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Merc 7.4 to Volvo 8.2 SWAP

DOVO3058

New member
I have a new-to-me 1991 30ft Bayliner Command Bridge with a possible seized 7.4 Gen 4 due to a failed exhaust manifold. The motor original and looks virtually new with a great maintenance history, so I haven't given up total hope that it can't be revived. Mated to the Mercruiser is a relatively new Bravo 2 outdrive that had 20 or 30 hours a year with documented annual maintenance.

I was practically handed a complete nose-to-tail Volvo Penta 8.2GSi (GEN 6 Fuel Injected) and Penta DP-S Duo Prop outdrive that were removed in running condition according to the previous owner, he had crated everything up several years ago and now just wanted to get the space back in his workshed. They are both in what appears to be good shape except the exhaust elbows were shot on the motor. My intent is to verify the 8.2 runs well with appropriate compression checks etc. on the ground and then backfit whatever I need to in order for it to replace the 7.4 Merc. Of course, I need to buy new exhaust manifolds and risers for the Volvo and a few bits and pieces.

My questions are:
1) Should I go with the original Penta Manifold/Riser setup or use the Mercruiser style?
2) Besides some wiring changes to support the fuel injection and electric fuel pump, what else do I need to consider moving from Mercruiser to Penta?
3) Would you switch to the Duo-Prop if you had all this stuff laying around?

Another option is to part out/liquidate the Volvo setup and stick with the 454. What are your thoughts?

Thanks for the help.
 
Personally I would see whether the lump that's in the boat will run. Doing a swap is a HUGE headache especially if the hole in the transom has to be reshaped, repositioned or redrilled.
Yeh you could just change engines. That too will have it's own set of issues. Fueling for one. You'll possibly have to replumb the whole supply/return hoses.
Bottom line is that if this boat is something you feel strongly about then do the swap. However, if the prospect of doing all this work with two questionable engines, then you should get out of this potential money pit before it eats you alive.
A duo prop equipped boat is like flying on a magic carpet.
 
Ayuh,...... Don't tryin do a Volvo/ Merc,.....

Either go full Merc,....
Or,....
Full Volvo,....
 
Is the VP a composite drive?

I'm unfamiliar with VP terms so I don't know about composite drive. Based on my Google search, I believe it is a DP280/DP290 as best as I can tell. I have not been able to locate the ratio, any help finding that would be awesome. Based on the little feedback that I've received, I'm feeling that liquidating the drive, props, and the 8.2Gsi Fuel injection set up to fund the 454 or 502 rebuild might the best course of action. I'm getting mixed feedback about carb vs fuel injection since the fuel injection would be new to the existing set-up and electronics. Here is the drive.... A33CD217-8C90-4AD6-9FA0-651BB4C5465A_1_105_c.jpg
 
...........................
I have a new-to-me 1991 30ft Bayliner Command Bridge with a possible seized 7.4 Gen 4 due to a failed exhaust manifold. The motor original and looks virtually new with a great maintenance history, so I haven't given up total hope that it can't be revived. Mated to the Mercruiser is a relatively new Bravo 2 outdrive that had 20 or 30 hours a year with documented annual maintenance.

I was practically handed a complete nose-to-tail Volvo Penta 8.2GSi (GEN 6 Fuel Injected) and Penta DP-S Duo Prop outdrive that were removed in running condition according to the previous owner, he had crated everything up several years ago and now just wanted to get the space back in his workshed. They are both in what appears to be good shape except the exhaust elbows were shot on the motor. My intent is to verify the 8.2 runs well with appropriate compression checks etc. on the ground and then backfit whatever I need to in order for it to replace the 7.4 Merc. Of course, I need to buy new exhaust manifolds and risers for the Volvo and a few bits and pieces.


My questions are:
1) Should I go with the original Penta Manifold/Riser setup or use the Mercruiser style?
If you go with the Merc drive, stay with the Merc exhaust system.

2) Besides some wiring changes to support the fuel injection and electric fuel pump, what else do I need to consider moving from Mercruiser to Penta?
Take the engine wiring harness with the Volvo Penta engine and integrate it into your hull and helm.

3) Would you switch to the Duo-Prop if you had all this stuff laying around?
Yes, absolutely.... especially with what I see that you have.

Another option is to part out/liquidate the Volvo setup and stick with the 454. What are your thoughts?
For me, I would go Volvo Penta.

Thanks for the help.

Bill said:
Don't tryin do a Volvo/ Merc,.....

Either go full Merc,....
Or,.... Full Volvo,....
I agree with Bill!

I'm unfamiliar with VP terms so I don't know about composite drive.
What you have is not a composite drive.

Based on my Google search, I believe it is a DP280/DP290 as best as I can tell.
No.... this is a DP-C1 or DP-C2, and IMO, is the best of the AQ series Volvo Penta drives.
These are main suspension fork/pivot tube geometry..... (no Gimbal system, no Gimbal bearing, no annual engine drive coupler alignment required, no thru prop exhaust, nice smooth cone clutch gear engagement, super easy drive shaft bellows R&R, etc.)

View attachment 22325

I have not been able to locate the ratio, any help finding that would be awesome.
Pull the props. Remove the ring anode. Take a wire brush to the AFT area of the bearing carrier. The reduction will be stamped into the carrier.
If this drive was fitted to the 8.2 engine, it will likely be 1.78:1 .

DO NOT loose the spacers that install just ahead of each propeller.

Based on the little feedback that I've received, I'm feeling that liquidating the drive, props, and the 8.2Gsi Fuel injection set up to fund the 454 or 502 rebuild might the best course of action. I'm getting mixed feedback about carb vs fuel injection since the fuel injection would be new to the existing set-up and electronics. Here is the drive....
Your call on that!

side note:
I do not see where the 8.2L GSi was fitted with the drive that you say came with it.
I would guess that someone may have done a re-power in front of the DP-C1 or DP-C2 drive.
 
Why would you install an antique in your boat? Parts for the AQ series drive are scarce and $$$$ Strip the exhaust off the 8.2 swap the bell housing, coupler and mounts , add Mercruiser exhaust and install it. The wire harness will plug right in if both have the 10 pin connector.
 
side note:
I do not see where the 8.2L GSi was fitted with the drive that you say came with it.
I would guess that someone may have done a re-power in front of the DP-C1 or DP-C2 drive.

You are correct, the original motor was a Penta 5.7, the PO said that they needed to change out the bell housing to make it work with that drive. I wouldn't have thought to ask, thank you
 
Chris, the Volvo Penta AQ series ran right along side of the Gimbal suspension SX and DP-S drives, up to 1996 with gasoline engines.
It also ran up to year 2000 with the diesel engines.

The OP's boat is a 1991, of which means that the Merc drive that came with it, would likely be 1991.
A DP-C1 or DP-C2 would be as new or newer than his 1991 Merc drive.
 
..............
You are correct, the original motor was a Penta 5.7,
OK.... the 5.7 would have required the 1.95:1 reduction.
I'm somewhat surprised that the 8.2 didn't eat that drive!
If it is a 1.95:1, I would NOT use it behind that 8.2L engine.


the PO said that they needed to change out the bell housing to make it work with that drive.
Actually, it would have been a change to the flywheel cover (bell housing in the Auto world) due to the larger diameter of the 8.2L flywheel.
It would not have been due to anything regarding the stern drive or the transom shield.

That's a nice thing about the AQ series...... all AQ series flywheel covers will work with the different AQ series transom shields.


I wouldn't have thought to ask, thank you
 
The OP's boat is a 1991, of which means that the Merc drive that came with it, would likely be 1991.
A DP-C1 or DP-C2 would be as new or newer than his 1991 Merc drive.

OP's was a 1995 and the 5.7 Penta with this DP-C was his original equipment, later converted to the 8.2GSi.

My boat is a 1991 and has the 7.4 Merc tied to a 6 year old Bravo2.
 
Leave it to Rick to add confusion and muddy the water.

As to which lump, f*** it, do as Chris says and delete the Mercruiser 7.4, replace with the Volvo flavored one. If at all possible to connect the Volvo outlets to the Mercruiser Y pipe, do it so you don't have to fool with the bracketry on the elbows (I find those 6-ply silicone connectors they sell on ebay are perfect for making transitions). Or put the exhaust out the transom straight and wake up the entire marina when you run it.

The only issue I can see is maybe you need to have a return line to the fuel tank? Otherwise it's kind of a no brainer and a straight up swap out.

PS you can always convert that B2 to a B3 and have your magic carpet ride. You change the lower on the drive, that's pretty much it. Oh and props. Don't have to take it apart much.
 
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Something worth mentioning here in this decision is the transom hole. If I am correct while the AQ may be of the same approximate age as the current merc drive the transom cut out is bigger. If you modify the transom now for a good but obsolete drive system, you cannot go back with out major transom work to a modern and currently available merc or VP drive system at a later date or future repower.

For me this would out of practicality push me to install as dockside mentions above using the current bravo II setup. Leaves options open in the future
 
Is short, Chuck the drive, convert and install the engine on your existing B2 drive and transom assembly.
I agree.... that would be the easiest and simplest thing to do.

Something worth mentioning here in this decision is the transom hole. If I am correct while the AQ may be of the same approximate age as the current merc drive the transom cut out is bigger.
Yes, the AQ series transom shield and transom cut-out are much larger.
Being larger, it places the stern drive torque (against the hull's transom) over a larger area.
This can be a benefit if there are any concerns about the integrity of the structural transom.


If you modify the transom now for a good but obsolete drive system, you cannot go back with out major transom work to a modern and currently available merc or VP drive system at a later date or future repower.
That is true. Once the transom cut-out is enlarged, it would be a rather major task to in-fill the structural transom and create a smaller opening.

Keep in mind that:
.... AQ series is not a gimbal suspension system.
....
no gimbal ring... no gimbal bearing.... no gimbal bell.
.... no rubber hub drive coupler that requires annual alignment checks.
.... incorporates fixed geometry between the engine and transom shield.
.... uses an all steel Borg Warner drive coupler.
.... single PDS bearing that can be replaced with simple transmission removal.
.... no thru prop exhaust routing.
.... very easy and user friendly drive shaft bellows R&R.


Although obsolete in terms of no longer being used as an OEM stern drive, C-1 and C-2 parts should be available for years to come.

But again, Chris is correct...... it will be easier to put it back together the Mercruiser system.
 
The power steering hoses should be the same. The control cable ends are the only question mark. Not sure what the ends are on the Volvo but the Mercruiser has a trunnion and a plastic eye fitting which is removable. There may be a Morse/Teleflex/Seastar adapter available if the cables need it.

One side note EFI systems are really happy when they are getting full voltage so this might be a good opportunity to be sure that your batteries and connections are all up to par. Clean and tight. Grease the heck out of the connections on the battery.
 
The power steering hoses should be the same. The control cable ends are the only question mark. Not sure what the ends are on the Volvo but the Mercruiser has a trunnion and a plastic eye fitting which is removable. There may be a Morse/Teleflex/Seastar adapter available if the cables need it.

One side note EFI systems are really happy when they are getting full voltage so this might be a good opportunity to be sure that your batteries and connections are all up to par. Clean and tight. Grease the heck out of the connections on the battery.
Would think the original merc shift plate would bolt up to merc manifolds like chris suggested using. Only issue is throttle cable would assume new bracket is required.
 
Right but then what about the Volvo ECU and all that? I am just guessing about the differences. OP will certainly have to work out these small deets.
 
Right but then what about the Volvo ECU and all that? I am just guessing about the differences. OP will certainly have to work out these small deets.

Keep tgat and the harness/sensors. I think the suggestion keep all the volvo items except manifolds and elbows, coupler, and flywheel cover. One issue maybe on some volvos I believe there are temp sensors in the elbows, which may need to be jumpered out if using merc exhaust.

Certainly some mix and match pains but makes best use of available components
 
Keep tgat and the harness/sensors. I think the suggestion keep all the volvo items except manifolds and elbows, coupler, and flywheel cover. One issue maybe on some volvos I believe there are temp sensors in the elbows, which may need to be jumpered out if using merc exhaust.

Certainly some mix and match pains but makes best use of available components

You can just delete the VP sensors. Leave the wires hanging, its a one wire sensor.
 
Keep tgat and the harness/sensors. I think the suggestion keep all the volvo items except manifolds and elbows, coupler, and flywheel cover. One issue maybe on some volvos I believe there are temp sensors in the elbows, which may need to be jumpered out if using merc exhaust.

Certainly some mix and match pains but makes best use of available components
Well I was referring to how the ECU box/holder/bracket thing might be attached to the elbow if at all. It might mean drilling new holes or some such thing in the bracket so it will bolt to the Merc elbowwwwwzzzz.
 
One side note EFI systems are really happy when they are getting full voltage so this might be a good opportunity to be sure that your batteries and connections are all up to par. Clean and tight. Grease the heck out of the connections on the battery.

Good to know.... 2 Brand new Interstate batteries and connections....
 
I think the suggestion keep all the volvo items except manifolds and elbows, coupler, and flywheel cover. One issue maybe on some volvos I believe there are temp sensors in the elbows, which may need to be jumpered out if using merc exhaust.

Manifolds and elbows were junk on both so I'm buying new Merc style from GLM/SIERRA as soon as I find a grand..... The old Volvo manifolds did NOT have any sensors, so that's one thing.

As I've looked at everything it seems like the only "Volvo Penta" parts that will remain on the 502 is the EFI system...
 
Although I have full confidence in the drive as it was re-sealed and pressure tested, the wise recommendations here including the incorrect gear ratio for the 502 make the mostly straightforward
Chuck the drive, convert and install the engine on your existing B2 drive and transom assembly.
the plan of action. Once I get the gear ratio confirmed, I'll sell the DP-C to fund the rest of the project.

The insight here is worth far more than the cost. Many thanks
 
I am a few months down the road and I need advice. Since the last update, the Volvo 8.2Gsi fuel injection was set aside as there were problems with the fuel pumps and I want a Mercruiser type set-up. We broke everything down and sent everything to the shop. Now, my Gen VI Engine is now a 502 bored .030 over with new pistons/rings, new rod/main/cam bearings all assembled into a short block ready for the bolt on goodies. Stock Roller Cam was checked out a-ok and re-installed.

This is where I need guidance.

I have a 1996 Mercruiser 502 Magnum EFI system with promising, yet not verified, history. Much of the advice I have been given by reputable builders is to go with an Edelbrock carb set up since wiring to the boat would be simple and these systems are reliable. Others have said the MPI would pay for itself in reliability and fuel savings, but since this boat will likely be run moderately and not looking for breakneck speed, I'm not so certain.

Since I am starting with a short block, I am working for relative simplicity so I like the idea of a carb set-up. The stock Mercruiser 7.4 from 1991 was carb and ran reliably for nearly 30 years, so I am expecting similar longevity.

Assuming the cost from now to install would be similar for MPI and Carb, what route would the group take; Used Mercruiser 502 Magnum MPI with unknown history, or for completely new Edelbrock carb'd set-up?
There would be wiring and installing an electric fuel pump, what else am i missing for MPI?
Do the carbureted and MPI 502 variants of the Gen VI use the same roller cam?
New Manifolds, Risers, and Cooling system are on the way. Hoping this can all come together in the next month or so.

What else am I missing?
 
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