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Stutter in upper rpm wot

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I have an 08 GT500, has a VMP 2.5 pulley, Ford racing twin 65mm TB, JLT big CAI, John Lund tune for 91 octane.

Finally got around to test the car tonight and noticed it stutters from 5000rpm all the way to redline.

I'm getting 12:1 AFR but as soon as it starts the stutter, it hits 11.8:1 so it's getting richer.

I'm thinking spark is suffering. I'm using TR6 plugs gapped at .035. I'm thinking of going down to .028 as I've read some guys are doing here.

I want to get rid of this stutter. It's not a bad jerking stutter, more of like a slight loss of something. It's not that smooth hard pull as you get from 3000-5000 rpm

You barely notice it when it happens but knowing it's there is just irritating more than anything.


Any thoughts?

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Well I gapped the plugs to .028 and still get the stutter. So I put in 100 octane and loaded the Lund race tune and no stutter at all.

So the 91 octane tune at 5500rpm to redline, the injectors are dumping fuel causing the stutter.

The AFR is going from 12:1 to 10.5:1 so way to rich up top.

Looks like I have to get a revision on this tune.

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Definitely check the coils. I was having this same issue as well but I did put in the Weapon X coils. Three of the coils ended up being bad. Swapped them out and put the stock coils back in, Fixed right up. It could also possibly be the tune. Though I know Lund does good tunes. But good luck and keep us update on what you find out.

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