Suzuki Burgman modification (aerodynamics + gearing mods)

[janvos39]

I am in the process of rebuilding the scoot to a 300 cc engine capacity. I like to share with you the reason for doing so. I looked into the MPG values form various scooters on the hubpages.com website and put the extremes per cylinder capacity together in the graph.
Of course there is a wide spread but the trend is very clear. I need a smaller engine to improve onthe FE (if all other circumstances stay the same like aero and gearing and the hypermiling).
So I expect to bring the fuel consumtion from just above the 3 liters per 100 km into the approx 2.8l/100 km.




Also you can see that only hypermiling can bring you a big step forward, but we all know that. However to get to the best result possible also aeromodification are absolutely nessecare. Look for instance at the results of Allert with his full fairing on the 125 cc Honda. And the VW 1l although this is a Diesel with atleast 15 % advantage in the engine efficiency.

[The Atomic Ass]

Originally Posted by janvos39

I feel like I belong.

[Cd]

janvos39 : Very useful thread !

Thanks !

[janvos39]

I made a first testdrive with the lower engine capacity on the Burgman. The gearing which is of the larger engine is still OK to get off the line quickly. Probably due to the long stroke. The gearing of a 250 cc Burgman is 14 km per 1000 rpm with a stroke of 59.6 mm while I have now on the 300 cc a 18.3 km per 1000 rpm but with a stroke of 71.2 mm. I am still using the large carburettor of the 385 because that has an ingnition advance switch coupled to the butterfly. I am prepairing a smaller carburettor of the 250 cc Burgman but have to build the ignition switch function to this carburettor as well.
I expect some more torque when I can change to the smaller carburettor.

[AaronD]

Originally Posted by janvos39

I am still using the large carburettor of the 385 because that has an ingnition advance switch coupled to the butterfly. I am prepairing a smaller carburettor of the 250 cc Burgman but have to build the ignition switch function to this carburettor as well.
I expect some more torque when I can change to the smaller carburettor.

I have heard about throttle response increase with a smaller carburator, persumably because there is a smaller volume of airflow that needs to change mixture before it reaches the cylinder. But I'm curious, why do you expect to see an increase in torque with the smaller carb?

AaronD...


P.S. Also I read above some claims that a "square" cylinder (same size stroke as bore) was best for efficiency. Where are you now with respect to Stroke:Bore ratio? Working backwards from 300cc and 71.2mm stroke I'm guessing you have a 73.2mm bore? That would make you VERY close to square at 0.97 Stroke:Bore ratio.

[janvos39]

AaronD Indeed the engine is now about square with the 73 mm bore on the 71.2 mm stroke. This should work for a 4000 to 5000 rpm range according to an earlier given article.
The smaller bore carburettor helps to get a higher velocity earlier in the rpm range. For good torque the speed in the inlet is to be as high as possible. For instance the Cosworth formule I engine which dominated for many years used a inlet track diameter of 26 mm to create 30+ horsepower with a 33 mm valve.

Jan

[AaronD]

Hmmm... that seems counterintuitive. I take your word for it because you've obviously done your research judging from the rest of this thread. But without knowing any better I'd assume that a higher inlet velocity would mean more friction for the incoming air at that RPM. Which would seem to say that less total air/fuel makes it to the cylinder, and lower torque or horsepower would be the result?

I guess you always have the option of opening the carbeurator further to get the same air/fuel charge as with a slower inlet velocity? Which then means that you simply have a lower top end because the maximum air/fuel mixture that can be let into the cylinder at high RPM is decreased?

I dunno, just trying to follow the logic. Any help appreciated. Thanks in advance.


AaronD...

PS. I guess I'm thinking of the flow test you did on the new 250 head on the previous page of this thread. Apparently there is enough more restriction in the head than the carb that the restriction added by going to a smaller carb is not noticed?

[janvos39]

I have been flowing and modifying and making heads for about 35 years now.
On the photo,s a cil. head for Honda 400 Hawk? I think. Made for dragracing in 1980/1981. And a head which I recently made for KTM racing with as you can see 4 inlet valves and two exhaust valves.
In every head design you have to choose the inlet channel so that at max. torque the inlet speed gets to mach 0.5 (approx 160 m/s at max pistonspeed position) for best performance. Honda made some publications on this subject with formulas added to calculate the best dimensions.





About the restrictions, you are right there. The main restriction is the inlet valve. Even at full lift this restriction is larger than the inlet track or carburettor.
Jan

[Frank]

I have been a motorhead for years and I believe that's the first 6-valve head I have ever seen!

[janvos39]

Frank, you missed some 6 valve heads in history. Maserati made in 1985 a 2.0 l -V6 with turbo and 6 valves. 3 inlet and 3 exhaust valves per cylinder.
Also an Italian fan of Ducati once made heads with 6 valves for the twin Ducati.I somewhere have a copy of an article about that bike.
And if I am not wrong around 1935 a France automobile company made a single cylinder with six valves for a car.
regards Jan