Topic: cb750f vs k head flow

[themotoworks]

I'm building a 750 motor over the winter and am going to get one of the cyclex 836 kits with the superflow valves, i'm planning on doing some porting based on info from axl's page, and what i want to know is:

do bigger valves and wider ports necessarily flow more air and exhaust where the motor makes power? 

I have both an f and k motor i could build, so i want to pick which one will respond best to the work and make the most torque at 6.5-9k where it'll spend a lot of its time

[Jerry Rxman Griffin aka MuthaF'er]

Which F are you referring to? Would you be a little more precise? We seem to forget there are 2 distinctive F's! The early F head is better than the K0 - K6. The K7 & K8 have the early F head. I have an early F head with larger intake valves (courtesy of MRieck). This could get confusing and you wouldn't get an appropriate answer.

[themotoworks]

didn't know that, it's an F2, vs a K7

[Jerry Rxman Griffin aka MuthaF'er]

Well, in that case..... 

The F2/F3 head has larger intakes and exhaust but is somewhat cursed in that it goes through/went through valve guides much quicker apparently due to more extreme valve angles necessary to keep those larger valves from hitting one another. The head design and cam towers are basically the same except for the chamber. It is said that the larger intake is the improvement while the larger exhaust is not as necessary. Honda did one last hurrah to try to get those few extra horsepower (5 maybe) out of it's design before introducing their double overhead cam engine. The K7 head you have is a -392 head ie it is originally from the F/F1 (392 is the middle component of the part numbers for the F/F1 changes) with an improved chamber design over the earlier K models. So YES, the F2/F3 head will flow more air but it too can be improved with porting work and replacement of the valve guides with more modern metal like what Big Jay at APE can do for you. Now if you wanted flow and reliability send the K7 to MRieck along with a big bag of cash and he can install 33.5mm intakes while leaving the exhaust at 28mm and port it. If you send a second bag of cash he can install special 34mm intakes with smaller lighter valve stems, ditto for springs which are even stronger, ditto for retainers, and 5mm stem exhaust. I really haven't looked into CycleX's Super Flow valves but if they are stock size that won't help much. If they are stainless they will be stronger and less likely to break. At the least go with stronger springs! (from someone that has obliterated a perfectly good F0 engine due to a missed 1 to 2 shift without letting off the throttle). Keep in mind if you use the F2 head you MUST use matching pistons for the F2/F3 head and the F2/F3 cylinders (due to matching oil return holes). While you could theoretically use K/F0/F1 pistons the chamber is higher and K pistons or F/F1 pistons will result in a DECREASE in compression which is counter productive.

Those are some thoughts to consider. The basic F2 will produce more than the K7 because of the larger valves but you gotta mix and match the right way. I think you'd notice the difference in the higher rpms due to the increased air flow. If you 836 the F2 just make sure you are using the correct 836 for the F2 and do NOT use a 836 for a K/F0/F1 engine. This isn't to say you can't make the K7 out perform the F2! Clear as mud now?     

[Retro Rocket]

Which F are you referring to? Would you be a little more precise? We seem to forget there are 2 distinctive F's! The early F head is better than the K0 - K6. The K7 & K8 have the early F head. I have an early F head with larger intake valves (courtesy of MRieck). This could get confusing and you wouldn't get an appropriate answer.

Sorry for the hijack guys but, Jerry, can you elaborate on why the early "F" head is good because i have one and would love to know....

[Terry in Australia]

I think Jerry's dreaming Mick, the FO, (which apparently stands for fcuking 'Orrible, ha ha, I never get sick of telling that joke..........) ran slightly higher compression pistons and a slightly higher lift cam than it's K6 brother, and that's what gave it the tiny HP improvement. (which it then lost again with it's increased weight)

In comparison, the F2/3 was a much better bike all round, and while it had it's mechanical faults, the parts are readily available to build a nice fast reliable black engine, and the finished bike will not only be more attractive than the fat maggot that the K7/8 is, but it will handle and brake much better too. Cheers, Terry. 

[Retro Rocket]

Yhanks Terry, i thought i was missing something, Thanks for clearing that up...
I am under the distinct impression that you aren't real keen on the K7-8's mate.....

[themotoworks]

thanks, i don't care too much about the f chassis, pretty much an engine to engine comparison, my plan is to build a foale type single backbone frame with a cbr f4 front end, monoshock rear, modern rims and tires, and alloy seat and tank, target weight for the bike is around 350lb dry, so that'll help, but i want to get the most out of the engine too

[MidnightLamp]

thanks, i don't care too much about the f chassis, pretty much an engine to engine comparison, my plan is to build a foale type single backbone frame with a cbr f4 front end, monoshock rear, modern rims and tires, and alloy seat and tank, target weight for the bike is around 350lb dry, so that'll help, but i want to get the most out of the engine too

If I can offer a suggestion on the front end, try to avoid the later frames with modern front ends. The increased rake of the later frames makes the trail a little excessive (above 5") using stock geometry. On my test runs so far with a modern front end (32mm offset triples), I've found that a good combination is to use a ~25.5" rolling diameter front wheel with  a 1 degree overall tilt forward to the bike, with a 1.5" swing-arm extension to pull the CG back slightly to accommodate. On an early K frame, this results in about a static trail of 4.8" and feels very nice on the streets (steering is a good compromise between stiff and stable). Also, watch out for fork length. Overall fork length means very little, as the modern top trees are usually flat, and therefore effectively pull the forks up higher than stock triples. Feel free to pm if you want some more info =]

As for engines, if I can offer any advice, I'd stick to the later castings on the crankcases and heads, and do an up-sized intake setup. The cycle X valves have thinned valve stems  in the port area, are available with oversize intakes, and are very light. I'd throw those on a later casting (but not an F2/F3 head) and get it ported and cut for a squish band. With the later F stuff, you run the issue that you only have one size of big bore without going wonky with the compression, and the price is higher than an equivalent K big bore from most places.

Just my $0.02 anyways... =]

[Jerry Rxman Griffin aka MuthaF'er]

Which F are you referring to? Would you be a little more precise? We seem to forget there are 2 distinctive F's! The early F head is better than the K0 - K6. The K7 & K8 have the early F head. I have an early F head with larger intake valves (courtesy of MRieck). This could get confusing and you wouldn't get an appropriate answer.

Sorry for the hijack guys but, Jerry, can you elaborate on why the early "F" head is good because i have one and would love to know....

Chamber design is cleaner without the "ridges". More hemi look to it. The chamber could very well be the only reason for the 0.2 bump in compression. Reason I say this is I have 2 distinctly different sets of -392 pistons that have different style domes. Both have "392" cast on them near the wrist pin opening. Either this or Honda lied about the compression of the 75 F and the 78 K both being 9.2. One from my 75 F and 1 from a 78 K. The only way to tell for sure would be to cc the chambers which I have not done. 

Now if Terry were correct in his rationale for compression by not including the chamber/head why would Honda have changed the chamber design?!   

[themotoworks]

cool, my theory is that at high rpms, where waves play a big part, bigger valves may not be better, but higher lift and a really finessed exhaust and intake tract is key.  I've got two k7 heads to work with, so i'll probably go that route, and build the frame so that disassembling the top end in the frame is possible