So just how fast am I going?

So after some upgrades to the 85SN 351W to include; new lifters, pushrods, Crane 1.7 Roller Rockers, GT40P heads and an Edlebrock Performer manifold, I took it out for a test run and had some incredible results. Before the upgrades I could barely push 4300RPM, 13X13 prop, BW 1:1 Velvet drive, which worked out to about 40MPH. Now I can swing 5500RPM, exact same drive train, but my speedos crapped out after 44. Am I really in the low 50's for speed? That's an unusal speed for an '85 SN. I didn't have a GPS unit this weekend, will have one for next.

Looks like a nice Mallory dist too.
Well done.

Hi. Please do not take this as an insult but more as advise.
I see in your photo a braided line going into the carb fuel inlet fitting and what looks like a hose clamp. If this is so hopefully it is only temporary and you plan on installing a correct fuel supply tube. I see many a older PCM motor where someone thought that the fuel supply inlet tube needed replaced with such as set up as a "cool" braided line and a hose clamp. This set up is a very dangerous situation in a inclosed motor and when they come into my shop this way they get changed at the expense of the owner.
Again only observation and advise.
Ill get off my soap box now!!

SN206 wrote: Jody, Could you or someone else please explain why? I am not that mechanically inclined.

   It's a fire hazard. It's supposed to be a steel line from the pump to the carb with a threaded fitting.

   Mike

JoeinNY wrote:    I think what you should do is bring it up here next weekend and take me wakeboarding, skiing, and maybe hydrofoiling and I will bring along my GPS and we will see what shes really doing. Something seems cruddy about those speedos or tach either way so we do some swapping and get it figured out. My 83's speedos really arent that far off in most situations.

Thanks for the words Joe, love to swing up through NY this weekend but I think that would be the final straw that would drag me into divorce court!
I'll get a GPS speed check this weekend and see what that says, even though the engine runs past 4500rpm to 5500rpm, it almost feels like there's more prop slippage than relative speed. To eliminate other variables, the tranmission has only 80 hrs on it and the prop is fresh back from a rebuild at Delta propellor.

81nautique wrote: If your truly turning the 13x13 to 5500 then I'd say you done good but I think something is not right here. A true GPS top speed will tell us if the prop is biting or slipping. Also make sure you check your trans fluid and all is well there. My prop calculator tells me that with an average of 20% slip your boat is running 53.9 mph, Joe knows what it takes to do that so I still think somethings up. The gps will tell. Those are some great improvements for that boat though , you won't regret it.

Al, I was told the tranny only had 40 hrs on it when I bought it last year and I put another 40 on it last season. Fluid level is good and prop is freshly rebuilt from Delta. Like I mentioned to Joe, as I power past 4500 to 5500, there isn't the corresponding huge change in speed, so my guess is maybe high 40's but probably not past 50. Definite GPS check this weekend.

You're right about the improvements. About $1500 total cost but an awesome holeshot now and a NASCAR-like sound at the higher RPMs. Just want to be able to surprise those bass boats once-in-a-while.

Kevin, Hardest thing now is tuning the prop, no need to spin it that high if it's not doing any good. I think a step up in pitch would be too much so it might be time for some more cup or borrow a few props from your buds and test em out.

They do sing a sweet tune at 5500 though don't they. I still run mufflers on mine and the tone this year with my mods is much more throaty than last year. The jump in Compression ratio is the balls in my book.

Al, I couldn't agree more with the need for more cup.......

Seriously, the prop is probably my limiting factor (limfac) right now. It does seem that the prop is just churning the water more above 4500rpm. As long as I can get a solid 42mph for footin' I'm pretty happy.

I noticed you post 300hp for your current setup. Is that dyno'd or a wag? Since you have a few more motor upgrades than I do, I am probably overstating my hp at 300, so maybe I'm closer to 275? 285? BTW, your rig is CHERRY, hope to approach that level of quality some day.

I do have a video of the run with good audio, but it's tape and I don't have good video capture software. Will try to post something soon.

Turning that prop 5500 whether the boat is moving or not is an indication of pretty good power. Actual Horsepower is hard to guess at, and doesnt mean all that much anyway since you have to get the boat perfectly propped to have your actual max rpm equal the rpm where your HP hits its peak. I think I have had 6 props on my 83 and I only got there once.

My guess is your running just below 50 which is pretty good. If you had that boat running in the 50's you would probably know it.

I kinda doubt the transmission is slipping, it would be hard for it to contiually slip a little, it would most likely degrade pretty quickly and your rpm would continue to increase. I just dont think the oj 3 blade is a great prop for the bigger boats past about 4500. All the resistance of the boat to going faster increases the forces on the blades, it is 20% more stress on the prop to go 50 on a big boat with 300 hp than it is to go 60 on a smaller boat with 300 hp. That stress increases the flex which exacerbates the cavitation. More cup will slow it down keeping the engine from making its max hp thereby reducing the flexing, but still keeping you from the theoretical max speed. The cnc 3 blade props tend to hold thier shape better and perform better all around but they are not available in higher pitches in general, the cnc 4 blades are less efficient as a rule than the CNC 3 blades but have more blades to spread the forces out upon...

You need a stainless CNC 3 blade in just the right pitch... good luck

Yeah alan I still have been attempting to comment on your thread but its long and involved and I never actually get the time to send it. The short story is the best cnc prop I ever ran on 83 when it was a 1:1 was the Acme 431. 13x13x1... problem is it is a LH and they dont make the right.. dont know why not. Best prop I ever ran on my 67 is a OJ xmp 3 blade, also a LH not available in RH. Why these guys are against us CC guys is anyones guess... Alan's situation is different then here, his boat is faster but he is starting to see the same limits. I dont know how much of the loss up high is flexing and how much is slippage, either way the problem remains too much water force on the same blade area. Makes a guy kinda want to run a well built and balanced engine with a single plane manifold and let it turn a bunch of rpm with a smaller prop pitch.

Sure is disapointing when 600 rpm gets you another 1.2 mph but I have seen it.

Well the weather held for a while on Sunday but I only had a GPS unit to verify actual speed, no shop tach to verify actual RPMs. I added a youtube link for a crude video, but decent audio.

Here are the results:

3000RPM = 30MPH
4000RPM = 38MPH
5000RPM = 42MPH
5500?RPM = 45MPH

From this data, it would appear that the stock OJ 3 blade prop is definitely cavitating proportionally more as the rpm increases, since the speed increase dramatically drops off after about 4000rpm.

Questions:
Would increasing the cup enable better bite at the higher rpms?

Would a better quality prop at 13 pitch hold the water better also?

I'm not looking to push past 50, but I would like the prop to be efficient in the low 40's for footin' once in a while. Thanks for all the great advice so far, the forums are great for helping each other out.

Definitely sounds like you could see some benefit with a better prop. I have the same hull and would see 50.2 at 5300 with the LH version of a 540. I would verify actual rpms first then save up some money for a new prop.

Md, you could try the 542 to see how the different blade shape/prop construction behaves on the RPM range.

Anyway it's not apples to apples since the Acme might cavitate less, creating more load and decreasing rpms, but it's 1 1/2'' shorter than what you have now.

It will set the bar to guess for the Acme props.

Well less cavitation should mean there's a more consistent load on the motor hopefully translating into better efficiency?

One more motor question Joe. Hopefully this is a short answer. Besides the obvious in water pumps and fuel systems, what are the big differences in a marinized motor? Specifically are the pistion wall and ring clearances different for a marine application? Will fitting a std. block with automotive pistons be just fine in a boat? I can't remember if you mentioned that in your engine build post. Thanks again for all the help!

MdFooter wrote: Well less cavitation should mean there's a more consistent load on the motor hopefully translating into better efficiency?

If the prop is cavitating then it's working on air, therefore spinning easier.
If it's pushing through water then it's working on load.