Replace Mercruiser 7.4 - 5L MPI or 5.7L MPI or 6.2L MPI

[Xplicitlnck]

thank you for your clarification, but I wanted to avoid changing the transmissions,
my 7.4 are given for 310HP at 4600RPH
the 350 MAG is given for 300HP at 5000RPM
the 383 is given for 350HP at 5200RPM
Do you think that with two 350s I will also have to change my transmission?
thank you for your help

No matter what motor you swap over to unless it's the the same 7.4 that you currently have, you will have to change out the transmission. But it doesn't end there. You will have to can a good shop and tell them what you previously had in the boat and what you now have in the boat and if there really really good at what they do they will know what to repitch and cup your props to. That almost never goes perfectly right the first time. You will need to take this reworked props put them back on your boat go for a ride make sure you're getting the rated rpms at wide open throttle. If your not pull the boat take the props off and send them back. And so on.
Where do you live and what is this tax your speaking of? Is hottodie correct in saying they tax you 5600$ a year just to have engines in your boat?!

 

[Chip S]

thank you for your clarification, but I wanted to avoid changing the transmissions,
my 7.4 are given for 310HP at 4600RPH
the 350 MAG is given for 300HP at 5000RPM
the 383 is given for 350HP at 5200RPM
Do you think that with two 350s I will also have to change my transmission?
thank you for your help

Mercury is no longer offering the 350 MAG for a 300 hp option. The 300 hp option from Mercury is now the 6.2L. I would stay away from the 350 MAG. 300 hp from a 5.7L engine was stretching the limit of the engine. Talking to people I know who have 350 MAG engines, the 350 MAG engines are not that reliable.

 

[scoflaw]

If I might ask, what exactly is the weak link?

 

[northern]

I would not replace the 7.4s with 6.2s and certainly not the 383s. The 330 is a heavy boat relatively speaking. The small block V8s cannot come close to matching the torque of the big blocks. They try and make up for it with higher revving engines. In order to move that boat with 6.2s you will be running small blocks at higher rpms for long periods of time, which cannot help but dent the life of the engine. In addition, the 383s specifically are not a good product. My mechanic hates them. They have a high failure rate and I believe Mercury is discontinuing them if they haven't already. I replaced my 7.4s with Mercury's new 8.2l ECT Horizons. They have the same footprint as the 7.4. As with all of these engines, however, the harness, throttle and shift, and the transmissions would have to be switched out. It's not a cheap or easy change. However, the 8.2l delivery great performance and decent fuel economy relative to the 7.4s (which in my opinion are under powered for the 340). If it's between the 7.4s and a small block V8, I would stick with the 7.4s for as long as you can. Change the oil regularly and in saltwater change the manifolds, risers and oil coolers every 3 years or so and they'll run forever.

If you do not mid how long did it take and what was the final bill for your engine upgrade. A friend replaced his twin 7.4 with diesels and a new diesel generator for 100K CDN that is about 80K US.

 

[dinopaul]

There are "CE" requirements for boats brought to or sold in Europe based on the yacht classifications A, B, C, D.
But there are also "CE" exhaust emissions standards for engines.
Are you being forced to meet the current standards to document your bought in France?
Even though it was manufactured in 1998?

Class B yacht (boat) is a vessel built to navigate on the offshore waters (200 miles and less) and can substain UP TO force 8 and waves UP TO 4 meters. (35kts/13ft)
Class C boat is a vessel built to navigate inshore such as lakes, rivers, bays and close to the shore and can sustain UP TO force 6 and waves UP TO 2 meters. (25kts/7ft)

A 330 would be 10 meter length, 3.5 meter beam and about 6,000 Kg with fuel and water.
I don't see how it could be more than a "C" class.
The cost you mention is about 4600 euro ($5500) per year, what is hull tax?

I ask not to be nosy but because there is no "simple" way to do this. It is unlikely anything will just drop in and fit and wire up. Not much of a big block will match a small block engine. You may need gear changes, prop pitch changes. This is very similar situation to people deciding to convert to diesel from gas. It can be done but it is not inexpensive and never cost effective.

My 1994 300DA is slightly shorter and slightly less beam but almost the same weight. I have twin Mercruiser I/O 5.7L/260HP and it performs "well" no race boat to be sure, 30kts is pretty much top WOT, 20-24 kts cruise.

If its costing you enough money to convert I would look at diesel vs smaller gas and how that impacts on your taxes.
That would maintain your current performance, save on fuel, improve the value of the boat (but not going to ever sell it for what it cost you to upgrade) and possibly pay for the upgrade by tax savings.

But given the effort involved in either of these solutions could you just sell it at a reasonable return? And buy something in France that doesn't have these issues?

Thank you for your reply,
yes absolutely Since June 16, 1998, all pleasure craft first placed on the market or in service in the European Union, whether they are new or used vessels, must bear the CE mark.
Whatever the navigation class, even in C and you are right it is inconsistent, for me it is only protectionism.
I studied for a diesel engine but as you said it will be necessary to change the transmissions and the propellers and in addition a new approval of the administration and at the end of the total a phenomenal cost. It is a solution that I forgot to interest me more for two 350 or 383.
According to the post I received the 383 will be better because the 350 will tire more, but you have two 357 for an almost identical boat and it goes well and anyway it is not a boat for water skiing.

 

[dinopaul]

My math says the "tax" on the engines alone are about $5600 per year, am I right plus tax on the hull. So if you spend $30,000 upgrading to new 6.2s how much would you save? I can't believe it would be worth it. If I were you I'd buy a sailboat (YUK).

your calculations are correct, I have a choice to make and it is complicated.
Thank you.

 

[dinopaul]

[QUOTE = "bfernald0, post: 1190608, membre: 55354"] Je ne remplacerais pas les 7.4 par 6.2 et certainement pas les 383. Le 330 est un bateau lourd relativement parlant. Les petits blocs V8 ne peuvent pas se rapprocher du couple des gros blocs. Ils essaient de compenser cela avec des moteurs à régime plus élevé. Afin de déplacer ce bateau avec 6,2 s, vous exécuterez de petits blocs à des régimes plus élevés pendant de longues périodes de temps, ce qui ne peut que réduire la durée de vie du moteur. De plus, les 383 ne sont pas spécifiquement un bon produit. Mon mécanicien les déteste. Ils ont un taux d'échec élevé et je pense que Mercury les arrête s'ils ne l'ont pas déjà fait. J'ai remplacé mon 7.4 par le nouveau 8,2l ECT Horizons de Mercury. Ils ont la même empreinte que le 7.4. Comme avec tous ces moteurs, cependant, le harnais, l'accélérateur et le changement de vitesse, et les transmissions devraient être désactivées. Ce n'est pas un changement facile ou bon marché. Cependant, le 8,2l offre de bonnes performances et une économie de carburant décente par rapport au 7.4 (qui à mon avis sont sous-alimentés pour le 340). Si c'est entre les 7.4 et un petit bloc V8, je resterais avec les 7.4 aussi longtemps que vous le pouvez. Changez l'huile régulièrement et en eau salée, changez les collecteurs, les colonnes montantes et les refroidisseurs d'huile tous les 3 ans environ et ils fonctionneront pour toujours. [/ QUOTE]

Thank you for your reply

 

[dinopaul]

[QUOTE = "Xplicitlnck, post: 1190708, membre: 24859"] Quel que soit le moteur sur lequel vous changez à moins qu'il ne s'agisse du même 7.4 que vous avez actuellement, vous devrez changer la transmission. Mais ça ne s'arrête pas là. Vous devrez faire un bon magasin et leur dire ce que vous aviez auparavant dans le bateau et ce que vous avez maintenant dans le bateau et s'ils sont vraiment bons dans ce qu'ils font, ils sauront quoi repiquer et mettre vos accessoires en coupe. Cela ne va presque jamais parfaitement du premier coup. Vous devrez prendre ces accessoires retravaillés, les remettre sur votre bateau pour faire un tour, assurez-vous d'obtenir le régime nominal à plein régime. Si vous ne tirez pas le bateau, retirez les accessoires et renvoyez-les. Etc.
Où habitez-vous et de quelle taxe parlez-vous? Hottodie a-t-elle raison de dire qu'elle vous taxe 5600 $ par an juste pour avoir des moteurs dans votre bateau?! [/ QUOTE]

Je viens de France et nous avons une taxe annuelle sur les moteurs et la coque, la taxe sur les moteurs est plus lourde plus la cylindrée est importante plus la taxe sur les moteurs est douloureuse.
Je vous remercie
[QUOTE = "Xplicitlnck, post: 1190708, membre: 24859"] Quel que soit le moteur sur lequel vous changez à moins qu'il ne s'agisse du même 7.4 que vous avez actuellement, vous devrez changer la transmission. Mais ça ne s'arrête pas là. Vous devrez faire un bon magasin et leur dire ce que vous aviez auparavant dans le bateau et ce que vous avez maintenant dans le bateau et s'ils sont vraiment bons dans ce qu'ils font, ils sauront quoi repiquer et mettre vos accessoires en coupe. Cela ne va presque jamais parfaitement du premier coup. Vous devrez prendre ces accessoires retravaillés, les remettre sur votre bateau pour faire un tour, assurez-vous d'obtenir le régime nominal à plein régime. Si vous ne tirez pas le bateau, retirez les accessoires et renvoyez-les. Etc.
Où habitez-vous et de quelle taxe parlez-vous? Hottodie a-t-elle raison de dire qu'elle vous taxe 5600 $ par an juste pour avoir des moteurs dans votre bateau?! [/ QUOTE]

Je viens de France et nous avons une taxe annuelle sur les moteurs et la coque, la taxe sur les moteurs est plus lourde plus la cylindrée est importante plus la taxe sur les moteurs est douloureuse.
Je vous remercie

 

[dinopaul]

Mercury is no longer offering the 350 MAG for a 300 hp option. The 300 hp option from Mercury is now the 6.2L. I would stay away from the 350 MAG. 300 hp from a 5.7L engine was stretching the limit of the engine. Talking to people I know who have 350 MAG engines, the 350 MAG engines are not that reliable.

Thank you for your reply.

 

[northern]

We have a Marine power 350 in our jetboat it has 330HP at 4,000 RPM. I have no idea if it would be an option for you
Look at this engine one marina said it is a good replacement for 454. t was 26K CDN = 22K US. They said it would work with the transmission in our boat now.
S270 Engine Hyundai SeasAll Diesel Engines - Reliable, fuel efficient, light weight marine diesel engines (hyundaiseasallusa.com)

 

[dinopaul]

I read on the sea ray specification that the sundancers 330 were delivered as standard with two mercruisers 5.7 260hp and the 7.4 were optional, someone knows it was the ratios of the transmissions and the sizes of the propellers that came with the 5.7?

 

[bfernald0]

No matter what motor you swap over to unless it's the the same 7.4 that you currently have, you will have to change out the transmission. But it doesn't end there. You will have to can a good shop and tell them what you previously had in the boat and what you now have in the boat and if there really really good at what they do they will know what to repitch and cup your props to. That almost never goes perfectly right the first time. You will need to take this reworked props put them back on your boat go for a ride make sure you're getting the rated rpms at wide open throttle. If your not pull the boat take the props off and send them back. And so on.
Where do you live and what is this tax your speaking of? Is hottodie correct in saying they tax you 5600$ a year just to have engines in your boat?!

 

[bfernald0]

Not so. I upgraded my 7.4s to 8.2s. Used the existing transmissions. The only thing that had to be changed were the shifting blocks to accommodate the dts. Tuned the propellers as well

 

[dinopaul]

I come from France and we have an annual tax on the engines and the hull, the tax on the engines is heavier the more the displacement is important the more the tax on the engines is painful 5600$ is right every year.

I sent an email to sea ray to find out when the boats were delivered as standard with two 5.7 260hp mercruisers which being the ratios of the transmissions and the sizes of the propellers provided, because the 7.4 were an option,
I will not fail to give you sea ray's reply.

 

[northern]

I come from France and we have an annual tax on the engines and the hull, the tax on the engines is heavier the more the displacement is important the more the tax on the engines is painful 5600$ is right every year.

I sent an email to sea ray to find out when the boats were delivered as standard with two 5.7 260hp mercruisers which being the ratios of the transmissions and the sizes of the propellers provided, because the 7.4 were an option,
I will not fail to give you sea ray's reply.

If we were to be taxed on engine size like you are I think there would be a revolution. It is like taxing people based on how much air they breath. I now know why small engines and diesels seem to be in most European boats one sees for sale. Question. If you were to go with a small gas turbo charged gas engine with the same HP would your taxes go down. IE use a Ford Ecoboost engine.

 

[Lazy Daze]

Paul, just a couple things to keep in mind that might be helpful... As you compare replacement engines, make sure you are looking at the "older" Mercruiser engines. Over the last few years, Merc has started to build their own engines from the ground up. Before that, the engines were a combination of GM truck engines and the modifications made by Mercruiser. For example, there is BOTH an "old" 6.2L MPI and a "new" 6.2L MPI engine. Although the displacement is the same, they are NOT the same engines. You're probably best to stick with the older engines as the cost of the engine and the installation of the newer ones would be even higher.

HP is not really the best way to compare engines. In fact, HP isn't even a "real" number. Torque is the real thing - HP is simply a math calculation based on torque and RPM. Torque is measurable, HP is not. This is why you're getting a lot of recommendations to stick with the 7.4's because of the amount of torque they develop. However, for the most part, HP is what is used to compare one engine to another - or at least to help differentiate within the same size displacement.

While it wouldn't be a rocket ship, I do think you'd do just fine with the 350MAG's (300HP). I've run a 310DA of the same era with smaller engines and it did just fine. The 350MAG's were a mainstay in the Mercruiser lineup for a long time for good reason - they were great engines - good power for the money and very reliable. However, as I'm trying to "think like you" and the way you would use your boat and your expectations (but with the mindset of "bigger is always better" when it comes to boat engines!)... I, personally, would lean towards the 6.2LMPI 320HP engines. These were designed as a replacement for the 7.4L engines and did very well in that respect.

Here's some info on props and transmissions for you:

V/D, MERC 300HP 350 MPI RWC W/RSR 1.52:1
V/D, MERC 310HP 7.4L MPI RWC W/RSR 1.56:1
V/D, MERC 380HP 454MAG MPI HRZN W/RSR 1.56:1

350MPI 300HP engines:
PROP, 16"X 16"X 1.5" L3BC BRZ DMX
PROP, 16"X 16"X 1.5" R3BC BRZ DMX

7.4LMPI 310HP engines:
PROP, 17"X 17"X 1.5" L3BC BRZ DMX
PROP, 17"X 17"X 1.5" R3BC BRZ DMX

454MAG 380HP engines:
PROP, 17"X 17"X 1.5" L3BC BRZ DMX
PROP, 17"X 17"X 1.5" R3BC BRZ DMX

All of this said... it was mentioned above, but bears repeating... could the better choice be to sell this boat and buy one that suits your needs better (with the engines it currently has)?

 

[Xplicitlnck]

Not so. I upgraded my 7.4s to 8.2s. Used the existing transmissions. The only thing that had to be changed were the shifting blocks to accommodate the dts. Tuned the propellers as well

Thank you for that. Good to know. I honestly did not think that would work

 

[dinopaul]

If we were to be taxed on engine size like you are I think there would be a revolution. It is like taxing people based on how much air they breath. I now know why small engines and diesels seem to be in most European boats one sees for sale. Question. If you were to go with a small gas turbo charged gas engine with the same HP would your taxes go down. IE use a Ford Ecoboost engine.

the best solution to have low taxes would be to change for two turbo diesel but the price would be exorbitant and it would be necessary to change the transmissions the propellers and surely many other things, I prefer to find a solution with two smaller displacement engines.

 

[dinopaul]

Paul, just a couple things to keep in mind that might be helpful... As you compare replacement engines, make sure you are looking at the "older" Mercruiser engines. Over the last few years, Merc has started to build their own engines from the ground up. Before that, the engines were a combination of GM truck engines and the modifications made by Mercruiser. For example, there is BOTH an "old" 6.2L MPI and a "new" 6.2L MPI engine. Although the displacement is the same, they are NOT the same engines. You're probably best to stick with the older engines as the cost of the engine and the installation of the newer ones would be even higher.

HP is not really the best way to compare engines. In fact, HP isn't even a "real" number. Torque is the real thing - HP is simply a math calculation based on torque and RPM. Torque is measurable, HP is not. This is why you're getting a lot of recommendations to stick with the 7.4's because of the amount of torque they develop. However, for the most part, HP is what is used to compare one engine to another - or at least to help differentiate within the same size displacement.

While it wouldn't be a rocket ship, I do think you'd do just fine with the 350MAG's (300HP). I've run a 310DA of the same era with smaller engines and it did just fine. The 350MAG's were a mainstay in the Mercruiser lineup for a long time for good reason - they were great engines - good power for the money and very reliable. However, as I'm trying to "think like you" and the way you would use your boat and your expectations (but with the mindset of "bigger is always better" when it comes to boat engines!)... I, personally, would lean towards the 6.2LMPI 320HP engines. These were designed as a replacement for the 7.4L engines and did very well in that respect.

Here's some info on props and transmissions for you:

V/D, MERC 300HP 350 MPI RWC W/RSR 1.52:1
V/D, MERC 310HP 7.4L MPI RWC W/RSR 1.56:1
V/D, MERC 380HP 454MAG MPI HRZN W/RSR 1.56:1

350MPI 300HP engines:
PROP, 16"X 16"X 1.5" L3BC BRZ DMX
PROP, 16"X 16"X 1.5" R3BC BRZ DMX

7.4LMPI 310HP engines:
PROP, 17"X 17"X 1.5" L3BC BRZ DMX
PROP, 17"X 17"X 1.5" R3BC BRZ DMX

454MAG 380HP engines:
PROP, 17"X 17"X 1.5" L3BC BRZ DMX
PROP, 17"X 17"X 1.5" R3BC BRZ DMX

All of this said... it was mentioned above, but bears repeating... could the better choice be to sell this boat and buy one that suits your needs better (with the engines it currently has)?

thank you for your informed opinion, I love this boat and it is the only one in France and I would like to keep it.

I do not yet have the answer from sea ray but I think for the moment to change for two 350 MPI (5.7 - 300hp) first generation as you advise me, moreover they are not expensive.

I have two 1.5.1 transmissions with 17x17 propellers it's very close to what you described, I don't want to do water skiing with this boat.
Do you think that two 350 + two 1.5.1 + two 17x17 will be consistent?