Re: Re: Re: Re: Benefits of higher compression?.

Posted by Neil on June 26, 2004 at 22:45:49 from (138.88.15.38):

In Reply to: Re: Re: Re: Benefits of higher compression?. posted by Jonathan on June 26, 2004 at 17:49:53:

Well, there's a lot to discuss in this post.

I'll start with thinner bearings. I think what you're actually referring to is thinner rod journals, which is the part of the crank shaft that the connecting rods attach to. It is definitely the case that a stroker crank puts the rod journals closer to the block. In some cases, that means that it is necessary/easiest to provide clearance between the rods and block by going to a thinner rod journal.

For example, many of the kits that are out there for stroking a Ford 460 to a 514 reduce the rod journal from the factory 2.5" diameter to a diameter of 2.2", which makes it easier to get the rods to clear the oil pan rails without doing any grinding. Does this have an adverse affect on engine longevity? Well, it's hard to imagine how it could improve the longevity. Thicker rod journals would make the crankshaft stiffer, which would reduce wear by keeping things aligned better. Also, thicker rod journals provide a larger bearing area to absorb the loads that compression and inertia forces put on the rods. The ultimate questions are: 1) how much will a given reduction in rod journal diameter affect engine longevity; and 2) how much reduction in engine longevity is acceptable to you to gain some power.

Most of the time what you have to do is consult with someone that has a good understanding and/or experience that will enable them to arrive at a durable combination of stroker components. Let's talk about the Ford 514 example again. The 2.2" rod journal that is used on the stroker cranks happens to be the stock dimension on a Chevy 454 crankshaft, which is an engine that is very similar to the Ford 460 in basic layout and power production. Most would surmise that if 2.2" is sufficient to provide adequate reliability in a Chevy 454, it's probably going to live pretty well in the Ford 514. Ford simply indulged in a little overkill with 2.5" rod journals.

The other thing to consider is that not all stroker combinations will require smaller rod journals. Sometimes there simply is a lot of extra room in the block. You can also gain clearance in some situations by going to rods that are made of a higher quality material and have less material surrounding the crankshaft. For instance, the big end of a steel rod can be made smaller than a cast iron rod because the steel is stronger. Thus, it is may be possible to go to a steel rod with less material around the rod journal and clear the block without having to go to a reduced rod journal.

The other way to avoid having to go to smaller rod journals is to have grooves in the block as you mention. That's not really a big deal - it just takes a little time in the machine shop. And it really doesn't have much detrimental effect on the durability of the block. The parts of the block ground on to clear a stroker crank don't typically see much stress. (I guess I should qualify this by saying that this is the case in car applications. In tractor applications where the engine block functions as the frame of the tractor, there likely is much more stress on the oil pan rails. But it's my guess that you'll also find tractor blocks have a lot more room in them such that you can likely get more stroke without grinding or reducing the rod journal diameters.)

I don't have any hard data on how stroker cranks and reliability, but I think your brother-in-law is overstating the situation. Of course I don't know if he is talking about people that put together sensible combinations or people that are pushing the ragged edge (as I understand it there are guys out there in antique tractor pulling with stroker crankshafts that are welded together - that's not good for longevity and they know it).

To make a long story short - if you talk to a competent engine builder about putting together a stroker combination, you'll probably be able to arrive at a substantial (at least 10%) increase in displacement without sacrificing much in the way of durability.

You also mention increased risk of overheating with a stroker engine. Well, yes, with the possible exception of increased compression ratio, almost any time you increase the power, you are also going to increase the amount of heat rejected by the engine and you'll need to think about upgrading your cooling system.

You also mention the possibility of making the cylinder walls too thin and weak. Here you are referring to boring the engine as opposed to stroking it. And, yes, one always has to consider how much extra material there is in the block when deciding how much boring can be done. With a lot of passenger car engines the default answer is 0.030" overbore is the max. A Ford 460 being a good example - the service manual says that the block can only be bored 0.030 over. So, a conservative individual would not bore the engine more than 0.030". There are plenty of engines out there, however, where there is plenty of material to allow boring the engine without causing durability problems. Heck, as best I understand many of the tractor manufacturers themselves offered larger sleeve and piston combinations when rebuilding engines.

As with deciding on whether/how much to stroke an engine, all you really need to do to find out whether/how much you can safely bore an engine is consult with a competent engine builder.

As for boring an engine creating overheating issues, like stroking the engine, increased power will cause increased cooling demands. Other than that I don't believe there are any peculiar cooling issues created by boring an engine.

One other thing to consider is that almost any increase in power is going to reduce engine longevity if you use it. An interesting bit of information I learned working at a heavy truck and diesel engine manufacturer is that the best predictor of engine wear, oil deterioration, etc. is not hours of run time, revolutions of operation, or miles logged - it's gallons of fuel burned. Of course the sharp advocates of increased compression ratio will point out that an increase in compression ratio would result in more power with the same or less fuel and thus, theoretically, more power and greater longevity. And that might be true, as long as you make sure to stay away from detonation. Detonation will easily reduce the life of an engine far more than a modest reduction in rod journal diameter, rod/stroke ratio, or piston compression height.

I guess the biggest thing I would want to make clear is that I'm hesitant to say that a compression ratio increase will yield "big" power and/or torque gains. I'll put it this way - if you and I take identical tractors, you do nothing but increase the displacement 10% and I do nothing but increase the compression ratio 10%, your tractor would whip mine.

Follow Ups:

• Re: Re: Re: Re: Re: Benefits of higher compression Jonathan 07:14:32 06/27/04 (1)
  • Re: Re: Re: Re: Re: Re: Benefits of higher compres Neil 08:59:23 06/27/04 (0)
• Re: Re: Re: Re: Re: Benefits of higher compression Ron 02:41:59 06/27/04 (1)
  • Re: Re: Re: Re: Re: Re: Benefits of higher compres Neil 08:37:31 06/27/04 (0)

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