Head flow testing.

Hi Jim here are some data sheets for my current RH10 head on the race bike that you could add to your collection as it is uneconomical to send you the head.
Regards Mike
This with an Amal 32mm Premier, standard manifold length and short screw on fluted bellmouth that comes with the carb from new.

This with an Amal long bellmouth with the mesh still in the bellmouth, same carb

This with an Amal long bellmouth with the mesh removed, same carb

This is with no carb or manifold, just a short exhaust stub.

Here are Mike's data from above in graphical form, where only intake #1 is shown.

It is obvious from the queries above that forum readers are curious to know more about the relationship of airflow to HP. To that end, a general relationship for correlating airflow to HP is given as HP = 25% of airflow at 28" WC, with the following caveats.

1. Airflow is measured with entire inlet tract in place (manifold, carb, air filter, etc)
2. Compression ratio is maximized
3. Correct cam design
4. Tuned EX pipe

To restate the above 4 points, this relationship of airflow to HP exists only for well prepared race engines.

Using Mike's data as an example, we would expect HP to be ~ 25% of 130 cfm * 2 cylinders, or ~ 65 HP. I believe the dyno results Mike posted in the past showed somewhere around 65 HP. However I don't know if this data was from that engine/dyno run, or what he had for air cleaners, etc. So bottom line, lots of unknown variables at this time. Nevertheless, thought it might be interesting to some.

So, if another 20 cfm @ 28" WC could be realized, another 5 HP/cylinder may be waiting out there… 0.25 * 150 * 2 = 75 HP. I thinks that's pretty close to the naturally aspirated holy grail of Norton HP (unless you are Axtell).

Come to think of it, where did I put that new meatier FA "special port" head that allows porting above and beyond the std FA head?

WZ507 said:

Come to think of it, where did I put that new meatier FA "special port" head that allows porting above and beyond the std FA head?

Click to expand...

I know where it's at.....

comnoz said:

WZ507 said:

Come to think of it, where did I put that new meatier FA "special port" head that allows porting above and beyond the std FA head?

Click to expand...

I know where it's at.....

Click to expand...

Man......you guys crack me up

It's taking a long plane ride tomorrow.........

Can anyone tell me, where in the UK I can get a motorcycle cylinder head gas flow tested :?:
The reason I ask is because I have just polished the ports in a twin cylinder head using nothing more than emery cloth to remove the rough casting in the inlet ports and I find that there is a difference of 1mm in diameter between the two ports. This is clearly a casting flaw.
One is also more a rectangle than the other, this was just lightly removing alloy, no cutters were involved.
I’m having a little bet with myself that the smaller port will be the better of the two :shock:

Bernhard said:

Can anyone tell me, where in the UK I can get a motorcycle cylinder head gas flow tested :?:
The reason I ask is because I have just polished the ports in a twin cylinder head using nothing more than emery cloth to remove the rough casting in the inlet ports and I find that there is a difference of 1mm in diameter between the two ports. This is clearly a casting flaw.
One is also more a rectangle than the other, this was just lightly removing alloy, no cutters were involved.
I’m having a little bet with myself that the smaller port will be the better of the two :shock:

Click to expand...

I am having small bet with myself that professionals measure the ports first and take such variations into account!

Sorry Bernard, but this is why I am nervous about embarking on such tasks......

WZ507,
yes you are right the original dyno chart I posted for this engine made 64 + HP at the back wheel , now that the volumes have been balanced , it makes 69HP at the back wheel, this time no air cleaner, just long horns( velocity stacks ) on the same 32 mm Amal Premiers with 300 main jets
Nothing else has changed.
The reason I ran an air cleaner originally, I ran out of jet sizes (was running 280's) while on the dyno so at the expense of a bit of performance, it made it a little richer so didn't lean out at WOT.
This time it marginally leaned out at WOT on the RHS carb due to a float level issue, although became richer when feathering the throttle at the higher rpm.
Keep up the good work.
Regards Mike

Brooking 850 said:

WZ507,
yes you are right the original dyno chart I posted for this engine made 64 + HP at the back wheel , now that the volumes have been balanced , it makes 69HP at the back wheel, this time no air cleaner, just long horns( velocity stacks ) on the same 32 mm Amal Premiers with 300 main jets
Nothing else has changed.
The reason I ran an air cleaner originally, I ran out of jet sizes (was running 280's) while on the dyno so at the expense of a bit of performance, it made it a little richer so didn't lean out at WOT.
This time it marginally leaned out at WOT on the RHS carb due to a float level issue, although became richer when feathering the throttle at the higher rpm.
Keep up the good work.
Regards Mike

Click to expand...

I'm not trying to rain on your parade, however does your bike lap quicker with 69 HP rather than 64+ HP at the back wheel ? If that is top end motor perhaps you might be faster with a bike with more mid-range and better gearing ? I'm not knocking your work with the flow bench, however there are many circumstances where the observed result goes nowhere near the predicted. I suggest that in the inlet ports and the exhaust system on a bike, you are dealing with sound waves and not laminar flow. So when you use the flow bench, you are making an assumption. When racing a bike there are at least six variables which have to be optimised and balanced to get maximum effect. The situation when this becomes most obvious is when you move from a small tight circuit to a large 'power circuit'. For myself, it is much more important to get the bike right handling and power-wise on small circuits. Every time I go to a race meeting, I see guys racing who have obviously got their gearing wrong, and I've been guilty of that myself. With one of my older bikes you could choose where you wanted to lose a race - in the tight stuff or at the ends of the longer straights. A high horsepower figure might give you bragging rights, however if it makes the bike more difficult to ride, where are you ? It is never as simple as adding up the factors and declaring yourself a winner.

Hi Alan, no you are not raining on my parade, perhaps you are just not paying attention to other parts of the forum.
The head I have shown results from was never flow tested when I built the bike, just put it on the dyno to get initial settings right.
I posted another thread that showed I cracked a DS case on this motor earlier this year, so while I was rebuilding it and had the head off I had it flowed to see what its potential is.
As I have now got the engine back together and it will be raced this weekend, I have no way of knowing where that extra HP will be helpful or not.
Nothing else has changed and the max. torque is where it was before , so no change there.
I am happy with the gearing and can change the rear sprocket quickly if necessary.
The figures I post are for general interest, as the RH10 I have has stock valves, so the info is for others to see what the potential that their own bikes have, certainly not about getting the most HP, those numbers are comparisons, plus it gives me some base line data to work with.
Regards Mike

SteveA said:

Bernhard said:

Can anyone tell me, where in the UK I can get a motorcycle cylinder head gas flow tested :?:
The reason I ask is because I have just polished the ports in a twin cylinder head using nothing more than emery cloth to remove the rough casting in the inlet ports and I find that there is a difference of 1mm in diameter between the two ports. This is clearly a casting flaw.
One is also more a rectangle than the other, this was just lightly removing alloy, no cutters were involved.
I’m having a little bet with myself that the smaller port will be the better of the two :shock:

Click to expand...

I am having small bet with myself that professionals measure the ports first and take such variations into account!

Sorry Bernard, but this is why I am nervous about embarking on such tasks......

Click to expand...

Re; “Sorry Bernard, but this is why I am nervous about embarking on such tasks......”

If you had reread my post you will realise that the larger /difference in port size was already cast into the cylinder head, there will be a difference in gasflow between the two ports as they both have the same inlet valve diameter.
If you are sceptical about the air flow testing on a cylinder head then you are missing out on getting the optimum performance from a cylinder. :shock:

Bernhard said:

SteveA said:

Bernhard said:

Can anyone tell me, where in the UK I can get a motorcycle cylinder head gas flow tested :?:
The reason I ask is because I have just polished the ports in a twin cylinder head using nothing more than emery cloth to remove the rough casting in the inlet ports and I find that there is a difference of 1mm in diameter between the two ports. This is clearly a casting flaw.
One is also more a rectangle than the other, this was just lightly removing alloy, no cutters were involved.
I’m having a little bet with myself that the smaller port will be the better of the two :shock:

Click to expand...

I am having small bet with myself that professionals measure the ports first and take such variations into account!

Sorry Bernard, but this is why I am nervous about embarking on such tasks......

Click to expand...

Re; “Sorry Bernard, but this is why I am nervous about embarking on such tasks......”

If you had reread my post you will realise that the larger /difference in port size was already cast into the cylinder head, there will be a difference in gasflow between the two ports as they both have the same inlet valve diameter.
If you are sceptical about the air flow testing on a cylinder head then you are missing out on getting the optimum performance from a cylinder. :shock:

Click to expand...

No, I read it. What I am saying is that given a head to rework I think you need to confirm the base you are working on before staring work and what is achievable with what you have. Of course that would be easier if you had a bunch of heads to measure and choose from....luxury...

I am not sure I could properly assess the base part to guage how to get the best from it. I think that takes experience most of us just won't have.

No, I read it. What I am saying is that given a head to rework I think you need to confirm the base you are working on before staring work and what is achievable with what you have. Of course that would be easier if you had a bunch of heads to measure and choose from....luxury...
quote]

Then can you explain to me and all and sundry how I can reliably obtain the inlet cylinder head diameter size when it previously had the “as cast” rough casting ( which is like extra rough sandpaper) from a smooth port hole :?:
In the ports rough state, I can only get a rough reading on my calliper’s :!:

You are clearly have taken this out of context :!:

Bernhard said:

No, I read it. What I am saying is that given a head to rework I think you need to confirm the base you are working on before staring work and what is achievable with what you have. Of course that would be easier if you had a bunch of heads to measure and choose from....luxury...
quote]

Then can you explain to me and all and sundry how I can reliably obtain the inlet cylinder head diameter size when it previously had the “as cast” rough casting ( which is like extra rough sandpaper) from a smooth port hole :?:
In the ports rough state, I can only get a rough reading on my calliper’s :!:

You are clearly have taken this out of context :!:

Click to expand...

Agreed, we clearly don't fully understand each other. No problem.

And agreed, the measurement reading from roughly surfaced port, will be well, rough! but even with only a rough reading you will know which is the larger, won't you?

Which will define the minimum achievable port size, roughly! and if you then simply smooth the surfaces (which is what I understand you did), you will be able to measure the size/shape discrepancy with less concern for measurement error (which is where I understand you are now). But at least the port shape and sizing and any discrepancy is now clear.

To get a matching shape and size for reach port, you will need to remove more material from the smaller one (or add material to the larger one!). Your concern seems to be that removing more material being the easier route, this means potentially degrading the flow of that port to the flow off the larger port, right?

But, and I think we agree, matching the port sizes as well as shapes seems like something you should do, in the same was as matching combustion chambers, valve seats/valve sizes/seat angles manifold lengths, jetting etc. is something you should do.

I wonder if Jim or Ken have any guidance or experience to share on this?

Without looking or testing it is hard to say what a rough spot in a port will do for flow. I have seen cases where rough spots can actually improve the flow.

Flow numbers and horsepower do not necessarily go hand in hand. You can nearly always make big flow numbers by making a port very large. A very large port will mean slow airflow and slow airflow will not create port plugging turbulence. But slow airflow will not have enough momentum to efficiently fill the cylinder and a power loss is usually the result.

One thing for certain is if one port has less volume than the other port then they will not tune at the same rpm and if the difference is large enough then the engine will feel unbalanced. Jim

Re; “To get a matching shape and size for reach port, you will need to remove more material from the smaller one (or add material to the larger one!). Your concern seems to be that removing more material being the easier route, this means potentially degrading the flow of that port to the flow off the larger port, right?”

After over 45 years of engine tuning, I know what works and what does not.
Just so that we understand one another I definitely am not contemplating removing metal from the smaller port to match the larger one. I was going to go down the harder route, which is bore out the larger port so that it is concentric, and then fit a split sleeve to reduce the port diameter to match the other one.

Doing anything to degrading the gas flow is the last thing I want to aim for.

Bernhard said:

Re; “To get a matching shape and size for reach port, you will need to remove more material from the smaller one (or add material to the larger one!). Your concern seems to be that removing more material being the easier route, this means potentially degrading the flow of that port to the flow off the larger port, right?”

After over 45 years of engine tuning, I know what works and what does not.
Just so that we understand one another I definitely am not contemplating removing metal from the smaller port to match the larger one. I was going to go down the harder route, which is bore out the larger port so that it is concentric, and then fit a split sleeve to reduce the port diameter to match the other one.

Doing anything to degrading the gas flow is the last thing I want to aim for.

Click to expand...

Let us know how you get on.

I tested an unusual head today. It is a factory racing, big valve shortstroke head. An RH7.

It looks like about the ultimate on a high rpm shortstroke 750 or a BIG longstroke motor. It has 43.1mm intakes and a 33 mm intake port. The velocity is very good.

Here is the CFM graph

the blue line is the RH7
the red is a Maney stage 3
the violet is a Maney stage 2
the green line is an Axtel head with standard size valves.



Here is the port velocitys

the RH7 is blue
the Maney stage 3 is green
the Maney stage 2 is blue



The Axtell still has the highest velocity
the Axtell is red
the RH7 is blue

Why is Axtell the speedest?

hobot said:

Why is Axtell the speedest?

Click to expand...

Because the port is the right size and shape. It allows the air to move fast without creating turbulence. Jim