# K.O.H.L.E.R COMMAND VTWIN CH750 based engine

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#### philr

##### Well-Known Member
I completely understand a port for 9000 rpm and 3200 will be different but the most critical part of a wet intake port is the short side radius and the cylinder heads on these engines have a sharp 90 there with casting flash hanging in the port at that location. I port and cam every engine I do taylored to the rpm it will make peak torque and HP. If you are stroking it and increasing piston speed and volume porting becomes more critical. Intake length and port speed are very good tuning tools.
Can you provide cam timings that are tailored specifically to 3200 rpm and a very nice running engines at all rpm levels up to that? This engine has to be good at low idle and respond well when punched but also run well at all rpm levels.

#### Chris Matheny

##### Well-Known Member
HBA Supporter
Just adding a stroker crank will increase torque but do so at a lower RPM unless the intake track, valve size and length was not optimized for the shorter stroke to begin with. Since HP=TQ*RPM/5252 you will likely make the same power but at a lower rpm. He may be correct though as the increase in stroke and swept volume will also increase compression ratio, very desirable in pulling, not as much for airplane usage.

#### TiPi

##### Well-Known Member
HBA Supporter
Log Member
Can you provide cam timings that are tailored specifically to 3200 rpm and a very nice running engines at all rpm levels up to that? This engine has to be good at low idle and respond well when punched but also run well at all rpm levels.
Hi philr,

I have mapped the cam lobes on my Briggs 49 and I think they are not ideal for the rpm range that we aim for. I'm looking at re-profiling them to the lobe profile that the Corvair guys are using, similar engine configuration (valves, displacement, power/cyl) and same rpm range. I would be very interested in the cam lobe profile that is used on the CH750 to compare to mine.

The B&S profile has too much lobe separation, compensated by very shallow ramps and a fairly large overlap. Going closer to the Corvair profile should improve low rpm (idle) quality and possibly a slight improvement in the max torque between 2,600 and 3,500 rpm.

#### blane.c

##### Well-Known Member
HBA Supporter
It seems to me that these industrial engines will all need propeller extension as crank ends to close to the block for most applications?

While obviously extreme, it shows that the propeller extension/extension area can be used for other.

(Franklin engine for Sea Bee)

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#### philr

##### Well-Known Member
It seems to me that these industrial engines will all need propeller extension as crank ends to close to the block for most applications?

While obviously extreme, it shows that the propeller extension/extension area can be used for other.

(Franklin engine for Sea Bee)

View attachment 113776
The crank from the big block is available with I think ~10" outside of the engine and full diameter. This should be cut as short as possible to minimize gyroscopic forces while clearing intake and so on.

#### Riggerrob

##### Well-Known Member
It seems to me that these industrial engines will all need propeller extension as crank ends to close to the block for most applications?

While obviously extreme, it shows that the propeller extension/extension area can be used for other.

(Franklin engine for Sea Bee)

View attachment 113776
Prop extensions serve three functions.
Since most automotive engines were designed to feed power straight into a transmission, they were never intended to absorb the extra loads imposed by propellers.
First, they allow mounting a (ball bearing) thrust bearing to absorb thrust loads. The best thrust bearings can handle both tractor and pusher loads.
Secondly, extra bearings strengthen the crank-shaft against gyro-scopic effects.
Thirdly, shat extensions can be used to improve balance. The Franklin engine illustrated above has a shaft extension to allow mounting the engine farther forward to cure a balance problem suffered by many light pusher airplanes. For comparison, the best Franklin engine replacement (for Sea Bees) is a Corvette engine with a shaft extension.