260hp I/o expectations

1224bret · #1 08-08-2013, 08:04 PM

Id like to start off by saying im new to the forum and new to the v20. Seems to be quite a wealth of knowledge around here! I recently purchased a 1983 v20 with a 260hp I/o 350ci engine I've had quite a time getting all the bugs worked out of it. I finally had a chance to run it this evening and I'm wondering if its running at its full potential. I was kind of thinking it would run high 40s low 50s tonight best I could get was 38 mph on gps granted that was with a full tank of fuel and three adults in the boat. Rpm was around 4200 which seems about right. It sounded good though I know it has some old gas. Does my speed sound about right or should I be going faster? I'm thinking of trying a different prop something along the lines of a mirage. Right now it has a random aluminum prop on it and I can't seem to find the pitch on it. Any imput would be appreciated.

garagenc · #2 08-08-2013, 08:14 PM

Welcome to the forum.

What kind of outdrive ie Volvo penta single or duo prop, mercruiser etc.

I have 91 V and mine has the Volvo penta duo prop, my rpm is 4200 and mine runs between 41-44mph, per my GPS.

At 3000 rpm mine runs between 28-31 mph.

Hope this helps.

Destroyer · #3 08-08-2013, 08:31 PM

If you're sure your engine is running the way it's supposed to, then I would look at the prop pitch and dia. as the next place to gain speed. I'm thinking that a 19 to a 21 pitched prop would be about right for 260 horses...depending on what you want your boat to do.

And in all cases, welcome to the site. Glad you found us.

1224bret · #4 08-08-2013, 08:37 PM

Well sounds like I might be close to where I should be I suppose. Mine has a mercruiser drive and a aluminium prop. Do the duo prop drives have a higher top speed than the old mercruisers? Not sure what my speed is at 3000 but at 3500 I was in the low 30s. I'm sure I could possibly get a little more by going to a better quality stainless prop. Don't want to seem like I'm only wanting top speed I just want to make sure I'm in the ball park if I'm seeing high 30s for a top speed when I'm fairly loaded. Thanks for the imput. What are the performance differences between the duo prop drives and jut the standard drives?

spareparts · #5 08-08-2013, 09:11 PM

start by checking the engine over good. Check the engine compression, look at or change the plugs, check the wires, cap and rotor. Once you feel the engine is sound, check the ration of the drive(should be on the side of the upper unit). It should be a 1.5-1. I'd say you should be running a 19 pitch prop, or even a 21p prop. A 350 should make that V fly, I'd guess in the 50 mph range turning a 21 p 4600 rpm

smokeonthewater · #6 08-08-2013, 09:29 PM

the FIRST step is to get rid of the old gas.... it won't run at full potential... with less energy per gallon you can be running lean as it won't be able to use up the oxygen available which can cause high cylinder temps and detonation and also it can cause a buildup in the engine which is most often noticed by sticking a valve open and smacking it with a piston

Notice I said GET RID OF and not MIX FRESH WITH... 10 gallons of bad gas plus 50 gallons of fresh gas equals 60 gallons of bad gas..... Kick the dirt, cry a little, and pump it all out..... Abuse a $100 lawn mower engine with it instead of a $2000 boat motor

1224bret · #7 08-09-2013, 05:23 AM

Yeah I was going to do some more checking on things when I get off work today. I really thought it would run close to 50 wot but Garagenc is saying his runs low to mid 40s with the duo prop set up. I looked at volvo's site and they claim 5% more top speed with the duo prop set up, that puts my speeds close to what he's getting when you factor in 5%. I'm thinking the prop is probably my biggest loss it looks like it is old tech even for a aluminium prop. I'm thinking I could possibly get 40 or better with a new style stainless prop. I will get out my gauges and check compression this weekend which would probably be a good time to change plugs. What are the correct plugs for this engine? The guy I got the boat from said that the engine had been replaced a few years back with another used engine I'm thinking that it could even be a 305 ill run the numbers on it and see what it comes up as. I also want to check for timing chain stretch since i cant seem to get it to run right with 8* timing at idle it wants more advance which makes me think maybe there is some chain stretch also. It runs great and has good oil pressure but I guess it could still be a little tired wont know till I do some tests. As far as the gas is concerned I know I need to drain it I just have to get it out of the lake and find a large enough container to put it in. Thanks for all the info! Does anyone else have first hand knowledge of what top speed I should be seeing?

wdlyle · #8 08-09-2013, 06:37 AM

I just purchased a 1986 V20 two months ago with a Mercruiser 260 I/O and it has the Alpha 1 outdrive with a 19P aluminum prop. Engine runs great everything is tuned properly. I get the exact same performance that you are getting, 4200 WOT and 38 MPH on the GPS so that may be what were going to get with this setup. It does get there pretty quick though.

spareparts · #9 08-09-2013, 06:42 AM

I forgot to mention, check your tach for accuracy against a known good tach. If that engine is the original, the factory engines had a real good timing chain, I rarely see them give any problems, if some one swapped in an automotive engine, the chain may be an issue.

1224bret · #10 08-09-2013, 08:03 AM

Well from the few that have this set up sounds like I'm pretty close to where I should be. I think the engine that was swapped in was quite possibly a automotive engine. The reason I'm thinking of chain stretch is that I can't seem to get the timing right. It won't run right without the timing light showing around 29* advance at idle I still have to check and make sure the pointer and balancer match up at top dead center, probably my first weekend project. I tried to set it at 8* but it doesn't want to start with tht timing and at 8* initial I can only get 27mph top speed and forever to get on plane. I don't seem to have any detonation where it is now and I think I'm actually still somewhat retarded as far as what it sounds like now. I will do some more checking and try to get some fresh fuel in it this weekend. I just rebuilt the carb but I need a new secondary metering rod hanger since mine looks slightly bent. This thing has been a can of worms since I got it. Thanks for all the help and if anyone has more performance numbers with this set up I'd appreciate it seems like not many 260hp I/o's are out there. Thanks agian!