Topic: Project CB750 Hemi

[kayaker43]

Just for refrence,... I just bought a CB650 head off ebay and noticed the chambers have a perfect Hemi shape with only the tiniest ridge around each valve about 1/8 from the seats. Its so slight I'm thinking its not intentional and may smooth it out?

[HondaMan]

I think the CB650 head was made that way to improve flow.

[MRieck]

Just for refrence,... I just bought a CB650 head off ebay and noticed the chambers have a perfect Hemi shape with only the tiniest ridge around each valve about 1/8 from the seats. Its so slight I'm thinking its not intentional and may smooth it out?

K7 & 8 and F2 heads are the same in regard to chamber shape.

[Pinhead]

So it seems that the CB650 head is shaped like HondaMan did his? If so, why aren't there any aftermarket parts for the 650?

[Jerry Rxman Griffin aka MuthaF'er]

Just for refrence,... I just bought a CB650 head off ebay and noticed the chambers have a perfect Hemi shape with only the tiniest ridge around each valve about 1/8 from the seats. Its so slight I'm thinking its not intentional and may smooth it out?

K7 & 8 and F2 heads are the same in regard to chamber shape.

Hey Mike,

Add the F0 & F1 to the K7 & K8 & F2 list. I just took my K8 top end off. The head has -392 (F0/F1) cast into it and K8 hand stamped next to that. It is also hemi shaped.

Jerry

[bill440cars]

So it seems that the CB650 head is shaped like HondaMan did his? If so, why aren't there any aftermarket parts for the 650?

 
                The CB650 had a four year run. It was similar to the 500/550 but yet, different. Just   
         because the head is shaped like HondaMan did his, has nothing to do with wheather aftermarket
         parts are made or not. There are a lot of reasons to produce aftermarket parts and apparently
         they just weren't there. It also doesn't mean that the bike wouldn't have been worth the effort
         either. It just wasn't as popular as the others.

                                                            Later on, Bill

[Pinhead]

So it seems that the CB650 head is shaped like HondaMan did his? If so, why aren't there any aftermarket parts for the 650?

 
                The CB650 had a four year run. It was similar to the 500/550 but yet, different. Just   
         because the head is shaped like HondaMan did his, has nothing to do with wheather aftermarket
         parts are made or not. There are a lot of reasons to produce aftermarket parts and apparently
         they just weren't there. It also doesn't mean that the bike wouldn't have been worth the effort
         either. It just wasn't as popular as the others.

                                                            Later on, Bill

I realize that the combustion chamber has no direct correlation to aftermarket parts. But also I'd think that since Hondaman's mod added significant horsepower, and the 650 head was "like that from the factory" there would be less work to be done for the horsepower. Sure, it has less displacement, but that's what big bore kits and stroker kits and hot cams are for. They just weren't made for the 650, and that seems strange to me. Besides the short 4-year life of the bike, I simply can't see why they weren't more popular in the aftermarket. Perhaps there are some downfalls that I'm not aware of...

I may have gone a bit off topic, but it seems that this topic is about combustion chamber design, and Hondaman's basically mimicing a 650 combustion chamber on a 750. Correct me if I'm wrong.

[bill440cars]

                                  Well, I believe popularity has something to do with it too and, I believe that,
               if there was thought to be a market for those parts they would have been there. Don't
               get me wrong I think there should be some. I have thought that there would have been
               some produced like cams, big bore kits, exhausts and ignition parts at least. And there may
               have been some back then, I don't know. I do know that I'm interested to learn what
               parts can be mixed between the 500/550 & 650 and that's why I'm keeping up with the
               thread that's about them. Only thing I can figure (as far as the head design goes) is Honda
               just didn't have the intentions of the 650 being anything more than something to get
               around on and maybe to try and carry the SOHC design a little further, who knows? Unless
               someone has some facts, all we can do is guess, right? Just my 2 cents worth, not trying
               to ryle anyone in doing so. So, if I did rattle you some, sorry bout that.  Later on, Bill

[Pinhead]

There were definately more 750's than 650's and I can see why there's a plethora of parts for the 750 models. However, after seeing the mods that are done in this post regarding the 750 combustion chamber, and reading some info on the 650, It seems that the 650 was technically ahead of most 750's. However, the unavailabilty of aftermarket parts seems to suggest otherwise.

I'm not trying to start an argument of 650 vs 750, I'm just trying to find technical aspects of the differences between the two and ultimately combine the best of both.

[Ibsen]

I think it had to do with bad timing. If Honda had launched the 650 a few years earlier, at the same time as Kawasaki launched their 650, things might have been different. But when they put the 650 on the market, nobody else used a SOCH engine anymore, and they also usead a strange mix of parts, that also was used on other models, when they put it together. The only real upgrade done to the engine during the periode between 1978 and 1983 was when it got CV carbs. And the chassi upgrade done on the Nighthawk never got very populare in Europe.

And Bill, I have had a 50's Jawa as well. A 1952 350 twin.

[MRieck]

I think it had to do with bad timing. If Honda had launched the 650 a few years earlier, at the same time as Kawasaki launched their 650, things might have been different. But when they put the 650 on the market, nobody else used a SOCH engine anymore, and they also usead a strange mix of parts, that also was used on other models, when they put it together. The only real upgrade done to the engine during the periode between 1978 and 1983 was when it got CV carbs. And the chassi upgrade done on the Nighthawk never got very populare in Europe.

And Bill, I have had a 50's Jawa as well. A 1952 350 twin.

The Kawasaki 650 was light years ahead of the Honda 650 especially in regard to valve train design. I loved the 1978 650SR...Siamese pipes, 5.10 X 16 rear tire etc, beautiful tank etc. It screwed too.

[Sam Green Racing]

[quote  Sure, it has less displacement, but that's what big bore kits and stroker kits and hot cams are for. They just weren't made for the 650, and that seems strange to me. Besides the short 4-year life of the bike, I simply can't see why they weren't more popular in the aftermarket. Perhaps there are some downfalls that I'm not aware of...

Pinhead, aftermarket tuning parts for bikes (not just Honda) in the 650 category fell out of favour just before Honda started to produce the SOHC4 range of bikes.

The next class in racing after the 500s was the unlimited class, mainley 650s, the odd 750 and the big 1000cc bikes.

When the 750 class was up and running the 1000s became the unlimited class and the 650s fell by the wayside.

When the Honda SOHC4s came along the 750 class was up and running so the tuning shops turned their attention to these, hence no 650 parts.

Sam.

[bill440cars]

And Bill, I have had a 50's Jawa as well. A 1952 350 twin.

   
           You know, as time goes on, I'm finding that there are several people who have either had an old Jawa or knew of one that a family member or friend had. I've never ridden a 350. I have the remains of a 350 Californian that I'm slowly trying to gather parts for and I'm trying to get enough parts to fix up the 250 Jawa to be like the one my Dad built back in the early 60"s. As you know, Jawas weren't meant to be fast, they were kinda like VW's (slow, dependable and didn't change much through the years). My dad's old 250 wasn't like that. He ported that engine and made other Factory recommended modifications that made it RUN! It would outrun the CL72 Hondas and give the 500 Triumphs fits. I'd better go before I get any more wound up than I already am. Glad to know that there is another ex-Jawa owner though. Later on, Bill

[JerrodR]

I ended up on this thread because I just changed the Head gasket on my 74, and I thought that the combination chamber's looked awfully Hemispherical, So I asked Google and it dropped me off here. I understand theres more to it then just shape, but how close are these 750 Heads to the Legendary Chrysler Heads?

[JerrodR]

And I also wanted to say, that yes Honda knows what there doing, be we have learned alot since the 70's. 2nd,  They weren't exactly building performance parts, they have alot to consider, sound, fuel economy, vibration exc, The CB-750's factory airbox that everyone swears by, does infact leave quite a bit of airflow on the table. Alot was done with that piece to monipulate the sound of the bike, canceling out what Honda felt was undesirable. The CB-750's we're not a factory race job, great Bike's, but not race Bike's.

[MRieck]

And I also wanted to say, that yes Honda knows what there doing, be we have learned alot since the 70's. 2nd,  They weren't exactly building performance parts, they have alot to consider, sound, fuel economy, vibration exc, The CB-750's factory airbox that everyone swears by, does infact leave quite a bit of airflow on the table. Alot was done with that piece to monipulate the sound of the bike, canceling out what Honda felt was undesirable. The CB-750's we're not a factory race job, great Bike's, but not race Bike's.

That's true though you can increase NA power about 110% with about 35% increase in in CC's. There's some other things that have to be done  but it is possible and it is streetable.

[HondaMan]

The stock heads on the post-1976 CB750 are close to hemi shape, and the earlier ones can be converted to it (like mine is). The HP in these bikes is not restricted by the airbox, but by the cam and types of carbs (before the PD series came out in 1977). I have personally seen 90 HP versions of this bike with the OEM 1974 airbox attached, with no more mods than what are found in my book and a Megacycle cam, with CB750F3 pistons (4th oversize) in a 1974 K4 engine.

Remember: the engine can only breathe as much air as the size of 1.25 intake valves will allow: also remember that there MUST be 3 pressure zones, atmospheric (bowl), bell (intake) and venturi (sometimes called vacuum) in order of declining pressures, in order to make these pulse-feed carbs work. Forgetting that will leave you with carbs that can only idle or else run above 5000 RPM with correct mixtures, and very fouled plugs in normal street usage.