CHO Combat ports on a 920?

The basic lore I know about CHO 32 mm heads is early ones were over hogged out in the factory and have a bad bend to turn so may be Cdo's worse flowing head for a 750. Question, would fitting CHO head on 20% bigger sucker take up some the sluggish flow badness of CHO ports? In other words is the CHO bad rap got more to do with the size of the ports per size of the jugs or more to do with the sharper bend? I know what a small port can do for a 2S cam and tuned exht, so do not desire that level temptation on my stock Combat thankyou. Also what's the port size recommended in Norton head or Fullauto for a racer 920?

thats a 750 short stroke head . above . , below is a 71 ' big valve ' head . reangled inlet guides & seats .



from http://atlanticgreen.com/nhth.htm

suposed to be the performance mods spec sheet , but not coming up , at the moment .

40 pumper Dellorttos would be good budget carbs .

dunno if this thing works ; http://filetram.com/mediafire/norton-85 ... 8884436590

Cor

Thanks Matt but does not deal with what I'm asking. I am half crazy d/t small head wake up call on past Peel Combat so wondering if the Combat head might work similar benefit on bigger 920 pistons drawing thought its other wise too big tubes.

One way to find out . :wink: Bolt it on . Sounds like all Combat Heads wernt created equal .

Depends if it was the apprentice that did them , or the master . states " SOME " have the to tight lower radius .

Wouldnt be impossable to make a cardboard template of the lower edge , to see it ' on paper ' .
Other way would be fill it with Silicone , let it dry . Then get it out . :lol: to have a copy of the orifice .

The guide / streamlineing , and other details can have a exaggerated effect , either good or bad . Devils in the Detail .

found a ' fullauto ' one page site yesterday . Not Today though . had 5 pictures , on a ' sectioned ' head , through ports .

A reasonable test of its compatancy , usually conducted whilst flushing after checking mateing of valves , with the garden hose .
Have it near full bore , finger over the end , so itll squirt say six foot . Hold about 8 to 10 inches from the port / head face , aimed
directly down the center of the intake port . At full lift , or from about 1/4 in open , the blast should come out directed straight
down what would be the cylinder , concentric with the center .

At Low Lift , the initial opening , it should be a asymetric cone .

As at that lift , the piston will be near T.D.C. , the presumed ' base ' of the cone should be concentric to the bore , in regard to the
approxomate piston crown position .

If nothing else , on a hot day , this is good fun . And removes all grinding paste residue . Or course particular attention has been paid
to flushing the paste in the guide area , etc , before fitting any valve .

The exhaust of course if sprayed from outside the head is working in revers , so will not get similar ' flow patterns , doing it from inside
the head , at the chamber / valve head ; Youre Liable to get WETTER . :shock: 8) :wink:

Perhaps someone can post a attempt on U - tube .

750 heads need holes moved and chamber bottom opened to before just bolting on a 920. The Woverhaven [sp] early heads were basically hogged out as verified by my Dreer kit installer taken measures telling me they were over done with sharpish bend. I think the later Combat heads got more refined port treatment.
Guess will just wait and find out myself if natural inhalation plenty enough for sane thrills. If I live long enough may get to try Fullauto head which may kill me.

Were a few that thought the assymetric ' squish band ' head to be capeable of superior combustion to the full hemisphere ' Norvil ' & Dunstall types .
The olde stratowhosit effect , giving a bump in increase rate of compression at ignition and the ' shock wave ' from that discharge to main chamber .

Nasty little two strokes , the squish band width , angle , and clearance to piston can be crucial for optimum performance . To tight a clearance
causeing detonation & disintigration . But with the little 2.5 cc & 5 & 10 cc things at 20.000 rpm plus , the chamber shape is crucial to combustion - performance .

On a 4- stroke , any pocketing - recessing of valves useually stuffs up flow at full wellie , and is a poor mans way to try to get low rpm cleanliness .
With a hot cam , youre on suferance untill its in the power band . Pulling the ignition back ( stronger springs ) at low rpm helps here .

Butif youre to high a C.R. for the fuel used , retarding the ignition is going to cost power , and isnt going to get it to run cooler .

Try marking the barrel / cylinder position on the underside of the combat Head for the 920 / 960 cylinders , get the hose out , And VIDEO to U tube .

:twisted: :mrgreen:

Let's face it.The best heads on earth ,with 0.450" valve-lift,won't flow much more than a 28 mm round port can.Any increase in port diameter is just a decrease in velocity (not an increase in flow).The valve is the limiting factor,not the port diameter.
Less velocity = less grunt.
It won't be so bad on a bigger engine.

Ok xfile that's satisfying to take valve size in account. Peel CHO head was lost for over a year so put on plain Jane 28.5 mm head [forgot the code] just for break in and was amazed as were strangers, then put on the CHO with 6mm valves and she lost her power edge. I'm thinking about non boosted performance as well as boosted so conflicted/confused on valve size. Heat transfer on sustained hi boost is main reason I'm starting off with std valve size, less mass/per rim area to flow heat into seats and lower detonation proneness while boost will over come head flow issues. Unboosted don't want such touchy throttle response it difficult to throttle up on THE Grit roads and off beaten paths. Unboosted is likely Pee'sl daily set up so thinking small valves might turbulize more on low throttle and not hinder much power till top end which is fine for daily life as Ms Peel real thrills is not slowing for turns so can stay legal in 30 mph going 60 mph at 50' lean for my acceleration fix.

Steve,

this might just be another case of my failing memory, but I don't recall what you mean by "CHO head". Could you enlighten me? All the references I have for Commando heads list them by factory part number, or stamped designations RH1 through RH10, or factory descriptions like "AMA Homologated Combat 750", "Woverhampton Combat", "Standard 750", "High Compression 750", etc.

Ken

Ken, You got the head that has been determined by measure by Micheal Starkey of British Toys in NC that put Dreer's K/W valve kit in Peel's head its definitely a hogged out early March'72 750, "Woverhampton Combat" aka RH3 . Been too long since seeing the casting code. Both you and he agree its big and bent enough not to bother flow testing it. It was a let down when last run on 750/2S Peel but happened to still be on Peel before going back to std head, when throttle stuck and a horrorific amazing thing happen after an instant of hesitation in rpm zing about 7200, it caught such a second breath the engine kicked back so hard on the iso's it knocked my adrenalized grip hand back off throttle by shock to throw me back a couple steps so it REALLY ZING'D Out OF Sight RPM FlOW! In tin shed sounded like fabric of space ripping as full orchestra dropped through the roof that rang a bit after Peel stopped. So with that in mind I'm thinking best stick with small valves heat flow for boost and just live more tamely cheaper unboosted. The other quandry though is boosted engines mainly gain by opening up exhaust side, which is where the heat focuses. ugh.


To simplify, would small valves help tame the Norris D cam in start and idle zone or would bigger ones flow more there too?

I know I've got the head Steve, buy you didn't answer my question. What does CHO stand for?

FWIW, I think that head will work just fine on your 920 engine, particularly with the supercharger, but that's just my personal opinion.

Ken

Hm, I don't know how i came to call it CHO but seems I saw it cast in. I thot it meant Combat Hogged Out. Personal opinion is not objective so hope we both are proved right. i just don't know what I can live with by escalating power adders. All references say what helps heads flow naturally pays back even more with boost which of course implies a FullAuto or a Yellow Peril head big valves or even a one off 3D metal printed. What I think will happen with the small valve RH3 w/o boost is fairly easy kick starts with a bit of cam lope idle that responds ok w/o bog to smooth throttle up till upper 4000's, then wakes up throttle on D+ cam for 2500-3000 more upper rpm fun power band up to 130-ish. Except for off road steering and parking lot stunts can't see much use for much low down power and none of yoose racer guys would dare let rpms drop more than couple-3000 off red line would ya. So really only question in Peel then is heat flow w boost and how big of valves she could get away with as more valve flow means even more heat flow. If I have my durthers I'd like to dyno at Texas Big D flat lands then up at Jim Comstocks Colorado Mt high. Another way I think of this is how much mixture flow can a head support, similar to carb CFM, weather the engine can actually suck in their full potential at least its there, if not so big its boggy the whole power band.

Related data points on valve size and flow

Valve lift is somewhat related to intake valve head diameter. An engine with an intake valve head diameter of 1.400in to 1.500in will generally perform best with a valve lift of 0.395in to 0.475in; an engine with a larger intake valve head diameter of 1.750in to 1.875in will generally perform best with a valve lift of 0.425in to 0.550in; and an engine with a large intake valve head diameter of 2.000in to 2.250in will generally perform best with a valve lift of 0.475in to 0.650in. But these are just rough guidelines; ultimately you will need to take some gas flow readings on a flow bench to determine the best valve lift for your particular engine.

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http://www.howrah.org/camshaft.html

hobot said:

Another way I think of this is how much mixture flow can a head support, similar to carb CFM, weather the engine can actually suck in their full potential at least its there, if not so big its boggy the whole power band.

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..................Correct me if I'm wrong Steve but with the supercharger there is no "sucking" as with a carb, seems like the blower will make this old engine / head design flow about as much as is practical with or without big valves, I mean you could spend more $ on oversized valves and trick ports, but will it actually make that much of a difference once you've got your boost on? Cj

..................Correct me if I'm wrong Steve but with the supercharger there is no "sucking" as with a carb, seems like the blower will make this old engine / head design flow about as much as is practical with or without big valves, I mean you could spend more $ on oversized valves and trick ports, but will it actually make that much of a difference once you've got your boost on? Cj

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NO ONE CAN MAKE ANY SENSE OF ANYTHING i POST ABOUT PEEL W/O THE TRIPLY* GOOD* TRI-LINKS*CONSTANTLY* ALWAYS* AND 8FOREVER8 MORE AT THE VERY* PEAK* OF MY COMMANDO AND LIFE , THROW IN THE OLE TOWEL AND WRAP IT UP. so...

I am only seeking excess power so to show the superiority of the isolastic Commando over all others [& get psychedelic hi w/o chemicals] by enough accelerations out of turns 200 mph bikes can't catch up in time for the next wee bit of leaning that upsets the poor babies so bad they can't even wheelie in a bee line w/o wind gusts w/o stumbling all over their balloon feet!

So... no question you are correct Ms Peel's dragster engine with spiffed up Drouin will definitely blow right through any intake restrictions beyond red line tolerance to just short of mach port speed jet shock wave choking for dramatically increased injection velocity and mix vaporized turbulence. Calc's show small valves reach mach ~0.7 at 8000 rpm w 56,000 rpm impeller exponential boost-blower CFM rise. Sonic choking don't start till about then and sonic choke don't actually choke, just stifle further flow increase as much per rpm rise as prior, in other words torque slope tappers off but not stopped - till impeller cavitates or blows up. Kompressors stay at set flow rates per rpm so work best down low but go flat above some threshold but not a centrifugal blower which both ups its pressure and flow volumes as rpm rises.
The issue with boost in air cooled engine on sustained boost is heat soak so there is some conflict in what valves work best unboosted and what can survive boost. Using only 7000 rpm power predictions says enough power/mass to break under 10 s 1/4 m. Peel will be traction limited more than anything else so can't really fully load engine till ridiculous speed resistance and then how long on a open tract section can ya stay at full blast before cooling let offs to set up for sharp turns>?
What blown Peel really needs is sodium filled exhaust valve. Peels engine with welded nitride crank and JMS pistons/rods and lightened small valve train should exceed Steve Maney 7200 rpm 920 red line to actually tolerate 8000 for a time so tempted to try it on a dyno and then on the run. Drouin early ads ran it to 8000 and Peel's had Bruce McGregor upgrades to take even more rpm.

So back on point, if the early 'CHO' head was too big & slow ports and bent for what a Combat can suck in below 6800, maybe it'd be more effective on 20% bigger natural inhaled engine draws not to be too boggy down low even with the over lapped cam? In daily life I don't think I want masses of blower and crash on it cage nor water/methanol/nitro-methane mix jug for 116 octane/cooling nor TTI tranny to take the torque spikes I couldn't resist. I know enough of what it takes to race even though I ain't a racer to know its torque that gets ya into, through and out of turns while hp just lets ya pull to higher speeds in the opens. No one I'm tested with will risk 140 mph here and unblown Peel should hit that so blower not needed to wipe smug sneers off pretty quickly and I can go on to have my real fun which is always alone with Peel. I hope unblown 920 feels like normal peppy Combat till 4000ish, then feel like 3rd piston of Combat 2S @ 6800+ kicking in.

Sorry Steve, I thought I was making sense of what you're doing with your bike, I've followed along with many of your posts and been glad to see pictures of your up grades. I was just thinking that which ever head you bolt on, you're gonna get about the same performance once the supercharger starts "doin' it's thang". Cj

Oh ugh cj above is as much wishful thinking as reality as I've made rather less dramatic item changes in past Peel and she flat fell on her face even to just leave my garage pad. On the other hand we've seen/heard some engines so tuned for top end they die unless constantly blipped into mid range rpm. My piggy back alternator gets in the way to put a carb on the impeller intake or even a straight out manifold like Bruce did, so will attempt to fix the flat slide Lake Injector death throttle and profile needles by hunt and peck and scratch method and see what happens. If I can not get Drouin to work out for me, lighened Peel will weigh less tri-linked with a potent Maney big block I can just follow recipe book on. Oh and air muscles of course can forget them. My P11 built 2" low in front would run level with 2-3" lift through 9000 rpm top outs in 1,2,3, chrome button touch WOT shifts and mid way through 4th before easing down. If my dreams come true I's like to learn to flick forks contracted 3" suddenly at same time as careful throttle up so it appears Peel just lifted the front tire level with a sky hook > right out of there. Past Peel could put me on tail light if not set first but P!! would break race seat loose, whiplash my neck to crack or even leave me hanging on bars like a flag in the screaming in the wind... I expect New Peel to do that while hooked up about low sided off boost or on... then idle back to 400 with the Drouin panting lightly into its nostrils. Then i can go peacefully.

I know that this is easier said than done but why don't you bolt up what you have and see how goes, you don't have to wring the snot out of it......well not at first anyway :wink:

Uh I don't have enough old or new Peel parts on hand but for mock up frame stuff. Home/car etc fates recoveries set fun stuff back a few years. I have considered putting the Powerarc ignition and Drouin on Trixie to learn their ropes but my R brain took over for the more faulty L brain to make me L hand dominate so the R hand has a mind of its own. Lake Injector sticks so must invent adapt push pull throttle and likely have to run 91 or 93 octane to boost up some on. Besides Trixie is shut down till KS fixed and cases resealed from last year's grit inhaled ring blow by job. I'm days away to finish the finish of my backed into modern suspension modified balloon race tire shod SV650 corner cripple that flat blows away normal Cdo handling so no time-desire to do double trouble on Drouin and ignition on something that just can't take it like I like it. Then a deer whizs across bow so fast I cry out in shock and wonder at my sanity to keep thinking about riding anything. Take care yourself whizing around on what every ya risking life flights on.