View Full Version : Question on power and engine life tradeoff

01-12-2012, 03:43 PM
Starting a new thread so I don't hijack.

Pismo posted a note about relatively cheap, reliable HP. The dyno #s for the setup are at this link.


All of which makes me wonder something, though I'm not sure anyone is likely to really know. When an engine is hopped up like that (from say, a stock 502 making 415 HP @ 5000 RPM to about 500 HP @ 5000), how large is the cut to engine life if you only CRUISE the motor and don't run it wide open?

An example makes it clearer. Let's say you propped your 415 horse stock 502 for max speed at 5000RPM, as most people would. 65 mph, say. Then, suppose you propped your 500HP 502 with a steeper pitch, so that it ran that same 65 mph down at 4100 RPM, where it makes 415 horsepower. And you never ran it above 4100. (Dream on, I know...)

1. If you stayed below 4100, and ran at the same speed profile (x hours at 6 mph, y hours at 35 mph, z hours at 50 mph, t hours at 65 mph) with the two engines, is there a big difference in engine life? Which engine wins? (The 500 will have more power behind each explosion, and thus more loading on some parts. But it will have about 20 % fewer explosions, running at 4/5ths the RPM ALL the time.)
2. Are you likely short on torque at low rpm with the 500HP motor, such that getting on plane with the steep prop could be a challenge?

Put another way, can your performance upgrade dollars be used to add engine life instead of extra speed, such that you can drive the boat the same as with the stock motor, but make it last longer? Or is that only (or much better) done by adding cubes, dropping rpm further, and burning more gas?

Just Say N20
01-12-2012, 04:02 PM
This ought to be interesting. I have some thoughts, but not so much first hand experience, so I will let the more qualified jump in.

Standard thinking has always suggested that it is bad to lug an engine. Everything I have ever read from Mercruiser, Crusader, etc., has said the boat/engine/propulsion package should always be propped so that the engine will reach the WOT rpm range for that engine, when the boat is "normally" loaded. They also state that a prop that prevents the engine from reaching that range, will harm the engine. I have read the same thing in most of the Boating magazines too.

I guess that being said, building a more powerful engine (500 vs 415 in your example) that will turn the correct max rpms using a higher pitched prop, when run at 4,100 rpms and the higher speed generated by the higher pitched prop, would NOT be lugging. I would further hypothesize that it would be more fuel efficient at that speed than the 415 hp engine because it wouldn't require WOT to hit that rpm.

And yes, the steeper pitch prop would make it much harder for the boat to get on plane, etc for the 415 hp engine because of the lesser torque/hp in the lower rpm range.

Am I close?

01-12-2012, 06:20 PM
I believe I agree w/Bill.
The easiest way to increase power is to increase engine speed, more pulses per second!
Our problem is the speed limitation of the outdrive. I think it's kind of like aircraft engines where the limiting factor is prop tip velocity. A 400 HP reciprocating, direct drive, aircraft engine will have quite a large displacement due to the maximum speed of 2,700 RPM, or so.
So maybe a very modestly tuned 572 is the answer.

Or, maybe find a very short planetary gear reduction box, then just crank it up faster.

01-12-2012, 06:55 PM
I don't think it's a matter of the HP you get as much as it is the load vs rpm underway

an engine at 1-3" of vacuum is under much more load than one at 10"

01-12-2012, 07:02 PM
Many marine V-8 factory engine owners/operation manuals that came with some of our boats over the last 50+ years stated that the boat should only be propped to allow the engine to reach 4400-4500 maximum RPM at wide open throttle.
The factory manuals also said to limit sustained wide open throttle operation to only short brief bursts.
Several also mentioned to avoid over-propping & engine overloading & lugging.

Running this factory suggested way I suspect will give you maximum engine life span.
I guess we have to always keep in mind that these basic V-8 engines are really very old 50s & 60s automotive & truck designs ?
Pushing them beyond their original design intentions will surely limit their runnig life span.
The famous Rolls~Royce Merlin V-12 aviation engine, used in fighter & some bomber warplanes during WW II, got gradually pushed to higher & higher horsepower as the war continued on.
As the engine's horsepower went up throughout the war~~~The engine's total running hour lifespan dropped dramatically.
The very last models of the R~R Merlin used in the P-51 Mustang fighter only lasted a few hundred total running hours before needing a total engine re-build.

ralph crocker
01-12-2012, 08:40 PM
My 2 cents. As most of you know there is no such think as true HP rating it is a calculation of RPM and torque which are both able to be quantified. In my engine world (pro stock and top sportsman) we rarely talked HP.
All motors are basically pumps and when you move into high performance all elements must be considered from cam selection to heads to exhaust and yes carburation. Although marine is new to me principles remain the same. I might suggest that once a motor is built it be run on a dyno to achieve max performance and provide a specific baseline to work from. It is well worth the $500 to assure motor is well sealed and provide good info to work from.
We often built motors and if they didn't hit dyno numbers desired they never went in car and were pulled apart for changes.
But have a good shop do it I have seen $80,000 motors grenade on the dyno. Lots of smoke.

01-13-2012, 12:39 PM
This is exactly why I like overpowered boats. Not for top end but for cruising all day below 3000rpm where the engine will live forever (at good speed due to the larger prop) . Got over 3000 fresh water hours out of my last boat engine with no internal work because of this.

01-13-2012, 01:58 PM
Honestly I think engine builders know the secret to this puzzle. There is a reason why Merc makes a 502 415hp and not 450 or 500. One is cost, other is longevity and warranty. There is a reason why a Teague engine carries such a limited warranty yet a 496 has a 3 or 5yr. I understand your thinking about a cruise from say 36mph at 3k to a cruise at 42mph at 3k due to a larger prop. My money is still on the 415 lasting longer even if run at a higher rpm to make 42mph. I also know a 500hp engine at 3k is going to eat much more fuel than a 415 at 3k or even 3300. Look at cars. If I run a 6cyl at 65mph in a SUV it may get 20mpg. Put a V8 in it and you may drop to 17 at the same speed. Punch that to 500hp and you may get 12. This is real world although many use the "theory" that a car going 65 uses the same hp to maintain that speed no matter what engine and therefore should do the same MPG. Same goes for boats. Run a 16' OB with a 4cy 115 at 40mph and do the same with a 225 Pro-max and even though the 225 is "loafing" it will burn more than the higher reving 115. Which lasts longer...the one with less cyls in my opinion.

I cruise at 2900 or so which is 35-38mph depending on load. At this rate it should live forever, the salt I think will take her before she grenades however. I know many who have over 3000 hours on cruiser engines and in my opinion they run harder than ours do. My bud's 454's run at 32-3500 all day long pushing 30,000lbs. They redline at 4200. That is equiv to you guys running over 4k all day long. Most marine engines die from water intrusion or lack of maintenance, not from hours of operation.

01-13-2012, 03:17 PM
Well, I have tried lots of different props on two different 454 engines in the past couple of years in my '88 22C. When I bought the boat it basically had the stock engine with a BIG Holley on it and a 27P which ran it up to close to 70 mph at about 4,800 rpm. This engine, which had lord knows how many hours on it blew on me while running it hard with a 23P prop at somewhere in the 5,400 rpm range. Bought a Bill Dennis built 454 claimed to have 490 hp, put a 26P+ prop on. Ran it through a break in process of never exceeding 2,500 rpms except for occasional up to 4,000 rpm. Within a month after the 20 hour break in period I was running WOT to avoid a pontoon that suddenly moved into my path, hit a big wake, went airborne ( prop completely out of the water I am told ) and within a few minutes this engine locked down with a broken piston. Obviously whatever I end up with this time, it will have some sort of rev limiter. Anyway, my choices right now are a 454 stroker at 482 CUI and a 540 CUI, both well built. I am thinking that as someone above correctly noted, these are all basically truck engines and should not be run at high rpms for other than short periods that either should work, but should use a prop that will get the rev limit back in the 4,600 - 4,800 rpm range. Lower than that is said to really put a load on the stock Bravo One outdrive, but I am definitely not an expert on that. Have read on this site that the Bravo One in stock form is good for up to 600 hp. Key does seem to be keeping the rpm down on these big blocks, at least running WOT for very brief periods. Just my observations after trashing two engines in a couple of years, which is not an inexpensive proposition...

joseph m. hahnl
01-13-2012, 05:49 PM
I think that heat and detonation are the major factors.Higher Hp has more of both. Higher octane fuel and high shear oil can counter act some, but there will always be more mechanical heat stress in a higher Hp Motor. The combustion charge is hotter, therefore that extra heat is dissipated through the block and all of the mechanical parts.A lower Hp at high load also creates heat.So at some point there would be a sweet spot on were both would be generating the same internal heat dissipation ratio.But detonation will always be more of a factor as Hp is increased.

Now if you cryo all of the parts and block, heat would not be a factor

01-13-2012, 07:34 PM
I think for the same reason a SBC marine engine with light internals would be a poor choice to SC.
I would also think, that the "heat" generated to an engine with strong internals would have better
longevity....regardless of being over-propped with higher rpm's........Boy!!...if they can only make
lightweight diesel!

01-13-2012, 07:59 PM
Lots of interesting comments, thanks. Don't hold back on more if you have them.

The more I read, the more I keep coming back to the idea of building about a 600+ cu in big block with a really modest cam. Maybe 450 HP. Something that would rev down there with the 385 HP 502. Just needs a drive that can handle the torque. FWC. Long life. Not so much top end, but effortless FAST cruising forever.

01-13-2012, 08:07 PM
to picking an engine. It's a way to compare different engines for a given application and to look at the loads w/o installing the engine in question and looking at the manifold vacuum like Jim suggested (Jim knows what he's talking about.....at least he does about engines:wink:).
The reason it's so useful is because it's completely calculated.
Here's an article describing the principles, and there's some interesting anecdotal information a bout actual engines.


- Brake Mean Effective Pressure -

BMEP: An important performance yardstick

We have presented the topics of Thermal Efficiency (http://www.donzi.net/forums/thermal_efficiency.htm) and Volumetric Efficiency (http://www.donzi.net/forums/volumetric_efficiency.htm) as methods for estimating the potential output of a given engine configuration.
Brake Mean Effective Pressure (BMEP) is another very effective yardstick for comparing the performance of one engine to another, and for evaluating the reasonableness of performance claims or requirements.
The definition of BMEP is: the average (mean) pressure which, if imposed on the pistons uniformly from the top to the bottom of each power stroke, would produce the measured (brake) power output.
Note that BMEP is purely theoretical and has nothing to do with actual cylinder pressures. It is simply an effective comparison tool.
If you work through the arithmetic (presented at the bottom of this page), you find that BMEP is simply a multiple of the torque per cubic inch of displacement. A torque output of 1.0 lb-ft per cubic inch of displacement in a 4-stroke engine equals a BMEP of 150.8 psi. In a 2-stroke engine, that same 1.0 lb-ft of torque per cubic inch is a BMEP of 75.4 psi.
(The discussion on the remainder of this page is with respect to four-stroke engines, but it applies equally to two stroke engines if you simply substitute 75.4 everywhere you see 150.8)
If you know the torque and displacement of an engine, a very practical way to calculate BMEP is:
BMEP (psi) = 150.8 x TORQUE (lb-ft) / DISPLACEMENT (ci)

(Equation 8-a, 4-Stroke Engine)
BMEP (psi) = 75.4 x TORQUE (lb-ft) / DISPLACEMENT (ci)

(Equation 8-b, 2-Stroke Engine)
(IF you prefer pressure readings in Bar rather than PSI, simply divide PSI by 14.5)
(IF you are interested in the derivation of those relationships, it is explained at the bottom of this page.)
This tool is extremely handy to evaluate the performance which is claimed for any particular engine. For example, the 200 HP IO-360 (360 CID) and 300 HP IO-540 (540 CID) Lycomings make their rated power at 2700 RPM. At that RPM, the rated power requires 389 lb-ft and 584 lb-ft of torque respectively. (If you don't understand that calculation, CLICK HERE (http://www.donzi.net/forums/power_and_torque.htm))
From those torque values, it is easy to see (from Equation 8-a above) that both engines operate at a BMEP of about 163 PSI (12.25 bar, or 1.08 lb-ft of torque per cubic inch) at peak power. The BMEP at peak torque is slightly greater.
For a long-life, naturally-aspirated, gasoline-fueled, two-valve-per-cylinder, pushrod engine, a BMEP over 200 PSI (13.8 bar) is difficult to achieve and requires a serious development program and very specialized components.
For comparison purposes, let's look at what is commonly believed to be the very pinnacle of engine performance: Formula-1 (Grand Prix).
An F1 engine is purpose-built and essentially unrestricted. For 2006, the rules required a 90 V8 engine of 2.4 liters displacement (146.4 CID) with a maximum bore of 98mm (3.858) and a required bore spacing of 106.5 mm (4.193). The resulting stroke to achieve 2.4 liters is 39.75 mm (1.565) and is implemented with a 180 crankshaft. The typical rod length is approximately 4.016 (102 mm), for a Rod/Stroke ratio of about 2.57. These engines are typically a 4-valve-per cylinder layout with two overhead cams per bank, and pneumatic valvesprings. In addition to the few restrictions stated above, there are the following additional restrictions: (a) no beryllium compounds, (b) no MMC pistons, (c) no variable-length intake pipes, (d) one injector per cylinder, and (e) the requirement that one engine last for two race weekends.
At the end of the 2006 season, most of these F1 engines ran up to 20,000 RPM in a race, and made in the vicinity of 750 HP. One engine for which I have the figures made 755 BHP at an astonishing 19,250 RPM. At a peak power of 755 HP, the torque is 206 lb-ft and peak-power BMEP would be 212 psi. (14.63 bar). Peak torque of 214 lb-ft occurred at 17,000 RPM for a BMEP of 220 psi (15.18 bar). There can be no argument that 212 psi at 19,250 RPM is truly amazing.
However, let's look at some astounding domestic technology. The 2006 Nextel Cup engine is a severely-restricted powerplant, being derived from production components. It uses a production-based cast-iron 90 V8 block and 90 steel crankshaft, with a maximum displacement of 358 CID (5.87 liters). A typical configuration has a 4.185" bore with a 3.25" stroke and a 6.20" conrod (R/S = 1.91). Cylinder heads are similarly production-based, limited to two valves per cylinder, but highly developed. The valves are operated by a single, engineblock-mounted, flat-tappet camshaft (that's right, still no rollers as of 2007) and a pushrod / rocker-arm / coil-spring valvetrain. It is further hobbled by the requirement for a single four-barrel carburetor. Electronically-controlled ignition is not allowed, and there are minimum weight requirements for the conrods and pistons.
How does it perform? At the end of the 2006 season, the engines were producing in the neighborhood of 825 HP at 9000 RPM (and could produce more at 10,000 RPM, but engine RPM has been restricted by means of a rule limiting the final drive ratio at each venue). (NOTE: As of early 2010, those same engines are now exceeding 860 BHP, with RPM restricted by the same "gear rule".) 825 HP at 9000 RPM requires 481 lb-ft of torque, for a peak-power BMEP of nearly 203 PSI (14.0 bar). Peak torque was typically about 520 lb-ft at 7500 RPM, for a peak BMEP of over 219 psi (15.1 bar).
THAT is truly astonishing. Compare (http://www.donzi.net/forums/comparison_of_cup_to_f1.htm) the F1 engine figures to the Cup engine figures for a better grip on just how clever these Cup engine guys are.
To appreciate the value of this tool, suppose someone offers to sell you a 2.8 liter (171 cubic inch) Ford V6 which allegedly makes 230 HP at 5000 RPM, and is equipped with the standard OEM iron heads and an aftermarket intake manifold and camshaft. You could evaluate the reasonableness of this claim by calculating (a) that 230 HP at 5000 RPM requires 242 lb-ft of torque (230 x 5252 5000), and (b) that 242 lb-ft. of torque from 171 cubic inches requires a BMEP of 213 PSI (150.8 x 242 171).
You would then dismiss the claim as preposterous because you know that if a guy could do the magic required to make that kind of performance with the stock heads and intake design, he would be renowned as one of the pre-eminent engine gurus in the world. (You would later discover that the engine rating of "230" is actually "Blantonpower", not Horsepower.)
As a matter of fact, in order to get a BMEP value of 214 PSI from our aircraft V8, we had to use extremely well developed, high-flowing, high velocity heads, a specially-developed tuned intake and fuel injection system, very well developed roller-cam profiles and valve train components, and a host of very specialized components which we designed and manufactured.

The definition of BMEP (as stated at the top of this page) is: the average (mean) pressure which, if imposed on the pistons uniformly from the top to the bottom of each power stroke, would produce the measured (brake) power output.
Putting that definition into mathematical format, therefore,
HP = BMEP x piston area x (stroke / 12) x RPM x power-pulses-per-revolution / 33000
Working through that equation in terms of a single cylinder engine, BMEP (in PSI) multiplied by piston area (square inches) gives the mean force on the piston during the power stroke. Multiplying that force by the stroke (inches divided by 12 changes the units to feet) gives the net WORK (http://www.donzi.net/mechanical_engineering_basics/energy_and_work.htm) (in foot-pounds) produced by the piston moving from TDC to BDC with the BMEP exerted on it throughout that motion. (Clearly this is not an attempt to represent the reality in the combustion chamber. As previously stated, BMEP is simply a convenient tool for comparing and evaluating engine performance.)
Next, power (http://www.donzi.net/forums/power_and_torque.htm) is defined as work-per-unit time. Therefore, multiplying the WORK (ft-lbs) by the RPM, then multiplying by power-pulses-per-revolution (PPR) gives the net (brake) power (foot-pounds per minute in this example) produced by one cylinder. (In a single-cylinder engine, PPR is either 1 for a 2-stroke engine or 1/2 for a 4-stroke engine.)
Since one HORSEPOWER (http://www.donzi.net/forums/power_and_torque.htm)is defined as 33,000 foot-pounds-of-work-per-minute, dividing the WORK (ft-lbs) by 33,000 changes the units from foot-pounds-per-minute to HP.
Since it is clear that piston area x stroke is the displacement of one cylinder (in cubic inches), then the equation can be simplified to:
HP = BMEP x (displacement / 12) x RPM x power-pulses-per-revolution / 33000
Horsepower is also defined as:
HP = Torque x RPM / 5252
Substituting that equation into the preceding one gives:
Torque x RPM / 5252 = BMEP x displacement / 12 x RPM x PPR / 33000
Reducing that equation gives:
BMEP = (Torque x 12 x 33,000 / 5252) / (Displacement x PPR)
Evaluating the constants, 12 x 33,000 / 5252 = 75.39985, which can safely be approximated by 75.4. Simplifying the equation again gives:
BMEP = (Torque x 75.4) / (Displacement x PPR)
It is also clear that because the equation includes PPR, it applies to engines with any number of cylinders by using the total displacement, total brake torque, and correct PPR.
Suppose, for example, that you measured 14.45 lb-ft of torque from a 125 cc (7.625 CID) single-cylinder 2-stroke engine at 12,950 RPM, you would have 35.63 HP (285 HP per liter, quite impressive indeed). The BMEP would be:
BMEP = (14.45 x 75.4) / (7.625 x 1) = 142.9 psi (9.85 bar)
That BMEP (9.85 bar) is an impressive number for a piston-ported 2-stroke engine. However, suppose someone claimed to be making that same torque from a single cylinder 4-stroke 125 cc engine at 12,950 RPM. The power would be the same (35.63 HP, or 285 HP per liter). The power density would not necessarily set off alarms, (the 2008 2.4 liter F1 V8 engines approached 315 HP per liter), but the BMEP would cause that claim to be questioned:
BMEP = (14.45 x 75.4) / (7.625 x 1/2) = 285.8 psi (19.7 bar)
That BMEP (19.7 bar) is clearly absurd for a normally-aspirated engine. Professor Gordon Blair stated that exceeding 15 bar of BMEP in a N/A engine is virtually impossible, but that was a few years ago. NASCAR Cup engines are now approaching 15.6 bar
Clearly, the difference between 2- and 4-stroke engines is simply a factor of 2, because of the fact that a 2-stroke cylinder fires once per revoultion whereas a 4-stroke engine fires only once per two revolutions. The equations can be simplified further by incorporating that PPR factor in the constant 75.4 and eliminating PPR from the equation, therefore making the constant for a 4-stroke engine 2 x 75.4 = 150.8. That produces the equations shown at the top of this article, which use the full engine displacement and measured torque.
BMEP = 150.8 x TORQUE (lb-ft) / DISPLACEMENT (ci)

(Equation 8-a, 4-Stroke Engine)
BMEP = 75.4 x TORQUE (lb-ft) / DISPLACEMENT (ci)

(Equation 8-b, 2-Stroke Engine)

01-13-2012, 08:16 PM
now, take that same Nextel engine and bump the c/r about 4 to 5 points more...


Phil S
01-13-2012, 08:25 PM
[ QUOTE=$originalposter]{$pagetext}[/QUOTE]

...sheesh...have I taught you nothing ?? I have at least 10 cases, shrink-wrapped, palletized, and otherwise properly stored of "Slick 50"....

Call me.

It's all about "liquid" ball-bearings...


01-13-2012, 08:27 PM
I'd like to see the BMEP numbers on a modern ProStock motor. Those guys have been over 100% VE for some time.

Phil S
01-13-2012, 08:39 PM
...I enjoy this discussion immensely..

Actually, btu / therms discussion is enjoyed as well. Flame spread, and the rapidity of same, type of fuel (chemical make-up) that enhances the release of carbon electrons, as well as the pressure under which it is ignited, all intrigue me. The mechanics of creating pressure somewhat bore me though. Flow and flame spread, and the creation of higher therms per fuel source interest me.

Just sayin',

With kind regards,

Phil S.

01-13-2012, 08:54 PM
Actually, btu / therms discussion is enjoyed as well. Flame spread, and the rapidity of same, type of fuel (chemical make-up) that enhances the release of carbon electrons, as well as the pressure under which it is ignited, all intrigue me. The mechanics of creating pressure somewhat bore me though. Flow and flame spread, and the creation of higher therms per fuel source interest me.

That stuff keeps me interested for hours on end. Check this out when you've got a minute.

Phil S
01-13-2012, 09:21 PM
...read two lines...gonna be up for hours dammit !

(Thanks Maddad ! :) ) I think... ;)


01-14-2012, 07:04 AM
But, to get back to Mike's original question about load and longevity, if you feel comfortable w/a BMEP of 150, or, maybe as low as 130, pick a large displacement, low stressed engine that will give the required power at the chosen BMEP.
I think a detuned 572 would be perfect.

01-14-2012, 08:27 AM
take a crate 572-620 and de-cam it a tad, and you'll flip over a cruise ship...

something Costa Cruises could use this morning :boggled:

01-14-2012, 11:07 AM
Skippy - "Boy!!...if they can only make lightweight diesel!"

Well, how light-weight do you want?

How about a twin turbo FWC diesel V8 weighing in under 1000 lbs dry? (992 lbs w/no running gear)
370 hp @ 3800 rpm (red line)
4.46 L / 272 CID
(don't have the torque number handy)

Continuous duty rating is 3683 rpm, so you can spin it at near red-line all day long.

It's an off the shelf Yanmar 8LV-370. Pretty run of the mill stuff.

The thing is, I wouldn't expect this sort of light weight/high RPM diesel to offer the long life of the old heavy weight/low rpm diesels. Just as discussed above it doesn't matter if it's gas or diesel; if you squeeze more power out of a give displacement, longevity goes down.

For comparison, Merc's website lists the 8.2 Mag ECT as follows:

1072 lbs
380 hp (@???)
8.2 L / 502 CID

4400-4800 rpm red line; no recommended cruise listed on the website, but I'll guess 3600.

Granted, Merc has the HO version of the same engine at 430 hp/5000 rpm, but trying to stay close on power outputs the Yanmar is actually lighter. Though oddly, the specs say it's larger despite the much smaller displacement. I attribute that mainly to the packaging of the dual turbos.

01-15-2012, 12:25 PM
I doubt the B1 could handle the torque of that Yanmar, probably 600 range. Most Yanmars are rated for about 5k hours before rebuilds, not worth it in my opinion when you can get 1/2 that easy on gas for 1/4 the price. The 12cyl Mercedes in my bud's head boat go almost 25k before rebuilds. The last year or 2 he NEVER shuts them down as they won't be easy to start again due to being worn the hell out.

Would be cool as hell to drive an oil burning classic. Then again they did make them but only in 300 or 315hp version 6cyls.

01-15-2012, 01:24 PM
I remember reading in several boating mags quite a number of years ago where Yanmar, & Donzi, got together and installed several Diesels in a few boats to test the concept of performance boats equipped with a diesel.

I believe the idea at that time was to develop a high-speed yacht tender that ran on the exact same fuel as the mother yacht.

What were the final conclusions that resulted from this diesel powered performance boat test experiment ?

Was it successful ?

How about overall performance ?

Any Diesel Noise issues ?

01-16-2012, 01:27 PM
I believe the Yanmar Donzis were built with/by Mastery Engine or Mack Boring (?)

Seems like they used the 6LP 315 hp turbo in-line 6 cyl. Don't remember what drive, but probably the B1.

These days the 6LP has gone away - doesn't meet current emissions regs. The new 8LV V8 motors are filling that hole in the market and a little more as they go up to 370hp. I would think they'd be a better fit in a Donzi as well since they are the 'right' shape.

As for drives, Yanmar offers their own sterndrive so that wouldn't be an issue.

All the same, I can't see it ever being popular in the US. Maybe there would be a very limited market for yacht tenders or in Europe where diesel is the preferred fuel even in small boats, but I can't imagine they'd ever offer the diesel as a regular build option.

I would expect it to offer good cruising speed at decent economy, but a low top speed. Plus it just wouldn't have the sound expected of a hot rod boat. Engine life should be better than a gas BB, but really who puts that many hours on a boat like this anyway? And of course the purchase price would go up.

01-16-2012, 03:39 PM
Not an expert by a long shot and will never pretend to be but I have always found that a longer stroke is a good thing. (In so many ways.:shades:) Also lower RPMs equals longevity. As long as you're not lugging.

01-17-2012, 01:00 PM
Considering you can replace about 4 or 5 502's for what the diesel option would cost initially, I agree it will not be a big seller. Since 16-22C's are now being built as ordered, it would be interesting to see how many people custom order a diesel.

01-18-2012, 03:55 PM
Another thing to look at is how the power is being made. Compare a hopped up BBC making 500 HP to a Merc HP 500. You'll probably find that the hopped up 454 has increased compression to run on premium fuel. This means the valve train on the hopped up engine won't go through near the stress and will be more reliable.

It's all about balancing the engine out. You can easily make a bunch of power camming the hell out of an engine but that doesn't mean it is going to last. Same goes for boost, nitrous, etc. A more balanced engine in overall build will tend to be the more reliable one, when performance comes to mind.

You can compare factory engines and come to your own conclusions too. Check out the specs on the factory 6.2 (377 Mag) vs. the 350 Mag. Only 20 HP more, so the question is why and which engine would be more reliable?

The Hedgehog
01-18-2012, 09:32 PM
It it is all in the way you make it.

Several good points have been made. Namely buiz and the stress and vett and the nitrous/cam points.

It is also in the quality of the build.

Some tout n/a. That can be good but an engine that is set up to pull 800 hp without boost will have a nasty cam and head profile. You will only get so much time out of that, regardless of the rpm. With a boosted engine, it is driven by how much time you spend in boost.

A crate engine assembled may or may not be good depending on how it was toleranced.

Then you get into parts.....I know one guy that will have a billet cam core from here on out!