Straight cut or dog box
Discussion
I am building a new engine /gearbox over the winter for a third year of sprinting.
What is the informed opinion of the best gearbox to use - straight cut synchro or the ultimate , but very expensive, dog engagement gearbox? Engine will be around 125 bhp.Appreciate your views, gents.
Edited by rww on Thursday 16th November 13:24
id say its horses for courses.
a dog box is stronger and offers a greater choice of ratios, but rebuilds will be expensive due to the sacraficic way the gears engage. there is also a level of skill required, harsh use will destroy the dogs or in worse case the whole kit very quickly
finding a consistant manufacturer who also has spares back up may be difficult now a days.
a normal s/c box is far cheaper, but the ratio choices are limited, rebuilds are far cheaper and the box is more user friendly.
a dog box is stronger and offers a greater choice of ratios, but rebuilds will be expensive due to the sacraficic way the gears engage. there is also a level of skill required, harsh use will destroy the dogs or in worse case the whole kit very quickly
finding a consistant manufacturer who also has spares back up may be difficult now a days.
a normal s/c box is far cheaper, but the ratio choices are limited, rebuilds are far cheaper and the box is more user friendly.
I gave this some thought and it's down to this.
I'd like a 5 speed box with a 1:1 5th, Kad do it in dog engagement only. It is expensive.
SCCR normal boxes are about as good as you can get for a 4 speeder. 4 speed dogboxes have gears as close as a challenge set 2.19 first IIRC and get closer. Designed more for rolling starts, etc. Not sure what final drive you're pulling but if you fancy toasting a clutch when you choose a 3.44 FD for your turbo mini, everytime you pull off. So, do some calculations, what do you want your 1st gear to be like? Or what do you want your top speed to be like 70mph at 5500 rpm?
So, SCCR box, easy for your mum to drive, good ratios 2.54 first and a 3.44 FD on 10" wheels.
4 speed dog, best you get are challenge ratios, better with 3.7 or 3.9 FD, starts needing revs to go really fast. Very close gears favour very narrow power band, you have one of those?
5 speed dog, the best box but for the money, you could build a full steel turbo engine and make do with a 4 speed box and about 70hp more!
Dog gears demand careful use, they are hugely strong but wear if engaged badly, you have to do lightening fast changes and double clutch the down changes which can be done fast or risk wearing gears.
The quaife kit 4 speed dog looks strong. I'd avoid secondhand ones as most people will just jam them to change gear!!!!
What's your spec?
I'd like a 5 speed box with a 1:1 5th, Kad do it in dog engagement only. It is expensive.
SCCR normal boxes are about as good as you can get for a 4 speeder. 4 speed dogboxes have gears as close as a challenge set 2.19 first IIRC and get closer. Designed more for rolling starts, etc. Not sure what final drive you're pulling but if you fancy toasting a clutch when you choose a 3.44 FD for your turbo mini, everytime you pull off. So, do some calculations, what do you want your 1st gear to be like? Or what do you want your top speed to be like 70mph at 5500 rpm?
So, SCCR box, easy for your mum to drive, good ratios 2.54 first and a 3.44 FD on 10" wheels.
4 speed dog, best you get are challenge ratios, better with 3.7 or 3.9 FD, starts needing revs to go really fast. Very close gears favour very narrow power band, you have one of those?
5 speed dog, the best box but for the money, you could build a full steel turbo engine and make do with a 4 speed box and about 70hp more!
Dog gears demand careful use, they are hugely strong but wear if engaged badly, you have to do lightening fast changes and double clutch the down changes which can be done fast or risk wearing gears.
The quaife kit 4 speed dog looks strong. I'd avoid secondhand ones as most people will just jam them to change gear!!!!
What's your spec?
Thanks gents. All very useful. Spec. of engine will be 649(or similiar) cam , steel flywheel/backplate, forged 73.5mm pistons, 45 Weber, and the usual race inlet/exhaust manifolds probably on a 3.9 or 4.1 final drive ratio.
Current spec for last two seasons has been 1.5 ratio rockers, 286 cam,MG Metro head with limited mods by Swiftune, std 1275 Metro crank, rods , pistons, lightened flywheel/backplate on std drop gears and gearbox and 4.1 FD.With 45 Weber this produced around 105bhp at Gerald Dale's RR and revved to 8000 rpm (chrono r/c) with no driven waterpump (electric) or alternator.The flexibility of the cam allowed us to virtually use only 3rd and 4th gear at most circuits. Pulled just over 7000rpm in 4th at Goodwood Lavant Str. (with a slight misfire) which, I think, is about 95mph.
Any contributions/opinions appreciated.
Current spec for last two seasons has been 1.5 ratio rockers, 286 cam,MG Metro head with limited mods by Swiftune, std 1275 Metro crank, rods , pistons, lightened flywheel/backplate on std drop gears and gearbox and 4.1 FD.With 45 Weber this produced around 105bhp at Gerald Dale's RR and revved to 8000 rpm (chrono r/c) with no driven waterpump (electric) or alternator.The flexibility of the cam allowed us to virtually use only 3rd and 4th gear at most circuits. Pulled just over 7000rpm in 4th at Goodwood Lavant Str. (with a slight misfire) which, I think, is about 95mph.
Any contributions/opinions appreciated.
Edited by rww on Thursday 16th November 15:55
A dog box is much stronger than the 4 speed SCCR syncro box and a should last a lot longer in a race environment. I ran a SCCR box this season and it was perfect. I was told the other day by a supplier that with a slight lift clutchless gear changes are fine and some drivers are flat shifting. The only downside as far as I can see is the cost around £350 for SCCR and £1150 for the dog, but that might even out if the sycro box is in need of constant maintenance. Oh the temptation! Richard maybe we could get a discount if we buy two!
If you are dead set on a Dog box take a look at the Kit that Swiftune are distributing/marketing. This has been proven in 2006 season by loads of the Miglia frontrunners and they are saying that they are bomb proof (Dogs show little wear after a seasons hard racing) and that the shift quality is amazing.
Part availability shouldn't be an issue as it is a new design.
Flat shifting seems brutal. I've used my Jack Knight involute dog boxes (I use the clutch!) for nearly 5 years now but bits are getting scarce and expensive.
Part availability shouldn't be an issue as it is a new design.
Flat shifting seems brutal. I've used my Jack Knight involute dog boxes (I use the clutch!) for nearly 5 years now but bits are getting scarce and expensive.
Involute dogs, especially the ones with lots of them are more prone to damage. The Qualife has 4 dogs per cog and the swift tune has about 7 IIRC, the quaifes are flat faced and like jellybeans whereas the S'T are trapezoidal, and the wrong way up so as soon as a gear is engaged, it guides itself in. As they wear, they become more like the quaife profile.
If you consider the weight of the transfer gears and the clutch plate, it is a good idea to double shift. No matter what steel they are made out of, the dogs won't like it!
If you consider the weight of the transfer gears and the clutch plate, it is a good idea to double shift. No matter what steel they are made out of, the dogs won't like it!
The 4 kidney shape dogs on the Glebe/Quaife set up are not as robust in engineering design terms (seem prone to accelraeted wear) IMHO as the Trapezoidal shape on JK and Swiftune products.
First sign of Dog wear and problems on the Quaife type set up is the box/car beginning to jump out of 2nd gear. Soon after that usually comes an expensive bill.....
First sign of Dog wear and problems on the Quaife type set up is the box/car beginning to jump out of 2nd gear. Soon after that usually comes an expensive bill.....
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