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Has anyoNe ever made any Bigger Fuel Tanks for these? Maybe a Seat Tank out of Aluminum. I have a 1972 292 TNT. I was looking at some Aluminum Tanks on the web a while back and got me to thinking. A 2.75" x 16" x 36" is about 6.86 Gallons. This is just a Concept, I haven't measured my Seat yet. You could make it either Square with just rounded edges or make the Top more Oval which would give you more room for more Gas. Use some Tabs on the Side to attach.

If say Avg Trail Speed is 35mph.

1970 Skidoo 292 TNT 3.75 gallons = Range ??? Miles?

--------------------------------------------------------------------------------

1971/1972 Elan 250 4.38 gallons = Range ???? Miles?

1971/1972 Oly 335 6.25 gallons = Range ???? Miles?

1971/1972 292 TNT 6.25 gallons = Range ???? Miles?

Something like this.
 

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Your 72 TNT should already have a 6 gal tank (5 imp)
Fuel burn at 3/4 throttle - will yield 2.75 hours
30mph X 2.75 = 82 miles

If you are going to make a seat tank make sure it is low enough to allow easy access to the carb and doesn't block any airflow. A junk yard Arctic Puma/Lynx 3.5 gallon tank might be low enough, but not sure on width. Could be a cheap easy install if it works

9.5 gallon fuel at 2.2 gal /Hr = 4.3 hours = 130 miles at 30mph.

All of this assumes your butt will take 4 hours at 30mph on a 1972 suspension
 

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Your 72 TNT should already have a 6 gal tank (5 imp)
Fuel burn at 3/4 throttle - will yield 2.75 hours
30mph X 2.75 = 82 miles

If you are going to make a seat tank make sure it is low enough to allow easy access to the carb and doesn't block any airflow. A junk yard Arctic Puma/Lynx 3.5 gallon tank might be low enough, but not sure on width. Could be a cheap easy install if it works

9.5 gallon fuel at 2.2 gal /Hr = 4.3 hours = 130 miles at 30mph.

All of this assumes your butt will take 4 hours at 30mph on a 1972 suspension
Yes, 6.25 US Gallon = 5.2 Gallon Imp. I'm wanting an extended Tank. 35mph x 5hrs = 175 miles. Probably about 10-11 Gallons needed.

With 15/34 Gearing and using the Calc it says at:

292 TNT 15/34 Gearing. Skidoo HP/Time Chart doesn't make since. 6500rpm x 75% = 4875rpm, 6500 x 50% = 3250rpm, 6500rpm x 25% = 1625rpm.

Max 6500rpm 20hp 2hr 15min = 48.1 mph <--- 48.1 x 2.15 = 103.4 miles.
3/4 6000rpm 15hp 2hr 45min = 44.4 mph <--- 44.4 x 2.45 = 108.7 miles.
1/2 5500rpm 10hp 5hr = 40.7 mph
1/4 5000rpm 5hp 8hr = 37.0 mph
4750rpm = 35.2 mph Trail Speed
3500rpm = 25.9 mph Low Trail Speed
 

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Yes, 6.25 US Gallon = 5.2 Gallon Imp. I'm wanting an extended Tank. 35mph x 5hrs = 175 miles. Probably about 10-11 Gallons needed.

With 15/34 Gearing and using the Calc it says at:

292 TNT 15/34 Gearing. Skidoo HP/Time Chart doesn't make since. 6500rpm x 75% = 4875rpm, 6500 x 50% = 3250rpm, 6500rpm x 25% = 1625rpm.
The chart doesn't make sense to you you because you are thinking in linear terms. Snowmobile drivetrains are not linear.

1) Throttle position does not equal RPM.
In rough terms - Throttle position / load = rpm. Greater load = Lower RPM
This easily demonstrated since dyno runs are made at FULL throttle, then vary the load to get different RPM.

2) Your gearing calculations assume full shift of clutches (1:1) ratio
Clutches are torque sensitive. The clutch ratio changes as load varies.
An Oly 335 towing a skid of firewood on a trail at full throttle will be going slower than the same sled with no load blasting across a glare ice lake - even though they may both be turning 5500 rpm

One way to see the effect of the clutch shift is to look at the Oly and Elan fuel burn charts
Full throttle and 3/4 throttle are BOTH 5500 RPM but Hp changes. That's because the clutches are trying to keep the RPM high by back shifting to a lower gear ratio as the engine TORQUE drops off.

You can easily see the effect yourself. Jack your TNT track off the ground. Run the engine and get the track spinning. Slowly increase the throttle. The clutches wont fully shift out till you get quite a bit of throttle in it. The load of the drive train itself is enough to keep the clutches from shifting to full 1:1

Personal experience with a 12hp Elan is that at full throttle on snow its somewhere around 25-30mph depending on conditions - even though gearing charts would tell you 35+ mph

For your 292 I chose 3/4 throttle burn and 30mph because that is what I estimate you will "conservatively" average on snow trails.

At 2 3/4 hours per tank, the 15hp at 3/4 TNT vs 12HP WOT for the Elan are virtually the same fuel burn / hr so mpg should be close too. - as always "Your Milage May Vary"

What kind of RPM/Speed/Fuel burn are you currently getting on your TNT ?
 

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Discussion Starter · #8 ·

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The chart doesn't make sense to you you because you are thinking in linear terms. Snowmobile drivetrains are not linear.

1) Throttle position does not equal RPM.
In rough terms - Throttle position / load = rpm. Greater load = Lower RPM
This easily demonstrated since dyno runs are made at FULL throttle, then vary the load to get different RPM.

2) Your gearing calculations assume full shift of clutches (1:1) ratio
Clutches are torque sensitive. The clutch ratio changes as load varies.
An Oly 335 towing a skid of firewood on a trail at full throttle will be going slower than the same sled with no load blasting across a glare ice lake - even though they may both be turning 5500 rpm

One way to see the effect of the clutch shift is to look at the Oly and Elan fuel burn charts
Full throttle and 3/4 throttle are BOTH 5500 RPM but Hp changes. That's because the clutches are trying to keep the RPM high by back shifting to a lower gear ratio as the engine TORQUE drops off.

You can easily see the effect yourself. Jack your TNT track off the ground. Run the engine and get the track spinning. Slowly increase the throttle. The clutches wont fully shift out till you get quite a bit of throttle in it. The load of the drive train itself is enough to keep the clutches from shifting to full 1:1

Personal experience with a 12hp Elan is that at full throttle on snow its somewhere around 25-30mph depending on conditions - even though gearing charts would tell you 35+ mph

For your 292 I chose 3/4 throttle burn and 30mph because that is what I estimate you will "conservatively" average on snow trails.

At 2 3/4 hours per tank, the 15hp at 3/4 TNT vs 12HP WOT for the Elan are virtually the same fuel burn / hr so mpg should be close too. - as always "Your Milage May Vary"

What kind of RPM/Speed/Fuel burn are you currently getting on your TNT ?
When we rode the River on my Yamaha V max 600, I was Avg 45mph. That's why I picked 35mph for ther Skidoo 292 TNT.

Never had enough Snow yet to ride the Skidoo yet other than down the street and back.. At 35mph I would think the Clutch would be cut in at 1:1 at that Speed.

Yes, your going to lose so much Speed depending on Snow/Ice conditions. Also everyone weighs different. Mine was stripped of Tach and Speedo, but I got a Tach that I have to install yet. I was going to just use my GPS for Speed.

If 6.25 Gallons gets me 2:45 at 6000rpm, and Speed is yet to be undetermined if we can't trust these Calc.

I'm wanting an extended Tank for Avg 35mph x 5hrs = 175 miles. Probably about 10-11 Gallons total needed. 11 - 6 = 5 Gallon for Extended Seat Tank.

With 15/34 Gearing and using the Calc it says at:

292 TNT 15/34 Gearing. Skidoo HP/Time Chart doesn't make since. 6500rpm x 75% = 4875rpm, 6500 x 50% = 3250rpm, 6500rpm x 25% = 1625rpm.

Max 6500rpm 20hp 2hr 15min = 48.1 mph <--- 48.1 x 2.15 = 103.4 miles.
3/4 6000rpm 15hp 2hr 45min = 44.4 mph <--- 44.4 x 2.45 = 108.7 miles.
1/2 5500rpm 10hp 5hr = 40.7 mph
1/4 5000rpm 5hp 8hr = 37.0 mph
4750rpm = 35.2 mph Trail Speed
3500rpm = 25.9 mph Low Trail Speed

This (1) route, on south leg is Hwy Ditch to North/South Legs both on Rivers, and North leg on abandoned Railway Line about 165-170 miles.
 

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Never had enough Snow yet to ride the Skidoo yet other than down the street and back.. At 35mph I would think the Clutch would be cut in at 1:1 at that Speed.

Yes, your going to lose so much Speed depending on Snow/Ice conditions. Also everyone weighs different. Mine was stripped of Tach and Speedo, but I got a Tach that I have to install yet. I was going to just use my GPS for Speed.

This (1) route, on south leg is Hwy Ditch to North/South Legs both on Rivers, and North leg on abandoned Railway Line about 165-170 miles.
Back in the day (70's) Chassis dyno's were the norm for TUNING snowmobiles. Clutching, gearing, jetting etc. were tuned based on TRACK load not bare engine HP. This took all the driveline efficiencies out of the equation.

Hartzell Dynos were one of the more popular models. We tuned our race sleds using one. Drive the snowmobile on and rev it up. Tune/clutch/gear for best MPH at preset load. Nowadays reputable speed shops still use chassis dynos but because of the high the HP/Speed of modern sleds the dynos are no longer drive on, but still run off the track.

Below is the worksheet chart Hartzell used to determine the HP delivered to the track based on recorded load and mph.

Snowgoer magazine tested sleds in 1972 using a chassis dyno -
https://snowgoer.com/vintage-sleds/snow-goers-wyoming-tests-1973-snowmobiles/2470/
Unfortunately, because the article is so old the photos' and graphs don't show anymore.

However - there are two very interesting findings in the article

DRIVELINE EFFICIENCY
Probably the most interesting fact about snowmobile dyno tests is that even the best snowmobile on the market today is only about 35% efficient in delivering rated horsepower to its track. This means that even the best snowmobiles today lose 65% of rated engine horsepower through the power train, and that if your snowmobile's engine is rated at 20 horsepower, the most you could possibly expect to reach the ground would be 7 horses

SNOW CONDITIONS VS LOAD
The six psi loads SNOW GOER used in Wyoming roughly w e r e equivalent as follows: 250/psi-ideal hard pack; 500/psi-compact snow trails; 750/psi-breaking trails in light snow; 1000/psi-braking trails in powder or wet snow, and 1250/psi-breaking trail and climbing.

Your route of ditches and rivers probably runs in the 500 psi range
 

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Using Snowgoers 35% efficiency rule of thumb (for the very best sleds which a TNT is not)
6500 rpm = 20 x .35 = 7.0 hp.
6000 rpm = 15 x .35 = 5.25 hp
5500 rpm = 10 x .35 = 3.5 hp
5000 rpm = 5 x .35 = 1.75 hp

Now look at the second line on the chart - 500psi (sorry hard to read)

At 500lbs
6500 rpm = 7hp = 48mph approx. ( just like your full shift calc)
6000 rpm = 5hp = 35mph approx. ( 9mph less than calc)
5500 rpm = 3.5hp = 22mph approx. (19mph less than calc)
5000 rpm = 1.75 hp = 12mph approx ( 25mph less than calc)

So my 30- 35mph is a good ballpark number for your 6000rpm - NOT 44mph from the calc

175/35 mph = 5 hours x 2.2 gal/hr = 11 gallons
175/30 mph = 5.8 hours x 2.2 gal/hr = 12.6 gallons (conservative guess)

You might want to consider shooting for 13 gallons onboard as a safety margin. Kinda like how the FAA requires a 30 minute reserve fuel onboard for a scheduled flight.
 

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