gear ratio 1.5 vs 1.3
 

I have read mixed reviews on gear ratios. which would be better a 1.5 ratio spinning a 34 prop or a 1.3 ratio spinning a 30? with the slip factor the same prop calculator shows them being within 2 mph. 6000rpm 1.5 34 pitch .085slip 117.8 vs 6000 1.3 30 pitch .085 120mph. (8.5% slip is what I have right now with 30pitch and 1.5 gears) is a lower pitch more efficient30 to 34? AND is the 1.5 harder on the drive than running a 1.3ratio with a smaller prop? I have heard thru the grape vine that a 1.5 with a big prop (34) is harder on the drive and more likely to break the drive shafts and or gears. any thoughts? btw big power (1100hp+)
and I know that any bravo style drive is very likely to break at some point with that hp, just wondering if one is a better choice than the other.

i like the 1.5 ,,it will get you from point a to point b quicker but 1100 hp.... yes the 1.3 would be stronger..you will also get more slip numbers with a 1.5,,brakes the prop loose easier..

What drive has a 1.3??? You are about of prop on Bravo style drives at 34" and I would swap to a 1.35 for more options.

With the 1.36 gear having less teeth then a 1.50 there is more strength. You got to find 1999-2000 desiel gears. These were the strongest gears produced. With 1100hp i would definitly choose the 1.36, after all with that kind of power your not going to b e able to take advantage of the torque a 1.50 gear offers unless you get sixs or arnesons.

A bmax does 1.3 and 1.5
Imco makes 1.35 and 1.5

Im also in the same situation accept only 900hp but currently running a zx w/1.5 & a 34p @ 6000rpm with about 12% slip. Was told that Im not efficient with this set-up and need to go to a 1.36 ratio. Can someone explain to me about efficient ??

the larger pitch props have a lot more slip then the smaller pitch props such as 28's or 29's. Back in the day before they came out with xr drives all the race boats ran bravo 1's with a 1.36 gear ratio. Having the power to turn a 34 pitch props puts a great amount of stress on the drive. Puts a huge amount torque threw the drive. Spinning a 1.36 gear allows to to spin a smaller wheel faster with less stress then spinning a large wheel slower. Also with a 1.36 gear you will notice a big difference in idle speed due to the fact your blade is spinning faster

I run a 1.5 on my boat a 26 DCB Mach with a 32 bravo 1 and I can tell you it is a dog out of the hole, slips big time, tach will go to 35-3800 getting on plane and I need a 1:35 ratio also or a better prop. But in my mind weather you put a 34 on a 1.5 or a 32 on a 1:35 the stress on the upper would still be the same correct. Now if you went down to a 30 or smaller pitch on the 1:35 then I would say you are reducing the stress on the upper verses the 34 on a 1:5.

when running big hp in front of bravos its a good idea to try to do everything you can to keep it alive, every little bit helps. With more material on the 1.36 gear compared to the 1.50 along with the ability to spin a smaller wheel faster can only increase your chances of more boating hours.

I went from 1.5 to 1.36 last year. I was limited to 5600 rpm from cam and heads, was running out of prop. The 30P had poor characteristics, and the 32P brought the RPM down but wasn't any faster. Only way for more speed was to turn RPM higher which I couldn't, or gear down and re-prop.

Now I run a 1.36 and a 28, fastest combo yet. My boat didn't seem to like the bigger pitched props.

28' Nordic, 540/Vortech

Isn't the ratio changed in the lower? It's not the lowers that usually go.

when running big hp in front of a bravo it,s a good idea to have some spare bravos.the standard bravo drive will be short lived with big power no matter what you do!

Old school upper gears changed the ratio, since the XR, only one ratio for the upper, so you change the lower gears to get the ratio when running the breakable 16/19 XR upper gears.
The 1.36 upper gear set has shorter teeth on the driven gears.
I generally use the 1.5 upper set and use the 1.35 lower set for the XR to get the ratio. I have been told, but not tried it, a 1.65 upper gear set combined with a 1.25 lower set gives 1.3812 ratio. Giving you bigger thicker gear teeth on both sets. Or the 1.65 upper with the 1.35 lower gives you 1.47.
So many things to try out.. :) Did everyone follow that?

Dick

Seems like the best bet is to go with the 1.3bmax or 1.35 with imco for higher hp. I did some searching around and read that generally a 28 bravo one prop is about the most efficient. Props smaller and especially bigger start losing efficiency. Looks like I will be going up in gear ratio to 1.3/1.35 instead of going huge prop. Wish I could find more info on the bmax lower case design especially for a small cat. I'm sure it's about the same with larger vbottoms but wonder about its efficiency pushing a 24ft htm cat. I would love to see a couple of pics (bmax lower) with it sitting next to an imco lower. I really like the bmax but looking at pics of its lower it just looks more blunt shaped. In my opinion! And is the lower case stronger than an imco? Think I would like to keep my imco lower and just change the shafts/gears in lower to work with the bmax upper. what do you think dick?

I have a friend that had an HTM, 24' I think and he went 120mph in it a few times before he sold it. I have talked to Imco about putting an Imco lower on the Bmax and if I remember correctly the vertical shaft on the Imco is too long to mate up with the Bmax upper. Too short and it could be dealt with, but too long makes it difficult. I choose the SCX with SCX lower at 1.35. running a 26 Maximus and can't hold the motor back at top end. Haven't even been to WOT yet and it still spins the motor too high. Back in the day there was usually a prop that worked better than the other pitches in that family, so gearing to that affect was a good move. I have not kept up on what props are better in a family, so if your research shows the 28 B1 is best, then it still holds true.
With the SCX lower you can run the Maximus prop without worry of lower gear failure. The Merc lower gears (which the SC and Bmax lower use) don't like to the load from the 5 blade props.
I made my choice and I am very happy with it so far.
Dick

Mr G. I thought the BMax ran a totally different vertical shaft and needed to be changed in the lower in order to work with the bmax upper?

Bones,
Not sure what you are saying here?? The Bmax does run a different vertical shaft that seems to be shorter than the Merc or Imco. So mixing a different lower on the Bmax is not an easy thing to do. Not sure where John has taken the lowers on the present day Bmax's.. I am basing this info on old data, by a couple of years.
But the Bmax upper will work with the Bmax lower to answer your question.. ????? :) Or are you talking about the gear ratios on the Bmax.. they would be changed in the lower, I am pretty sure there is only one upper gear set. ???

Dick

Sorry Mr G. What I was referring to was mating a non bmax lower to a bmax upper. For some reason I thought I read or was told that a bmax vertical shaft was required in order to put on say an imco or xr lower. Not sure but figured you would know something about that. For the tune of 1100 bucks for a vertical shaft from bmax and labor (if you can't do it yourself) might be better with a bmax lower as well. I don't know that why I rely on you guys for the answers.:)

I have never done a swap like that, but I was told that the Bmax shaft is shorter, so a shorter shaft would need to be installed in the off brand lower, be it Merc or Imco, or?. The length of the shaft that extends from the lower housing is too long and would need to be shorter. I am not sure if a shaft is available to do that swap. You might be able to install a -3" shorty with spacers to accomodate the difference in length. It would depend on the difference of the two shafts. I am not talking from experince here, just hear say. So if anyone else knows of a shaft that fits the Merc or Imco housing, please speak up.
It would be nice if you could bolt an SCX lower on a Bmax. That would give you the advantage of the larger lower gears. But there are compromises no matter how it is setup. Like you say, it might be better to stay with a Bmax lower.
Thanks
Dick

I just read this post and I'm wondering doesn't the weight of the boat have to do A lot with what gear ratio you use?

Weight and power are factors in the equation. End result is WOT rpm's the motor is rated for. You prop and gear for an end result. In my opinion, the prop plays a big role in which gear ratio you choose. If your combination requires say 5500 rpm, and you have a light boat, the prop will be a larger pitch number. Heavy boat will require less pitch to aquire that 5500 rpm. Lets say you can turn 6000 rpm, now that light boat may not pull it with 600 hp with a 34 P prop. But if you have 1000 hp, you may end up with a 38 P prop and might consider a gear change to 1.35 ratio to keep the 34 P. My questimations are not proper math (Not implying that the switch from 1.5 to 1.35 is handled by 4" pitch change, etc).
I have always considered the prop that may work better at a maximum rpm and see where the gear ratio lands. If it ends up a 1.35 puts me in the sweet spot for a certain type of prop then so be it.
Now a lot of people have experimented with different gear ratios and props and come to different conclusions. To say that one ratio works better than another depends on a lot of factors in each instance. But a heavy boat versus a light one, will move the numbers down the scale ( lower ratio/smaller prop). More power versus less will move the numbers up the scale.
So yes, weight does play a factor in the equation. Will one ratio have more bottom end than another, I think it depends on what prop you use with the different ratio. Are there issues with the size of the gears, maybe so. I would suspect some people that have experimented can speak to that. How much difference there is with ratios, could be compensated with the right prop in most cases..
Just my .02..
Hope that helps your thoughts. :)
Dick

I know reviving a very old/dead thread but someone may want this info for the future. You can 100% run a bravo shop lower, a imco sc lower, or a bravo standard blunt nose lower on a bmax. The vertical shaft must be bmax specific in either std length or -2 length. So if you want to run a -2 imco sc on a bmax upper you need the -2 bmax bravo shop vertical shaft, same for std length imco sc on a bmax, need the std length bmax vertical shaft. You MUST drill a secondary water hole to feed the upper cap bearing or it will grenade in short order due to chasing oil out of the upper cap bearing. 100+ hours on the std length sc lower on a bmax 1.30 upper with std length bmax vertical shaft with added water passage hole.