De-stroke a 400 to a 377 for a boat?

[maverick2]

Does de-stroking a 400 sbc to a 377 have the potential to significantly improve on the engine’s durability if used to run relatively high sustained loads? I’m building an aluminum jet boat, and picked up a good 400 block (2 bolt mains, casting #330817) with the intent of boring it .030†over and making a 406. With a goal of 475-500 ft-lbs around 4500 rpm, I started looking around for build recipes, and quickly found (thru grumpyvette and multiple other places) that the torque potential of a 406 far exceeds what I need (sounds like 530-550 ft-lbs is pretty attainable with a good builder). At the same time, I’ve also had quite a few boat gurus caution me that high sustained loads typical of a marine application can be tough on a 400 block because of the lousy rod:stroke ratio coupled with the thinner cylinder walls. One of the things I’ve contemplated is de-stroking the block to a 377 to address that potential weakness, and not worry about trying to wring every last drop of torque out of the block. I know reducing displacement tends to fly in the face of conventional wisdom, but if I can still meet my torque goals (based on the performance of a comparably sized 383, I believe this is very do-able) and improve the potential durability of the engine, what would be the downside? Keep in mind, there is no benefit for me to build an engine with any more torque than 500 ft-lbs, as the additional torque is essentially un-usable unless I jump up to a bigger, significantly more expensive pump.

 

In order to eliminate a few questions/confusion up front, here’s a little background on the application. A jet-drive for a boat is basically a direct-drive pump bolted to the back of the engine – no gearing, no clutch, no transmission. There are a handful of different jet pump manufacturers out there, but really only two that make a good pump for whitewater applications (must be able to handle aerated water well). As you can see below, the power demand rises exponentially with engine speed, and it becomes difficult (as well as counterproductive) to turn one of these pumps past the 4500 to 5000 rpm range if they are equipped with the whitewater-capable impellors I want. Top speed (both boat speed and engine speed) is dictated by the point on the engine’s TQ/HP curve where the TQ/HP demand of the pump surpasses the output of the engine. Pump loads are dependent upon the particular impellors chosen, and are pretty well known for any given engine speed. As an example, TQ and HP demands by the pump would be pretty typical of the following for the pump/impellor combination I’m looking at:

 

Jetboat Pump TQ and HP Demands

RPM TQ HP

2000 118 45

2500 147 70

3000 219 125

3500 285 190

4000 374 285

4250 432 350

4500 466 400

4750 500 452

 

What this means as far as operation for a boat of the size that I’m building, is that it will hop onto plane at about 2800 rpm (absorbing approx 200 ft-lbs and 100 hp from the pump) cruise at 3200 rpm (absorbing about 250 ft-lbs and 155 hp), and top out at whichever engine speed the engine hp/torque curve crosses the pump power absorption curve. My goal is to build an engine that will spin the pump between 4500 and 4800 rpm. As a result, I’d like to build an engine with a peak torque between 475 and 500 ft-lbs, and have that peak torque hit right around 4500 rpm. This would give me an 1800-2000 rpm split between planing speed and top speed, and a 1400-1600 rpm split between cruising speed and top speed. These splits are head and shoulders above what the commercially available marine engines can do with these pumps/impellors, and are a lot more fun to drive. As a side note, it’s very difficult to over-rev one of these engines, since the pump loads gets extremely high once you get above 5000 rpm.

 

As you can see, this application is a bit different from that of a performance engine for a street car:

• I’m looking to achieve strong torque in the mid-range engine speeds (rather than the lower rpm range that street cars often shoot for) with very smooth operation from idle to peak torque.
  
• I really don’t need/want a big rpm split between peak torque and peak Hp since the engine will not ever see speeds that are more than 500 rpm above the peak torque once I get the right impellors on board. Most street engines seem to sacrifice some torque in order to achieve a relatively large rpm split between peak torque and peak HP – I don’t need to worry about that…
  
• Engine durability needs to be an important consideration. Boats tend to operate for relatively long periods at constant loads between 3000 and 4000 rpm (skiing, climbs thru a long set of rapids,…) while street engines tend to fluctuate engine speed and load pretty regularly. I need to build this engine with that “sustained load†in mind, hence my consideration to de-stroke the 400…

What are your thoughts – is my logic flawed, or am I heading in the right direction?

 

As a follow-on question, as I look at build recipes for the 377, would it make sense to consider a head, manifold, and cam combination used in a 383 buildup? (Basically adopt those components from one of many 383 builds that makes the torque/performance characteristics I’m looking for…) Or is the airflow thru a 377 different enough that this approach doesn’t make sense? (I am NOT having much luck in finding published info and torque/hp curves for a 377 except for those that are built to rev thru the roof…)

 

Thoughts?

[280zwitha383]

 

It sounds to me like the easier approach would be a custom (or well-matched) cam.... Seems to be a lot less expensive and a lot less headache. If I were you I would start playing around with desktopdyno or a similar simulation program.

[dr_hunt]

IMO use a 3.75 stroke, 6 inch h beam rod and your set. There is no such thing as just enough HP and TQ. Build the 406, the 377's have never really been impressive even when compared to the 383's.

[maverick2]

The basic question I'm asking is whether or not there's an engine longevity/durability advantage in de-stroking to a 377 in this particular application... If so, the sacrifice of torque doesn't bother me a bit, since the only thing it will buy me is 200 rpm (roughly 5 mph) on the top end, where the boat spends almost no time. (In order to take advantage of that torque increase within the normal operating zone, I'd have to move up to the next larger pump size, which is an additional $3-5k, and not yet in production...) On the other hand, if there is no durability/longevity advantage, it's a no-brainer to stay with the 406...

[BRAAP]

The basic question I'm asking is whether or not there's an engine longevity/durability advantage in de-stroking to a 377 in this particular application...

 

... On the other hand, if there is no durability/longevity advantage, it's a no-brainer to stay with the 406...

 

The short and simple answer is NO!

 

All things being equal, including HP per Cubic-Inch, for comparably built engines, 377 and 406, the larger engine will make power at the same RPM, AND/OR the same power at a lower RPM, whichever scenario you choose to take advantage of. A smaller engine has to rev more to produce the same power as a larger engine. The more you have to rev the engine to get the power you need, the shorter its life will be, again all things being equal. Some might say, “Well then just use better parts in the 377 to make it more reliable!” I say, “Then why not use those same higher quality parts in the 406?” I’m sure you get the point.

 

Sure, a shorter stroke does take some of the stress off the pistons, rods and crank versus a longer stroke engine, but the valve train still has work harder at the higher RPMs to produce the same power as the larger engine. Also, bearing wear is greater at higher RPM’s, regardless of the stroke.

 

In short, parroting what Doc already said, build as big as you can to make as much power as you can, and the engine will live a longer healthy life. Remember, the less it has to work at producing the power you require, the easier it is on parts which will extend your TBO, (Time Between Overhauls). So in this instance, bigger really IS, better!

 

 

Good luck,

Paul

[jasper]

Years ago I was a tech rep for Volvo Penta. As an example, we had a line of marine diesels. The EXACT same engine was available in different H.P. ratings. The only difference was the injection pump output. More fuel=more H.P. More H.P. = less longevity. (rated in hours)

[maverick2]

All things being equal, including HP per Cubic-Inch, for comparably built engines, 377 and 406, the larger engine will make power at the same RPM, AND/OR the same power at a lower RPM, whichever scenario you choose to take advantage of. A smaller engine has to rev more to produce the same power as a larger engine. The more you have to rev the engine to get the power you need, the shorter its life will be, again all things being equal. Some might say, “Well then just use better parts in the 377 to make it more reliable!” I say, “Then why not use those same higher quality parts in the 406?” I’m sure you get the point.

 

Sure, a shorter stroke does take some of the stress off the pistons, rods and crank versus a longer stroke engine, but the valve train still has work harder at the higher RPMs to produce the same power as the larger engine. Also, bearing wear is greater at higher RPM’s, regardless of the stroke.

 

I understand what you're trying to say, but I don't think this line of logic works in this application. Consider these two options: build a 377 capable of a peak torque of 480 ft-lbs at 4500 rpm, or build a 406 capable of a peak torque of 520 ft-lbs at 4500 rpm. With the impellors that I want to use (selected to give me the performance I want from the 2500 - 4000 rpm operating range where the boat spends 95% of its time) the pump will limit the top speed of the 377 to about 4700 rpm, while the top speed of the 406 will be about 4900 rpm. (The pump will prevent both engines from turning any faster, so the references to additional wear and stresses by turning higher rpms with the 377 is moot.) Assume each engine is going to spend a lot of time running at 4000 rpm. Mated to the same pump, both engines will have to make 375 ft-lbs, which is the Tq demanded by the pump at that rpm. Which engine will live longer? I believe the 377 will live longer which is why I'm considering this route. The 377 will have to generate a little higher cylinder pressures to make 375 ft-lbs at the same rpm as the 406 (making up for its shorter stroke), but at these engine speeds, that additional pressure should not be a big factor on rotating assembly stresses, and makes no difference on valve train stresses. Where I think the 377 excells over the 406, is in the lateral stresses on the cylinder walls. With a significantly better rod:stroke ratio, the 377 is going to have a lot less side thrust into the cylinder wall, meaning less friction loss, cylinder wear, and cylinder wall flex in a block that already has relatively thin walls. In a marine application, I think this can be a very real scenario because of the sustained high loads that are typical of boat operation. Does that help to explain my line of logic? Thoughts?

[dr_hunt]

I understand what you're trying to say, but I don't think this line of logic works in this application. Consider these two options: build a 377 capable of a peak torque of 480 ft-lbs at 4500 rpm, or build a 406 capable of a peak torque of 520 ft-lbs at 4500 rpm. Which engine will live longer? I believe the 377 will live longer which is why I'm considering this route. The 377 will have to generate a little higher cylinder pressures to make 375 ft-lbs at the same rpm as the 406 (making up for its shorter stroke), but at these engine speeds, that additional pressure should not be a big factor on rotating assembly stresses, and makes no difference on valve train stresses. Where I think the 377 excells over the 406, is in the lateral stresses on the cylinder walls. With a significantly better rod:stroke ratio, the 377 is going to have a lot less side thrust into the cylinder wall, meaning less friction loss, cylinder wear, and cylinder wall flex in a block that already has relatively thin walls. In a marine application, I think this can be a very real scenario because of the sustained high loads that are typical of boat operation. Does that help to explain my line of logic? Thoughts?

 

You'll never get the 377 to make 520lb ft of torque IMO unless your running 14:1 + on alky. Your still using the 400 block as it is, so why not have cubes, but it sounds like you already have your mind made up so build what you want.

[maverick2]

You'll never get the 377 to make 520lb ft of torque IMO unless your running 14:1 + on alky. Your still using the 400 block as it is, so why not have cubes, but it sounds like you already have your mind made up so build what you want.

Actually my mind is not made up -- I'm trying to determine if downsizing to a 377 effectively addresses a potential weakness of the 400 block for this type of application. When my builder threw the concept of a 377 on the table, I immediately discounted it because I didn't think it could/would get the torque I want. After doing some looking, I think that 480 ft-lbs is attainable with 10.5:1 CR IF the components are well matched. I'm not expecting (nor needing) to get torque comparable to a 406 out of it, so if a 377 buys me some durability/longevity AND meets my torque goals, WHY NOT build it?

[BRAAP]

I understand what you're trying to say, but I don't think this line of logic works in this application. Consider these two options: build a 377 capable of a peak torque of 480 ft-lbs at 4500 rpm, or build a 406 capable of a peak torque of 520 ft-lbs at 4500 rpm. With the impellors that I want to use (selected to give me the performance I want from the 2500 - 4000 rpm operating range where the boat spends 95% of its time) the pump will limit the top speed of the 377 to about 4700 rpm, while the top speed of the 406 will be about 4900 rpm. (The pump will prevent both engines from turning any faster, so the references to additional wear and stresses by turning higher rpms with the 377 is moot.) Assume each engine is going to spend a lot of time running at 4000 rpm. Mated to the same pump, both engines will have to make 375 ft-lbs, which is the Tq demanded by the pump at that rpm. Which engine will live longer? I believe the 377 will live longer which is why I'm considering this route. The 377 will have to generate a little higher cylinder pressures to make 375 ft-lbs at the same rpm as the 406 (making up for its shorter stroke), but at these engine speeds, that additional pressure should not be a big factor on rotating assembly stresses, and makes no difference on valve train stresses. Where I think the 377 excells over the 406, is in the lateral stresses on the cylinder walls. With a significantly better rod:stroke ratio, the 377 is going to have a lot less side thrust into the cylinder wall, meaning less friction loss, cylinder wear, and cylinder wall flex in a block that already has relatively thin walls. In a marine application, I think this can be a very real scenario because of the sustained high loads that are typical of boat operation. Does that help to explain my line of logic? Thoughts?

 

 

Then change the pump!

 

I don't understand. If you are shooting for a certain HP at a certain “impellor” RPM, which this impellor you are talking about just so happens to fit your 377 mold, why lock yourself into only one impellor? If you are locked into that one impellor and have sold yourself on the 377 filling the role of “best power plant for that impellor” for the all the reasons you stated, why entertain other engine options?

 

I am pretty sure a different combination of impellor and nozzle is available, or at least attainable, that would allow you the same thrust and speed characteristics you desire, though at a lower engine RPM, allowing you to build the larger engine.

 

If the engine longevity is, or at least one of, your main criteria in the design of this project, Impellor pitch and diameter coupled with the nozzle sizing acts similarly to gearing in car. In other words, focus on your main criteria of building the longer TBO power plant and then figuratively speaking, “gear” the impellor to deliver thrust and speed commensurate with the engine, or use the impellor you are talking about as your main criteria and build the power plant around that, which you feel the 377 fits perfectly, then build the 377 and enjoy.

[OlderThanMe]

For the amount of power you seek, more cubes should get to that attainable power with lower stress on the pistons from a lower SCR.

 

The Doc knows his stuff... listen to him(and the other guys).

[maverick2]

Thanks for sticking with this thread – I know it’s gotta be frustrating the hell out of you.

Then change the pump!

 

I don't understand. If you are shooting for a certain HP at a certain “impellor” RPM, which this impellor you are talking about just so happens to fit your 377 mold, why lock yourself into only one impellor? If you are locked into that one impellor and have sold yourself on the 377 filling the role of “best power plant for that impellor” for the all the reasons you stated, why entertain other engine options?

 

I am pretty sure a different combination of impellor and nozzle is available, or at least attainable, that would allow you the same thrust and speed characteristics you desire, though at a lower engine RPM, allowing you to build the larger engine.

 

If the engine longevity is, or at least one of, your main criteria in the design of this project, Impellor pitch and diameter coupled with the nozzle sizing acts similarly to gearing in car. In other words, focus on your main criteria of building the longer TBO power plant and then figuratively speaking, “gear” the impellor to deliver thrust and speed commensurate with the engine, or use the impellor you are talking about as your main criteria and build the power plant around that, which you feel the 377 fits perfectly, then build the 377 and enjoy.

 

Actually, options for pump, impellor, and nozzle are pretty limited. (There are really only 2 pump mfrs suitable for whitewater jetboats, and both offer similar performance/characteristics to one another.) In each case, you CAN re-size your impellor and/or nozzle to drop the rpm at Wide Open Throttle and take advantage of the 406’ inherent ability to make more torque at lower rpms, but it comes with serious tradeoffs. Both pumps seem to be most efficient above 3000 rpm, such that changes to an impellor or nozzle have a noticeable effect on performance characteristics above 3000 rpm, but make little difference to thrust and operation below 3000 rpm (which is generally lousy). As a result, if you set up a pump to run WOT at 4000 (which is most typical in order to match the peak torque of most commercially built marine engines of 3700-3900 rpm), you are still stuck with a cruising speed between 3000 and 3200 rpm. With a 600-800 rpm split between cruise and WOT, most boats leave a lot to be desired as far as “fun factor” and drivability. About the only way to increase the number of options you have for impellors and nozzles is to increase the point at which peak torque occurs on your powerplant. My desire to get peak torque at around 4500 rpm should allow me to achieve around 4700-4800 rpm at WOT and cruise at 3200-3300. These splits are a lot more fun to drive – the boat is more responsive to throttle changes, and mid-range acceleration and handling takes a big step forward. It’s pretty hard to attain that with a whitewater jet boat by decreasing the engine speed at which peak torque occurs...

 

When I started this process of building a boat, I figured out the most limiting factor of the boat build (which is the pump), determined what kind of performance and rpm range I wanted with the pump, and then started shopping for engines which met the needs of the pump for that kind of performance. (There are a lot more commercial engine options available than pump options, and the options for custom built engine performance is nearly unlimited in comparison…) The answer I walked away with was the sbc 406 – it has the potential to meet and exceed my torque needs across the desired rpm range while coming in at the least weight. As I started looking at the torque potential of the 406, the question came up about the value of sacrificing some of that torque potential in order to improve upon engine durability/longevity for THIS APPLICATION. (I won’t even consider building a 377 over a 406 IF I don’t think I can get to the torque numbers I want, but right now I think I can, as does my engine builder). Assuming for a moment that I CAN get there, my question is whether or not it’s worthwhile in terms of durability – does the 377 (with it’s superior rod:stroke ration, longer dwell, and better quench conditions) offer some inherent durability/longevity advantages over the 406 GIVEN THE OPERATING PARAMETERS/SCENARIO I’ve laid out? I think it does – and that’s what I’m trying to pick people’s brain about. Bottom line is that if I can’t go into an engine build thinking the 377 will make the torque I want AND thinking the 377 doesn’t have some durability advantages, then I’ll simply build the 406.

 

With all that said, I’ll ask the durability question again: Assume each engine (a 377 and a 406) is going to spend a lot of time running at 4000 rpm. Mated to identical pumps, both engines will have to make 375 ft-lbs, which is the Tq demanded by the pump at that rpm. Which engine will live longer, and why? What problems would you expect to show up first?

[BRAAP]

Thanks for sticking with this thread – I know it’s gotta be frustrating the hell out of you.

 

 

At first this thread seemed destined as a “377 is the only option” thread. I applaud your patience in keeping it going and not just giving up. This is a GREAT technical subject.

 

 

Rod to stroke ratio will have an impact on combustion efficiency and sensitivity as you stated. Now does the 377’s greater rod to stroke ratio offset the 406 displacement? That is a good question and I honestly don’t know for sure, but I am willing to bet it will get you part way there at least. I do feel strongly that the shape and design of the combustion chamber will have even greater impact than the rod to stroke ratio itself. Also, the piston dome/dish matching the chamber shape and squish will help in taking advantage of the attributes a longer rod to stroke ratio offers.

 

The 406 can make 375 lbs of torque at that RPM at a lower BMEP than the 377, due to it displacement advantage, which we all have agreed to and have beaten to death.

 

So basically, as I understand this, the question is, does the rod to stroke advantage of the 377 working at a higher BMEP, to produce 375 lb feet of torque at 4000 RPM going to offset the shorter rod to stroke ratio 406 working at a lower BMEP, in regards to TBO?

 

Hmm… I’m inclined to think we are splitting hairs and personally would lean towards the 406 from a practicality stand point, but from a “wanting something a little less common”, might lean towards the 377.

 

 

 

So what sort of “white water” jet boat are you building? A race boat?

 

[G.I.jonas]

I have to ask this even though i know very little on the specific subject.

 

Is there a reason why turbocharging cant be involved?.Get the power where and when you need it and do so with an engine you build with you ideal parameters.

I guess looking at the boat above makes me wonder ?

[maverick2]

So what sort of “white water” jet boat are you building? A race boat?

The photos in this website are pretty typical. It's a recreational boat, built for conditions that range from Class 3 and 4 whitewater rivers, to 4-6" of water in shallow streams. My interest in "durability" is do to the fact that VERY, VERY BAD THINGS happen if you lose an engine in a class 4 rapid....

http://www.bentzboats.com/recreational_photo_album.htm

[G.I.jonas]

Nice!!

[dr_hunt]

Actually my mind is not made up -- I'm trying to determine if downsizing to a 377 effectively addresses a potential weakness of the 400 block for this type of application. When my builder threw the concept of a 377 on the table, I immediately discounted it because I didn't think it could/would get the torque I want. After doing some looking, I think that 480 ft-lbs is attainable with 10.5:1 CR IF the components are well matched. I'm not expecting (nor needing) to get torque comparable to a 406 out of it, so if a 377 buys me some durability/longevity AND meets my torque goals, WHY NOT build it?

 

Well, this is turning into tool shed material IMO cause you state the weaknesses of the 400 block but then your advocating using one anyway and seem to be stuck on one build. Rod to stroke ratio using a 6 inch rod on a 3.75 stroke crank is 1.6, a 350 chevy with a 5.7 rod is 1.63, so your right there. Running higher static compression is going to put more stress, but the two builds are very comparable cost wise and durability wise, just the 377 will top out at 4500 and the 406 will probaby pull that impeller to 5000 or even 5200 and will rev quicker under load and last longer IMO since you won't be running it flat out all the time like you would with the shorter stroke motor. If you were really into smaller cubes, the 383 is the way to go using the 350 block which would get away from the 400 block all together. Then 500tq and hp are attainable with 10.5 compression and some AFR heads.

[pparaska]

The rod/stroke ratio of the engine, in my opinion (based on reading things from Vizard, and many other tech writers) is only an issue at the extremes. The difference in rod/stroke ratio has been seen worthy by many writers only as it pertains to what you are looking at - reliability in terms of piston side loading. I've seen articles where "experts" think that once you get to 1.6:1 or below rod/stroke ratio, then you need to be concerned about the side loading issue. Here's one example: http://www.musclemustangfastfords.com/tech/mmfp_0711_ford_small_block_stroker_engine_build/piston.html

 

However, other net-wisdom is pointing to a 1.7:1 ish or above ratio as

"good" for piston side loading, etc.:

http://www.victorylibrary.com/mopar/rod-tech-c.htm

 

The stock 400 had 5.56" rods, 3.75" stroke - that's a 1.48:1 rod/stroke ratio. This has been deemed as "not good" by many, as a "too short rod" setup.

 

A 350 or 377 SBC has a 5.7" rod, 3.48" stroke - that's a 1.64:1 ratio

A 383 or 400 with a 6" rod, 3.75" stroke has 1.60:1 ratio

A 350 or 377 with a 6" rod, 3.48" stroke has a 1.724:1 ratio.

A 327 with a 5.7" rod and 3.25" stroke has a 1.754:1 ratio.

And "The 350 Chevy should have built" has a 6.209" rod, 3.25" stoke has a 1.91:1 ratio.

 

But, I'm thinking that a good forged 1.125" compression height piston with a 6" rod using the 400's 3.75" stroke will be a good choice, as you can back down on the compression and cam and make that needed hp/tq with less combustion pressure and a better response at the lower end cruise. It still gives a 1.6:1 ratio, almost the 1.64:1 of the stock rod (5.7") 350 that never seemed to have a reputation for excessive piston side loading wear.

However, the 1.724:1 ratio of the 377 with a 6" rod does lower the rod angle by a degree or so.

 

Check this out:

http://www.hotrodders.com/forum/longest-rods-std-deck-454-bbc-33043.html

 

From what I've seen, the kind of power and torque you are talking about will be a bit of a stretch for a 377. You'll need to have optimum everything to get it out of the 377 in my opinion. It's going to be easier to do with the 400. Just another reason I'd not worry so much bout the slight difference in the rod/stroke ratio difference between the 1 or so degree difference in rod angle between a 377 and 400 with 6" rods.

 

Just my opinion

[maverick2]

Is there a reason why turbocharging cant be involved?.Get the power where and when you need it and do so with an engine you build with you ideal parameters.

No, I could add a turbo if needed to get to my torque goals, but if I can't get there with a 377 (or find that there's little/no durability benefit to doing it with a 377 rather than a 406) I'll simply build the 406. There are quite a few different recipes for a 406 to get the numbers I want w/o adding the complexity of a turbocharger.

[grumpyvette]

build the 406 SBC we talked about, youll have the required 500 ft lbs over a wider rpm band and youll have it available both lower/earlier in the rpm band and over a wider rpm band and be able to use a lower octane fuel to obtain the necessary tq.

look tq is a basic result of effective use of cylinder pressure and efficient use of total displacement, compression and leverage due to stroke do have an effect but in this case your worried about durability and the differance is almost non-existant, look at air flow as a guide, at 4900rpm a 377 in theory moves 534 cubic feet of air thru the engine per minute , a 406 at 4550 rpm moves the same voluum so should make similar tq, BTW thats at identical fps in piston speeds , so stress is nearly identical, BUT the volumetric efficiency favors the lower rpm 406 simply because the valves are held open longer and the drop in cylinder pressure in the cylinders that allows the intake ports to flow builds sooner and faster in the intake stroke, plus the valve train stress levels are lower