73 Small Engine Super Scat

cfh

Well-Known Member
#12
horsepower does not really change top speed. RPM determines top speed. horsepower is just how fast you get to that top speed. All tecumseh 3hp to 5hp motors are rated at 3600 rpm. it's just how much punch you get. obviously more punch with 5hp than 3hp. Gearing is really what dictates top speed.
 
#13
horsepower does not really change top speed. RPM determines top speed. horsepower is just how fast you get to that top speed. All tecumseh 3hp to 5hp motors are rated at 3600 rpm. it's just how much punch you get. obviously more punch with 5hp than 3hp. Gearing is really what dictates top speed.
You couldn't be more wrong. The more horsepower available, the taller gearing (lower ratio) the engine can handle. Like many YouTubers, you are using "fast" when "quick" would be the correct term. Quick relates to acceleration while fast relates to velocity. Torque gets you moving, horsepower moves you fast. A 797 HP Dodge Challenger Hellcat Redeye can hit 200 mph even though it's a brick, a 305 HP Challenger SXT can't get anywhere near 200 mph, it can't pull the gearing needed to reach that speed. Back to minibikes, a more powerful engine can use taller gearing (lower ratio) than a less powerful engine. If you gear a 5 HP engine to where it is just capable of hitting 3600 RPM, and use the same ratio for the 3 HP, the 3 HP would not be capable of revving to 3600 RPM and thus would not be capable of reaching the same top speed.
 
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old shed finds

Well-Known Member
#14
Basically that bike is really neat.
I should have said this earlier today.
Your engine should have the lighting coil for the lights.
Its a fun bike.I would put 100 buck bet at top speed 35 mph to 38...
 

cfh

Well-Known Member
#15
Charles exactly. you proved my point. 3600 rpm, regardless of the HP, is still 3600 rpm. you end up at the same top speed. it's just how fast you get there. your point is CHANGING THE GEARING. that wasn't the question here. that part of the equation was a fixed, as is the rpm. which means the top speed is the same. really the only variable is time to get to that speed.
 
#16
Charles exactly. you proved my point. 3600 rpm, regardless of the HP, is still 3600 rpm. you end up at the same top speed. it's just how fast you get there. your point is CHANGING THE GEARING. that wasn't the question here. that part of the equation was a fixed, as is the rpm. which means the top speed is the same. really the only variable is time to get to that speed.
You're still saying "how fast you get there", which in incorrect, it should be quick/quickly. Fast relates to velocity/speed, not elapsed time/acceleration. Quick relates to elapsed time/acceleration. A vehicle can be quick but not fast; a vehicle can be fast but not quick; some are both quick and fast. Yes, I'm critiquing your terminology, which some would consider rude, but if you want to be clear and avoid confusion, you need to use correct words/terms. It's like people who call a high ratio a low ratio and vice versa, a common mistake, and it causes confusion. This is constructive criticism, not an insult, and I hope you are not offended by it.

No one said they had to run the same ratio, but if they did, as I said in my previous post, the 5 HP engine may be able to reach 3600 rpm while the 3 HP may not and thus not attain the same top speed; in that case, the 5 HP would be faster. Also, with the same ratio, the 5 HP, mainly due to more torque, would be quicker (reach a given speed in a lower elapsed time). Consider that many people change their final drive to a lower ratio, usually by installing a smaller rear sprocket, when they put a larger/more powerful engine on a minibike; it's quite common. Summary: More HP = faster minibike.
 
#17
Guess you guys try to get people to blow up so they get tossed out
Some members think you are Tommie (AKA: oldschools, AKA: Speedway mini bikes, AKA: Carljohnson) who, let's say, was somewhat lacking in social skills and thus didn't make many friends on here; any friends he may have made probably didn't appreciate it when he went off on the whole forum (You can see a small sample in the thread linked below. He also posted two similar but lengthy rant threads which have been deleted.) Because of that, some members may treat you badly and others may just ignore you, unfairly if you are not that person. Your posts are somewhat similar to Tommie's and it is somewhat suspicious that you have been following Tommie and oldschools even though both accounts have been shut down, and oldschools has been following you. Personally, I'm not concerned about it, just providing some information for you in case you are not that person. If you are not the same person as Tommie/oldschools/Speedway mini bikes/Carljohnson, I don't know how you could convince those who have decided that you are, so good luck with that.

https://www.oldminibikes.com/forum/index.php?threads/speedway-scarab.167745/

Screenshot_2019-11-21 Brad Dodge.png

https://www.oldminibikes.com/forum/index.php?threads/oldschools.168025/
 
#18
Charles exactly. you proved my point. 3600 rpm, regardless of the HP, is still 3600 rpm. you end up at the same top speed. it's just how fast you get there. your point is CHANGING THE GEARING. that wasn't the question here. that part of the equation was a fixed, as is the rpm. which means the top speed is the same. really the only variable is time to get to that speed.
With the exact same gearing, exact same tire OD, and at the exact same engine RPM, the speed would be equal, theoretically, but that is not a realistic scenario.
 

cfh

Well-Known Member
#19
actually it is a realistic scenario. i had a speedway shrike that i used as a test motor bike. i would put tecumseh 3.5, 4, 5 hp motors in the bike. it was a great test vehicle. and i would run a gps speed indicator on it with the test motor. bottom line... top speed was the same regardless of the tecumseh. it's just how long it took to get to that top speed. with a h35, it took some time to get to the 35mph. with an hs40 or hs50, that time line was much shorter. but in the end, the top speed was the same.
 

old shed finds

Well-Known Member
#20
But... This is tested on flat ground?
Up in the hills the 4 or 5 HP would pull whip cream off a pie at speed.
The 3 HP would be trying all its poo to try and make a show....so yes dropping the rear sprocket small would make the bike get a speeding ticket ..5hp.
 

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