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MinxRaider77
05-19-2008, 06:30 PM
Hey, I'm new to this site even though it says I registered a while ago I haven't really posted anything. I have a 1986 Donzi Minx that has a rebuilt 350 with an older B&M twin screw supercharger. The motor was just rebuilt and is running perfect but I cant seem to get above 50 kts. I should also mention that the drive was changed to a Bravo I (not the XR) and a new 4 blade performance Revolution 4 prop was put on. The oil gets amazingly hot (290 F!!) according to the gauge and I was wondering if this might be a problem or maybe why its not getting the speed it should. I put the Amsoil high performance synthetic racing oil in it so its supposed to survive high temps but I didnt think they would get this high. I have heard things about the Minx hull but im not really sure whats true. I would really appreciate it if someone could help me out. Im also wondering if anyone else with a minx has this problem. Thanks.

MOP
05-19-2008, 06:45 PM
50 is way slow, a decent running stocker will run 60. That oil temp is high, are you running an oil cooler? It should be one of the 12" ones blower motors make a fair bit of heat above a stocker. A blown 350 should be running well into the 70's maybe more, what is your top RPM at the 50 you are running?

Phil

MinxRaider77
05-19-2008, 07:25 PM
Right now I'm running on the stock oil cooler. I want to change to a Hardin high performance 3x5in one but i haven't had the time. Top speed im running at 5500 rpm which is pretty much where it should be with the prop thats on it.

gcarter
05-19-2008, 07:27 PM
Check seawater pump for condition and the rest of the cooling system for blockage?

MinxRaider77
05-19-2008, 07:41 PM
Sea water pump was just replaced last summer and the engine is flushed with fresh water after every use. Water flow seems normal. I forgot to mention earlier that mufflers were added to lower the sound could that hurt performance that much?? The mufflers are by Rex Marine but they discontinued them so I cant remember the model name or anything. I put them in when i got the boat a few years ago to make the boat "legal".

MOP
05-19-2008, 07:50 PM
Ok my thoughts you may be hitting the rev limiter, a roots blower makes it power down low I would increase pitch on the prop. I would shot for right at 5,000 maybe even a tiny bit less, these drives do not like being spun past 5k for long they give up and die!

MOP
05-19-2008, 07:52 PM
Just how hot is the engine running, with a blower I would run a 140 or maybe even no Tstat.

MinxRaider77
05-19-2008, 07:57 PM
The engine runs pretty cold I'm pretty sure that we took the tstat out. The only thing that really gets hot is the oil. Im really worried that the motor isn't "breathing" as well as it should because of the mufflers. The original specs on the muffler claim that it lowers top speed by only around 5mph or so but who knows what to believe.

MinxRaider77
05-19-2008, 08:05 PM
There is a Tstat it is a 140.

MOP
05-19-2008, 08:31 PM
Do you know the pitch of the prop on the boat now? You may be able to borrow a few to test get the RPM down.

gcarter
05-19-2008, 09:04 PM
How about trying a mechanical oil temp gauge just to verify the temps you're seeing.

mrfixxall
05-19-2008, 09:16 PM
minx ,how many qts of oil does your engine hold? how much total timing do you have in the engine?

MinxRaider77
05-19-2008, 09:22 PM
Wont increasing the pitch decrease the top speed? We have tried 3 different props already 23, 21 and currently 17. 17 was recommended by the two mechanics that worked on the engine as well as mercruiser. They all claimed, with the setup on the boat, that this was the best prop to go with. With 21 we were seeing well less than 50 kts we couldnt even break 45mph. At this point I really don't believe the prop is main thing causing the top speed to be lower than it should be.

MinxRaider77
05-19-2008, 09:30 PM
Timing im not sure of off hand I can try and check when i put it in the water this week. I believe it takes around 7 qts of oil.

mrfixxall
05-19-2008, 09:38 PM
theirs somthing seriously wrong here,,with that much power you should be turning at leaste a 25 pitch prop. i have a 355 in my x18 and a 24 pitch labbed prop and im turning over 6 k at 70+ mph..

Not to be mean but you need to find another boat tech that knows what hes doing.

count the teeth on the blower pullys and let me know what they are,under drivin? over drivin?what size carb? it can go on and on..:)

What mfr of props are you using? is their vent holes in the prop?

MinxRaider77
05-19-2008, 09:52 PM
The gear ratio of the drive is 1.65:1 as opposed to 1.5:1 which is why the prop pitch is so different. The prop has vent holes but they are plugged manufacturer is merc. Again, i really dont believe the propeller is causing the problems. I just looked up what i changed the timing to recently and its 28 deg above tdc, but the distributer is a newer MSD with 22 degrees difference or something so it turns out to be around 6 deg above tdc. I think we tried it at 8 deg too but it didnt make much of a difference. Im pretty sure we left it at 6 deg. As for the blower im not exactly sure of the pulley i know its pretty small though.

MOP
05-19-2008, 10:14 PM
Something is way out of whack! Go over your spark plugs & wires, what color are the plugs? Make sure no wires are out of sequence, if I remember right switching 3 & 5 it will still run fair but way off on power.

mrfixxall
05-19-2008, 10:22 PM
on a blower motor the intinal timing should be 16-18 deg,and depending on the boost lets say 7 lbs (you do have a boost guage? i hope) you should not exceed 32 deg and you want the advance to kick in between 2500 and 3000 rpm.remove your cap and rotor on your msd dist and make sure you have the silver springs in it..also what spark plugs are you running? if stock change them to a colder plug which will keep cylinder temps down which will make your oil cooler..

mrfixxall
05-19-2008, 10:24 PM
and what mop said:wink: 18436572. port side 1357 starbord side 2468

Cuda
05-19-2008, 10:40 PM
It was 50 knots, not mph, Someone else can do the conversion, but it's faster than 50.

MinxRaider77
05-19-2008, 10:41 PM
still not even 60 mph, Thanks to mrfixxall and MOP for all the tips, ill check everything over again and who knows maybe its something stupid

VetteLT193
05-20-2008, 07:52 AM
The gear ratio of the drive is 1.65:1 as opposed to 1.5:1 which is why the prop pitch is so different. The prop has vent holes but they are plugged manufacturer is merc. Again, i really dont believe the propeller is causing the problems. I just looked up what i changed the timing to recently and its 28 deg above tdc, but the distributer is a newer MSD with 22 degrees difference or something so it turns out to be around 6 deg above tdc. I think we tried it at 8 deg too but it didnt make much of a difference. Im pretty sure we left it at 6 deg. As for the blower im not exactly sure of the pulley i know its pretty small though.

You need WAY more prop. Do the math on the prop, and no matter what you do it won't go over 60 MPH. Check this site out if you don't believe me: http://www.rbbi.com/folders/prop/propcalc.htm

Drive ratios are reduction ratios, so you are being reduced more. This means you need even more prop than stock.

If you can't spin a 21 inch prop, you have something way off someplace else. With oil that hot and speeds that low I'm guessing you are running REALLY lean. I'd check the air/fuel ratio first. 2nd I'd check the engine alignment.

Sweet little 16
05-20-2008, 08:04 AM
bottom paint??

MinxRaider77
05-20-2008, 08:19 AM
Running colder spark plugs right now, i believe boost is around 7-8psi according to the gauge. Bottom paint yes. So your saying the timing needs to be much higher?? Ill check the dist and get back to you. Even with the blower i didnt think you needed to have the timing so high. Vette, the alignment is ok when we put the motor back in we triple checked it so i think thats ok, I do feel that there isnt a good air fuel mix like you were saying is there a good way to check it?

vonkamp
05-20-2008, 08:26 AM
Can you post some pic's? Perhaps a visual will help diagnose. :yes: These guy's here are real sharp and I've seen them find & solve problems with a photo. Just ask Donziweasel. :yes: :wink:

Sweet little 16
05-20-2008, 08:28 AM
50 knots is 57.6 mph

it has been said that bottom paint will scrub about 5 mph

cutwater
05-20-2008, 09:08 AM
You need WAY more prop. Do the math on the prop, and no matter what you do it won't go over 60 MPH. Check this site out if you don't believe me: http://www.rbbi.com/folders/prop/propcalc.htm

Drive ratios are reduction ratios, so you are being reduced more. This means you need even more prop than stock.

Hi Raider - I don't see how it gets any simpler than what Vette posted. I think you are getting exactly what you should be getting. Probably hitting the rev limiter at 5500. If you assume 10% slip you should be in the neighborhood of 49 MPH. I've got a stock 350 Minx with 1.31:1 gears and turning a 21p. I would think you should probably go with at least a 25p or 27p prop with your setup. You probably won't turn it to 5500 rpm, but like MOP said, you don't want to turn your drive much past 5000 anyway.

Good luck. What color scheme is your minx? Where do you boat?
John

BigGrizzly
05-20-2008, 09:18 AM
First problem is with a 1.65 and a rev 4 which only go to 25 pitch is off. Max speed will be in the low 60s if there. I would go to a 27 three blade to start and that may bring it up to 65 with the frontal area of that boat. The oil temp is too high you probably have a small oil pan. Amsoil is an advantage use itin my race bike (air cooled) and dropped oil temp 15 deg.- it does work.

Rootsy
05-20-2008, 09:19 AM
Speaking of rev limiter... don't "assume" it has one just because he said it has an MSD....

What exactly does the MSD setup consist of in this case? Distributor? box (which one)? Marine or automotive? and so on and so forth...

Something is "off" if it is what it is stated to be...

bottom paint surely is not helping.

VetteLT193
05-20-2008, 09:19 AM
Running colder spark plugs right now, i believe boost is around 7-8psi according to the gauge. Bottom paint yes. So your saying the timing needs to be much higher?? Ill check the dist and get back to you. Even with the blower i didnt think you needed to have the timing so high. Vette, the alignment is ok when we put the motor back in we triple checked it so i think thats ok, I do feel that there isnt a good air fuel mix like you were saying is there a good way to check it?

Read the plugs first. Pull one (or more) of the spark plugs and check the color.

I just googled and found this site for reference: http://www.4secondsflat.com/Spark_plug_reading.html

My guess is your plugs look similar to example #2 with the white electrode. They should be tan all around if you are in spec.

mrfixxall
05-20-2008, 09:21 AM
Running colder spark plugs right now, i believe boost is around 7-8psi according to the gauge. Bottom paint yes. So your saying the timing needs to be much higher?? Ill check the dist and get back to you. Even with the blower i didnt think you needed to have the timing so high. Vette, the alignment is ok when we put the motor back in we triple checked it so i think thats ok, I do feel that there isnt a good air fuel mix like you were saying is there a good way to check it?

if your running 8 psi of boost then leave the timing where its at.blower motors like alot of intinal timing..o and get rid of the bottom paint if its that crap that keeps the green stuff from growing on your hull.

MOP
05-20-2008, 12:15 PM
Do some research on the timing that is super important, if the top timing goes to high you will puke the engine.

MOP
05-20-2008, 12:24 PM
You guys get off on bottom paint, if it is smooth without chunks missing it does not do much in the way of killing speed. I was involved commissioning I bet close to a thousand new boats over the years, all with no bottom paint at first. We kept extremely good records, speed RPM etc. The figures had little if any variance until the bottom paint started going to hell as age crept up. Another point a very smooth bottom is slower then a stippled or sanded bottom, I went to a stippled and won one of the most coveted sail boat races boat over all on the east coast. I invite anyone to view the trophy at Block Island, smooth is BS equating to Boat Slow!

BigGrizzly
05-20-2008, 03:42 PM
The bottom paint won't cause that kind of speed loss

mjw930
05-20-2008, 03:59 PM
The bottom paint won't cause that kind of speed loss

No, it won't and it's not likely we're going to be able to find a problem this big over an internet forum. This is going to take someone going over the boat and motor from front to back who knows what they are doing.

Where are you located? I'm sure there's some mechanic within driving distance who can sort this out because it seem obvious the person doing the work now is not up to the task.

I'm also not understanding a 17" prop. It sounds like you have a 1.36:1 ratio drive. Even so, you should be able to turn a 23" prop well onto the 5000 RPM range and be running in the mid to upper 70's with a properly built blown 350.

cutwater
05-20-2008, 05:29 PM
What's the fuss here? Besides the oil temp reading (which may or may not be accurate), I don't see any problems other than he's got the wrong prop on. Do the math:

5500 rpms, 17p prop, 1.65:1 gears, somewhere around 10% slip = about 50 mph. This is exactly what he's getting.

Like BigGrizz said, try a three blade 27p and see how she runs.

RedDog
05-20-2008, 05:30 PM
Wont increasing the pitch decrease the top speed? We have tried 3 different props already 23, 21 and currently 17. 17 was recommended by the two mechanics that worked on the engine as well as mercruiser. They all claimed, with the setup on the boat, that this was the best prop to go with. With 21 we were seeing well less than 50 kts we couldnt even break 45mph. At this point I really don't believe the prop is main thing causing the top speed to be lower than it should be.

NO - this makes no sense. A 17 inch prop might be on the money if you are only putting out 150 HP - or you want to use it as a barge tow boat.

theoretical per a prop calculator yields the following:
RPM Pitch Drive Ratio % Slip Theoretical Speed
5000 27 1.65 10 69.7
5000 23 1.65 10 59.4
5000 21 1.65 10 54.2
5000 17 1.65 10 43.9

a much bigger prop is going to be in order

cutwater
05-20-2008, 05:40 PM
theoretical per a prop calculator yields the following:
RPM Pitch Drive Ratio % Slip Theoretical Speed
5000 27 1.65 10 69.7
5000 23 1.65 10 59.4
5000 21 1.65 10 54.2
5000 17 1.65 10 43.9

a much bigger prop is going to be in order

Looks like I beat your post by a few seconds, Tim! Exactly my thoughts, though. Thanks for the table.

VetteLT193
05-20-2008, 06:15 PM
what really makes no sense is he's running a 17 because he couldn't turn a bigger wheel. That is where the problem lies...

MOP
05-20-2008, 07:09 PM
I thought I would bring his prop?speed post back:

Wont increasing the pitch decrease the top speed? We have tried 3 different props already 23, 21 and currently 17. 17 was recommended by the two mechanics that worked on the engine as well as mercruiser. They all claimed, with the setup on the boat, that this was the best prop to go with. With 21 we were seeing well less than 50 kts we couldnt even break 45mph. At this point I really don't believe the prop is main thing causing the top speed to be lower than it should be.

There is a major oddity going on, I would be digging into things and not running it until I found the cause! I asked about the plugs color earlier, plus I would at this point check compression. I have seen where a guy ran high compression with a roots and ruined the pistons and burned the valves, there were probably other contributing factors. Plain fact a blower needs to be setup properly for longevity, I have only run one so my knowledge is sparse and very stale. How stale, a 471 on a 265 Chevy do the math!!!

Phil

CHACHI
05-20-2008, 07:11 PM
Was the bigger wheel a rubber hub? Was the hub good?
Ken

BigGrizzly
05-21-2008, 08:59 AM
I just saw the 17 inch prop thing. If your tech recommended that prop run don't walk to another guy!! That is what a old barge will run. I am starting to think the only thing wrong is the tech not the boat. This is not a 3,500 Rpm diesel.

mjw930
05-21-2008, 01:30 PM
Of course there's no mention of maximum RPM's the motor could turn with each of the props he tried but if we had that information we actually could calculate how much HP that motor is making.

We also do not know the ratio in his outdrive, I made a guess based on the limited information he provided, it was nothing more than a guess.

So, since we are working with virtually no information why not guess a little more? I'm guessing that based on his numbers that "blown" SB is putting about 200 HP to the prop....... Or, in other words, LESS HP than a well tuned Normally Aspirated 5.7L SB :(

Again, based on the little tidbits of information he's provided I would suggest that the motor is broken and since the current brain trust working on it thinks the solution is a 17P prop that he needs to run, not walk, to a real marine professional and have them figure out what's screwed up.

IMHO, This isn't something you're going to fix by asking questions on an internet forum.

I think his original reason for posting was to see what the hull was capable of because I think the "brain trust" he has working for him is blaming the boat rather than their work (the motor). There are others here with far more experience than me with these boats but based on what I've seen and read a Minx with a 260 HP SB should run in the high 50's, maybe even 60mph. A Minx with a 400 hp (conservative) Supercharged SB should run in the low 70's (mph).

cutwater
05-21-2008, 01:35 PM
The gear ratio of the drive is 1.65:1

This gear ratio information from MinxRaider is how we did the calculations w/ a 17p prop. 10% slip is an assumption, but reasonable.

All I'm saying is that from his 1.65 gears & 17p prop setup, he is getting exactly what he should. There might be something wrong with his engine, but my first action if I were him would be to try a good 25p or 27p prop.

mjw930
05-21-2008, 05:32 PM
This gear ratio information from MinxRaider is how we did the calculations w/ a 17p prop. 10% slip is an assumption, but reasonable.

All I'm saying is that from his 1.65 gears & 17p prop setup, he is getting exactly what he should. There might be something wrong with his engine, but my first action if I were him would be to try a good 25p or 27p prop.

Sorry, I didn't see where he posted the ratio but he's also confusing knots with mph because 50 knots = 58 mph and he ain't doing 58 mph spinning a 17P prop @ 5500 rpm.

He also mentioned he couldn't even get 45 mph out of a 21P prop. Going bigger, regardless of the "quality" isn't going to solve his problem which is a DRASTIC loss of power. The high heat and lack of speed all indicate that the motor IS NOT running right.

IMHO It's not a boat problem, it's not a prop problem, it's a motor problem exacerbated by a mechanic that apparently doesn't know his ass from a hole in the ground.

cutwater
05-21-2008, 05:59 PM
Yep, I think we all agree on the mechanic! :wink:

MOP
05-21-2008, 06:18 PM
Curious what kind of crank case pressure he has, I'd be yanking the plugs and doing compression & leak down. The motor needs to be gotten into!

joseph m. hahnl
05-21-2008, 07:16 PM
It almost sound like he's using aluminum props.

My Minx alpha one Mag 350 1x1:5 21p Stainless prop.

Straight pipe 60MPH @4800rpm Fresh water.

With Dapars And Gibson mufflers 58MPH @4600 Fresh water.

"LEGAL IN ALL 50 STATES":salute:

MinxRaider77
04-29-2009, 09:26 PM
Wow thank you to all that contributed to the problem I apologize for not getting back to you guys, we've tried a couple different props with no luck. we had a problem with the seal between the carb and the supercharger something happened with the gasket and it would leak a lot. this is only part of the problem however we were getting some bad noises and it turns out the cylinder heads were shot so we're currently in the process or rebuilding again using better parts (even though we had it build by a supposed racing boat professional who is now no where to be found). I will update to let you know the numbers we get when we get it back in the water but as of now we are keeping our fingers crossed that everything will be alright. It does seem that the final drive ratio is keeping the speed down as well so we will have to play around with the prop again. Again thank you for all the input.

BigGrizzly
04-30-2009, 08:45 AM
I have heard this too many times before.

Donziweasel
04-30-2009, 09:34 AM
Just ask Donziweasel

Pics are a great resouce as Barry said. I fought a head problem for a year and a half. Buizilla finally figured it out, wrong heads, ports and gaskets were not lining up.


IMHO, This isn't something you're going to fix by asking questions on an internet forum.

I don't know, I have solved some pretty big issues with this and other forums......

Glad you found the problem. Still would like to see some pics of the Minx and engine.