Un-stroked vs Stroked Displacement vs. Crank Angle, etc.

[nm9stheham]

This is just some 'FWIW' info that might be of mild interest to this pretty smart/active group of gearheads. I've read that different strokes and rod lengths change the angles at which peak displacement occur, etc., and that might be a factor in cam profiles, lobe angles, etc. So some spreadsheet work was used to compute piston speeds and displacement versus crank angle (both actual displacement and % of total displacement) for 3 different engines:

• 340 SBM
• 350 SBC
• A 273 SBM stroked 4" to 333 ci (which is why this all started....)

NOTE: All crank angles are with 0 degrees being at TDC.

This 1st is graph is for actual piston speeds on the intake stroke, in feet per second at 6000 RPM:

• Grey line is a standard 340 SBM rod and stroke: 3.31” stroke and 6.123” rod.
• Blue Line s a standard 350 Chevy SBC: 3.48” stoke and 5.7” rod.
• Red line is stroked out to 4.000” with a 5.95” rod length. (273 stroked out to approximately 333 ci.)

The 2nd graph is % of displacement versus crank angle; the data is expanded around the 50% level for clarity:

• Light green line is a standard 340 SBM rod and stroke: 3.31” stroke and 6.123”
• Orange line is a standard 350 Chevy SBC: 3.48” stoke and 5.7”
• Blue line is SBM stroked out to 4.000” with a 5.95” rod length. (273 stroked out to approximately 333 ci.) Note that this line closely represents any 4" stroked SBM, even with a 6.123" rod length.

This 3rd graph is actual displacement rate, in c.i. versus every 2 degrees of crank angle. This is with all 3 engine equalized to 340 ci by adjusting bore, to make the difference in the angle of the peak displacement rate to be easier to see. (The 'per 2 degrees of crank angle' is just the way it was computed. If you want to know it per 1 degree of crank angle, just divide the vertical axis numbers by 2.)

• Light blue line is a standard 340 SBM rod and stroke: 3.31” stroke and 6.123”; bore = 4.04"
• Red line is a standard 350 Chevy SBC: 3.48” stoke and 5.7”; bore = 3.94"
• Light green line is SBM stroked out to 4.000” with a 5.95” rod length; bore = 3.675". A 340 or 360 stroker graph will have the same shape and peak angle, but the peak will be about 20% higher.

Initial impressions: IMHO.... the last 2 graphs show that there is not a lot of difference in the angles at which peak displacement occurs. There is only about a 2+ degree spread in the displacements vs. the crank angles across these 3 stroke & rod combinations. So it seems that, for the range of common small block V8 designs from the 50's to 80's, there would not much changing in the cam lobe angles. You might want to use a 2 degree wider LSA for a 4" stroker SBM, or so it seems. But, hey, there could be more to this than I realize.....

Edit to add: Ooops... I reversed the intake and exhaust strokes in my brain! The effect would seem to be to NARROW the LSA with the 4" stroker, not widen it. The peak displacement rate on the exhaust would occur a bit later and the peak rate on the intake occurs a bit earlier.

 

[RustyRatRod]

Pretty interesting. And not to say your info is not useful.......it certainly is and thanks for posting........but to me this further shows that rod length has a minimum impact on what an engine does. You'll never feel the difference on the seat of the pants......or see it on a dyno other than perhaps a different curve.

 

[justinp61]

Just out of curosity, why a 5.95" rod in the little stroker?

 

[QuickDart360]

I always have been curious about more or less stroke motors. An example my dad has a 1976 318 ramcharger. I have a 360 dodge dart sport. His truck seems to rev up quicker and easier sort like a 340 (sadly I have not had the pleasure of driving a 340 car ). My car feels very different as compared to the truck. Hard to explain but that is my experience.

 

[QuickDart360]

I guess what I mean is as I rev car it pulls in a much different manner but hard to put in words. Probably more snap down low but revs are not as dramatic I guess. I do enjoy both!!! Nothing like these older machines. My other car my daily is a Toyota Venza. Even though it looks kinda sporty it is just like a stock slant 6 car. It takes you from point a to b. Nothing exciting.

 

[toolmanmike]

Just out of curosity, why a 5.95" rod in the little stroker?

Yep. 273 has 6.123 rods.

 

[rumblefish360]

If your going racing for big bucks, then this stuff becomes important to you and the cars performance on a specific track for a specific reason. And there is some good in it when you start splitting hairs and such when looking for that certain something out of the engine and car.

On rod Length, it has been a debate on what length to use for the ratio sought for the engine. IIRC, MoPar likes a long rod in the drag cars. This is where the 340 becomes a favorite of the company to run. A good example is there 440 create engine that stills runs a 4.00 stroke but makes up the displacement through a huge bore size. Which also has big bennifits. Going this route, bigger bore for the 4.0 arm crank helps limit the rod angle from being to severe. This, in there opinion, is the way to go. I like it myself.

For us street bound guys, big bore from a stock block just isn’t possible. So we go to a larger stroke. 4.15. 4.25. This increases the angle and side load.
But it will give you the inches your looking for. Also, 99% of the street guys have a limited budget. Let’s be honest. You don’t have $100,000 to play with on just an engine development team. The plus side is more torque power at lower RPMs rather than higher RPM for HP. In the street, this is good.

 

[nm9stheham]

Uh-oh... I realized I made a mistake in my conclusion about the LSA. My brain was putting the exhaust stroke after the intake stroke ....WHOOPS! So, with the exhaust's peak displacement rate occurring later in the exhaust stroke, and the intake's peak displacement rate occurring earlier in the intake stroke, then it seems like NARROWING the LSA by a couple of degrees the thing to do.

I've edited my conclusion in the initial post.

Pretty interesting. And not to say your info is not useful.......it certainly is and thanks for posting........but to me this further shows that rod length has a minimum impact on what an engine does. You'll never feel the difference on the seat of the pants......or see it on a dyno other than perhaps a different curve.

Yep, I cannot argue with you, based on this. All this started with some info that cams would change with different SB designs, and I just got to wondering 'how much' does it change, and it seems like not a lot.

Now as far as a complete answer for rod length effects, I don't think that has been 100% answered here. If you changed the rod length a significant part of inch, instead of the mere .4123" here, then things might start looking different. For example, the change in rod lengths for a /6 stoker design that has been around for a long time change the rod length about a full inch, and that may make more significant changes.

 

[nm9stheham]

Just out of curosity, why a 5.95" rod in the little stroker?

Good question.... I've seen some 273 stroker discussions here and no one seems to have come up with one that is easy to do. So I have been working on that. My first design used Holden pistons and shorter rods (some Eagle Chevy rods IIRC).... and this graphing was done for that initial design.

I have found some different off-the-shelf pistons now that look like they can be used for a 4" 273 stroked out to around 333 ci, and that uses the stock 6.123" rods that are minorly modified. If there is interest, I can put it out for discussion. I actually would like some feedback, in case I am missing something. But it looks pretty doable.

 

[moper]

I apologize if I'm missing something here. I even Googled it and it didn't help... What do you mean by "peak displacement"? I think I know, but I wouldn't call it that because it implies the volume of the cylinder changes - and that's not the case.

 

[rumblefish360]

I didn’t understand that completely ether.

Also, lobe separation, closer is nearly always better. However, you may not like the idle quality. Also, tail pipe smog, if that was a concern, would go up quite a bit with the unburnt fuel.

 

[Wyrmrider]

a couple of degrees here a couple of degrees there it all adds up
always a suggestion to advance a cam 4 degrees
a couple of things
The change of dwell - piston movement per crank degree around tdc long rod is slower than short rod- short rod gets the piston moving quicker- affects overlap TIME
piston movement around bdc short rod is much slower than long rod- cam timing for intake close and ex open does not make as much difference
so we get to piston flow demand around TDC , capturing power with exhaust opening- can be later with long rod and dynamic compression- more critical intake close with long rod
only a few degrees but at 4 different places
now throw in cam acceleration with the larger mopar lifter
cheers

 

[nm9stheham]

I apologize if I'm missing something here. I even Googled it and it didn't help... What do you mean by "peak displacement"? I think I know, but I wouldn't call it that because it implies the volume of the cylinder changes - and that's not the case.

No, you're not missing anything..... I just did not know what else to call it besides 'displacement rate'. It's the change in volume in the cylinder as the piston moves down..... in essence, the 'flow rate' if there were were no restrictions in the path of the flow in or out. The peak is the maximal rate of volume change, and is simply the rate of change in the piston position... better known as piston speed!

 

[Wyrmrider]

and where you want the cam open
but gas velocity keeps going up even as piston speed starts going down (after the point where the rod is tangent to the crank throw- which is before 90 degrees ATDC more like 70 short rods earlier, long rods later (gives you more time to get the valve open so you do not have to start as far BTDC- which gives more overlap)
Thanks NM

 

[greymouser7]

I didn’t understand that completely ether.

Also, lobe separation, closer is nearly always better. However, you may not like the idle quality. Also, tail pipe smog, if you that was a concern, would go up quite a bit with the unburnt fuel.

Rumble, I thought that GM guys run an LSA at 114 or greater on their LS engines (say trucks
For example). @rumblefish360

 

[rumblefish360]

Stock 340 cam is on a 114.

 

[fishy68]

Rumble, I thought that GM guys run an LSA at 114 or greater on their LS engines (say trucks
For example). @rumblefish360

I have a buddy that's into Chevy's and I noticed that too. I assumed it's because most (if not all) are computer controlled and the less overlap the better to keep the puter happy

 

[nm9stheham]

I have a buddy that's into Chevy's and I noticed that too. I assumed it's because most (if not all) are computer controlled and the less overlap the better to keep the puter happy

Yes.... reversion from a narrower LSA can really play heck with manifold pressure, and with so many ECU's using MAP as an input to judge air flow, it will mess up the ECU. Plus, 114 LSA is common on stock cams and on cams that are made for mileage and low RPM torque. And it's not that you can't make good HP on a wide LSA......but you won't reach the highest PEAK HP on that.

I wanted mileage and low RPM torque on my 351C and ended up with a 114 LSA cam..... did all I wanted (daily driving, general street hooliganism, towing, 18-19 mpg on the interstate) and still would outrun all sorts of highly cammed, 'lumpy' sounding engines. OBTW, .050" durations were 190/200.....! (Yes, I still have the cam card.) It was all in the engine breathing.... heads, intake, headers, high rocker ratios. Which I suspect is pretty much the same for an LS engine design....

 

[Wyrmrider]

nm summarized
It was all in the engine breathing.... heads, intake, headers, high rocker ratios. Which I suspect is pretty much the same for an LS engine design....
narrow lsa pumps up the mid range and hurts the top end- usually it gets the intake closed earlier all else being equal which bumps up cylinder pressure and DCR
usually a crutch with chevies cuz they have to run a too big cam to get the lift/ area under the curve they need resulting in an otherwise late intake close so they squeeze the lca and end up with big overlap- like a thumper
work with the 4 individual timing points and let lsa just be a result whatever it is

 

[dibbons]

Want to factor in a longer SBM rod (6.298") just for fun (from my 305 build)?

 

[Wyrmrider]

Going the other way is the 4 1/8 bore 400 chevy with the 3 3/4 stroke and 5.565 stock short rod or a 4 inch bore "383" chevy
In my experience the 5.7 rod worked better and a 6" rod better yet (both for rod angle and piston weight)
NM I did my grandfathers 400M with Cleveland heads- not the super big ones but the smaller ports with one size bigger valve- stock HP manifolds off something else
full size 4 door with cast iron ATX trans (the ONLY place to use Type F except in some Ford power steering) Used a similar short Crower .875 Cam rocker ratio makes a big difference with short cams- I think it was a late 60's or early 70's basically a stock rebuild as far as he could tell but really woke it up
Engine masters later showed that motor can really be made to work- Oh I put a TQ on it did not want to learn Autolite
dibbons- that rod should really cause re-evaluation of cam timing compared to say a 4" stroke and 6.125 rod or a BBM MOPAR combo

 

[moper]

No, you're not missing anything..... I just did not know what else to call it besides 'displacement rate'. It's the change in volume in the cylinder as the piston moves down..... in essence, the 'flow rate' if there were were no restrictions in the path of the flow in or out. The peak is the maximal rate of volume change, and is simply the rate of change in the piston position... better known as piston speed!

Ah, ok. Peak piston speed is not the point of peak cylinder filling - so I would advise that any conclusions brought about by simply looking at that angle will result in huge gains would be a mistake. The rate of acceleration away from TDC is a factor in camshaft design, but the angle of peak piston speed in and of itself isn't a big deal. I use mean piston speed for parts choices but that's it.
It's very well covered in Don Terrill's "The Horsepower Chain". Engine Pro also calculates it easily, although it is a simplified formula according to Don. He breaks the 4 strokes into 7 processes and I think it's a much clearer overall view. Both intake ramming and intake pumping are affected by piston speed, but peak cylinder filling is only distantly influenced by peak piston speed. It's much more complicated than that.

 

[nm9stheham]

Yes, I am pretty aware of most/all that, but glad you brought it up. I really just wanted to see 2 things when I did this:

• Max piston speed vs RPM so I could stay under the recommended max's for this stroker design
• Look at the difference in angles at which the peak speed occurred in relation to info as to cam timing of some different engines and rod length (SBM vs SBC)...to see if I could see why from this data.

Yes, flow rate really does not track piston velocity (unless it is at very, very low RPM's or cranking.) Buuut, that does bring up one thing..... there is a real difference in how the flow works between low RPM's and high RPM's.... explains why having a lot of overlap helps at the high speeds for the exhaust 'pulling' the intake in, to overcome that pesky mass that the fuel-air mixture has... but hurts at low RPM with reversion, which is the undesirable side effect of overlap. Compression ratio comes in more at the low RPM's in how well the cylinders are cleared and re-filled, where IMHO the flow moves (relatively) closer to the simple (static) model of piston position.

I'm gonna have to find your reference and read that, which I suspect accounts for the lo/hi RPM flow differences; I am sure I will learn. Many thanks for that!

 

[moper]

It's a good read and it sounds like you would get some from it. The book goes into a lot about head flow and interpreting results. It's heady... but at least it's not a veru long book...lol. Also if you don't have it, Larry Meaux's Pipemax os a great excel-based program to play with, and inexpensive to buy.