Topic: CB750 max RPM?

[Jinxracing]

I remember reading a post or two where stock redlines were mentioned in passing, but couldn't fine a thread about it.

I've got a no-number "replaced case" CB750 engine in my project bike with an early head casting (based on photos and info in Duckworth's book). Because of that I'm assuming that it's a "K" and not an "F" engine. It turns over smoothly and I don't have any plans to dig into it unless something goes blooey when I try to start it. On the other hand, it did come out of a '70s-era chopper so a part of me is dying to find out if there are any internal goodies in there.

So here's my question...what RPM am I safe running this thing to? I know that max safe engine RPM isn't an exact thing, especially when the motor is of unknown origin and wear, but assuming that the motor is sound (and stock), what should I be aiming for?

Thanks!

[mlinder]

I ran my K0 at 10k frequently. It's supposed redline was, what,  8.5? 9.5 is safe, in my opinion. 10 is safe in my opinion, but I guess 9.5 is a safer safe.

[754]

Safer safe..LMFAO... :

 If it is stock not much will happen after 8500, but if cammed you will know it, it will want to go, rather than Yawn..

[mlinder]

Safer safe..LMFAO... :

 If it is stock not much will happen after 8500, but if cammed you will know it, it will want to go, rather than Yawn..

754, my (far as i know) stock K0 pulled well above 8500. I dunno bout later models.

[steam-powered man]

redline on my 78750 starts at 8500.  have never gone past 7k 'cause of unknown camchain age.  new camchain/upper gaskets planned over during winter refurb. 
bobp     

[andy750]

mlinder didnt you just blow your engine up?  Could it have been something to do with running it at 10k all the time? 

cheers
Andy

[mlinder]

mlinder didnt you just blow your engine up?  Could it have been something to do with running it at 10k all the time? 

cheers
Andy

No, it's from running it at 13k plus. 10k was fine.

[BobbyR]

Well let's see. You have a motor with unknown mileage, no history as far a maintenace and oil changes. No history on how it was ridden before. I would say run it conservatively using only the RPMs you really need. If you don't mind rebuilding it, run it as hard as you want. I am an old fart and I now enjoy riding more than wrenching. 

[Sam Green Racing]

I remember reading a post or two where stock redlines were mentioned in passing, but couldn't fine a thread about it.

I've got a no-number "replaced case" CB750 engine in my project bike with an early head casting (based on photos and info in Duckworth's book). Because of that I'm assuming that it's a "K" and not an "F" engine. It turns over smoothly and I don't have any plans to dig into it unless something goes blooey when I try to start it. On the other hand, it did come out of a '70s-era chopper so a part of me is dying to find out if there are any internal goodies in there.

So here's my question...what RPM am I safe running this thing to? I know that max safe engine RPM isn't an exact thing, especially when the motor is of unknown origin and wear, but assuming that the motor is sound (and stock), what should I be aiming for?

Thanks!

8,500.

Sam.

[mlinder]

Hey Sam, the velocity stacks made it here. Sending you money tomorrow.

[TwoTired]

The Cb750F in 77-78 had a 9500 red line.

Yeah, I know, who cares.

Cheers,

[Sam Green Racing]

Hey Sam, the velocity stacks made it here. Sending you money tomorrow.

No problem Mark, did you like them?

Sam.

[fuzzybutt]

the stock 75 750f motor (with a .75mm overbore) i'm running now wants to pull like a freight train even over 8000 rpms, i try to keep it to 9 though i sometimes hit 10, but not often. only mods are the overbore, dyna s with 3 ohm coils and a kerker pipe and pod filters.

[Sam Green Racing]

the stock 75 750f motor (with a .75mm overbore) i'm running now wants to pull like a freight train even over 8000 rpms, i try to keep it to 9 though i sometimes hit 10, but not often. only mods are the overbore, dyna s with 3 ohm coils and a kerker pipe and pod filters.

OK, as Lloyd said, 9,500 for the Fs.  If you think about changing at  8,500, by the time you have done it you should be at 9,500rpm.

I've told Chris to do this but he still misses it at times, the pick up after 7,000 is unreal and you can soon see 11,000 on the tach.

Sam.

[Terry in Australia]

I run my newly rebuilt stock K2 engine (in my K1 bike) to 10,000 RPM quite frequently, when I rebuilt it I fitted some RC heavy duty studs, new HD cam chain etc, and so far, so good.

I've always been told that if you want to rev a Honda SOHC engine hard, make sure you use HD studs or you'll blow head gaskets.

Like has been said before, there's probably no gains to be made at those kinda RPM's from a stock engine though, I just wanted to see if it'd do it. Cheers, Terry.

[andy750]

I agree Terry! I ran my Rieck-built engine (top end rebuild) with stock studs and blew the head gasket at high RPM - must have been when I was seeing how hard it could pull  Pulling the engine in a few weeks (once gets too cold to ride) and have some HD studs put in this time.....fingers crossed for a leak free engine in the Spring!

cheers
Andy

[Terry in Australia]

You'll be fine Andy, and I torque my head down to 25 ft lbs with those HD studs, let it sit over night, loosen 'em off and re-torque 'em again, and the only engine leaks I should have is when one of my stock rods punches a hole in the crankcase, ha ha! (OOPS! )

[Sam Green Racing]

One thing I can't understand is Chinese writing, another thing is why people are obsessed with wanting to rev the bollocks out of motors in the areas that there is no power available.
Unless you have a cam and an engine set up that moves the motors abillity to make power at higher revs, then you are just waisting your time screwing it.
If Mr Honda says your motor makes power at 8,500 revs, then thats where it will be making it.
Allthough you will be going faster with the high revs, you will get there quicker if you change up at 8,500rpm.
Why put your motor under stress when you can just change gear ?

Sam.

[754]

Exactly.. it can rev higher, but there is no gain..

[Sam Green Racing]

But people still run the risk of having ventilated crankcases.

Sam.

[MRieck]

10,000 is when the Fat Lady sings unless you have super long duration cams etc.

[paulages]

paging mlinder: how are you gonna pull off 12K RPM's?

[mlinder]

Why, headwork and cam. How else?

[paulages]

Why, headwork and cam. How else?

  i figured that much.. just with all this talk about redline and power thresholds, i'm curious how you're gonna keep making power at those RPM's. these guys all seem to think you can't get much above 10K, unless i'm understanding wrong, which is why i'm curious about the details. strongs rods & bolts are obvious as well as heavy duty everything you can manage, but i'm wondering where the limitations really are concerning breathing, and where the weak link eventually is (2 valves? single cam?) i'd sure as hell love to see it though.

[mlinder]

It's my opinion that most people who increase the performance of their CB750's do so by changing the basic characterisitics of the cb750 sohc engine.
I think these changes that are made lessen the ability to make power over 10k.
I'm not saying the way I'm looking to do this is right, and other ways are wrong. Just different.
Power is Work over Time. You can increase the amount of work per cycle capable, or you can change the number of cycles in a given amount of time. I'm choosing to try the second option.
I'm hoping that by not actually changing this  engines characterisitics, but enhancing them, I will continue to make power over 10k.
Many people are saying that the engine stops making power at 8.5.
Now, since I have no idea whether my engine is stock or not, I can't say for sure if that's the case. All I can say is that there is absolutely no reason for me to shift at 8.5. I'm still pulling hard in 3rd at 10. I can guarentee that the engine in my bike is making as much power (or more) at 9.5 as at 8.5.
If I'm wrong, I'll get a cam that works for 10k redline rather than 12k redline.
It'll cost me 250 bucks to make the change, in that case. Not a huge deal.