Topic: Many moons ago

[Jinks]

In the '70s I built a K2 with 900 kit full race head & 3C cam the mech said I'd get 13k revs, my question is with nothing done to the carby's, what would be the top speed?

[MRieck]

In the '70s I built a K2 with 900 kit full race head & 3C cam the mech said I'd get 13k revs, my question is with nothing done to the carby's, what would be the top speed?

13,000 RPM?? Let us know how that works out.

[simon#42]

And where the piston landed

[newday777]

Welcome aboard Jinks
Good question. With the stock carbs you'd probably be pretty limited by fuel flow ability to make much above 11K, most big bore racers went with CR or Mikuni carbs for more juice to feed their need for speed.
Hondaman Mark recently wrote of some sprint cars he had seen had chopped off 750 cases(transmission removed) with big bore kits in the jugs and cam in the ported heads that turned out 15,000 and more with modifications. Hopefully he'll see this and chime in.

Mike I thought you might have a bit more input....

[RAFster122s]

Hi Jinks! Hope that motor has stout rods.
David

[MRieck]

Welcome aboard Jinks
Good question. With the stock carbs you'd probably be pretty limited by fuel flow ability to make much above 11K, most big bore racers went with CR or Mikuni carbs for more juice to feed their need for speed.
Hondaman Mark recently wrote of some sprint cars he had seen had chopped off 750 cases(transmission removed) with big bore kits in the jugs and cam in the ported heads that turned out 15,000 and more with modifications. Hopefully he'll see this and chime in.

Mike I thought you might have a bit more input....

Stu.....I really didn't think I needed too. I did just build 2 Yamaha R6 engines and they do redline at 16,000 RPM.
 CB750 make power up 9,500 to 9,750 RPM unless they are radically modified. Good porting and large valves will hold peak power past those points for another 1000 RPM's or so but that's about it....they just run out of charge.....not enough port or valve area.

[MRieck]

And where the piston landed

Now that's funny.

[scottly]

Stu.....I really didn't think I needed too. I did just build 2 Yamaha R6 engines and they do redline at 16,000 RPM.
 CB750 make power up 9,500 to 9,750 RPM unless they are radically modified. Good porting and large valves will hold peak power past those points for another 1000 RPM's or so but that's about it....they just run out of charge.....not enough port or valve area.
[/quote]
And the larger the displacement, the sooner they run out of breath.

[Sam Green Racing]

In the '70s I built a K2 with 900 kit full race head & 3C cam the mech said I'd get 13k revs, my question is with nothing done to the carby's, what would be the top speed?

Jinks, welcome to the forum.

If you look at the top of this page, you will notice we have 2 moderators, myself and mlinder.
Mark Linder was a nice guy but constantly bragged about how his motor would rev to 12,000 to 13,000rpm with ease.
Mark has not posted on here since the 2nd January 2013, that was the day his motor exploded.
We kept on telling him it would happen, but he just didn't believe us.
Take notice of the people that know what they are talking about.

Regards, Sam Green.

[Jerry Rxman Griffin aka MuthaF'er]

Mine did 13,000. Once. Missed a 1 to 2 WFO power shift. The broken valve head left a BIG hole in the top of the piston back in March '76. I've been working on it ever since.  However, the rods held up. I still have the rod connected to the piston as a souvenir of my indiscriminate youth.

[paul_cb836]

Our 915 race bike running CR31s on methanol puts out over 100 rwhp. Dyna 2000 ignition cut out set at 9,500. Even at Sydney Motorsport Park (Eastern CK) which has a reasonably long main straight, we don’t quite get to those revs in top with race gearing. Exceeding those revs just pushes passed peak power and results in slower lap times.

I remember reading a comment years ago where Gary Nixon apparently refused to ride the M3 CR750 unless revs were limited to 10k. Apparently he was not keen on extracting a piston from his fundamental orifice. And that was an engine that was said to be rebuilt after every race. Either way, work it out for yourself. Standard 18/48 gearing gives 15mph per 1000 revs. You just need a very deep mine shaft😀.

[MRieck]

I apologize if I came off like a jerk. I've been working with these engines for decades....as have others.

[Sam Green Racing]

Our 915 race bike running CR31s on methanol puts out over 100 rwhp. Dyna 2000 ignition cut out set at 9,500. Even at Sydney Motorsport Park (Eastern CK) which has a reasonably long main straight, we don’t quite get to those revs in top with race gearing. Exceeding those revs just pushes passed peak power and results in slower lap times.

I remember reading a comment years ago where Gary Nixon apparently refused to ride the M3 CR750 unless revs were limited to 10k. Apparently he was not keen on extracting a piston from his fundamental orifice. And that was an engine that was said to be rebuilt after every race. Either way, work it out for yourself. Standard 18/48 gearing gives 15mph per 1000 revs. You just need a very deep mine shaft😀.

Great post Paul and well said Gary Nixon, 10k is plenty for these old motors. My 915 makes similar power to yours but to 10k but unlike yours, mine is a 2 speed Hondamatic and geared way down, 16-57 for drag racing.
I would leave the line, hit high gear just before half track, and cross the finish line with 10k just showing at between 116 and 118 mph depending on which way the wind was blowing.

Ride safe my friend, Sam.

[Jinks]

Thanks for the info.

[PeWe]

My 970cc (JE 70mm) flatten out just after 9000rpm. 
As the 1005cc setup (JE 71.25mm) did too.

The torque point downwards, hp just flatten out.
I guess 9500rpm is max.
It is a street bike so I do not rev it that much.
It can happen very quick on 2nd and 3rd gear if not shifting in time

I told the Dyno guy to rev up to 9500 rpm.
Megacycle 125-75 cam give good even power from low and up.
33.5/28.5mm valves.


No methanol, Shell V-power.
17:48 gearing
130/80-18 rear tire

Torque in Nm.



It runs better now with correct needles and matching needle jets, before too rich on that dyno up to +6000.
Probably not more max, but better from idle to 6000 rpm.

[MRieck]

Nice work Per. It's all over between 9 and 9,500 RPM unless you straight port the head and go up to at least a 36mm intake valve.
 When you think about 105 RWHP (approximately 100% increase in OEM RWHP) accomplished with an approximate 40% increase in displacement it's impressive. That and the fact the intake valve that does it fits on the stock seat and only has 1.5mm larger OD.

[PeWe]

Thanks Mike.
Finding the good compression and cam timing made around 10whp compared to earlier setup.

The inlet spigots should be wider. They are less than 30mm inside.

Measured my old K2 head, 34/28mm valves that gave 101whp with 1005cc, not more than 29mm. That combo had thick gaskets so thinner should have  kicked up a few hp more.

My old ported K6 head spigots 29.5mm something. Carving up to +30mm must go thru the alu. Ports are really opened in that head.

32mm as the size of the carbs might help to increase the flow.

Too much to modify to bring it to next level.

I like it as it is. Revving more will cause more wear anyway.

The clutch another weak point. I replaced the K6 clutch with a later to use double metal disc. Clutch feels more stable and sounds better.
EBC CX1145 fiber discs as before.
Billet plate
https://good-bits.co.uk/product/cb750-sohc-clutch-actuating-hub/


I must use the harder Barnett springs for CB500 and other smaller CB's despite glass beaded metal discs. 66 (lbs) in the part number.

I tried the typical Barnett CB750 springs. (56 lbs)
It felt like wheel spin on 3rd gear (at ca 4000-5000rpm) when giving some throttle. Like it did last time I tried.

Back to the harder springs and good again.
17t sprocket front for making life easier for the  clutch.

[Don R]

 Caution, old guy story,
 My racing friend bought into the Hot Rod Mag article about using twin Chinese made turbos on a junkyard LS motor. He learned they will make a thousand HP. but just a time or two.  It's the same as spinning extreme RPM's, they might but not for a long time.

[HondaMan]

Hondaman Mark recently wrote of some sprint cars he had seen had chopped off 750 cases(transmission removed) with big bore kits in the jugs and cam in the ported heads that turned out 15,000 and more with modifications. Hopefully he'll see this and chime in.

Indeed they did! The guys were running midget cars in south and west Illinois, circa 1972-3 when I had my shop there.
Those guys had to be magicians, IMHO. The one I knew best called his the 'slow one' in the bunch, as he only got 12k RPM from it. But, he spent much less time wrenching on it between heats that did the other guys. One guy could get 16k RPM from his in 1st and 2nd gear, and it sounded like it, but he stated [often] that it had a 'flat spot' between 14k and about 15.5k RPM after which it tended to just jump up to 16k, but could easily drop back below 15k RPM, so it was difficult to run there in the races. He called it the "15k RPM wall in these engines" and his spark advancer, easily the most complex 'gadget' I've seen in such a situation, had 2 little pointed, wasp-waisted "fingers" (his words) that advanced downward through 2 slots in the (larger, but flatter) advancer weights above 14k RPM (in the usual place on these engines). These "fingers" would retard the timing a bit (at his "RPM wall" speed) above 15k RPM which, in conjunction with a larger throttle opening, would make the engine 'jump up' to just past 15k RPM, and let him control it to 16k where it wouldn't go any faster. It sort of sounded like an ON/OFF switch when he demonstrated it, but was one sweet GP-like sound!

They weren't 750cc, but "something bigger" (I think the Class limit was 1500cc), and these engines were all sans transmissions, with the oil pumps driven off the bolt-end of the cam. The box-shaped oil pumps were piped to/from their oil tank(s) and cooler(s). The alternators were removed and (I was told) the end of the cranks modified to engage the 4- and 5-speed manual trannies they had (5 cars total), manually shifted. The intakes were all fabricated manifolds with various carbs on them, mostly 2-bbl Carters IIRC. The thing that was amazing to me at the time: they ALL ran points! The points had 2 of the flat springs on them, hand-built by just one of the racing club's members, and the opening ramp of the spark advancers was flatter to prevent severe points wear. One had a hand-built transistorized ignition (not the 16k RPM car) that was triggered by the points (probably similar to the ones I make now), but he often had a misfire around 3000-4000 RPM that could be heard when driving slowly.

Those guys were all farmers, and geniuses IMHO. The midget cars were their hobby after the fields were planted, and they were the "Honda guys" who were against the "Offy guys", the latter whom ran some sort of Offenhauser engines. I never got to look at those (wasn't really interested, to be honest about it...). Their engines were running up to 10K RPM, and they had started the "club" years before. They were NOT happy about "the new Honda guys" and they positively hated the the "Kawasaki guy" who had one car with a Kawi 500cc triple (plus unknown mods) in it. They were forever calling insults at him for the smoke, but the Honda guys thought it helped with the mosquitos, so they didn't give him a hard time at the meets, or at the local bars. He wasn't competitive in the races, but had a good time with it from what I could tell. I heard some years later that the Offy guys forced new rules against the 2-strokers to get him/them off "their" tracks, though, after I heard he went to a 750cc triple after I left the area. Some people's kids...

[newday777]

I would have loved being there to see them.

[HondaMan]

I still wish I knew what rods they were using? One of them was once talking to another of them within my earshot, and he was talking about 'lightening his crank some more' for the next year's mods. This got me wondering about the crankshafts, too, as while they are strong, they ARE heavy for the engine size, and I have seen them lightened for bike roadracing, but those bikes were not doing anything like the RPM these midget cars were. The highest typical RPM they would do was like 10k, if the carbs were bigger. The Carter carbs I recognized on these cars were in the 500-ish CFM range (Ford small block engine types) and their manifolds fed 2 cylinders per bore (250 CFM possible, 125 CFM per cylinder?). So, they had a 45-ish cubic inch engine being fed with 500 CFM possible, holy cow!

But then again, there were no primary chains to drag along, nor constant-mesh gearbox mass, so maybe I [still] don't understand how their magic builds went. No alternators except on one of them (that was the 'slow' guy who could 'only' do 12k RPM), running total-loss on battery for each race. Fun to watch run, though!

[willbird]

Caution, old guy story,
 My racing friend bought into the Hot Rod Mag article about using twin Chinese made turbos on a junkyard LS motor. He learned they will make a thousand HP. but just a time or two.  It's the same as spinning extreme RPM's, they might but not for a long time.

Hot Rod magazine did a series they were calling the "big bang motor"...they took a "5.3" stock bottom end with added ring gap on the dyno and added more and more boost. They got to 1200HP and held that level for a decent number of runs. The engine is still alive and running at Westec. later they discovered it was a 4.8 not a 5.3. They stopped at 30 lbs of boost because they only had a 2 bar map sensor.

https://www.motortrend.com/how-to/hrdp-1109-stock-gm-ls-engine-big-bang-theory/

After eclipsing 1,200 hp, we kept going. Raising the boost beyond 27 psi resulted in what we suspect was an ignition failure. The motor was still in perfect shape internally and worked well up to 26 psi, but unfortunately we were out of available dyno time. The motor survived more than 60 runs, the majority of them more than 1,000 hp. We hammered this motor like we stole it, with repeated back-to-back pulls, but the little LS never missed a beat and was ready for more.

[newday777]

Caution, old guy story,
 My racing friend bought into the Hot Rod Mag article about using twin Chinese made turbos on a junkyard LS motor. He learned they will make a thousand HP. but just a time or two.  It's the same as spinning extreme RPM's, they might but not for a long time.

Hot Rod magazine did a series they were calling the "big bang motor"...they took a "5.3" stock bottom end with added ring gap on the dyno and added more and more boost. They got to 1200HP and held that level for a decent number of runs. The engine is still alive and running at Westec. later they discovered it was a 4.8 not a 5.3. They stopped at 30 lbs of boost because they only had a 2 bar map sensor.

https://www.motortrend.com/how-to/hrdp-1109-stock-gm-ls-engine-big-bang-theory/

After eclipsing 1,200 hp, we kept going. Raising the boost beyond 27 psi resulted in what we suspect was an ignition failure. The motor was still in perfect shape internally and worked well up to 26 psi, but unfortunately we were out of available dyno time. The motor survived more than 60 runs, the majority of them more than 1,000 hp. We hammered this motor like we stole it, with repeated back-to-back pulls, but the little LS never missed a beat and was ready for more.

Nothing to do with the CB750 question at hand.....

[willbird]

I still wish I knew what rods they were using? One of them was once talking to another of them within my earshot, and he was talking about 'lightening his crank some more' for the next year's mods. This got me wondering about the crankshafts, too, as while they are strong, they ARE heavy for the engine size, and I have seen them lightened for bike roadracing, but those bikes were not doing anything like the RPM these midget cars were. The highest typical RPM they would do was like 10k, if the carbs were bigger. The Carter carbs I recognized on these cars were in the 500-ish CFM range (Ford small block engine types) and their manifolds fed 2 cylinders per bore (250 CFM possible, 125 CFM per cylinder?). So, they had a 45-ish cubic inch engine being fed with 500 CFM possible, holy cow!

But then again, there were no primary chains to drag along, nor constant-mesh gearbox mass, so maybe I [still] don't understand how their magic builds went. No alternators except on one of them (that was the 'slow' guy who could 'only' do 12k RPM), running total-loss on battery for each race. Fun to watch run, though!

Oval track type cars which includes Sprint cars IMHO kinda get away with some stuff because it is a series of 1/8 mile or less drag races run back to back. From what the RKI say boat engines may put the heaviest load on a high performance engine, if the boat is setup right it is a dyno that is holding the engine right at peak horsepower for a really long duration...longer than Bonneville probably typically.