Dyno'd 20" Shelby Razors
 

Hi everyone --

I read a lot on this forum but seldom contribute -- don't feel smart enough I guess -- but maybe this post will be helpful.

I had my car tuned and dyno'd last october (on a superflow). Best run was 390 rwhp. At the time I was running the stock 17" wheels. I participated in the "dyno day's" at the same shop last Saturday and ran at 388 rwhp with my new 20" shelby razors (20x9 w/ 255/35 all four).

I read many posts regarding the pros/cons of 20" wheels and decided the look was worth it even if I did lose a little power. Granted not everyone will have the exact same result, but I'm happy with the look and it's worth the loss of only 2 rwhp.

Here's a picture with the new wheels and summit lowering springs (1.5" rear, 1" front).

Cheers!



[IMG]local://upfiles/64302/C3E00653E41442D3804B3C7CD26C8892.jpg[/IMG]

RE: Dyno'd 20" Shelby Razors
 

Rims look great! Don't ever feel too stupid to post here. I'm a IT - Networking Administration major, a complete nerd,who hardly knows anything mechanical, and yet I still post(much to the dismay of the lady of the house).

RE: Dyno'd 20" Shelby Razors
 

1+ boochy........ I have no idea how much HP I lost with my 20's but could care less....very happy with my dyno numbers and the rims;) Car looks great and the loss of HP is marginal and probably not noticeable at this point (unless you put the 17's back on;))

RE: Dyno'd 20" Shelby Razors
 

The car does not live in RWHP!

Rwhp is a measure of power, not the moment of inertia of the tires. The reason your HP went down is because you went with stiffer tires. The 20" wheels hurt your acceleration, NOT your RWHP. It's like when people install a Light Weight Drive Shaft. They gain 3 tenths of a second in the quarter mile, but the dyno only shows 1 HP and 3 toques.

Yet 3 tenths of a second is equivalent to 30-35 hp in gains!

Premium auto manufacturers have known this moment of inertia and rotational mass thing for a long time. The mustang's drive shaft weighs 65lbs for christs sake when it only needs to weigh 25 at only slightly higher price which can be seen by looking at other ford models or competitors. That's 1% of the car's weight in bloat alone on that part alone and this just shows you how bad things are so think about the other parts of the car. Anyway premium auto manufacturers can get so many gains from the rotational mass thing. The BMW 335i which can be best compared to the Saleen Supercharged, has the same 0-60 time as that car, with similar gearing, but the 335i produces about 340 hp vs 430 AND the BMW weighs 150 lbs more (worth 20 hp).

So with 23% less equivalent HP the BMW can get just as good speeds as the Saleen. That's the power of rotational mass.

RE: Dyno'd 20" Shelby Razors
 

great explanation Legion.... Thanks!

RE: Dyno'd 20" Shelby Razors
 

Why would you lose power (or acceleration as noted above)when going to 20's?

Assuming the OD of the tire is the same, the only thing I can think of is added weight (are 20's really that much heavier?). I have not been able to find the posts on the subject and would appreciate a short explanation. Thanks in advance.

RE: Dyno'd 20" Shelby Razors
 

I agree with you, but the stock drive shaft does not weigh 65lbs. It's closer to 45 and mine weighed 43 on my scale.

RE: Dyno'd 20" Shelby Razors
 

It depends on the rims, but any weight added to the wheels is going to make you slower. 5-10lbs per wheel is not uncommon. The stock 17's weigh 22lbs and 50lbs with stock tires. You never see rim manufacturers listing the weight of their wheels, unfortunately.

RE: Dyno'd 20" Shelby Razors
 

Moose,

Tire rack lists weights on some wheels. American Racing Shelby bullits for sure. 25lbs for the 18x9.

I agree that adding 20's should not hurt your rwhp. The difference you saw on the dyno could be for any of 100 different things, like air temp, dyno calibration, cool down time etc. So the change isnt significant enough to make any conlclusions, you would have to run the stockers back to back with the 20's on the same day.

The problem with 20's comes from 2 factors, the amount of sidewall, and the added rotational weight.

The first as was said is the lack of flex in the sidewall that makes them more prone to spin than hook, since the sidewall cant take as high of a load.

For every pound of rotational weight you add or lose is like 4 lbs of static weight. So if you add 25lbs of wheel and tires to the car, it is like having a 100lbs of cement in the car.

Now what does all this mean? Well it means 20" wheels arent as good for performance as 15" wheels. But so what if you like them and understand the tradeoffs, have fun.

And dont every worry about posting, it is the best way to learn.