Topic: RE: CB750 Max hp with 736cc or 760cc & ported head

[chewbacca5000]

What is the max HP that I can expect out of a stock cb750 or a cb750 1mm over with a ported head?  I have been very interested in getting increased performance lately from tuning vs higher displacement.

The main goal is a bike that accelerates very quickly.  My K1 engine with an early K0 head is a monster.  I am not sure what the displacement is, but is has breathtaking acceleration which makes it so much fun to ride.

I mean it accelerates like a fuel injected bike at any throttle position, and I want another!  Is that too much to ask?

There is no feathering the throttle or anything.  If you get a handful it lets you know it.

[rotortiller]

Not much.

[seanbarney41]

This is pretty much my engine combo...it was my first porting attempt.  I used hondamans book extensively.  Basically I built a k6 motor to k0 specs including early advancer and carb bits and 17/48 tooth sprockets.  I feel like I gained both mid and high rpm power, but not a mind blowing amount.  Its fun.  Next motor will be 810cc at least.  From your description of your k1, stock bore aint gonna get it.

[bwaller]

There is so much that could be done, just not sure that's what you're asking, but in case. There are +1mm oversize pistons (non stock) that can really bump CR. Then porting can be extensive and a suitable cam to match. Oversize, thin stem valves? You could push HP into the upper half of the 80's. Just add money. 

[slikwilli420]

This is the arrangement I'm looking at for an AHRMA race bike. Thinking ported head, 12:1 compression on stock bore, high rpm cam with a bottom end to support all that.

[scottly]

If you're asking how much HP you will gain on a totally stock bike by porting alone, the answer is about 6-7 HP. The stock bikes produce about 53 HP at the rear wheel, so figure 59-60. The 1mm over-bore is good for maybe a couple of HP.   

[chewbacca5000]

If you're asking how much HP you will gain on a totally stock bike by porting alone, the answer is about 6-7 HP. The stock bikes produce about 53 HP at the rear wheel, so figure 59-60. The 1mm over-bore is good for maybe a couple of HP.

What about in addition to the porting throwing in a megacycyle 125-65

[slikwilli420]

Im guessing stock compression is part of the problem. Most of the bang from an 836 kit comes from the increase in compression. The Cruizin Image kits are 836 but stock comp. and reports are more power over stock bore, but nothing like a true hi-po 836 kit. A nice 10.5:1 piston in a 0.5 overbore would be nice for the street.

[MRieck]

If you're asking how much HP you will gain on a totally stock bike by porting alone, the answer is about 6-7 HP. The stock bikes produce about 53 HP at the rear wheel, so figure 59-60. The 1mm over-bore is good for maybe a couple of HP.

What about in addition to the porting throwing in a megacycyle 125-65

That will make a big difference but you need valve reliefs which can handle that cam. You are looking at a forged 836 kit at that point. that's a nice cam.

[bwaller]

Wiseco's got some 62mm pistons that ought to work Mike? They offer in finished domes.... or unfinished so reliefs can be "adjusted"

[PeWe]

Ported head, higher cr pistons with deeper valve relievs, good cam means at least 80 rwhp. Deeper pockets can add some more over 90.

[Wingnut74]

I just finished a CB750 K4 with +1mm and a new head from Mike complete with valve job and some porting/breathing and although I couldn't tell you the HP, I sure feel like it rips it up pretty good! Also the head from Mike was a work of art - the prettiest part of the engine for sure.

[chewbacca5000]

I just finished a CB750 K4 with +1mm and a new head from Mike complete with valve job and some porting/breathing and although I couldn't tell you the HP, I sure feel like it rips it up pretty good! Also the head from Mike was a work of art - the prettiest part of the engine for sure.

What stage porting are we talking?  Did you do anything to the lower end?

[Tews19]

750 stock is 53, huh. That's interesting. My 605 is at 44hp to the wheel and that was with the carbs all out of whack and jetting not upto snuff.

[chewbacca5000]

750 stock is 53, huh. That's interesting. My 605 is at 44hp to the wheel and that was with the carbs all out of whack and jetting not upto snuff.

The makes a difference and all the other little stuff I guess.

[SOHC4 Cafe Racer Fan]

750 stock is 53, huh. That's interesting. My 605 is at 44hp to the wheel and that was with the carbs all out of whack and jetting not upto snuff.

Tews, Is your 605 a cruzinimage (non-high-compression) kit or a Wiseco? 

[chewbacca5000]

750 stock is 53, huh. That's interesting. My 605 is at 44hp to the wheel and that was with the carbs all out of whack and jetting not upto snuff.

Tews, Is your 605 a cruzinimage (non-high-compression) kit or a Wiseco?

Will high compression give more acceleration?  The main thing I want to get is the ability to pass cars with confidence.  So if I loose some top end I don't care.

What about a lightened crank will that help me accelerate faster at speeds of 50 to 100 mph?

[Tews19]

It's the cuzinimage. My carbs weren't syncd when I did the dyno run. Member Hellraiser commented on the ride. His 650 at the time had more torque but mine revved a lot faster.

[SOHC4 Cafe Racer Fan]

750 stock is 53, huh. That's interesting. My 605 is at 44hp to the wheel and that was with the carbs all out of whack and jetting not upto snuff.

Tews, Is your 605 a cruzinimage (non-high-compression) kit or a Wiseco?

Will high compression give more acceleration?  The main thing I want to get is the ability to pass cars with confidence.  So if I loose some top end I don't care.

What about a lightened crank will that help me accelerate faster at speeds of 50 to 100 mph?

I will defer to the gurus like Mike Rieck, but my experience from my Wiseco 836 kitted bike with a ported head is that I have a substantial torque and power increase that helps launch the bike off the line and with rolling acceleration.  The higher compression kit should give you better gains in hp and torque than a displacement increase alone.  That combined with a ported head and better cam and you get even more improvement.

[PeWe]

Higher compression needed for hotter cam with more overlap leaking pressure. The dynamic compression must be good enough. Thats why hot cams cams specify high cr pistons. Works the other way too, 12.5:1 pistons for really hot cams open 24/7 :-)

[chewbacca5000]

Higher compression needed for hotter cam with more overlap leaking pressure. The dynamic compression must be good enough. Thats why hot cams cams specify high cr pistons. Works the other way too, 12.5:1 pistons for really hot cams open 24/7 :-)

Thanks always wondered about that.  So that is why I would need 10-25-1 for the megacycle 125-65.  Makes sense to me.

[PeWe]

Type of fuel is involved too. High compression need higher octane to avoid pinging. Race bikes do not use std tap fuel. Street/touring bikes have therefore not the higher cr. High compression give a hotter running engine.
My CB750 ran OK with slightly lower CR since the head chambers were opened for RC 836 pistons with wider domes, 10.5:1. Pistons got holes and replaced by Action Fours 836 pistons fir std head chambers. CR lower.
Engine ran really well anyway, less prone to pinging, not same response when twisting the throttle but same top speed. Better for touring when it ran slightly cooler. Cam was a moderate cam
 like webcam 41.

[seanbarney41]

Chewie, it's just like the old adage that still rings true..."Cheap - Fast - Dependable...pick 2 and only 2"

[Duke McDukiedook]

Ported head, higher cr pistons with deeper valve relievs, good cam means at least 80 rwhp. Deeper pockets can add some more over 90.

You better do some frame reinforcing while you are at it if you are going for that kind of HP.

[jonda500]

It's the cuzinimage. My carbs weren't syncd when I did the dyno run. Member Hellraiser commented on the ride. His 650 at the time had more torque but mine revved a lot faster.

Carb syncing will not noticeably improve full throttle performance...
John