2024 Mustang Dark Horse — Whipple Stage 1, 10R80 — Persistent flutter/stutter + downshift clunk. Anyone seen this? (Taking it to Ford next week)

[SC Dark Horse]

The car: 2024 Dark Horse, Whipple Stage 1 (Ford-warrantied), 10R80 automatic, 11k miles, transmission/converter original and unmodified. Whipple's dual air intake system is part of the kit.

The symptom:

• Flutter/stutter under load at constant RPM and speed — initially most noticeable around 1,980 RPM, but I've since confirmed it happens across all gears, not just that one spot.
• Resolves instantly the moment I touch the throttle, either direction (on or off gas).
• Gets worse when the transmission is fully warmed up — not cold, which seems to be the opposite of what a lot of 10-speed shudder complaints describe.
• Progressively gotten worse over time.
• New: a clunk on downshifts, and both the flutter and clunk feel like they're coming from the middle of the car, right in front of the dashboard — roughly where the transmission/front driveshaft area sits.

Timeline / what's been ruled out:

• Flutter was present before I had the rear end replaced (unrelated repair). The rear end swap did produce a new squeak, but the flutter itself predates that work entirely, so it's not caused by that.
• Driveshaft/pinion angle was measured by a shop after the rear end replacement — came back within spec, so I've ruled that out as the cause of the flutter.
• Flutter has been present on both the stock Ford tune AND the Whipple Stage 1 tune. Whipple confirmed directly (twice, by phone) that their Stage 1 tune is engine-only — it never touches transmission/TCC shift or lockup calibration. So the transmission calibration has been the stock Ford cal this entire time, across every configuration.

Diagnostics done so far:

• Whipple reviewed the engine/supercharger datalogs twice (two different calibrators) — found nothing abnormal, called the most recent log clean.
• A local speed shop's story shifted a few times. First said AFR was "insanely rich," then said trans looked clean with no TCC oscillation in their data, then landed on recommending a full engine+trans custom tune. I never got to see the underlying data for any of these claims, so I've held off on that route for now.
• I pulled a longer datalog myself and had it analyzed in detail. Findings: the "rich" AFR readings were all tied to normal DFCO recovery enrichment after coasting — transient, not sustained, doesn't support the "insanely rich" claim. But there IS a real, repeatable pattern of sustained knock retard (spark pulled up to ~5°) recurring in top gears (9th/10th) right around 1,600–2,000 RPM under boost, and the magnitude increased as coolant temp climbed over the course of the drive — which lines up with my "worse when warm" observation. Sent this to Whipple, who reviewed it and said it looked clean/fine.
• No TCC-specific PID data (slip speed, duty cycle) has been captured yet by anyone — that remains the one piece nobody's actually pulled and shown me.

Current hypotheses I'm weighing:

1. Torque converter — clutch material/lockup behavior struggling to hold clean partial lockup under the added torque from the blower, possibly compounded by heat as fluid thins when warm. Would fit the all-gears symptom and the warm-vs-cold pattern.
2. TCC calibration hunting — stock Ford lockup/PID tables potentially not well suited to the modified torque curve, even though the tune itself came from Ford.
3. Something mechanical at the trans/front driveshaft — mount, U-joint, or slip yoke play, given the downshift clunk and the localized feel right in front of the dashboard.
4. Engine-side knock retard may be a contributing factor but probably isn't the whole story given the symptom is present in every gear.

Taking it to Ford next week for a proper transmission diagnostic (still under warranty) — trying to gather as much real-world input as possible beforehand so I can point their techs in the right direction.

Questions for the forum:

1. Anyone had a similar flutter/shudder at cruise on a Whipple Stage 1 / 10R80 combo, especially one that's worse warm than cold?
2. Anyone dealt with a downshift clunk localized to the trans/front driveshaft area which turned out to be mount, U-joint, slip yoke, or something inside the trans itself?

Thanks in advance!

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[marcus50]

The clunk when downshifting at slow speeds seems to be not uncommon:
https://www.mustang7g.com/forums/th...-when-slowing-down.161604/page-2#post-3311352

 

[Ninjak]

@SC Dark Horse : Question: When you say shudder/flutter, do you mean that it feels as if the car is hesitating, sort of going, not going, going? When you accelerate, it goes away completely; when you cruise it goes back to hesitating again, or as a stutter/fluter/shudder?

If so, I had this issue: it was the TB positioning and the TQ it expected to see. My next question is: did you get the v2 software/tune update from Whipple? This is supposed to fix that issue. The difference for me is that I have a 6 speed, so it did it in 4th and 5th, and that I also have a custom tune. My Whipple calibration tool was lost... (do not ask how....) I still do not know how it happened, but it was the dealer's fault.

When I explained this to my tuner, We went back and forth as they tried to find the problem. It does not appear clearly in the logs. He then asked me when I did my Whipple install, and I told him. He looked it up and told me they needed the Whipple V2 calibration file. When they did this and sent me a tune, my issues went away. To test this, I even installed the old tune back, and the issue came back.

May not be the same, but that could be your issue, of course, if what I mentioned in the beginning is actually happening.

 

[ZXMustang]

Reflash your whipple tune, it clears out the trans adaptives. Then go right out and beat the dogshit out of it. This is what I do after a flash. Works every time and gives nice smooth part throttle shifting for a very long time until the adaptives trend back down and shit gets goofy again. Then rinse and repeat.

 

[SC Dark Horse]

@SC Dark Horse : Question: When you say shudder/flutter, do you mean that it feels as if the car is hesitating, sort of going, not going, going? When you accelerate, it goes away completely; when you cruise it goes back to hesitating again, or as a stutter/fluter/shudder?

If so, I had this issue: it was the TB positioning and the TQ it expected to see. My next question is: did you get the v2 software/tune update from Whipple? This is supposed to fix that issue. The difference for me is that I have a 6 speed, so it did it in 4th and 5th, and that I also have a custom tune. My Whipple calibration tool was lost... (do not ask how....) I still do not know how it happened, but it was the dealer's fault.

When I explained this to my tuner, We went back and forth as they tried to find the problem. It does not appear clearly in the logs. He then asked me when I did my Whipple install, and I told him. He looked it up and told me they needed the Whipple V2 calibration file. When they did this and sent me a tune, my issues went away. To test this, I even installed the old tune back, and the issue came back.

May not be the same, but that could be your issue, of course, if what I mentioned in the beginning is actually happening.

Thanks for the reply, and sorry to hear about your flash tool. I don’t know how to describe it outside of it feeling like I’m driving over sticky tape when at a constant RPM/cruise speed. Slight hesitations or flutter. Disappears as soon as I add or remove throttle. The tune I’m running is the v2 cal. I did flash the ford performance tune as well, per Whipple’s recommendation, with the issue persisting through both tunes. I’m now back on the Whipple tune. I will mention TB to Ford as well. Thank you again for the reply. Genuinely helpful!

 

[SC Dark Horse]

Reflash your whipple tune, it clears out the trans adaptives. Then go right out and beat the dogshit out of it. This is what I do after a flash. Works every time and gives nice smooth part throttle shifting for a very long time until the adaptives trend back down and shit gets goofy again. Then rinse and repeat.

I’ll try this tomorrow. Thanks for the recommendation. I just reflashed it a couple days ago, but didn’t do any meaningful pulls immediately following flash.

 

[Ninjak]

@SC Dark Horse, do you still have your CATS? It may not be this issue, but even with the stage 1 Whipple, your CATS will not hold up. How long have you had this installed?

I have seen where CATS started to fail and will throw no code. If it starts to block your exhaust flow, and maybe at acceleration you are forcing more air into the system.....just trying to help troubleshoot this with you. It may be worth looking at.

 

[SC Dark Horse]

@SC Dark Horse, do you still have your CATS? It may not be this issue, but even with the stage 1 Whipple, your CATS will not hold up. How long have you had this installed?

I have seen where CATS started to fail and will throw no code. If it starts to block your exhaust flow, and maybe at acceleration you are forcing more air into the system.....just trying to help troubleshoot this with you. It may be worth looking at.

I’ve had it on for about a year. It’s low mileage on that year - probably around 2.5k miles. Still have cats. Do have open air box kit and resonator delete. Because it’s consistency across all gears, I’m not ruling out an airflow issue at all.

 

[Ninjak]

@SC Dark Horse I see. You may still want to look at changing to Hi-flow CATS or deleting them. The stock CATS will not hold up over time, even with the stage 1 tune.

 

[rush014]

OP -- Ford did a recall on 2024 -25 GTs and Darkhorses with the Ford performance Whipple.......

Per Ford ---- The software tune in affected kits can disable secondary safety monitors and trigger a wrench or check-engine light, leading to erratic throttle behavior or hesitation. A free PCM re-flash update from an authorized dealer corrects this.

I hope you can get it figured out, because I'm sure that DH rips when running correctly

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