[jakksincorpse]

k so my 2000 v6 has the stock 3.4 stroke.

just to clear things up is the stroke the length of the connecting rod or is it the width of the crank from the connecting rod when the combustion cycle goes thru its power stage at 90 degrees? idk which ever one all i know is the stroke plays a big role in torque output.

anywho here is my question. if my connecting rod is at (x) length and my compression ratio is 9.3:1 how much bigger of a connecting rod would i need to turn that into 11:1 and vice versa to 8:1. is there any calculation that is out there that can solve this? besides using a dyno lol.

[MineralIronHors]

Stroke is the length the piston travels in the cylinder. In order to change stroke, you need to change your crank as well, because just changing the rod wont effect it. Also, when you change compression, you need to have it tuned and will probably have to run high octane fuel, depending on how high the compression ratio is.

[green4money00]

the compression ratio changes with a change in piston design (pop up, flat top, dished...) or cylinder head combustion chamber volume

[jakksincorpse]

why would you ahve to change out the crank too? i mean it seems like you wouldnt have to do much to make the compression higher so wouldnt a longer rod push the cylinder up there higher? like a 3.4 stroke with 9.3:1 wouldnt a 3.8 stroke turn it to like 12.5:1? thats only a 5 centimeter difference. i dont think it would mess with the block or crank. but like i said idk how that works. thats just kinda what came to me with common sense.

[b1lk1]

IT cannot be generalized that you have to change the crank to change the stroke. You could theoretically change only the connecting rods for a longer or shorter stroke. You would be required to clearance everything to make sure there was no interference though.

Also, changing pistons is the least likely way to change compression. You'd get the least amount of gain that way and usually the only time you start changing piston design is when you have clearance issues.

You can't just generalize the inside of an engine, there are too many factors to take into consideration. I'd have to say that there is no method to raise or lower compression without taking in the whole rotating assembly into your thought process.

[green4money00]

visualize this: if you get rods that are 1 inch longer (just for an easy number pretend this would work) yes the piston will be 1 inch higher at TDC, but it will also be 1 inch higher at BTC. therefore you gain nothing.
compression ratio is how many times the combustion chamber volume at TDC will fit into the entire cylinder and combustion chamber combined at BTC. if you get pop up pistons you create a smaller combustion chamber therefore increasing your compression ratio. if you change out your heads for ones with smaller chambers or even if you have stock heads machined it will change your compression ratio. you change out your crankshaft to increase the stroke. the stroke isnt how long your rods are, it is how far the crank is pushing the rods.

[shinerstang]

Originally Posted by jakksincorpse

why would you ahve to change out the crank too? i mean it seems like you wouldnt have to do much to make the compression higher so wouldnt a longer rod push the cylinder up there higher? like a 3.4 stroke with 9.3:1 wouldnt a 3.8 stroke turn it to like 12.5:1? thats only a 5 centimeter difference. i dont think it would mess with the block or crank. but like i said idk how that works. thats just kinda what came to me with common sense.

Weren't you the guy who told us that you just needed to change your timing to increase/decrease compression and that we didn't know what we were talking about?

[b1lk1]

Originally Posted by green4money00

visualize this: if you get rods that are 1 inch longer (just for an easy number pretend this would work) yes the piston will be 1 inch higher at TDC, but it will also be 1 inch higher at BTC. therefore you gain nothing.
compression ratio is how many times the combustion chamber volume at TDC will fit into the entire cylinder and combustion chamber combined at BTC. if you get pop up pistons you create a smaller combustion chamber therefore increasing your compression ratio. if you change out your heads for ones with smaller chambers or even if you have stock heads machined it will change your compression ratio. you change out your crankshaft to increase the stroke. the stroke isnt how long your rods are, it is how far the crank is pushing the rods.

Changing the stroke on the crankshaft it would accomplish the exact same thing as changing the length of the rods.

[jakksincorpse]

wow i had a huge freakin explaination for all this but it signed me off right when i finished and lost all this.

anywho this is what i said

ignition timing does play a role in compression cuz that number is just a number at tdc, what really counts is when the plug fires after tdc cuz if it fires right on tdc you'd just detonate your pistons.
so obviously if all that air was measured at tdc and your plugs fire at 20 degrees after tdc the distance for your pistons and valves would be greater than tdc nonetheless creating less actual compression.
i didnt say timing was everything

for bl1 and green money. we accomplished that stroke was how far the piston moved in the cylinder right? so yes changing your rod would not change the ratio it would just move the piston closer to the valves, if you have the right clearence from the block. cuz compression ratio is how much air you can squeeze in tdc (or your firing degree if you want to get technical) vs. how much air is in the cylinder at the bottom of the intake stroke. so in actuality the piston would indeed be closer or farther from the valves but you would need to force more air into the chamber to increase the ratio

this is kinda what i was getting at because my stang is turbo'd and is at 9.3 if i got shorter rods the pistons would be farther from valves, yet you need a low ratio to begin with so by the time all that forced air comes in you wont be at 14:1 ratio and explode your heads. 9.3 ratio is perfect for what i'm running now i was just curious as to changin my rods so my rpm can be higher without all the wear and tear a longer rod would get.

actually if i'm forcing air in like that and now my ratio is around 11.7:1 with my piston at that distance from the valves, wouldnt it be better if i put shorter rods on to where my ratio will be lower but then boost more psi? right now i have the stock jectors and pump only pushing 5 psi.

it seems like i'd get more rpm and have the same hp if i match that ratio with the shorter rods=]

and this is a question for shinerstang cuz we have history about this little turbo dispute. i'm looking at a 255walboro pump is that strong enough for pushing 10psi? and what injectors would i need for that. i was thinking 42's but idk i remember you said somethin about you pushing 11psi and had 72's i think??

[jakksincorpse]

mineralironhors i guess another reason why i stated this was because i didnt want to lose torque if what i thought was actually true. thanks for clearing up stroke meaning its just the distance of how far the piston travels.

like i said i thought it was that multiplier effect that the crank and rods play roles for. which for a NA vehicle changing the crank is basically all you need to do to change the stroke, not adding in more air flow, piston design, ect..

if torque is really measured upon the force of the piston X the length of the crank arm for the connecting rod, how do you measure the force that your piston is supporting?

this is gonna sound gay but i remember working on a volkswagen 4 cyl in shop class and we had to measure the crank journals and thier length, they were roughly around 3.2 inches long and the motor had 110 ft of basic torque, so my calc says each piston had 34.4lb of force per power stroke. is this correct?? sorry i just wanna know everything about motors cuz i think talk is cheap, after markets are over paid, and only true knowledge is gonna help you in the long run.

[b1lk1]

You do not and cannot change compression ratios with ignition timing. It simply does not work that way. The compression ratio is the same no matter when the spark plug fires. Changing pistons, piston rods, crankshaft throw, headgasket thickness, planing the heads, etc... can and will change the compression ratio. Compression is a mechanical value and ignition timing absolutely and positively can never, even for a split second, ever change actual mechanical compression ratio.

Your math is also wrong. If you divide 110 by 4 you get 27.5. I am not certain how you could mathmatically measure each cylnders power output since the overall torque/horsepower numbers are of all cylinders working together. Many times when it comes to engines, it is not as simple as diving the total output by the number of cylinders. In theory it is possible, but I would bet money that if you could sever on cylinder off that engine and run it alone the number would not be 1/4 of the total of the entire 4, it would almost certainly be less.

Lower compression ratios are beneficial to supercharged/turbo engines as well.

[jakksincorpse]

Originally Posted by b1lk1

You do not and cannot change compression ratios with ignition timing. It simply does not work that way. The compression ratio is the same no matter when the spark plug fires. Changing pistons, piston rods, crankshaft throw, headgasket thickness, planing the heads, etc... can and will change the compression ratio. Compression is a mechanical value and ignition timing absolutely and positively can never, even for a split second, ever change actual mechanical compression ratio.

Your math is also wrong. If you divide 110 by 4 you get 27.5. I am not certain how you could mathmatically measure each cylnders power output since the overall torque/horsepower numbers are of all cylinders working together. Many times when it comes to engines, it is not as simple as diving the total output by the number of cylinders. In theory it is possible, but I would bet money that if you could sever on cylinder off that engine and run it alone the number would not be 1/4 of the total of the entire 4, it would almost certainly be less.

Lower compression ratios are beneficial to supercharged/turbo engines as well.

you dont measure the ft lb of torque per cylinder you measure it by the crank throw.

think of it this way. you have 4 cylinders. you got that right so far. but correct me if i'm wrong but each cylinder doesnt fire at the same time,which means that the motor i was stating in your mathmatical equation only has 27.5 ft/lb torque.

obviously you wont get the right power output of each cylinder cuz your measuring the cylinder itself. not what the power is transferred to. aka. the crank throw. what you put up there is basically a generalization of how much power is coming out of each cylinder. which is what i use to think when i was younger but now realize that it doesnt prove anything.

you cant use what you said also because the piston doesnt go directly down on the crank. the pistons power doesnt travel at a vertical line into the crank, its on that throw so its power is mulitplied

which is why i said 110tq/3.2in throw theoretically should equal 34.4lb's of force to the cylinder head per power stroke.

the 34.4 isnt a measure of actual torque in the motor its pressure being applied to the piston. which is what i want to know if thats how you measure it.

[green4money00]

Originally Posted by b1lk1

Changing the stroke on the crankshaft it would accomplish the exact same thing as changing the length of the rods.

those are two different things. like i said, one inch higher at tdc is one in higher at bdc. if you use stock rods and a stroker crank the piston is going up higher at tdc and down lower at bdc

[b1lk1]

Originally Posted by green4money00

those are two different things. like i said, one inch higher at tdc is one in higher at bdc. if you use stock rods and a stroker crank the piston is going up higher at tdc and down lower at bdc

Actually, if I had used my brain I would have seen that you are absolutely correct on this point. Although I do believe longer connecting rods alone would still increase compression since it would cause the combustion chamber to be smaller, I also believe it would almost certainly cause piston to valve clearance issues as well. Thanks for pointing that out.

[green4money00]

Originally Posted by b1lk1

Actually, if I had used my brain I would have seen that you are absolutely correct on this point. Although I do believe longer connecting rods alone would still increase compression since it would cause the combustion chamber to be smaller, I also believe it would almost certainly cause piston to valve clearance issues as well. THanks for point that out.

after i thought about it more i do agree with you that longer rods will increase compression ratio. but i have never heard of anything changing the rods to do so. i always hear of people changing the pistons and and heads. i know they change the rods out for stronger ones in forced induction and racing apps.