Squishy Rod Ratios

So as I wait on my head to be machined, I've started to turn my thoughts towards what I'd like to do with the block. So the items that I'm looking to address in this game are:
1) Rod/Stroke Ratio
2) Rod Angle
3) Squish

Now, the first 2 really go hand in hand and can probably be considered one item. But for arguments sake I'm going to leave them separate. My end goal is to increase the Rod/Stroke ratio, decrease the Rod Angle, and shoot for a nominal 0.040" squish band.

To start things off, I created a tool that I can use to plug and play different aspects of my engine build to see what the expected results will be. This script reads a text file input so I can save different configurations off. Right now, it just spits the output onto the tool workspace. In this case, I'm using Matlab but this should also run in other math programs such as Octave with no modifications to the syntax.

I ran the numbers on the stock 200. For this exercise, let's assume we're using a stock large log head with the 62cc chambers and a coreteco gasket with a 0.44 compression height. This is where the "average" refreshed engine would be without machine work. Now bear with me on the cam specs. I haven't completely validated those numbers, so the DCR could be off. The output looks like this:

================================================
INPUT - BUILD PARAMETERS
================================================
Block Specifications:
Cylinder Bore = 3.68 in
Rod Length = 4.715 in
Stroke = 3.126 in
Piston Dish Volume = 6.5 cc
Piston Compression Height = 1.511 in
Cylinder Head Specifications:
Combustion Chamber Volume = 62 cc
Gasket Diameter = 3.81 in
Gasket Crush Height = 0.044 in
Camshaft Specifications:
Intake Duration = 252 deg
Lobe Separation Angle = 102 deg
Ground-in Advance = 0 deg
Camshaft Installation Advance = 0 deg
Resulting Intake Closing = 48 deg
================================================
OUTPUT
================================================
Engine Displacement = 200 in^3
Rod to Stroke Ratio = 1.51:1
Rod Angle = 19.36 deg
Resulting Deck Height = 0.019 in
Squish Distance = 0.063 in
Static Compression Ratio = 7.80:1
Dynamic Compression Ratio = 6.99:1

So what I'm doing right now is attempting to identify a tradespace where I look at available rod lengths and compare to available pistons. The critical parameters I have to take into account are:
1) Cylinder Bore: 3.68" - 3.72"
2) Piston Compression Height
3) Piston Pin Diameter
4) Rod Length
5) Rod Width
6) Big End Diameter
7) Small End Diameter

One of the references I've been looking at is this: http://victorylibrary.com/mopar/rod-tech-c.htm

Based upon this, I'm trying to creep up to the "Best breathing with small ports" line. Obviously, we're not going to get there with these short blocks. It looks like 18* might be the limit to getting there with COTS parts. So far I've gotten to 17.9* with a 1.63:1 ratio, but it blew my squish up to ~0.10, which is completely unacceptable. I could mill the deck to clear that up some but it won't get me where I want to go.

Obviously I could just go the route where I throw money at it and make it work with custom parts, but that's not going to happen.

So based upon all this, am I wasting my time or does this avenue have some merit?

I am all kinds of lost on what you are talking about lol
But I am trying to learn.

There's a whole mess of information here. My problem is I read stuff, then start trying to figure out how I can do that stuff.

http://www.strokerengine.com/RodStroke.html
http://victorylibrary.com/mopar/rod-tech-c.htm
http://www.hotrod.com/how-to/engine/131 ... ch-height/
http://www.superstreetonline.com/how-to ... oke-ratio/

That being said, "readin ain't doin" and just because it works out on paper doesn't mean it makes sense from a practical application standpoint. I have basically zero/nil/nada real-world experience with this stuff. So that's why I'm lookin for some advice in this respect. Realizing of course that I might be dipping into someone else's secret sauce so I'm not necessarily expecting precise responses. I'd rather not walk into my machinists shop with a build and a stack of parts and be laughed out of the building because I'm doing something stupid....

Ok. I see what you are doing now. Personally I am not messing with my rods at all, mainly for budget reasons.
I do the same thing with reading, but have learned to not over think it.

I think your best bet after doing some reading is focus more on a good quench area and improve breathing.

Well, i'm already doing a port and polish job, installing larger back cut valves, and doing a direct mount for a 1.14 Autolite 2100. Not much mure I can do for better intake other than a multicarb setup. When I read that a larger rod to stroke ratio can help engines with small ports, it got me thinking. I think I'm really close to a solution, but then i reckon that if I couldn't find someone else having succeeding at this, then I must be missing something.

I figure from your math with a 1.63 rod ratio you are using a 302 length rod to get that number. The problem with this combo length-wise is that you end up with a really short compression height piston which is undesirable for long term dependability for several reason. Because of the 200's wide B/E rod width, its going to be hard to find another rod that short and wide to allow an easy swap. Custom may be the only route to find a happy medium for rod length and good compression height.

Yeah. I was looking at the 302 rods. I assumed that the smaller 6 would have a smaller rod width and I could just have them turned down. Looks like I'll have to dig some more. It's interesting research anyway. I ran the numbers with a hypothetical piston with a 1.155 compression height. This zeros the deck (maybe 5 though above, I don't recall offhand) and gets me in that sweet spot for squish. What would be considered a "safe"compression height? I've read some conflicting info on that front...

That being said, am I at least walking down the right path in terms of how I'm laying out the problem? I realize thjs doesn't take into account the crank balance, but so far this has made sense to me from a really high level.

kind of depends on your planed use but .030 is about as tight as you would dare go on quench with a steel rod engine and .035 is pretty safe. To accomplish that you could offset grind the crankshaft to get the piston deck height another .025 higher (piston would only be .006 out the hole) that would fix the quench height to .038 with the Cortex head gasket and the stroke increases would give you a bonus of slightly more Displacement. Though this would also change your Rod Ratio, and Rod Angle a little bit in the wrong direction but it may not matter all that much unless your planing to twist it too really high RPM's. If you are building more of a race motor then you might might be able to have the stock rods bushed on the small end and when they are being rebuilt (don't forget to use a set of ARP Rod bolts too) you could gain a little bit of length that way as well as getting them all the exact same lenght, and with full floating pins, too. Good luck in your quest

If you want go crazy you could use the smaller honda pin. It will give you more room for a better ring package ,shorter CH, lighter piston and more room to offset bush the rod. You may think that will not hold up ,they do it in 1400 hp at 11000 rpm pro stock and I believe cup engines also.

Played this game with the 200 and 250 for better rod ratios.
Here are some rod/piston combos I've come up with for the 200:

Fiat Punto 1.4L rod: Between Centers – 128.50mm (5.059"), Big end – 48.63mm (1.914"), Small end – 22.00mm (0.866"), B.E Width – 25.40mm (1.0").
2000-2002 Chrysler 3.3L V6 flat top piston +1mm: Bore - 94mm (3.701"), CH - 1.167", Pin Dia - 0.901"
The rod journals on the 200 crank would have to be ground down to accept the Fiat rods, and the small ends of the Fiat rods would have to be enlarged for the Chrysler pistons.
This combo would yield 202cid, a rod ratio of 1.618, and a piston recess of 0.013"

Ford Pinto Custom 2.0L rod: Between Centers – 5.050", Big end – 2.2486", Small end – 23.17mm (0.912"), B.E Width – 25.40mm (1.0").
2000-2002 Chrysler 3.3L V6 flat top piston +1mm: See Above.
The Pinto rods have the same size journals as the 200, the pin hole in the Chrysler pistons would have to be enlarged (or bush the Pinto rods).
This combo would yield 202cid, a rod ratio of 1.615, and a piston recess of 0.022"

Ford Pinto Stock 2.0L rod: Between Centers – 4.977", Big end – 2.2486", Small end – 23.17mm (0.912"), B.E Width – 25.40mm (1.0").
SBC 305 standard piston: Bore - 3.736", CH - 1.261", Pin Dia - 0.9272", Piston Recess - 5cc.
The small ends of the Pinto rods would have to be enlarged to accept the Chevy pistons.
This combo yields 205cid, a rod ratio of 1.592, and a piston recess of 0.007".

All these combos use readily available off-the-shelf parts (even the custom Pinto rods).

I'll wait for a later date to talk about rod/piston combos for the 250 using VW Jetta rods with SBC 334 pistons, or SBC rods with SBC 334 pistons, or 300 I6 rods with Chrysler 3.3 V6 pistons yielding rod ratios of 1.585-1.590.

Hope this helps.

Dan, thanks good info.

Pinto big end dimensions corrected.

Part of the reason that engines like this have those kinds of stock rod ratios is to actually help breathing thru wheezy cylinders heads.

If you look at the angular acceleration of the rod and piston, you'll see that those short rods cause the piston to approach and depart TDC much faster than a long rod. A low lift, short duration cam is better suited for that dynamic, especially when higher lifts only net you a stalled airflow thru a restricted port. Quench is less a factor because the piston dwell time at TDC is shorter so the time for quench to occur is less.

Considering the original design parameters of low speed torque and limited rpm it makes perfect sense. The Ford engineers had some method to their madness in that regard.

Sounds like the perfect reason for me to start making some overlay plots that show piston velocity as a function of crank angle. The overlays will be with different rod ratios. I like maths...

And this is exactly the info I need to do it!

https://en.wikipedia.org/wiki/Piston_motion_equations

Frankly I think they did what was cheap . Stylist wanted a low hood line ,compact block . 144 had a 1.94 rr .People wanted more power so they stroked it twice resulting in the 1.5 rr. That is just my opinion.

The 200 is the maximum displacement possible with the original designed block, and keeping the rod/stroke ratio above 1.5.
The 200 is a very compact block allowing for a lower hoodline.

When Ford designed the 250, they wanted to keep as many common parts with the 200 as possible (head, pistons, distributor, cam). They couldn't increase the bore, so they had to increase the stroke. Increasing the stroke meant the deck height had to be 0.392" taller than the 200 to accommodate the additional crank throw. Ford wanted to keep the rod/stroke ratio above 1.5, so the rod length went from 4.715" to 5.88" (1.165" difference). Since Ford wanted to use the same pistons and head as the 200 the decision was made to increase the deck height an additional 0.104" to keep the CR down. So adding up all of these numbers (0.392+1.165+0.104) to the original 7.808 deck height of the 200 gives a 9.469"deck height to the 250. Increasing the stroke also caused the relocation of the cam in the 250.

Look at all Ford had to do to the 200 block just to get 50 additional cubes.

Ford of Australia took a different approach by designing the 250, then destroking it to get their 200.
The downside of doing this is the loss of the compactness of the US 200.

So I got all the geometry stuff plotted out for 3 different rod ratios: stock, 2.0L, and 302. The results are interesting. There are definite improvement to piston velocity/acceleration through the mid-stroke for the longer rod ratio. I would translate that to better vacuum on intake push on the exhaust. I don't know that I'm convinced of significant low- lift improvement on the intake side for the lower ratio. I would interpret the data as a broader power/torque band. I'll try to share the data tomorrow.