Topic: My old Dresda Honda CB500 race head and piston...

[MRieck]

not so easy to find them when they get spat out though . fine for a road bike if you are sensible but i would avoid them in a racing engine . if you over rev and get valve bounce they can dislodge . this happened on my sohc road bike while being stupid .

I had 1 lash shatter BUT it was machined by a local company and I don't think they had a handle on heat treating. I know a number of fellas that ran/run lash caps in racing engines in Europe and North America....road and drag. The oil forms a suction on the cap which helps retain it. I don't know....check your lash, use a rev limiter and don't over downshift....I'm confident they will be OK.

[gschuld]

not so easy to find them when they get spat out though . fine for a road bike if you are sensible but i would avoid them in a racing engine . if you over rev and get valve bounce they can dislodge . this happened on my sohc road bike while being stupid .

I had 1 lash shatter BUT it was machined by a local company and I don't think they had a handle on heat treating. I know a number of fellas that ran/run lash caps in racing engines in Europe and North America....road and drag. The oil forms a suction on the cap which helps retain it. I don't know....check your lash, use a rev limiter and don't over downshift....I'm confident they will be OK.

Mike, can you define “over downshift”

George

[simon#42]

george i sometimes wonder if you have ever ridden a motorbike !

[MRieck]

not so easy to find them when they get spat out though . fine for a road bike if you are sensible but i would avoid them in a racing engine . if you over rev and get valve bounce they can dislodge . this happened on my sohc road bike while being stupid .

I had 1 lash shatter BUT it was machined by a local company and I don't think they had a handle on heat treating. I know a number of fellas that ran/run lash caps in racing engines in Europe and North America....road and drag. The oil forms a suction on the cap which helps retain it. I don't know....check your lash, use a rev limiter and don't over downshift....I'm confident they will be OK.

Mike, can you define “over downshift”

George

Downshift 3 times instead of 2 for a tight corner and revs go to the moon. The rev limiter will not help you in those situations

[gschuld]

george i sometimes wonder if you have ever ridden a motorbike !

What’s a motorbike?

🤣

Thanks Mike, I understand it just fine. I just don’t recall the words “over downshift” specifically

George

[Sam Green Racing]

Or motorbike, hahaha.

[bwaller]

Mine uses lash caps on the intake side due to shortened stems. The caps are thin and begin to show wear early under the adjusters when rev'd to the moon consistantly, so consider them "consumables" also! 

[MRieck]

Mine uses lash caps on the intake side due to shortened stems. The caps are thin and begin to show wear early under the adjusters when rev'd to the moon consistantly, so consider them "consumables" also! 

.040 crown thckness

[Dresda500]

Will the +1mm valves fit a stock seat or do you need to put in seats as well? Wondering if I have time to go ahead and put in some new valves in a stick head and a mild port for this season...

[MRieck]

Will the +1mm valves fit a stock seat or do you need to put in seats as well? Wondering if I have time to go ahead and put in some new valves in a stick head and a mild port for this season...

+1mm intakes fit on the stock seat. I increase the seat ID to 90% of the OS valve OD.....it looks after that but no failures yet after many, many street and race miles.

[Dresda500]

Will the +1mm valves fit a stock seat or do you need to put in seats as well? Wondering if I have time to go ahead and put in some new valves in a stick head and a mild port for this season...

+1mm intakes fit on the stock seat. I increase the seat ID to 90% of the OS valve OD.....it looks after that but no failures yet after many, many street and race miles.

Awesome, thanks Mike. I guess this means the exh needs seats to go bigger?

[bwaller]

Mine displaces 572cc so is just for your information and may not translate perfectly.

In year two with stock valves but good valve seats and porting and 1:11.5CR, tight squish, tight head gasket bore, o-ringed sleeve flanges and finally an ignition that worked...we were @ 65.5rwhp. The next season the only difference was 1mm O/S valves and Mike further ported & bored the intake throat to 90% which gave us a 6.4hp difference. Then I improved the piston and increased CR, reduced cylinder friction, then went up & down in valve size to get some more juice.

Just to say I'd suggest the o/s valves and some porting, it's worth 6+hp alone!

[Dresda500]

Mine displaces 572cc so is just for your information and may not translate perfectly.

In year two with stock valves but good valve seats and porting and 1:11.5CR, tight squish, tight head gasket bore, o-ringed sleeve flanges and finally an ignition that worked...we were @ 65.5rwhp. The next season the only difference was 1mm O/S valves and Mike further ported & bored the intake throat to 90% which gave us a 6.4hp difference. Then I improved the piston and increased CR, reduced cylinder friction, then went up & down in valve size to get some more juice.

Just to say I'd suggest the o/s valves and some porting, it's worth 6+hp alone!

Thanks for the info Brent! Much appreciated- You're last sentence...Reduced piston friction...are you using dry film lubricants on the skirts and/or cutting down the skirts as well? I'm a big fan of coatings....I thought to start a thread on it's own on this topic as I imagine there are lot's of guys using special coatings and lots of guys interested as well. I became very into coatings when we started racing at Bonneville at WOT for 5 miles.....and then your last words..."then went up &down in valve size" ...did you reduce one to be able to increase the other? 

[bwaller]

Yes/yes and yes! 

[MRieck]

Yes/yes and yes! 

That's funny.

[Dresda500]

Yes/yes and yes! 

That's funny.

Haha! then we are all aligned!

[Dresda500]

Just adding info as I come across it. I found that these are CB250 pistons, with modified domes for larger valves and a bit of a haircut it looks like on the top. The CB250 twin has the exact bore and stroke as the CB500 so it's a drop-in as far as compression height and bore, just a chitload of head mods to make it into a hemi chamber to get the big valves in there. As others have said as well, I question the efficiency of this mod in regards to squish. if done properly you still get it from the sides of the dome to chamber, but getting accurate squish distance, by hand, would be a feat in patience and skill.
 I'll get around to CC'ing the chamber and piston at some point to see what CR these give.

And an FYI I measured the throat in the intake port to compare with the bigger valve and looks like they are right about 86% of the valve. I'm always worried about old ported heads as many times they are not very good and often ported too big. This looks quite OK. and looks like they even favored the offset side a bit on the porting to give extra flow on the outer side that promotes swirl on our offset  port to cylinder centerline. Looking at the straight spigots and comparing to stock they are offset higher and to the correct side as well. 30mm ID on the spigots and port entrance. They just might have known what they where doing back in the day.. The intake ports are quite polished though, a bit more than I like to see, but very common back in yesteryears and that can be sorted.

[Dresda500]

Finally back on this one....I got busy road racing a CB250 twin this season and the Dresda was put on idle and working on its brakes and forks etc. So I finally got the flowbench up and going in the new shop and threw a stock head and the Dresda head on there the last couple days. The ports arent perfect by any means and some nasty ridges right outside the valve seat on both Int and Exh, but this is just as is from the 70s. I just did a quick lap to see the seat width and make sure they seal when testing for leakage. Basically these heads need some port evaluation/modification as at .350 lift it just about flat lines indicating the valve is no longer the restriction, but the port. There is a funky bump in the bowl(blue lines in pic) that needs be fixed,fix the ridges in the chamber, backcut the intake valve and surely more once I dig into it a bit more.

32mm intakes and 28mm exh with 6.55mm stems. Throat is at 89% on the intake. Big fat seat on the intake. The Straight inlet adapters at 30mm ID and it runs CR29 carbs.

What kind of RPM do you guys turn n the track and whats your powerband? Honestly I think this could be a bit on the big side, looking at the Ave cross section area of the port. I'm afraid it could be a bit lazy even with as high CR as possible and a conservative cam but pull up top.

FYI, when looking at the CFM numbers, the 0 lift is showing 2.8cfm, that is the leakage, so subtract that from the numbers below. I don't think this is already done in the program, but I would think it should be. You can see it just stops increasing flow at .350 or so. This is basically saying it's not the valve that's the issue anymore, but port limited.

I did a test on a stock head and it maxed out in the high 60s, which matches with what I've seen from other guys

The pic of the chamber shows an arrow pointing to considerable ridges and in the port the blue lines are a sort of hump on both sides right in the middle of the bowl. I'll likely clean them up just a hair and retest and see if it makes any difference

[bwaller]

Depending on whether I was battling for a position kind of determined my shift point. LOL My rev limiter kicked in @ 11K, and I expect power likely tailed off above that. The dyno showed it was also pretty flat between 10.5 & 11K. So racing hard pushed the rpm's up, otherwise why bother, as there wasn't any extra juice or likely finishing position to be had!

I always found, and then my son who rode it later, that the power delivery made it easy to ride fast. That makes it fun, and as you know fun makes us faster.  Between 8K and 11K mine has 15HP available. But it would pull from 7500.

[bwaller]

Thoughts on your chamber. The plug extends so much deeper since the chamber was machined. It would be good to shim it. I feel the exhaust valve is way bigger than needed plus it will likely develop a crack from the plug thread toward the exhaust seat. Shrinking the exhaust valve would allow a for a larger intake valve but when does it become too large if the bore size doesn't change? I can't say. I also cannot offer an idea about flow with greater valve lift, maybe turboguzzi played with that.

[grcamna2]

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[Dresda500]

Depending on whether I was battling for a position kind of determined my shift point. LOL My rev limiter kicked in @ 11K, and I expect power likely tailed off above that. The dyno showed it was also pretty flat between 10.5 & 11K. So racing hard pushed the rpm's up, otherwise why bother, as there wasn't any extra juice or likely finishing position to be had!

I always found, and then my son who rode it later, that the power delivery made it easy to ride fast. That makes it fun, and as you know fun makes us faster.  Between 8K and 11K mine has 15HP available. But it would pull from 7500.

Thanks for the insight Brent. While I haven't raced this bike yet, I've been racing a CB250 this past season which is basically half of the CB500 but with GIANT valves and ports. Those have the head from the CB350 so get the same 34/28mm valve package and port volume. The CB250 is very peaky , especially after a big cam, and too little CR due to valve pocket machning. Same experience there though, I think it quits making( or increasing HP) in the 10500 range, but will pull well past 12k if you dont stop...as you said , when fighting for a trophy, you extend that rpm a bit. They say anythign after 11k is hard on cranks on the twins. That bike is not easy to ride due to narrow powerband and trying to keep it between 9-11k only. I'm hoping I can build the 500 to have a bit wider torque band

[Dresda500]

Thoughts on your chamber. The plug extends so much deeper since the chamber was machined. It would be good to shim it. I feel the exhaust valve is way bigger than needed plus it will likely develop a crack from the plug thread toward the exhaust seat. Shrinking the exhaust valve would allow a for a larger intake valve but when does it become too large if the bore size doesn't change? I can't say. I also cannot offer an idea about flow with greater valve lift, maybe turboguzzi played with that.

I was just looking at the pics of the chamber and said the exact thing to myself about the plug...you don't want threads in the chamber! They will get shimmed. at 32mm intake I honestly think its already on the big side for a max 57mm bore. I have to stay under that to be legal and a 32mm intake is already56% of the bore....too big in my opinion. Some very well known head developers/designers say that once you go over 53-54% percent on the intake to start to actually lose power across the needed range, even if flow number go up and a possible increase at peak HP/rpm. Again...totally agree on the Exh valve...but they are much more forgiving as far as sizing compared to the intake since they have hot high pressure gas pushing the gas out vs the intake only having atmosheric pressure pushing it in for the most part, other than at valve overlap. I like the exh to be 75/80% of the intake....and right now its at 86%....but I'd have to cut out both seats, weld it up, install new seats just to decrease the size....that ain't gonna happen...So I'll run the valves at the size they are now. If I want to go crazy, Ill start with a new head. With as much as this head flows, or will flow, I plan on trying the CB650 cam since I don't need a lot of lift to get decent CFM and the big valves flowing more at low lift make the mild cam "seem bigger". It much like a modern sprtbike with tons of valve area and flow, then stick in a mild cam to make midrange ( and milage, emmisions), but still makes killer top HP due to flow, even with a mild cam.
On a sidenote, I'm going to completely weld up a CB250 twin head(or 350 actaully) and go down in size from 34/28mm to 31/25mm valves and a shallower combustion chamber...I think those are pretty much spot on for sizing with a smaller port volume as well to get velocities up and Coefficient of discharge where I want it. We'll see if theory can win on the track....
Anybody got lift and duration specs on the stock CB650 cam?

[johno]

Hard to tell from the pics but it looks like the valves haven't been back cut.  That prevents the Venturi effect and kills the flow at low lift.
Mod number one in any port job

[Dresda500]

Hard to tell from the pics but it looks like the valves haven't been back cut.  That prevents the Venturi effect and kills the flow at low lift.
Mod number one in any port job

You are correct, the Int wasn't backcut. I just wanted to get a baseline flow test with the heads exactly as they were modified back in the 70s. I have now backcut the Int valve(not the Exh), and have begun fixing a few other areas. I flow tested after just the backcut and cleaning up the small lip in the chamber and it did pick up a few cfm in the low and mid lift as expected.