Mercruiser 5.0 TKS vs 4.3 MPI

[Scott Danforth]

The 5.7 4 barrel is 260hp. 300hp from the mpi. No 4 barrel of those days was ever as powerful as the mpi slightly later version. Apples for apples.

you really should look up the specs for the volvo penta AQ311a and B, 572 a and b and 574 a and b.... all with a carb....and all prop ratings... and many with flat tappet cams, etc.

 

[QBhoy]

you really should look up the specs for the volvo penta AQ311a and B, 572 a and b and 574 a and b.... all with a carb....and all prop ratings... and many with flat tappet cams, etc.

Different to what’s being discussed. But I’ll add that if we throwing Volvo hp ratings in the mix. Their 4.3 gxi (mpi) is 225hp. 5.0 gxi 270hp. 5.7 gxi 320hp. Haha. This isn’t helping your case.

 

[QBhoy]

you obviously think I dont know that. however Im glad you like your MPI engine.... you still cant make more power with EFI than you can a carb. the combusion mix of the engine (dynamic CR, cam profile, rocker geometry, combustion chamber and of course bore and stroke) is what determines an engines power.

Think we on a different thing now. You compared tbi with carb for fuel efficiency. Not relevant here. Since we talking about mpi. That’s all I’m saying

 

[Scott06]

Hi
A 4 barrel 4.3 in those days was about 190-210 Hp at best. The mpi 220. The 5.0 4 barrel was from memory about 230/240 (I think)...5.0 mpi is 260hp. The 5.7 4 barrel is 260hp. 300hp from the mpi. No 4 barrel of those days was ever as powerful as the mpi slightly later version. Apples for apples.

I think your memory is off 4,3 vortec 2 bbl is 190 hp, no 4 bbl offered , 5.0 Vortex 220 is 2 bbl Again no 4 bbl , 5.7 vortec 260 hp is a 2 bbl.

 

[Scott06]

Hi
A 4 barrel 4.3 in those days was about 190-210 Hp at best. The mpi 220. The 5.0 4 barrel was from memory about 230/240 (I think)...5.0 mpi is 260hp. The 5.7 4 barrel is 260hp. 300hp from the mpi. No 4 barrel of those days was ever as powerful as the mpi slightly later version. Apples for apples.

Again memory is off a tad 4.3 vortec2 bbl is 190 hp.. the 230 4 bbl you reference is non vortec rating, mpi you reference is a vortec, merc never Made a mpi non vortec so Not apples to apples

 

[Lou C]

MPI runs better cold and has a steadier cold idle for sure. More power? Debatable but ok maybe true in some cases. More important: ease, cost and speed of repair. Carbs win that one every time.....

 

[QBhoy]

Again memory is off a tad 4.3 vortec2 bbl is 190 hp.. the 230 4 bbl you reference is non vortec rating, mpi you reference is a vortec, merc never Made a mpi non vortec so Not apples to apples

So is has there ever been a 4 barrel carb as powerful as an mpi from factory ? I don’t think so. I think there was recently a reman quicksilver 4.3 carb that claimed 220hp. But that was only last few years or so. I think the 2 barrel vortec is only 250hp, by the way.

 

[Scott06]

So is has there ever been a 4 barrel carb as powerful as an mpi from factory ? I don’t think so. I think there was recently a reman quicksilver 4.3 carb that claimed 220hp. But that was only last few years or so. I think the 2 barrel vortec is only 250hp, by the way.

You made my point for me ... merc no longer offered new 4 bbl engines in 4.3, 5.0, 5.7 sizes. Not that carbs wouldn’t make that much power, but rather because few would pay the $ for an mpi that made similar or same hp as a 4 bbl carb version could.
yes if you look at merc remanufacturing you can find a 4.3 4 bbl at 225 hp and a 5.7 bravo 4 bbl at 325 hp which is evidence that carb engines can be right there with mpi in hp generation.
To take it one step further price these 4 bbl reman Engines vs the same engine with mpi About $2000- 2500 diff carb vs mpi.

 

[Scott Danforth]

there is no power benefit from EFI, there is no fuel economy benefit from EFI.

the benefit comes from its ability to adjust for differing atmospheric conditions and to constantly monitor combustion events to pull back timing and fueling to prevent detonation

as @Scott06 and I stated, Mercruiser and VP stopped the 4 barrels when they launched MPI because no-one would buy the higher cost fuel injection vs a motor with a carb for $2-$2500 less.

its easy as heck to get 300-350hp out of a 350 with a carb in a boat. you can get well over 550hp out of the same motor in a car. the top EFI 6.2 from mercruiser is the 350hp black scorpion. yet I have built a pair of 383 carb motors that on the dyno ticked just over 450hp

 

[QBhoy]

Ok. Let’s all go back to carbs then...what possible reason would there not to be ??
Someone had better call GM, Ford, Honda and all the world’s engineering top dogs...tell them they’ve got it wrong. Unless....it’s all a big anti Carb Conspiracy?
C’mon guys. Reality check here. You can’t be serious ! I’m as nostalgic as the next guy about old motors. But this is a ridiculous suggestion.

 

[QBhoy]

there is no power benefit from EFI, there is no fuel economy benefit from EFI.

the benefit comes from its ability to adjust for differing atmospheric conditions and to constantly monitor combustion events to pull back timing and fueling to prevent detonation

as @Scott06 and I stated, Mercruiser and VP stopped the 4 barrels when they launched MPI because no-one would buy the higher cost fuel injection vs a motor with a carb for $2-$2500 less.

its easy as heck to get 300-350hp out of a 350 with a carb in a boat. you can get well over 550hp out of the same motor in a car. the top EFI 6.2 from mercruiser is the 350hp black scorpion. yet I have built a pair of 383 carb motors that on the dyno ticked just over 450hp

You can get anything you like out these engines. Not because they are carb though. It’s the rest of it you need to add to. The same power could be taken from a fuel injection version of the same engine. This is silly now.

 

[Lou C]

the power/economy argument is not the most important thing to boaters. The most important things are:
my boat won't start what do I do?
my boat won't run right what do I do?
my ECM or module is fried and the bean counters at Volvo Penta or Mercrusier made it NLA, now what do I do?
this is where the simple carb engines excel. Got a stubborn Quadrajet, or leaky Holley? bolt on a new Edelbrock and you're back in business!
I prefer to do my own but most people don't have the patience.

if you keep your boat a long time like many here do, be aware that by 10-15 years certain OEM parts become NLA, with simple carb boats the aftermarket will keep you going nearly forever, the OEMs want you to buy their ultra expensive and complex cat converter direct injection engines, I say no thanks, I can take care of business on my own just fine thanks.
a $50 rebuild kit, 2 hrs of my time and back to a smooth running Quadrajet, the next best thing to EFI. Just ask Cliff Ruggles!

Cliffs High Performance Quadrajets – Quadrajet Rebuild Kits, Cabruretor Parts, Quadrajet Rebuilding, Quadrajet Parts, Bushing Kits, Carb Tuning

No $1500-2000 ECMs, or $1100 fuel pump modules, if you have an older engine that takes a mechanical pump an $85 mechanical Carter fuel pump is all you need and they last forever. At most you're going up to $450 or so for a new Edelbrock 1409 or Holley 4160 whatever is your preference if what you have is too corroded to rebuild.
If you look around, there are a lot of threads complaining of hard to repair running issues with older EFI systems on both Volvo and Merc boats. For me its all about repair-ability. I could care less about the last 10-15% extra power or economy.

 

[JimS123]

If you want to hear a fresh young mechanic say "huh?", tell him you need him to dress your points. I suspect that carburetor maintenance will one day parallel the advent of electronic ignition.

Its fine if you have the time, patience and knowledge to do your own work, but for those that rely on marine mechanics I fear that one day there won't be one left that can work on an engine without a laptop.

The second best thing I ever did was install a Pertronics. Now the motor ran the same all the time. I didn't need to carry emery cloth with me to fix the points every so often.

The first best thing I ever did was upgrade to a new boat with MPI. After sitting all Winter long, click-zoom and the motor is purring like a kitten. You would have thought it was running just an hour before.

I guess I'll take my chances with NLA or DOA. My kicker and TowBoat membership offsets the convenience and performance.

 

[Lou C]

Modern vehicles and boats are meant for a 10 years life span as far as parts availability.
So if that fits into your buying habits its fine. But if you buy old (ie the first gen of throttle body and EFI boats) that are 15-20 years old be forewarned. NLA will stop your boating season right quick.

 

[Scott Danforth]

If you want to hear a fresh young mechanic say "huh?", tell him you need him to dress your points. I suspect that carburetor maintenance will one day parallel the advent of electronic ignition.

points ignition and carb used to be part of IC engines 101. they quit teaching that about 4 years ago. todays "ASE certified mechanics" learn less in year one than a 1985 high school kid in Autos class.

 

[Lou C]

If you want to hear a fresh young mechanic say "huh?", tell him you need him to dress your points. I suspect that carburetor maintenance will one day parallel the advent of electronic ignition.

Its fine if you have the time, patience and knowledge to do your own work, but for those that rely on marine mechanics I fear that one day there won't be one left that can work on an engine without a laptop.

The second best thing I ever did was install a Pertronics. Now the motor ran the same all the time. I didn't need to carry emery cloth with me to fix the points every so often.

The first best thing I ever did was upgrade to a new boat with MPI. After sitting all Winter long, click-zoom and the motor is purring like a kitten. You would have thought it was running just an hour before.

I guess I'll take my chances with NLA or DOA. My kicker and TowBoat membership offsets the convenience and performance.

While a lot of people have no idea how to set points, carbs in both the collectible car market and the hot rod aftermarket most certainly are not going away. There will be someone who knows how to rebuild a Holley or an Edelbrock for many years. Those of us who know the Quadrajet, however, might be an endangered species!

 

[QBhoy]

While a lot of people have no idea how to set points, carbs in both the collectible car market and the hot rod aftermarket most certainly are not going away. There will be someone who knows how to rebuild a Holley or an Edelbrock for many years. Those of us who know the Quadrajet, however, might be an endangered species!
View attachment 327492View attachment 327496

Love seeing stuff like this Lou. I’ve saved these pictures. Definitely be useful. Thanks again.

 

[tpenfield]

As the discussion on MPI vs. Carb continues, would it be fair to say that MPI has advantages over carb at lower RPM since it does a better job 'vaporizing' the fuel and puts it right where it needs to be? It seems to me that once the velocity of intake air increases based on higher RPM that carbs vaporize better than they do at low RPM (i.e. low air velocity)

Also wondering if there are significant torque curve differences between the 2 types of intake, particularly below 2,000 RPM.

Asking for a friend . . .

 

[Scott Danforth]

As the discussion on MPI vs. Carb continues, would it be fair to say that MPI has advantages over carb at lower RPM since it does a better job 'vaporizing' the fuel and puts it right where it needs to be? It seems to me that once the velocity of intake air increases based on higher RPM that carbs vaporize better than they do at low RPM (i.e. low air velocity)

Also wondering if there are significant torque curve differences between the 2 types of intake, particularly below 2,000 RPM.

Asking for a friend . . .

the carb motor curve will have a higher peak to the torque curve vs the EFI having a flatter curve. this is partially because the cam LSA changes for the induction (reference the Mercruiser HP500 in both carb and EFI.... same HP rating at WOT, slightly different torque curves) the EFI motor has a very very slight advantage about 1800 RPM, the carb motor has a noticeable advantage at 4000 RPM at 5200 RPM they are about equal

manifold plenum volume and runner volume change the curve more so than what is squirting fuel and metering air into the mix. larger volume plenum and longer small cross-section runners gives you a fatter low RPM and mid-range. shorter runners with a larger cross-section push the fat part of the torque curve up the RPM range.

since most boat motors are not in the 6000 RPM or higher, a big plenum and longer runners works best for a good torquey mid-range.

on go-fast boats, big runners and small plenums help when the RPM's are closer to 10k

 

[Lou C]

Love seeing stuff like this Lou. I’ve saved these pictures. Definitely be useful. Thanks again.

These things are in our blood. They were on all the GM cars we drove back in the '60s, '70s and well into the '80s. While Holley always had the performance reputation for hot rodders etc, the majority of GM muscle cars from Chevrolet, Pontiac, Oldsmobile and Buick all came with Quadrajets. And if your family was not into muscle cars, but your dad or uncle had a big Chevrolet, Pontiac, Buick or Olds the top option was a big block (427 and later a 454 for Chevy, 455 for the others) with of course a Quadrajet. In fact the spread bore design was so good (small primaries, with advanced booster design) that Carter imitated it with the Carter Thermoquad that was used on many Chrysler products. And ironically even Chrysler used the Quadrajet on some models in some years.