How is Counter Rotation done in 1988 7.4 Inboards?

K

Ken Wolkens

Active Member
May 22, 2014
154
Melbourne, FL.
Boat Info
390
Engines
Gas
1988 Mercruiser 7.4 inboards in a 390 EC. I'm curious if one of the engines actually spins backward or if it's done in the transmissions?

Thanks, Ken
 
If you have access to the boat you can just pull the coil wire, have an assistant crank the engine at the helm while you see which way she spins.
 
Last edited:
1988 340 SDA is in the engine. Does not make sense to me as it could easily been done in the tranny. I suspect it was an outsourcing issue between sea ray and Borg warner in my case
 
To follow up, My Engines are equipped with the Borg Warner direct drive trans. I can pull a tag number off them the next time I'm up there.
 
Lot more than firing order to accomplish reverse rotation. One has timing chain the other has timing gears, different starter,
different cam grind, different raw water cooling pump hose connections, transmission pump indexing is reversed
 
Happy Hour IV said:
Lot more than firing order to accomplish reverse rotation. One has timing chain the other has timing gears, different starter,
different cam grind, different raw water cooling pump hose connections, transmission pump indexing is reversed
Click to expand...


This is a more accurate explanation. My 1986 390EC has Crusader 454's. The starboard engine is the standard (left hand) rotation motor connected to a BW reverse 1.91 ratio transmission to make the prop rotate RH for the starboard side of the boat. The port engine rotates opposite (right hand) and use the same BW 1.91 reversing transmission the make the prop rotate LH for the port side of the boat.

In the mid 1990's the transmission manufacturers developed transmissions that would make reverse rotation engines unnecessary.
 
What about the distributor on the backward spinning engine? Does it spin backward also?

I'm trying to plan for the day that I need to replace the distributor because the Thunderbolt modules are no longer available.
 
My counter rotation thunderbolt module quit so I replaced my ignition with a Davis Unified Ignition "DUI" specified with a reverse rotation gear.

I like this system so much I will be doing the other engine soon.
 
The answer to your original question is, yes. One engine actually runs the opposite direction from the other. If I recall, the serial# specifies rotation....or the model #.
 
Do the distributors actually turn the opposite way or is it just the drive gear? Wouldn't that drive the oil pump backwards?
 
Thornton69 said:
Do the distributors actually turn the opposite way or is it just the drive gear? Wouldn't that drive the oil pump backwards?
Click to expand...

OK I know it gets confusing but in the reverse rotation (RH) the crankshaft rotates clockwise and the camshaft, distributor and oil pump rotate the same as a standard (LH) rotation engine. That is accomplished by the camshaft being driven by two gears (no timing chain) off of the nose of the crankshaft. That way the camshaft rotates in the same direction of a standard (LH) rotation engine. The camshaft is machined to open and close the valves in the opposite order of the standard rotation engine while allowing the distributor and oil pump to rotate in the same direction (clockwise when viewing from the top). The plug wires are the installed in the opposite order in the distributor cap to coincide with reverse firing order. I know it's confusing and I even know experienced mechanics that can't grasp the concept. The main reason I get it is because 2 years ago I pulled both engines and had then totaled redone with mild performance camshafts to yield 380hp. I did the complete R&R of all components and both engines fired instantly when I turned the keys. That reverse rotation motors was fun to figure out.
 
hottoddie said:
OK I know it gets confusing but in the reverse rotation (RH) the crankshaft rotates clockwise and the camshaft, distributor and oil pump rotate the same as a standard (LH) rotation engine. That is accomplished by the camshaft being driven by two gears (no timing chain) off of the nose of the crankshaft. That way the camshaft rotates in the same direction of a standard (LH) rotation engine. The camshaft is machined to open and close the valves in the opposite order of the standard rotation engine while allowing the distributor and oil pump to rotate in the same direction (clockwise when viewing from the top). The plug wires are the installed in the opposite order in the distributor cap to coincide with reverse firing order. I know it's confusing and I even know experienced mechanics that can't grasp the concept. The main reason I get it is because 2 years ago I pulled both engines and had then totaled redone with mild performance camshafts to yield 380hp. I did the complete R&R of all components and both engines fired instantly when I turned the keys. That reverse rotation motors was fun to figure out.
Click to expand...

Thanks for the thorough explanation.:thumbsup:
 
Thanks for the explanation. Haven't been into reverse rotation gassers before. Assumed the cam was same direction as crank.
 
Thanks Hottoddie!

This was exactly the info I was looking for!

So if I understand this correctly, I should order two standard rotation distributors because it's just the wires in different positions on the cap that are different. Correct?

Thanks,

Ken

hottoddie said:
OK I know it gets confusing but in the reverse rotation (RH) the crankshaft rotates clockwise and the camshaft, distributor and oil pump rotate the same as a standard (LH) rotation engine. That is accomplished by the camshaft being driven by two gears (no timing chain) off of the nose of the crankshaft. That way the camshaft rotates in the same direction of a standard (LH) rotation engine. The camshaft is machined to open and close the valves in the opposite order of the standard rotation engine while allowing the distributor and oil pump to rotate in the same direction (clockwise when viewing from the top). The plug wires are the installed in the opposite order in the distributor cap to coincide with reverse firing order. I know it's confusing and I even know experienced mechanics that can't grasp the concept. The main reason I get it is because 2 years ago I pulled both engines and had then totaled redone with mild performance camshafts to yield 380hp. I did the complete R&R of all components and both engines fired instantly when I turned the keys. That reverse rotation motors was fun to figure out.
Click to expand...
 
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