Topic: Is 34mm the largest intake valve for K head?

[gschuld]

Just curious,

Given a k head without relocating the guides or replacing the factory seat and retaining the 28mm exhaust valve size.  Is a 35mm intake valve possible? 

I’m talking specifically about valve to valve clearance with high lift cams like a megacycle 125-75. 
And whether there is enough meat on a factory seat to allow opening up the throat enough to make use of the larger intake and allow a quality high flow seat.

410 heads sink the valves a bit to allow larger valve diameters.  35/30 (from 34/31) has been done with success on a 410 head.  I’m wondering if 35/28 is possible with a k head....🤔

It seems the intake flow is the main limiting restriction with K heads on racing or high performance big bore engines, at least on the top end of the rpm range anyway.

George

[Tracksnblades1]

George, you always ask the right question. While I'm thinking about it. I find you have already asked it...😎   I watch and read.

[NitroHunter]

Dale Walker and Tommy Bolton told me they had 36mm intakes in the first SOHC pro stock head they built, they said it was a real badazz for it's day. I seem to remember it had stock seats, but not positive now... 

[MRieck]

 35 will fit but I'd go down to a 27 exhaust to get valve to valve clearance without sinking the valves a lot. You have to change the exhaust seat to  fit the 27 valve.

[gschuld]

Interesting.  And the 35mm intake will still work on the factory seat?

I recall reports that the factory cr750 racing heads had downsized exhaust valves in favor of larger intakes.

George

[MRieck]

Interesting.  And the 35mm intake will still work on the factory seat?

I recall reports that the factory cr750 racing heads had downsized exhaust valves in favor of larger intakes.

George

Seat gets thin BUT not as thin as a 550 eat with O intakes and the ID bored. Turboguzzi thought they would fall out BUT Brent has won titles with the set up so I think that answers that question.

[bwaller]

Shrinking the exhaust in favor of a larger intake has proven to work in my case.

[MRieck]

Shrinking the exhaust in favor of a larger intake has proven to work in my case.

Yeah......I heard that somewhere.

[gschuld]

I’ve seen the Team Waller blue/silver cb550 rocket raced in anger.  It is FAST😉

Well, it seems the Honda engineers though so too for the 750 Daytona race engines in 1970 if my info is right.

I don’t recall reading about what the finished valve sizes were, but I specifically recall mention of the exhaust valve size being reduced in favor of a larger intake, with the extra suction from the tuned 4 pipe mega exhaust providing plenty of exhaust flow at the smaller valve/seat.  Hondaman wrote about this at one point I think.

I’d love to find out more if anyone knows.

Has anyone tried a 35/27 valve combination in a k head?

George

[bwaller]

It may have been tried in some crazy science lab shop somewhere...but when bigger is always thought to be better George...maybe not.    My vote is for you to be the first!

[PeWe]

Interesting.  And the 35mm intake will still work on the factory seat?

I recall reports that the factory cr750 racing heads had downsized exhaust valves in favor of larger intakes.

George

CR750 kit pdf attached in an older post
http://forums.sohc4.net/index.php/topic,144005.msg1637724.html#msg1637724

page 3:7
IN 33.5, EX 27mm
Piston and cam data also shown on this page

[gschuld]

PEWE,

Thanks for posting that.  It’s interesting that they did not go bigger than 33.5mm on the intake after reducing the exhaust by 1mm to 27mm.  33.5/28 was the most common upgrade back in the 70s, often with the factory exhaust valve left unchanged.  So I’d think that 33.5/28 combination could handle any cam lift/duration combination and not worry about valve contact.  I would have assumed they would have gone up in intake size further searching for more flow potential.

Another thing to note was the 9.5mm intake lift, 8.5mm exhaust lift cam.(.374 .335). That’s pretty mild by modern standards, especially the exhaust.  I can imagine how a 410 head modified for larger 35mm intakes (from 34) and smaller 30mm exhausts (from 31) could be made to outflow a 33.5/27 valves k head easily.

George

[bwaller]

Just as a guess I suggest it was to manage engine reliability considering it was a RR based kit and in those days they weren't exactly sprint racing. (200 miles at Daytona as an example)

[TurboD]

I'll chime in with my opinions, and we all know what that's worth..

One factor/question that must not be over looked is the fact that, if you are able to merely install larger valves will the port be able to be modified enough in terms of flow to take advantage of it? Simply installing a larger valve at the end of a restrictive port does little to nothing to improve performance.


Secondly It is my opinion that too many SOHC cams are grossly oversized for these old engines with restrictive heads, again trying to use too much of something within a racing engine is not going to force the disired results. Trying to rationize things by stating " by todays standards or modern standards" is not going to change the facts about these old engines.

[gschuld]

That’s a very good point Brent.... we are not talking 5-7 lap sprint races here.

Dick Mann pretty much limped that #2 bike over the line in 1970 for the win.  Racing hard for 200 miles is a punishing ask for late 60s technology motorcycles.

Reportedly there was just a little bit of oil left, not surprising as it was smoking pretty good out there on video.  That race was a war of attrition more than anything.  Many of the faster factory bikes in the field had mechanical failures and retired.  Peak HP wasn’t the primary concern.  Hanson reportedly made Mann ride the #2 that was set up a bit more conservatively than the other Honda’s.

My info might be wrong here as I certainly am not a CB750 road race historian or anything.

George

[MRieck]

I have found the valve to be the restriction in these engines (and every engine when you think about it). You can get enough volume out of the port to take advantage of a larger valve. You bore the intake seat to .9 for a 35mm intake and you should see increased CFM over a 34. I also think the shape of the valve has an effect on flow especially in the F2 head (which has kind of a "lazy" port looking at velocity on the flow bench) compared to the K head.
 I have never been a fan of long duration cams on the street. I would rather get a good flowing head and use lift (around .400)/reasonable duration. Keeping compression up makes for a much more fun street engine.

[TurboD]

I agree, over caming is a quick way to kill any usefulness of any engine. I too like to stick with a lower duration with these engines street or racing.

Do you have any flow numbers (@10) of your SOHC heads?

[gschuld]

TurboD,

I understand what you are saying.  Certainly relocating the intake guide and enlarging and offsetting  the seat for 37/38mm intake valves in a K head with an otherwise stock intake port would likely be a mistake.  The valve/seat would outflow the port supplying it. 

I’m trying to get a sense for head combination options that could produce a good balance of flow between the intake port, intake valve/seat, chamber shape, exhaust valve/seat, exhaust port.  Carbs, exhaust, cam, compression, piston crown shape, etc are part of the overall picture but I’m limiting thoughts to the head at the moment.

From what has been discussed, once a k head get up to Mike’s stage 4 level(34/28 thin stem valves with extensive porting) the power limiting factor ...seems...to be in the limit of intake valve/seat flow.  Mike has suggested as much to DoctorD with his 836.  The dyno results showed HP rising quickly and essentially flattening out for about 2500 rpm.  The assumption is that a power curve should be just that, a curve.  When the curve doesn’t have a definitive peak it’s commonly suggested that there is a restriction somewhere in the system.   The RS-34 carbs would not be the likely cause.

Unless I misunderstood the post, Mike had suggested that a 1mm larger diameter intake (35mm instead of the 34mm) would likely improve the peak HP.  I take that as a suggestion that the 34mm valve presented the greatest restriction in a k head at that porting level.

EDIT:  Mike and TurboD posted a response before I managed to see it when typing this out.

George

[MRieck]

I agree, over caming is a quick way to kill any usefulness of any engine. I too like to stick with a lower duration with these engines street or racing.

Do you have any flow numbers (@10) of your SOHC heads?

Sure do. I can post them but you have to give me a day or so.

[MRieck]

TurboD,

   I take that as a suggestion that the 34mm valve presented the greatest restriction in a k head at that porting level.

George

The valve is always the restriction in any port.

[TurboD]

I understand, many times while I am typing (slowly) other comments are added.

What is your intended use for this head/or basic question, street, drag, road racing?

Because it was mentioned, My actual hands on back to back flow bench results when comparing the stock tulip shape valve to a undercut valve has ALWAYS resulted in lower flow numbers. meaning anytime that you move away from the stock tulip shape valve, you will always have to increase the valve size and or porting work to gain the lost flow back.


Mike that would be great, I would love to see them. I know that you invest more work into these old SOHC heads than I do thanks.

[TurboD]

TurboD,

   I take that as a suggestion that the 34mm valve presented the greatest restriction in a k head at that porting level.

George

The valve is always the restriction in any port.

I cannot completely agree on this one, I have tested heads that after a given lift and even at times or removing the valve completely resulted in little to no flow increase, sometimes you simply "run out of port" Lol.

[gschuld]

TurboD,

My specific interest is in a head for a very large displacement STREET engine.  Ideally, 10-10.5:1 compression with maximizing squish.  K heads with the tight chamber shape offer a slight advantage in total perimeter flat squish area I think allowing a flat top piston for maximum ideal squish area control and good flame travel.  Basically what Mike has done with his latest version of the million dollar CB engine, but for a noticeably larger displacement engine than his 1005cc 71.25 JE piston build.  Larger squish area, flat pistons, under 30 degrees total advance, high lift/ modest duration cam(125-75, CX-7, etc) all things that, on paper, add up to a strong performing pump gas engine.

Admittedly, I enjoy the theorizing and learning about the details more than most.😉

Mike,

I’m glad that I understood your previous posts about valve restrictions.

George

[TurboD]

I am not saying that It is not doable but a very large street engine with any compression is going to be a handful to start and will be pretty demanding in the cylinder head department to supply it. I have a 1000cc NA SOHC bike that makes no sense to try to street ride.

I can pretty much say that that I feel my budget stock valved 836 streetbike will roll around any big cc NA SOHC street bike on the street. It is mild mannered will run on any fuel if needed and will go any distance. Its hard to beat the turbos Haha.

[gschuld]

TurboD,

Bear has managed an impressive amount of power out of a dohc crank(69mm stroke) and 72mm pistons(+/- 1132cc) 410 head (125-75 cam) with 36mm/30mm 6.5 stem valves installed.  This is with pretty high compression.  A similar sized engine with 10-10.5:1 compression shouldn’t be much of a stretch, certainly with a 410 head as a base.

So out of curiosity, sticking with a K head and N/A only applications, what is the general order of flow improvement beyond a Stage 4 34/28 thin stem valves head?

Say you went up to 35mm intakes, reduced the exhaust to 27mm. Could this be an improvement or would the added intake flow be offset by a reduced exhaust flow?

The next step at some point is the more dramatic surgery of relocating the intake guide and offsetting the larger intake valve seats to handle a larger intake valve and retaining the factory exhaust size and position.  What could be installed for an intake in this splayed intake configuration.  36/37mm intakes with 28mm exhaust?  This would seem to have great potential assuming the intake port could keep up with the rest.

Once contemplating moving the intake guide and seat, one logically assumes the 410 head is the ideal starting point as the both the intake and exhaust ports are larger to begin with.

And you can install a 36/30 valve combination without needing to alter the guide seat geometry on a 410 head.  Bear had raced with that for years.

Splaying the guides and seats on both ends allows truly huge valves.  38/34 valves is the biggest I’ve heard on welded straight port intakes.  That’s WAY beyond my interest level😳.  But bad ass for sure👍