Has anyone had any first hand experience with Mick Hemmings big valve conversion??? Does the new valve (26*) angle and changing the combustion chamber hemisphere make a significant differance? Most of all He states that he installs a brass valve seat....I have never seen brass used as a valve seat before in any engine I worked on. Is it tough enough to withstand that kind of heat especially with an over laping race cam??? The valves are both open at the same time for a few degrees, thats what causes the blowback that makes your slides click in there bores.. If it's a good upgrade I might like to do it while the exchange rate is down. Wish there was some way to beat the excesively high international air freight shipping rates. Bought a MARK3 rear sprocket from england and it cost about a $100. to ship.
I believe it is a clone of the dunstall reangle valve job and recontour of the chamber. dunstall head A local club member had the hemmings job done and primarily it pushes the torque way up the RPM range. (into crank breaking region) Yes it had boy racer surge when it came on, but the reduced low RPM torque would allow low RPM hi-torque bikes to pull away, then he would be playing catch up... I own the head in the pix and I might try it someday but after seeing it perform I am not so inclined to try it except for a bike with a 5 speed (or 4 speed close ratio) and a nourish crank.
dynodave BSA 3 1961-1963 Ducati 3 1992-2002 Norton many 1951-1975 87 Serv-Equip 100HP MC brake dynamometer,
Hi Alan, Yes, I've got a few big valve heads on my racers. It does work. OK, I've done a lot more to the motor too, but 58ft/lbs torque flat from 2-4.5k rpm, and 70bhp at the rear wheel ensures it's quick off the line, out accelerating rob north triples easily. The motor is so torquey that it would be really good fun as a road bike too!
They're not brass valve seats, they're Ampco45 bronze. Yes, they'll hold up.
You don't need to re-angle the valves if you just use the larger intakes, which is what Keith Johnson recommended for a "hot street" motor anyway. For about $1200 he ported, cc'd, new guides, new valves, surfaced my lifters, etc.
Bike runs great and pulls like a freight train from 2500 rpms to redline.
Thanks .....Dave, I had a head that was milled as bad as the one looks in the picture and in my opinion it was RUINED and trashed it. I played around with compression once by omiting the base gasket, it's about .015 thick. I don't know if some one previously milled the head a bit on that bike (didn't look like it) but you had to use all of your ass and an eyebrow to push it through. City driving was OUT because it ran so hot you could smell the oil cooking in the head. It was a bit faster but definately not worth the dis advantages, I pulled it apart and replaced the gasket. I was planing to have the BIG VALVE job done for my MARK 3 which is set up as a road bike and is the most dependable NORTON I have ever owned. If it raises the torque band higher up in the uper R's that would defeat my purpose. I have always felt that the 850 engines were choking on too small of valves for their cc's.....what else would make such a power difference between the 750 & 850 the 750 putting out more power?
750 motor, standard bore, 10.6:1 compression, big valves, Pazon ignition (slightly shortened advance curve) SET @27 DEG full advance. I use a dash of octane booster, easily available at most garages. 70 BHP @7,200 rpm.....motor often revved over 8 in the heat of the moment. Can't tell you what cam I'm using, as it's not commercially available, but not really wild...the bike can be left ticking over @800rpm all day, just like a street motor. If I was to build one for the street, I'd do all of the above and use a PW3 cam. I remember Mick Hemmings setting the fastest lap of the weekend on his PRODUCTION Commando at Snetterton at the Race of the Year meeting a few years ago...(thats the fastest lap in any class!)...and that had production pipes/peashooters, iron barrel etc.(Snetterton is an all out power circuit, long straights!) As for MOT, well, race bikes over here have to be silenced, both my 2-1 and 2-2 systems are quieter than the avaerage poser on his Hardley Ableson with after market pipes. Kick starting, not an option with a Quaife box, but six steps pushing form cold, 3 steps when warm fires her up. the last bike I had that had a kick start was a 850 weslake with 10:1 compression, no big problem,my mate still kick stsrts his 960 weslake with 11:1 comp ratio, as he's kept the standard 5 speed triumph box!
'I have always felt that the 850 engines were choking on too small of valves for their cc's.....what else would make such a power difference between the 750 & 850 the 750 putting out more power?' Alan, it won't be just valves, what I see is if you up the engine size and the head and everything else associated with the breathing; cam, pipes, carbs, stay more or less the same, it will make its maximum torque at lower RPM, and it will likely make quite a bit more at that point, though the HP isn't so great because HP is related to work done which involves RPM. The 750 won't max out the breathing till higher RPM and then though it's torque peak may be less, the HP will be up because of the RPM multiplication. It may only match the bigger motor's HP but it's higher in the rev range and it will have more power say at 7,000RPM pushing the bike to a higher top speed where the bigger motor cannot breath enough to have any power beyond 6,300 perhaps, so the bigger bike dies there and the smaller zooms past. My view is HP is what you really want if the bike is going to zoom, so if the bigger motor is going to out perform the smaller it needs to be able to pump proportionally more air through the head etc, then it will thrash the smaller motor everywhere because it will again rev out and have power at 7,000 , provided it will take the extra stress and someone doesn't optimise the smaller motor better though in the end they both have the same RPM limitation.
I've always felt that to make a Commando engine rev harder to make more power is a waste of what this engine is all about. Low end and mid-range power is what they do best. Engine mods to enhance the lower parts of the rev range are what is more effective on the street. As I recall, the 850 motor makes maximum HP at 5800 rpm rather than the 6500 rpm of the 750 motor. My MK1 850 rarely gets revved over 5000 rpm because I don't feel the need. Mechanical sympathy for that rubbery crank also has a hand in it.
Regarding valve sizes, I would think that the standard sizes are quite adequate for just about any street application you could think of. The ports are another matter. Jim Comstock tells me that smaller, better designed ports flow better and make more power than standard ports. Hence, his port designs have found their way onto my new heads. More photos to follow.
So is that a new casting Fullauto? Very cool if so. What you quote illustrates my point with why the different feel between 750 and 850 motors though, and performance at higher RPM, which is what Alan's question was about. Some people race these and naturally need top end, plus some street riders want that as well, the big valve modified heads look like they sure deliver that, 70HP at the wheel is a heck of a lot. And if that is a new casting you have, you could re-angle oversize valves and do all sorts of things in the initial machining, having that metal in the floor of the port would also be a very cool thing to make a very high performance head. The 'rubbery' crank, once again people race them, and an other alternative I like to comfort myself with when I'm thrashing my Norton cranked BSA is that twisting a pin around 90deg with a new centre flywheel theoretically takes a lot of stress off the crank.
Fullauto is this considered the d-shaped port? One year at Bonneville I was sitting with some friends that know Jerry Branch and heard him talk of ports and exhaust. If you know who he is you will also know when Mr. Branch talks people listen, He said people are fooled by bigger ports and exhaust and it all comes down to being tuned and sometimes bigger is worse. He was the guy who developed the flow bench, With this he was able to work out the best shaped design for many types of motors. I had heard he even worked on Norton heads at one time. Do you guys know him? http://www.motorcyclemuseum.org/halloffame/hofbiopage.asp?id=378
The 850 is down on compression. That is one of the reasons it has less power. It has BIGGER ports than most 750. Bothe RH4 and RH10 is more port... is not better in this case. This is why several years ago Marino (MAP)and I chatted about 850 compression and the need for higher comp pistons for 850's. As a result he had some (IIRC) 9.5 comp pistons made. While I know 10.5 or 11 would have been my choice, I suggested moderation for a generation of mechanics that can't even figure oput why boyers ping and would have immense trouble sorting out the ignition and therefore would have never ending complaints..... Keep in mind that a norton 850 is actually only 10% bigger than a 750...it is only 828cc.
During the 92 rally I assisted in the dyno runs and it became clear that the whole fleet of commandos were 38-44RWHP. Only my almost stock combat did more @47rwhp. At the time it actually had a 28.5mm 1970 head. Awsome factory short port from the seat to the guide. Putting the combat head back on did not improve the dyno runs...even with the higher compression.
dynodave BSA 3 1961-1963 Ducati 3 1992-2002 Norton many 1951-1975 87 Serv-Equip 100HP MC brake dynamometer,
Higher comp will not move where the power occures in the rev range much, that seems controled by the complexity of what is the breathing, I went from 818cc to 883cc with my BSA and had this problem of loosing power at revs with the bigger motor, until I increased valve lift then it shifted back up about 1,000RPM, all with the same 44.5mm inlet valves and 38mm carbs. It may respond to even more lift but then the valves may start hitting each other and wear and other things become a problem. On my last rebuild I filled the floor of the intake ports 'D'ing them, I'd read this is a dead area for flow, and it has made quite a difference, its much more noticably powerful, as far as having bottom end midrange and top end its like having cake and eating it to. I cannot do it with the exhaust ports because the epoxy filler won't take the heat it would need welding which looks impossible or a new casting with metal there. I also find it has more power with chrome molly pushrods than with the std alloy ones.
Yep, that would be a D shaped port. Jerry Branch is a legend, well known even in this far flung part of the universe. Mark, that is indeed a new casting. My first head should be in my hot little hands within the week. Thence, onto my '73 MK1 850. Exciting times. You would have heard of Harrop Engineering in Melbourne. They have done the whole exercise from go to whoa. A very professional team. Regarding reshaping of the ports, Jim Comstock actually mills out the ports and uses JB Weld ( like Devcon from the description) to "glue" in his machined aluminium inserts with his desired port shapes. Don't try this at home kids. Interesting what you are saying about the pushrods. The Chev V8s had a similar problem with aftermarket cams. The standard pushrods were flexing which means that your lift isn't what it should be. Stiffer pushrods meant more HP. Your experience tends to confirm this.
FULL AUTO....I fell inlove with the head casting the minute I saw them! Much better than the origional castings. Back 40 years ago a few of us guys bought some G15CS norton scramblers, the one I bought had MATCHLESS labels on it. That bike was extremely fast....just crack the gas (no clutch) and it would stand straight up and go right over if you didn't back off on the throttle. One of my buddys liked the way the bigger ATLAS pipes looked and put a set on his G15CS. The bike instantly turned into a dog! That was my first experience with exhaust tuning. Seems the smaller pipe filled completely and the momentum of the expeled gas going out helped pull fresh gas in when the valves over lap. I see guys selling "BIG BORE EXHAUST KITS" 1.5" pipes...does the extra 1/8" help or hurt performance? The length of the intake manifold makes quite a differance also...the momentum of the flow does the same thing pulling fuel in. I think it a was 2.25" extended manifold I made up between the carb and the head, we did the math on it...can,t remember what the tuned exhaust length was. It was 40 years ago.
From what I've read and gleaned from those who know, larger diameter pipes with no other changes will harm performance rather than enhance it. Reading some old articles I came across the fact that the Gus Kuhn racing Nortons used standard sized exhaust valves. Jim Comstock has added material to the exhaust port and it flowed the same, confirming that shape is more important than size. I was talking to Matt Rambow this morning and he wanted a dimension from my heads. He is having some pistons made for a customer's bike (with the first of the new heads on) and needs the dimensions to get the compression ratio correct for what he wants. He told me that he has never measured two the same ! The dimensions of the new heads will be demonstrably the same with small tolerances. The combustion chambers will be CNC machined and hence the same volume. They were designed so that all standard Norton stuff will go straight on, saving heaps of prep time (and money!) on old heads. Like I've said before, I love the way Nortons make their power from low in the rev range and any mods I do to my own Commandos will be aimed at enhancing this power, not moving the power further up the rev range.
I am interested in seeing the results you get from the smaller ports. I am OLD SCHOOL (you can drink more beer from a glass than through a straw)Kind of guy. From the picture it looked like about a 30% or more restriction in the exhaust port. I have read the articles on porting and got a diferent view. I understood the D style port as a way of getting a better flow around the large valve guide which restricts flow and in that area only, not the whole length of the port. I envy you, you have the unique oportunity to change the castings any way you like.
But its not liquid, its air, on a flow bench the bench is drawing the air so its a little different, a bigger intake pipe always supplys more air, on the motor its more complex. Question: How much air goes into a motor through a given pipe size if you double the speed its travelling? Power bands if you want to call it that start when the speed of the air in the pipe reach enough to pack into the chamber without being blown back out while the inlet is still open. Everything effects that, type and size of exhaust, cam, engine displacement, and size of the inlet pipe, carbs, valves. Valves and carbs need a relationship that seems to optimise at 85% of the intake valve head to work best. If the intake pipe is smaller the speed in the port is higher earlier so power zone comes in earlier. The floor of the port where it turns under the guide seems a dead area for flow, so if its filled the ports are smaller but the flow stays the same, what's the point? The pipe is smaller so gas speed in it is higher earlier in the rev range, packing air in before the inlet closes, so power starts earlier and stays up into RPM as well.
I intend to have my pretty much standard MK1 850 dynoed before and after the head swap to see the difference. If it needs new rings or whatever I'll note it in the post showing the differences. I'll stick with the standard cam and compression ratio etc. so that I have a point of reference. Damn, this is going to be interesting. Alan, the design will be refined along the way if needed to optimise performance, and yes, it is a unique opportunity which I am grateful for.
I have tried to find some info on the shape and design of the heads on formula 1 race cars, with no success....now those are the guys that realy have it together getting the MAX HP PER CUBIC IN. Look at HARLEY'S V-ROD....PORCHE designed that motor and it gets 125 HP out of 1100 CC. Doesn't seem like they like to share their secrets with the rest of us. They say if you could vaporise a cup of gasoline properly it would have enough energy to move the empire state building a foot.