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  #41 (permalink)  
Old 01-20-2021, 04:01 PM
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Quote:
Originally Posted by joyridin' View Post
It is higher than 9:1. I think 12.5:1 is not realistic either, but 10.5:1 or even 11:1 with what he has is very realistic. He said he had 12.5:1 pistons, but did not actually say the cr was 12.5:1. I doubt if he could massage those "factory castings" (head) as he states a few times, to get the cr down to 9:1. They don't have enough material on them. If he cranks that blower to 22 psi, he better start mixing that C16 with Nitro. Guys are running what he has in LS engines right now getting about the same HP if not more, but they have about a zillion intake/cam/head options.

Read the postscript I added to my post for more about the why for aspects while you were posting.


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  #42 (permalink)  
Old 01-20-2021, 05:14 PM
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Quote:
Originally Posted by eschaider View Post

p.s. We have often heard the old saw that an engine is just an air pump. In fact that is why cams and head selections carry the significance they do. We are attempting to optimize the pumping efficiency of the engine. the short block is only about reliability.

As luck would have it it takes 10lbs of air to produce 100 horsepower. The interesting thing is it is fairly constant across fuel choices, i.e. gas, methanol, ethanol but not nitro because nitro is a monopropellant.

Once we achieve a 100% Ve the only way (with out boosting) to increase power is ram tuning of the intake which tends to be rpm specific and not a uniform increase across the operating rpm band. So where does c/r fit into the picture?

If you run too much timing the in cylinder pressure rise is too steep (quick) and the mixture literally explodes at or very near TDC creating the engine killing knocking sound we all recognize and try to avoid. Increasing octane slows down the burn rate and allows a wider window in crank degrees for the burning mixture's pressure to work on the piston.

A Internal Combustion Engineer (ICE) tries to engineer his engine design metrics and ignition curve for the most commonly available fuels to produce the advertised horsepower target. He does this by manipulating the in chamber rate of burn (measured in feet per second).

Once you have fit the completition of the burn event into that 7 to 10 crank degree window ATDC you have not only done all you can for that charge but it has provided all it can for engine power production. If, for example, you were to increase c/r for this event in this engine there would be no increase in power.

Why, you say. Well you have already previously converted all the fuel and air through their normal oxidation (burning) process into pressure energy. There is no more fuel to convert therefore no more power to be made.


The 10 lbs of air per 100 horsepower is an excellent rule of thumb to tell you when you have gotten all there is to get for a particular mass flow of air. At that point fiddling with timing or c/r will bring no benefits to the table. If you want more power you need to process more air it's really that simple. The rest of the dials will have no effect until you process more air. That means either a bigger engine, higher engine speed or a supercharger.

Ed
Well, the compression ratio directly affects the efficiency of converting heat energy into mechanical energy. The higher the CR, the higher the efficiency, the greater the mechanical energy output for the same amount of air/fuel. This is why diesels are so much more fuel efficient at lower throttle positions as compared to gasoline engines. The diesel is converting more of the heat energy into mechanical energy. So you want to run as high of a CR as possible for the fuel you're using to get the most power output. Lower octane fuel actually has slightly higher energy potential than high octane fuel, but you will get more power from the higher octane fuel assuming you are running a higher CR, as the increased energy conversion efficiency of running a higher octane fuel with higher compression is greater than the higher heat content of the lower octane fuel with a lower CR. Yes, the engine is an air pump, but I think there is alot more things at play in determining how much mechanical power an engine is able to put out still consuming the same air/fuel quantity. Stroke/rod ratio, combustion chamber dynamics, quench, etc, I'm sure a lot more than I know. Granted, you won't be doubling the power output, but these little things do add up to a degree.
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  #43 (permalink)  
Old 01-20-2021, 07:24 PM
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You are missing some things that are important here Anthony and confusing others.

Quote:
Originally Posted by Anthony View Post
Well, the compression ratio directly affects the efficiency of converting heat energy into mechanical energy.
Gasoline engines do not convert heat energy into mechanical energy. They convert chemical energy into mechanical energy by oxidizing the fuel and using the expanding products of combustion (oxidization) to push against the top of the piston.

The products of combustion take up more space than the fuel and air components prior to oxidization (ignition). The increased volume attributable to the combustion process at high pressure, push on the piston which is part of a crank slider mechanism that translates linear motion into rotary motion.

Quote:
Originally Posted by Anthony View Post
The higher the CR, the higher the efficiency, the greater the mechanical energy output for the same amount of air/fuel.
This is a yes and no statement. Higher compression ratios produce a faster burn rate. Remember the firecracker gunpowder on the ground vs wrapped tightly. Sooo, in that context higher compression ratios produce faster burn rates and allow for the oxidization of more fuel and air in the same period of time.

Intuitively the natural instinct is if a little is good more is better. That is true to a point. The point is once you have burnt all of the fuel and air as fast as possible without detonation. Further increasing compression at this point does not increase power output. What it does do is introduce detonation which saps power and destroys parts. Once you have burnt all the fuel /air in the chamber by 7 to 10 degrees ATDC you have done all that is possible to do.

Quote:
Originally Posted by Anthony View Post
This is why diesels are so much more fuel efficient at lower throttle positions as compared to gasoline engines.
Diesels are fuel efficient partly because compression ignition and spark ignition engines are different. The Diesel will have a much greater expansion ratio, what you are thinking of as compression. That is necessary for ignition because spark ignition of diesel fuel is not practical. The primary reason for the better fuel economy is the energy content of the fuel, it is much higher than gasoline.

This is why when you run any of the ethanol fuels while you can produce good power you can not produce the economy of the gasoline fueled equivalent engine. The ethanol fuel's energy content is just lower. All the other stuff is just interesting window dressing.

Quote:
Originally Posted by Anthony View Post
The diesel is converting more of the heat energy into mechanical energy.
Diesels do not convert heat energy into mechanical energy. They convert chemical energy into mechanical energy when the fuel is oxidized, just like a gasoline engine. The difference is the ignition process, diesels use compression ignition and gas engines use spark ignition. Once past the ignition event it is a chemical to mechanical conversion through oxidization of the fuel .


Quote:
Originally Posted by Anthony View Post
So you want to run as high of a CR as possible for the fuel you're using to get the most power output.
A basically correct answer for the wrong reasons. You want to run the highest compression ratio (in a spark ignition engine) as possible without incurring detonation. You are attempting to maximize the down force on the piston between 7 to 10 degreed ATDC to optimize torque.


Quote:
Originally Posted by Anthony View Post
Lower octane fuel actually has slightly higher energy potential than high octane fuel, but you will get more power from the higher octane fuel assuming you are running a higher CR, as the increased energy conversion efficiency of running a higher octane fuel with higher compression is greater than the higher heat content of the lower octane fuel with a lower CR.
Basically the right answer for the wrong reasons again.

Lower octane fuel can have a higher energy content. The problem is burning the entire volume of combustible fuel in time to maximize the pressure on the piston crown between 7 to 10 degrees ATDC. The fuel can not burn fast enough without detonating to produce the maximum down force on the piston between 7 and 10 degrees ATDC.

The burn rate of the high octane fuel is actually slower and more manageable than that of the lower octane fuel. By choosing the proper ignition point it is possible to optimize the down force on the piston between 7 and 10 degrees ATDC at a much higher level than with the lower octane fuel.

All the energy conversion, energy conversion efficiency, heat content etc. terminology sounds good but is being misused and is not informative nor helpful in describing or understanding the processes occurring in the combustion chamber.


Quote:
Originally Posted by Anthony View Post
Yes, the engine is an air pump, but I think there is alot more things at play in determining how much mechanical power an engine is able to put out still consuming the same air/fuel quantity.
Not really. It is as simple as using an acetylene torch. If you need more heat you use a bigger tip. Same thing with the engine. Think of the engine displacement as the oxygen control on the torch. If you need more power what do you do? You make the engine bigger or run it at a higher speed and add more fuel! If you want the torch to be hotter (more energy) you use bigger tip add oxygen and more acetylene. Same thing with the engine except now it is gasoline instead of acetylene and it is not a continuous burn it is staged incremental burns after each ignition event.


Quote:
Originally Posted by Anthony View Post
Stroke/rod ratio, combustion chamber dynamics, quench, etc, I'm sure a lot more than I know. Granted, you won't be doubling the power output, but these little things do add up to a degree.
This is not how you make power. This is how you fine tune the mechanism to minimize internal power losses.

If UHF TV's were around when you were growing up you will remember they had two tuning dials. Essentially a coarse and a fine adjustment. You would get the channel with the coarse adjustment and fine tune its video quality with the fine adjustment.

What you are talking about are the equivalent fine tuning controls on the engine and they should not be confused with the big dial that brings the horsepower.


Ed
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Last edited by eschaider; 01-21-2021 at 02:03 AM.. Reason: Spelling & Grammar
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  #44 (permalink)  
Old 01-21-2021, 10:15 AM
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A few more factoids for the inquiring minds.

It appears that they have a engine building company (Willis Performance Enterprises) that is offering these engines in completed form for sale to enthusiasts. This is their website => WillisEngines.com

Joyridin's sharp eyes caught the C16 fuel annotation on the Dyno chart. For those that missed it here it is again;



Although they have a provision for a description of the blown version of the engine on their website, it is not yet populated with any data. When you review the piston offerings on their site for the engine, the do have a 10:1 c/r offering. This would argue for joyridin's suspicions about a 10:1 c/r offering for the blown version the engine.


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  #45 (permalink)  
Old 01-21-2021, 06:11 PM
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I contacted Brian Wolfe to get an answer to the compression ratio question because it was an itch that needed scratching. His answers were interesting.

The blown version of the engine does in fact run 12.5:1 compression with C16 fuel. To avoid detonation they back the timing down to a "safe" number but, you really do have a 12.5:1 c/r in a Whipple blown Godzilla.

This approach is similar to what John Mihovitz started doing a number of years ago in Modmotors. John ran high compression (only about 10:1 at that time not 12.5:1) and pulled timing (or added timing depending on your point of view) until he found the engine's knock threshold. John got some fairly impressive power numbers out of the engine, 962 hp at 7000 rpm — from 281 inches. The engine definitely wasn't a wimp.

John used 18 psi of boost (2.2 BAR) intercooled and 10:1 c/r for an effective c/r of 22:1. Because of a late closing intake valve (for upper rpm performance) the dynamic compression was closer to 8.5 to 9 to 1. Making the effective c/r 18.5 to 19:1. John ran C16 fuel like Brian does.

The lower compression build model will be easier on engine parts, especially if the engine is street driven. High ambient air temps and high engine temps can encourage detonation.

It appears Brian Wolfe pursued a similar build model with his Godzilla. I need to give him a fairly significant attaboy for the success he has had with the build. However, it is important to recognize that while he used C16, which didn't hurt his performance, if you use any of the many varieties of pump gas available today, you will need to exercise a bit more caution because of the lower octane fuel's predisposition to detonate

My personal preference is still the 9:1 c/r solution particularly with a 93 octane grade of gas. Fuel becomes dramatically less expensive, readily available, and capable of quite stunning power levels. While the following statement is true of almost any supercharged engine, if you experience a whoops on a high c/r blower motor it is much more devastating than a whoops on a lower compression equivalent engine.

The trick (if that is the right word) to getting the high c/r engine to live and work is using just enough timing that the fuel air mixture has fully or nearly fully burned by that 7 to 10 degree window ATDC. Too much spark and the cylinder pressure rises too soon occurring "in front of" the target 7 to 10 degree window ATDC. Too little timing will have the max pressure point occurring after the target 7 to 10 degree window ATDC. Either way it will cost you power. One way to will cost you parts also. Experimentation will be necessary to find that optimum ignition timing for that 7 to 10 degree ATDC window.


Ed


p.s. The high compression and reduced timing window size changes and moves around as rpm and load change so a programable EFI system is necessary to "follow" that optimal timing window as it moves around throughout the rpm range.
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