Vire 7 - Increasing the Torque

Webmaster's Note: - Mikael Wesala in Sweden sails an RJ85, an 8.5 metre yacht, fitted with a Vire 7. This, in his own words, is his account of how and why he souped-up his engine. The translation is mainly his, with a little editing from me. If anything is unclear, please E-mail the Webmaster or Mikael with your enquiry and we will do our best to help.

Introduction

As I sometimes use the engine in rough weather, I find the Vire 7 too weak, especially at low revs. Therefore, I converted the cylinder head to a modern shape, with a high compression ratio (11:1) and a squishband. This was done by welding and machining .

The other big improvment was a very long intake manifold. This works on the principle that it improves the airflow to the engine. I tested many different types and the one in the pictures is 43cm (17 inch) long and works very well. At 1500 rpm it increases the torque by 90%. At higher revs the increase drops and at 3300 rpm it is only about 10% more then standard.

Unfortunately this intake made a terrible noise, so I had to make an intake silencer, which you can see as a square-shaped box in front of the engine.

Vire 7 with extended intake manifold and silencer

To measure the torque I built a testbench of an old hydraulic engine driven by a chain from the Vire engine. Then I put a "load" on the Vire engine by closing the outlet from the hydraulic engine. The hydraulic engine was mounted free floating and braked by an one meter long lever. To the end of that lever I fitted a spring balance (one my brother uses for weighing fish). In that way I could see how much torque my engine was producing at different rpms.

Test Rig

I tried to calculate the horsepower and I was a little bit disappointed that it reached only 8 hp. The reason for this is probably the fact that I not have increased the maximum revs, just the torque at lower revs.

Vire 7 - Relative torque for standard and modified engines

Im now very satisfied with the engine. It feels much more powerful in heavy seas and does not slow down as much as previously.

By the way, I have not done a real 12 hp conversion. I have tried to keep the maximum rpms as standard to avoid having to change the gear ratio. As well has having a different carburettor, the Vire12 also has different port timing to let it rev higher, and for that reason they have to change the gearing, which is expensive. The only thing on my Vire 7 which is similar to a Vire 12 is the combustion chamber with the high compression ratio.

If you want more power it's better to build a long intake manifold as on my engine, but don't make it as long as mine. The shorter it is, the higher it revs. My modified engine delivers peak torque at 2150 rpm with a 42 cm long intake. I guess that a 30 cm intake would give peak torque at about 2800 rpm.

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