Straight cut have more surface area per tooth of contact. Straight cut gears also have more meat in the base area of the tooth. Helical may have up to 3 teeth engaged at the same time, they are smaller teeth with shallower base area. The helical cut also adds side load forces on the gear box which adds heat and soaks up some power. I am sure Matt will have a way more detailed answer but this what I was told and was good enough to sell me on using straight vrs helical, wait there is also different pitches to helical cut too...

 

The sound of straight cut gears are like the smell of napalm in the morning. You either love it or hate it.

 

Good morning 6speeders...

I've been around this business for the better part of a decade now, and I'm going to be honest with you... Porsche people are funny. What I mean is that Porsche owners have a certain OEM is best mentality. They tend to think what the engineers at Porsche did has to be the best possible thing in the world. I couldn't for the life of me sell a straight cut synchronized gearset until about 5 years ago when the sequential cup car gearbox came out. That was the first time in Porsche's history that the factory used straight cut gears. Otherwise, for the previous 40 years of 911 existence, all factory racecars had helical gears. However, this was not for engineering reasons. It was for homologation reasons. Porsche put helical gears in their racecars because every racecar they've built since the 917 was discontinued has been a production based GT class car that required that the racecar run the same gears as the street variant that was used to homologate it.

Meanwhile in rally and other forms of motorsports, Subarus and Mitsubishis and all manner of other racecars outside of GT classes where synchronization was required, were running straight cut gears. They are stronger.

The load on the helical gear tooth moves from one side of the tooth to the other as it engages. Picture the phone book analogy, where it's impossible to tear a phone book in half from the bound side, but from the other side it can be done if bending the phone book and beginning with just a few pages.


Factor in that any bore clearance on the helical gears idler will clock the idler ever so slightly (out of perfect mesh) during moments of extreme loading. This sets up a repetitive tearing motion from one side of the tooth to the other.


The straight-cut gear on the other hand, engages evenly across the width of the entire tooth as it engages, so the load is spread out. Bore clearance has no effect on this equal engagement across the entire width of the tooth.


When a tooth breaks off a helical gear, self destruction usually occurs. When a tooth breaks off a straight-cut gear, that gear amazingly enough will often still finish the race. This is exactly what happened this year at Daytona. The #24 Alex Job Racing/McKenna Porsche owned by Bob Faieta lost a single tooth on 2nd gear due to a bad downshift sometime around dawn. The car just kept racing and took 14th in class. With a helical gear that would have never been possible.

I have attached two pictures. One is a GT3/Turbo gear. The other is a 915 2nd gear that was part of a special run we just did. It's in a factory SC-RS that's running the hill climb at Goodwood this weekend. The vintage rally guys in Europe have really started to warm up to the idea of straight cut synchro gears in the old 915s. We've been doing a lot of those by special order this year. For the GT3/Turbo gears, we generally keep the more popular ratios for those in stock.

 

Great input guys, im leaning towards straight cut gears! I'll keep you guys posted.

 

Matt, don't you guys also make a set of helical gears with a lower toothcount (i.e. larger, stronger teeth)? Something in-between a stock gear and a straight-cut race gear, regarding strength and noise levels?

I thought I read a post from you about that, once.