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Discussion Starter · #1 ·
Hi All
After getting my 08 EFI Thruton TuneBoyed ,I have no rev limiter , I'm doing a track day tomorrow and was wondering what was thought a safeish max RPM .
 

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8000 is ok.you might can go higher but on a stock motor you wont be making much power over 7500
 

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Discussion Starter · #3 ·
Thanks . It is a stock motor but has K&N filters , TORs , AI removed and dynoed to suit.
 

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Thats true but closing on another bike to pass you have the inclination to wait on the shift. The bike is pulling hard and you just wring it out hoping to get a bike length or two ahead.

I destroyed a perfectly good old twin that way many years ago. Take the redline seriously.
 

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Discussion Starter · #6 ·
Thank you all for your comments. I'll keep it below 8k today. Next time I'm at the dyno I will get a limit put back on.
 

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One thing to keep in mind, your tach is probably not accurate. Mine hits the limiter at an indicated 7500 rpm and it's supposed to be set at 8000. If this is true I would actually be turning 8500 when the tach reads 8000 if I had no limiter.
 

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My newly reprogrammed ignitor by Triumph Twin Power has an 8500 rpm redline, which I bumped up against the other day, and it generally is considered the upper end for operating with stock components. The racing guys run the stock rods up to 8,500...for sustained operation over this rpm, they recommend going to carrillo rods. Not sure what the limits of the stock valve spring are...

FWIW, I've seen 9K on my tach with my procom ignitor. Dunno why it doesn't kick out before this level since it is supposed to have an 8400 rpm limiter in its programming. Come to think of it, I've never had the limiter kick in on it...

My carbed thruxton with the stock 865 cams made its best power between 7,500 and 8,500...definitely had to keep it up over 6,500 to stay in the power band. Unless you've disabled the rev limiter, I'd simply let the limiter do its job...if you have disabled it, then keep it below 8,500.

Cheers,

--Rich

ok, duh, ...I see that you don't have a limiter. Then you'll need to pay attention & keep the tach in view...
 

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Sweat,

real dyno...the butt dyno agrees with you though!

check out the dyno I posted earlier this week...the baseline (blue line) is my bike with 865 cam and procom ignitor. Like I said, the procom doesn't seem to have a limiter...the dyno operator cut it off at about 8,500.

While the torque curve for that cam drops steadily from 6500 & up, the power gained via revs slightly outweighs the torque drop & allows the power to continue to build. Unlike the 790 cams, you never feel it come 'on cam'...its a slow amble upwards, so I can see exactly why you'd think the power is falling off. With growing wind resistence with speed, the effective increase in speed is even less dramatic.

My solution to that was going to be dropping the front sprocket to 17 teeth so the bike would pull 5th to redline, but now I'm going to stand pat on gearing until I get a better feel for what the 790 cams bring to the table...initial feedback sez they don't seem to need the extra help!

Cheers,

--Rich
 

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Gordon Jennings (RIP), who among other things wrote the 2 Stroke Tuner's Handbook, recommended mean piston speeds not exceeding 3500 FPM for max. reliability. In a stock stroke 865 that calulates out to just under 8000 RPM (I don't recall exactly, it's been awhile since I ran it); according to Jennings, once mean piston speeds approach 4,000 FPM the motor is on thin ice. All that said, that formula was derived from 60's technology. I'll post the calc when I get home later...
 

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Gordon Jennings (RIP), who among other things wrote the 2 Stroke Tuner's Handbook, recommended mean piston speeds not exceeding 3500 FPM for max. reliability. In a stock stroke 865 that calulates out to just under 8000 RPM (I don't recall exactly, it's been awhile since I ran it); according to Jennings, once mean piston speeds approach 4,000 FPM the motor is on thin ice. All that said, that formula was derived from 60's technology. I'll post the calc when I get home later...

That's interesting stuff!
I wonder if the 3500 FPM number was decided on empirically, or what.
Do you think it is mainly and issue of friction at that speed, or the starting and reversing of that mass that fast that is the biggest factor?
What other things limit an engines theoretically top running rpm?
Valve travel and springs? Oil supply?
I bet there's whole textbooks out there that only address engine rpm limits!
Fascinating stuff, tell us more!
-K
 

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That's interesting stuff!
I wonder if the 3500 FPM number was decided on empirically, or what.
Do you think it is mainly and issue of friction at that speed, or the starting and reversing of that mass that fast that is the biggest factor?
What other things limit an engines theoretically top running rpm?
Valve travel and springs? Oil supply?
I bet there's whole textbooks out there that only address engine rpm limits!
Fascinating stuff, tell us more!
-K
Yep, plenty of stuff out there (SAE alone), however keep in mind the Jennings book was published 36 years ago.

The mean piston speed formula is:

Cm = .166 x L X N

Cm = mean piston speed (feet per minute)
L = stroke, in inches (to convert mm to inches divide by 25.4)
N = crank speed (in RPM)

You do the math! :D
 

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Discussion Starter · #16 ·

Hi Rob
The Thruxton handled like a dream , a bit lacking in power on the main and cooper straights but I was with the quicker runners around the bends. The longer ZRX rear shocks helped with the ground clearence as I got to the edge of the tyres with only one touch of the pipe.
It does have a slight oil leak on the head gasket ,we noticed it before the track day ,and it has not got any worse. I will take it down the local Triumph dealer in the morning to see if I can make a warrenty claim .
All I need now is more POWER !
 

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It does have a slight oil leak on the head gasket ,we noticed it before the track day ,and it has not got any worse. I will take it down the local Triumph dealer in the morning to see if I can make a warrenty claim .
All I need now is more POWER !
you probably mean valve cover gasket, right?

looks like fun!
 

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Go ahead and flame away... but how do longer rear shocks provide more ground, or exhaust pipe clearance?
 

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If my memory serves me correctly, I believe Jojje recommended limiting these twins with stock rods to 8300 rpm. He has a LOT of experience with these motors on the race track. You might enjoy visiting his web site at http://www.stabbarps-auto.com/welcome.html

I would definitely recommend running with a rev limiter programmed in the ECU.

On my '03, the peak hp is at 7800 rpm; however, at 8100 rpm it is still producing between 65 and 66 hp--so the hp doesn't fall that drastically after it reaches peak. My igniter's rev limiter is set at 8100 rpm.
 

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Go ahead and flame away... but how do longer rear shocks provide more ground, or exhaust pipe clearance?
By slightly lifting the rear of the main chassis or frame of the bike further off the ground, the angle of the swingarm in relation to the ground will have changed slightly. I noticed a similar effect when I fitted a slightly larger rear tyre to my bike - it's a little harder to scrape the footpegs now than it was with the standard sized tyre because I've gained a little extra ground clearance.
 
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