Topic: Exhaust flanges need fins?

[Greyhound]

Wondering how necessary fins are on the exhaust flanges.

Does anyone know know if the fins help with cooling? Or are the aftermarket billet okay to use without negating any cooling functionality?

[KennyRedman]

I doubt they do much as small as they are.

[TwoTired]

Cooling fins add surface area where heat can be exchanged into the air that passes over them. They are there for a reason.  Probably to help keep the chrome exhaust from discoloring as well as remove heat from the head.  The exhaust is one of the highest heat areas of the motor.

Unlike water cooling where there is a thermostat to maintain even temps.  Air cooling is usually designed to save the motor and oil when encountering the most extreme heat conditions predicted.   This means it is over cooled  when the other extreme, cold, is encountered.

If you only drive in winter cold, you can probably remove all the fins on the motor without much issue.  If you are in hot summer traffic sitting in still air even adding more fins is unlikely to keep the oil from overheat damage, with engine seizure the next item in sequence.  The engine fins, and the entire outer surface of the engine is a heat exchange vehicle.  The more surface area, the more points exist for heat to leap off into passing air so long as the air temp is different than than the leap off point.  The greater the differential, the more heat flow occurs. 

The fins are not just a visual affectation.  They are a functional design element.  If you reduce cooling ability, the trade off is applying more attention to your oil condition. And possibly increased frequency of oil changes.  When the trans starts shifting clunky, change the oil, regardless of what the time or mileage is.  Or,  live with shortened life of the motor and cost of replacement components.  What price, custom?

Cheers,

[MauiK3]

+1

[Don R]

  I doubt you will notice much difference. I always consider the complexity of the part. The more it cost to make the more Honda thought it was needed.  They loved to simplify and save per unit cost.

[TwoTired]

  I doubt you will notice much difference.

I agree.  At least not from the seat. :-)  But, our bikes aren't instrumented with temp gauges... anywhere... Oil or Cylinder head.   We just trust their engineering to make a long lasting machine.

Of the era, didn't the British bikes have those same style finned flanges right at the exhaust, where the greatest heat would occur?  There is speculation that Honda took their design cues from those when making the SOHC4, cause they were selling pretty well at that time.

From a heat transfer perspective.  Heat flows greatest/fastest when there is a large differential from source to destination.  As the exhaust flange location is almost certainly the very hottest part of the machine, don't discount smaller fins doing a lot of transfer, simply because that's where the largest differential is located and in direct path of oncoming airflow.  If the heat doesn't transfer off machine at that point, it gets conducted to other parts of the machine for cooling.  Exhaust gasses are in the range of 1300^ F.  Anything next to that is going to absorb heat.  Getting it into the passing air as quickly as possible seems a pretty good idea to me.

If the part change is for that racy look.  Remember that racing machines are usually expected to last for the duration of a race.  After which any damage is repaired or replaced.  Track bikes are seldom very reliable street machines.  Hot rod or raced machines often have far more shortened operating intervals without maintenance than those made solely for street use.  Of course, if you like working on the bike more than riding about, just about any mod is justified.

Cheers,

[Greyhound]

I figured as much as well, hence why I asked… why else would Honda add them. I rarely (if ever) would choose form over function, so depending on how clean up efforts go with my flanges, will likely replace with finned ones.

[754]

Many folks run smooth ones.. some  bikes come with smooth ones.

[Tim2005]

All models of CB400Fs had fins, except the 1977 North America version, which didn't.  I've never heard of them making any difference to anything between those models

[754]

 XL 350 Never had fins, and a lot of Kawi fours came with smooth ones.

[tlbranth]

I have a MAC 4 into 1 exhaust. The flanges have no fins on them. I've had no problems with this setup. But I will admit to not running them by Einstein's equations.

[754]

Yeah and you only had your bike for what ? 51 years..  yes, long enough.

I did wonder why the question was asked... coming from a custom bike standpoint I see reasons why some folks like smooth ones.
 Do finned ones hurt anything, of course not. But if they helped a lot, I bet there would have been a few versions of bigger ones.
 I would say a big reason they have fins is tradition.. lets see what Goldwing used as they are not visible. Off to look.

Back from looking, well Wing ones appear to be flat unfinned, but it is watercooled. Hmmmm
 Then I thought, a CX 500 is also water cooled, but clamps/collars are highly visible ... lets see what they use.,  well they have fins, but sort of mini fins. Hmmmm...... what does that suggest ?

[TwoTired]

If you are interested in heat flow mechanics, here are two major factors in heat exchange into air.  One is area or number of leap off points for molecular exchange and the other is the differential in temps.  The exhaust fins and pipe have a nearby heat source of 1300F or more.  The rest of the motor should not be much more than 300F, if you want oil to survive the experience.  So back of the envelope calc says that when fins are supplied with, say, 100 degree air in constant replacement, a small area fin at 600 degrees will shed twice as much heat as a larger area fin at 300F.

The actual calculations are far more complex, of course.  Each material type has its own thermal conductivity factor.  And the air temp rises as heat is absorbed from a nearby fin surface.  There is also density and humidity of air.  When you drill down into the fundamentals, there are many and they are interactive. 

When calculating needed fin area for power supply survival and realiability 30 years ago, I used spread sheets to determine if semiconductors would survive all the places the unit was intended to live in climates around the world.  The aluminum heat sync selected was 10 inches long, by 4 inches wide, and had five fins along its length one inch tall.  The ten heat generation devices were place equidistant on the flat surface of the heat sink.  Outside air was fan forced at measured velocity across the fins.  I had to make an environmental chamber to put the entire unit inside, then instrument various internal points with a data logging temperature recorder.  For my power supply design, I had to change device locations and add baffles for air routing/concentration in order to have all the components keep their temps down into the survival and longevity acceptable regions.  The heatsink locations farthest from the air supply would be hotter and get heat absorbed air, reducing its effectiveness at keeping devices within temperature tolerance.  A factor I would later have to add to my spreadsheets.

Yes, engineering is tedious and fraught with details which are hard to factor in at the beginning of the design process.  But, when you are making 100000 or more copies of the design and they must all be guaranteed to function where advertised.  The details make that happen when they are all identified before production happens.  And that is where the good engineer’s salary becomes justified and even economical for product profitability.

I’ll be keeping my exhaust fin flanges on my bikes.  Good luck with your bike!

Cheers,

[TwoTired]

I have a MAC 4 into 1 exhaust. The flanges have no fins on them. I've had no problems with this setup. But I will admit to not running them by Einstein's equations.

Might I point out that the Mac head pipes are single wall?  These would heat transfer to passing air faster than double wall pipes.

Has the chrome on them blued?

Cheers,

[754]

So does this mean , you woul like Bigger fins on your exhaust collars ?

[TwoTired]

So does this mean , you woul like Bigger fins on your exhaust collars ?

In winter cold, probably not.  Doing 80-90 through Mojave summer, yes.  Would be nice to have dynamic fins size control. HA!

Much of the reason for liquid cooled motor popularity, is the stability of the engine component temperatures and expansion control.  Air cooled motors generally require larger tolerances to accommodate the wide variance of temperatures in which they operate.  They run hotter in heat, and cooler when its cold.  Metals expand and contract accordingly.

The fins and outer surface area presented are sized as a compromise, they over cool in really cold climes and may well under cool in hot climes, especially when high power demands are placed on them.  The engines are not efficient energy converters.  60-70% of the fuel energy used by the motor is wasted as heat.  The more fuel you are burning the more heat is produced.  It is either absorbed and builds up in the motor, or it it expelled by cooling components into the air passing around it.  There is no controlling regulation of the cooling system in our motors.  Fans in the garage, forward movement outside, is the only heat control options we have for the fixed fin surface area designed into our motor units.  A lot of the heat wasted comes out the exhaust, and those components often experience the most heat extremes.  Conducting heat into air there nearest the engine seems a pretty good means of keeping the rest of the engine unit from absorbing any of that heat and nudging up the oil temps.

Cheers!

[maxheadflow]

Not sure fins on the exhaust pipe clamps are any good.  Seems to me that all they do is kill/heat flow to the cylinder heads, making the motor hotter. Better to keep the heat in the pipe and use the heat to control exhaust flow.  While I'm not a fan, some wrap exhaust pipes to keep the heat in.   

[ekpent]

Wonder if they keep the exhaust valves ever so slightly cooler ??

[carnivorous chicken]

But if they helped a lot, I bet there would have been a few versions of bigger ones.
 

I'm agreeing with you, but I have seen some Triumphs with ridiculously long fins.

I've had a 550K since '92. When I bought the bike it had a Kerker 4-1 with smooth clamps. Had it since then with no problems, but it's also been in Seattle where (until lately) it doesn't get that hot.

[754]

Big fin exhaust clamps on Triumph are aftermarket , far as I know... been around a long time.. look good on some bikes..

[Bodi]

The 400F I have has smooth clamps. I have a set of finned ones that were on the bike but got the smooths on a spare engine. I think the smooths look better. No problem in a few decades.

[754]

I ran smooth collars , high pipes blocking airflow, ran without aircleaners a lot, and my 836  still went around 25 years..
 Now running aircleaner and finned clamps I cant get 3 years out if a topend... not sure why.
 Oh and I  forgot  corner of head fins milled way back shorter. Plus chromed engine covers, plud fins ground off vakve cover.

[SOHC4 Cafe Racer Fan]

My buddy Isaac Siegl makes the smooth ones with the holes under the brand Speedy Siegl - https://www.speedysieglracing.com/

I had them on my Triumph Thruxton.  Cosmetically, they look great.  They did not appear to affect functioning in any significant way.  The exhaust did not discolor near the flanges.