compression

[jdo]

can anyone translate for me how much compression would i have at 9.5 to 1 ratio how about 11.3 to 1 ratio, how does this translate to compression pounds. I need help with this as i dont want to get my compression pounds to high

 

[Drew]

There is absolutly NO WAY to translate a mechanical compression ratio into cranking PSI. Why are you worried about cranking PSI being too high? You can have the same mech. ratio, change the cam timing, change the valve lash, etc. etc. and you will change the cranking PSI.

There are simply way too many variables to even begin to compare cranking PSI numbers to mech. compression ratio numbers.

 

[gtx1967jewison]

i would guess because he has a 2cyl jd & too much means not able to roll it over with a starter. nothing like a red tractor.

 

[CJ_1080]

If he has a green two lung machine shouldnt he have compression releases? Even if he has a power block it can be drilled and tapped. I guess I dont understand his concern about compression.

 

[jdo]

better question; it is a john deere which is better 9.5 to 1 or 11.3 to 1 ratio

 

[CJ_1080]

the more the better, i would say. but when you get over 10:1 depending on timing, engine temp, load, ect. you may end up needing premium or higher octane. or come up with a way to suppress detonation.

 

[Dick W.]

Or the other green!

 

[Anonymous-0]

nothing is exact..but i would say that if you had 220psi..it would be about the most you would want generally speaking...some have more...some less..i have the best luck longevity/power balance at or around 200-220psi cranking full throttle.....maybe this will help. i don't do the ratio thing..but most builders tell me 9 or 10:1...

 

[Anonymous-0]

I&T shop manual shows 5 to 1 as 100 psi. Break that down and that means 20 psi equals 1 to 1. 5 x 20 is 100.

On the other hand, Drew has a good point. Engines that need overlap in the camshaft, need to deflate the cyl pressure to more of a likable, tolerable range. In the end, low end torque suffers, but you gain higher rpm torque!
When a camshaft has overlap and put into a stock engine, its not good..... you will see a drop in cyl pressure, so the readings will be off by being lower at cranking speeds.
V8"s do it cause you wont run a V8 at 240 psi at 7500 rpm very long without shaking apart, or blowing the head gasket, or bearing life is not so good.

Then there are cams that COULD build cyl pressure by doing the opposite. Pushing the lobe timing to the point of building up a higher incoming air charge, by delaying the intake from opening, and the pistons are pretty much on theior way down, and then the valve opens and WHOOOSH! A more aggressive slurp off the carb, better air speed, and better fuel mixing thru out the port, and should help PULL air into the cyl and creating a higher cyl pressure reading. In a stock engine, that would be a significant gain in power, but a stroker with naturally high cyl pressures wont gain, it will hurt if they are already above 250psi, hence the reason for a camshaft with overlap.

I&T shop manual says 5 to 1 is 100 psi in the book I have, which is older than I am! ChadS

 

[Anonymous-0]

(quoted from post at 08:01:59 04/27/09) I&T shop manual shows 5 to 1 as 100 psi. Break that down and that means 20 psi equals 1 to 1. 5 x 20 is 100.

On the other hand, Drew has a good point. Engines that need overlap in the camshaft, need to deflate the cyl pressure to more of a likable, tolerable range. In the end, low end torque suffers, but you gain higher rpm torque!
When a camshaft has overlap and put into a stock engine, its not good..... you will see a drop in cyl pressure, so the readings will be off by being lower at cranking speeds.
V8"s do it cause you wont run a V8 at 240 psi at 7500 rpm very long without shaking apart, or blowing the head gasket, or bearing life is not so good.

Then there are cams that COULD build cyl pressure by doing the opposite. Pushing the lobe timing to the point of building up a higher incoming air charge, by delaying the intake from opening, and the pistons are pretty much on theior way down, and then the valve opens and WHOOOSH! A more aggressive slurp off the carb, better air speed, and better fuel mixing thru out the port, and should help PULL air into the cyl and creating a higher cyl pressure reading. In a stock engine, that would be a significant gain in power, but a stroker with naturally high cyl pressures wont gain, it will hurt if they are already above 250psi, hence the reason for a camshaft with overlap.

I&T shop manual says 5 to 1 is 100 psi in the book I have, which is older than I am! ChadS

OVERLAP helps scavange the cylinder which helps with any motor to get a better charge on the intake stroke . rod length also effects the effective comp ratio. reasons to read up on static comp. vs effective comp. effective is what the engine sees running and the motor octane requirements it needs to keep from detonating.

 

[Anonymous-0]

Is that AC alive yet? ChadS

 

[buickanddeere]

As previously stated, it depends on the cam.

A long duration cam can be installed in an engine with 11 to 1 mechanical compression ratio and have for example 160psi cranking pressure.
The other wise very same engine with 8 to 1 mechanical ratio pistons and a very short duration high torque cam. May have a cranking pressure of 180psi.
Anything over 200psi cranking is going to require good fuel. In particular engines with open instead of quench combustion chambers.

 

[Anonymous-0]

15.1 ratio j@e flat tops due in from californ i a this week . wont be long after they get here.

 

[Anonymous-0]

(quoted from post at 00:08:28 04/28/09) As previously stated, it depends on the cam.

A long duration cam can be installed in an engine with 11 to 1 mechanical compression ratio and have for example 160psi cranking pressure.
The other wise very same engine with 8 to 1 mechanical ratio pistons and a very short duration high torque cam. May have a cranking pressure of 180psi.
Anything over 200psi cranking is going to require good fuel. In particular engines with open instead of quench combustion chambers.

AS previously stated rod length also has to do withit .

 

[Anonymous-0]

read john earbs article effective vs static on kb silvolites web page under tech. articles it is a verry good detailedexplanation.

 

[Anonymous-0]

Rod center to center does have effects on cyl pressure, and piston speed, which can affect how aggressive that engine can draw thru the intake and carb from changes in the piston dwell time at TDC. Same trick added with a cam with a LSA that will help build cyl pressure, not deflate, would be a good combo in a stock, or lower compression engines. Same goes with different ratio rocker arms.

Most dont have more than what it came with from the factory. You have to keep in mind that when your not changing alot of factory parts with aftermarket, some rules that V8's follow to make them run, does not work when applied to a low rpm, low compression tractor engine, and truthfully, thats what most are pulling with today, starting out. So we have to figure out the old school technology, and help build.

Tractor engines have different combustion chambers too. Some are high cyl pressure freindly, and some are not. Some combos create the perfect cyl pressure, some create too much or not enough. Its what you have in the end, and what techology works with the combo. I know I seen a WD blow the ring lands right of the pistons and LIFT the head from 320 psi with a SMALL stroker on 115 fuel. Ive seen farmalls with 300 psi and run on 105. Just food for thought,,ChadS

 

[Anonymous-0]

15 to 1 and flat tops in a wedge engine?? U sure nitro will hold the detonation? Chad

 

[Anonymous-0]

(quoted from post at 09:47:41 04/29/09) 15 to 1 and flat tops in a wedge engine?? U sure nitro will hold the detonation? Chad

OH yeah they are bbc promod pistons . they are heavy though at 1100 grams. methanol loves compression . just run enough nitro to keep the intake from freezing up and me from tearing up. its hard to drive down the track while crying.

 

[Anonymous-0]

We ran 14.2 static on 110 motor octane the last two years. no detonation troubles at all ngk -9 race plugs 32 degrees timing. the ferra pro comp. valves show no signs of being hot either . tore it down cause the stock sleeve material couldnt handle the chrome molly rings. started sucking oil back. oh yeah on a stock blue felpro head gasket and 1/2 head studs.

 

[Anonymous-0]

As long as your sleeves are thick to hold the heavy pistons in line, you should be fine. High cyl pressure, heavy piston weight, increased piston speed, should work together and be worth a few extra FT lbs of torque, from the weight of the pistons alone. Heavy pistons can help keep momentum going in a low rpm engine. Chad

 

[Anonymous-0]

(quoted from post at 07:02:12 05/01/09) As long as your sleeves are thick to hold the heavy pistons in line, you should be fine. High cyl pressure, heavy piston weight, increased piston speed, should work together and be worth a few extra FT lbs of torque, from the weight of the pistons alone. Heavy pistons can help keep momentum going in a low rpm engine. Chad

Sleeves are custom dardin . tarbil didnt have enough interference fit on the block bore to the sleeve thats why he had water promblems . my sleeves are 1/8 thick at the thinest part. I think I can cut some piston weight from the under side of the piston to get the pin and ring package around 1100 grams instead of the piston itself . 4 3/4 just a big piece of aluminum . I like throttle responce never like waiting on a engine to come up in rpms.