Contour Enthusiasts Group Archives Forums Modifications Duratec Performance Acceleration with 18's

They were E1 wheels with bfg kdw on them 215/50/16.....brian

Dude, Pascal! You still didn't change those plugs and wires? Change them to at least Taylor wires and Bosch +4 Platnimum plugs, and you WILL see differences. BTW, I still think your 18" slowed you down quite a bit because of bigger moment of inertia.

Benjxn mentioned maybe the diameter is larger. That would definitely have an effect. A larger diameter means larger circumference which means slower time due to altered taller gearing. Conversely, a smaller diameter with a smaller circumference would lower the gearing and improve acceleration at the expense of increased odometer readings. Just because you have a plus 1 or plus 2 tire/wheel combo does not mean there is no difference in overall diameter.

Again on the weight thing, rotational inertia increases with the square of the radius or something like that. The fact that the wide heaviest part of the rim (where the tire is mounted) is farther from the axle than a 16" wheel is what causes slower times.

I never knew that the stock SVT rim with the tire weighed 43 pounds each. Is this correct. Or is is 43 pounds for all 4 rims and tires. I put 18's on my SVT and I went with light weight racing rims that were 19 lbs. each without the tire and have noticed no difference in being slower. If anything I think pickup it better. I know I can tell that I have better traction in take off. I thought that the stock SVT rims weighed about 16 lbs. each with out tire. Can't wait to here from ya all.
peace

Silver98.5SVT: 43lbs was for 1 single rim w/ tire mounted on it. For most any rim you choose, the tire you mount will probably be heavier. 19lbs rims with tire mounted would yield a ~40 to 42 lbs combo. I also have much better traction on take off, no more feathering the clutch, just punch it and dump it .. It's past 60mph that things aren't so bright. Jeff in his mystique passed me real bad when we put it in 4th on the highway.

AirKnight: I'm gonna get on the plug's case as soon as I have some time off. Will clean the upper manifold and TB at the same time. More on that later.

SVTCole: I dunno about the difference rolling diameter would make. I've lined up with a stock Mystique V6 at 120kph and both our speedos were smack on, even with my bigger rolling stock. I'm guessing the difference is in the low 1% area, not enough to hamper acceleration.

Am I the only one thinking that my problem is related to the high speed drag that my tires induce because of:
1- Increased frontal area hence more drag through air (poor Cx on those 225's)
2- Double the size of the rubber contact patch with the road (more friction)
3- Add a softer coumpound to boot (even more friction)
... and you get a slower car that drinks more gas on the highway! I think if my tires were 215's of the FZ4 / A520 variety the effect would be lessened. The T1-S is a LOT grippier than those.

Still somewhat puzzled. You guys that do the drag strip, doesn't anyone have times with stock vs upgraded wheels?

What wheels are you running? It is rare to find an 18 wheel/tire combo that is going to be lighter than the svt rims (19 for E1 and 20 for those prior).
I went from 17s to 16s last year (the 17s were 24lbs and the 16s were 15.5) and then did some 0-60 runs w/ my g-tech. I dropped 3-4 tenths from my times with the 17s. Also note that tires(of the same size) do not weigh the same and when you go to a larger tire, the tire is also going to weigh more.

Simple physics regarding larger wheels/tires. The larger the outside diameter, the further you are from the center of the axle. Torque (what gets you going out of the gate) is Force times distance. Consider that you have a constant amount of torque availabe. If you increase the distance from the center of the axle to the road, then the force your tire exerts on the road MUST decrease. Also regarding the Toyo tires - at the race this past weekend (Mosport), the Toyo tires cost racers 2-3 seconds from last year (BFG's) because they are a harder compund and don't quite grip as well. On the other had, the racers liked the fact that they could race 3-4 times on these tires instead of the 1-1.5 times on the G Force rubber. They may be less sticky, but if everyone has to use the same tire, who cares, right?

Neither of these things should be an excuse for losing to an SE in 3rd, though.

A couple things to clear up.

225/40R18 = 637.2mm diameter or 2001.82mm circumfrence
205/55R16 = 631.9mm diameter or 1985.17mm around

0.8% error, hardly noticable. When plus sizing, you don't actually increase (ideally) the diameter (or circumfrence) of the tire. Larger wheels does not mean larger tire as well. Get this out of your heads! Usually, a larger wheel will weigh more and that is where inertia comes into play. Not the case for Silversvt.

Possibly a softer compound and more surface area could contribute, but the plugs are going to be the biggest difference that he will notice.

Thanks a lot guys, this is starting to make sense. Plugs & intake / TB cleaning are next on my list, and maybe a swap to my original wheels before the next race to see if I do differently.

Oh well, I know all 'tours weren't created equal, Is it possible that I have a weaker engine from the factory? Production tolerances don't seem to be Ford's strong point, as I remember seeing dyno plots of stock SVTs that show more power than modded ones.

emm... inertia increase ALSO when it's further away from the center of the circle. You have have more inertia even if you have less weight if it's further away from the center. Just think about how most of the weight (the rim) is on the outter most part of the wheel.