Chrome355z

iTrader: (1)

Hey all,

I was bored and reading this thread and it caught my attention.

I laugh at the people here that say because it has a bigger stroke it will make more torque period... in a static world that would possibly be true.

Now I know NOTHING about engine design. I'm a degree'd engineer from Purdue, but specialized and work in the Hydraulic industry so i'm not going to sit here and claim I know jack **** about engine design. But I thought I would throw out a few things as I have recently taken a interest in the field: (and wish I would have taken those engine design classes... )

1) At the instantaneous combustion "time", it doesn't matter if you have a 3.48" stroke or a 5' stroke; as you technically have no leverage at this point. If you have a given cylinder bore the downward force will be the same no matter the stroke. But in the example of a "fixed" displacement the larger bore will have a larger force at that "moment" then the smaller bore.

2) As the cylinder travels down the bore the pressure will decrease and the force exerted on the piston will decrease as well. But the larger piston will still have a greater force then the smaller piston.

3) As the cylinder travels down the bore the longer stroke engine will gain more "leverage" in a larger moment arm then the smaller stroke.

4) More of an interesting point of conversation... at the point of the next cylinder firing (90 deg); the moment arm of the "stroke" will be at it's greatest point even though the pressure in the cylinder is at its lowest. How does this torque compare to the cylinder that is firing at that exact same moment?

5) What is the frictional losses of moving the piston faster down the bore compare to a slower speed?

To these answers I will admit I have NO CLUE. I would have to do countless hours of research, testing, etc... to even give somewhat of a "educated" estimate. I just thought I would throw some of these out there to people that believe this is a cut and dry "longer stroke=more torque" static type of situation. This is a VERY dynamic system, and not just dynamic on one front; but multiple fronts.

So I suggest listening to the people that have the knowledge AND the experience in giving the best answer for this type of a question as this is nothing that can be even remotely explained on an internet message board.

1 FMF

iTrader: (1)

few things that caught my attention reading this,

#1 454cid 4.125" bore 4.250" stroke
#2 454cid 4.185" bore 4.125" stroke
#3 422cid 4.030" bore 4.125" stroke iron block more tq/hp than #4
#4 428cid 4.125" bore 4.000" stroke alum block

a#1) the comparison made between the 2 engines 422 and 428 cid. the main focus was on bore & stroke and a general statement of everything else was the same was made. Everything else was not the same... the cam specs were never mentioned and only the statement was made that they were both in the 230's duration range.

b#2) And i'm assuming it was a chasis dyno giving the output numbers of the 422 vs the 428 since you mentioned both had M6 transmissions. Biggest problem I have with going off any kind of dyno numbers is they are not accurate and almost never comparable, there are too many variables never accounted for, oil temps can be a big factor and they are never monitored, no mention was made to when the dyno's of the 2 motors happened, if they weren't back to back then what was the temperature, humidity, barometer difference? Was the 428 done in the afternoon, the 422 in the evening after the temperature dropped?

c#3) the engine tune get's it's own line, the tune was never mentioned and do you know what the a/f ratio was for each engine? At this point I simply can't put any faith in your claim the 422 (greater stroke) made more power than the 428, and I certainly would not draw the conclusion the stroke was the cause of the extra power !

d#4) disputing Mr. Reid and Making Stroking Power page 8, yeah theoretically there's an increase in leverage so you can say a greater torque or twist. Does he bother to do the math and show that a 1/8" increase in stroke is only 1/16" an increase in moment arm? tq = f * d means it's a linear relationship, on a 4" stroke vs a 4.125" stroke that's a moment arm change of 2" vs 2.0625", only 3.125% and 3% of 400 lb-ft is only 12 lb-ft ! sad, i don't think i'll be buying that book, this just goes to show how nobody thinks anymore. In fact, if you do the math comparing this change to the change is piston surface area (bore change) you will find another linear relationship happening: pressure (pounds per square inch). Using a 4" bore and 4" stroke, going up X" in stroke and decreasing same X" of bore (neglecting actual piston surface area) they cancel out, how about that! Hmm, maybe other things are happening influencing power output and it's not just about stroke and bore?

e#5) the engine with the longer stroke will end up making more power because you have the same amount of pressure from the air/fuel being burned pushing down on the piston. completely neglects the whole point of mean effective pressure (keyword mean) because as the piston moves downward volume increases and pressure decreases, also see d#4.

f#6) the iron vs aluminum block may very well matter, I don't know how relevant it is at 500 hp levels which was stated, but if you get cylinder distortion you will definitely lose power and I think it would be more likely in an aluminum block with liners vs an iron block.

g#7) and 20 more horsepower @ 2600 rpm being a RCH over 40 lb-ft of torque is correct, however 20 lb-ft of tq @ 2600 rpm is a BCH under 10 hp. A red hair is thicker meaning greater, a blonde is thinner meaning smaller.... you ***

joecar

I'm no engine builder, but we can look at this from a math/physics POV:

- always keep in mind that force = pressure x area where pressure is proportional to displacement and VE (is that right...?) and varies as combustion progresses;

- the force component along the connecting rod may be less than the force on the piston because the rod may be at an angle from vertical; when the rod is vertical, the force component along the rod equals the force on the piston; when the rod is angled, the rod's force component quickly reduces as the crank turns further from TDC;

- when the force on the piston peaks, the rod is not perpendicular with the crank arm, which means the crank moment arm is effectively shortened; when the rod is vertical, the moment arm has zero length; when the rod is angled, the effective moment arm increases with crank angle, but the force component has already reduced quite a bit;

- increased stroke produces greater peak piston velocity (and mean pistion speed also) (see my example graph derived from applying calculus to crank-rod geometry), which improves VE if the heads can flow (would you agree...?); and improved VE produces greater peak combustion pressure; this would be greater than the pressure produced by displacement alone (i.e. same stroke or bigger bore).

- something to consider is that increased piston velocity means increased frictional losses since friction increases with velocity, but as you can see in the graph the increase is more substantial closer to the velocity maxima/minima (i.e. when velocity is furthest from zero crossings) and is not as substantial near TDC/BDC;

What can we conclude: torque is not influenced by crank radius moment arm [stroke] as it is by displacement [stroke x area] and VE.


(I can also graph for offset pistons, those are interesting.)




Angular velocity (omega) [rad/s] = w = 2 * pi * rpm / 60

To convert velocity [inch/rad] to velocity [inch/s] mulitply by w.
To convert acceleration [inch/rad^2] to acceleration [inch/s^2] multiply by w^2.

billc5

iTrader: (1)

You mentioned piston offset, I was wondering why it is seldom talked about?
so in your example;

if L=6.125" the 4.125x4.25" stroke, with a .25" offset

how would that compare to

L=6.125" the 4.185x4.125" stroke, no offset.

Does piston offset have any benefit for power/slap/friction, ??

joecar

Here's a graph I previously made showing what offset does to piston velocity and acceleration (+/- 0.5" is a bit extreme):





K > 0 is toward "after TDC" (i.e. same direction crank spins);
K < 0 is torward "before TDC" (i.e. toward major thrust side);

I'll make you a set of graphs tomorrow.

matt_paco2010

iTrader: (3)

Yep the biggest thing to remember is power is a function of the stroke length, bore size, RPM, and the average cylinder pressure during the combustion stroke. The longer stroke has a greater influence on the average cylinder pressure.

Also don't forget, if you double the power a motor makes you'll only increase the peak cylinder pressure by about 20% so that's some food for thought. As the piston travels downward it's still burning the fuel mixture. Timing and the type of fuel used can greatly alter these charicteristics

topend

displacement = torque /more displacement = more torque

If 2 engines have the same displacement but stroke is different , both engines will make the same torque.

tecmec

iTrader: (1)

Wow, just wow. Did you read any of the posts before yours? That statement is a little simplified, don't you think?

Old SStroker

Nope. It's pretty damn close, and summarizes things quite well.

You are a big-ender as Jonathan Swift would say. Sorry for the obscure literary reference, but I think it applies. Gulliver's Travels (big hint) is one of my favorite stories.

Jon

DanO

I agree.... but will add that displacment = torque... or basically the ability to make torque.

Bascially just because you have a certian displacement doenst mean you will make a certain torque value.. but you only have the abilty to make so much torque NA per displacemnt.

Most engine builders dont have the ability to achive maximum torque vs. displacement.. but there is a limit.. and its around 13 bar bmep.. i have seen more and i have seen less...

Old SStroker

I think it's higher than that, DanO. Current F1 NA engines have a BMEP about 15.1 bar at power peak rpm, and somewhat higher at torque peak. The real stars are the 500 cube ProStock NA engines with over 16.5 bar BMEP at power peak rpm. Even Engine Masters Challenge engines achieve about 14 bar @ 6500 almost every year.

Maximum NA BMEP (@ power peak rpm) hasn't improved a lot in the fairly recent past even as hp/L (or per cubic inch for those of us who don't think metric) continues to improve as engine folks are able to hold the BMEP at ever higher rpm. That indicates (pun intended) to me that IMEP is being improved, or at least friction and pumping losses continue to be reduced.

There is a considerable gap between the theoretical ability to produce torque (BMEP) and that actually produced by most builders.

I think we are preaching to the choir here, however. The rest aren't buying it. More's the pity.


Jon

DanO

Agreed... thats why i said i have seen more.. but more often i see less than 13bar. You are talking about engines that are well made, there arent alot of average engines above 13bar

and yes all of the advancements are trying to get that high BMEP at higher RPM.. those F1 guys always amaze me. although, personally, i wish they would go back to boosted engines... maybe drop down to 1.0L? but i digress..

anyways.. we need to keep in mind the fuels being used... many different energy contents and air fuel ratios... cant really engines if one is running nitromethane and the other is running 87 octane.. For sake of this discussion, i'd stick to 93 pump gas

blackz93

iTrader: (2)

How in the hell are we still arguing about this?!

STOCK1090

iTrader: (1)

You guys are racing calculators. Kind of like racing dynos.