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4 Valve / 896cc Conversion

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    4 Valve / 896cc Conversion



    I’ve embarked on building a new engine for my Bros. Like my current engine, it is based on the long stroke engine from the Africa Twin XRV750 and the main changes I’m making are to fit the 4 valve top end from the NT700-V and 89mm pistons to take capacity to nearly 900cc.


    The main advantage of the 4-valve heads is that Honda deleted the second spark plug and fitted much larger intake valves, 32mm compared to the 26mm items in the 3-valve heads (see pic). There seems to be some agreement that the standard valves are one of the major limiting factors on power output from the standard engine.

    539017AF-E323-445F-A0E7-B5AA225C53D9.jpg


    Happily used NT700V parts are cheap and plentiful in the UK - I bought a complete and sound engine for a bit more than 200 UK pounds and already had the remains of a blown up AT engine that I’d bought earlier for the heads. Other than a broken crank, the rest of it is in good order.


    Perhaps the main technical challenge presented by the 4 valve conversion is that one of the 10mm cylinder studs doesn’t line up, the other 5 studs do and the cam chain tensioner is common to both models. On the AT engine, the oil way to the head also lines up - I suspect that a conversion of the 650 Hawk/Bros motors with the external oil feed would need extra work in this area but otherwise would also accept the 4 valve head if the stud problem is resolved.


    The way that I did this was to find the centre of where the stud should go on the crankcase and then to bore out the case at that point to accept an aluminium insert that was then pressed in and welded. A friend of a friend kindly did the welding for me and I machined the cases for the insert by adapting my mini lathe to hold the crankcase halves. I also used this set up to drill and tap for the 10mm stud and to clean up the joint face. I finished off the joint face with some emery cloth sandwiched between the cylinder barrel and the crankcase.


    With this critical stage done and the 4 valve barrel and head sitting on its studs the next thing will be to bore out the cylinders and crankcase mouths for the bigger liners. I’m expecting that I’ll have to farm this bit out to the professionals or at least the fitting of new liners and the bore and hone. I’ve got some 89mm pistons from JE which I’m hoping should do the trick. They take the same pin diameter as the AT and the other critical dimension is compatible. They are for a four valve engine although, when I get to that stage, I shall have to check carefully that there is clearance between the valves and the piston crown.


    CA88CB80-2A67-4D53-8961-6631F0A710E0.jpg 17699E12-F872-4F85-A41B-C3191DB63324.jpg 329925FF-052D-4A1B-A1EA-4ED864420B34.jpg 78FA3700-CA75-41F6-BD39-12E880ADC8ED.jpg







    #2
    A couple more pics, showing a barrel and head loosely assembled on the studs.

    DB4E0E22-CF12-49D1-8A6E-D483F7A7E505.jpg F1C6B278-8AC0-4F83-8A48-0913C735AF21.jpg

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      #3
      Wow that’s some project. 900cc in a Hawk chassis that’s going to be some bike !

      Congrats on having the nouse to work it through. Can’t wait to see how it turns out.

      Comment


        #4
        A most awesome attempt. Very best of luck and I hope everything aligns correctly!

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          #5
          Originally Posted by DrPMC View Post
          A couple more pics, showing a barrel and head loosely assembled on the studs.

          DB4E0E22-CF12-49D1-8A6E-D483F7A7E505.jpg F1C6B278-8AC0-4F83-8A48-0913C735AF21.jpg
          Not sure where you are in the UK, but I’d love to see the finished article some day

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            #6
            Subscribed!

            Very cool. Very very cool!! Can't wait to see it take shape.

            What do you have for power estimates?
            Don't spend money and buy, spend time and learn.

            Comment


              #7
              I love it.

              WW/R
              Life is a journey, not a destination.

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                #8
                I am in awe.
                Flock of Hawks | '13 Tacoma | '69 Falcon (currently getting reassembled!)
                I've spent most of my money on women, beer, cars and motorcycles. The rest of it I just wasted.

                Comment


                  #9
                  Originally Posted by 6 View Post
                  Subscribed!

                  Very cool. Very very cool!! Can't wait to see it take shape.

                  What do you have for power estimates?
                  Thank you Sir!

                  I haven’t really thought much about numbers, being more is proof of concept mode. However, Honda claim 66 BHP for the NT700V, which one independent test on-line finds to be 55BHP at the rear wheel. Presumably the extra 200cc, somewhat higher compression and free flowing exhaust and intake must be worth a significant dollop on top. I would imagine that a skilled engine builder (not me), with easy access to a dyno would be able to get to the the 100BHP per litre benchmark, so 90BHP. I’m not sure what the crankshaft would think of that!

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                    #10
                    Do the NT700-V heads have available aftermarket cams?
                    Flock of Hawks | '13 Tacoma | '69 Falcon (currently getting reassembled!)
                    I've spent most of my money on women, beer, cars and motorcycles. The rest of it I just wasted.

                    Comment


                      #11
                      Originally Posted by 69Falcon View Post
                      Do the NT700-V heads have available aftermarket cams?
                      That's a good question and one which I've started to look at. The simple answer would seem to be that there isn't anything available off the shelf - the two bikes that share the 4 valve head are the Deauville and the Transalp, neither of which has a sporting pedigree. Putting to one side how expensive aftermarket cam profiles are (Megacycle etc.), I shall need to investigate whether it is practical to have the profile from another performance cam (perhaps a Hawk one?) replicated onto a reground NT700-V cam.

                      If this happens, this would be a way down the line - I shall probably aim to see what the motor does with just the 4-valve heads and big bore before trying anything else.
                      Last edited by DrPMC; 03-14-2022, 04:23 PM.

                      Comment


                        #12
                        Love this kind of shit. Great job so far! Can't wait to see more progress.

                        Cheers!
                        "I couldn't afford NOT to buy it!"

                        Comment


                          #13
                          Originally Posted by DrPMC View Post

                          I haven’t really thought much about numbers, being more is proof of concept mode. However, Honda claim 66 BHP for the NT700V, which one independent test on-line finds to be 55BHP at the rear wheel. Presumably the extra 200cc, somewhat higher compression and free flowing exhaust and intake must be worth a significant dollop on top. I would imagine that a skilled engine builder (not me), with easy access to a dyno would be able to get to the the 100BHP per litre benchmark, so 90BHP. I’m not sure what the crankshaft would think of that!
                          if nothing else I bet you find the gains you get to be very useable gains. And even if they are minimal, not often do we get to see things this different. If you gain 0 this is still an awesome project!

                          As far as the crank I have read that the limiting factor really is the RPM, that the guys that implode cranks are revving to 10,2 or more.

                          I have also read that it's the tq that gets them and building a lighter rotating setup and spinning it faster is the way to the "safe" power.

                          How's that for conflicting info???

                          I'm no engineer or engine builder. But we do have someone here who could probably definitively answer the crank question (for probably the 50th time.. my memory is not my most robust quality).

                          Hordpower Mr JD sir?
                          Don't spend money and buy, spend time and learn.

                          Comment


                            #14
                            I admire your work but don't think it will be a long lived engine. I would keep a bucket handy to keep all the engine shrapnel in.
                            ​​​​​

                            Comment


                              #15
                              Originally Posted by Fastmongrel View Post
                              I admire your work but don't think it will be a long lived engine. I would keep a bucket handy to keep all the engine shrapnel in.
                              ​​​​​
                              If it blows up it won't be the end of the world - I've still got the 750 engine to put back in. We'll all end in a metaphorical (or actual) bucket sooner or later so it would be a pity not to try these things out

                              However, I am cautiously optimistic, the big bore conversion has quite a pedigree - the German firm African Queens have been offering a similar conversion for the XRV for years (see http://www.africanqueens.de/EN/MOTORCYCLES/legend.html). Obviously my project takes it a step further with the 4 valve head, but the 4 valve conversion doesn't have any obvious reliability implications - the physical modification of the engine needed is relatively minor.

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