Effective Compression Calculations: Installment One

[xctasy]

Here is a little info which will perhaps help you if you are into reading camshafts rather than tea leaves or hops. It covers what happens to the pressure in the cylinder when you wind up the camshaft duration.

These are some tables which I have compiled which are based on the calculations used in many engineering manuals on recipricating engines. The are part of the calculation of Brake Effective Mean Pressure, which governs how much horspower an engine can develop. Since you can't cheet the laws of physics, and the formulae used are written elseware in books like Practical Gas Flow and Phil Irvings "Tunning For Speed" book on motor cycle engineering, I want go into the trigonimetery used to calculate it...yet. People like StrangeRanger and XT500, Aussie7Mains and others will go over this with a fine tooth comb, so I'll refrain from shoving my hands in a mouth that has two feet in it already

Consider yourself spared from more dribble.

MORE DRIBBLE:

Case One :Using a 250 cubic inch Aussie 2V or similar head with:

*a slightly bigger than normal 62 cc combustion chamber,
**12 cc piston dish,
***41 thou thick composite gasket (7.15 cc),
**** a 25 thou (4.36 cc) below the cylinder deck register
{which is much closer to a zero deck than the US 250},
*****and the stock 681.49 cc piston displacement

you get an 8.97:1 static compression ratio.

Given that the stroke is 3.91 inches, and the hypothetical "stock" cam has a valve event of 15/53 52/19 for the otto cylce, the effective compression the piston actually sees can be calculated. You use a formulae which works out the distance of effective compression stroke using trig, the stroke, the valve event, and the length of the rod (which is irrelevent...a 6.275 inch rod verses the stock 5.885 rod makes a 0.2% lower effective compression). It's always less than the total stroke. You then plug this calculated value into the compression ratio formulae, and presto, you got the effective compression. The red value is the one used for calculation purposes, I think. (It's been a while since I've used it, and the reference manual is at work).

"Stock" cam 15/53 52/19, 182/185, 248/251 means intake cylce is 15/53 degrees, exhast is 52/19 degrees, 50 thou valve duration at intake is 182 deg, exhast duration is 185 deg, and total duration is 248 intake, and 251 exhast.

This has an effective stroke of 3.264", and gives an effective compression of 7.654:1. This is a benchmark figure which shouldn't be gone under for longer duration cams. The static compression needs to be raised to ensure the on cam characteristics are maintained. If you go more than 20% below 7.654:1 for any subsequent cam combo on this engine, you risk unfavorable brake specific fuel consumption characteristcs, and performance and economy will go down hill.

So I have just grabed at random six hydraulic cams, and three mechanical cams. Here are the amounts of compression that need to be added for each one:-

"Stage 1 Stock hyd" cam 15/53 52/19, 182/185, 248/251, 7.654:1 effective, okay as it is at 8.97:1 already. The Baseline
"Stage 2 hydraulic" cam 27/65 68/30, 202/208, 272/278, 6.781:1 effective, 13% loss, needs to be 10.125:1 static to run best.
"Stage 3 hydraulic" cam 32/70 73/35, 212/218, 282/288, 6.475:1 effective, 18% loss, needs to be 10.603:1 static to run best.
"Stage 4 hydraulic" cam 36/76 79/39, 222/228, 292/298, 6.089:1 effective, 25.7% loss, needs to be 11.275:1 static to run best.
"Stage 5 hydraulic" cam 40/80 81/41, 236/238, 300/302, 5.958:1 effective, 28.5% loss, needs to be 11.523:1 static to run best.
"Stage 6 hydraulic" cam 42/83 83/47, 243/248, 305/310, 5.823:1 effective, 31% loss, needs to be 11.791:1 static to run best.
"Stage 4 mechanical" cam 37/77 79/41, 234/240, 294/300, 6.089:1 effective, 25.7% loss, needs to be 11.275:1 static to run best.
"Stage 5 mechanical" cam 43/85 88/46, 244/250, 308/314, 5.483:1 effective, 39.6% loss, needs to be 12.523:1 static to run best.
"Stage 6 mechanical" cam 46/86 89/49, 254/260, 312/318, 5.414:1 effective, 41.4% loss, needs to be 12.682:1 static to run best.

Summary: Any hi-duration cam will need bulk compression increases to run best. It's unlikely any one would consider using race fuel to run such high static compression ratios. It is clear that a compromise between the theory and the practice will have to made. Any thingover 18% loss should be countered with increased compression, so long as detonation doesn't result.

Case Two :Using a late 200 cubic inch US Log 1V or similar head with:

*a normal 62 cc combustion chamber,
**zero cc piston dish,
***41 thou thick composite gasket (7.15 cc),
**** a 69 thou (12.00 cc) below the cylinder deck register
*****and the stock 544.847 cc piston displacement

you get an 7.714:1 static compression ratio.

Calculations for the same cams above with a smaller engine, and less compression. Close to a stock post 71 engine. Stroke is 3.126, connecting rod is 4.71 inches long.

"Stage 1 Stock hyd" cam 15/53 52/19, 182/185, 248/251, 6.747:1 effective, okay as it is at 7.714:1 already. The Baseline
"Stage 2 hydraulic" cam 27/65 68/30, 202/208, 272/278, 6.065:1 effective, 11.1% loss, needs to be 8.581:1 static to run best.
"Stage 3 hydraulic" cam 32/70 73/35, 212/218, 282/288, 5.818:1 effective, 16.0% loss, needs to be 8.946:1 static to run best.
"Stage 4 hydraulic" cam 36/76 79/39, 222/228, 292/298, 5.504:1 effective, 22.6% loss, needs to be 9.456:1 static to run best.
"Stage 5 hydraulic" cam 40/80 81/41, 236/238, 300/302, 5.395:1 effective, 25.1% loss, needs to be 9.647:1 static to run best.
"Stage 6 hydraulic" cam 42/83 83/47, 243/248, 305/310, 5.285:1 effective, 27.7% loss, needs to be 9.848:1 static to run best.
"Stage 4 mechanical" cam 37/77 79/41, 234/240, 294/300, 5.504:1 effective, 22.6% loss, needs to be 9.456:1 static to run best.
"Stage 5 mechanical" cam 43/85 88/46, 244/250, 308/314, 4.999:1 effective, 35.0% loss, needs to be 10.410:1 static to run best.
"Stage 6 mechanical" cam 46/86 89/49, 254/260, 312/318, 4.941:1 effective, 36.6% loss, needs to be 10.534:1 static to run best.

Summary: Any hi-duration cam will need bulk compression increases to run best, as before. In this case, the engine runs a low compression. In this instance, it is quite likely someone reconditioning an engine may elect to use these recomended figures with the Stage 4 and below cams, with no detonation setting in. Again any thing over 18% loss should be countered with increased compression, so long as detonation doesn't result. The cam maker suggested just this. Low stages don't need modification. Above Stage 3, compression upgrades are needed.

 

[MandarinaRacing]

This all very interesting. This exactly was I was trying to research for my 200 "log headed" motor. I'm planning a kick ass engine that would not see street action very often. My plans call for a "mega" ported 1980 log head, a 500 cfm carb and a 280H cam (I hate adjusting valves) but the thing that I still cannot get right is the CR. I know it's very complex to calculate an effective CR. I plan on running a race gas blend with pump gas so I'm guessing that detonation would not be an issue. From the looks of it I think that killer CR (11:1) is the only way to make decent power on our little sixes (Naturraly aspirated of course. Later on hopeffuly N20 can be added to the equation). Since I flunked math in high school.....How much CR would you recommed based on what I just posted??????
Also take into account flat top pistons, zero deck and a composite gasket.
Hope I can get some feedback!!!!!

Alex

 

[xctasy]

Plan to use the 41 thou thick composite gasket you can get anywhere for 200's. When you go to n20, look around for a 63 thou gasket to lower the CR down.

Ensure that there is no heating to the intake, and then dial a Chevy forged, Tempo piston which is taller, or deck the block. (Check with the Ford Six manual. I suspect a 50 thou block deck is possible without hurting the deck integrety.

Option 0ne:+56 Thou overbore, stock size Chevy 229/305 forged pistons (3.736"), 1.56 tall with 4.5 cc for the eye brows these pistons came with, gives about 3369 cc, or 206 ci. 10.88:1 is the best you'd get, unless you build some pop top pistons. Go for the a chamber of 45 cc or so, achieved by decking the head if needs be .

Option Two:+40 thou Tempo flat top pistons (3.72") with a zero deck. I don't know how much cc's these have...hopefully none! The 255 V8 pistons won't handle the piston speed you'd be looking at, and they may be dished too. Use a 48 cc, and you'd get 11.07:1. However, cast pistons may not like too much compression. Advice anyone?

Option Three:+40 tou ACL flat top pistons (3.72"), ex Aussie. 11.07:1 with a zero deck. These are not just normal cast pistons, they can handle some punishment.

This may be the option for you. Go for 63 thou gaskets, and option one two and three drop about 0.7 points, 10.2 and 10.4:1 for the last two.

I'd be going forged. With the Ford six, you save on all the hassle of doing a v8, seems a shame to skimp. But if you junked a piston, then maybee the last two a better options.

With a 280 hydraulic cam, you'd easily get the desired effective compression. If the closing event is 70 degrees, I'd say you'd only need about 10 :1 to make the came work. With a good racing gas like VP Red, there would be no issue with detonation, and if you used funny gas, you'd get a good extra 75-100 hp out of it with the deaper gasket before something horrible happened, I think.

 

[Anonymous]

http://cochise.uia.net/pkelley2/DynamicCR.html


-=Whittey=-

 

[MandarinaRacing]

I'll probably go for the flat top tempo pistons, 0.040 over, decked block and a 48 to 50 cc chamber. This should give me the desired CR. I was thinking all the time about detonation but overlooked the fact the cast pistons don't like the heat! A set of custom pistons is out of the question right now ($600 a set from ROSS). I'll gather as much info as I can before doing anything....I'm running out of un-milled heads .
Thanks for the info!!!!!!!
Alex

 

[xctasy]

Whittey, your the guy...who noticed that its the intake closing phase which governs Dynamic, or Effective, compression, not the exhast. So my maths err on the side of caution.

Man that's a good website, tho'!

Thanks.


PS MandarinaRacing, as long as you don't hit the head with the piston, and run a race gas I'm sure you won't get detonation. If running on the street, try water injection like MustangGeezer used to.

 

[Anonymous]

Interesting concepts Dino

I'm assuming these ratios were derived by using standard octane fuel huh

How would these numbers change while under a boosted condition?

 

[xctasy]

Hullo Magnus. Love the new Avartar. Do you like mine?

The boost ratio, which is in pounds (which I love) is equal to boost+sea level gage pressure, all over sea level gage pressure.

For a 18 pound XE Falcon, thats 18+14.7{at sea level} all over 14.7. Boost ratio is 2.224 times what it is nomally aspirated. Off course, tha theats the air something wicked, and you don't see even a two fold boost, even with an intercooler. This factor depends on heat gain, and is called the Effective Boost Ratio. In this case, we'll say its 2.00.


On a stockish XE Falcon with 164 hp EFI, and just an intercooled TO3 or TO4 Turbo at 18 pounds, and all little water injection or a compression ratio drop to stop the dreaded knock, you'd get around 328 hp. If the stock cam has had no compression change form the stock 8.8:1, but has had a head plane, and rates at the same level as the 250 2V engine below

"Stage 1 Stock hyd" cam 15/ :arrow: 52 52/19, 182/185, 248/251, 7.654:1 effective, okay as it is at 8.97:1 already. The Baseline.

Then the effective of Dynamic Compression is in fact 7.654:! times the Effective boost ratio...which is 15.308:1. Ouch, knock city. If you fitted a longer duration cam that didn't end up shoving most of the boost out the exhast, you could bleed off a lot of that pressure, and likley get less detonation and more power .

This is exactly why Mike Vine uses Methanol/water injection on his Falcon Turbos rather than spacer plates and two gaskets to lower the compression ratio.
Avoid the Big Bang

 

[Anonymous]

lol.. just got back from Mexico, theres a big market for Convenience Stores on backs of trucks
Interesting avitar, but the bouncing smilie was sorta... hmm. Not quite you, but getting close. Perhaps one jumping up and down, then exploding

hmm, interesting. I wonder what effect H2O and CH3CH2OH injection into the intake charge would have on the maximum allowable dynamic compression ratio.. I'd imagine it would skyrocket. I'm also running on the premise that a different head material, namely Alloy, would also have an effect on the dynamic ratio running in a motor, either raising the total allowable ratio runnable.. Or would it reduce the dynamic ratio you're getting, since its so good at siphoning the charge's molecular kinetic energy?

hmm. its 2AM already. I guess its a little late to form a poignant statement that I'm thinking about regarding the universal gas constant laws, but I hear ya on the big temperature increase in the charge.

but something in my head doesn't quite ring true about having a longer duration cam that doesn't bleed boost out the zorst.. I'm thinking that the boost pressure would be more in the intake manifold - fitting a larger cam would allow more into the cylinders, reducing the pressure differential to the atmosphere, versus a smaller cam, which would keep the differential high... and even then, I'm also suspecting that the difference would be kinda small, since we're talking effectively somewhat twice the atmospheric pressure getting pounded in... Either way, a turbo is set to flow a certain amount of air, my understanding of such is that boost occurs when a motor cannot effectively pump away said amount of air, which builds up in intake manifolding, intercoolers, etc, forming a pressure differential.. boost

feel free to enlighten me (14.7lbs off the shoulders perhaps? ) if I'm misled in here somewhere.. its a somewhat new concept to me.. I'd also be interested to know what dynamic compression can be run with LPG, if someone out there has numbers on that

 

[xctasy]

Hangin out with the Mexicans...people from SA or Victoria if your in Queensland, NT, ACT or WA?

Hey, I never gave it much thought. Of course a the boost ratio would go up with a slightly bigger cam...as long as the turbo doesn't run into surge or reversion at low speed as the thing comes on cam.

Gosh it's bl**dy complicated.

The alloy head would take more compression before detonation sets in. Then LPG has a higher octane ratio but no rich cooling like gasoline/petrol does. And when you use water, power drops, but if you use water and alcohol (methanol/ethanol) together, peak power goes up when the compression ratio is increased.

One thing is that Anti Detonation Injection will work as a cost effective intercooler. It's just that getting suplies of ethonol or methanol makes you cop-bait for sure. Last time I worked in a Materials Laboratory, I had to make up some phenophaltlien solution using ethanol as the base. We were out of stocks (id actualy used the last lot in my Falcon when it was pertol as a fuel conditioner), and Smiths Biolab told us we'd have to get a new permit to store it...the new cottage industry drug supply market was kicking in. Another good one was CD cleaner...which one guy in Christchurch or Timaru sold throuh the back door as a meth e ingredient. So watch out if you intend on using it. "Yes, officer, my car likes a big drink to stop knock, I never use it myself"

I'm no help. We need XT500!

 

[Anonymous]

Interesting. I suppose using LPG injection to help this process along a bit would make for some exceptional gains.. I cant help but think that liquid propane in a manifold or a port would enhance an engines performance via reduced intake temperatures (exothermic reactions take place better in an endothermic environment), and reduced detonation characteristics due to what probably would be a sub-zero intake charge..

Converting the EA CFI injection system to LPG would be Very Interesting indeed

Amyl Nitrate being used in the process to cook up 3,4 Methylenedioxymethamphetamine? hmm, its news to me, I've heard of it being abused on its own, but either way I suppose thats a story for the bluelight.nu forums

heh. yep, bring on the people who can get this stuff down without thinking too hard