Author Topic: 10 deg downdraft ready (Read 12710 times)

turboguzzi · « **on:** March 20, 2007, 06:49:59 AM »

Hi all

The most difficult piece to design and manufacture in my cb500 racer project is ready at last

quite a bit of CAD work was needed to get the twin angles on the short stubs to match the 10 degs and the light convergence

The 10 degree downdraft matches the natural shape of the ports realy nice.

turboguzzi · « **Reply #1 on:** March 20, 2007, 06:51:38 AM »

+ having the carb straight in front of each port (well, almost) cant hurt either ;-)

turboguzzi · « **Reply #2 on:** March 20, 2007, 06:55:43 AM »

last, being able to see the cylinder from the carbs entry means its quite a straight shot.

valve seats are the shinny ring at the bottom of the throat.

going to sleep well tonight

TG

bwaller · « **Reply #3 on:** March 20, 2007, 07:32:00 AM »

Interesting work dude. You'll be painting a black stripe out of the corners with that thing!

peter c · « **Reply #4 on:** March 20, 2007, 11:51:18 AM »

HeYossef,
looks good to me,I'll need to visit Italy to have a head to head on the race circuit with my simple non downdrafts. Got make a difference.

Peter

maksuttt · « **Reply #5 on:** March 20, 2007, 03:12:15 PM »

Nice work. Did you touch ports yet?

turboguzzi · « **Reply #6 on:** March 20, 2007, 03:52:10 PM »

Hei mak

did already all the port entrance as I had to match the bigger manifold tubes and enlarge the holes on the head up to get the downdraft. basically the floor of the port remanined at the same height (save for clearing the casting core ridge) but the height of the port is now up to 27.5 milimeters.

Now that the manifold is sorted I am going deeper. In order to reduce the guess work i did a silicon casting of the port and its quite amazing how bad it is shaped.... at one point, just before the seat diameter goes down to 22 mm! should post pics of the silicon casting soon. it sure clarified where do i need to work and also that Hondaman was spot on in his recomendation to carve a big pocket above the valve.

turboguzzi · « **Reply #7 on:** March 21, 2007, 01:44:36 AM »

theses are 28 mm mikunis. 27.5 at the port is because I built a little bit of taper into the manifold. parts availability seems to be good indeed as basically these have pretty much the same body as 29VM mikunis. looking at the sudco parts catalog, looks like it shouldnt be too hard to convert them into full 29 smoothbores if I'll need.

cheers
TG

turboguzzi · « **Reply #8 on:** March 21, 2007, 05:52:54 AM »

thanks for the input, really useful. Yes I know that mine arent smothbore really, studied quite closely the drawing in the sudco catalog. My idea was to (maybe) remove that shorter block and machine/find a SB jet block, fit it inside and use the cylindrical sleeves. Sudco still sell them the smoothbore sleeves. I still have to consider if its worth going trhough all this trouble and not buy 29 CR's straight away... Dont know how much sudco charges these days for a set but I remeber they were quite expensive. In comparison, my VM's cost 40$, hard to beat that :-)
As you sound quite experienced with them VM's, can you confirm if the reduced jet blocks in mine are indeed removable or will I have to machine them off if I want to do the above?

tnxs
TG

turboguzzi · « **Reply #9 on:** March 23, 2007, 11:52:22 AM »

very informative, thanks. I did noticed the extra air passages but as I am also quite involved in mass production techniques (not carbs though) I had the feeling that mikuni had used the same basic tooling for producing the 27-29 VM mm range, only changing some internal cores according to the desired features. but seems like I am wrong.

Interesting enough, many of the cb500 racers here use the 29 CR's but they seem to me way too big for 125 cc cylinders. mikuni has got a good size against hp graph in their tuning handbook and 28's should provide up to 20 hp per pot. wishful thinking of course! I'll be more than happy with 16 equaling 64, that's more than a good todd henning tuned CB500T.... peter who races a cb500 in the UK got that kind of figure with 28mm F1 carbs, so that gives me hope.
Let's see how these 28 work, with the straighter downdraft manifold I am hoping for good results. Cant wait to strap the thing to the dyno!
if you have some experience with 28 mm VM's in a cb500/550 too, i would be more than interested to hear your jetting /needle recommendations.
cheers
TG

turboguzzi · « **Reply #10 on:** March 23, 2007, 04:30:14 PM »

hi daniel

checked out your site.... very very nice work ! good stuff and obviously, the page full of carbs made my eyes pop.

complimenti!

TG

Kikemon · « **Reply #11 on:** March 23, 2007, 06:54:42 PM »

I'm in way over my head here, but I guess there is no reason not to make an idiot out of myself for the sake of an education... Are these carbs downdrafts instead of sidedrafts just because they are angled 10 degrees downward? I know modern bikes try to get the intake tract as straight as possible to minimize the amount they ask the intake air to turn, but is the performance gain of reducing the turn by just 10 degrees really worth all the effort you put into this? If it is a significant gain, why didn't Honda design it that way to begin with? It doesn't seem like the slight change in angle would affect packaging of the engine/fuel system enough to change their frame design all that much. Thanks, sorry if these questions sound stupid - I'm still catching on here!

MRieck · « **Reply #12 on:** March 23, 2007, 07:33:11 PM »

Quote from: Kikemon on March 23, 2007, 06:54:42 PM

I'm in way over my head here, but I guess there is no reason not to make an idiot out of myself for the sake of an education... Are these carbs downdrafts instead of sidedrafts just because they are angled 10 degrees downward? I know modern bikes try to get the intake tract as straight as possible to minimize the amount they ask the intake air to turn, but is the performance gain of reducing the turn by just 10 degrees really worth all the effort you put into this? If it is a significant gain, why didn't Honda design it that way to begin with? It doesn't seem like the slight change in angle would affect packaging of the engine/fuel system enough to change their frame design all that much. Thanks, sorry if these questions sound stupid - I'm still catching on here!

Good questions. 10 degrees is still very much sidedraft. Frame clearance was, and still is an issue for sidedraft bikes. Straight tubes were cheaper back in the day. Perimeter frames made downdraft easier though GSXR's refused to give up top rails for a long time. Downdraft really didn't work to it's full potential until FI IMO.

LoopsAndLogic · « **Reply #13 on:** March 23, 2007, 08:04:55 PM »

Could someone explain why I can't view the attached photos that turboguzzi posted???

I'm a member and I've already signed in.

Already did a search and I even went back to me Profile setup, but no luck.

turboguzzi · « **Reply #14 on:** March 24, 2007, 08:54:04 AM »

good question but there's a good answer too....

the basic picture of a sectioned head (hope speed racer doesn't kill me) shows the original 90 degree side draft in red. not bad, but I guess that the 10 degree of so called downdraft in blue will flow a little bit better. I was quite surprised how this little change straightened every thing up, especially after grinding out the little green triangle.

the 10 deg difference might not sound like much but when you consider that the air is traveling at around 200-300 mph inside at 10k, that light kink (at the visible core ridge) will create a lot of turbulence, specially on the bottom face of the port as the air will not want to stay close to the surface.
Why was it designed that way? that's a long story, Mrieck is spot on + remember that back in 69-72 these bikes 500-750 were regarded as very powerfull anyway + production techniques (the cb500 was built to a very tight budget) meant that many features couldn't be done economically. then add the contorted S shape of the manifold (viewed from top) and you end up with a motor that runs out of breath at 9-9.5 K. The S shape was required to keep the carbs close and give the CB500 the thin waist line it's famous for.

BTW, just slotted at last the motor in and the carbs required lifting the gas tank a good 1" or so at the back mount. will have to redo the throttle pull cable too as that one DOES hit the frame. tight but solvable. now that everything is in place I'd say that 10 degs is the max you can do with the current frame mainly because of the throttle linkage hitting the frame smack in the middle.
Cheers
TG

MRieck · « **Reply #15 on:** March 24, 2007, 09:09:58 AM »

Nice pic. A lot of people think that moving air is bendable....like plastic. When air is moving at almost supersonic speed it only wants to go straight. Combined with the fact that a valve is only open for a millisecond at about 10,000RPM you want to make access as clean and easy as possible.

Kikemon · « **Reply #16 on:** March 24, 2007, 03:26:10 PM »

Thanks for the great answers to my questions. I really like the cut-away head - it all makes sense. One thing that last photo makes me wonder about is why the intake tract appears to be more constricted than it needs to be. It looks like the constriction where the top part of the tract arcs down to support the valve stem is purposely larger than it needs to be. Were they trying to speed up the airflow there for some reason? Does the valve guide just need a lot of support? Or is it just some economic or casting consideration that came into play there? Thanks again. I feel like I ought to be paying tuition - which I guess I can with a donation to the site!

scunny · « **Reply #17 on:** March 24, 2007, 05:52:06 PM »

in the course of my work, we do a fair bit of air sampling in duct systems, measuring velocity, pressure and volume of moving air. you would be surprised to see the laminar effects a slight change of direction can make. straighter the better.
nice wee project ...

speedracer741 · « **Reply #18 on:** March 24, 2007, 06:30:48 PM »

Keep up the good work! I'm glad the picture could get some good use. Now I don't feel as guilty about cutting a cylinder head in half

I think you engine is going to perform very well. Not only will the downdraft help but the carbs bieng more in line with the ports will also yield improvements.

turboguzzi · « **Reply #19 on:** March 25, 2007, 04:55:22 PM »

yes speed, your pic did help a lot!
kikemoon, the cut was done exactly in the middle of the ridge that supports indeed the valve guide. behind that ridge the port rides much higher albeit as two narrow grooves on each side of the valve guide.

the picture bellow shows a silicon casting i've done of the port, essentially the port in negative and you can clearly see the dip in the middle which is the valve guide support ridge and the two narrow passages around it. I did it to have a better idea as to where should i put attention while porting.

TG

Swiss · « **Reply #20 on:** November 30, 2007, 10:04:20 AM »

Major Kudos to TG on his intake and port work. We got into a discussion on a different list that he was visiting and are now discussing some of the older 500-550 Honda engines. I have some of the older Yoshimura catalogs where Yosh was actively developing and racing the early 500. The idea of the 29 being too big for the engine is really a problem of air flow in the head and manifolds. TG did a good thing in straightening out his intake manifolds AND lifting the port angle by 10 degrees. The factories often begin an engine design outside of a frame limitation and then have to make adjustments to get everything to fit. The budget dollar often gets the final say in the design.
Yoshimura liked running the 31mm Keinin CR carbs and they probably matched the full race designs that Yosh worked with in their porting and hc pistons and race cams. The new Honda 150 single is running a 32mm FCR carb, but it also has a 4-v cylinder head and bore/stroke specs of 66 x 43.7. But understand that this is a dirt bike and not a highly tuned RR bike. Carb size isn't always the entire problem behind loss of botom end. The entire system needs to be balanced in order to make use of the big carbs. that means the ports, cam, and exhaust system needs to be optimized to work together.
Looking at TGs pics of his carbs, there is a small problem with the inlet design. Now, understand that this is a Production design in order to mount a rubber connector between the carb and the air filter box and it has a straight tube at the outer edge of the small diameter bell inside the inlet of the carb. The bell is nearly correct, and it would flow decent if it were cut off right there. But the bell is actually too small for the venturi diameter and a bigger bellmouth would work better. Of course, you get into problems with the various holes for idle circuits and bowl vents etc.... So basically it is a compromise that disturbes the flow. The non-smooth-bore design also disturbs and restricts the flow. There ARE inserts for some of the Dirt bike type Mikuni carbs that screw into the bottom of the slide and smooth the flow under the slide. Website for one of the products is here http://www.startinglineproducts.com/catalog.cfm?pageID=detail&catalogID=1&catID=50&productID=892 Jetting adjustments would have to be made for the increased flow. There are some good articles out there on flow and bellmouth design. Here is a link to an article by Dr. Gordon Blair on intake Bell design http://www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf
It will be a little technical for some but there are illustrations to show how it all works.

Hope that some of these ideas help.

Swiss

turboguzzi · « **Reply #21 on:** November 30, 2007, 09:59:22 PM »

hey swiss
good to see you here! good links too. Funny enough had a similar idea about those carb flow smoothers but now see that such a solution exists. My carb's pistons look just the same so think I'lll order those soon. Looks to me like a simple mod for many SOHC4 carbs.

I am aware of the non optimal intake in my carbs and have already produced a tool to make the speed stacks, just didnt got around to do them. time time time.... designed them around some formulas made by paul edmonston of Lectron carb fame.

you can see a 3D CAD screen shot of them in the second page of this post

http://forums.sohc4.net/index.php?topic=27474.0

cheers
Y

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Author Topic: 10 deg downdraft ready (Read 12710 times)

peter c

Swiss