Topic: Project CB750 Hemi

[HondaMan]

Ever wonder what a full hemi head could do for the CB750? Me, too.   

I semi-hemi-ed mine in 1974, after I quit roadracing, because I wanted some more low-end pull for touring 2-up. It made a noticeable difference, so much so that retuning was required. I posted that picture here a few weeks ago.

Now, I'm going for the full hemi, like a Kawasaki (can I say that here without starting a fight?) 900 engine. Here's some pix.

The first shows the stock CB750K2 head, HCP chamber and shrouding. Compression ratio is 9.0:1. You can see severe valve shrouding issues at the center and near the outside edge of the intake valve. The exhaust isn't much better off. This really reduces breathing in the low-end, which is where I want more this time around. The changes will increase "blow-past", which is the unburned fuel that sneaks out the exhaust valve during overlap, so I will not be advancing cam timing to go with it, lest I end up with blued pipes from the heat. If you leave that ridge in the middle intact on yours, this won't be so severe.

The second pix shows the areas that are being reduced.

The third shows the type of tool you should use: dip it in beeswax or Chapstick to reduce bit clogging.

The fourth shows the most important part: polishing. This must be done to prevent hot spots of carbon from collecting in the divots you will probably make in these soft aluminum heads. Start with 100 grit, work down to 150 and then 200 or finer. It takes a lot of Coke to cool you down during this part, so set aside some time here.

When reshaped, the compression ratio will be too low, on the order of 8:1. So, the head will have to be milled. I'll re-post here later with how much milling is needed. I intend to end up at 8.8:1 to 9.0:1, with each head CC-ed to a new spec, which I'll also post here. I want to be able to run on mid-range gas instead of premium this time, 'cuz it's gonna be my working ride.



Let's see how it goes!

UPDATE: 7/4/06.
It's running! Zowie, did this chamber change the volumetric efficiency (as hemis often do)! The first problem: in 15 minutes of running, all 4 plugs are soot black! This, with 105 mains where 115 used to be, stock in 1972. Carbs' float levels are all OK, and it idles fine, but as soon as the needle jet starts working, it launches like a dragster! (That's the over-richness kicking in, though...). What a hoot! Gotta go work on the carbs - anyone got some 90 or 95 jets? I could use 4... 

[HondaMan]

I couldn't post all in one post, so here's the other pix. This is the cutting-sanding part.

[cben750f0]

just a question, how much difference would this make, i dont know alot about the intrinsic links of fuel, flame and air, but this is very interesting..... you wouldnt mind explaining what you are hopeing to acheive.... i probably wont understand, but i think , me and the internet could figure it out:P... thanks mate...peace

[mrbreeze]

Right On CBEN........

[mutters]

Mate , I (we all I suspect ) really appreciate the work being put in here,with the photo's and all .

 Thanks mate 

[WHALEMAN]

Hondaman, Do you really think you and rotozip can improve upon the Honda R&D engineering staff? If you are just playing around that is great, but to really expect better flow,swirl and performance is asking a lot.

[Tim.]

Hondaman, Do you really think you and rotozip can improve upon the Honda R&D engineering staff? If you are just playing around that is great, but to really expect better flow,swirl and performance is asking a lot.

Yeah, I guess all those guys porting heads all these years had NO idea what they were doing and never realized any benefit. 

[cb650]

Hondaman, Do you really think you and rotozip can improve upon the Honda R&D engineering staff? If you are just playing around that is great, but to really expect better flow,swirl and performance is asking a lot.

Yeah, I guess all those guys porting heads all these years had NO idea what they were doing and never realized any benefit. 

I want to start a new religion that worships the honda engineers.   If they did it its gospel!



                              Terry

[WHALEMAN]

I knew I would get in trouble. I think you have to wreck a lot of heads to find out what really works. Porting and chamber design is more art than science. What looks good ie.shiny polished intake tracs are not what really works. I have a degree in Mechanical Engineering and the study of flow in a pipe is more complicated than it looks. I remember asking my internal combustion professor in 1971 how to modify the header of my Yamaha 305 2 stroke to get more power and he laughed. The factorys build a lot of heads changing just one aspect and graph the results. After a couple hundred heads they start to see what works. The odds of hitting a better head design the first time is slim. Just my opinion. Also, just building a head for full throttle is easier than one that flows well at all speeds.

[Jerry Rxman Griffin aka MuthaF'er]

Hondaman, Do you really think you and rotozip can improve upon the Honda R&D engineering staff? If you are just playing around that is great, but to really expect better flow,swirl and performance is asking a lot.

One, you have to know what you are doing.
Two, Hondaman has the bio to back it up
Three, Honda made a somewhat crude MASS PRODUCED head in the day
Fourth, The stock head is not the hand built R & D race head
Fifth, Honda R & D built more reliability rather than high performance into stock heads
Sixth, Building a good head is not asking a lot. It's putting one finishing touch on a basic engine. Can be done and will make a noticable difference. Not for the faint of heart who would ruin a perfectly good head.

Jerry
 

[dakeddie]

Has anyone read David Vizard's "How to tune an A-Series engine"?  You'd probably only ever have read it if you owned and Austin Mini.  Anyways, it's an excellent book on engine modification and much of what he talks about can be applied to any engine. 

Many of the head modifications he talks about make a lot of sense, like reducing the shrouding of valves, but every single modification he makes is backed up by flow bench tests and dyno'd engines so you know quantitatively how much of a difference they make.  I know that the SOHC4 is by far more advanced than the ancient A-series in a mini, so mods will probably make a less of a difference, but if Hondaman has tried this mod and was able to feel the difference, then I'm sure it's a good one.

Hondaman, thanks for posting your mods.  I think they're great and I'm hoping to make to mods to my head when I rebuild my engine this winter.  I've seen slightly larger intake (+1.5mm) and exhaust valves (+0.5mm) are available on www.cbrzone.com and I'm wondering if they are worthwhile modifying the head to use...  or is it just a racetrack kind of mod?

[bill440cars]

                 Those Honda engineers built products to sell with some decent reliability and
         performance. Guys with experience in building engines, to perform, do so with good results.
         All kinds of engines have been modified for more years than some of our members have been
         on this earth. People like HondaMan have the experience to do some modifications that will
         benefit performance. 2 stroke porting was mentioned previously in this tread. My Dad took an
         old 50' Jawa 250 motor, ported it and modified it to perform. I took that 50' Jawa and was
         blowing the wheels off of the Honda CL72 250 Scramblers with no problem. Before that Jawa
         was modified, you could probably take a smaller bike and put the Jawa away. The old 50's &
         60's Jawas were slow but dependable bikes. Guys like HondaMan are the ones who can
         provide us with alot of technical info when we ask. Just listen to them and learn. I always find
         it interesting when one of these guys brings up a subject with detailed info. There are a lot
         of guys on this site with gobs of info on all kinds of things. Listen and you might be surprised
         but, you'll also learn why this is the best site going.

        HondaMan,     Keep up the good work, we're (at least most of us are) listening and wanting
        to learn.                              Later on, Bill

[cben750f0]

i have a head at home with a buggered spark plug thread, i am getting a solid insert put in to replace the thread, i dont trust helicoils, ... the head has already had on in it and it actually screwed itself into the combustion chamber!!!!, which then let the gasses past and stuffed the head more, it is an F head, so was really interested in what was/is being done, how, and what are the basic tool/philosophies behind the combustion chamber shape, and the material removed... BTW i am a fitter and machinist by trade, and have worked many years in plant and milling equipment, so have a better and good idea around a tool box..... hit me with the technical stuff, and iwill try to put it into a practical application... thanks in advance hondaman....peace

[Tim.]

I knew I would get in trouble. I think you have to wreck a lot of heads to find out what really works. Porting and chamber design is more art than science. What looks good ie.shiny polished intake tracs are not what really works. I have a degree in Mechanical Engineering and the study of flow in a pipe is more complicated than it looks. I remember asking my internal combustion professor in 1971 how to modify the header of my Yamaha 305 2 stroke to get more power and he laughed. The factorys build a lot of heads changing just one aspect and graph the results. After a couple hundred heads they start to see what works. The odds of hitting a better head design the first time is slim. Just my opinion. Also, just building a head for full throttle is easier than one that flows well at all speeds.

Well now if you'd said all that in the first place, I would have kept my smart-ass comments to myself! 

[MRieck]

  I've seen slightly larger intake (+1.5mm) and exhaust valves (+0.5mm) are available on www.cbrzone.com and I'm wondering if they are worthwhile modifying the head to use...  or is it just a racetrack kind of mod?

They work very well on the street with other mods.

[bguerr]

Hondaman -
sounds like a wicked project, good luck, i'd love to hear how it turns out

[HondaMan]

Here's my intent: I have lost most of my old razor-edge roadrace reflexes, and praise God I'm smart enough to know that. So, I am rebuilding this ol' roadracer into an around-town torquer that can still smoke the highway if I need to.

The hemi head will increase torque in the lower RPM ranges. In town, this will be valuable, as you all know. However, it will introduce some tuning issues at about 6500-7500 RPM, because the overlap action will change. That part should be fun, like the old days, to figure out again. Overall, though, the breathing should be considerably better on the intake stroke because the shrouding will be gone (and somewhat so on the exit). We'll see what happens.   

[chung]

The Chambers on these motors suck.

But it is already a real hemi head. IMHO
Bigassedpistons help with the compression loss.

[MRieck]

 Rc Engineering and Branch did the same modification to all the CB heads they ported (and that goes back about 35 years).

[dusterdude]

every racer knows the hemi head is the ultimate in combustion chamber design

[Greg]

let me ask a dumb question --
what is "shrouding" in this context?
thanks!
greg

[MRieck]

every racer knows the hemi head is the ultimate in combustion chamber design

Mark...it was the ultimate in chamber designs back in the day. Now with 4 valve designs and narrow valves angles it is not. Most new stuff is 22 to 25 degree included angles with small chambers and dished pistons. The smaller the chamber and the flatter the piston the faster the burn and you can increase compression without resorting to piston domes. The shallow angle also allows the use of larger valves and higher lift/increased duration cams without worrying about valve interference.

[dusterdude]

well ill be

[MRieck]

well ill be

You are...therefore you be.

[volz1fsu]

I'll have to go with WHALEMAN on this one.  Porting and Polishing a head will always help somewhat but modifying the combustion chamber is another.  You can do a lot more to decrease the effectiveness of the design than you can to improve it.  There is a lot of R&D on those stock cumbustion chambers already to say the least.  Honda took a lot of time and many different designs to get it right.  Maybe if I had an engine dyno, an unlimited supply of gaskets, heads, and time; I might try it.  Otherwise you might be kicking yourself in the head after the damage is done,  Just to put it lightly.  Go for the more simple ways to make power first: Porting, Polishing, bigger carbs, higher compression, Timing adjustments......