Loki

iTrader: (1)

Looks like pieces of cat that have been rolling around for a while. How does the cat itself look?
The speedo being 5-7% short on stock tires is news to me. The couple of speeding tickets I have are pretty spot on

Steve Dallas

Yeah, that looks like bits of broken cat. If you still have a cat, you definitely need to pull it down and inspect it. Put a small LED flashlight up its butt and look for signs of clogging. Clogged cats kill engines quickly, sometimes within a few days.


I am also surprised by the speedo being off on stock tires. Mine is spot on with several GPS devices. I just mentally account for the difference on my smaller diameter track tires.

Leo13

So if it is clogged am I in for a BHR pipe and $1k on a new factory cat? Different tires do vary in diameter and maybe my high quality Tigerpaws run small. I have been using the radar signs to check the speed but will down load a GPS app for my phone. I want to say all, but will say most manufactures have a minus tolerance on the calibration of their speedo's. I have never seen a one on the plus side. It increases the calculated MPG, is safer and gets a vehicle out of warranty quicker.

I'm not sure if a 235-45-17 on a 9" wheel is better then a 225-45-18 on a 8.5 wheel. Maybe the lower profile 18 is better. Also what is the general opinion about the effect of larger tires on reducing acceleration? How big is to big on a NA engine? I guess I could go for a 255-45-17 on a 9.5" wheel or a 245-40-18 on a 9" wheel.

Loki

iTrader: (1)

The midpipe is the cat, so either a stock replacement OR a BHR midpipe (with or without the cat they supply, you can choose at that point).

If you're planning to track the car, go for the 17x9 wheel. It's the better choice in weight, cost and tire cost and sidewall amount. If you're planning for acceleration, you have the wrong car. This one is about momentum, so get all the rubber you can get on there. 255-45-17 is not at all crazy, provided correct wheel offset. And they're exactly same diameter as stock to boot.

I have to say, all these points will determine the last tenth of a second in your laptimes. Your choice of rubber and seat time will determine the minutes and seconds.

Leo13

After close inspection it is cat material. My thought was I need to smog it every two years so I need a cat but I dont want to run one if it has such a short service life.

I know the 8 is not the fastest but I don’t want to add insult to injury. Although one thing I have noticed with the rotary is that they don’t produce high torque numbers but they make a relative high level over a broad range. The intergral of the torque curve is good.

Steve Dallas

17x9 wheel with either 235 or 245 tire. 255 just adds weight and cost and requires higher tire pressures to keep it from squirming. I actually run a 225 on a 15x9 on my Miata.



If you track the car, you probably need a catless midpipe like BHR. Put the stock cat back on temporarily for smog.

Leo13

Thank you for all of the replies, especially Loki & Steve.

I down loaded a gps speed app for my phone and it showed 58 at 60 which is 3%.