Landracing Forum
Tech Information => Technical Discussion => Topic started by: fordboy628 on February 10, 2013, 09:39:48 AM
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To anyone searching this thread,
I've deleted all of my original content on this older thread. Sorry for any inconvenience this may cause you.
It's now the time for others to take over this thread.
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Ah, if we ever get to just below the cheddar curtain my wife and Mrs. Fordboy could compare notes.
I see the words static and dynamic have reared their ugly heads again. Boys, it's a dynamic world inside that engine that is far, far different than your static thinking.
Very good description. The dynamic view of compression ratio is the only one that matters.
One of the terms mentioned is BMEP, Brake Mean Effective Pressure. It is the singular measurement that allows us to compare equally the performance of the small block Chevy to that Briggs and Stratton.
Read here:
http://www.factorypipe.com/t_brake.php (http://www.factorypipe.com/t_brake.php)
I did a write up about fuel a few years ago:
http://www.landracing.com/forum/index.php/topic,2308.0.html (http://www.landracing.com/forum/index.php/topic,2308.0.html)
It is also effective to talk about what exactly we are compressing and why.
The combustion cycle
Air + fuel + ignition = heat = pressure = hp
More air + more fuel + ignition = more heat = more pressure = more hp
More air + more fuel + ignition + compression = even more heat = even more pressure = even more hp
Air:
On a nice summer night you wander outside and gaze up at the stars. That clear view is through 75 miles of air. There is more nothing than there is air.
At 62 miles you have passed through 99.99997% of the atmosphere. 75 miles effectively puts you in outer space.
Let’s put some of that in our theoretical single cylinder 1000cc engine. Remove the head and rotate the piston from TDC 180 degrees to BDC. You now have 1000cc of air.
But air is a highly flexible substance. It is 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases. It’s the 21% oxygen that we work with, the rest is just in the way. And don’t forget the 0.0000325% of nitrous oxide.
At sea level that 75+ miles of air in a 1 sq in column weighs 14.7 pounds. That 14.7 pounds per square inch is what is pushing the air into the cylinder.
But that 14.7 is a variable. At 6,000 ft it’s 11.8 pounds. 50% of the atmosphere is below 18,000 ft so gaining altitude cuts the amount of available air to the engine in a hurry.
Heat is another variable. To simplify thinking without going into the science, think of an atom as vibrating and pushing the other atoms away the more it vibrates. As the heat increases there is less air in the same volume.
But wait! There’s more! Air also contains a variable amount of water vapor, on average around 1%. Humidity is the term, and the amount of water vapor in the air displaces the air and makes less available to the engine.
High, hot and humid = poor hp
Low, cool, and dry = very nice hp
Either through on-site calculations from your weather station, or internal calculations from the air/fuel ratio sensors you have to adjust the fuel to match the air.
Let’s put the head back on our engine. We are going to replace the intake with a soda straw. Now when we rotate the piston 180 degrees we still get 1000 cc’s . . . eventually. That is the problem you face when you are trying to stuff the maximum volume of air into a space in a fraction of a second through the restrictions of the intake and exhaust system. The faster you try and do it, the less air actually ends up in the engine. Air is the limiting factor on producing horsepower - until you reach the end of the mechanical limits of the engine. The bulk of the work on a racing engine is to provide more air.
In addition to porting, cams, valves, etc. additional air can be inserted through intake and exhaust length tuning to take advantage of standing pressure waves in the system. More air can be obtained by a ram air system. At 200mph 0.7 psi can assist in pushing the air in the engine. The ultimate is to put a supercharger or turbocharger to push pressurized air into the engine. All of these are discussions for another topic.
Fuel:
Ok, now we have air. The next step is fuel. Gasoline, methanol, nitro methane, diesel, etc. For this discussion the type of fuel doesn’t matter. What you do with it does.
Gasoline requires approximately a 14.7:1 air/fuel ratio by mass. Methanol requires exactly a 6.4:1 air/fuel ratio by mass. Nitromethane requires exactly a 1.7:1 air/fuel ratio by mass.
This is the stoichiometric ratio. In a laboratory setting all of the fuel will consume all of the air and there will be no excess fuel or air left.
Notice the word “approximately” for gasoline. For gasoline fuel, the stoichiometric air–fuel mixture is approximately 14.7; i.e. for every one gram of fuel, 14.7 grams of air are required (the fuel oxydation reaction is: 25/2 O2 + C8H18 -> 8 CO2 + 9 H2O). Any mixture less than 14.7 to 1 is considered to be a rich mixture; any more than 14.7 to 1 is a lean mixture – given perfect (ideal) "test" fuel (gasoline consisting of solely n-heptane and iso-octane). In reality, most fuels consist of a combination of heptane, octane, a handful of other alkanes, plus additives including detergents, and possibly oxygenators such as MTBE (methyl tert-butyl ether) or ethanol/methanol. And you have no idea what you are actually getting . . . ever.
But we are not in a laboratory setting. You have thousands of a second to get the fuel and air into the cylinder and ignite it. Kiss the laboratory goodbye and welcome to the dynamic world inside the cylinder.
The head is back off and the piston is at bdc. In the 1000cc’s of air we are going to put 1 oz of gasoline in the bottom. 100% of the fuel is at the bottom, 100% of the air on top. Neither one burns by itself. At the interface between the two is a very small area of intermixing that we will ignite. You can do this in a tall jar if you want to experiment. Gasoline is flammable. Do it outside with a fire extinguisher. The ignition will produce a small flame and then it will turn sooty black and go out, long before any amount of fuel is consumed. As the air is consumed it is replaced by exhaust products that displace the air. Just having the two in the same space isn't enough.
To get maximum effects you have to combine 100% of the fuel with 100% of the oxygen molecules in the air. This is never possible, there isn’t enough time. The liquid fuel has to be atomized and then vaporized so that the gaseous vapor combines with the oxygen molecules at a molecular level. Everything happens to individual molecules, not big clumps of them.
On a float carbureted engine the venturi in the carburetor produces a partial pressure that sucks the fuel out of the bowl and spits big drops into the air that is rushing into the intake manifold. Turbulence and heating help to atomize and vaporize those drops prior to ignition. There are still large quantities of fuel that are not attached to any oxygen molecules and go out the exhaust unburnt.
Fuel injection certainly helps this problem. The fuel is pressurized and injected through a nozzle and the reduction in pressure when it is sprayed assists atomization. Turbulence in the design of the head is a major factor.
This is why you are always running richer than stoichiometric. That oxygen sensor in the exhaust is telling you about all of the wasted air you didn't ignite.
Ok, on our test engine the head is back on, the piston is at bdc and the cylinder has the optimal ratio and mixing of fuel and air. If we ignite the fuel at this point we will get 100% of the heat of combustion. Now refill and then rotate the piston 180 degrees. At a 10:1 compression ratio we have compressed the mixture to 100cc’s. If we ignite the fuel at this point we will get >100% of the uncompressed mixture. How can that be? What did we gain? Compression is all about time and distance. At bdc the molecules are far apart and the advancing flame front is cooling rapidly between molecules. When the mixture is compressed the molecules are compressed together and the flame front travels faster. The more compression the better. There is no end to the benefits of more compression . . . until you find the limits of detonation.
And you wonder why there are very big books written on engines.
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WOW!!! This is some great info.!!! Thanks guys!!! :cheers:
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This is for next week when I have some time and the inclination to absorb the information supplied. Great work guys! :-D :-D :-D :cheers: :cheers:
Pete
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One of the key things that Fordboy has done for my engine development is to take all of these engine theories and practices, things I’ve been reading about since high school in publications like Hot Rod, Popular Mechanics and David Vizard’s book, “Tuning the A-series Engine”, and put them in their proper order and sequence. It’s his experience that has set the proper order of things, and has dispersed many of the misconceptions I’ve had for years.
There’s no way I’d be as far along as I am without his insight on this.
I’ve said it before, I’m at best an amateur builder, but in our numerous conversations over the last year, Fordboy has turned my scattered knowledge into acute insight, and that has been crucial to my build.
Professor Fordboy, I’ve said it before, and I’ll say it again – Thank you.
:cheers:
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Fordboy & Dean: Two of the best and most concise posts on this or any forum! :cheers:
There's certainly more to suck, squeeze, bang, blow than meets the eye! I subscribe to the 7 event cycle myself. :-o
Don Terrill of SpeedTalk.com has just put together a very good book "The Horsepower Chain" on how his ENGINE Pro software works! The math and practical advice! Check it out guys! :cheers:
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Big thanks to Mr.Boy and Mr.L.A. as I am in process of yanking the engine in Amy's car to bump up the gratuitous production of heat. These posts seem to filter the useless chatter that frequently accompanies the search for info to help guide in the decision making process.
Frank
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Some numbers for street/race would be useful for two valve engines with hemi style heads and domed pistons vs four valve pent roof heads and flat pistons. This is a nice thread.
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For a frame of reference:
Normally aspirated engines : Maximum BMEP in the range 8.5 to 10.5 bar (125 to 150 psi), at the engine speed where maximum torque is obtained. At rated power, bmep values are typically 10 to 15% lower. A 4" piston has 12.57 square inches. At 150 psi that's 1885 lbs of force.
BTW: 15/15.6 bar (217 to 226 psi) is generally accepted as the upper limit of what can be achieved with normally aspirated engines, regardless of rpm range.
Honey, you ain't neva gonna see them numbers.
Blown engines : Maximum BMEP in the 12.5 to 17 bar range (180 to 250 psi).
Top Fuel dragster engines: 80–100 bar (1160 to 1450 psi).
Compression ratio is the computation of the ratio between two volumes. Pressure ratio is the ratio between the starting pressure and the ending pressure. We assume that an adiabatic compression is carried out (i.e. that no heat energy is supplied to the gas being compressed, and that any temperature rise is solely due to the compression). We also assume that air is a perfect gas.
| Compression ratio | 2:1 | 3:1 | 5:1 | 10:1 | 15:1 | 20:1 | 25:1 | 35:1 |
| Pressure ratio | 2.64:1 | 4.66:1 | 9.52:1 | 25.12:1 | 44.31:1 | 66.29:1 | 90.60:1 | 145.11:1 |
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Dean;
My guess is that the compressed gas actually transfers some of the heat of compression to the piston, cylinder, & head, so the process isn't perfectly adiabatic--- not isothermal but not really adiabatic either, somewhere in between.
Regards, Neil Tucson, AZ
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What, no multi-step tutorial on how to calculate displacement?
Somewhere between perhaps 1,000 RPM and the beginning of the torque range DCR approaches meaning zero. Exactly where varies with the IVC point, VE, charge inertia in the manifold, etc.
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Thanks for this fun reading. It explains some answers I was given, some years ago when I started my little pushrod motorcycle project. I was talking with Larry Slutter and Dave Phillips about compression ratio. I asked them what I should try for. Larry said, "a lot". I asked, "maybe go to 13:1?". Dave laughed out loud and said "no...a LOT!".
They were sure right, and if I could get higher than 14.5:1, I would.... but I run 1.4:1 bore/stroke ratio and am at the limit of clearances (650cc pushrod twin).
Would you have any guidance on 90% distillation temp targets for those of us running 10-11,000 rpm on gasoline? Your lessons are starting to explain why some of us have had trouble with heavy gasolines and high rpm. I have been staying with the lowest 90% temps, but suspect I am leaving some money on the table.
Thanks again,
JimL
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Um.............. if Beer is 3.2% and Whiskey is 80% and Nitro is 90%, why am I Drunk, Broke, and Hung over on just a "few" drinks?
Actually, although I am not smart enough to really follow and understand what Ford, Dean, Mantra, etal are saying, I percieve just enough to realize how damn stupid I am, and how Amazed I am that I can even make my Race Car start!
All in all, I appreciate (even if I can't comprehend) what you Deep Thinkers say, but then again, If I was a Rhoades Scholar, I sure as hell wouldn't have spent every dime I have on a Race Car............................
Bob :-D :roll: :cheers:
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Bob, you are a "roads" scholar.
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Would you have any guidance on 90% distillation temp targets for those of us running 10-11,000 rpm on gasoline? Your lessons are starting to explain why some of us have had trouble with heavy gasolines and high rpm. I have been staying with the lowest 90% temps, but suspect I am leaving some money on the table.
ERC is the vendor for gasoline at Bonneville.
Here is their fuel comparisons chart. No fuel vendor provides as much information as they do.
http://www.ercracingfuels.com/sxs1.htm (http://www.ercracingfuels.com/sxs1.htm)
The lowest 90% distillation temperature is A19. It also has the highest Heat Release per unit weight, and that's what I'd be looking for in a fuel.
What works best for your particular engine and tuning is best to determine on the dyno.
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Thanks....now I feel better. A19 is what I have had good results with. I will try other improvements.
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Thanks....now I feel better. A19 is what I have had good results with. I will try other improvements.
Is A-19 one of the choices at Speedweek? :?
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Yes it is. I first tried this in 2000 when our little turbo roadster. It only went 102 on the first pass with A8C (had been running 150 at ElMirage on Shell Red). Went back to the fuel truck and switched to A19. We went straight back to the line without touching a thing. Car went 183.
I have been worried that I am playing it too cautious, with my small bike engine, but I believe Dean has straightened me out.
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In my 16-1 motor the A19 made 226-227hp, 110 made 229, and VP Q16 made 235.
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Excellent discussion regarding compression ratios as well as fuel choices.
Don't want to hijack this thead but has anyone ever wondered about or checked running compression pressures? From the simple screw in compression gauge - with corresponding cylinder misfire - or pizeo - to perhaps some exotic calcuations? WOT at max torque - at max HP? At idle RPM?
Hmm.
Chime in folks-
Best - -
KenB
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The best results I have had is to vary the spark advance angle until I get optimum power with a gasoline, and then to try another blend and find the best spark advance angle for it. Then I compare horsepower. It seems that each blend has its own best advance curve setting.
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This is gonna' get ugly...you might want to stop right here.
Active compression check at idle is used on modern production engines to identify ring, valve, guide, or valve spring problems. A cranking compression check can provide enough air pressure to help sealing. When the engine is idlng, the air volume available per cylinder is much smaller for a shorter time. We often see 50-80psi across all cylinders, at idle. A weak valve spring, or worn guide , will drop that cylinder to 35 psi or so. Not that much performance issue noticeable, but a check engine light turns "on".
A cranking compression test on this problem engine might show 175-180 across the board. The trick for a proper test is to keep idle air control fixed while testing each cylinder. The more cylinders the engine has, the more useful this test can become. I have seen V8 with a broken inner valve spring, making no noise because the pieces had screwed themselves together, that only this test was able to pinpoint a reason for P30# misfire code. This really gets nerve wracking when you've seen a slow to disengage starter Bendix setting engine misfire codes due to crank dampening during the slow engagement release!
There are some ways to get a diagnosis by looking at the electrical "ringing" following spark event, but this can be caused by a dirty injector. If that is suspected, an oscilloscope must be used on each injector signal. A dirty or damaged injector will have more active back-emf ringing pattern (after injector closing) than other injectors, due to change of range/rate of pintle movement in the coil. That takes more time, and has more potential for harness damage, than starting with a simple active idle compression test.
With the reduced rpm range of the newer designs, allowing very light valve spring pressure, diagnosis judgement can get hard to call. Even the cam lobes are only a little over 1/4" wide, these days. Loads are so light, the chains look like jewelry material...and dont seem to wear at all. It's all about narrowing the rpm range to keep catalyst volume down, by adding more gears in the trans. The bigger the engine, the more gears you need....especially if it is making good power. Electronic throttles allow the ECM to reduce the number of throttle transitions through 15:1 a/f ratios (transition between fuel on and off), which tend to notch up cat temps in steps. At about 1850 degrees, degradation begins. When a/f ratios start leaning beyond 15:1 there is not enough fire left to overheat the cat...its the trip getting there that puts in heat, needing time and air flow to get out.
So now we are back to the delicate little valve stems and springs, and low tension thin rings, which have much less tolerance for "trouble" than the old stuff. We need them to reduce friction, extend oil life, and still have great response in a narrower engine speed range. All modern prodction cars start with the selection of cat size to do the target job... then you build a car around it.
So we teach the techs to do active idle compression testing before they replace a gazillion dollars in warranty parts for naught. Now there is a twisted, tortured trail on the way to one answer to your question!
Welcome to the Brave New World. :-P.
This was the torture I lived with for 20 years until Social Security tapped me on the shoulder and said,"clock out, buddy, time to go home.". I told you it was ugly.....
JimL
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"Information - we want - information"
Now we have transitioned to the area where we say, "Ok, I now have active cylinder pressures recorded from a dyno run. Absolute cylinder pressure/vacuum Vs crank angle
absolute inlet manifold pressure/vacuum Vs crank angle, absolute exhaust manifold/header pressure/vacuum Vs crank angle......"
Having the data and doing something with that data is a whole nuther world. If you touch ANYTHING you have to run the tests all over to see what the effect was. When you look at engine data you may peel off 20 things that you could do to improve the situation. Each affects the other.
Sometimes it's better to shoot the engineer and run the damn thing.
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Sorry to slip off track, fellows....shouldnt be blogging with a fever and medication! I hope no one thinks I dont stand in wonder of the company engineers....that is an overworked, underpaid profession. Despite the obstacles, they solve the problems. :cheers:
The spark advance puzzle is really tough for those of us using unusual engine designs with no known historical data and no budget to test. Many of us just try to find something that starts working, and then spend years gaining very small improvement. Its a hobby, so its ok.
Regards, JimL
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Thanks Dean.
I've run a bunch of different projects over the years and I've always considered my self more of a chassis guy. I prefer to have the serious engine guys build the lump and I'll make the thing get round the corners. I have no problem making the necessary adjustments for weather conditions and altitude but I prefer to leave it to others to do the really fancy stuff on the internals. At the same time I've really enjoyed this discussion and others relating to how the horsepower is actually made as it gives a little further insight into what actually makes a racing project of any sort successful.
Keep moving forward guys. The learning never stops!!! :cheers: :cheers: :cheers:
Pete
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The spark advance puzzle is really tough for those of us using unusual engine designs with no known historical data and no budget to test. Many of us just try to find something that starts working, and then spend years gaining very small improvement. Its a hobby, so its ok.
And in no other sport are you going to find a wider variety of unusual engine designs.
One of the things we're looking at is to essentially start from scratch on an advance curve. Not that the BMC A series is an uncommon engine, but with the shortened stroke and long rod-to-stroke ratio, the long relative dwell at TDC with respect to the fairly constant burn rate of the AF mixture is going to give us a very narrow window of maximized advance.
Many variables . . .
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Excellent discussion regarding compression ratios as well as fuel choices.
Don't want to hijack this thead but has anyone ever wondered about or checked running compression pressures? From the simple screw in compression gauge - with corresponding cylinder misfire - or pizeo - to perhaps some exotic calcuations? WOT at max torque - at max HP? At idle RPM?
Hmm.
Chime in folks-
Best - -
KenB
Actually, your question is inter-related to C/Ratios, (all of them), pressure ratios, and pressure decay rates.
Oem's and very serious racing engine developers, and others, record and analyse many esoteric bits of data, among them being:
absolute cylinder pressure/vacuum Vs crank angle
absolute inlet manifold pressure/vacuum Vs crank angle
absolute exhaust manifold/header pressure/vacuum Vs crank angle
among others......
They don't do this just because they are able to do so.
No one who has access to this type of info for racing engines is going to publish it on the web. It's just too expensive to acquire and too valuable to give away. There are some collegiate texts on internal combustion, which cover the basics of this. Check out my reading list.
There MAY be some way to acquire this data for your engine, IF, you can convince a professor at a large university (and his graduate students) to take an interest in your project.
Also, I know that there is a cylinder pressure transducer that fits under a spark plug. I have no experience with it and no idea how accurate it may be. It does indicate though that there may be some transducers available at a reasonable cost. You would still need someone to analyse the data though.
:cheers:
Fordboy
edit was to correct spelling, sorry. :-(
Check these guys out: http://tfxengine.com/
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Reserecting this thread,,
the discussion to date has been about the single stick camshaft,,
What are the advantages/disavantages in relation to compression/performance rpm range, of double overhead cams, where the lobe centers can be set indepently, thus affecting overlap and duration ?,,,
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What are the advantages/disavantages
You don't have to manufacture new cams quite as often.
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"some guys are spoiled by the simplicity of one cam, though,"
this comment says it all,, given the infinete settings that can be achieved with double overhead cams,,
my experience to date has shown that changes in exhaust cam timing really dont achieve much, however messin with the inlet can show noticable improvements or losses,,
exhaust design is directly related to exhaust cam timing, i have often wonded what improvement would occure if one was to change their exhaust system in line with exhaust cam timing,,,
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. . . i have often wondered what improvement would occure if one was to change their exhaust system in line with exhaust cam timing,,,
I can't speak for adjusting exhaust cam timing alone, but I can say that during dyno work over the last two years, we evaluated 5 different headers and 2 different cam timings.
It's complicated - as Fordboy is fond of saying - and in the case of the 5 port, the predictions and calculations were counter to what actually delivered the best power band.
And notice I didn't say best peak horsepower.
Our final iteration used a header that experience and calculations indicated shouldn't work, but instead, it wound up widening, extending and smoothing the power band right where we needed it.
So yes, I'll say there are improvements to be had in the exhaust system. None were huge, but the final result gave us a much more drivable and flexible combination.
I would think on a bike at speed, flexibility would be preferable to high hp that was peaky.
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We also found a horse hiding in the valve adjustment.
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What are the advantages/disavantages in relation to compression/performance rpm range, of double overhead cams . . .
Also, a valve drive train without rocker arms allows faster valve opening and closing.