powerhead tuning

[smyers1115]

next question. this block is used to generate up to 140hp. what needs to change in order to go from 100hp to 140hp. sorry, its a 1979 evinrude/johnson 100hp powerhead now.

 

[Chris1956]

Re: powerhead tuning

You can check part numbers between the two HP models and see what is common and what is unique. Usually the differences are port timing (part of the block) exhaust tuning and carb + jet size.

 

[Dhadley]

Re: powerhead tuning

You can change the block to the same as the cf 140 (powerhead rated - 110 to 115 propshaft rated) pretty easy once it's all apart. However, you'll not gain much in torque. Torque is what moves the boat. Don't ever expect a crossflow to produce the torque of a V4 looper. Even the 60* 90 looper will eat a 140 cf alive. And burn less fuel doing it.

That being said, in most cases you can improve performance (efficiency) by maxing out your set up. Work with that first.

 

[smyers1115]

Re: powerhead tuning

thanks all for the info. i want the most power and torque from this block that i can get. its going to get a complete tear down any how before being placed in service. i am a hot rodder, and understand tuning on car engines and am comfortable working on marine engines. where can i find specific tuning info, and interchange info in order to gain the best performance??

 

[Dhadley]

Re: powerhead tuning

I doubt you'll find it written anywhere. The porting in your block can be made to the same as a 140 cf style. It's not that far off now. The exhaust is pretty easy, just machine out the bottom of the center rib to match the 140 style gasket and install the inner & outer covers. You don't need the (expensive) exhaust filler blocks.

The torque isn't going to change all that much. What's your set up like now?

 

[smyers1115]

Re: powerhead tuning

dhadley, the engine now is stock as far as i know. just purchased on ebay for $5.00. was listed as locked up. a little penetrating oil and its free and turns easily. seems like it has good compression on all cyl. it will be completely torn down to check all internal components and dimensions when i get time. also want to let you know its been a pleasure receiving info from you. you're always a great help.

 

[smyers1115]

Re: powerhead tuning

Dhadley, where did you go??

 

[Dhadley]

Re: powerhead tuning

Got busy with (real) work yesterday. Just don't expect too much with the mods. A lot depends on what boat (hull design) you're going to put it on.

 

[smyers1115]

Re: powerhead tuning

i know the feeling, dhadley. the boat is a 16ft aluminum hull starcraft, 1970 model. i am currently running an 85hp johnson. the boat is rated for 85hp. it performs quite well, but i want more power while maintaining the 85hp serial number on the mid section, if you know what i mean. currently running a 3 blade 13 1/4" x 17p prop. it launches good and tops out at around 43mph. a little slow bringing a slalom ski up, but not too bad. my boy wants to learn how to barefoot and i would like the extra power. the changes you have explained sound like simple port matching to the 140hp gaskets. am i correct on this assumption. of course i will have to jet carb. thanks again for your help.
p.s. my goal is a respectable 55-60 mph with a powerful hole shot. i love experimenting with props to find just what i need.