winsome

Hi,

Can someone give me some idea as to the best ratio to regear a 03 gearbox?

Many Thanks,

Winsome

Geoffrey

Best ratios for what? What purpose and what budget?

winsome

I am looking for more aggressive acceleration and focus more on track use. Top speed around 230-240 will be sufficient.

Thanks,

Winsome
94 993 w/ G50-03 gearbox

Geoffrey

Ok, so here is how I would approach it given an undefined budget. First I would determine the top speed I wanted to reach in 5th gear at redline - in this case, assuming a stock redline, that makes 240kph (150mph) @ 6750 and assuming a 25.5" diameter tire. So, with a stock 3.444 R&P, this means a gear ratio in the 1:1 range. Now, first gear is part of the main shaft, so unless you want to spend $1500 on a new main shaft, you'll just use the stock first gear ratio of 3.5:1. Now, you need to pick a 2,3,4th gear that works within the spread you have selected.

Obviously, this is a very simplified way of selecting gears and you'd probably want to consider the torque/HP curves in selecting the gear ratios. Here is what your gear chart might look like.

winsome

thanks geoffrey. actually regearing is one of two options i am thinking of. The other one is to go the supercharger way, then i wil leave the gearbox alone. understand you have some experience of the sc kit, will you recommend it for track use also?

thanks

Winsome

Geoffrey

I hesitate to post my opinion on superchargers since I don't want to touch off an argument...but here it goes.

I have found that the installation of a supercharger creates issues with elevated intake air temperatures that must be dealt with. Now, this is nothing new, every supercharger appliction does this. However, with the Porsche being air cooled already has a stressful time properly managing engine temperature and I'm not just talking about engine oil here, but the whole package. The supercharger becomes a large heat sink right on top of the engine which is not good. With the MoTeC setup, compression reduced to 9.0:1, 6lbs of boost, and an intercooler, I could not get manage the air intake temps below 160 degrees F in a track environment which is too high.

The TPC kit seems to work, however the design they use to make it work is a compromize. It uses a 7th injector which sprays fuel into the manifold to cool the air charge, and manifold down. They also run the air fuel ratios down into the 10.8:1 range which is VERY rich, and include a box that reduces the igntion timing as well. I know that Automobile Associates has a number of these kits on 964 and 993 engines with 3+ years of track time and no issues, so the design works. Since the Motronic system does not do data logging, I have no data to determine what the various temps are in this configuration. An infrared gun shows the manifold at 130 degrees F.

I have read posts on Rennlist of cars with superchargers pinging from time-to-time as well as hammer codes showing knocking. The 911 engine is fragile and more prone to detonation damage than many other engines. I know for a fact that you can break rings in a matter of 7-10 seconds of mild detonation, so it doesn't take much.

If you asked me which I would do, I would choose another method other than supercharging in a track environment. For the street, I might consider it. Would I buy a car that already was supercharged, I doubt it.

winsome

Thanks again Geoffrey. Glad to hear from someone with technical background like yours. As you understand the temperture in HK can get pretty high 34c and high humidity can get to 90% so thats not helping either. I was hoping the intercooler from tpc may be able to put the temp under control? now that doesn't seems to be the case.....

May be a close ratio box will be the only option.......but 400hp can be soooooo tempting!

Winsome