More Power

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Nov 21, 2004
:D Traditionaly speaking any performance modifications start with pipes, mufflers or if the wife allows expensive carbs, this time we will go against the grain straight to the breathing heart of any engine, the CYLINDER HEAD. if your intrested read on. We took an early 70's Norton 750 nothing special, dyno tested it to achive a baseline torque and horsepower at the rear wheel. I a Saturday afternoon we swapped out the stock head and bolted a ported and flow tested head back on, off to the dyno. When the dust was settled we made 8 more horsepower and 10 foot-pounds of torque between 2700 to 5800 rpm. With just cylinder head work the torque gain was 24.7% and horsepower was 18.9% over stock baseline results. When we took it out for a spin the first impressions are of surprising mid-range power and continues pull in the upper rpm band, and in a drag race against an 850, the 750 was three bike lengths ahead and holding. Not bad for just cylinder head work !
Do you just play with nortons on Saturdays ???????
You even just joined this forum just to brag I bet !

Well done, you are obviously pleased with yourself.
Any of us would be, now if I can just work out how you did it........

Did you copy a diagram, is it in a book anywhere?
Please tell us all how you sprecifically achieved it.

With Thanks & Good Job !!

:D Norton Fan, Hows it going eh ? No I don't just play with Norton's on Saturdays and above all I didn't mean to insult anyone by bragging, this project has truned out remarkable that's all. No i didn't copy this off a diagram or book and I will tell all, no secrets here !

I'm a trades man, Licensed Journeyman Automotive Technican. For the last 15 years I've worked rebuilding engines but my specialty is cylinder heads and PERFORMANCE. For the last 8 years I've concentrated on flow testing and porting, testing and porting into the wee hours of the night, I've pissed my wife off more than once. She states that cylinder heads get more attention than she does, one thing I've leared in marriage , women are always right, this time she is and I'm not one to argue Ha Ha, back to heads. The intake port has a 28.5mm opening and a 1.560 valve and the exhaust is a 1.360 valve. The flow numbers and swirl are outstanding, we have a 39.4 cfm gain on the intake and 32.1 cfm on the exhaust over stock flow numbers. At the dyno we went up two sizes over the previous amal settings and tried three different pipe sizes. The best overall combo was with 1-5/8 pipes, there's more work to be done with pipes and carburetion, we need to find the SWEET SPOT ! If anybody has any more question just ask.

Have you ever analyzed a '74 Norton cylinder head? This 850 head has 30mm intake ports whereas all other 850 heads have 32mm ports. The smaller ports obviously increase the velocity of the air/fuel mixture entering the combustion chamber. However, it's not clear to me that faster velocity makes more horse power in this case.

As the 30mm head was produced for only one year, I suspect that some aspect of this design was a failure. Any information or theories is greatly appreciated.


Hi Jason,

Sounds like you have a RH 10 head, M2A mid year change up.
1974 was the year they started major changes to the 850 line, as with any manufacture incorporating new parts into assembly lines means old stuff has to go first, hens the mid year change up. The 30mm port isn't a design failure, its better enginerring for street use, and Norton realized that. In the world of induction enginerring ( heads & intakes ) having smaller intake ports and tuned intake manifolds results in broad power curves and thats what you wont for the street. Having peaky race style power curves suck on the street going from intersection to intersection. You asked about velocity, I hope this dosen't come out to technical but this is what we've leared over the years. There are two velocities at work here, port and swirl, tumble. Port velocity is the design of the intake track 28.5, 30, 32mm, size is the deciding factor. Swirl and tumble is the measurement of velocity entering the combustion chamber, swirl is for 2 valve heads and tumble is for 4 valve heads. The trick is to maintain velocity and swirl in the desired rpm band for the application. We have enough experience know that one can predict real world performance from what is observed on the flow bench. I hope this answered your questions.

Johnny Rocket
30mm port 32mm carb.

HI Johnny

My bike also have the RH10 head with 30mm ports but the carb. is 32mm,
would it be an advantage to cone the first lets say 2-4mm of the port to avoid the flat face in the flow direction?


There should not be a flat surface in the way. If there is, you have the wrong manifolds. Special manifolds were made for this head that measured 32mm at the carb and tapered to 30mm for the head.
If you need em, they are available through 'Norvil' and others.

If the RH10 is such an excellant street head,which it is, why on earth would Norton go back to the RH4, 32mm, for the 75? This has always puzzled me.
Hi lucky,

How's it going eh ? Check the intake manifold first where the head and manifold meet. From what I've seen the manifold at the base were the carbs sit is at least equivalent to the carb size. Usually the manifold tapers down to the size of the intake port in the head, but don't quote me on that, nothing is ever the same from production runs year to year. Try and maintain some venturi effect were the manifold and head meet 29.5, 30, 30.5mm you get the picture. The venturi promotes off idle torque to the mid- range of the rpm band a must for street use, also if you do any grinding or porting finish with a 80 or 60 grit stone. This polishing the ports is bad, bad news, we haven't done that in 15 years. Rough is thumbs up ! I hope this answered your question.


Are you the owner since 1975 of the bike ? In reference to the INOA Tech Digest 30mm is correct, but in the final years who is to say what was on paper is what went on the bikes, my experience's with companys closing there doors is anything goes they have to liquidate old stock, it makes business sense.

Johnny Rocket
Hey Johnny,

You haven't insulted anyone.....Thats was just a bit of ozzie sledgin !
No doubt there are many of us norton fans following this topic with a lot of interest.

Still watching the comments with lotsa interest :!:

It is great to c stuff like this talked about.
:D Here are the Flow numbers on the 750 project, please note that all
flow testing is done at 28" of H20 not the traditional 10"

-Intake-Valve lift----100---150----200----250----300----350----400
Stock CFM (1.490)--47.5--73.9---96.1---114.3--120.9--122.5--124.3
Ported CFM(1.560)--54.2--82.5--106.5--129.6--147.6--157.3--163.7
Gains in CFM--------+6.7--+8.6--+10.4--+15.3--+26.7--+34.8--+39.4

-Exhaust Valve lift---100---150----200----250----300---350----400
Stock CFM (1.300)---37.5--55.1---66.9---74.1---82.5---86.8---94.5
Ported CFM (1.360)--44.9--63.5---78.8--98.9--105.7--117.7--126.5
Gains in CFM---------+7.4--+8.4--+11.9-+24.9-+23.3-+30.9--+32.1

I've had people ask about reliability would the bottom end take the power
so we took an 850 and turned it into a 920 on the dyno she makes 64.8 horsepower and 56.2 foot-pounds of torque, this bike gets flogged , and 2 years later A OK. The whole point is to get more power under 6000 rpm , the less one rpm's an engine the longer they last. So here's a question, would you buy a performance kit , carb , manifold, head , pipes, mufflers that are matched, dyno tested and proven on the street or would you like to guess at what makes power and have douts in the back of your head ?
I would prefer a package myself, I'm tired of hunting for knowledge and parts from the past, so we're doing somthing about it from our corner.
Johnny Rocket
Ok I'm interested/curious.
My stock 750 combat made 47 RWHP and you say it takes a 920 upgrade with tuning to make 64.8hp. It should make 57 just based on the size increase. You're doing good for sure.
What compression do you run and what cam specs(stock cam/2S??)?
For this head work and other much$$ for 7.8hp
Your bike is running still, but have you actually torn it down and inspected the crank and cases? How may times have you drag raced or road raced it?

But in all reality, if I need to go over 110MPH, I leave my antique touring bike (commando) at home and take one of my Ducati's .
Otherwise I do like following tuning efforts and the results obtained. However personally I'm interested in power gains all the way off idle up to about 4500rpm.

I'll have to check the tech digest but I didn't think they say 75s had 30mm heads. Otherwise I will try and have that corrected, since most everyone knows they came with RH4/32mm heads.
tech digest contributing editor
My 75 has 32mm ports and my tech digest lists RH4 for the 75. I was just curious if anyone had any explanation as to why Norton went back to 32mm in 75.
I have two theories. One is Norton didn't really understand the smaller port concept at the time seeing how the Mark III kept the same black box and bean cans.
The other is cost related in that the 32mm was more cost effective to produce. Why I wouldn't know but I do know a lot of decisions are made based upon costs.
Combat RWHP


Do you have any dyno graphs for the combat? How many combats have you put on the dyno? Just curious to see if there is much variation.
OK just measured the manifolds 32mm at each end, seems like I must have the wrong manifolds!
The intake port is 30mm so there is a flat.
Would it be ok just to grind the first 2-3mm of the intake port so it will be conical to fit the size of the carb or schould I go for new manifolds?


I am certainly not a head meister but I would recommend NOT grinding anything unless you absolutely know what you are doing. While the head you have is not necessarily rare it is in deed not common. sells the correct manifold for 24 lbs., english, which is about $40.00 U.S.
If you want the bigger head, RH4 heads are available all over the place.
Lots of people would be willing to buy your RH10, (me for instance) but my recommendation would be to keep it.
[/b] Henrik,

I must agree with Mike on the manifolds, do not grind the 30mm
cylinder head and find the appropriate intake manifolds, you wont to maintain that taper in the intake track for torque purposes it makes a difference in the lower rpm band.

Forum Apology,

In our Norton Community we have a member that owed a1975 850 with the RH10 head and to his knowledge this is factory original. In reference to the Tech Digest I should have read closer and 3 or 4 lines above is the RH4 head, again I apologize, the 1975 850 must have rolled of the assembly line in late 74 for the 1975 year. The bike has since been sold , I would have liked to look at it again. Again my apologies.

Dynodave ,

Later tonight I'll post the 920 specs and figure out a dollar value on the head package.

Johnny Rocket
:D Dynodave,

Here are some of the specs on the 920, She was build by Brent Fraser the " resident Engineer " of our Norton community to be the famed "Hog-Slayer".

Johnny Rocket head--large valves, port work, rockers lightened, titanium retainers, high tensile valve springs.
Crank--lightened, balanced, Nitrided
Pistons-- lightened, complete with lightened gudgeon pins
Camshaft--Megacycles 56-000
Compression ratio 10:7.1
35mm Kehin flat slides
Dual 1-3/4 pipes with aftermarket mufflers
64.8 HP at 6000rpm
56.2 foot-lbs at 4700rpm to 5500rpm
All dyno testing performed on a Dynojet.

Brent Fraser is an avid member of the our Norton group and I'm sure he'll
be willing to share his build experiences but before I give out his contact info I'll have to have his permission, you now client confidentiality


Here is the info on the cylinder head work and final cost.


1. strip & jet wash
2. primary inspection of cylinder head and copmonents
3. chemical crack check of combustion chamber
4. bead blast entire head
5. secondary inspection of cylinder head ( look for stripped threads or external cracks or funnys )
6. R&R valve guides
7. Hone valve guides to individual valves
8. performance valve grind
9. port work
10. final assembly


1. phosphorus bronze guides
2. performance nitrided valves intake and exhaust
3. intake seals
Please note: this package dosen't include springs, at the time of the rebuild the customer will have to choose a camshaft and the proper springs to be installed. Also this dosen't include any thread repair or welding external cracks or broken fins.

In this package you will receive :
1. a before flow test ( stock )
2. an after flow test ( port work)
3. a detailed condition on arrive report
4. a blueprint on the rebuild process
5. final documentation booklet ( for your future reference )

Final Total $ 550.00 Canadian , + shipping

Johnny Rocket
:D Bill and anybody intrested in the " Hog Slayer 920 "

Brent Fraser the owner has compiled a three page portfolio on the build, from why he did it to all the upgraded parts. This is real good reading for anybody intrested, you can reach Brent by email at If Brent dosn't get a chance to respond, you can also reach me at and I'll email a copy.


Johnny Rocket

I advise against modifying your head to match the larger 32mm intake runners. Instead, you should try to find some 32mm X 30mm intakes. However, these may be difficult to find because of their limited production run, '74 model year only.

If you can't find these special intakes, purcahse some used 30mm X 30mm intakes and open up the carburetor side to 32mm. You can use a dremel tool with a drum sander to make a gradual taper from 30mm to 32mm.


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