Scott what timing numbers do you recomend for a 260 in the whiplash i need to rebuild one of my engines.

@ 155 intake, 178/180 exhaust, stock transfer height works very well over a broad rpm and still retains great torque.

Quickdraws have been made with both the standard and full circle crank. Our 109 mph record engine had a standard crank. Our 100 mph one way sport hydro had a full circle crank. I think other factors were more important in the engines' power.

Intake and exhaust timing have been steadily climbing over the years. I think you can run 188 degree duration exhaust with over 160 degree intake duration along with all the other standard modifications and still pull start the engine. Big high rpm gains are possible with increased intake timing. Start by trimming a little off the intake side piston skirt. Somewhere in the .030 to .050 area the engine will get hard to pull start. In the .050 to .080 area the engine really looses low end torque and won't pull your standard prop, depending on what hull you are running. The particular pipe may influence this as well. My testing is with an M&D pipe.

Lohring Miller

So it looks like to change the intake timing your doing this by trimming the piston and not the intake port.If this is true are you basicly widenly the intake port?I have been running 150 and 180 for the timing numbers with the transfer stock hieght.My whiplash never was a real fast hull best run being around 65 with the 2716/2 blade.Most of the time running right around 60.How much squish do you recomend?

Try and keep my squish clearance within .015" to .020" range.

I will note that as you increase intake duration your engine will rev out better, but to allow engine too do so requires a slightly reduced prop loading. If you maintain the same RPM once you have trimmed piston, only increased spitback will be your reward ... Not increased power >

Bruce,

There is so much more to getting those last few MPH out of ANY hull than just the engine ..... Hydrodynamic drag plays a HUGE part, thats Riding surface drag, Rudder & Turn fin drag, Stuffing tube & strut drag, and lastly how Sharp and accurate prop blades are.

Then you get the Aerodynamic drag .... Frontal area that sees the wind, cowl shape and any cut outs or scoops that grab air cause drag.

I am not one who builds wild timed engines and have NEVER found the need too, but have always found those last few MPH with careful attention to the points mentioned above

Scott

Trimming the piston skirt on the intake side only is an easy way to raise the intake timing. The M&D cast cylinder uses the stock piston with a higher intake port in the cylinder. It amounts to the same thing. I like tighter squish clearances than Scott, about .010 to .015. I bring the transfer port heights back up by the amount I take off the bottom of the cylinder (.020) to lower the stock squish. The other standard mods are lightening the piston and opening the carb bore. It's all a package and the best engine builders have it down very well. Each modification only helps a little, but everything together gives you a 6 to 6 1/2 hp engine compared to a little over 4 hp with the same carb and pipe on a stock cylinder. Remember, the power required goes up as the CUBE of the speed. This means that a mod engine will only be 1.18 times as fast as a stock one. However, if a stock engine powered boat goes 50 mph, the mod engine could push it 59 mph.

Lohring Miller

Lohring I would agree this best, but actually doing accurately is VERY difficult to do !!

IMO, you have more to lose screwing up the flow pattern than gains made in duration of transfer cycle.

To the layman engine tweaker, NOT something I would recommend attempting.

Regards,
Scott

I use a milling machine to cut the edges of the transfers and a ball end mill with a tilted dividing head to extend the rear transfers over the intake port. I do hand grinding to blend the cuts into the rest of the transfer. I know some people can do a good job by hand grinding, but I'm not one. My exhaust cuts are mostly done on the milling machine as well with some hand touch up to bring up the center of the port. Below is a picture that also shows the M&D cast cylinder.

Lohring Miller

http://www.modelgasboats.com/media/kunena/attachments/legacy/images/2709_P1010037_JPG916ed75eaf14ff34db3b48c647988e90

Thanks for the picture Lohring!Would you mind showing pictures of the carb and exhaust ports?Im going to mod a engine that needs rebuilt over the winter.I have never bought a modified engine just trying on my own.Any help is great!My first mod is still running but could be better.The more i see the mods you and Scott has shown is a great big help!!Thanks for taking the time to explain and show pictures of how its done.