260hp I/o expectations

[1224bret]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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!

[step up here]

Look for the serial number on the block Mecruiser has them on the right rear side of block. Providing its a merc.

[1224bret]

Wdlyle I agree it gets there pretty quick acceleration is great I guess I just expected more top speed. I don't run wide open very often but it's fun to get on it now and then. Pushing 40 isn't bad but 10mph faster would be nice. Either way I'm really happy with my v20!

[wdlyle]

I agree, I expected a little more out of my 260 when I bought it seeing what others are getting with lower HP engines.

[smokeonthewater]

weirdness with timing is more likely to be in the harmonic balancer than the timing chain... tho 29* is a LOT... you ARE checking timing on the correct cylinder arent you?.... had a buddy who always had fords try to check timing on the right front of a chevy once and it didn't work for him LOL


If you pull out #1 spark plug and use a screw driver to feel the top of the piston you can determine if it is at TDC and then mark the balancer in line with the tdc mark....
You can check for slop in the timing chain by turning the crank back and forth by hand with the dist cap off so you can watch the rotor.... there WILL be a little slack but shouldn't be a lot.

I haven't had a 260 in a V20 but I did have a '78 merc 898 (198 hp 305) in an arrowglass siesta... 20' deep V cuddy... It ran 40-42 mph with a 14"x19" old school 3 blade stainless prop.

[step up here]

Pretty tough to beat a 200hp or more outboard. Especially a new four stroke,you need a worked 350 with around 500hp see back post under Ubhistory He built one put in a V-20 went around sixty mph I believe.:fight:

[1224bret]

Yeah the timing has me baffled. My bet would be I'm giving up a few ponies just by the timing being off. I have some timing tape and have thought about making sure it was at tdc compression and making my own mark to go by. I don't know how the balancer would be that far off either though.... I've tried two timing lights and got the same result. I may have a little time tomorrow to look at it. I'm sure I'm looking at the right plug wire it's the front left wire if your standing behind the engine correct? Would a better prop turned at the same rpm give different results? I have quite a bit of checking on stuff left but I think I'm on the down hill ride now. I know the outboards have a better power to weight ratio but I would have thought the 260 would have cranked out a little more...

[smokeonthewater]

yep left (port) front if it's a chevy

4200 is a bit low too but could all be in the motor..... 4400 would be better

Get that fresh gas in first and then get the timing right


The balancer can be off just from being old and moving on the rubber... It can be off from not having a key in it... It can be off because the timing indicator on the timing chain cover can be in the wrong place IE mismatched parts.

[reelapeelin]

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!

Spare...do they make the electronic (Petronix?) ignition for this motor...that might help him out some here...

[step up here]

It should the thunderbolt IV system which is very good!

[spareparts]

It should the thunderbolt IV system which is very good!

what he said. If you look on the box, the last two numbers tell you haw many degrees of advance it has in it, check that against your light. I have seen them act up and keep all the advance locked in, but otherwise a pretty good set up


btw, check your tach!!!!!

[garagenc]

Spare...do they make the electronic (Petronix?) ignition for this motor...that might help him out some here...

Reel yes they do I converted mine over this year to the pertronics distributor and haven't had a problem since.
My old one fried and when I checked into it there was only 52 of the electronic ignition systems left in the world so that is why I converted mine over.

[1224bret]

Well I think I have it dialed in better now. I found that part of the old pointer was broken so I was using the wrong notch as a reference it looked to be a aftermarket deal. I used some msd timing tape and centered it to tdc. Runs like a champ now still getting right at 40mph which is fine. Thanks everything guys its much appreciated. What is this patronix conversion? I know about their points conversion thing but these already are breakerless ign correct? Are you doing away with the thunderbolt module all together? Is there any issues with the thunderbolt IV? Anything else I should look at doing as far as reliability is concerned?

[garagenc]

Well I think I have it dialed in better now. I found that part of the old pointer was broken so I was using the wrong notch as a reference it looked to be a aftermarket deal. I used some msd timing tape and centered it to tdc. Runs like a champ now still getting right at 40mph which is fine. Thanks everything guys its much appreciated. What is this patronix conversion? I know about their points conversion thing but these already are breakerless ign correct? Are you doing away with the thunderbolt module all together? Is there any issues with the thunderbolt IV? Anything else I should look at doing as far as reliability is concerned?

Mine was very simple; pull the old distributor and drop in the new one and connect the wires.

[1224bret]

So you keep the thunderbolt module with the new dizzy? Is the patronix unit a marine unit? Or is it just a self contained 1 wire distributor style?

[spareparts]

Pertronix isn't going to be any better than the Thunderbolt, a pertronix is great if you are replacing points, but its not needed for the Thunderbolt . If something goes wrong with teh Thunderbolt, a pertronix distributor may be a cheaper alternative, but I prefer the Thunderbolt as it has an electronic curve, rather than mechanical. Keep the set up you have, check your total timing to make sure its coming all teh way in. The V6 boxes are the hot tip on v8s as the timing comes in faster, but I doubt you would be able to tell a differnce

[garagenc]

So you keep the thunderbolt module with the new dizzy? Is the patronix unit a marine unit? Or is it just a self contained 1 wire distributor style?

It's a marine set up

[1224bret]

Ok one last question regarding the performance of my v. I just ran the numbers on my prop and it is a Michigan wheel aluminium 17p. I'm getting 40mph at 43-4400 rpm doe that sound right? I know when I first made this post a few of you guys said it should be turning a 19-21 pitch. If I want to go stainless I know they normally recommend dropping a few inches off the pitch which would put me at a 15? Sounds awful short on prop for a 260hp inboard to me but I'm not used to the weight of these boats. What are you guys with the v8's running as far as pitch?

[spareparts]

check your tach against a known good tachometer, then report back

[smokeonthewater]

are you running ALL fresh gas yet?
have you done a compression test?
you only drop pitch when going from 3 blade to 4... from al to ss you will likely stay the same

You want to sort out any and all engine issues before messing w props

Also are you trimming up for max speed?


My 28'er has old school 14.5x17" al props on twin 260's and runs 48 mph

[1224bret]

Well just as an update I finally had some time to get some work done on the boat. Put fresh fuel in, the 17p aluminum was pretty beat up so I installed a 19p mirage, I'm now getting 41-42mph at 4200 rpm which was with 4 people and a dog in the boat no issue getting on plane. I'm pretty happy with the performance of the boat now I'm right on par with garagenc's numbers. Boat cruises great at around 30mph. So much smoother now with the mirage prop. We spent several hours cruising around lake Kissimmee yesterday afternoon and I'm really digging the way these hulls ride and perform!