19lbs of Boost Gearing?

[RX-1 Moose]

I need some advice.
I was wondering what gearing you would sugest for me.
I have an 03 Rx-1 Bender rear mount Turbo running 19lbs of boost. 159 camo track, Fabcraft rear skid and 8 tooth extrovert drivers. I ride from 8,000ft to 11,000ft.
I am 6' 7" & about 330lbs with my gear on.
I have been using the 21/40 stock gearing the sled came with but I would like to get everything I can out of it. I have done alot of tuning and I am very happy with the way it runs and responds, About zero Lag now. I can beat the sage II Superchargers out of the hole and kill them in Mid and Top. I ride in the montains and have to race alot of people up hills & on the flats, you know keep proving the 4 stroke are the poop! I have not been beat yet but some 2 smoke turbos are getting close. I feel there is alot more there for Top end, it's like I hit a wall sometimes and she wont go any faster but wants too.
I have the same gearing I was using when I was pulling 10lbs, 12lbs to 14lbs of boost, (gearing is the only thing I have not messed with) and now I am getting more boost she stops pulling on the Top end & feels like she wants to go. I want the most out of the set up I have. I don't want to go into the motor and install a bigger Turbo at this time.
Has anyone tried the stock short track gearing of 24/38 with 8 tooth drivers?
Thanks for your help,
Mike

I have stock Mt gearing of 21/40 with 8 tooth drivers.

 

[snowy1]

I am running 20/42 9 tooth drivers,works good, unless you just want top end I would not gear up, in the mountains. I run 18 to 25 lbs. what kind of top speed are you looking for. mine will due 100 mph in a very short distance and thats fast enough for me in the mountains, if you want to lake race then the higher gears would help. but if you want to hill climb and drag race up the hill stick to the lower gears. by the time I gear out I am usually so far ahead of anybody else that it doesn,t matter the race is over.

 

[rxrider]

 

[Boyko]

8 tooth drivers will gear it down quite a bit, I do not think you are going to be bumping off your rev limter at that altatude, boost and the gearing. To low a gear will heat up your cluthes on a long pull. You might want to try to carry a temp gun and measure you clutch temps. Keep a log of your data and try to be as consistand as you can when you take the temps. Then you can tune your gearing, heat is the enemy, the cooler you can run your clutchs the less power wasted. If your sled has never lost to anything and has to haul that kind of paload your tuning is probbaly pretty much as good as it gets. LOL

 

[HAMMER]

03 RX-1, 8-tooth extrovert drivers, 159 track, 8,000ft to 11,000ft, 6' 7" 330lbs, 21/40 stock gearing

RX-1 (1:1 Clutch Ratio)
31/37-gear reduction (21/40 stock chain case)
9,800 x 83.7% = 8202 clutch rpm (82.20mph)
10,000 x 83.7% = 8370 clutch rpm (83.89mph)
10,200 x 83.7% = 8537 clutch rpm (85.56mph)
10,400 x 83.7% = 8704 clutch rpm (87.2mph)
10,600 x 83.7% = 8872 clutch rpm (88.92mph)
10,800 x 83.7% = 9039 clutch rpm (90.6mph) (97mph .75:1 overdrive)

it's like I hit a wall sometimes and she wont go any faster but wants too

19lbs at 11,000ft is like 25lbs at sea-level... You're most likely dropping off the back side of the compressor map (efficiency) on that rear-mount Bender, have a boost leak, going rich/lean, etc-etc...

A recent article in DynoTech showed a front mount GT28RS (capable of 495-CFM) made 233hp at 12.5lbs boost on an Apex with 9:1 JE pistons... That’s a 55% increase in horsepower or 6.8 horsepower per pound of boost... At 26lbs boost (max short run) it made 338hp, which is 2.28 times more horsepower than stock or 7.3 horsepower per pound of boost... Savvy tuners are very aware that dyno horsepower is only the POTENTIAL horsepower and does not directly translate to on-the-snow performance.

"The fact that a larger turbo can flow 500-CFM and that a smaller turbo can only flow 400-CFM does not mean the larger turbo can actually flow any more air than the smaller one into the engine at 15-PSI plenum pressure. So what’s important when you are running high-boost applications is how much air the turbo will flow above 15 PSI. This is revealed only in the compressor flow maps."

"In any case it takes about 150-CFM per 100-HP....so for 300-HP you need a turbo that puts out about 450-CFM. You should be thinking airflow and not pounds of boost."

"Keep in mind that it is airflow that we are concerned about. Do not confuse pounds of boost with airflow...it is CFM that makes the torque and power, not pressure alone."

Compressor flow maps indicate: (approximate depends on trim)

MPI Stage-I (310 CFM @ 65%)
BoonDockers GT25 (405 CFM @ 60%)
CPR GT25 (405 CFM @ 60%)
MCX TDH04-16T (435 CFM @ 65%)
MPI Stage-II (450 CFM @ 65%)
Bender Stage-II (450 CFM @ 65%)
GT28RS (495 CFM @ 60%)

 

[Turbo Tim]

Good point Hammer .........All boost is , is resistance to air flow. An engine with less exhaust restriction and better intake flow will perform better at less PSI than with a restrictive intake and exhaust.

 

[snowy1]

like HAMMER said it may not be your gearing, He just has a better way of saying it. I personally would not gear up until you know everything else is working properly, at that elevation and your size you will need lower gearing to get the most out of your clutches efficiently, for it to flatten out it could be any of the things HAMMER mentioned and I would think its more likely than the gearing at this point.

 

[RX-1 Moose]

Well I think I will leave it alone. It really runs well now and I am not a flatlander so flat out speed is not important unless I get beat. I just have to remember this is a Mountain Sled not a Top End Race Sled.
But it is pretty fun having a sled that does very well in both places in Long Track form.
My Clutches stay cool, My Air leakes are all gone (that is when the sled really ran good is when this was done) & it climbs like an animal. I think I will put my sled and myself on a diet this year and see what happens. I really am having fun with the sled, I have no Idea why I want to go faster on it.
Thanks for all the info. I hope the Turbo is not (dropping off the back side of the compressor map (efficiency)). The Sled runs clean and crisp no popping & crackling and my air fuel read as it should, My Pyro is reading 1050 to 1200 deg. I have zero problems climbing the only time I felt I ran out of gear was on a long straight away
Thanks, again

 

[Boyko]

Hey Moose,
It's good your sled is tuned and ya just have to wait for snow and have have a trouble free year, Goto like the Yamaha, Just wondering were do you ride at 11000 feet, I get pretty winded at 8000, I ride in BC, I'm from Alberta the locals are great in BC. One more question, were do you have your pyro tapped into?

Boyko

 

[RX-1 Moose]

I ride at that elevation in Colorado & Wyoming. The Snowies in Wyoming, (Snowy Range) where I live we unload our sleds at 9,200'.
I put the Pyro right after the Turbo.
The 4 stoke Turbo/Supercharger is incredible and it is just amazing how reliable they are.