Can someone explain displacement to me? - LS1TECH

# Can someone explain displacement to me?

 02-17-2017, 05:11 PM #1 Launching! Thread Starter   Join Date: Aug 2015 Posts: 252 Can someone explain displacement to me? Just so I have a full understanding, if the AFR is at Lambda, is displacement the measure of how much air and gas is worked after one full engine rotation (or after four?) which means you could derive how much fuel is spent per rotation if you are at perfect AFR?
 02-17-2017, 06:08 PM #2 8 Second Club iTrader: (9)     Join Date: Dec 2003 Location: vancouver, WA Posts: 580 Displacement = bore x stroke as far as what you are getting to, yes. The PCM determines how much air is flowing in to the engine And determines how much fuel to add for a requested A/F ratio. So yes, you can do some math and determine how much fuel you need for a given displacement at a given rpm for a given a/f
 02-21-2017, 01:19 PM #3 TECH Veteran iTrader: (21)     Join Date: Nov 2001 Location: NW Florida. Land of no 1/4 mile tracks. Posts: 4,148 I think you may be confusing displacement with volumetric efficiency.
 02-21-2017, 01:49 PM #4 Launching! iTrader: (23)   Join Date: Jul 2008 Location: Cushing, OK Posts: 293 Per google: Engine displacement is the swept volume of all the pistons inside the cylinders of a reciprocating engine in a single movement from top dead centre (TDC) to bottom dead centre (BDC). It is commonly specified in cubic centimetres (cc or cm3), litres (l), or (mainly in North America) cubic inches (CID).
 02-21-2017, 04:28 PM #5 TECH Junkie iTrader: (2)     Join Date: Nov 2001 Location: Houston/Alvin, TX Posts: 3,818
 02-22-2017, 03:10 AM #6 TECH Senior Member iTrader: (2)   Join Date: May 2008 Location: South Florida Posts: 10,450 I know the more "cid" you have....the more power you will make. Thats all I know for sure. lol .
02-22-2017, 04:57 AM   #7
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Quote:
 Originally Posted by AgFormula02 Displacement = bore x stroke as far as what you are getting to, yes. The PCM determines how much air is flowing in to the engine And determines how much fuel to add for a requested A/F ratio. So yes, you can do some math and determine how much fuel you need for a given displacement at a given rpm for a given a/f
It's (pi * bore ^ 2) / 4 * stroke, you take the area of the piston, times the stroke. Volume of a cylinder is pi * radius squared, * height which is stroke. Displacement is how much volume each cylinder has, times number of cylinders. For a standard small block Chevy, bore is 4" and stroke is 3.48", so half bore is 2", squared is 4", times 3.14 is 12.56 inches squared is the area of the piston. Multiplied by height of the stroke is 12.56 * 3.48 is 43.7 inches cubed being the volume (displacement) of 1 cylinder. Multiply that by 8 cylinders is 349.6 inches cubed, which is ~ 350 cubic inches, small block Chevy. That's how much air a 350 cubic inch engine can injest. Since combustion (the bang part) requires air and fuel, the more air and fuel you can injest, the more power you can make.

The air in an engine is sucked into the cylinder, as the piston goes down on the intake stroke with the valves open and throttle body open it creates a vacuum and sucks in the air. When your throttle blades are only a tiny bit open, but you are at 2000 rpm, it can only suck in a tiny bit of air, and you do less laundry (cruise steadily at 30mph because you are only taking in a little air and therefore making small horsepower from a small bang). If you go wide open throttle, you can suck in more air since the the hole to get more air from, the throttle blades, is bigger, so you make more power and accelerate. 350 cubic inches is the ideal conditions amount of air your motor can injest, but it's restricted by the flow path. That's why adding bigger heads or a bigger cam that leaves the valves open longer makes more power, it allows more air to fill the cylinder to get you closer to the ideal 350 cubic inches of air (volumetric efficiency).

Last edited by bufmatmuslepants; 02-22-2017 at 05:22 AM.

02-23-2017, 11:19 AM   #8
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Quote:
 Originally Posted by SouthernRex Just so I have a full understanding, if the AFR is at Lambda, is displacement the measure of how much air and gas is worked after one full engine rotation (or after four?) which means you could derive how much fuel is spent per rotation if you are at perfect AFR?
Not exactly. Displacement is simply the volume of space the piston moves as it travels from TDC to BDC and vise versa. Lambda isn't based on volume, it's based on mass. The mass of oxygen consumed by the engine varies throughout the RPM range and across different atmospheric conditions. That's why sensors are used to measure it.

 02-28-2017, 11:08 PM #9 Launching! Thread Starter   Join Date: Aug 2015 Posts: 252 Thank you for the replies guys, that was super informative.
 03-01-2017, 04:40 PM #10 Teching In     Join Date: Jul 2016 Location: "U aint frum round hur ar ya boy" Texas. Posts: 36 If you dont mind rex, id like to hijack, this since it looks like you got your answers, with a pretty dumb question. What is the importance of displacement? A lot of the threads i see about making power, its all bolt ons or modding a regular 350. As it has been explained to me for my wants and needs i should go 383. But the 350 threads say they can make the same if not more power with a 350 rather than going 383. Now with stroking you get more torque yes. But my feeble mind cant find why thats important for the NOT drag racing applications. The powerband seems to be all up on the high side of the charts, tq usually following the hp line. With the 350 it starts low then climbs rapidly. Whats the importance of having that difference in powerbands. Again this is probably engine 101 stuff, but im feeling like this-
 03-01-2017, 06:32 PM #11 12 Second Club iTrader: (3)     Join Date: Mar 2007 Location: Warrenton, VA Posts: 3,051 More displacement gives more POTENTIAL for power. Same size heads though, same cam, on a 383 vs a 350, not much difference in power. Just because you have a bigger washing machine doesn't mean you are doing more laundry, you have to put more laundry in there. You CAN do more laundry with a bigger machine, that's the POTENTIAL part, but if you put 1 weeks worth of clothes in a normal washer vs 1 weeks clothes in a super jumbo washer, you still only did 1 weeks worth of laundry. Same size heads (area for air to enter through) and same cam (time valves open and lift), and you are bringing about the same amount of air into the cylinder. There's a slight difference, very tiny, due to the increased vacuum of a little more area, but really you are sucking through a straw. If you had a midget and a giant have a contest, who can inhale the most air through a McDonald's straw in 1 second, it would be about a draw since the straw is the restriction, neither gets a full breath. Give them each a paper towel roll and the giant will win since he'll finally get a full breath. A STOCK motor is restricted by a straw called emissions, noise, and fuel economy. Since we don't care about those, give your motor a paper towel roll before you make it a giant breathing through a straw. Midget with a paper towel roll gets more air (and power) than a giant breathing through a straw. Last edited by bufmatmuslepants; 03-01-2017 at 06:37 PM.
 03-01-2017, 09:11 PM #12 11 Second Club     Join Date: Jan 2010 Location: New Orleans, LA Posts: 1,498 I like how you broke that down ^^ 100%
 03-01-2017, 09:13 PM #13 11 Second Club     Join Date: Jan 2010 Location: New Orleans, LA Posts: 1,498 But 350 vs 383 giving all things are the same the 383 should make roughly 30hp more.. I don't have any proof.. Just going off word of mouth..
03-02-2017, 06:45 AM   #14
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Quote:
 Originally Posted by NewOrleansLT1 But 350 vs 383 giving all things are the same the 383 should make roughly 30hp more.. I don't have any proof.. Just going off word of mouth..
thats partly what im missing. yea if you stroke an engine but not change a damn thing else youre wasting your time. but as you say, if you built them equally, your gain might be 30 more horses. the hells the point then. the point im told is tq and the difference in powerband. fantastic. again, why would that be important. you take a 350, mod the hell out of it, then rev it up higher than the national debt and you got lots and lots of power. Stroked engines dont rev high. i have seen exceptions but in general, there not for reving. so i dont know why going through all that work to have POTENTIAL but you can get the same out of a 350.

03-02-2017, 10:13 AM   #15
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Quote:
 Originally Posted by TheUrbz thats partly what im missing. yea if you stroke an engine but not change a damn thing else youre wasting your time. but as you say, if you built them equally, your gain might be 30 more horses. the hells the point then. the point im told is tq and the difference in powerband. fantastic. again, why would that be important. you take a 350, mod the hell out of it, then rev it up higher than the national debt and you got lots and lots of power. Stroked engines dont rev high. i have seen exceptions but in general, there not for reving. so i dont know why going through all that work to have POTENTIAL but you can get the same out of a 350.
It all goes back to the formula for HORSEPOWER:

HP = TQ x RPM / 5252

The number 5252 is a constant, so the only way to make more horsepower is to increase torque and/or RPM. When you increase displacement, you increase the overall torque output, so the horsepower throughout the RPM range actually increases too.

If the camshaft and/or cylinder heads are not changed to compliment the larger displacement, the peak horsepower stays the same, but you WILL go faster because the overall power between gear shifts has increased. Driving around on the street is nicer too because you have more power available at lower RPM so you don't have to change gears and buzz up the RPM just to pass a car on the street.

The stock LS7 small block from GM has a 4" stroke crank and has a 7k RPM redline. Many 4" stroke 408ci combos turn well into the 6500-7500 RPM range in street applications. Drags racers are even turning over 8k with almost 5" of stroke in some classes (IHRA PS for example). Those aren't exceptions, that's the norm! Saying "stokers are not for reving" is one of the most false statements I've ever read!

03-02-2017, 11:36 AM   #16
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Quote:
 Originally Posted by KCS The stock LS7 small block from GM has a 4" stroke crank and has a 7k RPM redline. Many 4" stroke 408ci combos turn well into the 6500-7500 RPM range in street applications. Drags racers are even turning over 8k with almost 5" of stroke in some classes (IHRA PS for example). Those aren't exceptions, that's the norm! Saying "stokers are not for reving" is one of the most false statements I've ever read!
damn yea. i havent really looked into what their making with what now, i was just going off of some of the things ive read about keeping a 383 N/A. i tried to get into ford stuff for awhile but then i started on GM so my small view comes from reading about baseline LS/LT1's. so come to find out im the exception. preciate the info.

Quote:
 Originally Posted by KCS If the camshaft and/or cylinder heads are not changed to compliment the larger displacement, the peak horsepower stays the same, but you WILL go faster because the overall power between gear shifts has increased. Driving around on the street is nicer too because you have more power available at lower RPM so you don't have to change gears and buzz up the RPM just to pass a car on the street.
i will happily agree with anyone who will call me a damn idiot, but ima do my best to have it make sense to me. peak HP will stay the same but i will go faster. so that being said its the acceleration of the vehicle that gains when you change the displacement due to the higher availability of torque leading to the higher average beginning availability of hp when starting from a low rpm. ...did that make sense? so where is that in applicable comparison to a strung out 350? would the 350 be faster in the top end? a higher top speed?

i dont get a lot of GM experience where im at. youtubes great but its just TV basically. there is no opportunity for me to physically see or experience these comparisons. all my douche friends drive/want lifted trucks. im the only one that wants to "drag ***" as they say.

03-02-2017, 03:33 PM   #17
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Quote:
 Originally Posted by TheUrbz i will happily agree with anyone who will call me a damn idiot, but ima do my best to have it make sense to me. peak HP will stay the same but i will go faster. so that being said its the acceleration of the vehicle that gains when you change the displacement due to the higher availability of torque leading to the higher average beginning availability of hp when starting from a low rpm. ...did that make sense? so where is that in applicable comparison to a strung out 350? would the 350 be faster in the top end? a higher top speed?
Since we're on an LS site, let's compare typical LS displacements. A 450hp 383ci LS1 (3.905" bore x 4" stroke) will accelerate faster than a 450hp 347ci LS1 (3.905" bore x 3.622" stroke) because of the extra overall power, and it might be able to manage a higher top speed because you can use a numerically lower gear. I'll admit, I'm not really sure about that one though.

03-02-2017, 09:29 PM   #18
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Quote:
 Originally Posted by TheUrbz Now with stroking you get more torque yes. But my feeble mind cant find why thats important for the NOT drag racing applications. The powerband seems to be all up on the high side of the charts, tq usually following the hp line. With the 350 it starts low then climbs rapidly. Whats the importance of having that difference in powerbands. Again this is probably engine 101 stuff, but im feeling like this-
Im going to make this really simple, torque moves the car, horsepower is just a human calculation for work.

Quote:
 tq usually following the hp line.
This is backwards. An engine dyno reads torque then calculates hp from it. Torque is what is real and is what you feel, your cars acceleration follows the torque curve exactly, and gears multiply torque.
Example, why is 1st gear so much more punch than 2nd gear? Because 1st gear multiples torque more than 2nd, 2nd more than 3rd, 3rd more than 4th and so on.
And if you are making more torque at 3500RPMs than 4500RPMs, then you will accelerate harder at 3500RPMs even though hp is lower. *note* this does NOT mean you should shift at 3500rpms or if you have a CVT it should stay at 3500rpms.
Then what does hp "do"? Thats where gearing comes back in to play, its about maximizing the gearing available. Remember how 1st pulls harder than 2nd? Well you don't want to shift early and loose all that extra torque at the wheels from the previous lower gear. You don't shift a peak torque because you want to maximize what that gear for the torque you make, and peak hp tells you when the maximization is best for the gear.
Another easier way to think about it is when riding a 10 speed bicycle. 1st gear gets you going really easy (lots of torque multiplied) but you don't want to shift early as its harder to peddle then (lost some of the torque multiplier) but if you are peddling crazy fast just to maintain speed you didn't maximize that gear (hp) and should have shifted already.

03-03-2017, 07:51 AM   #19
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Quote:
 Originally Posted by JD_AMG This is backwards. An engine dyno reads torque then calculates hp from it.
i honestly could have said HP following the TQ line, thats just what i typed first.

so although the "power" is the same, the ability to lay it down is what the advantage is for stroking. so witha 350 and a 383 having the same HP, they will have the same top end, but the 383 will get to it faster, right?

03-03-2017, 03:06 PM   #20
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Quote:
 Originally Posted by bufmatmuslepants More displacement gives more POTENTIAL for power. Same size heads though, same cam, on a 383 vs a 350, not much difference in power. Just because you have a bigger washing machine doesn't mean you are doing more laundry, you have to put more laundry in there. You CAN do more laundry with a bigger machine, that's the POTENTIAL part, but if you put 1 weeks worth of clothes in a normal washer vs 1 weeks clothes in a super jumbo washer, you still only did 1 weeks worth of laundry. Same size heads (area for air to enter through) and same cam (time valves open and lift), and you are bringing about the same amount of air into the cylinder. There's a slight difference, very tiny, due to the increased vacuum of a little more area, but really you are sucking through a straw. If you had a midget and a giant have a contest, who can inhale the most air through a McDonald's straw in 1 second, it would be about a draw since the straw is the restriction, neither gets a full breath. Give them each a paper towel roll and the giant will win since he'll finally get a full breath. A STOCK motor is restricted by a straw called emissions, noise, and fuel economy. Since we don't care about those, give your motor a paper towel roll before you make it a giant breathing through a straw. Midget with a paper towel roll gets more air (and power) than a giant breathing through a straw.
BUT....I had the same top end that you see in my sig on my first LS6 stroked engine, which was a 436ci... (4.1 bore x 4.125 stroke)....basically terrible 1998 LS1 heads flowing 304cfm @ .650, terrible stock 1998 TB and ported LS6 intake. 1 7/8" LTs.

I put all of this on my new engine....390ci. My 436ci blew up.....I had a 390ci built and installed. Absolutely no other changes except the cam.

So...Only change.....different cam. The 436ci had a rather mild, stock-like, smooth idle: 242/242 .610/.610 114 lsa. Nothing big at all.

390ci has the new cam in my sig.

So, same air flow capability.....but the 436ci had A LOT more power to pull air through faster.....like being force-fed without boost. More CIs did the work on the 436ci.

Same exact 4L60E tranny, same rear-end, gears, wheels/tires.

390ci can only make 410 RWHP.

So...its apparent that if absolutely nothing changes on the air-flow potential side of things (TOP END FLOW ABILITY).......the more CIDs you have, the more power you can make through brute strength. You don't really need to have a better top end capable of more air flow to make more power.......

More powerful pistons will pull the air through faster to make more power.

It won't be as efficient as the 390ci...but more power will result with just increasing CI's.

So I guess if I understood your post correctly.......I would add that you don't necessarily have to do more laundry to make more power....you can also move the laundry through much faster to make more power also.....

I think.....Now I'm getting confused.......

.

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