496HO - Best RPM for Max HP
 

OK motor heads, I have a question for you. Throttle Up has my props right now for reworking. They are in pretty bad shape as suspected. Anyway, the question came up regarding the best rpm for max horsepower. For instance, I have noticed that my top speed does not increase when rpms increase from say 5100 to 5250. Matt called a racer buddy of his who has a lot of experience with our engines. His said that his experience was that best speed (in the 38 Scarabs) was attained at 4750 to 4850.

What are your opinions on this? And I know this group has an opinion!

thanks

Not sure where max hp @ rpm is, my single runs 5100 at top end.
You should be hitting the limiter at 5250? I've bumped it a few times myself when its cool out. I would like to know, 66 on gps is the best I can do. john

Your best HP rpm is dependent on many factors. The cylinder head flow is one, cam profile, intake, carb(s), exhaust, intake manifold...........If you give me more info, I can get you in the ballpark for what you have now. I can also help you set up for whatever power band you want.

blown1500, that's way more info than I am capable of providing. I am NOT a motor head. That will have to come from merc or someone else. My 496's are completely stock at this point.

fountain, yes i was bumping the rev limiter at 5250 in fall air. my typical summer rpm's were between 5000 - 5100 after labbing Bravo 28's.

It seemed to me that the motors ran (and sounded) much better after labbing as opposed to before when I was in the 4700 - 4800 range.

Macklin,
I am curious why you are having your labbed props done.
What problem are you trying to correct from the original lab job and who did that job.

Thanks,
Rick

If they are still stock give Mercury Racing a call in Wisconsin. I talked to a guy in the prop lab and he asked the weight of the boat drive ratio and engine model and then told me what the ideal max RPM was for my boat. BTW for me with HP500EFI and 1.5 ratio Bravos on an 11,500 lb hull he said the closer I can get to 5300 the better.

Fountain27ho496
I'm a bit surprised as to why you are only getting 66gps with the 496HO in your 27? With a full bag of gas and four onboard I'll slow down to 66. With a half bag of fuel and just myself onboard I've seen 69.2 gps...

BTW: 2002 Sunsation 288 -w- 496HO stock 24p Bravo-1

rv, after labbing I lost a solid 5 mph off my cruise speeds. NOT acceptable. Plus the props were not matched when they were labbed. They are 100 rpms apart which means I am always having to work the sticks to get the rpms matched up. I don't want to say who labbed them, let's keep this on subject. I will say that everyone agrees my props were bad out of the box.

I will make these my spares and am trying to decide which props to buy......the reason for this thread.

The 4750-4850 is probably the torque peak on the motor based on the Scarab comment. I know I am starting something with that comment. You might try running a higher pitch prop that the boat can turn at that RPM and see what the speed is. The larger pitch would also help the cruising speed. If you have the opportunity, it might be worth a try.

Just for some comparison figures...I have a 2001 Nordic 28' with the 496HO, 24p Bravo. The best I got was 67 with two people and 1/2 tank gas, rpms just below 5000. I had the prop labbed and repitched to 25 and lost cruise and wot. Total bummer. I'm trying a friends 26 labbed by Bronson Hill next summer. Finding the right person to tweak the prop is difficult and costly I'm finding out. This will be the last prop I deal with, if it doesn't work out, I'll just go back to a stock 24, these single engine 496 boats aren't fast enough to warrant the prop headaches in my mind...save your time and money :mad:

I have a 28 Thunder with a 496HO. I have a Merc. labbed 24 Bravo and like it. My cruise speed went up a couple but no improvement on top end. I would liked to have seen a little on top but the cruise is more important anyway, isn't it? :D

Macklin,
Check your PM or email.

Rick

Rick, I sent you a reply. Did you not get it?

It would really be nice if Dustin (or somebody else who has dyno'd the 496) would post some numbers here.

Macklin,
No I did not receive it. I checked both my email and the private messages on OSO. I just sent you another private message on OSO with a different email address. Maybe if you just replied to the private message that would work.

Rick

Reckless, I'm suprized too..
 

Sunsation is a faster hull I believe, maybe lighter as well.
Fountain says this boat runs 71 (on website) never happen.
My performance report says 68, I'm only hitting that on speedo
gps is 66. I'm already at max rpms, can't belive you guys are
running 24's? I'm running 26 bravo 4 blade, 1.5xz. The 24 should put me at 5500 prms? Don't want to spend a fortune testing
props, did that last time. Also have the larger shaft to deal with.Still have a 23 mirage that worked on my last boat but can;t try it. If any one has something that actually works better on this combo let me know. thanks John

I'm not sure if the Merc 496 is much different from the marinized GM 8100, but here's the basic motor curves from GM. Can anyone confirm that this would apply to the merc offering....I don't know if they change anything after receiving the engines.

http://www.gm.com/automotive/gmpower...1mhp_tc_lg.jpg

http://www.gm.com/automotive/gmpower.../81hpspecs.htm

Fountain27ho496...Your right about the 71 Fountain said you'd get...never happen. Same with my manufacturer, although they gave me a range and I got within 1mph of the low end. Your real world numbers are right in there where they should be without spending a fortune on props and testing, even then, you'd not be looking at huge gains. I almost bought your model boat and the dealer (one of the realistic/honest ones) said 68 tops. I've come to the realization that the only thing that will make me go consistently faster is more hp. Don't forget that you'll get more hp based on the conditions, more so than any prop work. Somebody in my exact model boat got 70 out of it, but it was a 50 degree day!! My speeds will vary 3-4 mph based on temp, humidity, and chop. I don't even bother with WOT runs when it's 90 degrees with 75% humidity.

Regarding your comment of not increasing speed from 5100 to 5250 is because of slip. You're probably trimmed to far out and losing grip. That rpm where start to lose or at least maintain speed is the optimal trim angle for your hull, anything more you are just gaining rpm through greater slip. Didn't you hit the rev limiter at 5150 anyway?

Baja Daze, interesting. The chart indicates max hp at 5000 rpm then it levels off. This is consistent with my limited experience with my engines above 5000 rpm. I am thinking that I want my new props to be in the 4900 - 5000 range in normal (not blazing hot) summer temps.

Indy, I attribute the increased rpm more to temperature than trim. Those rpms were achieved last fall with the 5250 day being the coldest. It was just starting to tap on the rev limiter at that point.

Macklin,
Ya, there's some good info there. Looking further, a comparisson shows:

496 HO
GM: 420HP @ 5000rpm
MERC: 425 HP MAX, WOT RPM Range is 4600-5000

496 Base
GM: 375HP @ 4800rpm
MERC: 375 HP MAX, WOT RPM Range is 4400-4800

I would think that going much past 5000 isn't going to net too much of anything advantageous.

Macklin...following that line of logic, if your rpms increased from 5100 to 5250 and you saw no increase in speed, you've got a big slip problem, or trim might have come into play. I suspect a GOOD prop person would have a better idea, but even with a high slip factor, you should have seen some increase in speed, abiet a small amount. That's why I was focusing on the trim angle. I know in my boat I can hit max rpm if I just trim to the moon and throw a huge rooster, but I acutally lose speed. But hey...I'm no prop engineer :eek:

***lurk mode off***
I've got a 2002 Potato chip boat (Velocity 280) with a 496HO spinning a labbed 26 bravo one. The best gps speed is 74.3 mph 1/2 of fuel with the wife aboard and we were bouncing the rev limiter (puget sound area = cooler weather) the temp outside was maybe 60 degrees. I was told to try a labbed 28 bravo one (Scott from velocity). so I got one, I'm able to spin it about 4800-4950ish(damn near 5000) depending on the weather. With the labbed 28 (it's a pretty thick lab job) the best we've seen it 78.6 mph on a cold 56 degree november day. both day we had about 8-10 inch even wind chop. I run the 28 bravo most of the time unless we are loaded down. Meaning more than three poeple total. I must have a magical 496 Ho.. Or it could be that I have a thin chip...:eek:

***lurk mode on***

I'm jealous
 

Wanna trade? I know.
That is a fast hull, velocity has always been 5-10mph faster than many others. I like that 28, wish I wasn't trapped in this one.John

velocity, that is one fast 28 footer with stock power!

Indy, I'ld say I have a prop problem. Here is a pic of a properly labbed Bravo 28 sitting against the pitch block.

Now here is a picture of one of my labbed 28's in the same pitch block.

I think I'm pretty lucky to get those number out of that set-up myself. Or Velocity just did a extra nice job on everything because it was one of the Miami boat show boats. But All the velocity's I've seen at the local dealership (7 total) have been really nice and put together very well and solid. Although the 280 that was on powerboat TV was sold by at the same place as mine. But I don't remember the number he was running.
I noticed the same thing when I took my labbed 26" bravo to get worked on. All the blades were at a diiferant pitch. They were able to true it up, re-balance it. clean up the edges after the summers running and Hopefully I'll have good luck with it and I'll take my 28" up for some more work.

Here is a separate question... How many of you think running 100-200 rpm's below max RPM on a full computer motor is bad??

You can literally profile an engine's HP curve changing props. All other things being equal when a higher pitch lowers increases RPM and speed drop you have gone beyond max torque and HP. Drop the pitch to where speed goes down and you are below max HP. If you change prop designs it is another story.

Re: 496HO - Best RPM for Max HP
 

Funny, I was just out today seeing what is wrong with my set up. I am a new boat builder "Triple000ught", the first boat is a 28.5' Triple000ught. All aluminum and weighs about 3000lbs with the 496HO in it, no interior etc. yet. It has an 8.5 beam and mild steps. I thought with the low weight I would run close to 80, I can only pull 72.6 with low fuel and 70.6 with almost full fuel. Kind of slow.. I am running 1.36 ratio and a stock 26P. I just got the Smartcraft hooked up and found out I am stuck at 4430rpm at WOT, 70mph. I noticed that I could pull 68 with only 4200rpm. I was thinking I need to go to a 24-25P?? I really don't know? I would think with the low weight I would be able to outrun the Velocity. Any ideas?? to see the boat goto www.gofasts.com search Triple000ught. thanks

Re: 496HO - Best RPM for Max HP
 

Your prop size sounds okay, but you have to go to a 1.50 gear ratio to run this setup. I have tried 1.36's and they do not work well in a mild power boat. In my opinion, you should only use these when you run out of prop.
The 1.50 will give you better response and mid range and not lug the engine,
plus get the engine rpm up to the peak range.

Re: 496HO - Best RPM for Max HP
 

Fountain27 (john),your still more than welcome to try my 28 that was worked to a 27 by hydromotive,Smitty

Re: 496HO - Best RPM for Max HP
 

I've got a 496 Mag HO in a Velocity VR 1 and a 26" Bravo 1 runs 69.6 mph GPS at the rev limiter (5150). A 28" Bravo 1 only runs 67 GPS at 4800 rpm. This was within a half hour of each other, on a hot summer day and calm water. I think my mufflers (exhaust tips) are knocking a bit of stuffing out of the engine.

Re: 496HO - Best RPM for Max HP
 

Our Dyno runs on the stock 496HO motor on a merc ECM with no modification shows that peak horsepower develops at about 4900rpms and peak torque develops at 3900 rpms. Generally speaking the best overall speed with a 1.5 ratio in the average non-ventilated bottom boat will probably be had at about 5100 rpms which is just below the magic 5250 number in fresh water. This is however dependant on a lot of variables like x dimension, hull type, weight, prop type, prop tuning and trim condtion. I don't think their is one rule that works for all boats, experimentation is the only real answer. Just one little trick for some of you 496HO owners, remove your stock stainless risers off your merc manifolds and remove the restrictor plate. We have found that this is good for about 15 -20HP on the dyno. Might give most of you a mile an hour with the right prop work. Hope this info helps!
Ray @ Raylar

Re: 496HO - Best RPM for Max HP
 

Raylar, what hp were you seeing at the crank on the dyno? Thanks.

Re: 496HO - Best RPM for Max HP
 

Hey Ray: I'm intersted in your comment about removal of the restrictor plate. This year I removed the riser from the manifold on one side to make the removal of my exhaust diverter a little easier. I know there was a riser gasket that I replaced, but I don't recall seeing any type of "restrictor plate" that I could have removed. Was I in the right spot or is the plate to be found somewhere else? Interested in knowing as some extra hp might get me to a milestone. Thanks alot.

Re: 496HO - Best RPM for Max HP
 

We saw about 418 to 430HP on the dyno at the crank on the 496HO at between 4900 and 4950 corrected. the torque came in between 470 and 486 lb/ft. between 3800 to 3900 rpms corrected. This was run with 87 octane pump gas and with stock Merc 496 exhaust system. the motor would pull about 5200 rpm and hit the rev limiter about 5225rpm. Our best dyno runs with our BP103 kit ( heads, intake, and cam) came in at 533HP at 5000 rpms and the torque at 601 lb/ft at 4100 rpm corrected with the stock Merc ECM and 80 MM throttle body and a set of our Tri-Y stainless headers. The restrictor plates are found mostly on the 2001 and 2002 496's. I think they were designed as some sort of anti-reversion plate. they were in between two gaskets at the riser base to manifold and with openings tapered out towards the riser. On our first stock engine dyno pulls they were in and they killed about 15 -20 HP between 4500 and 5000 rpms. Mercury may have opted to delete these in later motors, I am not sure, but I have talked to several 496 owners who have found them installed. If you don't have them, all the better.

Re: 496HO - Best RPM for Max HP
 

Thanks for taking the time to respond and help us out Ray.

Re: 496HO - Best RPM for Max HP
 

i was running 28's and was consistently right under the rev limiters at 5100-5150, changed to 30's picked up a couple mph gps... and boat runs around 4850 now...engines seem to like being a little overpropped... my acceleration is better now and hole shot the same..
Don

Re: 496HO - Best RPM for Max HP
 

Is this nascar? I don't think that is really considered a restrictor plate. Yes it may reduce exhaust flow out the motor. However it should help keep that water from running back and sitting nice and warm next to your exhaust valve. The idea is to steam off any water that will get that far back into the exhaust, before it gets to the cylinder head. I would probably say for 15 hp this is not a good recommendation. However, If you are doing other upgrades to your 496ho not a bad idea. If you go that far though you might as well go for a good set of exhaust headers. (CMI,CMI,orCMI)

Re: 496HO - Best RPM for Max HP
 

Update, I hit 68.5 gps with 2 people with a 28 bravo @ 4600 On mullet this summer. Wasn't able to do it again, even alone. Put my 26 back on and ran 67 gps @ 5100.
I guess the 5100 is ok.
Smitty I'll have to wait until spring, I'm putting it away. I do think it will work.
I don't think worrying about reversion is worth 1 mph unless someone is sure it wont hurt, and how big of a job? John

Re: 496HO - Best RPM for Max HP
 

The thought was the the very light weight would put less burden on the motor and the 1.36 would get us up to higher speeds, no so as I found out. Would putting the 24P on get me to the same place while keeping the 1.36 or should I swap the gears and it that a big deal> thanks.

Re: 496HO - Best RPM for Max HP
 

The thought was that with the light weight less burden would be on the motor and the 1.36 would bring higher speeds, not true as I found out. Would using the 24P get me to the same place or should i switch the gears and is that a big deal?