ITS Wheels/Tires Question
 

This is for the ITS Racers out there.

On a 15x7 inch wheel, what is the maximum outward offset you can use, while staying within the ITS rules?

9.1.3.d.7 -

a. "any wheel/tire may be used within the following limitations..."

4. Tire tread .... "shall not protrude beyond the fender opening when viewed from the top perpendicular to the ground"....

Doesn't matter on the offset, so long as it's within the fender. I think it will also depend on the tire size and make, I assume a 225/50 but the width varies slightly from manufacturer. It would also depend on if they were shaved square or rounded if using something like a Toyo. There is a specific test for compliance with the above as it's stated in the GCR.

I'm pretty sure a 0 offset won't work like found on the Circle wheels that EP guys use. I think the "ideal" offset is around 25 for IT prepped cars (so I've heard). I just ordered some Team Dynamic wheels and even custom, they could only do a 30. I went with 35 as they were in stock and will use a 3/8" spacer on ARP studs.

hmm....i know there's other ITS guys on this board...+25? +30?

I'm thinking going even shorter and going 225/45 also.

You'll need about +25-27 to clear a 225 Hoosier. Most wheels are in the 35-37 range and guys use spacers. If you can find wheels with the correct offset so much the better. If you run spacers you'll need longer studs. You will want to run a 225/45. The 50 series tires are too tall and give up gearing you'll need.

Isn't rear end gearing and track a factor on tire size? I've ran out of gear with a 225/50 and 5.12 rear before. I would have run out of gear sooner with a 225/45. I could not imagine running a shorter tire, I think I'd actually prefer a slightly taller tire or a tad higher gear ratio.

well, everything's interdependent, how much were you topping out at with the 225/50 and the 5.12?

Is there anywhere other than Lowe's that the 5.12-225/45's won't be enough? They work at Daytona...

What rpm are you shifting at?

Texas World Speedway, 2.9 CW

Shift light set at 7500, peak HP around 7200, 5.12 rear, 225/50 Toyo. See it here...

http://www.youtube.com/watch?v=AdsMY4eSAzM

Generally speaking I tend to run a track in 3rd/4th though. I haven't run the full Texas Motor Speedway track yet, just the infield but imagine the same thing as above will happen there.

You have plenty of over-rev potential past peak power. There are a couple areas at different tracks where I'll see 9k in 3rd or 4th to avoid a quick up/down shift. If the engine is built correctly this is not a problem. The tracks in the midwest won't even come close to running you out of gear. I've heard of Lowe's and maybe Watkins Glen needing a taller gear. If that's the case there are lots of options for those tracks.

I'd say 7500 in 5th isn't running out of gear...at all. Even with power peaking at 7200, which is very typical, I would ultimately prefer to see 8000+ at the end of the longest straight. With proper tuning the power fall off is not that bad above peak and a shift point of around 8500 maximzes average power. If you're shifting at 7500 you're probably giving up a good amount of acceleration.

Hopefully the offset question got answered.... I'll add that I just put on some new 15x7 +35 wheels tonight with 225/50 Toyos. I used a 3/8" spacer and have less than a pinkie finger between the front spring seat and tire (there is maybe 1/4"). With the spacer and +35 that should equate to a +25.5 or so. I'm going to get and try a 1/2" spacer. In my case, a +25.5 was not enough to comfortably clear suspension.

Off topic and back to what Chris said, interesting information you wrote. I'm looking at my dyno right now, if peak is about 7200 -167rwhp and by the time I'm at 7800 RPM rwhp has dropped to 158 (opposite end at 6200 RPM I'm at 158 rwhp). So, based off what you said one of two things is going on - either I shift way too early at 7500 and should shift much later (based off the above where? my light is at 7500 but I ride it past sometimes for up to a couple seconds) or my power fall off is bad and the car is not tuned right.

On the offset question I ran a 225/50 Victoracer on a +35 wheel with no issues at all when I first started years ago. The same wheel with a 225 Hoosier rubbed. When they did rub it was on one side only (you'll have room at the rear, they'll only rub at the front) and it wasn't a real bad rub. I've seen a stock FC run 235/45 Michelins without rubbing on +35 wheels. What guys can stuff under the car seems to vary pretty widely.

What I ended up running for several years was a +38 Kosei with a 1/2" spacer (+25.3 offset equivalent). No issues at all with 225/45 Hoosiers and 225/50 V710s or Toyos.

Now on the shift point discussion, what you are trying to do with each shift is maximize average horsepower. Since the gear spacing gets tighter as you move up through the gears you will have a different optimum shift point for each gear. In practice it's easier to pic one point unless you have a gear dependent shift light.

Running some numbers from a dyno chart I have I came up with the following for the 3-4 shift. The car peaks @ 7400 with 170 whp. Not stellar but it'll work for discussion. If we shifted at 7500 (170 whp) we'll fall to 5500 in 4th (126 whp). We end up with 148 whp average. By extending the shift point to 8000 (165 whp) we fall to 6000 (135 whp) and we have 150 whp average. But at 8300 (156 whp) we fall to 6300 (147 whp) and have 151.5 whp average. This particular sheet only goes to 8300 so I can't accurately extrapolate on out. By taking a WAG and drawing the curve on out at the same shape I'd guess we'd see about 150 @ 8500 and we have 150 @ 6500 so we'd have 150 average and we'd hit the point of deminishing returns. Optimum shift point is thus in the 8300 RPM range.

This example also does not take into account rolling friction and air drag that will drop you lower in the rev range between each shift and exagerate the numbers even more. This is also a vastly simplified explanation of something that can involve a ton of math if you want it to. Carroll Smith's books have a good write up on the subject.

Since the gear spacing is tighter on the 4-5 upshift the shift point will be closer to peak power in theory. In practice rolling resistance (assuming downforce) and air drag is higher at the higher speeds so that would need to be accounted for as well.

Another way to look at it is like this.

At 7500 this example engine made 119 lb/ft of torque. In third gear (1.366) that is 162.6 lb/ft at the output shaft. If you were to shift at that point and drop to 5500 in 4th (121 lb/ft * 1.000 = 121 lb/ft) you have much less torque at the output shaft. By shifting at 8300 (98 lb/ft * 1.366 = 133.9 lb/ft) you still have more torque at the output shaft than you would of by shifting earlier and after the shift (6300 rpm 122 lb/ft * 1.000) you still have the same torque available as you would of shifting at 7500.

This example only works for this particular engine and every engine and gearbox configuration needs to have the calculations made to optimize it. However, since most ITS engine fall into a pretty narrow performance window you may make generalizations and say shifting at X rpm is best. Tires and rear gear don't play much of a factor (unless you're really into the nitty-gritty and want drag numbers) so they are pretty much a constant and left out of the arguement.

And SCCAITS, you need to talk to me about the ITS legal programmable ECU we're working on. I can make that past peak part of the power curve even sweeter for you. We've seen close to 18 whp gains in areas just past peak and solid 10 whp gains from 3000 to 6500 rpm over cars with fuel pressure optimized for peak power on a stock ECU.

Thanks for all the info on shift points, so much to learn. Generally speaking is the RPM drop calculated around 2000 rpm?

Using your information and my dyno sheet I came up with this....

Peak - 167hp @ 7200

Shift @ 7500 - 164hp, fall to 5500 - 144hp, avg 154hp
Shift @ 7800 - 157hp, fall to 5800 - 152hp, avg 154.5hp
Shift @ 8000 - ? hp, fall to 6000 - 154hp, avg ? (8000 RPM looks like it would be 154hp)

Some torque numbers - 5500/131lbs, peak 5700/132lbs, 7500-115lbs

According to this, the 3-4 shift should happen around 7800, not 7500, right?

I don't have data over 7800 so I'm limited. I assume your dyno numbers were off a 2nd Gen, what's interesting is my hp is significantly higher at the lower RPM's than in the example you used. The numbers above are an S4 motor.

I've read about the ECU you are working on, it's for an S5 motor and I have an S4 motor and car. I'm not changing my motor quite yet as it has less than 20 hours on it. I will probably look into doing an S5 motor next time when this current one no longer does it's job.

In my expereince the optimal shift point is when you drop right to the torque peak. This typically delivers the largest "area under the curve" in terms of available power.

-Trent

Ooh...ITS legal programmable ECU? I'd like to get in on that.

Just wondering how you guys are making such high power numbers without any induction modifications.

Drop me a PM and I'll get you hooked up.

I remeber when i dynoed my car just after i put the new exhaust on it.....my motor mad 178rwhp and 167ftlbs.....and i have a 5.12 rear end currently running 225/50 RA1s and i usually shift about 8400rpm just to keep the car in the power band range witch is on my car anything above 6000rpms. and the wheels i have on my car i dont need spacers the offset is perfect i just dont know what it is.

We actually used V6 mustang wheels on our car. They're 5x114.3, 24 offset and 15x7. They work perfect for our rain tires and they don't look terrible once you take the centre cap off them. And they don't need spacers.

The Volks that we have with a 30 offset need a small spacer or the inside wall of the tire rubs on one suspension bolt a little bit (but we run wide slicks -- not ITS legal car haha), and the Enkeis with 35 offset we need a spacer or they won't fit.

So to boot, 225/45/15 on a 15x7 +25 is the widest you can go to maintain legality?

are you sure these weren't the numbers from the 20b? 167 lb/ft out of an NA 13b with no porting???? naaaahhhhhh

im just saying what the dyno read but then again it was 100 degrees in the dyno bay so maybe that had something to do with it.......but i have been wanting to take it to another dyno and see wht it reads....

hmmm......threadjacking my own thread but on an unrelated note:

is it legal under ITS rules to remove your 5th/6th port actuators?

Yes. You do have a GCR, right?

Yes, you can remove the 5/6 port actuators and either a) wire them open, or b) remove the inserts completely from the block. You cannot modify the inserts (eg. Pineapple Racing).

IIRC, technically you are not allowed to modify the VDI valve, but most people seem to just go ahead and wire theirs open anyway.

I do have one, I just didn't have it on me and was hoping for a quick answer.