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Small block ~v~ big block (and....did you know a marine 427 has more than 300-hp? )

February 7 2008 at 4:10 PM
Paul  (no login)

I received a long and somewhat rambling email from Mike, regarding this subject matter today, and I returned the note with a reference to check the forum here, for some comments. I think the topic is valid, and it deserves some exposure, and I didn’t want the info to be limited to just an email response. Mike, I hope this answers some of the concerns you indicated, and if you have any other comments, please post away!

Everyone who knows anything about the 427 motors and their history, knows the solid lifter valve train is capable of much higher rpm than the 4,000 rpm limit set by Chris Craft. The FoMoCo street version, as offered to the general public, had a RPM limit of 6,000, and this was with a pretty aggressive camshaft (which may have also used higher valve spring rates to avoid valve float). We all know the marine 427 is detuned for longevity, and it uses a torque cam to provide the kind of power delivery needed in the marine environment. Automotive specs, means, and methods just do not apply to the marine context.

Something that may surprise many Chris Craft 427 owners: your 427 actually produces more horsepower than 300 just as it sits in your boat now. All you have to do to get more power, is rev it higher. I’m not recommending this, I’m just stating a fact. Chris Craft set the 4000 limit to assure the motors would last a long time and to control callbacks from customers who, as members of the auspicious group known as “the general public”, could not be trusted to use restraint when restraint was due. They propped the boats to maintain the 4000 rpm engine speed, although some are capable of exceeding this.

If you put a Chris Craft 427 on a dyno just as it sits today, you will get more than 300 horsepower if you run the motor to 4500 rpm. Motors are air pumps, and the faster they spin, the more power they produce. I won’t speculate on how much more power you’ll see, or how much additional rpm the torquey marine cam will take, but Chris Craft could have easily called this motor a 320 or 350 by setting the rpm level higher. They chose to keep things down to a limit that would assure long time durability.

This must be taken into consideration when comparing small block motors that have seemingly equal or higher horsepower ratings to the marine 427. That marine 427 is grossly detuned and under-rated for a reason, and it’s called “longevity”.

Speaking of torque, often left unstated, the 427 produces a massive rating of 438 footpounds at an astounding low 2,900 rpm. That, gentlemen, is a number that even new bigger displacement computer controlled motors will find tough to match.

So now let’s take a look at an automotive small block example rated at an impressive 320 hp and producing 380 footpounds of torque. Those numbers look strong at first glance, until you realize the motor has to use higher compression AND it must spin at 5000 rpm to reach the power level quoted. In addition, it must also spin at 4000 rpm to reach the torque number. What looks like a “comparable” and lower cost power plant to the detuned marine 427 at first glance, would simply burn up early trying to do the same job as a lower compression and slower turning 427.

What is even more telling, is the fact that few of us will run a 427 at 4000 very long, and we therefore don’t need or use 300-hp much at all. Most of us run the 427 motors at 2500 to 3000 for cruising, and this equates to an actual utilization of 215 to 250 real horsepower. What we do use frequently, however, is that awesome torque number at 2900 rpm, and we use it often. Many of us will cruise at or near this number, in order to stay at the range where the motor is efficient and comfortable.

Using a 38’ Commander as a baseline for comparison, taking that high powered 320-hp small block down from it’s 5000-rpm power spec, to a comfortable cruising range, will put it in territory where the boat has a tough time staying on a plane. As the rpm is lowered to a comfortable cruising rpm, torque will drop considerably, and the ability to spin a 23” diameter prop with 2.5:1 gear reduction will also drop. Some of this could be compensated with a different gear ratio or different prop pitch, however, the smaller motor will be working harder at any given rpm, to stay with a comfortably cruising 427.

Where weight factors are reduced, the small block motors are great choices. They are not good choices when it comes to making a 20,000 pound hull move efficiently through the water at planning speeds. Yes you can buy new SBC motors with impressive power ratings, they will do the job up to a point, and they’ll burn themselves up trying to do the same job a big block can do easier.

Chris Craft offered a 327F marine engine in the early 38’ Commander. These are very fine solid lifter small block motors rated at 210-hp. These motors were undboutedly propped with smaller pitch and diameter than the big block alternatives, and although they would be fun boats to own, and would be able to get onto a plane, they are underpowered for some kinds of open water running at Express speeds. Few of these boats still exist, few were probably sold with the 327F, and most would have been repowered by now. I know of none that exist today with original 327F power.

My 38’ Commander is a bit over-propped now with 4-blade props that have one inch more pitch than I should probably have. The jury is still out on this, and I don’t notice any reduction in performance, it’s just the math I’m considering now. At this level of propping, 23x24 in a Dynaquad, the boat will run on a beautiful plane at 2500 rpm, using 430-footpounds of torque. I would think a higher horsepower SBC producing 325-hp at 5,000 rpm, and 380 footpounds of torque at 4000 rpm, would not even be able to achieve a plane at 2500 rpm. It would take considerably more throttle and rpm to do so, and this is why I say they’ll burn out faster. More piston travel per minute, more bearing rotation per minute, more camshaft rotations and valve train action per minute, and the life of the engine will be affected in an adverse manner.

There are some SBC combinations that produce better power ratings than the ones I’ve used. Some of the 383 strokers are impressive, but the cubic inch displacement is being increased due to stroking of the crankshaft. This increases piston speed at a given rpm, and although you can try to make a big block out of a small block, you may be better off in the long run by just going to the big block and being done with it..

Here are a couple of photos showing a similar 35' hull, one with 470-hp (twin 327Q motors) and the other with 600-hp (twin 427 motors). The purpose in posting these photos is to illustrate the capability of larger torque, to propell heavy weights, and the importance of torque in evaluating potential repower options.

Here is a photo of a small block powered 35’ Commander running with several guys aboard. I took these photos a few years ago, and one was published in the Brass Bell. I discussed the performance of the boat with a friend who was aboard at the time, and he indicated “the boat felt like it was plowing, and working hard to maintain a plane”. As you can see from the second photo, there were several people aboard. A pair of small blocks is the perfect choice for this Express, but once it's loaded with this much additional weight, you can see what happens. I've experienced the same issue with my 35' 327F powered Sea Skiff years ago, when it was loaded with people, curling a huge wake and working hard to do it.

By comparison and contrast, here is another 35’ boat, weighing in at considerably more weight than the Express in the previous photos and with even more people aboard. It’s running easily on a fast plane while using 75% throttle. I know this because I'm the guy in the photo wearing the red hat, and I was observing the tachometer during the entire trip. The motors never broke into a sweat, even with this many people aboard. The difference is in the available torque, and the ability to spin large props efficiently. This boat, belonging to Mike Schrage, was running right on the top of the 427 torque curve at this speed, producing a full 876 footpounds into twin 2.5:1 gear reduction Paragons.

I am reasonably sure a pair of healthy hopped up small blocks would propel this boat at the same speed as shown in the photos. In order to do so, however, I think they would be spinning at at least an additional 1000 rpm, maybe more, and in doing so I would wonder how long they could maintain the pace working that hard without longevity issues. This is where understanding the weight of the boat, and the necessary torque numbers, really helps a lot in picking the right choice. Nothing stated here should be construed to be negative about the small block motors, they're very very fine motors and I have two of them. They are the "perfect" choice for many boats, offering lighter weight and fuel economy, and lower cost in some cases too. When you're getting up into the larger boats, careful consideration should be given to torque numbers, because that's what produces the nice photos included on this posting. The more torque the motor will make at lower rpm, the better suited it is for a long life in the marine environment. Chris Craft knew this when they specified the compression ratios and cam profiles for their marine motors, large and small. I would beware of some of the numbers being advertized today, becasue some of those motors may be a lot better suited in a ski boat working at a rate of 3 to 5 miles per gal of fuel, rather than less than one mpg in a cruiser!

These are my thoughts, and I hope they help put some of these issues into perspective. I welcome other opinions.



Edit comment: NEW INFORMATION ADDED: The overloaded 35' Commander was originally powered with 327F motors, and was repowered with a pair of 350s. No data on the source of the motors or their power rating is available.

I have added the (uncropped) photo below to give more of a perception of this photo event. This photo was taken seconds after the frontal image showing the boat coming toward us. We hovered a while waiting for better photos in our fast 250-hp powered center console photo boat, but the 35 never got any farther out of the water until it reached the dock and unloaded the beef.

This message has been edited by FEfinaticP on Feb 8, 2008 10:33 AM

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