Hypothetical Gas Engine Question

[catalinajack]

The answer to this is fairly simple. The cheapest repower for big blocks is another set of big blocks. If you don’t want the speed but want economy, you can rejet the carburetor and advance the timing leaning out the engine at 2200 rpms. Now doing this will likely increase your fuel economy by 50% but you will want to put in a throttle restriction. Pushing a leaned out advanced engine beyond 2200 rpms will likely blow it up. What you have done is optimized the engine for the low end giving up ever being able to access the higher end. No one does this but it has been done in the past.

Tilrider1, you just gave me a big laugh. "No one does this but it has been done in the past". A classic Yogi Berra line if there ever was one. Thanks for the smiles.

The answer to this is fairly simple. The cheapest repower for big blocks is another set of big blocks. If you don’t want the speed but want economy, you can rejet the carburetor and advance the timing leaning out the engine at 2200 rpms. Now doing this will likely increase your fuel economy by 50% but you will want to put in a throttle restriction. Pushing a leaned out advanced engine beyond 2200 rpms will likely blow it up. What you have done is optimized the engine for the low end giving up ever being able to access the higher end. No one does this but it has been done in the past.

 

[Pete Meisinger]

Nope, there is no such thing. Besides, don't do it. Even if you find a free 50 to 100 hp diesel, don't make the conversion. You will have just squandered the resale vales of your boat to nearly 0.

pete

 

[Bill V]

A 15 minute WOT run on a seal trial of any boat without continuous rated engines is an absolute non-starter in my book. Even more so with most marine gas engines, where max continuous power is a heck of a lot less than full power. 1 - 2 minutes at WOT to check temperatures, RPM, etc. is fine, but anything more is just abuse. For a 454, for example, max continuous is typically regarded as 3400 RPM, which is only about 180 - 220 hp depending on where in the 4000 - 4400 RPM range it sits at WOT with the current props.

There's nothing inherently bad about a marinized car engine, but how well it works really comes down to how good the marinization parts are, if anything was modified or done differently for better durability at high load, and whether the basic design is suitable for the desired max continuous power output (regardless of peak power output). Some engines (like the 454) translated pretty well into marine use, for example.

Totally agree that anytime you go to WOT is to verify correct props, mechanical parts in good working order and engine in good state of tune. 15 minutes is a looong time at WOT.

Having purchased two new Crusader 350/310hp freshwater cooled engines in 2005 and having paperwork stating "fasr cruise" was 4000 RPM maximum I cruised at times 3600 and blew up an engine at 900 hours. After disassembly I found they were nothing but a normal car/pickup engine with a marine spec camshaft. Pistons, bearings, valve train and oil pumps were the same used in most car engines. My failure was a piston coming apart at 3600. I have little faith in "production" marine engines now.

My present engines were built by a former NASCAR engine builder with nothing but "bullet proof" components for RPM's to 7000, carry 8 qt. oil pans, high pressure oil pumps, oil coolers, freshwater cooling, windage trays, NASCAR rated bearings, forged rods, forged pistons, and forged stroker crankshafts. Roller marine camshafts, timing chains and rockers. They were both run after 25 hour break in installed in the boat at 4000 RPM for 10 miles at 33 - 34 knots with no problems. WOT is 4900-5000 @ 38-40 knots. I would never buy another Crusader engine ever. My present engines cost about $4500 each for parts and labor to build and I installed them. They were test run on my shop floor for 3 hours at 2000 -3000 to verify settings and no leaks.

 

[rslifkin]

Is there a problem switching fuels in the tanks?

Should be as simple as clean and inspect the tanks, add return ports if needed. The (very rare) diesel versions of my boat used the same tanks. Only difference is that my gas powered version has plugs in the return ports on the tanks.

 

[rslifkin]

Totally agree that anytime you go to WOT is to verify correct props, mechanical parts in good working order and engine in good state of tune. 15 minutes is a looong time at WOT.

Having purchased two new Crusader 350/310hp freshwater cooled engines in 2005 and having paperwork stating "fasr cruise" was 4000 RPM maximum I cruised at times 3600 and blew up an engine at 900 hours. After disassembly I found they were nothing but a normal car/pickup engine with a marine spec camshaft. Pistons, bearings, valve train and oil pumps were the same used in most car engines. My failure was a piston coming apart at 3600. I have little faith in "production" marine engines now.

My present engines were built by a former NASCAR engine builder with nothing but "bullet proof" components for RPM's to 7000, carry 8 qt. oil pans, high pressure oil pumps, oil coolers, freshwater cooling, windage trays, NASCAR rated bearings, forged rods, forged pistons, and forged stroker crankshafts. Roller marine camshafts, timing chains and rockers. They were both run after 25 hour break in installed in the boat at 4000 RPM for 10 miles at 33 - 34 knots with no problems. WOT is 4900-5000 @ 38-40 knots. I would never buy another Crusader engine ever. My present engines cost about $4500 each for parts and labor to build and I installed them. They were test run on my shop floor for 3 hours at 2000 -3000 to verify settings and no leaks.

A build along those lines is probably what I'd do to my 454s if I were building a new set. As far as I know, the primary reason for the 3400 max continuous limit on the older 454s like mine is the cast pistons and the valvetrain. Fix those and the 2 bolt mains will probably become the next limit. My big challenge if I were to build the 454s is that I either need to keep the stbd one as reverse rotation, or I'll need new transmissions, as I can't just reverse the direction of my current Velvet Drive 72Cs.

Durability wise, one of my engines got a long block at around 1400 hours after a previous operator (in the family, but not me) neglected to change some aged oil cooler lines and had one fail while up on plane. Didn't get it shut down fast enough and wiped the main bearings. Still ran fine and sounded fine, but had no oil pressure at hot idle and a bunch of metal in the oil filter.

The other engine is still original with around 1600 hours on it, plenty of those turning 3200 - 3400 rpm up on plane. Still shows good oil pressure (better than the rebuilt one actually), compression is still good and I can't tell a difference in how the 2 engines run. Oil consumption over this season was about 1/8 quart in ~90 hours for the newer engine, a little under 1 quart in ~90 hours for the older one. Mine are Mercruisers with the Thunderbolt IV ignition.

Building new, I'd either keep the Quadrajet carbs and update to Thunderbolt V ignition (adds knock sensors and a much more efficient light load timing curve), or I'd go for something like the Holley Sniper Marine EFI and let that also do timing control. If I were feeling like more effort and money, I'd consider a fancier multiport EFI setup, but that increases the number of connections containing pressurized fuel by a lot, so I'm hesitant there.

Out of the box, a lot of marine gas engines make a few too many compromises in my mind. They're aimed at keeping costs down and providing a reasonable lifespan while being operated by idiots. That leads to dumb decisions like Merc putting 140* thermostats in my fresh water cooled engines, no oil cooler thermostats, etc. I've updated to 160* thermostats for better efficiency and less wear (and have considered going to 170s, as they run 175 - 180 on plane anyway, so being a little closer to that at low speed would be fine). I also intend to monitor oil temperatures at low speeds and decide if they'd benefit from thermostats on the oil coolers to allow things to warm up better when not being run hard.

 

[moparharn]

2 miles to the gallon on a gas big block is fantastic. Those numbers are trawler like in my opinion. Many single engine trawlers will range 2-6 gph at 7 knots and 4gph is right in the middle of that. I suppose if the fuel system and ignition were optimized for low rpm this could be a viable option. Actually the ideal option. This boat has fuel scanning and the consumption was about 42 gph at 4200 rpm.

As for the WOT run times. I agree. 15 minutes is too long, but many surveyors ask for it or suggest it at least. I think this length of time is appropriate for a diesel. Far less certain for gas. The justification (I think it was Pascoe) is that if you encounter a bad storm front a 15 minute wide open throttle run to a secure anchorage or harbor is more than likely. This is not the time to find out that your engines are not up to the task.

Another interesting thing I learned from reading a Pascoe article and from wrenching over the years is that Crusaders and Mercruisers are NOTHING alike outside of their cubic inch displacement. Crusader uses a different truck spec engine than the Mercruiser light duty 7.4. The heavy duty version used by Crusader has a 4 bolt main versus 2 bolt, heavier rods, pistons, and cam spec. This would address some of the desires expressed in this thread. The engines I sea trialed were Crusader. EFI with computer controlled ignition and timing might really be a nice option. Finding someone to install either a Holley or FITECH system is difficult. High pressure fuel needs are also a concern.

The overall concern about good ongoing maintenance is legitimate. Thanks again for the posts.

Mopar, I will give you some real numbers of our last eight months of cruising exactly how you see yourself.
We are 40’ and 50’ over all with dinghy. 30,000 full fuel, water, gear! We have twin 454 ‘s and cruise religiously at 7 knots. And always hoping on a following current.
We are getting 4.1 gal/hr for both engines, we know this mileage well, because we start every trip full of fuel and end topping off tanks. So we have a pretty good accurate fuel burn with no fancy gauges.
Buuut if we screw up and have to push current or wind or running late for a passage, well hell yah we can burn a couple dinosaurs. We like this option in our back pocket knowing we can beat a weather front or just get to where we need to be.

 

[moparharn]

Very interesting ideas. Electric motors or a single driving two props is not to be dismissed off hand. Thank you.

Greetings,
Mr. m. Just noticed the typo in my post (#14). Should read steam instead of stream...BUT, what about the option of a single diesel driving the two prop shafts? Potentially cheaper than 2 independent engines, increase in ER space and IF the correct size, planing may be possible.

One last "out of the box thought" Single diesel generator and twin electric motors.


Edit: Just had to do that "Captcha" thing to post this.

 

[Art]

I have a couple of specifically well built engines. One in my 1985 vintage Chevy Cheyenne truck [350 cid 325 hp.] One in my classic 1967 Buick Wildcat [430 ci 400 hp] - Both carbureted.

I also have three, carbureted, standard-build engines in boats [350 ci with 255 hp +/- each].

And, a standard-build 1995 454 tbi Chevy engine in our RV.

Points I want to make.... regarding keeping engines alive without "catastrophic breakdown"

1. NEVER keep any engine revving tooooo high in rpm for extended time spans.

2. Keep ALL fluids, hoses, pumps, belts and cooling apparatus in top condition.

IMO: Follow those two basics and engines last a loooong time!

 

[rslifkin]

Another interesting thing I learned from reading a Pascoe article and from wrenching over the years is that Crusaders and Mercruisers are NOTHING alike outside of their cubic inch displacement. Crusader uses a different truck spec engine than the Mercruiser light duty 7.4. The heavy duty version used by Crusader has a 4 bolt main versus 2 bolt, heavier rods, pistons, and cam spec. This would address some of the desires expressed in this thread.

On the basic 454s, yes, the Crusaders used the 4 bolt blocks and a little better internals. But they still recommend 3400 rpm as max continuous and about the same WOT RPM, so they didn't consider their continuous power output to be any higher. My standard build Merc 454s are 2 bolt main, cast crank and pistons, but forged rods. Realistically, the pistons are the biggest weak point of that build. In the real world, either version lasts fine if cared for, propped to turn 4200 - 4400 at WOT and not cruised above 3400.

Merc did make a higher output version of the 454 which used fully forged internals, better flowing heads and the 4 bolt blocks (the 454 Magnum). Those rev a little higher at WOT and I'm not sure what they consider to be max continuous, but they make an extra 30 or 40 hp and are probably a little more durable.

If we're talking 80s Mercs vs Crusaders, it's a mixed bag. Crusader was still using points ignition back then, as they were always very tech-shy and didn't trust electronics. Mercs had a very reliable and good functioning electronic ignition system in those years. Packaging wise, the Merc exhausts are a little better, but design wise, the Crusader ones are a little better. Neither is really a good design though, IMO. Internals on the Crusaders were better than the standard level Mercs. Both used the same carbs AFAIK. Both are more work than ideal for an impeller change, at least for the 454s.

 

[cabinetman1955]

I had a Chevy in line 6 292 cu in in a 1967 pickup truck. It was a low rpm torque monster. It liked 1200 to 1800 rpm and towed trailers better than my son's Chevy 350 powered early 80s pickup.
I have seen boats with them but don't know how they performed.
The old Ford 300 in line was pretty torquey as well, but don't know if it was ever marinized.

Those old 6 cylinder 292 truck motors were beasts I have no idea why GM stopped using them unless the fuel economy was really poor?

 

[David Ess]

Moparharn....why not go with electric motor by diesel genset? Lots of torque.

 

[rslifkin]

Those old 6 cylinder 292 truck motors were beasts I have no idea why GM stopped using them unless the fuel economy was really poor?

Fuel economy and production costs. Cheaper to use an engine that already has other applications.

 

[Art]

Those old 6 cylinder 292 truck motors were beasts I have no idea why GM stopped using them unless the fuel economy was really poor?

In Maine, early 70's I had a 1967 1/2 T, full length fleetside, 3 on the tree with a 292 straight 6. Used her a work truck. Even with 2 bbl carburetor was not fuel efficient. Strong beast though.

 

[Benthic2]

If the goal is efficiency, it is presumably to save money. The expense of having to change everything to mount a different motor is going to be MUCH more expensive than any fuel savings you hope to achieve, and loss of resale when you try to sell a planing boat that doesn't plane.

 

[CharlieO.]

Ford 300 straight six.

 

[friz]

Greetings,
Mr. m. You'd probably be better off with one stream engine driving the two props. Almost unlimited power with steam.

Possibly we were in different Navies? Adm. Rickover convinced us of the benefits of nuclear propulsion. 10 years between fuel stops. No need to be followed by a coaler.

 

[friz]

Lets say you own a 45-50 gas powered planning hull flybridge cruiser that needs repower, or will some day. If you had no real interest in going faster and could live with 7-9 knot cruise and 10-11 knot WOT could you consider re-power in some kind of unique gasoline engine? If you did not have the money to spend for diesel conversion, and could remain with gasoline everything. Is there such a thing as a high torque low rpm gasoline engine. A Gardner in gasoline so to speak. High displacement, high quality, good longevity, high torque, lower rpm? Replace a big block Chevrolet with some kind of farm or off-road gas engine that would keep the fuel bill low yet still spin a good size prop at trawler like performance. Can you re-power a gas boat and get some of the trawler like benefits of diesel and keep your cost down. I realize a transmission is part of the change and you still have the potential fire hazard of gas. Is this a well hashed out question already?

If you own a 50' boat, can afford a repower and are content with trawler speeds, I would repower with diesel. I don't understand your loyalty to gas. I have a trailer trawler, 30' with a gasser. It gets up on plane, at WOT around 28 mph. I can get there quick but can't go that far. I am in the process of re-powering with a diesel about 50 hp less. The reliability of a diesel engine, range increase with my 150 gal fuel tank, for me, far outweighs the drawbacks.

 

[jimfrens]

The boat that you trialed sounds like a disaster waiting to happen. Financially speaking, you are better off finding the same model of boat that's already been repowered in the last 10 years, whether gas or diesel with the obvious preference for diesel.

 

[Bill V]

Ford 300 straight six.

In the late 80's while living in Montana I got into circle track racing at a local dirt track. Built me a 66 Mustang coupe and ran a 300 Ford 6 with forged pistons, 4bbl carb and manifold with a high lift cam and headers. Ran that engine 5000 - 6000 RPM all season with never a problem, In the class I was in 307 was max cubic engine size and many cars ran GM 302-305 V8's, Ford 302 V8's and MOPAR slant 6's or 273 V8's. The Ford 300 won a lot of races due to torque and light front end weight which gave great handling. Ran that engine 2 seasons with only a hone job and new rings plus a valve job before "retiring" due to family obligations. Best dang engine I ever had.

 

[Bryant]

Gas engine

Lets say you own a 45-50 gas powered planning hull flybridge cruiser that needs repower, or will some day. If you had no real interest in going faster and could live with 7-9 knot cruise and 10-11 knot WOT could you consider re-power in some kind of unique gasoline engine? If you did not have the money to spend for diesel conversion, and could remain with gasoline everything. Is there such a thing as a high torque low rpm gasoline engine. A Gardner in gasoline so to speak. High displacement, high quality, good longevity, high torque, lower rpm? Replace a big block Chevrolet with some kind of farm or off-road gas engine that would keep the fuel bill low yet still spin a good size prop at trawler like performance. Can you re-power a gas boat and get some of the trawler like benefits of diesel and keep your cost down. I realize a transmission is part of the change and you still have the potential fire hazard of gas. Is this a well hashed out question already?

Actually, there is an option that is way out of the box thinking, but I've seen it done successfully on several different boats and it was quite interesting and cool. This solution though is totally dependent upon what type of waters you boat in and how long you intend to keep your present boat, because this solution will make it very difficult to sell unless you find one of the 10 or 20 other people in America who can see the beauty of this set up and are willing to buy it.

Remove the motors, transmissions, shafts and all connected equipment from your engine room. Glass up the through hulls. Put a really strong outboard mount/swim platform on your transom and select an appropriately powered 4 stroke outboard that will push your boat at 10 kts at an affordable fuel burn rate and RPM. You can keep your gas system, free up lots of area in the engine room for storage, engine is much more simple to work on, 4 stroke mechs are everywhere, and re power is a breeze when you need too. A quality, well maintained 4 stroke outboard will last 20 years, If you do this, you (and I for spewing this blasphemy) will get all kinds of pushback. It's anathema to trawlers, but the owners of the boats that I've seen that have made this alteration couldn't be happier.
Believe it or not, if I were 20 years younger, based upon our usage of our GB 36, I'd do that in a heartbeat and just keep the boat forever then pass it on to my sons. I'd just put up with the ridicule and then laugh at the neigh sayers while they are suffering the tortures of the damned with diesel/gas rebuilds, replacements et al.

 

[Art]

Heck... with many boaters now discussing, seriously looking into or even accomplishing the installation of outboards on extra sturdy swim step; for replacing inboard direct drive or outdrive power trains:

If one were to go that route... Seems pretty simple and relatively inexpensive to pick up three [or more] good condition used, mid power o/b's [75 5o 100 hp each] and install them. I can only imagine the magnitude regarding COG difference and altered handling capabilities in rough seas that that altered power engine layout would produce.

I'd think following seas would become a real bitch. And, close quarters handling may be clumsy - Unless the two furthest outside o/b's were somehow capable of having separate forward/reverse and throttle lines than each other as well as to any of the others.

My preference is inboard direct drive twins!

 

[rslifkin]

For what it's worth, you can add me to the list of people who wouldn't buy a de-powered planing hull at any price. They're fine for running in calm water, but when the going gets rough, many planing hull designs rely on the extra power to maintain good handling.

 

[High Wire]

Slow speed sheltered waters only. Agree with bolting on a pair of widely spaced fuel injected 4 stroke outboards of 70-90 hp. Get the longest shaft length available to keep the powerhead away from the water.
Realize they would cost more than 2 rebuilt 454s but what the heck.

 

[Bryant]

repower

Heck... with many boaters now discussing, seriously looking into or even accomplishing the installation of outboards on extra sturdy swim step; for replacing inboard direct drive or outdrive power trains:

If one were to go that route... Seems pretty simple and relatively inexpensive to pick up three [or more] good condition used, mid power o/b's [75 5o 100 hp each] and install them. I can only imagine the magnitude regarding COG difference and altered handling capabilities in rough seas that that altered power engine layout would produce.

I'd think following seas would become a real bitch. And, close quarters handling may be clumsy - Unless the two furthest outside o/b's were somehow capable of having separate forward/reverse and throttle lines than each other as well as to any of the others.

My preference is inboard direct drive twins!

I agree with you. My caveat was the area in which you use your boat and how long you intend to keep it. This option is obviously not desirable for any rough water or long distance cruising. It's only feasible for use in bays, the ICW and other protected waters. Even then you have to take COG consideration into account and provide for ballast but at 10 kts that's not much of a concern in mostly calm waters. It's an option for people who use their boats under these conditions.

 

[Bryant]

Repower

Slow speed sheltered waters only. Agree with bolting on a pair of widely spaced fuel injected 4 stroke outboards of 70-90 hp. Get the longest shaft length available to keep the powerhead away from the water.
Realize they would cost more than 2 rebuilt 454s but what the heck.

454s drink more gas than sailors drink beer in Hong Kong after a six month cruise.

 

[moparharn]

I thought this thread was going to die a quick death. I was wrong. Many good thoughts here and I appreciate everyone sharing them. The boat I am thinking of is 48 feet long and has a three foot swim platform on the back. I think outboards would not work for two reasons. 1. The length and weight of the boat would likely require a much bigger prop to be able to control the boat in tight and windy spaces. 2. For me a critical feature of a boat is the water access for swimming with family and dog-outboards screw this all up. The outboard idea is a good one for many people and applications, just not for me on this one.

Rebuilding the 454's with low rpm in mind and a quality fuel injection and ignition management system is likely to reap the best of all worlds for me in this application.

However: RT Firefly did mention something interesting. One diesel split into two outputs. I saw the converse of this in the "adventures of Slyphide" thread where a decrepit 90 foot steel boat has twin 671 naturals fed into a single screw output. The motors could be run together or singly for maintenance. One into two for the boat we are discussing, or two into one for other boats I might encounter. This ability I have no knowledge about, but it sounds interesting, and VERY expensive.

Finally, RSLIFKIN makes a very important point that I had some worry about for this issue. If your planning hull requires varying speeds for varying conditions you might have created a monster that is unsafe in bad conditions. Large gas powered cruisers are getting VERY hard to sell. The "depowering" concerns might not be outweighed by the "economy" delight and thus make a difficult boat to sell an impossible boat to sell. Thank you again for the input.

 

[Benthic2]

.... This option is obviously not desirable for any rough water or long distance cruising. It's only feasible for use in bays, the ICW and other protected waters......

Why would this not be suitable for rough water ? There are lots of serious off shore fishing boat with outboards.

 

[rslifkin]

Why would this not be suitable for rough water ? There are lots of serious off shore fishing boat with outboards.

There's a big difference between a boat designed for outboards and one that's been converted.

 

[moparharn]

I think this is a future concept that we will see more of. On this application the cost would be pretty high and I am starting to think that the hull might not handle slow speeds well in bad conditions. Sometimes you need a little UMPH to get on the front or back of a wave, or simply stay on or change course. Rebuilding the gassers for low rpm is the way to go in my opinion. It is going to take a certain amount of BTU's to make things operate right. Diesel has the BTU game won big time. If I went with some special in line 6 cylinder I would likely use the same amount of fuel and newer purpose built 454's.

Moparharn....why not go with electric motor by diesel genset? Lots of torque.

 

[moparharn]

OK. Now to confuse this even more. There are now some new diesel outboards coming on the market. Hang one on each corner as far outboard as possible and put the biggest darn prop you can on it. Put a new swim platform in-between them and you are getting closer.

I still do not think you will be able to maneuver this nearly 50 foot flybridge cruiser in the slop with two smaller outboards. In fact you won't. If you add more outboards you completely defeat the whole point as far as I am concerned-economy. Both in repower cost and operating.