Author Topic: RPM limits (Read 7086 times)

afkrejci90 · « **on:** July 06, 2014, 12:46:49 pm »

I've been thinking about this for a while now. The 78K has a redline of 9,500 or 10,000 rpm I believe. Having a lightened and balanced crank to 14,000 rpm along with lighter valves, is the engine able to rev higher when all assembled? Is there anything that would limit rpm's in the top end such as the rockers?

Old Scrambler · « **Reply #1 on:** July 06, 2014, 05:22:37 pm »

YES..............the ROCKERS..............along with a few other things with long skirts (pistons), curls (springs), and odd-shaped weights (crank) that cause flex and stress the bearings. The cam-tower fasteners and bearing-caps need re-enforcement to hold lumpy cams...............and any amount of valve-float with a Hi-Po motor will usually grenade it. That said, a lot of builders have gotten a lot of power and performance from these motors when ridden or driven to their limits

.......and not beyond

Quite a few riders have learned that the way to go faster is to up-shift

Run-ups to 12,000 rpms in lower gears are common..............but falling back to 10,000 -11,000 or so is typical in top gear. I personally have hit those numbers and have never blown a motor in close to stock condition. I see the carnage in the race-track pits...........and I'm about to find out for myself with a somewhat out-of-the ordinary build. Let's see now................14,000 rpm in 5th-gear with an 18 x 38 sprocket combo will go HOW FAST???

Bill/BentON Racing · « **Reply #2 on:** July 06, 2014, 05:30:42 pm »

Midget motors with no trans(cut off/plate added) would wind up to 14-16,000 rpms! K.

Bill

bear · « **Reply #3 on:** July 06, 2014, 05:51:48 pm »

I have learnt the hard way, that these motors do not like to have the crap reved out of them.
Their just not built for that.
In my opinion, there's more to be gained working with their strengths.
For their age, these motors can handle very high loads.
We chase hp through high cylinder pressure, cylinder volume, combined with coaxing as much flow as possible through the heads limited capability.

Cheers,
Brian

Retro Rocket · « **Reply #4 on:** July 06, 2014, 06:07:47 pm »

Ok then, who's going to put their hand up and make us some roller rockers...

BobbyR · « **Reply #5 on:** July 06, 2014, 06:59:33 pm »

The stock 78K redlines at 9K. I have reached that on mine a very few times and so far so good. I do not race so I don;t need to rev that high. As Bear said, there is no advantage to overdoing it.

Jim F · « **Reply #6 on:** July 07, 2014, 10:57:59 am »

Quote from: bear on July 06, 2014, 05:51:48 pm

I have learnt the hard way, that these motors do not like to have the crap reved out of them.
Their just not built for that.
In my opinion, there's more to be gained working with their strengths.
For their age, these motors can handle very high loads.
We chase hp through high cylinder pressure, cylinder volume, combined with coaxing as much flow as possible through the heads limited capability.

Cheers,
Brian

Hey Brian
what kind of CFM have you achieved with the heads?

bear · « **Reply #7 on:** July 07, 2014, 06:03:57 pm »

G'day Jim,

Our heads usually test in the high 70's, not all of them have been flowed though.
Could never break through the 80cfm mark.

Hopfully Uncle Johno may be able to help us with that later in the year with a bit of luck.
I think we probably need around 90cfm for these motors to reach their potential.
Their a little asthmatic at the moment.
.
Possibly one of your hair dryers could solve our problems.
But the powers to be won't let us run them.

Cheers,
Brian

Bill/BentON Racing · « **Reply #8 on:** July 07, 2014, 08:51:31 pm »

Very fortunate to have a friend like Mike Rieck, my (his) K head flows 79 @.400 lift, 82 @ .500....I know ,no .500 cams! Can't wait to install it! K.

Soon!

Bill

Doctor_D · « **Reply #9 on:** July 09, 2014, 05:07:47 pm »

Even with a well prepped, hot-street motor, power starts to fall off pretty quickly after 9k. Balanced, lightened, shot-peened, cryo-treated, etc., and etc. internals, and I've still set my limiter to 10.5k, and I could probably drop that to 10k and never know the difference.

Like Bear said, these motors don't need to scream to make power (paraphrasing). Square bore to stroke on 836's means they make great torque over a broad and very usable rpm range. And, really, when I did ping it off the limiter in 3rd (and promptly rolled off the throttle and trail-braked out of "lose your license" territory) - I hit 108mph. Shoot, I hit 81mph in 2nd - and didn't feel the limiter. More than fast enough to merge with traffic on the highway. What's 11k in 3rd, 112mph versus 108 at 10.5k? My opinion is that when compared to up-shifting and getting back into the meat of the torque curve - the extra rmp's aren't the answer on these bikes.

johno · « **Reply #10 on:** July 12, 2014, 01:17:54 am »

Hi D.
I use your bike as the benchmark in judging old honda's, its pure class mate.
I am curious though, Have you ever given it a decent squirt in top gear ?
cheers johno

Doctor_D · « **Reply #11 on:** July 12, 2014, 03:42:57 pm »

Quote from: johno on July 12, 2014, 01:17:54 am

Hi D.
I use your bike as the benchmark in judging old honda's, its pure class mate.
I am curious though, Have you ever given it a decent squirt in top gear ?
cheers johno

Top gear? No. I live in an urban area where there's little room for that sort of stuff. Thought about bringing it out for track-days, but I'd cry if I laid her down. Actually, crying's probably the least of it.

My top-speed recall has ticked 112 mph after a few seconds of drop-the-elbow, chin-on-the-tank foolishness. But, the wind noise and the feeling that I'm going to blown off of the bike starts to get to me around triple-digit speeds - so I back down pretty quickly once I'm there.

754 · « **Reply #12 on:** July 12, 2014, 06:46:07 pm »

I think it depends gearing..,( with my old 836) I often run17/54 gearing..And shift without tach at 10,5 to 11 Ish.. Run a tach occasionally to see where I am at rpm wise.
.just another tool to enable lower gearing, with relatively. Same top speed..
RC 315 Cam.. Stock head, triple cut stock valves, 30 mm Webers..

Greg H · « **Reply #13 on:** July 13, 2014, 01:08:06 am »

Quote from: 754 on July 12, 2014, 06:46:07 pm

I think it depends gearing..,( with my old 836) I often run17/54 gearing..And shift without tach at 10,5 to 11 Ish.. Run a tach occasionally to see where I am at rpm wise.
.just another tool to enable lower gearing, with relatively. Same top speed..
RC 315 Cam.. Stock head, triple cut stock valves, 30 mm Webers..

When I used to race a 970 in the street class I used to change gear when it seemed like the right thing to do

. The regulations stated that a working speedo must be fitted so I fitted one off a moped and dispensed with a tacho to save weight

. Air shifters weren't allowed so we just power shifted , which wasn't good for the "box" but was good for the times . I remember one instance where I'd rigged up a tacho and then proceeded to miss a gear , when I looked down at the tacho the needle was coming down at 12 k

. Result ,one bent valve

We live and learn

.

Old Scrambler · « **Reply #14 on:** July 13, 2014, 10:13:53 am »

Dr. D.................I know a place where the rules encourage you to ride your bike as fast as it will possibly go

YOUR INVITED

Doctor_D · « **Reply #15 on:** July 15, 2014, 09:13:19 pm »

Quote from: Old Scrambler on July 13, 2014, 10:13:53 am

Dr. D.................I know a place where the rules encourage you to ride your bike as fast as it will possibly go

YOUR INVITED

You think I'm putting this bike on the salt?

Ummm... no.

johno · « **Reply #16 on:** July 16, 2014, 03:41:37 am »

+1

you look after our forum child D

Bill/BentON Racing · « **Reply #17 on:** July 16, 2014, 04:41:15 am »

Haha, very smart D!

Bill

Doctor_D · « **Reply #18 on:** July 17, 2014, 02:03:35 pm »

Quote from: johno on July 16, 2014, 03:41:37 am

+1 you look after our forum child D

John, I love getting up to speed ludicrously quickly, and generally do. However, restraint is generally the better part of maintaining one's health (not to mention driving privileges). I've had a shoulder reconstructed as the result of a lay-down avoiding an apparently-sightless cager in 1991. Did everything right. Low-sided, got away from the bike, then I hit the curb... and, bam. Shattered my collar-bone, and tore everything else to hell. Getting back to the rugby pitch took two surgeries, six-months of fully dedicated rehab, and healing the way only young men can. Continuing the full-on thread-jack/side-note; Though my Doc said contact sports were over for me, about nine-months later I hooked in the Div 3 national collegiate tourney. And my knees... that could be a thread unto itself.

Tough as I am - I have, however, gotten a bit older in the interim. Likewise, a bit more cautious and restrained. Which is not to say that I'm not a fan of speed. While never eyeing my CX500 for the 500cc pushrod twin LSR, I do believe the current SCTA Speed Week MPS PG 500cc record is just shy of 127mph. Not that I've given it any thought, but I figure I'd need to get that 48hp slug up to about 81hp at sea level to hit 130mph with a partially streamlined CX. Again, not that I've given it a moments consideration, but the ratio of the ring and pinion on the shaft drive seems to be the most complicated bit of engineering involved.

johno · « **Reply #19 on:** July 18, 2014, 03:39:29 am »

Speaking vaguely of max RPM

D the ol CX 500 seems to have a cult following in the US salt scene and also here in OZ.

Thos bloke was at the salt this year, I'll show you how he solved the gear ratio problem.

johno · « **Reply #20 on:** July 18, 2014, 03:44:09 am »

Hacksawed the back part of the frame off, cut the frame off an old 500 jawa track bike ( speedway) which is rigid and put a chain drive on so he could swap sprockets easily.

nothing a 5 inch grinder and welder couldnt fix .

johno

dusterdude · « **Reply #21 on:** July 18, 2014, 04:12:01 am »

Johno,wasnt that bike shaft drive?

Sent from my iPhone using Tapatalk

johno · « **Reply #22 on:** July 18, 2014, 04:28:31 am »

Yeh , Was, he converted it to chain drive,

cheaper to swap sprockets than ring gears.

MRieck · « **Reply #23 on:** July 18, 2014, 07:04:23 am »

Crazy Australians.

bwaller · « **Reply #24 on:** July 18, 2014, 10:48:42 am »

I've seen a couple of those bikes with high mileage. They seem to be a tough engine. I always thought they'd make a good endurance racer.

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Author Topic: RPM limits (Read 7086 times)