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I have an 8.1 volvo penta on my 2006 chapparel. I think it…

I have an 8.1 volvo...
I have an 8.1 volvo penta on my 2006 chapparel. I think it is rated at 375hp but some of the the 8.1 are rated at 420hp. What can i do to get some more power out of the engine
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Answered in 8 hours by:
3/8/2010
Marine Mechanic: John White, Marine Mechanic replied 12 years ago
John White
John White, Marine Mechanic
Category: Boat
Satisfied Customers: 306
Experience: 11 years experience working on outboards, inboards, sterndrives & pwcs
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Do you have a model & serial number off the engine? There should be a tag towards the front of the engine that looks something like below. Just get me the information off the tag so I can see exactly which engine you have.

 

graphic

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Customer reply replied 12 years ago
Ok I will try to look for it tomorrow after work.
Marine Mechanic: John White, Marine Mechanic replied 12 years ago
Sounds good. I'll be back online tomorrow evening and look it up then.
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Customer reply replied 12 years ago
This is what I found. Some numbers were hard to read.
8.1 GI-G (NNN) NNN-NNNN XXXXXXXXXX
Marine Mechanic: John White, Marine Mechanic replied 12 years ago

There are quite a few differences between the 375 HP which you have and the 420 HP 8.1L. It wouldn't be too bad, but the differences are going to be pretty expensive parts. The only difference I can see internally on the engine is the camshaft. The intakes are the same, but the throttle bodies are different to allow more air into the engine.The problem is this is a fuel injected engine so once you change the camshaft and the throttle body the fuel / ignitions maps in the ECU would need to be changed to really see any gain from it. You can't actually change the setup of the ECU as far as I know so you would need to replace the ECU with one from a 420 HP model. Changing the ECU would also require the main wiring harness and fuse box to be changed. Some of the difference is also made up in the max RPMs of the engine which is 4,600 for your current engine and 5,000 for the 420 HP models. Once again the ECU comes into play here.

 

I have a list below with general pricing in US dollars on the parts. There may be a few other smaller parts that would also need to be changed, but it would take a few hours to look through all of them. You can bring up the parts diagrams on http://www.volvopentastore.com/ if you want to take a look for yourself. I was comparing to the 8.1GXi-GF.

 

Camshaft - $650.00

Throttle Body - $550.00

Main Wiring Harness - $800.00

ECU - $1,300.00

 

You would also end up having to change the prop or props if it's a duoprop. If it's a duoprop outdrive, which I assume it is, this can add another $1,500.00 or so to the bill. Another thing to consider is when you start adding HP you are usually going to reduce the lifespan of the engine and possibly the outdrive. On speed you may see a gain of around 5 MPH, if that much. I would have to say it really wouldn't be worth it unless you can find a blown engine with good electronics for a good price.

 

Do you know what RPMs the engine is currently turning when loaded, wide open and trimmed out? Why exactly are you wanting more HP, for speed or some other reason? Depending on exactly what you're wanting I may be able to give you some other recommendations other than the engine.



Edited by XXXXX XXXXX on 3/9/2010 at 4:11 AM EST
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Customer reply replied 12 years ago
Thank you for all the info. You definitely had the info. Most of the time I don't need the extra hp as it runs great and 80 % of the time I run in the ocean so I just cruise around 30 mph or we have our kids with us so we don't go to fast. The boat max out at 50. The only reason I wanted extra hp is one or twice a year I go to Colorado river without the kids and would like to get 2 mph faster or so. Sounds like changing it to 420 hp is a lot of work. It was easy to add extra hp to my truck and dirt bike so I thought there might be something simple I could do.

I don't want to change the props as those dual props are exspensive. Is there something I can do to give it a little extra for the weekend I am runing at the river?
Marine Mechanic: John White, Marine Mechanic replied 12 years ago

With automobiles, dirt bikes, 4-wheelers and even PWCs there are good sources for aftermarket modifications, but it hasn't really come around for boats yet. Outboard boats are a little different as they have the advantage that you can change the mounting height of the engine. With a sterndrive you're pretty limited as to what you can do cost effectively.

 

If you don't have a set installed already you may consider trim tabs. They are mainly used for planing and stability issues, but they will give you a little extra on top end (1-2 mph). They will also increase fuel mileage as the engine doesn't have to work as hard to plane the boat and can stay on plane at lower speeds. A good set would run around $600.00 - $800.00 + installation, so that may be a little more than you want to spend.

 

Other than that you come down to weight issues and prop issues. Try to reduce the weight of the boat as much as possible. If you have a water tank installed and don't use it then make sure it's drained dry. If there's a portable toilet, remove it if you're not going to be using it. Look around the boat and eliminate anything you can that's weighing you down. You don't want to get rid of anything that's important such as an extra battery, but you can probably find a few things that don't really need to be there all the time.

 

Propping is another way to get the extra speed, but it has it's limits. The pitch of the props does a few things. It gives you a slower or faster hole shot (planing), it gives you the top end speed & it acts as a governor to keep the engine RPMs within check. The idea behind propping is to get to a compromise with the planing speed you like, the top end speed you like, but without allowing the engine to over rev or under rev. Going down in pitch decreases the time it takes to plane, usually decreases the top end speed and increases engine RPMs. Going up in pitch increases the time it takes to plane, usually increases the top end speed and decreases engine RPMs.

 

Your engine is rated to turn between 4,200 - 4,600 RPMs when at wide open throttle. Depending on what your current RPMs are you may be able to play with the prop pitch a little. Most prop shops can change the pitch on stainless props by 1". Depending on your exact RPMs you may be able to go up 1" in pitch and gain a little more speed. It won't be a lot, but every little bit counts. Just remember any prop changes are going to effect you in both situations that you run the boat in, fully loaded or light weight.

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Customer reply replied 12 years ago
I already have trim tabs. I only use them to balance it out if it leans to much on one side. How do I use them to ballance for more speedv
Marine Mechanic: John White, Marine Mechanic replied 12 years ago

I assume they are adjustable trim tabs and not the fixed style. With the trim tabs you usually want them all the way down or close to it when the boat is off plane. The same applies to the outdrive trim angle. This will give you more lift for planing. As the boat levels off you can start to adjust both the trim tabs and outdrive trim upwards to gain more speed. It's really a matter of trial and error to find out which angle works best for your particular boat.

 

Normally at wide open you will want the trim tabs to be pretty level with the bottom of the boat. If the weight on the boat is balanced good then they should both be pretty even. If the boat tends to lean to one side you can either do some weight balancing or run one of the tabs up or down to level it off. Just keep a check on your RPM's and speed and adjust the trim tabs and outdrive trim until you are maxed out on speed with the engine RPMs at or under 4,600. It takes a little practice to fine tune things, but once you have it set look at your indicators for the trim tabs & outdrive trim and you can get pretty close every time. If you're not using a GPS for speed indication you may want to borrow one or buy a cheap handheld.

John White
John White, Marine Mechanic
Category: Boat
Satisfied Customers: 306
Experience: 11 years experience working on outboards, inboards, sterndrives & pwcs
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Customer reply replied 12 years ago
Yes they are adjustable at the helm. I normally don't use them to get off plane as the boat seem to get up easily with hardly any bow rise. Once on plane I normally try to trim the outdrive up to maximize performance but with your tip I will start playing with the trip taps at both slower and higher speeds. I never thought of trimming them at lower speeds but I think this will help me as sometimes I'm running at lower speed (20-25mph) and find that the boat rides better at 30mph because the bow drops down. So next time I run at those lower speed I will use the taps to see if it rides better. Thanks for all the info. It looks like I won't be doing any mods to the boat but you certainly gave me lots of info and I was pleased to find out the difference between the 375hp and the 420hp. Maybe some day if the engine needs rebuilding then I might to the upgrade. I am accepting your answer but if you can think of anything else let me know. Also do you know how often should I grease the gimmalberaing threw the Zerk fittingOh and yes I do use have a GPS and thats is what I use to measure speed as the speedo on the dash is never accurate
Marine Mechanic: John White, Marine Mechanic replied 12 years ago

Sounds good, just play around with the tabs and trim to see what you can get. Within a few years someone may come out with a modification for the ECU or a performance chip to add into the system and change the fuel/ignition maps. I know CDI can re-program most of the Mercruiser ECU's, but I'm not aware of anyone that does anything with the Volvo systems yet.

 

The outdrive should be pulled every 100 hours or yearly as regular maintenance to check the condition of the drive line including the gimbal bearing, u-joints, drive shaft splines, engine alignment, bellows, etc. At this time the gimbal bearing would be greased or replaced if needed. You can give it a few pumps of grease every 6 months or so, but it's not really necessary. You can see a copy of the complete maintenance schedule for your engine & drive package here. It's also in the owner's manual which you can find here.

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