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  • 4 Post By POwer tuners RAJ

Thread: Yamaha rx series, increase in compression with different heads of the rx series

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    Post Yamaha rx series, increase in compression with different heads of the rx series

    In an internal combustion engine, a piston compresses a large volume of a mixture of fuel and air into a very small space. The ratio of the maximum piston volume to the minimum compressed volume is called the "compression ratio." Compression is often measured in PSI through a compression gauge.



    Compression ratio in two strokes depends upon the design of the head and the bore of the engine. The slight change in engine packing or gasket can also affect compression ratio. Two strokes basically have primary compression ratio and secondary compression ratio. Coming to the head, two stroke engine compression mainly depends upon the squish band volume of the head. The bowl area is designed to get the desired compression. The bore of a two stoke engine also plays a major role in the compression ratio of the engine. The ports of a two stroke engine are designed to get the desired level of compression, increase in port size will lead to loss in compression but has other advantages like increase in power band etc. For ex- if you increase the transfer port size through porting you may get the desired level of increase in power band (for applications like top speed etc.) but there is decrease in compression ratio.


    I had imported an head from Thailand which is used on an RX-KING. RX-KING is an 18.5bhp engine. The RX-KING transfer ports are approximately 2mm larger and a larger exhaust port than our Indian RX-5SPEED. RX-5 SPEED is a 14bhp engine and RX-KING is an 18.5bhp engine. The increase in compression is basically due to increase in the size of the ports which is compensated by an HIGH COMPRESSION HEAD (LESS SQUISH VOLUME), bigger carburettor, better ignition and a close ratio gear box for making better use of the power.


    The engine head as I mentioned was form the RX-KING, I wanted to check the compression ratio on an Indian RX 4SPEED which is 12bhp with smaller transfer ports and exhaust port and an Indian RX-5SPEED with comparatively larger exhaust and transfer ports. The compression ratio of a 4 speed engine with stock head was 130psi and after fixing the RX-KING head on the same engine it was 170 psi!! and coming to the 5-SPEED engine the compression ratio on the stock head and bore was around 135psi and after fixing the RX-KING head the compression ratio increased to around 155psi. The difference in the compressions between 4-speed bore and 5-speed bore is due to the change in size of the ports. Right now building a strong engine base for my 5-speed engine by fixing up the 4y2series crankshaft which comes from the RX-KING as it is designed to take up more stress and beating as I want to use the RX-KING head on my 5-speed which will be great fun. Usually ported bores loose low-end rpm due to loss in compression, but any Indian RX-135s which are ported for higher power bands, the low-end can be compensated by using such high compression head. The RX-KING is a legendary engine and is the best of the RX-SERIES. I will share the experiences with my 5-speed once the build is done today. :-D :-D





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    Default Re: Yamaha rx series, increase in compression with different heads of the rx series

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    Default Re: Yamaha rx series, increase in compression with different heads of the rx series

    Interesting !!!! Would love to see your bikes perform, some videos would be of great help for learning enthusiasts like us.

    Cheers

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    Default Re: Yamaha rx series, increase in compression with different heads of the rx series



    sure.very shortly.

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    Riding a motorcycle is like being in the film


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    Default Re: Yamaha rx series, increase in compression with different heads of the rx series

    Quote Originally Posted by POwer tuners RAJ View Post
    In an internal combustion engine, a piston compresses a large volume of a mixture of fuel and air into a very small space. The ratio of the maximum piston volume to the minimum compressed volume is called the "compression ratio." Compression is often measured in PSI through a compression gauge.



    Compression ratio in two strokes depends upon the design of the head and the bore of the engine. The slight change in engine packing or gasket can also affect compression ratio. Two strokes basically have primary compression ratio and secondary compression ratio. Coming to the head, two stroke engine compression mainly depends upon the squish band volume of the head. The bowl area is designed to get the desired compression. The bore of a two stoke engine also plays a major role in the compression ratio of the engine. The ports of a two stroke engine are designed to get the desired level of compression, increase in port size will lead to loss in compression but has other advantages like increase in power band etc. For ex- if you increase the transfer port size through porting you may get the desired level of increase in power band (for applications like top speed etc.) but there is decrease in compression ratio.


    I had imported an head from Thailand which is used on an RX-KING. RX-KING is an 18.5bhp engine. The RX-KING transfer ports are approximately 2mm larger and a larger exhaust port than our Indian RX-5SPEED. RX-5 SPEED is a 14bhp engine and RX-KING is an 18.5bhp engine. The increase in compression is basically due to increase in the size of the ports which is compensated by an HIGH COMPRESSION HEAD (LESS SQUISH VOLUME), bigger carburettor, better ignition and a close ratio gear box for making better use of the power.


    The engine head as I mentioned was form the RX-KING, I wanted to check the compression ratio on an Indian RX 4SPEED which is 12bhp with smaller transfer ports and exhaust port and an Indian RX-5SPEED with comparatively larger exhaust and transfer ports. The compression ratio of a 4 speed engine with stock head was 130psi and after fixing the RX-KING head on the same engine it was 170 psi!! and coming to the 5-SPEED engine the compression ratio on the stock head and bore was around 135psi and after fixing the RX-KING head the compression ratio increased to around 155psi. The difference in the compressions between 4-speed bore and 5-speed bore is due to the change in size of the ports. Right now building a strong engine base for my 5-speed engine by fixing up the 4y2series crankshaft which comes from the RX-KING as it is designed to take up more stress and beating as I want to use the RX-KING head on my 5-speed which will be great fun. Usually ported bores loose low-end rpm due to loss in compression, but any Indian RX-135s which are ported for higher power bands, the low-end can be compensated by using such high compression head. The RX-KING is a legendary engine and is the best of the RX-SERIES. I will share the experiences with my 5-speed once the build is done today. :-D :-D





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    Superb bro... Really one of the most interesting and important knowledge u have shared...I was actually hunting on this topic.... Thank u very much....v would also like to hear or c more of ur bike....

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