Combustion Contraption

Ive been seeing alot of questions in this forum regarding "theory" and i thought i would make a post to maybe get some people thinking and interesting idea's thrown around. I'll start out with a classic bench racing scenario, and you can all chime in. (Lets keep this clean and light hearted, this is meant to be educational.)

2 engines of identical displacement.

Engine A= 1997cc Bore=85mm Stroke=88mm
Engine B=1997cc Bore=80mm Stroke=99.3mm

All other variables the same (R/S ratio, valve size, cylinder head flow, etc. etc.)

What would the characteristic differences between the two engines be?

NJIN BUILDR

The hard part is keeping "everything else" the same.The same head will typically flow better on a 85mm bore than a 80mm bore (as I am sure you know).I would expect the big bore engine to make a few more hp at a couple hundred rpm higher.The torque number maybe a few pounds less but probably because it happens farther above 5252 rpm.I vote for Engine C= 2254cc Bore=85mm Stroke=99.3.

RC000E

Engine 1- 80/99.3 or 3.14"/3.91" The "stroker" of the two
Undersquare//5213.3 fpm at 8000rpm

Head related:

1) Requires less piston dome volume to achieve desired compression
2) Less dome volume, with smaller bore will require less overall ignition timing due to faster flame development.
3) More piston to valve clearance for large overlap/high lift (and/or) cams (dome volume advantage).
4) Less exposed surface area of smaller bore-faster flame development leads to greater thermal and combustion efficiency. Less strain on cooling system.
5) Better emissions also generally result of smaller bore.
6) Effects on mixture quality and homogeny from high port velocity at low rpms...advantages and disadvantages based on head port work..etc...Discuss?
7) Detonation resistance higher over larger bore

Block related:

1) Requires better oiling...higher contact speeds at rod bearings for given rpm...discuss?
2) Piston speeds require more investment in order to keep intact as peak power rpm increases.
3) Increases torque production from larger stroke, leading to easier street driveability, less street driven wear and tear.
4) Requires stronger crank with more effective dampening for given rpm
5) Windage effects of larger stroke crank....argueable maybe?

Engine 2- 85/88 or 3.35"/3.46" The "RPM" motor of the two
Motor is generally considered square with bore/stroke of .95:1

Head Related:

1) Generally allows use of larger valves with unshrouding
2) More available quench area...good and bad I suppose.
3) High peak power and peak rpm potential


Block Related:

1) Better rpm stability
2) Lower piston speed reduces outrunning the flame front potential
3) Higher VE....arugueable...discuss?
4) Less pumping loss than longer stroke...discuss?


Very difficult comparison, hence why this has been an arguement for years. It still seems feasible that with enough money, all other factors being equal (rod/stroke, etc), the square motor is gonna make more power.

I'd have to say though, that an undersquare combo has alot of potential for a cheaper cost. It seems in my opinion that an undersquare motor will accel in quarter mile vs. something square. Then came along VTC....

Consider I am on 2 hours sleep and have worked 32 hours in the past two days, so if I said anything crazy...currently at work now in fact. Luckily crime is low in the morning on Sunday...that'll all change in about an hour though. Let's get the discussion going here.

Combustion Contraption

Great replies so far! Im in the middle of doing a head so i can only say real quick, that the use of larger valves in in this comparison is out, since we want to minimize variables. Be back later!

Combustion Contraption

Maybe more people will look @ the thread if theres a picture icon next to it.

Heres the pr3 casting ive been working on today.

*unfinished*

RedEj8

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Combustion Contraption &raquo;</TD></TR><TR><TD CLASS="quote">Maybe more people will look @ the thread if theres a picture icon next to it.

Heres the pr3 casting ive been working on today.

*unfinished*

</TD></TR></TABLE>

Im to ignorant to post anything about this discussion..But it is very interesting..And that surree iis a purdy picture ur got der .

sheepey

Good work there Steve Coming out good as usual

Combustion Contraption

Thanks, Alex.

Just to jump start the original discussion, would peak power and torque be identical on the two engines? Would they come in @ similar points in the RPM band?

D-Rob

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Combustion Contraption &raquo;</TD></TR><TR><TD CLASS="quote">Thanks, Alex.

Just to jump start the original discussion, would peak power and torque be identical on the two engines? Would they come in @ similar points in the RPM band?</TD></TR></TABLE>

I believe the 'stroker' motor of the two would reach a peak at an earlier engine speed.

I believe peak power/torque would be similar to one another.

Really, I'm a novice when it comes to theory, so I'm just in for the educational experience, and I'll chime in whenever I can.

- Derek

Combustion Contraption

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by D-Rob &raquo;</TD></TR><TR><TD CLASS="quote">I believe the 'stroker' motor of the two would reach a peak at an earlier engine speed.

I believe peak power/torque would be similar to one another.

Really, I'm a novice when it comes to theory, so I'm just in for the educational experience, and I'll chime in whenever I can.

- Derek</TD></TR></TABLE>

Glad to have you . The general accepted theory is that the longer stroke motor reaches peak power sooner. Why, though, is never discussed in much detail. Anyone?

D-Rob

Could it have something to do with the fact that the longer-stroke naturally reaches peak torque earlier?

91SiZ6

Taking a shot at that one. The longer stroke will provide more time for the cylinder to fill instead of just more area to fill as well as keeping the dynamic compression ratio higher.

Combustion Contraption

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by D-Rob &raquo;</TD></TR><TR><TD CLASS="quote">Could it have something to do with the fact that the longer-stroke naturally reaches peak torque earlier?</TD></TR></TABLE>

Why would a long stroke engine reach peak torque sooner? Do you subscribe to the theory that a longer crank throw equals more "mechanical leverage"/torque?

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 91SiZ6 &raquo;</TD></TR><TR><TD CLASS="quote">Taking a shot at that one. The longer stroke will provide more time for the cylinder to fill instead of just more area to fill as well as keeping the dynamic compression ratio higher.</TD></TR></TABLE>

I think you have it a bit backwards. Assuming both engines are operating at the same RPM, the piston speed in the longer stroke engine will be greater, therefore the cylinder wont have as much time to fill.

D-Rob

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Combustion Contraption &raquo;</TD></TR><TR><TD CLASS="quote">

Why would a long stroke engine reach peak torque sooner? Do you subscribe to the theory that a longer crank throw equals more "mechanical leverage"/torque?
</TD></TR></TABLE>

Yes, I do.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Combustion Contraption &raquo;</TD></TR><TR><TD CLASS="quote">
I think you have it a bit backwards. Assuming both engines are operating at the same RPM, the piston speed in the longer stroke engine will be greater, therefore the cylinder wont have as much time to fill. </TD></TR></TABLE>

Indeed. As engine speed increases, the pistons move away from TDC too quickly, therefore hindering power at higher engine speeds... Correct me if I'm wrong.

- Derek

Runnerdown

I'm thinking the the piston speed of the long stroke motor is causing it to peak sooner. the fast moving piston has more cylinder fill at the same rpm as the short stroke motor. then at higher rpm, the head and intake become very taxed and it begins to lose effeciency.

alterdcreations

The shorter stroke engine would have a higher torque curve but a lower rpm range. maybe a good boost engine with proper gearing
hear is a pic of a h22 head i have been working on (stage 2)

Combustion Contraption

Please resize that pic, and now that this convo has taken off, lets stay on topic.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Runnerdown &raquo;</TD></TR><TR><TD CLASS="quote">I'm thinking the the piston speed of the long stroke motor is causing it to peak sooner. the fast moving piston has more cylinder fill at the same rpm as the short stroke motor. then at higher rpm, the head and intake become very taxed and it begins to lose effeciency. </TD></TR></TABLE>

One of the problems with piston speed is it gets confusing when you think about it in relation to RPM. Think piston accelleration instead and it become more obvious. I dont think the head and intake become 'taxed' per say, but i know what your alluding to. Remember as piston speed increases, so does friction, so mechanical efficiency declines as does thermal.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by D-Rob &raquo;</TD></TR><TR><TD CLASS="quote">Yes, I do.

Indeed. As engine speed increases, the pistons move away from TDC too quickly, therefore hindering power at higher engine speeds... Correct me if I'm wrong.

- Derek</TD></TR></TABLE>

In "theory" the short stroke engine is still displacing the same overall swept volume per cylinder, so when that spark ingities the air fuel mixture in each engine, MEP utop the piston crown should be the same. Using this theory, you could also theorize that the short stroke engine is more mechanically efficient due to the fact that the power stroke has less of a distance to travel while pushing the piston down the bore.

Your spot on on the second point. The piston does indeed start outrunning the flame front in a long stroke engine @ high RPM.

91SiZ6

Ok. If the stroke is longer the piston would have to move faster to complete each revolution in the same amount of time. That makes sense.

Suck my DX

good thread...ill chime in when i can...

91SiZ6

Here's a nifty article I just found;
http://findarticles.com/p/arti...14621

RC000E

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Combustion Contraption &raquo;</TD></TR><TR><TD CLASS="quote">Maybe more people will look @ the thread if theres a picture icon next to it.
</TD></TR></TABLE>

Well, that's some pretty aggressive headwork. Looks like alot of widening of the short turn, alot of bowl work, and alot of work in the splitter.

I do have a question though, you stated all things being equal. Are you saying that despite two motors with different strokes (what'chu talkin bout Willis) you WILL be able to maintain the same rod/stroke (i.e. deck riser)?

It seems to me that head is gonna favor a big stroke, unless your spinnin that square motor really fast...just takin some early guesses though...can't wait for more info. Any details on the cams, intake, and exhaust setup and specs being used?


Modified by RC000E at 6:19 PM 8/27/2006

RC000E

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by alterdcreations &raquo;</TD></TR><TR><TD CLASS="quote">The shorter stroke engine would have a higher torque curve but a lower rpm range. maybe a good boost engine with proper gearing
hear is a pic of a h22 head i have been working on (stage 2)
</TD></TR></TABLE>

I hope that's not done, the ports look pretty inconsistent. Hope it's just the camera...but being far almost makes it easier to see.

Combustion Contraption

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by RC000E &raquo;</TD></TR><TR><TD CLASS="quote">Well, that's some pretty aggressive headwork. Looks like alot of widening of the short turn, alot of bowl work, and alot of work in the splitter.

I do have a question though, you stated all things being equal. Are you saying that despite two motors with different strokes (what'chu talkin bout Willis) you WILL be able to maintain the same rod/stroke (i.e. deck riser)?

It seems to me that head is gonna favor a big stroke, unless your spinnin that square motor really fast...just takin some early guesses though...can't wait for more info. Any details on the cams, intake, and exhaust setup and specs being used?


Modified by RC000E at 6:19 PM 8/27/2006</TD></TR></TABLE>

Sometimes a picture can be decieving. As the head belongs to someone on HT, i wont divulge any specs. I will gauruntee you though, that this port has not been 'hogged' out. In fact, just to reassure customers that i dont hog out their ports, sometimes i leave about .040" of the original casting seem on the port entry. I work the port divider to help 'direct' air to each valve. Theres alot of other tricks going on in there that you cant see. The sides of the short turn have been widened a bit yes. What this does is help slow the incoming charge down a bit to assis the air in negotiating the short turn. Remember, air has mass.

Back to the topic at hand, yes each engine in this theory has an identical rod/stroke ratio. Remember, we're not actually building these 2 engines, just theorizing.

RC000E

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Combustion Contraption &raquo;</TD></TR><TR><TD CLASS="quote">I work the port divider to help 'direct' air to each valve. Theres alot of other tricks going on in there that you cant see. The sides of the short turn have been widened a bit yes. What this does is help slow the incoming charge down a bit to assis the air in negotiating the short turn. Remember, air has mass.
</TD></TR></TABLE>

I understand the concept entirely. It just looks like an aggressive head that sacrifices a little velocity for overall flow. Granted though I can't see the port from a direct view to see what center divider manipulation you've done...just the bit of notchin at the base for the right valve.

The reason I assumed a bit of extensive work in the bowl is because B16 heads notoriously have alot of core shift which is visible in the bowl. I don't see any of the shift so I can tell there was a good bit of material removed. I understand though your trying to create high pressure at the valve entry. Head looks good, just seems aggressive is all I was saying.

I thought, by your earlier statement maybe you were building both these combinations for comparison. Good thread though

Combustion Contraption

Yes, this pr3 casting had some shift, youre right, most of them do. This particular cylinder head will be going on a high RPM, high HP "B". Knowing the setup specifications lets me know where to remove material, how much and most importantly, where NOT to remove material.

Also, i wish i had the time to actually build these two hypothetical engines for testing purposes.