Benefits of higher compression?.

In order to determine how much more power will be made we need to determine how much higher the pressure in the higher compression engine is on average during the power stroke. Which is the same thing as saying we need to determine how much greater the thermodynamic efficiency of the higher compression engine is.

Unfortunately, it seems I threw out my thermo book the last time I moved. Fortunately, the internet is a wonderful place and you can find an explanation of compression ratio and thermodynamic efficiency at the following website –

Link

As you can see there the equation for thermodynamic efficiency is:

1 – 1/(r^(k-1))

where:

r = compression ratio

k = Cp/Cv (which = 1.4 (you’ll just have to trust me on this one (thanks for the assist Jerry, I wouldn’t have remembered what it was otherwise))

Plugging and chugging, we arrive at the following theoretical thermodynamic efficiencies for various compression ratios:

4:1 – 42.6%

5:1 – 47.5%

6:1 – 51.2%

7:1 – 54.1%

8:1 – 56.5%

9:1 – 58.5%

10:1 – 60.2%

11:1 – 61.7%

12:1 – 63.0%

13:1 – 64.2%

14:1 – 65.2%

This is also shown graphically on the above-listed website.

Now, to find the theoretical increase in power from one compression ratio to another, one divides the thermodynamic efficiency of the higher compression ratio by the thermodynamic efficiency of the lower compression ratio. For example, to figure out the theoretical increase in power from a 6:1 compression ratio to a 8:1 compression ratio, one divides the thermodynamic efficiency at 8:1 (56.5%) by the thermodynamic efficiency at 6:1 (51.2%).

56.5/51.2 = 110.4%

Thus, the theoretical increase in power by going from 6:1 compression ratio to an 8:1 compression ratio is 10.4%.

The theoretical power gains for each of the 1 point jumps is listed below

4:1 – 5:1 = 11.5% power increase

5:1 – 6:1 = 7.8% power increase

6:1 – 7:1 = 5.7% power increase

7:1 – 8:1 = 4.4% power increase

8:1 – 9:1 = 3.5% power increase

9:1 – 10:1 = 2.9% power increase

10:1 – 11:1 = 2.5% power increase

11:1 – 12:1 = 2.1% power increase

12:1 – 13:1 = 1.9% power increase

13:1 – 14:1 = 1.6% power increase

It should be noted that the theoretical power increase is also the theoretical torque increase.

It is also worth mentioning that the equation for the theoretical increase in torque for a displacement increase combined with the necessary head and cam work to fill those extra cubes is considerably simpler - it is the new displacement divided by the old displacement:

New Displacement/Old Displacement

Thus, while a 33% increase in compression ratio (from 6:1 to 8:1) will yield a 10.4% increase in torque, a 33% increase in displacement coupled with the appropriate intake work will yield a 33% increase in torque.

Even back in the '60's during the muscle car era engineers got 10% torque improvement per point of CR increase. Like I said, it's a very general rule, subject to lots of factors. One factor that entered into this that 104 octane gas was available anywhere. As you moved up the CR scale, you got those excellent gains because the high octane prevented knock and therefore power loss. There were some muscle cars than ran CRs as high as 11.5:1.

Another example: the perfect blower motor has a static CR of 8:1 or so. By adding 6-8 lbs. of boost, most supercharger manufacturers will claim a 30% boost in torque... and it just so happens that 6-8 lbs. of boost raises the combustion pressure to about the equivalent of an 11:1 compression ratio. Remember, engines don't "see" compression ratio. They see cylinder filling and therefore combustion pressure.

If anyone has any GOOD dyno numbers (sorry guys I don't trust the stuff you wrote down off the bouncing gauge on the M&W while you were trying to read the conversion wheel in your left hand) on the matter I'd certainly be interested.

A fundamental thermodymamic characteristic of the Otto cycle (4 stroke, spark ignition) cycle engine is this: The higher the compression ratio (CR) the higher the "expansion ratio" (the opposite of the CR) during the power stroke. The higher the expansion ratio, the more energy that is extracted from each power stoke. The more energy that's extracted, the higher the engine efficiency. And the higher the efficiency, the more power that is produced from a given engine displacement. Hence all things being equal, higher CR = higher power.

The downside is higher CR's result in higher stress on the engine (both mechanical and thermal), increased problems with detonation, ect. Simply increasing CR to increase power is not exactly a "free lunch"!

Whenever you compress a gas (air/fuel mixture here) you heat it. The greater the compression, the more heat; the more heat, the more power. Remember, it's the expansion of the air/fuel charge as it burns that drives the piston. So yes, in that sense, it is more efficient. This is exactly how diesels work and why they are more efficient than gas engines.

As you have observed, the heat from higher compression ratios must be managed. Should the cylinder become too hot, spontaneous combustion (knock) occurs and power is decreased... if continued too long, you start breaking parts. Octane enhancement is only one way of managing spontaneous combustion. See>Link
for>Link more information.

Your point about horsepower is true but far more important is the enormous low rpm torque improvement in higher compression engines. I was a kid in the '60's when there were lots of cars sporting 400-450 ci. engines running as high as 11:1-11.5:1 compression ratios. Many of these engine made over 500 lb./ft. of torque and the acceleration was mind-blowing.

For tractors, torque is all that matters. And the more torque you can build at low engine speed the better. Higher compression ratios are the best way to make more low speed torque. An old rule of thumb is that for each full point you gain 10% in torque. For example, going from 8:1 to 10:1 should net you about a 20% torque gain.

Mark

I just have to say that in my opinion compression ratio is very over-rated for increasing power. As I alluded, I suppose it makes sense up to around 8:1 where you can get by on regular gas. After that, if it were me, I would take the extra money you would spend on race gas for a compression ratio increase and spend it on more bore and stroke or making your heads flow better or putting a better cam in it.

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.

A couple more points. While I come from the same school, I've learned over the years that it's usually far more cost-effective to replace an engine with one that has a larger displacement than get into boring and stroking the old. This is more true with car/truck engines where certain engine families have a range of displacements. Some tractors do to. Example: I can buy and rebuild a 454 Chev and swap it into any engine bay a 350 came out of for less than just the cost of a custom 350 stroker crank and machine work. And the bigger engine will always be more reliable than the smaller one.

Some tractors also present these swapping possibilities. Olivers are especially good. I realize than some sanctioning bodies have rules that preclude such swaps.

Actually there are some engines where boring results in huge overheating problems. Siamised cylinder engines already have cooling issues due to no coolant flow between adjacent cylinders. Boring them is usually not done. Most tractor engines are wet sleeve designs and most tractors have enormous reserve cooling capacity so they are good candidates for boring.

I agree with your statement about the 10% inprovement in general but there are other factors. I can shave the head of an inline 6 tractor engine for say $400. Because they are usually low compression engines, I can pick up 10% easily. To get a 10% increase with a bore/stroke might cost 10 times that $400. Lastly, there are about a dozen factors that enter into engine performance. They must all be balanced. For example, with a bore/stroke, the intake and exhaust systems must be modified to increase flow. This can get mighty pricey depending upon the exact situation. The one big advantage of compression ratio increases is that because they increase efficiency, they often produce excellent torque gains with no other modifications though some engine builders in some situations will run a longer cam to help eliminate the dreaded low-rpm part-throttle knock.

First, let me say you really do know your stuff, which makes this discussion so much more worth while than otherwise - there's nothing more frustrating than debating with someone that is too ignorant/dumb to understand what you're saying.

You mention that it's smarter to swap to an engine that has more cubes than to bore and stroke a smaller one where possible:

AMEN, AND THANK YOU!!

I never cease to be amazed at the desire to spend $15,000+ on a 454 small block. You can easily put together a better 454 big block for far less $.

To the extent that it's possible, the idea to swap to a bigger engine is the right one for tractors as well. Of course, my general modus operandi would be to find a bigger engine to swap in and then bore and stroke it.

As for boring and overheating, I'll admit I kinda glossed over that one. It's true that I am not aware for certain that it causes overheating or in what circumstances it will. On the other hand I've heard various rumblings that it will cause overheating.

I suspect most of the reputed problems are, as you mention, with siamesed bores. I'm guessing it's more of a problem with localized overheating of the material between the cylinders and not the engine as a whole. Of course, if the material between the cylinders is too hot, you've got a problem regardless of the temp. of the rest of the engine.

I don't know how big an issue overheating with siamesed blocks is, as I've never really had the opportunity to get good information on that subject (and boy would I like to). Maybe someday I'll find out as I'm currently lusting over GM's Ramjet 502, which uses a siamesed block, for daily driver use. I guess GM provides a warranty with it (though I don't know how long). My guess is that due to temperature gradients, the cylinders run a little more out of round than a "regular" block and cylinder wear is advanced. And I've not heard whether or not you can do a clean-up bore of 0.030 or so on that block.

I'd say that you're almost certainly right that compression ratio increases are usually the cost effective way to get the first 10-15% increase in torque. And I must admit that if I was building a tractor, one of the first things I'd probably do is bump the compression ratio to 8:1 or so - it just makes sense.

I wouldn't consider it a technique to get "big" power or torque gains, though.

Also, I say compression ratio increases are usually going to be more cost effective because it seems that displacement increase can be pretty cheap sometimes too. This seems to be particularly true where guys swap in larger cranks/pistons from the same engine family. The Minnie Mo guys come to mind first. It seems every time I'm on that board there is someone asking about putting X crank in Y block.

But compression ratio increases and forced induction are by far the best ways to increase low end torque and of the two, compression ratio increases are usually cheaper and less troublesome to obtain.

I definitely understand your point about bang for the buck. It's pretty darn cheap to have the head milled or put an lp head on or swap out pistons. And it's generally less troublesome indeed than forced induction.

I, however, come from the "no replacement for displacement" school. In my opinion, there's no better way to add low end torque than to add cubes. Yes, as far as the cost of parts and machine work goes, it's more expensive than a compression ratio increase. But, if you use the tractor for more than a few tractor pulls a year, you would want to factor in any additional cost for higher octane gas. With fuel costs factored in, one can imagine that there are certainly situations in which boring or stroking would be more cost effective than a CR bump.

Of course, in the case of most antique tractors the compression ratio can be raised from the factory level without requiring any upgrade in fuel. In that case it most certainly makes sense to increase the CR at least to the point where it is as high as it can go without needing to upgrade fuel.

At the end of the day I suppose it's all a matter of personal preference as to how one prefers to waste one's money. ;-)