On Monday I attended a private track rental at MIR with 12 other Mustangs. I was driving our 2005 4.0L/auto ragtop, which is strictly bolt-ons with a 7.5" 4.10's, LSD, LCA's, alum DS, tune, CAI, and alt pulley. I bolted up my 275/50R17 M&H DR's (28" dia) and made 8 passes. I adjusted my timing with the 93 tune, having added 4 gallons of 104 unleaded; played around with shift pressures, and shift points. I would stall up to 1800-2K, and I made several 15.0xx passes with my 60's around 2.15, but my speeds were down a few MPH's over my only other time at the track with this car.

I pulled off my DR's to give two other guys a try with them, and decided to make some runs with my 275/50R17 Nitto NT450 street tires (also 28" dia). Leaving at idle, I immediately knocked off 2/10th and gained 1mph. I ended up with a best ET of 14.779 @ 91.66mph with a best 60' of 2.106.

So why was I so much better with my streets than the DR's? Track prep was the best I've ever seen. Folks were dead hooking like they've never done before. Could the stickier track been holding my DR's back? Thoughts?

 

Maybe those DRs were just too much traction? That sounds a little ridiculous....

Were the wheels on both sets of tires the same?

 

Having used the the MH Racemaster's personally I would say your launch RPM/stall was holding you back. On my MT ET Street 28x13.50's I would leave the line @ 4500RPM's and cut 1.7x's all day long and on the MH's radials leaving the line at 100-200 RPMs less I would still cut 1.8's on the 305/35/17's.

Considering the track prep was phenomenal that night I bet you could have footbraked a bit higher without spinning the tires that night if the groove was nice and sticky.

 

+1

Also, the tires might be to tall for the car...more rotational mass= more weight and that slows you down...maybe a 26" tire with a little more air would've netted better times

 

+1. What RPM were you leaving at. Was the car bogging? Gotta leave hard on sticky tires.

 

I agree with the rest. Mayby too much tire for the car. What happened with the other guys running them? Were there times faster or slower ?

Launch RPM is deffinetly hurting you. I leave between 4500-4800rpm and cut consistant 1.6-1.7 60ft times on mickey thompson e/t streets 275/40/17(26" tire) and on stock wheels

 

Considering that the 60 foot times were very close, I would look at the weather being a factor first. As others have said a 28" tall tire is WAYY too tall for a bolt on 6'er, even with the 4.10's. With that tire you probably have a 3.50ish effective gear ratio.

 

The low leave RPM makes sense on the ET. As far as MPH, do the drag radials experience much side wall growth? My old ET Streets would grow enough to knockoff 400 RPM in the traps compared to my daily performance radials. The drag radials my be growing enough to knock off a little MPH.

 

I calculated my final effective gear ratio with the 28" tire & 4.10's to be 3.94:1.

You may be on to something there. Although both sets of tires are shown to have the same rotations per mile, I did notice a difference through the traps. With the DR's, I was going through the traps in 4th gear, but with the street tires the car would shift into overdrive at about 4,800 RPM's just before the traps (don't give me too much crap about being in overdrive - I'm used to racing stick shifts). My best runs were on streets with the OD off. So the DR's are a tad bit taller than my street tires.

 

try using your size tire with DR's on your car...i'm willing to bet you'll pic up a few .1's