Engine swap question - Merc 3.7 vs. 4.1

[brodie123]

Hi All;

I have twin Merc 3.7 L4 165 HP motors in my boat (27' cabin cruiser) that give me constant grief. (One seized on me this weekend)

In our 19' boat, we have a 4.1 L6 165 and I really find it to be bullet proof - very happy with that one.

I scored two Merc 4.1 L6 165 HP engines a while back and want to replace them in the cabin cruiser.

I did a quick measurement and it appears that it will fit fine but wanted to post here before I invest in rebuilding them prior to the install.

I'm not sure about gear ratio or anything else I need to think about.

Any thoughts on this?? What will the difference be between the two motors? I'm hoping a little more torque??

Thanks!!!

 

[redmen62]

My first thought is DO IT!! Chevy used to call that the "Iron Duke 6" and put them in everything from from novas to trucks. I've got one in my run about, and it's great... Super easy to work on (and fully rebuild if needed), very reliable, strong, and pretty efficient for it's age and size.

However, the 470's you have and the 165's you're looking at putting in have different ratios, and if I remember correctly I think I read somewhere that the 470's had a different gimbal assembly too (I haven't been able to find that again though). If you're up to the task and willing to switch the outdrives too, I think it'd be well worth it

 

[Scott Danforth]

The 470 and the 260 cubic inch would interchange ok. With the proper application of parts, you can easily get 200hp out of the 6 cylinders

 

[brodie123]

Thanks. So is it safe to say that without changing the outdrives, The 4.1 would be compatible?

Also, I thought the 4.1 was a "250" and the 3.7 was a "214".

 

[redmen62]

Thanks. So is it safe to say that without changing the outdrives, The 4.1 would be compatible?

Also, I thought the 4.1 was a "250" and the 3.7 was a "214".

3.7L is 225 cu in displacement, the 4.1 is 250 cu in displacement... Many GM guys will call the automotive version either the Iron Duke 6, or just the Chevy 250

They came from the factory with different drive ratios, the 470 outdrives are 1.84:1, the 165's are 1.65:1. Using the 1.84 will make the engine rev too high at WOT. With that in mind, there's hot rod guys that wind these engines out at 5500-6000 RPM, and will even throw a super or turbo charger on them with stock components... so you'll probably be ok. I'd try to find some 1.65:1 outdrives though

 

[Bondo]

So is it safe to say that without changing the outdrives, The 4.1 would be compatible?

Ayuh,.... Even though the 250 Chevy is also outa production, like the 470,....
It's alot more common, parts are still readily available,....

It's a Great swap, 'n the way to go, as ya probably can't fit V-motors in without movin' the drives,...

You'll probably need a new set of props,..... maybe not, sea-trails will tell ya,....

 

[achris]

What year are the 3.7's? The in-line 6 was last used in 1981, that means it only has MC-1 flywheel housings available. If the 3.7's are 1982 or later, then the inner transom plates, couplers, and the drive yokes are not compatible. The good news is that the later model 140/3.0 litre engine flywheel covers will bolt straight up to the old in-line 6 engines. All you then need is the couplers and shorter yokes.

Chris...

 

[brodie123]

Funny I had just measured the bell housing (distance of the mount bolts) and almost cried when I saw a 6" difference. I haven't pulled the motor so can't quite verify the bell housing will fit the L6 but I am so happy to hear it should.

If the housing fits the L6, will the old yoke and coupler off of the 3.6 not line up??

 

[achris]

I said the flywheel cover from the 3 litre, not the 3.7, would fit. The yokes you have will not fit for the in-line 6. You need the shorter (20.6cm) yokes. The yokes on your drives now are 24.3cm.... And will 'bottom out' if you attempt tp put them behind the in-line 6....

 

[brodie123]

So with a new bell housing for a 3.0L, coupler and yoke, I would be good to go with the L6?

Can you please help me confirm I have the right selection here:


Bellhousing:
http://www.ebay.ca/itm/Mercruiser-bell-housing-3-0l-2-5l-140hp-120hp-63530-/321806258932
(I couldn?t find a Sierra # so I assume I have to go to Ebay for this)


Yoke:
26363 as per the image you provided (20.6cm)
I ?think? it?s Sierra 18-2146 ?? I see it on Ebay but having trouble finding the correct Sierra # for 39385A1

Coupler:
Not sure which one I need here. Can you recommend one?
I ?think? it?s the same one I would have had on the L6 previously?? (Sierra 18-2171)

 

[Bondo]

Bellhousing:

Ayuh,.... That's the Wide motor mount flywheel cover, you need the narrow flywheel cover for an Alpha drive,....

I believe, if ya get the 3.0l Alpha flywheel cover, ya won't need the shorter input shaft,....

The In-line Chevy motors went to the standard length longer input shaft when the Alpha drives came out,....

 

[brodie123]

Hi All;

Thanks. I'm struggling to find the right bell housing. I 'think' one of these will be right? But I need a hand to know for sure. Can anyone tell me if any of these are the right one?

http://www.ebay.com/itm/301708604671?item=301708604671&viewitem=&vxp=mtr

http://www.ebay.com/itm/361638690516?item=361638690516&viewitem=&vxp=mtr

http://www.ebay.com/itm/262056048719?item=262056048719&viewitem=&vxp=mtr

 

[achris]

Hi All;

Thanks. I'm struggling to find the right bell housing. I 'think' one of these will be right? But I need a hand to know for sure. Can anyone tell me if any of these are the right one?

http://www.ebay.com/itm/301708604671?item=301708604671&viewitem=&vxp=mtr (should be correct)

http://www.ebay.com/itm/361638690516?item=361638690516&viewitem=&vxp=mtr (should be correct)

http://www.ebay.com/itm/262056048719?item=262056048719&viewitem=&vxp=mtr Not correct (is for a 470)

Chris.......

 

[brodie123]

Awesome, thanks guys! I'll order that up and the L6 is at the machine shop. I'll post how she goes.

BTW, I never ran the L4's at WOT as it was pretty easy to go over top RPM. With people on board and full of gas etc, she would struggle to get on a plane a little bit. The trim tabs help a lot obviously but I'm hoping that with the 2x L6's vs. the 2 L4's, I can up the props a little.

They are 4 blade aluminum but not sure of the pitch, I need to clean the paint off to see. Either way, I was going to go up 2 pitch and call it a day unless recommended otherwise here.

It's a 27' cabin cruiser - late 80's so nice heavy fiberglass. I 'think' the props are 15's so I was going to go to 17".

Any thoughts on this?

Thank you again - I love and promote this site all the time.

Chris

 

[achris]

The '470's were producing 165hp, and a good amount of torque. The in-line 6s produce 165hp and a good amount of torque. If you stay with the same drives (ie, ratios) I seriously doubt you will see much of a difference. Moving to a 'taller' prop will make getting on the plane even slower.

 

[redmen62]

I'd stay with what you have for a little to get a baseline... you might find those are about right (good planing time, in the RPM range, ect). No need to spend money unless you need to

I'm running 17's on my 83 Regal (30' LOA) with twin 305's, so the 15's would seem to be a decent starting place

 

[brodie123]

Thanks guys. I will take that advice and see how she runs for a while - see if I notice a change on getting to a plane. Especially since I am staying with the 2 barrel carb.

I have this same engine on our 19' and upgraded to electronic ignition with a 4 barrel carb simply by removing the reducing plate - that allowed me to go to a higher pitch and when taking tight turns, really holds the speed now.

Curious though. I realize that both the L4 and L6 are 165 HP but what is the difference of the two (with a 2 barrel carb)? Or is it simply that the L6 is far more reliable?

 

[Bondo]

Or is it simply that the L6 is far more reliable?

Ayuh,.... It's better motor architecture from the get-go,.....

Merc tried to reinvent the wheel with the 470, 'n failed miserably,....

 

[Scott Danforth]

The I6 was a GM truck motor the 470 is a Mercruiser only block casting and modified Ford 460 cylinder head.

The inline 6 is a much better motor with a much better potential.

Pistons for the 250 are cheaper if you buy a set for the 307 V8 as a kit

 

[brodie123]

I used the top end of my port engine (which was rebuilt last year) and moved it over to the stbd engine (which needed a new head) and the stbd engine now runs great.

But now that I have an L6 in the machine shop replacing the dead L4 (I will do the same to the stbd engine next year) please pleas please tell me that I don't need to worry about engine rotation and that the left-right rotation is in the outdrive - not the engine itself. Going by memory, I'm 90% sure the both rotated the same way but everything is at the shop and I can't double check. I can tell you the stbd engine is clockwise.