500cc speedway engine restricted to 250cc

[secsy1]

In Poland apparently they use R500 engines that are restricted to 250cc to save costs as the engine is then unrestricted and used as a 500cc.

Can anyone elaborate on this and how specifically the engines are restricted?

[phillwhitewasmad]

54 minutes ago, secsy1 said:

In Poland apparently they use R500 engines that are restricted to 250cc to save costs as the engine is then unrestricted and used as a 500cc.

Can anyone elaborate on this and how specifically the engines are restricted?

Its through an insert in the carb and inlet manifold to reduce the fuel flow from 34mm down to about 28 or there abouts it is becoming a class in the uk next year

[piston197]

1 hour ago, phillwhitewasmad said:

Its through an insert in the carb and inlet manifold to reduce the fuel flow from 34mm down to about 28 or there abouts it is becoming a class in the uk next year

The CC is worked out on bore and stroke, even if you sleeve down the inlet manifold the bike will still be a 500cc albeit with less power, you would need to sleeve the cylinder bore and fit a smaller piston to produce a 250cc engine, much like Matchless did in the day, the 350 Matchless and 500 matchless single had the same stroke, but the 350cc had a 69mm piston and the 500cc had a 82.5mm piston

[phillwhitewasmad]

5 hours ago, piston197 said:

The CC is worked out on bore and stroke, even if you sleeve down the inlet manifold the bike will still be a 500cc albeit with less power, you would need to sleeve the cylinder bore and fit a smaller piston to produce a 250cc engine, much like Matchless did in the day, the 350 Matchless and 500 matchless single had the same stroke, but the 350cc had a 69mm piston and the 500cc had a 82.5mm piston

Yep but the r is for restricted 500 so they are reducing it power out put by reducing the fuel input it's already happening. It's not about reducing the cc  to 250 it's about using the abundance of lower cost 500cc engines available 

In recent times the Speedway Control Bureau members have discussed the for's and against's introducing a new class to the British Youth Championship of the 500cc Restricted Class known as 500R. The 500R is where motorcycles with a 500cc engine use a 27mm carburetor restrictor which reduces the power to the level of a 250cc engine. The main goals for introducing this class is improve safety, lower costs and easier for riders to learn. The 500R engines have more torque at lower RPM's which makes the bike smoother and more stable to ride. Reducing the power with the restrictor does not effect the stability of the bike, ensuring the safe and smooth riding even for beginners. The 500R engine will last longer than a 250cc engine which reduces maintenance and repair costs, and with more 500R available on the market, this makes them cheaper for riders to buy. 500R motorcycles allow riders to move smoothly from Youth to Senior classes, after removing the restrictor, the engine can be used in full 500cc racing. Riders can use the same bike at different stages of their careers, which saves money on new equipment, the diameter of the carburettor can be easily checked by the Machine Examiner.  Therefore based on this information and with all of British Speedway trying to introduce more riders at the same time of keeping the cost affordable, for the 2027 season the 500R class will be introduced to the British Youth Championship for riders between the ages of 13 years old to 15 years old. As well as the 500R riders will have a choice to compete in the British Youth Championship in the following classes - 500cc, 250cc, 125cc/150cc

Edited 17 hours ago by phillwhitewasmad

[piston197]

10 hours ago, phillwhitewasmad said:

Yep but the r is for restricted 500 so they are reducing it power out put by reducing the fuel input it's already happening. It's not about reducing the cc  to 250 it's about using the abundance of lower cost 500cc engines available 

 

In recent times the Speedway Control Bureau members have discussed the for's and against's introducing a new class to the British Youth Championship of the 500cc Restricted Class known as 500R. The 500R is where motorcycles with a 500cc engine use a 27mm carburetor restrictor which reduces the power to the level of a 250cc engine. The main goals for introducing this class is improve safety, lower costs and easier for riders to learn. The 500R engines have more torque at lower RPM's which makes the bike smoother and more stable to ride. Reducing the power with the restrictor does not effect the stability of the bike, ensuring the safe and smooth riding even for beginners. The 500R engine will last longer than a 250cc engine which reduces maintenance and repair costs, and with more 500R available on the market, this makes them cheaper for riders to buy. 500R motorcycles allow riders to move smoothly from Youth to Senior classes, after removing the restrictor, the engine can be used in full 500cc racing. Riders can use the same bike at different stages of their careers, which saves money on new equipment, the diameter of the carburettor can be easily checked by the Machine Examiner.  Therefore based on this information and with all of British Speedway trying to introduce more riders at the same time of keeping the cost affordable, for the 2027 season the 500R class will be introduced to the British Youth Championship for riders between the ages of 13 years old to 15 years old. As well as the 500R riders will have a choice to compete in the British Youth Championship in the following classes - 500cc, 250cc, 125cc/150cc

Yes having spent my life working around motorcycle engines I was aware of that, the original post suggested the 500cc engines were "restricted to 250cc" by putting a sleeve in the inlet manifold, I was clearly stating this was not the case, it is a power restriction, not a cubic capacity restriction, it is much what the sports mopeds did to get around the law when a BHP and speed restriction was added to the law to prevent manufacturers making 70MPH "mopeds" by tacking pedals onto sports 50cc motorcycles , these were usually restricted by smaller inlets/carbs and restriction in the exhaust, easy to circumvent if you knew what you were doing !!!

[*JJ]

The present 250cc class uses reduced size GM engines, doesn't it?

[secsy1]

Thank you all for the information, it is much appreciated.

Is their a specific 27mm carburettor available or is it just an insert for standard carburettors?