RH4 vs RH10 cylinder head performance
- Thread starter APRRSV
- Start date
- Status
Fast Eddie
VIP MEMBER
APRRSV said:
I assume you've read the very long thread by Commoz looking at flow and velocity of various heads etc?
I think that's as good as you'll get. To have Dyno comparisons of the heads would mean someone dyno testing a bit with a known 100% stock RH10 and then swapping it for a known 100% stock RH4, without changing anything else and re-testing the same bike on the same dyno etc.
I'd be a tad surprised if anyone has done this...
I bought my '72 Combat new, which I've probably had apart fifteen times in the past forty-five years. I've also had an 850 which I ran with RH4 and RH10 heads--pistons were .040" over with the standard camshaft, carburetors and crossover exhaust, albeit with 32x30mm manifolds for the RH10.
The Combat is lighter and quicker with a higher top end than the 850's, although the RH10 seemed to have better torque--to me, it's just behind the Combat in street performance in my suburban setting. The RH4 head seemed to be down three or four BHP and was definitely breathless above 5000 rpm. In my opinion the RH10 head on the same engine is a plausible alternative to the Combat.
For the first time I'm installing an RH10 850 engine in my Combat machine while my 750 undergoes major repair from a broken exhaust valve. We'll see how that works out.
Tim Kraakevik
kraakevik@voyager.net
The Combat is lighter and quicker with a higher top end than the 850's, although the RH10 seemed to have better torque--to me, it's just behind the Combat in street performance in my suburban setting. The RH4 head seemed to be down three or four BHP and was definitely breathless above 5000 rpm. In my opinion the RH10 head on the same engine is a plausible alternative to the Combat.
For the first time I'm installing an RH10 850 engine in my Combat machine while my 750 undergoes major repair from a broken exhaust valve. We'll see how that works out.
Tim Kraakevik
kraakevik@voyager.net
Which head has the 30mm dia. inlet ports ? My theory is that bigger diameter ports operate better at higher revs because the gas velocity is high enough to give good cylinder filling. And if you are operating at higher revs, the number of firings per minute is higher, so you get more horsepower. So there are two choices - a torquey motor, or one with a power band which is higher up the usable rev range. In both cases you gear the bike to suit the intended use - i.e. a short stroke motor with big inlet ports operates well on a power circuit, the torquey motor works better on a rider's circuit. With the torquey motor it is possible to run with higher gearing on a short circuit. With a top end motor, unless you have 6 speeds, it can be a real pain on shorter circuits. With a 4 speed close box, you choose where you want to lose races.
I tested both heads on the same stock bottom end several years ago. I used the same 32mm carbs.
The maximum horsepower was basically the same.
The rh10 made more power below 5000 than the RH4. As I recall the largest difference was 3 or 4 horse at around 4000 rpm.
I also tried 32mm straight manifolds on the rh10 and tapered the ports in the RH10 to match. It then made less power than either stock head. Jim
The maximum horsepower was basically the same.
The rh10 made more power below 5000 than the RH4. As I recall the largest difference was 3 or 4 horse at around 4000 rpm.
I also tried 32mm straight manifolds on the rh10 and tapered the ports in the RH10 to match. It then made less power than either stock head. Jim
Good information Jim and thanks for sharing.
In the first RH10 test you cited where there was more power at lower RPM, were the manifolds tapered from 32mm at the carbs down to 30mm at the ports?
In the first RH10 test you cited where there was more power at lower RPM, were the manifolds tapered from 32mm at the carbs down to 30mm at the ports?
Fast Eddie
VIP MEMBER
comnoz said:
How dare you spoil the party by bringing facts to the table??
How counter intuitive is that ?!
I'll bet hundreds of heads have been 'tuned' exactly like that over the years !
comnoz said:
Comnoz - If tapering down the intake port from 32mm reduces performance and is the worst choice of the 3 port shapes as you say above. Then why did you design the FA head intake port with a taper down from 32mm?
FA head below showing ports tapered down from 32mm
Small port original Norton head with no taper.
There must be some taper for the raised FA port floor but it seems like you would have provided the FA head with 30mm ports and use tapered manifolds for either 30 or 32mm carbs.
How good is torque measurement on most dynos ? Are they responsive and repeatable enough to detect small improvements in midrange power ? My meagre knowledge of modern dynos tells me that the measurement is based on the rate at which a heavy drum spins up when driven by the rear wheel of the bike. Perhaps as the torque increases, so does the slip of the rear tyre ? How is the calibration maintained and is the calibration absolute or relative ?
Given the following caveat, i.e., that engine power is consistent from run to run and does not drop off with successive runs as heat builds or other factors influence engine output, several back to back dyno runs in a row provide a single overlaid line for both torque and HP curves on a well maintained and operated dyno. So yes, repeatability is very good and small changes in torque are readily observed and measured. And of course dyno readings are always corrected for temperature and pressure, i.e., changes in air density, as changes in air density obviously result in changes in tune and consequently HP.acotrel said:
Modern Dynojet or Superflow motorcycle chassis dynos are typically specified to measure up to ~ 750 HP, so our mighty Nortons at 50-100 HP are not exactly taxing these systems unduly.
I have found that when cam'ing and carb'ing up a Norton, as far as the head goes, less can certainly to be more.
A dumb ole RH1 has been an improvement over the Combat Head. Maybe this isn't saying much to those who work with these often, but for those who don't, I feel there is no reasonable need to search out the RH3. However, an RH6 would be nice to try.
As far as my dumb ole RH1 goes, I did mill the face and cleaned up the ports, including a short taper to smooth the transition of the 32mm manifold output.
I realize I am speaking of the 750 motor, but the I think the point being made here is that it is not hard to go to far with the soup up processes and end up losing ground. Sometime less is more.
A dumb ole RH1 has been an improvement over the Combat Head. Maybe this isn't saying much to those who work with these often, but for those who don't, I feel there is no reasonable need to search out the RH3. However, an RH6 would be nice to try.
As far as my dumb ole RH1 goes, I did mill the face and cleaned up the ports, including a short taper to smooth the transition of the 32mm manifold output.
I realize I am speaking of the 750 motor, but the I think the point being made here is that it is not hard to go to far with the soup up processes and end up losing ground. Sometime less is more.
A common mistake many Australian historic racers make is to over-port the inlet tract. My feeling is the gas speed needs to always be above a certain value. If the port is too big, the motor never reaches the revs at which the port operates best. A big inlet port probably only looks good. Does a flow bench ever really replicate what happens when the motor is running ?
'
Modern Dynojet or Superflow motorcycle chassis dynos are typically specified to measure up to ~ 750 HP, so our mighty Nortons at 50-100 HP are not exactly taxing these systems unduly.'
If I was doing the assessment of a Norton engine, I'd like a dyno with a range of zero to 100 BHP. Do you measure your big ends with a metre ruler ?
Modern Dynojet or Superflow motorcycle chassis dynos are typically specified to measure up to ~ 750 HP, so our mighty Nortons at 50-100 HP are not exactly taxing these systems unduly.'
If I was doing the assessment of a Norton engine, I'd like a dyno with a range of zero to 100 BHP. Do you measure your big ends with a metre ruler ?
Dances with Shrapnel said:
Yes, they were tapered manifolds.
I have since found out that if you are using larger carbs than the port in the head [which helps] then the tapered section needs to be before the straight section of the port in the head. The straight section of the port flows fast with no turbulence so leave it small and straight. It works best if the taper is in the last 1/2 inch of the manifold or a very short taper right at the start of the port in the head. Make the short taper like a radius similar to a bellmouth on a carb. Jim
The taper which matches my 34mm carbs to the 30mm ports is probably about the first 12mm of the port and the angle a where the taper ends is blended in to the parallel part. However I don't play with petrol - only methanol. I don't know how you guys ever get your jetting right. With methanol, the jets are 1.6 times the diameter to give twice the flow. So the effects of errors are halved. - Means it is easier to get it lean enough to get best power using normal jets and needles. Under those circumstances, the effect of an error in the taper would be minimal, - if you were using methanol instead of petrol.
- Status