Mercruiser 898 Upgrades

[greg82255]

Hello,

I currently have a 19' 1982 Checkmate that was in storage for 20 years. I pulled it out last summer and restored it, replaced everything that was needed, and got it running very well. The boat is equipped with a Mercruiser 898 (305 engine, type I stern drive) and currently goes about 52 mph. I would like to see if I can get some more speed out of it.
I was thinking of upgrading to a 4bbl carburetor, which would require a new intake manifold. Also I am going to switch to aluminum center-rise exhaust manifolds and create a through-transom exhaust. Obviously a new propeller would be needed as well.

Any suggestions?

I also want to remove the old Y-pipe. Can this be done without removing the engine? If not, is it a difficult task?

Any help would be appreciated. Thanks.

 

[havasuboatman]

Re: Mercruiser 898 Upgrades

Sounds like you are going in the right direction, but don't get carried away with the cfm on the carb. Just dumping more fuel into an engine wont jack up the HP. You have to be able to get it into the cylinders, and you have to be able to get the exhaust out. That means more cam.
The center rise exhaust manifolds are good, and so's the thru hull. Remember to remove the exhaust bellows when you put in the thru hull.
To get the Y-pipe out, the motor has to be removed, and a block off plate has to be put in it's place or the boat sinks.
The difficulty of the motor removal is relative. It depends upon how much you know about boat engines.
You can find a link to most of the merc. manuels at the top of this forum.

 

[greg82255]

Re: Mercruiser 898 Upgrades

Thanks for the help. I have never actually done this before, but I will hopefully have some help from a friend who is a mechanic. I have been looking around for a block-off plate, but it appears that the Y-pipe design was changed after 1982 and I have only been able to find a plate that fits the newer design. Any idea where I could find one?

As far as the cam goes, I was told that a performance cam was installed by the previous owner, don't know what kind though. Any suggestions?

 

[havasuboatman]

Re: Mercruiser 898 Upgrades

You can make the block off plate. Use the y_pipe as a template.
For the cam, You can check the lift of the valves with a micrometer. For the duration and overlap you will need a degree wheel to mount on the end of the crank. It's kinda' involved.

 

[jtybt]

Re: Mercruiser 898 Upgrades

If you're gonna pull the engine, might as well drop a 5.7(350 ci) in there. People are getting rid of their old boats.

 

[greg82255]

Re: Mercruiser 898 Upgrades

Do you think that buying a used 350 would be cheaper than doing all these upgrades? I am trying not to spend a lot of $, maybe no more than a grand.

 

[WizeOne]

Re: Mercruiser 898 Upgrades

To really wake that 305 up, you need to dump the dished pistons. Putting in flat tops (I would use a compression calculator) will boost your compression from 8.5:1 to just short of 9.5:1. Combine that with a well calibrated 500 cfm 4 bbl and you will have one heck of a rocketship and still be able to burn regular gas.

IMHO, this would be a lot cheaper than going to a 350.

 

[greg82255]

Re: Mercruiser 898 Upgrades

I never thought of that.. it sounds like a good idea. Again my only concern would be cost. I don't mind how long the work actually takes, just the cost of the parts. How much do these pistons go for?

 

[WizeOne]

Re: Mercruiser 898 Upgrades

........ How much do these pistons go for?

I've never priced pistons for a chev. You'll need to do a google search and check sites like jegs and summit. You'd also want to have your block checked for bore wear and taper to see if it needed to be bored or just honed. Keith Black pistons are one common and reasonably priced brand. There are undoubtedly others as well. Cast pistons are what you want. You do not need forged pistons.

Before choosing your pistons, you need to find out things like the cc volume of your combustion chambers, how far from the top of the deck the pistons travel, the crush thickness of the head gasket you use (should be marine) and then the CC value of the pistons you choose. Usually flat top pistons have no volume unless they are dished (like your stock pistons) or if they have valve reliefs cut into the flat tops. In the opposite, domed top pistons (which you do not want) have a negative CC value.

Here is a C/R calculator so you can see the elements that are needed to calculate a desired compression ratio. All of your volume values will be fixed, you'll have no alternatives once your block is ready. (stock or bored) So you will have to experiment with different piston CC volumes till you end up at 9.5:1 or slightly below

http://www.csgnetwork.com/compcalc.html

The piston volume description is confusing. You use positive values for any dish or valve relief and negative values for any protrusions or domes.