what Delta Force can do, in fact I had lunch with Pat a week ago today--sort of, we had hot dogs from the convenience store up the street from their shop.

Since the bogging occurred when the car was stock I agree that it's likely a transmission/tranny tuning issue.

[ramprat1983]

lol, well that's cool you know pat personally too lol he's a great guy but yeah i wasn't sure if it was the tuning or not, realistically we've had the trans pressure up, stock, a little boost and it's all the same - just might be the gearing INSIDE the transmission. unless theres something i'm missing or pat is missing i guess it'll have to be addressed when i go there tomorrow. just wanted to know if anyone else experienced it, guessing it wouldn't be there if it was a 5 speed

[lizzyfan]

Originally Posted by cliffyk

I am quite aware of what Delta Force can do, in fact I had lunch with Pat a week ago today--sort of, we had hot dogs from the convenience store up the street from their shop.

Since the bogging occurred when the car was stock I agree that it's likely a transmission/tranny tuning issue.

Sorry to the O/P, cliffy I had to put that in my sig!