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Crank timing advice, please? Still trying to wring more power out of a HPI .12R SS >

Crank timing advice, please? Still trying to wring more power out of a HPI .12R SS

Crank timing advice, please? Still trying to wring more power out of a HPI .12R SS

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Old 01-11-2017, 02:56 AM
  #61  
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Fair enough.
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Old 01-12-2017, 01:01 PM
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Thanks for your help, guys. I finally hit the goal I was aiming for. Not too shabby for a .12 cubic inch RTR engine, I think.

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Old 01-16-2017, 12:02 AM
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great job, and efforts!

Have you looked into changing pinions/spurs for a lower final drive to get higher top speed?

Only takes a few minutes and will make much more top end difference.

You'll only get so far with this engine before the internals start failing, a higher quality race engine is the way to go if you want a big difference.
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Old 01-16-2017, 04:54 AM
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Yes, in fact changing the 2nd gear pinion from 22t to 23t made the last little bit of difference to reach 50mph. However, gearing options are limited on this car (I'm already using a 16t 1st gear pinion "borrowed" from the R40 to get better acceleration at low speed), so there was no sense re-gearing if I couldn't at least get close to 50mph through engine tuning. 48mph was just close enough that re-gearing could bridge the gap.

Now that I know 50mph is possible with this specific engine in this specific car, I've replaced both spur gears with 47/41t instead of the 49/43t I was using before, which means I can go back to using the original pinion gears (because they're purple, and that matters to me) while maintaining the same top speed. In fact, it may be about 0.2mph faster in 2nd gear with the original pinion and the smaller spur, compared to the larger pinion and the original spur. Basically I'll know if that worked in real life because I'll see 50mph on the GPS more consistently than I did during the previous test. (my GPS doesn't measure tenths of an MPH, which is annoying -- or at least it doesn't display that info for me to see.)

Sure, using a race engine was always the easy option. But squeezing every last bit of performance out of this RTR engine, while keeping the convenience of the pullstarter, was more of a challenge. I have almost 40 RCs now, so I need to work on finding challenges that don't involve spending money. I have enough RC loans to pay off already, and I have plenty of spare parts stocked up for the .12R SS in case I destroy something.
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Old 01-16-2017, 08:45 AM
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You need to keep in mind that if the gearing is too tall you're going to lose top speed because the engine doesn't have the power to push it that fast, it bogs. You might get higher top speed with a lower gearing.
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Old 01-16-2017, 09:53 PM
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Indeed, you can't just gear-up nitro to get more guaranteed top speed like you can with electric. That's why I focused on engine tuning first and didn't re-gear until the very end.
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Old 01-18-2017, 04:16 PM
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As I predicted, the car hit 50mph more reliably with the original pinions and smaller spurs, meaning it goes a fraction of a MPH faster than before. It still has more than enough torque to spin donuts when I feel like being silly, though. I can finally say this project is done.
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Old 04-09-2017, 07:46 PM
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This was one of the most entertaining reads I have read. You did well. I have been messing with these little cars and nitro engines since 1969 (a Cox Buggy), and I love getting the absolute best out of them. I salute you! However, I do have a question; did you ever try cutting a little off the header? If not, I think that will help you a little more.

Originally Posted by fyrstormer View Post
As I predicted, the car hit 50mph more reliably with the original pinions and smaller spurs, meaning it goes a fraction of a MPH faster than before. It still has more than enough torque to spin donuts when I feel like being silly, though. I can finally say this project is done.
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Old 04-10-2017, 12:54 AM
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No, I hadn't tried that. The header and exhaust are held together with a silicone coupling, and they're already smushed together about as close as I can get them; the coupling is already pushed against the bend in the header and the widening chamber of the exhaust pipe, so if I were going to make any changes I'd need a shorter coupling to start with. Maybe someday, but I'm content with 50mph from this car.

It's coincidental that this thread should pop back up right now, because yesterday afternoon I made some of the same modifications to a Losi 3.4 engine (mostly just increasing the compression until I stopped seeing speed gains) that I have installed in a different RS4, and speed tested it:

+ YouTube Video
ERROR: If you can see this, then YouTube is down or you don't have Flash installed.




Based on my calculations (65mph speed, 66mm tires, 3.96:1 drive ratio), the Losi 3.4 was turning ~33,333rpm, which is very close to its rated maximum of 34,000rpm. I doubt I'll get more speed out of that car, maybe 1-2mph at the most. I might shim the sleeve up by 0.1mm to see if the extra port timing makes any difference, but I doubt I'll do anything to the crankshaft on this engine; this is already my fastest vehicle by a 30% margin.

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Old 04-11-2017, 07:43 PM
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That's a good running car there. The Losi 3.4 is okay as long as you don't add much power to them. The rod will let go fairly quick in them if you add a little timing and increase blow-down and other drag mods. They will absolutely scream but the rods are weak. He is a little OS .15 that I tweaked a little. I live in Denver (elevation is quite high) so the engine is down about 18-20 % up here.

https://www.youtube.com/watch?v=zFfJ_CDspXs
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Old 04-12-2017, 08:01 PM
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Increase blow-down?
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Old 04-12-2017, 09:09 PM
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Increasing blow-down time produces more peak horsepower and the engine will have a greater RPM potential. This can be accomplished by slightly raising the exhaust port, or by widening the exhaust port at the top of the port. Widening the top of the port doesn't necessarily increase blow-down "time" but, it does create a more efficient exhaust, which is needed with all the inlet/intake mods. What goes in must come out. However, I wouldn't advise doing it if you have not experimented with it. Here are a few before and after pictures. These are NOT professional ports. It's just what I do to my own engines along with block mods for drag racing. Some of the "after Pictures" are before I finished them, so there's shavings. They work. Sorry for not reducing the size of the pictures.















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Increase blow-down?
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Old 04-14-2017, 12:08 AM
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Oh. I know what you're talking about, I just didn't know that was the term used to describe it. The closest concept I could think of was blow-by, and I was pretty sure that wasn't what you meant.
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