need for speed

[problemchild]

hey,
thanks for all your help on my last problem, it was greatly appreciated. I have however another question to run by you guys. My '76 18' Donzi 2+3 has a 450+ hp chevy 400ci small block that only sees speeds around 65 on the gps, even with it trimmed out 'till little more than the outdrive and tabs are in the water. I know the 280 drive is bigger and a little slower then the mercruiser (although she'll probably handle the power better), but seems like it should move along alittle faster,
Thanks again
-the Problem Child

[Lenny]

Gear ratio, prop style and pitch and VERIFIED rpms please ASSUMING you truly provided the GPS speeds.

You have the 280T (trim version) ?

You should be doing a lot better than that... In a Merc version, with trim and verified DYNO torque/ponies, you would be in the 80's...

[problemchild]

The motor turns 5100 RPMS with a 25" pitch SS prop. I have the dyno sheets for the motor that say 456hp and 486.6 ft/lbs of torque (3200 rpm's) The outdrive was fitted with trim in '78 (my dad knew the guy who owned it at the time) and as near as I can tell it has 1.75 to 1 gears. And yes our gps shows about 65.7 mph.

[Formula Jr]

((5100/1.75)*25)/1056

You're theoretical speed is 69.

If you are seeing 65.7 your slip is only 5%.

Not bad. This rules out a lot of things dealing with weight and drag on the boat in the water. I was thinking since its a '76 it might be one of the foamed boats that got water saturated. But you are saying that she airs out.

Places I would look. Since your engine is spinning up to 5100 under wot load,
Something is stealing horse power by the time it gets to the prop. Are you running 20 weight or 95 in the drive?

Cam profile may be wrong. We will want a
scan, photo or list of the Torque/Rpm curve.
What is your torque at 5100?

Drive Gear Ratio on an 18 with that HP should be in the 1.3/1.5 range. You should be able to pull this ratio with 450 hp in an 18.
I don't have my volvo info any more. Don't know whats available.

With a 1.3 ((5100/1.3)*25)/1056 = 93 mph, 88.4 with slip

with a 1.5 .................................. 80, 76 with slip.

We still have to wonder why you are not over reving with the 1.75.


Oh, I just read your earlier post

http://www.donzi.net/forums/showthread.php?p=392917#post392917

I would venture that your drive is binding in some manner, and this is where all the horsepower is being lost. Is the drive hot to the touch after a run?
Sorry, this isn't a good outcome if the drive is binding. Not a quick or cheap fix. Yet with a rebuild you would be able to change the ratio. So there is some silverlining here.

[Pismo]

Less than 300hp is getting to the prop.

[BigGrizzly]

Your saying your tabs are in the water. this is a problem buy not the only one. you say you have 480 torque at 3200 rpms what RPM is the max HP at and 3200 is impossible in a 400cid without a blower. the other problem I don't remember a 1.75 to 1 ratio in a volvo. What prop are you turning, sounds like it is an Ultra with a 16* to one that would make the slip high enough to be an ultreprop. My Corsican is 67mph with one of my props with 400+hp 351 ford clevland with max HP about 5500 and torque abou5 3700rpms. youdon't sound that far off excep for the Dyno numbers.

[problemchild]

The motor turns 5100 RPMS with a 25" pitch SS prop. I have the dyno sheets for the motor that say 456hp and 486.6 ft/lbs of torque (3200 rpm's) The outdrive was fitted with trim in '78 (my dad knew the guy who owned it at the time) and as near as I can tell it has 1.75 to 1 gears. And yes our gps shows about 65.7 mph.

[Lenny]

The motor turns 5100 RPMS with a 25" pitch SS prop. I have the dyno sheets for the motor that say 456hp and 486.6 ft/lbs of torque (3200 rpm's) The outdrive was fitted with trim in '78 (my dad knew the guy who owned it at the time) and as near as I can tell it has 1.75 to 1 gears. And yes our gps shows about 65.7 mph.


VOLVO gear units were 1:34(A), 1:61(B) (which you have) 1:89 (C) and 2:15(D)

[Formula Jr]

Problem Child,
You have the dyno sheets.

Please type out your torque readings in 300 RPM intervals.

Plugging in the "B" drive ratio, your Theo speed is 75 MPH.
So you are at 12% slip.

HP= (Torque*RPM)/5252

(486.6*3200)/5252 = 296.5 HP.

We need to see the full dyno to figure out where the other 160 HP
is coming from.

[Pismo]

or tremendous drag someplace or weight/waterlogged. Hows the bottom? 456hp on a dyno should still yield some decent hp/torque in the boat. What exhaust?

[BigGrizzly]

Ther is no dragor weight problem. at 65 Mph there isn't much boat in the water. unless you arn't trinning it hidh enough. you still havn't answered the question of what rpms is the max HP nor what prop you are using. Inorder to get 70 MPH with a 24 ULTRa ( I know you have a 25) and a 1.61:1 drive you have to turn about 6000 RPMs and I don't care about the numbers in the formula ask Magic Bill. His tech has been there too. Since you say you have a stainless which one is it. Right now what I see is speed and RPM are where they should be with the torque you have but the HP sounds off. A boat gets heavier the faster you go. after 60mph it takes from 10 to 20 hp more for every mile per hour you want to go. So 65 is another hundred horse power over 60. I have 680+ HP in my Criterion at 3977rpms IT has 700lbs of torque and HP peeks at 5400. I can travel at 86+ with a 29 pitch. figure it out the numbers fit. My 18 Corsican also fits the numbers. Don't tell me the 280 drive is so inefience, becausr the numbers hydrodynamically don'y say that. I know look how fat it is. well the 1983 Mercedes 4 door has a lower CD than the 1983 Corvette and lookat thoes cares together. I think that the Dyno numbers are incorrect, If this engine is a 400 cid chevy and your using the merc center rise exhaust you will not get over 410 ponies out of it on a good dayi

[justleft]

It could be the motor has huge low end torque and suffers on top.

Great hole shot !?

Or maybe a fuel delivery problem ?

Great on the dyno but less in the boat?

Ya gotta look at what is different in the dyno pull as compared
to the real boat. Could be great motor with something holding it
back.

[Pismo]

Yes, great on the dyno (which can be setup to say anything it seems) and not so great in the boat. Common boating problem.

[BigGrizzly]

Just let me through this into the discussion. Ever wonder why you see 383 all over but not 400 cid. 302 Chevy run better than 307 Chevy etc. There is a reason, lets who knows.

[Pismo]

A dyno can say anything

read

http://www.offshoreonly.com/forums/showthread.php?t=134019


The 383 is a 350 derivative which is a great engine. The 400 was a short lived problem ridden siamese block which is not very common.

[BUIZILLA]

there are TWO different 400 blocks, one has 2 freeze plugs on each side, one has 3 on each side.... there is absolutely nothing wrong with a good 400 block, with the correct rotating parts as a package. Trust me, mine is destroked to a 392, turns over 8,000 rpm, I can drive it anywhere I choose, and it is the fastest small block stock appearing Chev in the world... :wink: :yes:

[problemchild]

dug up the results from the dyno and attached it to this reply, (think I did it right). Somebody brought up a good point about the exhaust, i am running a mercruiser exaust, while the machine shop where it was tested used a good pair of headers, how much does this affect the hp? I know on a car it usaully makes at best 15-20 hp. I also do think thatsomething in the outdrive is binding, as was also suggested earlier. thanks again, Problem child.

ps. I have a 25" pitch Quicksilver prop that has had a volvo hub pressed in and has been reworked at a prop shop here in Rochester some years back. Hope that answers the remaning prop questions, thanks again.