MPH gains from a vortec head swap and 4bbl carb

[jokergerm]

So i have 21ft milan with a 2bbl carbed mercruiser and it goes 50mph, I want to get to 60+

In my garage i have a set of vortec heads and a holley 4bbl carb if i was to swap those on along with a 4bbl intake, would this be enough added power to get to 60mph??

I dont want a letcure on how its stupid to try to go this fast in this boat.

I just want to know what i need to do power wise to make 60+

i was thinking of swaping to a 21p prop after the swap i am speeking of.

thanks guys

 

[jimmbo]

Re: MPH gains from a vortec head swap and 4bbl carb

What size is your present engine? 10mph is a hell of a big increase and a carb swap is not likely to do it. 5 or 6mph is reasonable(been there done that). Your best way to get 10mph is going to be a install a bigger engine or a super charger on the existing engine.

However, if you want to try a carb swap, that 4 bbl Holley you are thinking of using better be a marine carb. If it is not, don't even think of using it. No ands, ifs, or buts DO NOT USE. It
could cost lives.

 

[jokergerm]

Re: MPH gains from a vortec head swap and 4bbl carb

My current motor is a 5.7 mercruiser with a 2bbl

You really dont think that a head swap and 4bbl intake and carb swap would gain me 75hp?? and 10mph?

I know i have done this swap on cars and gotten 75 hp.

 

[mtnrat]

Re: MPH gains from a vortec head swap and 4bbl carb

yep go for the vortecs and get 30 to 35 hp and a qjet tuned right and get 20 over the 2bbl. You may get 55 but much better low end and likely better cruising gph.

 

[John_S]

Re: MPH gains from a vortec head swap and 4bbl carb

You end up with about 280hp vs 210 @ prop. I did that mod to my '97 last spring. I had similiar wot before as you. It is only going to get you about half way to your goal. A 383 stroker will get you closer to 60.

 

[TilliamWe]

Re: MPH gains from a vortec head swap and 4bbl carb

Now if you want to (and the block can accept it) install a roller cam in addition to the heads, intake, and carb, you might get closer. But the camshaft you have now is probably no slouch, so that might not do it either.
Can anyone remind me, what's the formula in a boat to make speed? 20hp to get 1mph? It's a lot harder to build speed in a boat than in a car, sorry.

 

[Mkos1980]

Re: MPH gains from a vortec head swap and 4bbl carb

I know in another post about adding an intake I had a link to where boating magazine had a 2bbl vortec motor putting out 250 HP and running 52 MPH. Just them adding a 4bbl intake ,edelbrock carb and a new prop got them 57MPH.

 

[QC]

Re: MPH gains from a vortec head swap and 4bbl carb

I can't remember who posted this, but I save the good stuff Amazingly applicable to this thread!

Here's the way to calculate it:

Drag is a function of the square of speed so take your old top speed, square it, then divide by the old HP. Take that number, multiply it by the new HP and then find the square root of the result.

For instance:

50 MPH top speed with 210 HP changing to 300 HP would be:

50 squared=2500, 2500/210 = 11.904761, 11.904761*300=3571.4283, The square root of 3571.4283=59.7614

The new top speed would be 59.7614 MPH assuming the new prop is exactly as efficient as the old prop and that the two engines were both accurately rated for power.

 

[180shabah]

Re: MPH gains from a vortec head swap and 4bbl carb

The new top speed would be 59.7614 MPH assuming the new prop is exactly as efficient as the old prop and that the two engines were both accurately rated for power.

OR 57.735MPH with the 280hp that the above motor will realistically produce. Of course this is all theoretical and an absolute best case scenerio. Propeller efficiency was already mentioned. How accurate is the current 50MPH? Is the hull sound or will it deflect more as speed increases? Will the boat be controllable with an additional 10MPH - Chinewalking will NOT impress your significant other.

 

[John_S]

Re: MPH gains from a vortec head swap and 4bbl carb

FWIW: My before numbers are with a 14.5 x 3 x 19 Black Max prop, and after is with 14.25 x 3 x 21 Black Max prop. The after numbers were confirmed with gps. The before numbers were with speedo only, but current speedo is amazingly within 1mph of gps.

Boat: 1997 Bayliner 2050 CL (this is the classsic bowrider style and has more deadrise than eurostyle versions)
Before: 50mph speedo approx from multiple wot runs, best trim.
After: 56mph gps from multiple wot runs, best trim.

Runs done with just myself and 3/4 to full gas tank, minor gear aboard.

Formula is rather close in my case. To hit 60, you will want to target 325hp @ prop, or approx 345-350hp at crank. This would give a little head room. The 383 stroker will easilly get in this range. A built 350 could get you there but may compromise on the low-end, out of the hole, performance.

 

[TilliamWe]

Re: MPH gains from a vortec head swap and 4bbl carb

So i have 21ft milan with a 2bbl carbed mercruiser and it goes 50mph, I want to get to 60+

In my garage i have a set of vortec heads and a holley 4bbl carb if i was to swap those on along with a 4bbl intake, would this be enough added power to get to 60mph??

I dont want a letcure on how its stupid to try to go this fast in this boat.

I just want to know what i need to do power wise to make 60+

i was thinking of swaping to a 21p prop after the swap i am speeking of.

thanks guys

Just put on a 24p like you seem to think will work in your thread in the prop section. Report back to us later this summer after you have spent $5000 on a $3000 boat and then get discouraged and give up boating.

I don't like to be so mean spirited and or sarcastic, but you are making the same mistake that every guy who thinks he knows how to make a car faster and then applies it to a boat. Silvertip kinda spelled it out to you in the prop thread about the prop being your rear axle ratio and your transmission all in one.

But please, put on the Vortecs, put in a high lift/high duration cam, change the intake (get a single plane while you're at it), put on a Holley 750 double pumper auto carb, go with an MSD super high powered ignition, put on a 25p cleaver prop, and after all that, you'll have a pig that won't plane, probably still won't run over 60mph, and will suck water up everytime you turn it off. Oh, and you're wallet will be a lot lighter.

 

[tmh]

Re: MPH gains from a vortec head swap and 4bbl carb

I dont want a letcure on how its stupid to try to go this fast in this boat.

/QUOTE]

So, as in his thread in the prop forum, Joker wants only advise that fits his desired outcome - which is to overpower his boat at a danger to himself and, more importantly, others. I think it is important to read the prop thread he started to put this in context before helping this kid mod his boat up to dangerous speeds. He and his buddies are obviously boating rookies looking for more of a thrill and ready to take chances to get that thrill. I have no problem with thrill seekers, even high risk takers.......just don't do it in the same arena that my family (and all of the rest of your families) want to plays safely in!

The financial costs involved in this are far less an issue than safety. Who cares if he spends more to mod his boat than a new go-fast boat costs! it's his money, I guess. The REAL cost is in having a thrill seeker in a family boat who has no experience at those high speeds and may have a boat not able to be safely handled at those speeds.

 

[TilliamWe]

Re: MPH gains from a vortec head swap and 4bbl carb

TMH, while I doubt that 10mph will make his boat an instant death mobile, I am reminded that dangerous drivers cause harm at even low speeds.

 

[wca_tim]

Re: MPH gains from a vortec head swap and 4bbl carb

wow... you guys are a rough crowd...

We all have different motivations and things that we get enjoyment from.

There's some good advice above, and in particular what John s is telling you is straight up.

I bet that the changes you're proposing will get you halfway there (ie 5 mph). Note that if the Holly 4bbl you're talking aobut isn't a marine carb, you're playing with fire.

If you do upgrade a little more cam would be helpful, but don't get out of hand or you can suck water back into the engine. for hydraulic tapet cams, the comp cams xm series are good. Something like a comp xm262h, 256h or equivleent crane with an edelbrock performer or performer rpm and a 600 or 650 cfm carb would probably run great. You would lose some of the low end torque so be prepared to run a little different prop for general use and pulling skiers than you would to get your best top speeds. Good exhaust manifolds will net a good bit too and become the bottleneck before long - and they're pricey for good ones.

btw... my current boat I went with the 383 and have a relatively mild cam for the engine build (comp cams xm270H), good aftermarket heads (wp sportsman) and a single plane intake (torker2) with a 750 cfm carb. Bottom end is all forged with pistons chosen and matched for best quench with the heads. Was built by a shop that specializes in marine builds. and yes I spent well over the value of the boat in getting the long block built. It hits peak hp about 5500 and will idle down to 500-600 rpms when warmed up. I am running top end manifolds and long risers and would expect reversion problems if not. Overall, I couldn't be happier with the performance... I'm currently running mid 70's with a 29" hydromotive prop and think with some more set-up and a little weight loss I'll be upper 70's or into the 80s . I'm running a 27" mirage plus when pulling skiers and boarders. out of the hole is slower with the hyrdomotive (but still not bad) and once up to cruising speeds, accelleration from say 40 to 65 feels almost instataneous. By goig with lighter exhaust and changing a few things to lose a little weight, I have aobut the same weight in the rear of the boat that I did with the v-6 and it handles even better than it did before...

I hope this is helpful.

 

[wca_tim]

Re: MPH gains from a vortec head swap and 4bbl carb

TMH, while I doubt that 10mph will make his boat an instant death mobile, I am reminded that dangerous drivers cause harm at even low speeds.

a moron behind the wheel can make almost anything unsafe...

 

[John_S]

Re: MPH gains from a vortec head swap and 4bbl carb

FWIW: I suspect most of the negative responses were geared toward the perceived boating ability of the poster, which was in a thread that I hadn't bothered to read. Anyway, my comments below are about the boat and not the guy behind the wheel.

The typical 21' V boat, that I have seen of Milan pics, would not suggest that a vortec head/4brl mod would make them a dangerous boat. I wouldn't be surprised if the factory offered a 300MPI option on the same hull. They might have even had a Big Block option. Don't know, but it wouldn't be hard to research.

A 383 stroker or HO 350 might put some additional stresses on a alpha drive, but might not nessarally cause major handling problems. The weight and balance of the boat remains virtually unchanged. If excessive porposing or chine-walk occur, there is a good chance it can be controlled with adding tabs. BTW, smart tabs eliminated the porposing that was an issue even with the 210hp output, in my case.

In my own boat, the modifications have not caused any loss of stability. It still scares me more to get in some of the older 18'/150hp bass boats.