straight cuts or helical

[spondonturbo]

So do you really need straight cut gears on an aircooled turbo motor that is mainly for street use with some occasional RWYB drag meets? They are a lot of money and someone who does a lot of turbo stuff recently said they were not necessary unless properly drag racing.

Opinions?

[dupersunc]

Striaght cut gears are more efficient, but not necessarily stronger, in fact the opposite can be  true. Helical gears are quieter an make a bike feel more refined.

 

[Gixer1460]

51 minutes ago, dupersunc said:

Striaght cut gears are more efficient, but not necessarily stronger, in fact the opposite can be  true. Helical gears are quieter an make a bike feel more refined.

 

The above is true but there is a trade off - angle cut teeth by design introduce side loading to anything connected and in our case its the clutch and input shaft bearing - evidence of issues occuring in excessively torquey motors - NA big bore included , so not just a turbo problem.

[Arttu]

Yep, the issue with helical gears is side load for the crank and clutch. Not strength of the gears itself.

I haven't any clear answer either when straight cut gears start to be necessary. I can just tell that I run my old turbo engine for quite long time on street with helical gears, about 200hp, no issues. And I have seen also more powerful engines with helical gears but I don't know much about their longevity. So for about 200hp street engine I probably wouldn't bother to invest for straight cut gears. But if you are aiming for closer to 300hp or above then I would consider them seriously.

[spondonturbo]

I do normally use them but was told recently that even some motors up at 250 - 300 hp run helical gears. I agree that it is the side load that is the issue. Another idea suggested to me was helicals but strengthen the back of the clutch.

With regard to hp I would hope to see 250 maybe on an old aircooled motor with turbo.

I ask because I am at the point of needing to get the crank done so need to decide whether to spend £400 plus on straight cut gears. Decisions, decisions.....

[johnr]

pip higham once told me that gs750 motors run straight cut primary gears and they are good for about 190bhp. ive a crank with a straight primary gear but no clutch gear to match it if you need one chris.

 

[Arttu]

6 hours ago, spondonturbo said:

 Another idea suggested to me was helicals but strengthen the back of the clutch.

You should do that no matter what gears you use. I wouldn't trust stock back plate on any seriously tuned engine.

[spondonturbo]

11 hours ago, johnr said:

pip higham once told me that gs750 motors run straight cut primary gears and they are good for about 190bhp. ive a crank with a straight primary gear but no clutch gear to match it if you need one chris.

 

what is your crank out of John?

[Reinhoud]

16 hours ago, Arttu said:

Yep, the issue with helical gears is side load for the crank and clutch. Not strength of the gears itself.

I haven't any clear answer either when straight cut gears start to be necessary. I can just tell that I run my old turbo engine for quite long time on street with helical gears, about 200hp, no issues. And I have seen also more powerful engines with helical gears but I don't know much about their longevity. So for about 200hp street engine I probably wouldn't bother to invest for straight cut gears. But if you are aiming for closer to 300hp or above then I would consider them seriously.

I had about the same, no probs around 200hp.  

 

7 hours ago, Arttu said:

You should do that no matter what gears you use. I wouldn't trust stock back plate on any seriously tuned engine.

I also didn't had any problems with this, but I had some really strong rivets in it, and that welded to the back plate, so not sure if this counts as stock back plate.

[spondonturbo]

think I will play it safe as the crank will have to be stripped and welded anyway so may as well get the gear done

 

[Duckndive]

the 750 straight cuts are a different ratio and spin the clutch slower and are preferred by some of the race boys and girls ....

 

[johnr]

but is it correct that as the oil pump is spun by the clutch, that will also provide a lower flow rate of oil, which is why the 750 oil pump had different gears, so going down thee 750 primary drive route will require the 750 pump gears to just give the same flow rate as the stock 1100 gears? i think..... this is very confusing for my simple mind....

 

Edited February 11, 2018 by johnr

[spondonturbo]

Never knew that about 750 gears. I use after market straight cuts. Oil pump gears will be getting changed for 750 gears for sure.

[Arttu]

It's worth of noting that also aftermarket gears are available at 750 and 1100 ratios. I learned this hard way when I bought "just straight cut gears" and later on found out that they were with 750 ratio. Luckily I managed to trade them to 1100 ratio ones.

Generally I would advice to stick with 1100 ratio unless you really know what you are doing. There are few disadvantages related to 750 ratio. Like John mentioned the oil pump will spin slower. The clutch gear is slightly too big for 1100 engine cases and some grinding is required to make it fit. Higher ratio also means that you need to use quite extreme final drive sprocket sizes to get total drive ratio sensible. And then at least in theory higher drive ratio makes clutch life much harder. More torque on clutch for the same crank/wheel torque and less rpm for lock-up mechanism.