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Author Topic: Safe Piston speed  (Read 25845 times)

nolimits

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Re: Safe Piston speed
« Reply #30 on: May 04, 2005, 07:29:48 AM »
HIRoller, that's all fine and well, but we're talking about a STOCK cbr600rr which makes the question somewhat pointless...

Did I miss the bit where he said it was going to stay a stock motor?

If he were tuning the motor to deliver it's power at a higher rpm he may need to raise the rev limiter. Knowing the max piston speed would be useful so you can set your theoretical rpm limit and design your tuning around that.

When I raced a 125 the max power was at 11,300 and the red line around 12,000. We tuned the engine and moved max power up to 12,800 with over rev allowed to go to near 13,500. This allowed the use of lower gearing (to give more acceleration) whilst the extra 1500 rpm allowed us to keep the same top speed. I forget what the max piston speed was but we were just inside the comfort zone. 2 strokes have larger heavier pistons (due to the piston porting) so have a lower max piston speed. Hence the lower rpm limit 12-13,000 for 2 stroke, 15-18,000 for four stroke. 18,000 being those little CBR250rr's with tiny pistons.

Zagan

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Re: Safe Piston speed
« Reply #31 on: May 10, 2005, 10:27:57 AM »
Well I was wanting to know so that I could work it out based on the gear ratios, no don't know if the gearing will effect this much or not but at times I end up at 10,000 to 14,000rpm end sometimes going into the redline in 1st or 2nd gears and instead of sitting that high up in the rpm range at such low gears, which to my thinking won't be great for the motor.

I would have liked to work out what the pistion speed would be for the different gears and see what was least damaging at least.

Anyway,  I asked here as a few people here seem to be racing 600RRs rather than simply going fast on open roads and thought they might have a number or and idea about it.

nolimits

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Re: Safe Piston speed
« Reply #32 on: May 10, 2005, 07:22:31 PM »
The rev limiter will stop your engine self destructing and I think your more likley to bend valves than break pistons due to excessive rpm.
There is a certain amount of overlap and at high rpm the one valve may not have enough time to withdraw before the other starts its way down. You are relying on spring pressure to close the valve and this can only happen so quick.

quain159

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Re: Safe Piston speed
« Reply #33 on: February 13, 2009, 05:45:10 AM »
where did you hear that that was correct for "racing"?  ive never heard of piston speed in relation to anything.


at the really high engineering level, what jeff points out is crucially important to oil viscosity and thermal reciprocity, and that also play a role in dertermining piston ring attributes...

am i mistaken jeff ???? ;D
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Jeff

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Re: Safe Piston speed
« Reply #34 on: February 13, 2009, 04:25:23 PM »
Shit, I have no idea...  I just have to laugh at this thread after reading it again from 4 yrs ago...

Good times they were...  good times...

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cejay_2k

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Re: Safe Piston speed
« Reply #35 on: February 14, 2009, 06:26:04 PM »
Safe piston speed is used to calculate max RPM for a given engine

for example;  Max. RPM = Mean Piston Speed (ft/min) x cylinders / by Stroke in Inches

around 4000ft/min for a motorcycle, Formula 1 is around 7500ft/min

Still a stupid qeustion as the piston speed is used to calculate the max RPM so it will never be less than the redline.

Chris G

cosmo

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Re: Safe Piston speed
« Reply #36 on: February 15, 2009, 03:57:53 AM »
dunno where i heard this, but i understood that f1 cilinders have a bigger bore and thus smaller stroke then day-to-day cars, and somehow that would mean the piston speed is actually less then a normal good-performance car, though the rpm is much higher. ??? just what i heard, i don't stand by it since i have no ideea on f1 mechanics. 'sides, it's not that i really have nightmares about f1 piston speeds :P
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Paulred/black

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Re: Safe Piston speed
« Reply #37 on: February 15, 2009, 06:06:35 AM »
I think that with F1 cars the rpm's are so much higher because of the tolerences in different components in the engine. They are so fine that the engine is actually kinda seized itself when its cold, that is why they need to use special techniques for warming them up. so because of these extra fine tolerences (and parts being so lightin the engine) you can rev it alot higher, its what they were designed to do.

whereas your average car is built iguess for comfort and putting the high miles on  :thumb:

Paulred/black

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Re: Safe Piston speed
« Reply #38 on: February 15, 2009, 06:09:15 AM »
i suppose an example of this would be different oils you can use like WATT's that can be used in minus temperatures, compared to oils that are 'designed' <- wrong word i know to be used at higher temperatures.

CallmeJoe

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Re: Safe Piston speed
« Reply #39 on: March 08, 2009, 11:51:10 AM »
dunno where i heard this, but i understood that f1 cilinders have a bigger bore and thus smaller stroke then day-to-day cars, and somehow that would mean the piston speed is actually less then a normal good-performance car, though the rpm is much higher. ??? just what i heard, i don't stand by it since i have no ideea on f1 mechanics. 'sides, it's not that i really have nightmares about f1 piston speeds :P

Hi cosmo,

Hopefully I can explain this to you. That is very true indeed and this is why. The max piston speed for any engine out there is 4,400fpm. If you think about it this is pretty crazy, imagine going back and forth at roughly 50mph, no stopping! So with this being said lets say we have two engines, both 1000CC. We will call these engine #1 and engine #2. Let's also say that #1 has a bigger bore than #2, but keep in mind they are still 1000CC. So what is happening is you are decreasing the stroke length of #1, also there is less "ground" for piston #1 to cover when going from bottom dead center to top dead center. Now when you combine the bigger bore with shorter stroke you are actually going to get a higher RPM and not a high piston speed. This because the piston doesn't have the stroke length to reach that magical 4,400 number. Now hopefully you can see why harleys rev out at 9k instead of 15k+ because they have a longer stroke. Hope this information was useful.

Joe

Paulred/black

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Re: Safe Piston speed
« Reply #40 on: March 08, 2009, 12:23:09 PM »
Hi cosmo,

Hopefully I can explain this to you. That is very true indeed and this is why. The max piston speed for any engine out there is 4,400fpm. If you think about it this is pretty crazy, imagine going back and forth at roughly 50mph, no stopping! So with this being said lets say we have two engines, both 1000CC. We will call these engine #1 and engine #2. Let's also say that #1 has a bigger bore than #2, but keep in mind they are still 1000CC. So what is happening is you are decreasing the stroke length of #1, also there is less "ground" for piston #1 to cover when going from bottom dead center to top dead center. Now when you combine the bigger bore with shorter stroke you are actually going to get a higher RPM and not a high piston speed. This because the piston doesn't have the stroke length to reach that magical 4,400 number. Now hopefully you can see why harleys rev out at 9k instead of 15k+ because they have a longer stroke. Hope this information was useful.

Joe


+1

cosmo

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Re: Safe Piston speed
« Reply #41 on: March 09, 2009, 08:46:55 AM »
yeah, that's something along the lines i remember it being talked about.
tnx for clearing it up ;D
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Re: Safe Piston speed
« Reply #42 on: January 20, 2010, 08:24:01 AM »
Does anyone know what the safe piston speed is for a CBR 600 RR 04 model is, or all of them I guess as it's the same engine in all of them.

it wouldn't be max rpm on the dial would it?

I've read 4500 feet per minute is safe in racing but that was around 98 or so.


Hi zagan,

Don't let these dunderheads fool you into thinking you don't know what you're talking about. Without my notes I believe you are correct with the 4500 fpm, give or take a few. It has to do with a variety of things though such as, bore size, stroke length, and the material the piston is made of. I recommend that all of you take a few tech classes at your local Community College.

PS- Zagan if you have a fax machine I'd be more than willing to fax you my notes pertaining to this subject from class.

Jeff

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Re: Safe Piston speed
« Reply #43 on: January 20, 2010, 11:03:02 AM »
Welcome aboard... 

Thanks for clearing up a 5 year old post. 
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Paulred/black

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Re: Safe Piston speed
« Reply #44 on: January 20, 2010, 12:21:59 PM »
LOL