454 big block difference

[kmarine]

I have mercruiser 310 hp 454 engines in my 1995 wellcraft Martinique. Are these the same as 330 hp engines with different carbs? can I change the carb jets or camshaft to increase hp without damaging the engines? my current wot is 4200rpm what could I do to get it to 4500 rpm without changing props. Should I consider different carbs than my Carter/weber stock carbs or rebuild them? Thanks for any input in advance

 

[MarkSee]

Re: 454 big block difference

Some really good tech person that knows all the differences should be able to answer that but interestingly, I have a model year 2000 454(7.4) MPI engine and that is listed also as 310 hp yet I thought the mpi motors put out more than carb models would.

Mark

 

[kmarine]

Re: 454 big block difference

seems we both are curious about the differences, I bet your MPI may get better fuel econemy than my carbs.

 

[Maclin]

Re: 454 big block difference

Optimize what you have first. That 300 rpm may be there hiding in the tuning of what you have. But if your engine is in a top state of tune then I think it would take more than 20hp to get another 300rpm.

As far as how to get 300 more rpm with no prop change and all else being equal, change the drive ratio

Ok, that is not a recommendation, more of a joke, but you asked...

 

[MarkSee]

Re: 454 big block difference

seems we both are curious about the differences, I bet your MPI may get better fuel econemy than my carbs.

Indeed I am interested in knowing.

Maybe gets better mpg but it sucks gas pretty quickly so I don't really want to know; it does start right up every time even if it has been sitting for a few weeks.

When the Pacific is relatively smooth I tend to have a heavy throttle hand and even in my 9000+ pound 270, that motor with a B3 gets it going over 40 mph; not even sure what WOT would be.

Mark

 

[TilliamWe]

Re: 454 big block difference

I have mercruiser 310 hp 454 engines in my 1995 wellcraft Martinique. Are these the same as 330 hp engines with different carbs? ...

Shouldn't be. The "old" 330hp engines were Gen IV engines, I believe. Very heavy, very stout. When they started calling them 7.4s and 300 or 310hp, that should be the Gen V engine. It's "light duty" in comparison.

Mark, fuel injection does not produce more horsepower, all by itself. It meters the fuel much more precisely, but adding fuel injection does NOT increase horsepower.

 

[pyrotek]

Re: 454 big block difference

I would question if you really are even going to notice a difference of 20hp in a boat. It seems to me that the effort involved does not equal to the benefit you will get. I would think you need at least 100hp to make a noticable difference.

 

[RogersJetboat454]

Re: 454 big block difference

There were differences between the Mark IV and Gen 5 heads as well. Gen 5 is to the big block as the LT1 was to the small block. Some parts interchange, but some things are different. A cam change, intake manifold change, and optimizing your carb tuning may net some more power. But as many have suggested, you may not even notice the difference in the top end.

Why are you so against changing props?

 

[picklenjim]

Re: 454 big block difference

I have mercruiser 310 hp 454 engines in my 1995 wellcraft Martinique. Are these the same as 330 hp engines with different carbs? can I change the carb jets or camshaft to increase hp without damaging the engines? my current wot is 4200rpm what could I do to get it to 4500 rpm without changing props. Should I consider different carbs than my Carter/weber stock carbs or rebuild them? Thanks for any input in advance

The horsepower difference is because they used to measure horsepower at the crankshaft. Now they are measured at the propshaft. Actually would be no real difference between the two. The Mark IV's were called 7.4's starting in '88. Gen V started in '92. You could go to an aluminum highrise dual plane intake and mild cam and probably pick up 300 rpm. Just keep it mild because the rest of the parts in that engine aren't designed to perform with a big cam change, ie heads-exhaust-cast bottom end.

As far as heavy or light duty between generation there is no difference. Here's a link showing the changes made with the generations.
Chevrolet Big Block Engine Generations - Chevy High Performance

 

[TilliamWe]

Re: 454 big block difference

ok, maybe the term light duty wasn't correct, but there are things about the Gen IV that make it somewhat more desireable than the later ones. Just not hte two piece rear main seal.

 

[MarkSee]

Re: 454 big block difference

Mark, fuel injection does not produce more horsepower, all by itself. It meters the fuel much more precisely, but adding fuel injection does NOT increase horsepower.

Maybe this apples and oranges between the 454/7.4 and the 4.3 regarding horsepower but here's why I am unclear:
My 2005 Maxum had the 4.3 mpi which was listed as 220 hp but the same exact year/model but with a carb was rated at 190 hp. So as far as I knew, the only difference between the 2 was the induction system which lead me to think a mpi would have more hp.

Mark

 

[kmarine]

Re: 454 big block difference

The reason I am against changing props is because I already have three sets of nibral props which cost about $1800 per pair I have 16x17,and a 16x16 thre blade set and a 4 blade 15x15.5 acme . I have found little difference in rpm the 16x17 will get me 4100 rpm the 15x15.5 get me to 4200 rpm as well as the 16x16 which. the 4 blade allows me to plane at 3600 rpm where the 3 blade requires about 3800.
These props were recomended by michigan wheel and acme propeller for the boat. I have already changed plugs cap and wires. I am looking for sugestions before I send the carbs for rebuilding. I currently am getting about 3/4 mpg and a top speed of 28 mph.

 

[kmarine]

Re: 454 big block difference

The reason I am against changing props is because I already have three sets of nibral props which cost about $1800 per pair I have 16x17,and a 16x16 thre blade set and a 4 blade 15x15.5 acme . I have found little difference in rpm the 16x17 will get me 4100 rpm the 15x15.5 get me to 4200 rpm as well as the 16x16 which. the 4 blade allows me to plane at 3600 rpm where the 3 blade requires about 3800.
These props were recomended by michigan wheel and acme propeller for the boat. I have already changed plugs cap and wires. I am looking for sugestions before I send the carbs for rebuilding. I currently am getting about 3/4 mpg and a top speed of 28 mph.

 

[spdracr39]

Re: 454 big block difference

Sell two sets you won't be using and get a set that works for you then pocket the difference for gas money. Save the third set for spares

 

[kmarine]

Re: 454 big block difference

I currently have the props up for sale, the newest props I have were custom cnc machined for the boat and I want to make sure that I have tried all alternitives before sending them back to the prop shop to have reworked. This would be the third set. the prop shop propulsion engneering tech does a great jod with me as I heve been refering customers there for 20 years.

 

[TilliamWe]

Re: 454 big block difference

Maybe this apples and oranges between the 454/7.4 and the 4.3 regarding horsepower but here's why I am unclear:
My 2005 Maxum had the 4.3 mpi which was listed as 220 hp but the same exact year/model but with a carb was rated at 190 hp. So as far as I knew, the only difference between the 2 was the induction system which lead me to think a mpi would have more hp.

Mark

No, I believe there is a camshaft difference as well. But even if there isn't, the MPI probably is a higher CFM than the 2bbl carb, that the 190 hp has on it. So it's a little like comparing a 2bbl carb to a 4bbl carb. So the MPI didn't add the power, the increase CFMs added the power. They just happen to be attached to injectors, not a 4 bbl carb. Clear as mud?

 

[TilliamWe]

Re: 454 big block difference

Guys is there a propslip calculator that would apply to this guy's setup? K, I'd hate to see you rebuild carbs, if all you need is the right props. But I understand at $1800 a set, it's not fun to experiment with props.

 

[kmarine]

Re: 454 big block difference

I agree, The prop shop is willing to make changes for me, and we already have. I wanted to make sure that I did not have any posible engine updates before giong further with more prop work. The carbs are due for rebuild anyway. I wanted to see if I needed to change anything carb related first. Carbs havent been touched probably since the boat was new. I also have other work I want to get to before spring. Thanks everyone for your input.

 

[Lyle29464]

Re: 454 big block difference

How wide are your trim tabs? Do you release them part way above planing speed?

 

[Fun Times]

Re: 454 big block difference

If you suspect an engine problem, then you may want to consider finding an marine service center that has an marine prop shaft dynamometer model type 4000 so that it could handle big block engines like yours to help ensure that your engine is putting out the normal amount of power the service center should be used to seeing with your engine model.

Boat Marine Dynamometer Dyno K G Marine Model 4000 | eBay
Boat Motor Dynamometers - International DYNO
PROPSHAFT DYNO @ BOOSTPOWER MARINE - YouTube