Converting a 1986 120 HP V4 Looper to a 140

[muchco]

Hi, my brother has a healthy 1986 120HP V4 Looper that he would like to get a little more out of performance wise. We are considering changing it into a 140. Has any one done this, and if so what were the results? We are assuming that the 120 and 140 have different carbs, jetting and reed cages, but we are not sure about the exhaust tuners and how they differ. Does anyone have any insight into this. Any other tips to hot rod a V4 Looper would be appreciated.

 

[Dhadley]

Re: Converting a 1986 120 HP V4 Looper to a 140

The carbs are a tiny bit different as well as the intake. I doubt you'll know the difference.

Lets start at the begining. We'll assume the compression is OK and the motor is basically healthy. What boat is it on? How does it run now? Jackplate? What prop, top rpm, speed? What are you looking for? Better holeshot? Carry a load better? More top end?

 

[muchco]

Re: Converting a 1986 120 HP V4 Looper to a 140

Dale,

The motor is mounted on a DC155 (15'5" 1650 lbs loaded) Larson with manual 5 1/2" jackplate. Current set up is propshaft is at 4 1/2" below the lowest part of the hull. Motor compression is 125 lbs on all 4 cylinders. Currently he's running a 22 X 13 1/2 Raker 5700 RPM @ 52.5 mph GPS 15 PSI water pressure. We have raised the motor higher but the water pressure starts to fluctuate. He's looking to get a little more top end speed out of the set up.
My brother has access to a out of commison 140hp power head. We were wondering if there would be any value to install the different carbs and intake without changing any exhuast components (example tuner). We have looked through a OMC parts book and see that the 120's and 140's have different exhuast tuners. Does it make any sense to change intake components without changing the exhaust components? Are there any modifications that you are aware of to hot rod his 120? Thanks for your time and interest.

 

[muchco]

Re: Converting a 1986 120 HP V4 Looper to a 140

Dhadley, any additional thoughts or suggestions?

 

[jimmbo]

Re: Converting a 1986 120 HP V4 Looper to a 140

If you put bigger carbs and intake to let more air in, then the other exhaust probably helps get the extra exhaust out better.

 

[kenmyfam]

Re: Converting a 1986 120 HP V4 Looper to a 140

Basic 2 stroke theory.
The faster the exhaust gets out, the faster and more fuel/air you can get in.
Proved that out a gazillion times on 2 stroke motorcycles.

 

[Dhadley]

Re: Converting a 1986 120 HP V4 Looper to a 140

At the kind of rpm we're talking here the different tuner probably won't matter anyway. If the 140 carbs and manifold are easily accessable, go for it. If the hull will respond to small increases in torque you should see something. I've never played with that exact hull so I don't know. Most likely if it's not a pad bottom hull it won't show much increase.

There's a lot you can do to that motor, including boring it .185" (3.500" to 3.685") to match the 1988 spec of the "big bore" V4 looper. However we need to have a hull that reacts to relatively small increases. If you have a hull that gets up and has very little wetted surface (drag), it'll respond well. The sectret of getting a boat to go faster is to reduce the drag. A pad bottom hull will get up and have almost nothing in the water, almost regardless of weight. At that point it takes very little horsepower and torque to keep it going.

On a standard V bottom hull we'll always have a lot of wetted surface or drag. You can play with props too. The Raker you have does have good lift, good choice.

 

[muchco]

Re: Converting a 1986 120 HP V4 Looper to a 140

Thank you to all for the responses. We will do a little expermenting and keep in touch via posting the results.