Idle smoothness of Cross Flow vs. Loop fuel scavenged engines.

Was just surfing the Chrysler site and noticed that Frank had a new toy, a shaved head for a triple. I noticed the size of the wedge was much smaller, indicating to me that a lot of material had been removed.

But that trigged a question and I may have heard it before, so if you choose.....

I heard that Cross Flow engines idle smoother than Loopers. Is there any truth to that, i.e. say I have 2 engines of similar capabilities, but one is one and the other the other? I know the little 40-50 hp Mercs built off the original 4 cylinder Cross Flow design were very smooth as were the 2 cylinder OMC's of yesteryear. I know the latter vibrated a lot at idle, but it had soft rubber mounts and the boat got very little if any of that.

Just curious and someone may want to use the answer in selecting their engine of choice.

Mark

Re: Idle smoothness of Cross Flow vs. Loop fuel scavenged engines.

Cross flows idle smoother than loopers---usually. But there is more to it than that. The crankcase design has a lot to do with it. That's why the old crossflows had crankcase drain valves. When they eliminated the drain valves in the early '70s, the motors didn't idle as good as their ancestors. Newer motors have better overall design, so I don't think one can make a blanket statement. But still, if I wanted a slow putt-putt motor, I'd select a crossflow.

Re: Idle smoothness of Cross Flow vs. Loop fuel scavenged engines.

F_R said:

Cross flows idle smoother than loopers---usually. But there is more to it than that. The crankcase design has a lot to do with it. That's why the old crossflows had crankcase drain valves. When they eliminated the drain valves in the early '70s, the motors didn't idle as good as their ancestors. Newer motors have better overall design, so I don't think one can make a blanket statement. But still, if I wanted a slow putt-putt motor, I'd select a crossflow.

Click to expand...

.

I agree. There is much more to it than a general statement.

The best idling, two stroke, I ever owned, was a 1973 Johnson 85 V-4. That engine was somewhat unique. It was not the 99.6" block that OMC built on, for years to come. I believe it was 96.9" cid. I maybe mistaken on the numbers?

That 85 would idle for hours, with no complaints. We trolled for hours, in Lake Michigan pulling downrigger weights and lures. It was attached to a 18' Starcraft SS. We always used the OMC oils and decarbed it yearly.

Other than water pumps, one Powerpack and regular maintenance, that engine logged over 1000 hours (ten years), confirmed. We worked with a dealer that insisted on an hour meter, at installation. We agreed and were forever thankful for the insistance of the dealer.

We maintained it well. Other than the Powerpack failure, it still ran and got us back to port, and served us for many years.

One of the best engines ever built, in my opinion.

The later 90's (99.6 cid), that I owned, did not idle as well. Less tolerant of endless trolling. We found the same to be true of a later 70 HP.

Re: Idle smoothness of Cross Flow vs. Loop fuel scavenged engines.

Once you understand the basic principles of each design, certain things will become clearer.

The cross flow design uses multiple ports located on either side of the cylinder for exhaust and intake (bypass). The piston has a baffle to direct bypass mixture up into the combustion chamber and sweep out residual exhaust gases. Sometimes there is what is termed a "Boost port" (Merc Tower of Power) to help direct incoming charge. Because of the baffle, the piston is rather heavy. This has one immediate consequence: Since the piston stops dead at both TDC and BDC, it must accelerate and decelerate twice through the 360 degree crankshaft stroke. Since it takes HORSEPOWER to accelerate and decelerate, the available horsepower from the engine is reduced. THUS a crossflow engine of the same displacement as a loop charged engine will USUALLY produce less horsepower. Note: "Always" generalities never apply because of the sheer number of variables.

A loop charged engine has ports spaced around the cylinder wall and specifically angled to direct gas flow. There is usually only one or two large exhaust ports . This design uses a flat topped piston. This has several benefits: 1. The piston is lighter thus less horsepower is wasted in acceleration and deceleration. 2. The angled ports usually ensure a better gas transfer with more exhaust exiting, less admixing of gases, and more intake charge delivered to the cylinder.
3. Since the piston is flat, the head can have a hemispherical combustion chamber for more uniform burning AND the head can have a "squish band" which leads to more turbulence and faster burning. This allows much less timing advance--usually on the order of 18 degrees at full advance versus 30 degrees for a crossflow. A squish band is a ring of material around the circumference of the head that at top dead center very closely approximates the top of the rim of the piston. This "squishes" the charge into the center hemi chamber. Since the burn is faster, more complete burning is accomplished during the power stroke and less raw fuel is expelled with the exhaust.

Because of the differences, a loop charged engine will usually outperform a crossflow engine in the delivery of power and fuel economy.

In an engine set up for general use, I would not expect major differences in idle performance. I do not have practical experience comparing the two. HOWEVER: ANY engine set up for maximum performance (racing) will ALWAYS suffer a poorer idle than a general performance engine. This is due to port timing and gas flow characteristics.

To the best of my knowledge, the loop charged engine design and tuned exhaust pipe were developed by an early 1960s model airplane racer called Ted Wisnewski. He was unbeatable. These concepts were copied by japanese model engine builders, the dirt bike manufacturers, and finally outboard engines.

Re: Idle smoothness of Cross Flow vs. Loop fuel scavenged engines.

Thanks guys for your time.

Quite a few guys ran the 96 cu in 85 OMC around here in the early 70's and on. I ran the '72 125 and had it for about 7 years (after running a 71 Chrysler 55 hp and later an 85) and never put a wrench to it other than to change plugs every couple of years or so. Gas was not bad as it had some in/out tuning which helped, making it much better than the original v4's, but it was cross flow. The Chryslers never had a maintenance item but I only owned them for a year and bought both new. The 85 Chrysler liked it's petroleum.

I had a "Direct Charged" Tower in 115 hp. A superb engine and ran it for me about 7 years. On the 17' Ranger Fisherman, speed and fuel economy were excellent......and smooth idling........ Replaced a leaking head water jacket cover and plugs over the years. Son in law some 13 years later still running it.

Frank, I figured you'd respond to this and that was my original intention. Did Chrysler/Force ever produce a looper? When did Force quit using the West Bend exhaust outlet and use the thru hub prop for exhaust as it was originally designed?

Later.

Mark

Re: Idle smoothness of Cross Flow vs. Loop fuel scavenged engines.

I believe that the Force line started to use the "thru-hub" exhaust after Brunswick (Mercury Marine) aquired them, but I could be wrong.

Re: Idle smoothness of Cross Flow vs. Loop fuel scavenged engines.

Chrysler and later Force never produced a loop charged engine. Even that beautiful 60 was a crossflow. Force did introduce the term "Swirl Charged" which was just marketing hype--It was the same old block.

Thru-hub exhaust was introduced --I believe-- in 1989. It was certainly present from 1990 to 1994 as --again, I think--the "D" type lower unit.

My experience with the thru-hub exhaust on my 15 foot test bed Glastron is that it does NOT produce any more power AND--it tends to ventilate easier. For that reason and because the gear thrust within the case is upward, I don't particularly like this unit and any of the two piece drive shaft lower units either.

Re: Idle smoothness of Cross Flow vs. Loop fuel scavenged engines.

Frank Acampora said:

Chrysler and later Force never produced a loop charged engine. Even that beautiful 60 was a crossflow. Force did introduce the term "Swirl Charged" which was just marketing hype--It was the same old block.

Thru-hub exhaust was introduced --I believe-- in 1989. It was certainly present from 1990 to 1994 as --again, I think--the "D" type lower unit.

My experience with the thru-hub exhaust on my 15 foot test bed Glastron is that it does NOT produce any more power AND--it tends to ventilate easier. For that reason and because the gear thrust within the case is upward, I don't particularly like this unit and any of the two piece drive shaft lower units either.

View attachment 124405View attachment 124406View attachment 124407

Click to expand...

Thanks.

OT sorta, Frank, did you ever run the white, alum. 3 bladed, OEM props that were supplied back in the '70's or so that had the reverse curve blade and experience cavitation erosion that I mentioned elsewhere?

Mark

Re: Idle smoothness of Cross Flow vs. Loop fuel scavenged engines.

from a theretical design point of view i would say the crossflows has better chances for a smooth idle, in practice all modern 2 strokes ( e.g etec) are loopers, and i think we can agree that an etec will outperform in regards to a smooth idle every old carbed crossflow, just because of it fancy fuel injection electronics which eliminate the basic principle disatvantage of a looper .

best regards !

Re: Idle smoothness of Cross Flow vs. Loop fuel scavenged engines.

Mark: If you mean the 18 spline, thrust pin props like these, yes, I originally had my 90 on the old two piece midleg and ran these props. They did sometimes develop "cavitation burn" at the blade roots. The paint would wear off and a rough area about 1/2 inch wide and the length of the blade root would appear.

Re: Idle smoothness of Cross Flow vs. Loop fuel scavenged engines.

Frank Acampora said:

Mark: If you mean the 18 spline, thrust pin props like these, yes, I originally had my 90 on the old two piece midleg and ran these props. They did sometimes develop "cavitation burn" at the blade roots. The paint would wear off and a rough area about 1/2 inch wide and the length of the blade root would appear.

View attachment 124680View attachment 124681View attachment 124682View attachment 124679

Click to expand...

Yes Frank, #2 looks like the likely candidate. Since you are into this brand, and I am just curious as I thought the lower unit/prop/conical hub were real cool at the time would be interested in what you did to either live with it or overcome it.

Thanks,

Mark

Re: Idle smoothness of Cross Flow vs. Loop fuel scavenged engines.



I always liked the later two piece lower unit with the long cavitation plate and the slope from the midleg to the plate. However, the only engine I had that used this lower was switched to a one piece midleg and lower unit to prove the Chrysler claim that the one piece was 3 MPH faster. I did a post on that and yes it is faster than the two piece.

I am contemplating using the two piece on the block I am currently modifying but then, I would not get a valid comparison .

As to the cavitation burn issue-- I really don't remember what I did or if I did anything at all, so I can't help you there.

Additionally, I have been told that the two piece lower "blows-out" at speeds over 60 mph. This means that the shock wave from the nose cone is larger than the prop diameter and all thrust is lost. It had never been an issue with me because I had never had a rig capable of those speeds. Now, however, with this blue boat I can hit 60 and am looking to get 65. Thus, a two piece lower would not be in my best interest. That is a shame because I do have a couple of O.J. racing bronze two blades for that lower.

Re: Idle smoothness of Cross Flow vs. Loop fuel scavenged engines.

One of the worst engine for idle was the 72-74 50hp Johnson.This was a not a true looper as it had a dome piston with pressure backed rings a true looper piston is flat. With this design it was not unusally to see this engine new with 130+ psi cranking compression.

Re: Idle smoothness of Cross Flow vs. Loop fuel scavenged engines.

Faztbullet said:

One of the worst engine for idle was the 72-74 50hp Johnson.This was a not a true looper as it had a dome piston with pressure backed rings a true looper piston is flat. With this design it was not unusally to see this engine new with 130+ psi cranking compression.

Click to expand...

Any idea as to why they did that when they had a successful 3 cylinder looper.....exhaust timing maybe that the triple had and it didn't?

Mark

Re: Idle smoothness of Cross Flow vs. Loop fuel scavenged engines.

Frank Acampora said:

View attachment 124712View attachment 124713View attachment 124714View attachment 124715

I always liked the later two piece lower unit with the long cavitation plate and the slope from the midleg to the plate. However, the only engine I had that used this lower was switched to a one piece midleg and lower unit to prove the Chrysler claim that the one piece was 3 MPH faster. I did a post on that and yes it is faster than the two piece.

I am contemplating using the two piece on the block I am currently modifying but then, I would not get a valid comparison .

As to the cavitation burn issue-- I really don't remember what I did or if I did anything at all, so I can't help you there.

Additionally, I have been told that the two piece lower "blows-out" at speeds over 60 mph. This means that the shock wave from the nose cone is larger than the prop diameter and all thrust is lost. It had never been an issue with me because I had never had a rig capable of those speeds. Now, however, with this blue boat I can hit 60 and am looking to get 65. Thus, a two piece lower would not be in my best interest. That is a shame because I do have a couple of O.J. racing bronze two blades for that lower.
View attachment 124717

Click to expand...

So what mil are you running for those speeds? Is the boat in the pic, apparently your blue boat a cat?

Re: Idle smoothness of Cross Flow vs. Loop fuel scavenged engines.

A stock Force 90 gave 55 MPH and a stock Force 125 gives 60. The prop is either a 12 X 21 Chrysler cleaver with full cup or a 13 X 21 ballistic. Strangely and surprisingly to me, while the Chrysler cleaver ran at 5900 RPM the Ballistic dropped the engine RPM to around 4500 and still gave the same top speed.

The hull is a 14 foot flat bottom of unknown origin BUT recently I saw an ad on ebay for a larger
OMC hull which looks amazingly like mine.
My hull is highly modified to resist the forces of the high horsepower and speed. Transom is built-up and reinforced with hard aluminum, stringers were replaced with all fiberglass ones, hull and deck were glued together, crossmembers were built into the new floor, and seats are bolted to a 2 X 4 frame solidly attached to two crossmembers. It used to be that I would run the boat for 1-2 hours then take it home and glue it for 8 hours. If I ever get a round tuit, I will modify the bottom some more in an effort to get to 65---then maybe some engine work. On the straightaway, it is surprisingly stable but I do need to slow down to turn. It is a boat that I only use when there is very little traffic on the river.