120hp vs 105hp (1974 HF's) differences???

[Glenn NZ]

Hey there everybody!

Ive got a 105hp short shaft in great condition, and a 120hp long shaft that I was looking at bolting onto my 105 leg.
What are the differences in the 2 engines??? ( as I may just swap some of the bits over if the 120 blows?)

Many thanks!

 

[Frank Acampora]

Re: 120hp vs 105hp (1974 HF's) differences???

If they are about the same year, there is almost no difference at all Porting and carbs. Tghe 120 will bolt onto your leg with absolutely no modifications. Depending upon ignition type, you may need to change one or two wires at the switch and the engine terminal board.

Other than that, you are good to go.

I didn't know that Chrysler made the 105 in short shaft version--that's a new one to me. But as long as you use the short shaft and lower unit, the splines are the same, so everything will mate.

 

[Glenn NZ]

Re: 120hp vs 105hp (1974 HF's) differences???

Hey Frank, Thanks for that! The compression seems higher in the data for the 120s, so I thought they mite have different pistons or head also?

 

[Glenn NZ]

Re: 120hp vs 105hp (1974 HF's) differences???

mmmm yep i had the engines side by side today & the lengths looked the same to me! they may actually be the same, will check it tomorrow

 

[Frank Acampora]

Re: 120hp vs 105hp (1974 HF's) differences???

Nope! as long as it is a Chrysler, the 105 and 120 blocks were the same--same size piston, 3.3125. Deck height and heads were the same so I can only suspect that compression was set by port height which was different. I know port height and size was different, but I never actually measured so I couldn't tell you how much. ALL 4 cylinder engines EXCEPT the 125 and 140 were 2.80 crank throw. The 125 and 140 were 2.875 stroke. They will swap into any block, but unless the block is correctly ported, it will not deveop the power. That is: if you swap a 125 crank into a 105 block, you will not automatically get 125 HP. Maybe 110.

EVERY Chrysler (with the exception of the 2 carb two cylinder 55-60-65) from the 20 That's right, 20, up to the 140 used the same connecting rods.

 

[Glenn NZ]

Re: 120hp vs 105hp (1974 HF's) differences???

Thanks Frank, Whats the carby difference? would the 105 get a few extra HP with the 120 carbs? (or just run rich?)

 

[Frank Acampora]

Re: 120hp vs 105hp (1974 HF's) differences???

The 105 came with 1 5/16 diameter venturi Tillotson WB carbs. The 120 came in two versions: 1 5/16 diam WB carbs and 1 5/8 diam Tillotson TC carbs. The main difference in the 1 5/16 venturi carbs is the diameter of the butterfly and carb outlet into the manifold. The 120 WB carb has slightly larger butterflies and outlet and thus will flow a little more air--if the engine is capable. It also has slightly larger jets to flow the extra fuel that a 120 needs. Remember: the high speed jet only functions to LIMIT the amount of fuel delivered at full throttle.

Using the 120 1 5/16 carbs on the 105 will yeild no extra power as it is primarily port size and timing that sets power output. Thus it can not pump the extra air through a WB carb that a 120 will. Since the airflow will not be as much as a 120, the high speed jets will not deliver more fuel to the engine than the smaller jets of a 105 carb. THUS: no power gain.

To use the 1 5/8 TC carbs which are physically much larger, the manifold covers must be changed, but that's all. Since the TC has probably 1/2 the air restriction of a WB carb, the engine MAY breath easier and you might get 5 to 10 HP more, might not. I do have a 105 that I substituted the TC carbs onto, but I have not yet tested it so I could not say for sure.

NOW: you may ask, haven't I contradicted myself about airflow through the carbs? No, not really. It takes part of the engine's horsepower produced to pump air into it. This "lost" HP is not accounted for in the rating. So, a 120 block may be producing 130 HP and using 10 HP just to pump air to keep running. The WB carbs are so similar that the airflow difference is minor and there is still a given restriction at the manifold and reeds. The TC carbs are so much larger, that the airflow restriction AT THE CARBS is much less. THUS: It takes less horsepower to induct the same air charge. This horsepower can then be used at the prop. Same thing if you removed a water pump impeller: The engine would have 15 extra horsepower at the prop--wouldn't run for more that 2 minutes but would have more horsepower.

 

[tx1961whaler]

Re: 120hp vs 105hp (1974 HF's) differences???

Does a water pump use 15 HP?

 

[Frank Acampora]

Re: 120hp vs 105hp (1974 HF's) differences???

Whaler: The 15 was a guesstimate fer example. However: As I wrote in a different post answer, in the 1980s when Chrysler changed from crankshaft rating to propshaft rating, they de-rated the 105 to 90 HP Thus: the water pump and lower unit gears absorb about 15 HP. And lower unit gears are more than 98% efficient so at least 13HP is being absorbed by the pump.