Swapping out a 305 for a 383.

[QC]

Re: Swapping out a 305 for a 383.

Some clarifications may help this thread . . .

I honestly have never understood this advice. Look at the torque curve of any marine engine and you'll see that it's lowest torque (force on the drive) is at the bottom of the RPM range and maximum torque is reached around 3500- 4000 RPM depending on cam grind.

This is kind of correct. The most total power is actually at peak horsepower, so for extended wear and tear, WOT (peak power) has the most power going in (and out), so the most total heat, wear etc.

Bubba, static friction takes more force to overcome than kinetic friction. And for every force there is an equal and opposite force in the opposite direction. If you understand those two concepts, then you'll realize why hammering the throttle from a standstill is so hard on an outdrive.

But maybe not the hardest. Yes that initial twisting point (torque) may break something, but WOT is a big deal too.

It also isn't anywhere near 400hp, more like 325, with Merc advertising the newer ones at 350. But not with stock exhaust manifolds.

Agree.

Hmm..looking at my manual, it doesn't look like Merc used an Alpha I (even Gen II) past a 260hp 5.7.

300 bhp Mag MPIs are sold with Alpha's even today. Propshaft horsepower. Stay tuned . . .

The 383 I'm looking at is available up to 400hp.

If you check closely the factory Merc 350 bhp ratings are at the flywheel, and 325 at the propshaft. Even Alphas eat about 8% or so going through the drive. Say 30 horsepower. The "Merc 260" of old is actually only a 230ish when rated as they are now. No way with a reasonable marine cam are you going to see 400. Either at the propshaft or the flywheel. Go radical street cam and wind her up then maybe. Look for no more than 350 at the flywheel and 325 at the propshaft for reasonable expectations. Take it easy with the right hand and you should be OK with the Alpha.

 

[DaNinja]

Re: Swapping out a 305 for a 383.

Thanks for the input QC!

 

[proshadetree]

Re: Swapping out a 305 for a 383.

Bubba is right on this on. An engine will not produce maximum torque and horsepower at a low rpm. Thas why racers use stall torques in their cars to get to the rpm range where they start makeing power. But in a boat I would assume that The effects of torque will be less after the craft is moving. That is unless the operator is running say 20 and floors it. And we never do that. Well hardly ever.

 

[TilliamWe]

Re: Swapping out a 305 for a 383.

Hmm..looking at my manual, it doesn't look like Merc used an Alpha I (even Gen II) past a 260hp 5.7.

The 383 I'm looking at is available up to 400hp.

In 1987 and 1988, before there was a Bravo series drive, Mercruiser installed Alpha Ones behind 454 Magnums, rated at 330hp at the cranshaft. My dad had one is his big ole' Sea Ray 268 cruiser. It took a 15 pitch prop to plane that thing with his usual load, but with over 800 hours on it, the Alpha One drive had never broken.

And yes, I determined that you have a Gen 2, based on the description of your boat in your signature line.

Ok, bubba, you are right about force on a shaft and everything else. But let me ask, when people have a spun prop hub or a coupler, how come it only shows up when they are trying to get on plane? Not when they are running wide open?

 

[telstar1]

Re: Swapping out a 305 for a 383.

I disagree. We may be I THINK ignoring the difference between available torque and torque actually used. Getting out of the hole, we floor the throttle and the engine goes to the max rpm it can get to at that time say 2500 or whatever your boats characteristics allow. At that time we have used peak torque AVAILABLE at that rpm. As the boat comes up on plane the rpms increase and we back off the throttle lever till we reach cruise at say 4000 . Sure the torque curve shows more torque AVAILABLE at 4000 but were not using it.Case in point now try dragging some skiiers or another boat etc,you can still reach 4000 but now you need more throttle.More torque required.In my opinion, the most torque used when doing NORMAL operation is coming out of the hole. Heck you can FEEL it cant you. Sorry I cant prove this with a bunch of graphs but Im pretty sure Im correct. The spun hub analogy is I think a good one. That is where they USUALLY go,due to the high torque at that time.
As I said in my prev. post Im not sure about wot. I dont have torque curves to refer to so couldnt say whether torque at that point,which WOULD be the max available at say 4800 or whatever,,is higher than the max available at hole shot time say 2500 rpm.

 

[TilliamWe]

Re: Swapping out a 305 for a 383.

Nice try telstar. But it's obvious that bubba is right about this no matter what physics facts or experiences the rest of has. And that's fine, he can be right.

The FACT is, a 383 from Mercruiser will NOT produce 400hp in a boat without exhaust and carb help. And when it's only producing 300-325hp in it's stock form, and Alpha One Gen 2 will handle it fine, as long as it's not abused.

 

[GLENN M]

Re: Swapping out a 305 for a 383.

you can not compare tourque to h.p,tq is ability to pull at a steady speed ie;hammer brakes on car floor it you will have lots of tq and not go anywhere,hp is ability to gain rpm.at 5252 rpm hp and tq allways on gas motor cross paths,look at any graph.hp is goin up tq is going down,so higher rpm equals less tq.jump on a bike start peddling in first gear you can only put max tq at low speed you can still pick up rpm.so lifting heavy boat on plane engine will use more tq,once on plane it will start putting out more hp because it will be picking up rpms.and torque is resistance to moving.you put more twist on a shaft that aint moving,the more it moves less twist you can exert,why do you think truck 454 have such small ports,tq falls off fast,dont care how you make it look you wrong.and under any circumstances engine will only use as much tq as it needs,and formula we use millwrighting for tq or hp also includes gear ratios

 

[DaNinja]

Re: Swapping out a 305 for a 383.

So should I stick with a replacement 305 or upgrade to a 383?

 

[TilliamWe]

Re: Swapping out a 305 for a 383.

So should I stick with a replacement 305 or upgrade to a 383?

Almost anything's better than a 305! You should upgrade, just don't expect you're getting a 400hp rocket boat. You will get a NICE upgrade in power, though.

 

[picklenjim]

Re: Swapping out a 305 for a 383.

I agree with the fact that the torque increases with the rpms but what no one has seemed to consider is load. Taking off from a dead stop the load is much greater on the drive even though the engine is not developing full torque yet. It's a much greater strain on the drive getting the boat rolling and on plane than it would be if you nailed it at 4000 rpms. Cruising at an rpm where max torque is achieved there would be less resistance against the torque. The wear factor in the drive is going to be determined by weight or load on the prop with a given input torque. Getting on plane is when the max load is on the drive regardless of there being less torque developed by the engine at that rpm.

 

[telstar1]

Re: Swapping out a 305 for a 383.

The load on the drive is only there if the prop is big enough to make that load.You cannot look at a torque curve and say that is the toirque on the drive at that rpm. To take a ridiculous but perhaps illustrative example, remove the prop completely. Run your engine at 2000 3000 4000, whatever, how much torque(same thing as load by the way)do you think the engine is producing?zero. A properly propped planing vessel will struggle to get out of the hole because the prop is loaded (torqued)right outat that point. When planing is achieved, its like getting to the crest of a hill in your car, the load (torque)is drastically reduced.
I will grant you this Bubba, if your torque curves show more torque at wot, I guess that is max torque. Havent seen the graph but Ill take your word for it I think. Personally I try never to run more than about 80% throttle due to the astronomical difference in fuel consumption above that. At that cruising rpm, the load(torque) being demanded/supplied by the drive/engine is WAY less than the curve says the engine will produce at that rpm I think.Guess this post has got way off track-sorry.

 

[proshadetree]

Re: Swapping out a 305 for a 383.

This is getting way over rated a alpha one and a alpha one gen two will in fact hold a jasper 383 if you dont build it beyond a race engine. All drives break, Even under low power, especially if not maintained. Run it like you do the engine you have. As stated it is a 383 not a 502

 

[DaNinja]

Re: Swapping out a 305 for a 383.

...Guess this post has got way off track-sorry.

I've found it educational and entertaining.
I'm not sure if I'm going with the 305 or 383, but I've learned a little on the way.

 

[QC]

Re: Swapping out a 305 for a 383.

you can not compare tourque to h.p,tq is ability to pull at a steady speed ie;hammer brakes on car floor it you will have lots of tq and not go anywhere,hp is ability to gain rpm.at 5252 rpm hp and tq allways on gas motor cross paths,look at any graph.hp is goin up tq is going down,so higher rpm equals less tq.jump on a bike start peddling in first gear you can only put max tq at low speed you can still pick up rpm.so lifting heavy boat on plane engine will use more tq,once on plane it will start putting out more hp because it will be picking up rpms.and torque is resistance to moving.you put more twist on a shaft that aint moving,the more it moves less twist you can exert,why do you think truck 454 have such small ports,tq falls off fast,dont care how you make it look you wrong.and under any circumstances engine will only use as much tq as it needs,and formula we use millwrighting for tq or hp also includes gear ratios

Uhhhh sorta, kinda, uhhhh . . . nope. Do a lot more reading on this topic.

I swear I don't get it. Voodoo beats established math calculations for sizing a gear box (out drive) and it's components? Not anywhere I've ever seen, except in this forum.

Relax my friend . . . I have been trying to explain torque and horsepower for much longer than I have been on this forum. Automotive advertising has ruined most forever. With that said, I do believe in this particular discussion, that both points are valid. Twisting up against load is one thing, sustained output is another. Both can lead to failures.

 

[QC]

Re: Swapping out a 305 for a 383.

Sorry but any comparissons to a car simply don't work.

You got that right!!

 

[GLENN M]

Re: Swapping out a 305 for a 383.

i should have said it like this,hp is work done,without looking it up i will say 33000lb moved so far in so much time=1 hp,tq =force on a shaft not meaning any work is being done,if a shaft needs 600 ft lb"s to turn and you apply 500ft lbs,you get nothing happining,no hp whatso ever.so how can you say 200ft lbs is 95hp and so on.there is no way to convert one to the other,and again it depends what rpm range your engine is designed for,ie; an engine designed for 3000 to 7000 rpm will take for ever coming out of the hole once it hits 4000 hang on,all depends how its built.thats why i said truck engines have small ports make gobs of tq almost zero hp.so a 383 with 305 2 barrel bad idea,to much tq,no hp, put big manifold and carb will kill bottom end tq [still more then 305 had]but top end power will be fun,go 383.no substitution for ci

 

[StevNimrod]

Re: Swapping out a 305 for a 383.

I'm running an Alpha Gen 2 behind my 383 and haven't had any drive problems in two years.

Consider a good intake manifold + carb. You're feeding 78 more cubes.

I went from 350 to 383 and now can cruise at about what what I was getting WOT. I use more fuel at WOT. Significantly more fuel. It's nice to have the extra power, but you might get tired of filling up every week if you run WOT often.

Overall I'm satisfied, but it's hard to say if I'd do it again if I knew then what I know now. It's a little more complex than just picking up "free" HP. I know of quite a few around here that are stuck with stroker powered small and big block boats they can't sell at half price because people get scared about prices at the gas pump.

Also keep in mind that compression ratio tends to get high quick with strokers and shops use various means to keep in down (dish pistons, pistons down the hole some borderline obscene amount, big cc heads). I ended up with big dish pistons to keep my ratio down using 64cc Vortec heads. All stuff you might want to find out from Jasper or keep open the possibility that you may end up at a local machine shop depending on how detailed you want to get.

 

[DaNinja]

Re: Swapping out a 305 for a 383.

I'm running an Alpha Gen 2 behind my 383 and haven't had any drive problems in two years.

Consider a good intake manifold + carb. You're feeding 78 more cubes.

I went from 350 to 383 and now can cruise at about what what I was getting WOT. I use more fuel at WOT. Significantly more fuel. It's nice to have the extra power, but you might get tired of filling up every week if you run WOT often.

Overall I'm satisfied, but it's hard to say if I'd do it again if I knew then what I know now. It's a little more complex than just picking up "free" HP. I know of quite a few around here that are stuck with stroker powered small and big block boats they can't sell at half price because people get scared about prices at the gas pump.

Also keep in mind that compression ratio tends to get high quick with strokers and shops use various means to keep in down (dish pistons, pistons down the hole some borderline obscene amount, big cc heads). I ended up with big dish pistons to keep my ratio down using 64cc Vortec heads. All stuff you might want to find out from Jasper or keep open the possibility that you may end up at a local machine shop depending on how detailed you want to get.

Thank-you! That's very helpful.

 

[GLENN M]

Re: Swapping out a 305 for a 383.

like anything you need to be able to tell machine shop what parts you want installed,and be able to make sure you get it.i had ford non adj heads done they didnt shim springs all valve stems were differant height,you want zero deck height with pistons,and 76cc chambers forpump gas with 383,vortech heads,and aluminum heads can take more compression.383 wont have to work as hard as 305 or rev as high,it is like free power.if your planing on selling boat build it stock people who dont know are scared of anything modified,as for a little more fuel consumption,you gotta pay to play

 

[StevNimrod]

Re: Swapping out a 305 for a 383.

Like Glenn is saying, it can be a lot more complex and time consuming than it looks...especially the first time around. Most just involves simple math, but it can be complicated. 76cc heads or 64cc heads, what deck height (mine is 0.001 so with a 0..039 thick head gasket my quench is 0.040"), how big of a piston dish, shape of the piston dish (if you plan on using the quench you'll want pistons with D-shaped cups - see attached pic).

Then you get into cam selection, and getting a grind the complements the entire system (compression ratio, head size, weight of the boat, what kind of exhaust, usage, dockside manners, etc.).

My boat is a bit underpowered and many of similar size have twin engines. My justification/rationalization, at the end of the day, was come time to sell I'd be competing against twin engines, single big blocks, or underpowered single small blocks. About the only boats (similar to mine) I can't keep up with or outpace are those with twin engines.

All that said, what I end up using the boat for most is fishing (usually 2-15 miles offshore). The power is nice to have available, but I'm generally in no big hurry to get to the fishing hole that an extra 5MPH is going to make or break my trip. It is nice, in general, to be able to turn less RPM and get the same speed as before and I'd venture to say the engine and drive will be "happier".

When I talked about "free" HP I meant this: you can build a 383 for not much more than a 350 so it might be essentially a wash in terms of getting the long block. But if you're going to use the horses you've got to feed them, and gas ain't free. Then once you're getting into intake manifolds and a bigger carb you can figure on tying up some more coin.

It's not rocket science by any means, but it's not too hard to clean up your wallet in the whole process.