Why is my Donzi so slow?

[flying tomato]

Hello to all,
Just restored a 1984 Donzi, Hornet III. Has a 496 ci engine that has to be getting about 400hp to the prop minimum. It does have the TRS drive on it. I have tried a 24p turbo prop and 22p bravo I prop. Best speeds are 60 mph, and 57 respectively. RPMs are 4800, and 4900 respectively. I am new to the performance boat game. Looking for some more speed. Any help would be appeciated.

Thanks, Flying Tomato

[handfulz28]

Has a 496 ci engine that has to be getting about 400hp to the prop minimum.

Got a dyno sheet? 400hp at the prop would be 500hp at the crank...assuming approx 20% loss for TRS/trans. A LOT of big ci/big power engines using dyno/street exhaust and no marine accessories magically turn into dogs once they're bolted in the boat. You can see another 10+% loss in those scenarios.

FWIW the numbers seem about right as far as speed/RPM I think.

And congrats again on the restore effort! Huge Hornet fan here. :D

[JimG]

FT, congrats! Boat looks great! Good speed, too...

[mattyboy]

boat looks good :)
your numbers seem to be on for a 1.6 drive ratio
was the X dimension changed at all

it may be a foot shorter but it is a much bigger boat than a 22 classic ;)

[flying tomato]

No. the X has not been changed. Is there any way to determine if gear ratio is 1.6 rather than 1.5. Motor dyno is 480 hp, 550 tq. The power seems to fall off the curve around 4600. Motor has stock iron, oval port heads. What do yall think about a set of aluminum heads to try and find some more top end???????

Thanks for the compliments.

[VetteLT193]

could be just a driving issue. do you have it trimmed up while running full throttle?

[handfulz28]

Forget the aluminum heads (for now) and get good exhaust on there. I'm going by the picture you posted in the other thread, that you're running the OEM manifolds and risers.
If those dyno numbers are with dyno headers, AND the curve is dying at 4600, sounds like carb or cam isn't right. Have you done a plug check to see if it's running lean?

I'm no expert when it comes to cams and carbs, but you need to post the details. Also, exactly which "oval" heads are on there? Need a casting number. If they're "peanut port" heads then they have to go.

[mattyboy]

No. the X has not been changed. Is there any way to determine if gear ratio is 1.6 rather than 1.5. Motor dyno is 480 hp, 550 tq. The power seems to fall off the curve around 4600. Motor has stock iron, oval port heads. What do yall think about a set of aluminum heads to try and find some more top end???????

Thanks for the compliments.


well put it this way I was using 10 percent prop slip as an average using your numbers so if you go with 12-13 percent slip then it could be a 1.5 gear ratio

marine engines live above 4000 most of their lives it is the range that they need to make all of their power
or you would have to prop it for 4500 before the power dies off

[Just Say N20]

As an aside. All those sail boaters like you fire that up in their midst? :eek:

[Ghost]

...marine engines live above 4000 most of their lives...

Another quick aside:

Not challenging anything else above in the least--simply was curious about this 4k number. Isn't the maximum continuous RPM rating on most smallblocks somewhere around 3800? My PCM 5.7s list 3800 as "Cruising RPM", which I interpreted as max continuous operation. Anybody know for sure if that's what's intended? -Mike

(I'd think typically for fast runabouts, they'd live around 2700-3300 most of the time, no?)

[mattyboy]

true I should have said Donzi/hi po go fast Marine not just marine engines they are not wot all the time but from what i have seen most run them at a fast cruise for extended time and we all know donzi owners are all "stock" ;)

[Ghost]

true I should have said Donzi/hi po go fast Marine not just marine engines they are not wot all the time but from what i have seen most run them at a fast cruise for extended time and we all know donzi owners are all "stock" ;)

Good point. (Mostly I just wanted to know whether 4k+ average was a pontoon boat thing. :))

[AndyDiSario]

I'm no expert, however from what I've seen with these boats, with a 24p prop, a true boat speed (GPS or radar) of 60 mph at 4800 RPM sounds about right.

Good luck and enjoy it!
-Andy DiSario

[HOWARD O]

Andy is right on by my numbers. I have a 22', TRS with the original 330 hp. Also a 24 pitch Turbo1. Most likely quite a bit less power than your 496, but I'm sure the boat isn't nearly as heavy either.

3200 RPM - 10gph - @ 38 mph
3500 RPM - 12.5 gph - @ 45 mph
4800 RPM - 63 mph

Now I'm not positive about my top RPM, I didn't look at it, but I am positive about the top speed. My drive is a 1.5 ratio.

Hope this helps some, but I doubt you are too far off. I had to trim up more than I thought. Also keep in mind that these trim pumps are quite slow. I also had to leave it up there for a while to get the top speed. Only mods to the engine are a Performer intake and Dana exhaust.

[Planetwarmer]

Hope this helps some, but I doubt you are too far off. I had to trim up more than I thought. Also keep in mind that these trim pumps are quite slow. I also had to leave it up there for a while to get the top speed. Only mods to the engine are a Performer intake and Dana exhaust.

Did those mods noticeably help?

[flying tomato]

I know how to drive a boat! This one is my 7th. Heads are oval port, not peanut. Exhaust is iron GLM's. Anyone know how good GLM exhaust is? They were new on the old boat. Carb is an Edelbrock 750 cfm.

Keep it coming, pls.

[JimG]

Tomato, you getting out on CL this weekend? I'll shoot you a PM...

What sort of speed were you expecting?

[BUIZILLA]

I'm trying to understand your parts stackup...

your 496 = 1/4" over stroked Gen IV 454?

stock mechanical fuel pump?

fuel line size?

anti siphon checkball still in tank outlet fitting?

no mention of cam choice.... if it's the stock 330 cam, (you say it dies at 4600, same as what the 330 does) this is a major choker in a 496.. meaning no extra lift or duration to feed 10% more cube load

no mention of intake choice.....

Edelbrock carb is okay, but won't make the best overall power..

stock unported oval port heads from a 330?

stock GLM's are another cork since you added 10% more inches and same or less room to evacuate the already inefficient gasses

no mention of ignition menu..

IMO, your choking the monkey here...

we need to start somewhere ..

[HOWARD O]

Did those mods noticeably help?

I don't know any different, got it that way from Cuda. But I believe he was getting 59 mph before the intake and exhaust.

[Cuda]

I don't know any different, got it that way from Cuda. But I believe he was getting 59 mph before the intake and exhaust.
You are correct. It got 59 on a two way average on the river with no trim. New exhaust, intake, carb, and distributor netted 5 mph. Speed don't come cheap. :)

[flying tomato]

Ok guys, here is the motor:
Gen V, 454 bored .060 over
Stroker crank 4.250 stroke, domed pistons, 9.5/1 compression
Heads are stock oval port iron heads off the 330/454 (no porting done)
Cam is Comp cams 226/236 .542/.542 Duration and lift respectively
Intake is Edlebrock RPM, Air gap
Carb is Edlebrock 750 cfm
Exhaust is GLM center riser, iron (no thru prop exhaust) Goes strait out the transom.
Fuel pump is Holly (Red), Fuel line is 3/8 inch, No anti-siphon valve
Additionally, full roller rockers
HEI hi perf ignition
Summit sells this motor which I copied and advertises it as 480hp/550 ft lbs of torque (The only diff is they put a 800 cfm carb on their's)

Am I just expecting too much!!!!! Just seems like it should go faster.

[BUIZILLA]

are the secondary's opening?

[flying tomato]

Secondaries are opening, no detonation. Motor sounds good.

[BERTRAM BOY]

Does anyone else think that there might be a reversion issue with that exhaust setup?

Oh, and how about a little bit bigger flame arrestor?

[handfulz28]

Heads are stock oval port iron heads off the 330/454 (no porting done)
Cam is Comp cams 226/236 .542/.542 Duration and lift respectively

FYI, there's two different "oval" port heads from Merc. A "large oval" and a "peanut port" or "round" port. If they're the original vintage heads from an '85 330, I'm gonna bet they're "large oval" which is good. But if they're the smaller ports, that would explain a whole lot. Did you see the intake, gasket and heads while apart? If the intake and gasket were the same as the heads, then that's good.

What about valve sizes? Did you stay with stock?
Is that the same cam in Summit's 480/550 motor?

I'm gonna bet your engine is running exactly as built. But you replicated a "street" motor not a "marine" motor and it shows. The heads themselves are good but if not already done you need bigger valves. Plus you need the cam to match that setup. Plus you need a proper aftermarket exhaust.

[HOWARD O]

Am I just expecting too much!!!!! Just seems like it should go faster.

Boy, I'd like to have that in my boat. :yes:

I think it should be faster, I agree.

[DonziBuoy]

Beautiful boat, well done! FWIW, I had a 27 ' formula with a 502 415 HP = not a light boat by any means, my top speed with a 23P 4 blade was 67 mph (GPS) @ 5000 rpm. So I think you are losing somewhere - if not the prop, has to be the engine.

[BUIZILLA]

Beautiful boat, well done! FWIW, I had a 27 ' formula with a 502 415 HP = not a light boat by any means, my top speed with a 23P 4 blade was 67 mph (GPS) @ 5000 rpm. So I think you are losing somewhere - if not the prop, has to be the engine. was this a 1.5 or 1.65 drive?

[flying tomato]

I am not sure about the drive. I just assumed it is 1.5, but if that is the case my prop slip % is up around 15-18 %. Can that be right?

Flying Tomato

[MOP]

I second & exhaust! The air cleaner that one is a choke at high RPM it is 1/4 the volume of the one I run on my 383, also that cam needs longer exhaust you are or have to be right at the edge of reversion.

[BERTRAM BOY]

The flame arrestor might not be an issue. I'm using one of similar size with more cubes and have noticed zero results by taking the flame arrestor off.

[flying tomato]

I took the flame arrester off and wired the choke valve open------NO DIFFERENCE!

What is reversion?? I am not a huge motorhead. That is why I love this site. I love all the suggestions.

This boat will go faster, sooner or later. Thanks again for all the help.

Flying Tomato

[gcarter]

You mentioned the HEI ignition, but what's the timing?
Has it been set? If not Buizilla can give you some guidance.

[mrfixxall]

i ran in to this problem on 2 occasions with a 496 build.
1) a 496 will produce more performance in the midrange and topend with a single plane manifold.
2) a 496 with stock 330 hp round port heads will produce alot of torque but over 4000 rpm's it stps producing power with the peak being 4400 to 4600 rpm's..
3) we got 39 more hp and 32 ft lbs of torque just switching to a 850 cfm holly carb.
the guy i bought my formula from had a issue with a 496 build, his engine produced 36somthing hp and 500 + ft lbs of torque,his customer wasnt satisfyed with the results so his engine builder installed a set of square port heads and a differant cam and the motor went up 100+ hp and torque..
if it were me i would call,RMBUILDER"/Bob Madara of Marine Kinetics in N.Y. 585-654-8583
he is vary technical and can give you te low down :)

Heres a pic of a stock 330 hp geniv cylinder head, is this the cylinder head your running?

[BERTRAM BOY]

I'm with Mr. Fix on this one. I'm sure a few of us would really like to see your cylinder head and intake manifold ports.

Also, I would have started with at least an 800cfm Holley.

[BUIZILLA]

those are the 310hp peanut ports.. true 330's didn't have the flat ridge on the bottom of the port and the ports were bigger as were the valves... the exhaust valve in the one pic is not an original 330 valve and is sunk in the seat... second chamber exhaust valve is original and has a thicker head

[flying tomato]

Ok guys,
Well, it seems like the consensus is pointing to the heads. That is what I thought from the start. I think I will order a set of performance aluminum oval port heads and see what happens.

Stay tuned for the results!!!

Thanks to all,
Slow Flying Tomato

[BERTRAM BOY]

Ok guys,
Well, it seems like the consensus is pointing to the heads. That is what I thought from the start. I think I will order a set of performance aluminum oval port heads and see what happens.

Stay tuned for the results!!!

Thanks to all,
Slow Flying Tomato

Whoa!!! Slow down. I didn't hear anyone say that you needed new heads. We just want to see the combination of heads and intake manifold to make sure they're compatible.

[BUIZILLA]

Edelbrock makes an oval port alum head for marine use with coated water passages... they flow pretty darn good out of the box

[mrfixxall]

those are the 310hp peanut ports.. true 330's didn't have the flat ridge on the bottom of the port and the ports were bigger as were the valves... the exhaust valve in the one pic is not an original 330 valve and is sunk in the seat... second chamber exhaust valve is original and has a thicker head

well they came off of a stoek 454 330 hp motor(serial # said so),mabe merc used what they had at the time:)

I know what your saying buiz about the ports not being oval! thats why i have them and theirs now merlin big oval ports on that motor now in the 496 configuration..

[mrfixxall]

Whoa!!! Slow down. I didn't hear anyone say that you needed new heads. We just want to see the combination of heads and intake manifold to make sure they're compatible.

double check the hole configeration on the edlbroch heads, your exhaust may not line up with the exhaust manifolds..if its the same head as the merc 525efi i know a stock 454-502 exhaust wont bolt up.

[Lenny]

(I'd think typically for fast runabouts, they'd live around 2700-3300 most of the time, no?)

Mike, I'd "say" I am mostly 3800-4000 for extended periods... 1/2 hour or so stuff.

[handfulz28]

double check the hole configeration on the edlbroch heads, your exhaust may not line up with the exhaust manifolds..if its the same head as the merc 525efi i know a stock 454-502 exhaust wont bolt up.

Edlebrock doesn't sell the 525 heads direct. They have their own heads with standard ports so like Buiz said, they're a great item.

BUT, like everyone else says, you need to figure out exactly which heads are on there now. But if you're itching to spend money, you need a good exhaust also. Otherwise you're still choking it.

[BERTRAM BOY]

Edlebrock doesn't sell the 525 heads direct. They have their own heads with standard ports so like Buiz said, they're a great item.

BUT, like everyone else says, you need to figure out exactly which heads are on there now. But if you're itching to spend money, you need a good exhaust also. Otherwise you're still choking it.


I think what Mr. Fix is saying is that many BBC aftermarket cylinder heads have the exhaust port raised by 1 inch to improve exhaust port flow, and that there may by issues with the exhaust pipe exit.

[handfulz28]

I think what Mr. Fix was saying is that Edelbrock cast the Merc 525 heads for Mercruiser, which have the raised exhaust ports. So if one were to order those "Edelbrock" heads, then a stock exhaust wouldn't work.

Edelbrock 61459 with "standard" exhaust port location is worth consideration.

[mrfixxall]

I think what Mr. Fix was saying is that Edelbrock cast the Merc 525 heads for Mercruiser, which have the raised exhaust ports. So if one were to order those "Edelbrock" heads, then a stock exhaust wouldn't work.

Edelbrock 61459 with "standard" exhaust port location is worth consideration.


I was refering to the flange bolt pattern, i dont know it the manifolds bolt pattern would line up with the bolt pattern on the exhaust manifold. i know with the 525 efi edlbrock casted merc heads that the bolt pattern is differant:)

61559 would be my choice if he goes that route,2.25 intake valves would flo better on a 496..

the 61459 has a 110 cc chamber and j&e makes a special piston for them,the flying tomato has dome pistons so those heads wont work for his application,piston will hit the head.

61559 has a 118cc chamber which should work with the pistons he is currently running,but he still needs to research the exhaust thingy :)

[BUIZILLA]

the 61459 has a 110 cc chamber and j&e makes a special piston for them,the flying tomato has dome pistons so those heads wont work for his application,piston will hit the head.
I think his *domes* are nothing more than .110 flat humps.... hardly a *dome*, if it was me, i'd use the 110's if he only has the humps..

at this point a pic of the intake ports and piston humps are required.. :yes:

[flying tomato]

I am going to try a couple different props next. It seems like the engine has tons of power below 4800 rpms. I'm going to try to put a larger pitch prop on and see what happens.

I'll let yall know the results after the tests.

Slow Flying Tomato

[handfulz28]

61559 would be my choice if he goes that route,2.25 intake valves would flo better on a 496..

61559 has a 118cc chamber which should work with the pistons he is currently running,but he still needs to research the exhaust thingy :)

Yeah, I'd like the 2.25 intakes also for the 496. I think the other difference between our two part numbers is Oval vs Rectangle. He supposedly already has an Oval intake, not a huge cam, so figure the Ovals would be better.

I remember a certain other member trying to bolt a stock exhaust onto the 525 heads....seems he decided against it...:wink: All that's needed is to confirm a new set of heads have the ports in the "standard" location, which both the 459 and 559 have.

:pizza::pizza:

[Pismo]

Seems like the carb swap to 850cfm would be the easiest first try. Good heads under a 750 may do nothing. same for the exhaust.

[mrfixxall]

If it cant suck then force it,,throw a whipple on it:biggrin.: