Crossflow vs. Looper

Hey guys! I wanted to get some opinions on repowering my boat. It is a 18'3" deep vee center console. I have a 1973(I think) Evinrude 85 on it now and it runs pretty well but it uses the s#@% out of fuel and it tops out a 30 mph. Of course if I throttle it down it gets better fuel economy but it is only running about 23mph or so. I have two motors in my garage that I have picked up over the past year. One is a 1985 Johnson 150 V6 crossflow and the other is a 1986 Evinrude 120 V4 looper. Which one would be a better motor for my boat? Top speed is not that important, but I also don't want to take all day to get to my fishing spots. I am mainly concerned with fuel economy as I only have 18 gal fuel capacity. I also do a little watersports in the summers so I still would like a good holeshot. Any suggestions would be greatly appreciated.

Re: Crossflow vs. Looper

looper, the crossflows are harder on fuel, but i'd do comp checks on both motors before I'd even begin to make a consideration.

Re: Crossflow vs. Looper

The smaller engine(V4) will get better economy than the bigger engine(v6) , but you have to assume they are both in the same mechanical condition.

To clarify these odd-ball 2-stoke scavenging terms, a looper is what is more properly known as Schneurle scavenging; the piston top is either slightly crowned, or flat, and there is a pair or symetrical transfer ports in the cylinder that are opposite each other. They direct the incoming charge at the side of the cylinder wall that is directly oposite the exhaust, and there may also be some booster and auxilliary ports aimed at a much steeper angle towards the combustion chamber to "loop" the charge up and over the combustion chamber to sweep it clear of exhaust gasses.

Older "cross flow" engines have a baffle on the piston to direct the incoming charge from the transfer ports upwards towards the combustion chamber. Without this baffle, the charge would go straight towards the exhaust port and not fill the cylinder as effectively.

Typically, only high performance 2-cycles use Schneurle "loop charging". They usually have the exhaust port taking up about 40-50% of the stroke, making their corrected compression ratio no greater than about 6.7:1 max. The combustion chamber is a Hemispherical type with an angled squish band to quench the charge into the hemi dome for more efficeint burning due to the turbulence this creates.

Cross flow engines by contrast have very low exhaust ports that usually are only about 30-40% of the stroke, giving a higher effective compression ratio, and a longer power stroke, but they do not rev as high due to the baffled piston being quite heavy. This design is great for low end torque, and low speed trolling, and quite fuel efficient when compared to a Schnuerle of the same size, but not as powerful.

Cross flow was around for years, originally developed by Scott, Schneurle was developed in the late 1930s in Germany by Dr. Schneurle. Typically cross flow engines are long stroke, or square, while Schneurle "loopers" are either square or short stroke.

Re: Crossflow vs. Looper

crossflows are fat

Re: Crossflow vs. Looper

You do better on fuel with your 85 than either of the others, by a significant %.

Re: Crossflow vs. Looper

The 85 hp engines of that year were only 92 cubic inches-not much for pushing that big boat. Cubic inches relates to torque, and the V6 has larger displacement at 149 cubic inches than the V4 at 110. Keep in mind the V6 will run faster than the V4. You can throttle the V6 down to run the same speed as the V4 would at full throttle to improve the V6 cruising efficiency/mileage compared to the V4.

Re: Crossflow vs. Looper

I would second the sugeston to check out each motors mechanical condition, then start to decide.

Re: Crossflow vs. Looper

I would also recommend a new tank. 18 isn't enough with either engine unless you boat on a very small lake.

Re: Crossflow vs. Looper

You def. need to know the compression of the other two motors. I will tell you your 120 is the same block as my 140, jsut carbeurated differently. WIth my 140 on my HEAVY 19 mako I top out around 45, but also burn good bit of gas - think around 13 gph or so. I can run half throttle around 30 or so and conserve quite a bit, and this power head is also the largest before stepping up to a V6, which would consume quite a bit more even at half throttle (UNLESS you got a newer fuel injected V6). You must also take into consideration the weight difference between the V6 150 well over 400 pounds and the 120 looper at or under 300 pounds (my 140 1985 johnson is 315). And last but not least propping... i agree above a bigger tank would suit you better for either of those engines, or at least carrying a spare! But i love my looper...
Hope this helps...

Re: Crossflow vs. Looper

DocShock said:

You must also take into consideration the weight difference between the V6 150 well over 400 pounds and the 120 looper at or under 300 pounds (my 140 1985 johnson is 315).

Click to expand...

My 80 115hp is 315 as well. His 120 will be the same weight.

Re: Crossflow vs. Looper

if top speed is not importand like you said your 85hp wil be the best option for fuel economy.

if you need to get a skier fast out of the whole the v6 will be your best option.

i would get the 120 if its in good shape and prop it out to pull a skier fast out of the hole.
so when you go fishing just dont go over 4400 rpm and you safe some gas in stead of running it wide open.

ist it more fuel efficiant than your 85??

i dont think it is.

does it have faster wholeshot?

ooh yes baby.

Re: Crossflow vs. Looper

I appreciate you guy's input. I realize that the 85 should get better fuel economy but as was stated by someone else before, I think that my boat is too heavy for that motor. I had a Johnson 88SPL on it and that motor ran circles around the 85. Also for the 85 to get the boat up to 30 mph the motor is turning(by my boat's tach) 6000-6200 rpm. I know it is not good to spin the motor so fast, so I pull back the throttle a little to about 5500 or so and I am only going about 21mph. The boat has 13-3/4 X 17 prop that was on the 88SPL and it worked great on the 88, but my tach didn't work then so I am not sure what the rpm's were. With the 88 the boat would plane in 2 seconds and top out at about 36. The 85 came with a 19 pitch prop and it was really a turd with that prop. My theory is that with the extra power either of these two bigger motors produces, I can run at about 4000-4500 rpm and still cruise about 30ish mph, thereby saving fuel. When I run the 85 at 5500, it gets about the economy as my 88SPL did. I went all through the 85 and it is tip top, runs great and has 150 lbs on all cylinders. Plus the 85 is going on my aluminum boat soon. Does anyone know what prop size I need for the bigger motor?, or is it the same pitch as the smaller motor's prop?

Re: Crossflow vs. Looper

Oh yeah, the boat has a 45 gal tank, but its got a pinhole leak, and I cant get it out the hull without modifying the floor a little and I have not done it yet, so I am running three of those 6 gal outboard fuel tanks.

Re: Crossflow vs. Looper

I run a 15x17 on my 140.. the 120 and the 150 should be somewhere close to that.

Re: Crossflow vs. Looper

my 20 foot mako has a 140 with a 13 3/4 X 19 and runs at 5600 rpm. stainless viper. my speed is around 40 mph depending on the chop in the bay.

i would try a 17 stainless on your boat with the 120hp.

you will deffenatly increase your speed an wholeshot.

Re: Crossflow vs. Looper

Well, I put the 150 on my boat, which by the way is really a 19'4" not a 18'3" (thats what the title said). It runs pretty well. I put a 14-1/2 x 19 prop on it that I had off of another boat and yesterday in a strong headwind and considerable chop, it ran 38-39 at about 4500rpm. I think that I need to raise the motor up on the transom a couple of holes because the water level when on plane is right about at the joint where the lower unit bolts up. It also got the same or even slightly better economy than the 85 and that was running wide open! The only issue I had was that after I ran it for a while, when I turned it off to fish and then went to restart it, it would usually start ok, but it would bog and die when I tried to take off. It was kinda hard to pinpoint the cause, but I think it may be the spark plugs. It has the "high speed plugs" that don't have a ground electrode installed and I am going to put the regular plugs in. I don't know how old the ones that are in it now are and I think that they are fouling out at low speeds. I am not real familiar with the "high speed plugs".

Re: Crossflow vs. Looper

you should run manufacturers suggested spark plugs


will pumping primer bulb solve this problem?

Re: Crossflow vs. Looper

The plugs that are in it are the manufacturer suggested plugs for "high speed operation". As far as squeezing the primer bulb, I don't think it helps any. I tried it one time and it seemed to help, but it could have been luck of the draw but every other time it did not seem to help any.