I've had this argument with all the bench racers already. Nobody understands the power of a properly setup turbo 20b. The purpose of the huge rear tires is so they are not spinning going down the straightaway. I've had to sacrifice all out corner balance to just being able to use the power. When I had my 13b, I had a square tire setup, its just not possible with the 20b. I would have to put equally huge tires on the front which is not only difficult but would be retarded. My first 2 track days with the 20b were with 295's all around. The car was impossible to drive fast, at least safely. It was literally spinning down the entire straightaway in 3rd gear and would only catch grip when it was shifted into 4th gear. And this is with Hoosier slicks so its just not possible to run small tires. Basically, it comes down to whats the point of having the power, if you can't use it. I'd actually like to be able to run a 305 tire in the front but I don't think I can fit it. I might try but budget is a limiting factor.

Unburnt gas, was pretty cold that day so exhaust probably wasn't getting hot enough to light it. But I've noticed my car is sadly not very flamy, I might have to add some lag to the fuel cut to give it a little more of that rotary signature.

thewird

 

Have you thought about running 17's in the rear and lowering the tire pressure? Run 18's up front.

the wider the tire, the worse traction you will get under acceleration. you want a low pressured smaller diameter wheel if you cannot grip.

The wider the tire the better for traction under cornering, but with small sidewalls, you are going to have less braking/acceleration traction.

The engineers at michelin had some good write ups about the testing they did on the porsche cup cars. They were of the sort to fit the tires with a stretch 11" 295/30/18 front would be good, and with a 12" rear wheel throw in a 315/35/17 tire (taller tire as well)?

Obviously you have been testing this more than I have on your car, just throwing out ideas.

general rules

1” of extra wheel width is worth ~ 1 sec per min of track time (front wheel)

A +1” in wheel width provides a similar improvement to moving up in tire classes, like an Extreme tire to R comps or R comps to Slicks.

Many people believe that wider tires improve grip. However, this is not true. You need to fit your tires to your wheels so they have a slight stretch. Fitting the widest tires possible on specific wheel sizes will deaden response and cause the car to roll due to unsupported sidewalls. Additionally this will decrease the performance and function of your dampers as you introduce a major damping mechanism into your system. Extra tire width is not used as the tire is pinched; creating a muffin top close to the wheel lip that is not in consistent contact with the ground. It’s important to note that none of the racing series above use tires that are wider than the wheels. Every team has their tire widths matched to the wheel width. On TV you will noticed a slight stretch of the tire as the wheel is slightly wider than the tire.

Just trying to get the information out there.

 

I'm currently running 17" rims and have tried lower tire pressures. The lower pressures makes the car feel boaty and unstable so while it seems to bite better, its slower overall. I run 33-34 PSI HOT on the rear and like 34-35 psi on the front. This is pretty much what Hoosier recommends for my weight and it is what works better. I have been playing with it all season as I'm trying to get the most out of my setup possible.

I know stretched tires work better. That is why I went from a 12" wide rim to a 14" wide rim with the 335's. I noticed I wasn't using the inside of the tire and a bit of the outside as well so I stretched them. I didn't notice improved grip and stability by stretching it for sure.

I am however moving to an 18" rim next year so I can move to radial slicks which have a better compound and they only have the sizes I want in 18". They also have the added benefit of having the 355 size. If there was a 335 size, I would probably run that to start as the compound is stickier but the only compound available in the right height would be a 320 width. Initially, I was planning to try the 320 on the rear but I know how I am and would end up at the 355 eventually so I'm just going to do it from the getgo so I can setup my fenders for it. I'm hoping I can fit a 305 tire on the front but I'm not sure.

Here's the difference in tires with the stretch...

12"



14"



thewird

 

I read this and thought of your car: SRT Viper TA edition specs and release date | evo

 

Maybe Ronin Speedworks fenders might work well for you. They look relatively stock and can stuff 315s under them.

FD Wide Front Fenders

Fantastic car!

 

Just looking at this pic shows me you are not using the wide tire to its full potential. Probably because you do not have the car balanced out with a wide front tire. With a wider front tire you will be able to carry more speed through the turns and brake later. Listen to what lOOkatme is telling you. You added font weight and are compenstating for it by making the rear tire wider you are going in the wrong direction. You want to be able to go faster around turns rather than drag race down the straights.

 

Marco, have you corner weighted the car yet? I have worked on getting a 5.4L Cobra R balanced and it is critically important that you undeerstand what the corner weights are telling you. The Cobra R had almost 650 hp and well over 450 lb ft of torque. It also was 54% FWBias and 46% RWBias. We had to stop for a second and see what the tires were telling us.

First thing to do is talk with the tire engineers. In almost all cases the rim should be the same width as the tread or contact surface of the tire. In some cases like the Michelins for the front of a Porsche GT3 Cup car the tire contact surface is narrower than the rim slightly as they are limited by rules to a certain tire size and they are looking to instantize the sidewall response to minimize understeer (gross shortening as to what is happening). In your case Hoosier will be able to give you a range of effective rim widths for a tire and the recommended rim width. I always go for the recommended rim width as it allows for some sidewall flex to allow the tire contact surface to remain in contact with the track surface.

Regarding width's, when you are not constrained by rules you need to keep adding tire width until you get to a point where your tire temps are not exceeding the maximum tire temps as recommended by the manufacturer. For example, a Toyo RA1 has a recommended tire temp range of between 180 and 220 degrees F. If your tire temps are below that number, you need to either drop tire pressures until you get into that range, but that may make the car unresponsive to steering inputs and give it a boaty feeling or you can drive the car harder to get more mechanical action on the compound so that tire temps go up. The third way is to increase spring rate or swaybar rate to increase momentary loadings on the tire to allow temps to grow. If the tire temps are too high then you again have 3 choices, increase tire pressures slightly within manufacturers recommendations, slow down a bit to reduce loadings or soften spring and bar rates. The extra option you have is to either change compound or tire sizes to accomplish the same goals.

You have to remember that the contact patch between the tire and the ground changes with width. The contact patch varies due to a pounds per square inch loading factor. You may think that adding more tire would increase grip, but if the tire is too big, the contact patch can actually not grow in a linear fashion to the wider tire as the weight per square inch is diminished when there is more surface area.

Therefore, if you will listen to me at all, I would recommend that before you spend a ton of coin, corner weight your car as it sits right now to determine what the weights are. try to rempove as much weight as possible to the rear of the car and also consider a driver seat setback along with a steering column extension, pedal and shifter setback. Secondly set the tire pressures in the manufacturers safe zone and then lift the tire off the ground and wet the bottom of it then drop it on a clean piece of cardboard. This will show you the contact patch. Then play a bit with pressures until you get the largest contact patch for your existing tire size. Then go out and drive the car hard for about 5 laps until you reach max tire temps. Ignore how the car feels at this point. After the session pull the car into the pits and check tire temps through the 3 zones of the tires. If everything is okay with the temps according to the manufacturers recommendations then you are good to go. If they are too high, then you need to look at changing compounds to a harder compound or increasing the size of the tire.

Take a look at this video that shows all the work that was done to the Cobra R to get the weight shifted backwards. It also shows the tire sizes and wheel sizes. This car is now fast and doesn't chew up tires in 3 hours, versus when we started we would get barely an hour off a set.

Eric

 

Dave from Mazdee's has been helping me setup my car every single event. The car has had some sort of suspension change before every event and has been corner balanced every time as well. I inspect the tires after every event and have been making adjustments. I show Dave the tires from time to time and both him and I agree they are wearing perfect now. The only thing they are telling me now is to add even more camber. I'm gonna have to modify that front left adjuster to get more then 4 degrees of camber and the left rear might have to be modified too. My car is setup to work on Ontario tracks so I have staggered camber to turn right.

When selecting the second rim size for the 335's I asked Hoosier what size to run and that is what they told me would be the ideal size assuming no fitment or regulation constraints so that is exactly what I did. I've been playing with tire pressures all season, I know what works for my weight and what doesn't and continue to experiment. I actually had to go up in tire pressure since I went 20b.

I need to mention though, I'm not having trouble in the corners. The last 2 events I got the car behaving proper and am able to comfortably steer the car with the throttle again. The reason I'm planning to go wider tire is because next year the car will have possibly 800+ rwhp with maybe 550-600 foot pounds of torque... Going by past experience, I'm gonna need more rear tire or having 800 rwhp is waste of time as it will just be spinning tires down the straightway. My initial plan was actually to go down in tire size to a 320 with the R80 compounds but since I'm adding power, I have to change that up.

Here's a picture of my final corner weight of the season. I took a picture of it because I spent 2 hours trying to get a perfect 50.00%. I know its not needed but I wanted to achieve it at least once and had finished everything else I needed to do on the car for that weekend so kept at it.



thewird

 

Perfect!

 

Hmm, I've met 2 separate RX-8 owners trying to put 20b's in their car in the past week. Next year might get interesting. Looking forward to see what they can come up with.

thewird

 

nice. Hope they are finished for next year.

 

My son absolutely loves your car we saw it at Cayuga.
Since then he can't stop talking about it.

Fabulous Build Marco, you keep this canadian forum alive..

I would like to build a track car but life always seem to get in the way.

 

My son is 4 years old and he remembers Marco and the car from the summer. LOL. He saw a picture of it in my DGRR album and goes daddy thats Marco's car. Not even sure how he even remembers Marco lol..

 

God damn! That thing just gets up and goes now!! Passed the Z like it was standing still!

Good to see the tranny situation got solved.

Going to a force to be wrecken with in the new season

 

Rear fenders, yes or no?





thewird

 

Rocket Bunny? Didn't think I would see one here so soon.

 

Yes for rear fenders, don't want to be flinging rocks at people behind you, lol.

 

I like the subtle fender flares better, like Shine's Spec-B:



probably won't be nearly enough fender though.

Don't know if you posted it, but what are you using for boost control?

 

I actually like those and it would match well with my 20mm wide front fenders but I want to run 355 tires so that definitely won't fit.

thewird

 

yes for rocket bunny!!!!!!!!!!!!!!!!!!!!

 

Since you mentioned traction issues, are you controlling boost by gear? If not you should invest in it, especially since you plan on increasing the power even more.

 

I've been running off the spring all season at 13-14 psi. I do have a mac valve wired into the Haltech but I haven't bothered hooking it up since I had more then enough power lol. That was the plan at the beginning of the season but the minimum boost is already too much. Next year it will be hooked up.

thewird

 

Yeah, that's what you need. Unless something exists that is wider...but I can't imagine that.
Go for it.

 

Don't we all love Canada?



thewird

 

I was in toronto last night. Lots of snow. Drivers out there were driving 20km everywhere. Why so slow
My 10th AE on saw snow once. I rust checked it and left it outside for one winter when I first bought it. My 89 saw snow when we drove back from DGRR.

Drove my 86 NA all year round. So fun in the winter.