Long rod/Short rod

[Anonymous]

Much has been writen and hours spent bench racing on this subject with little agreement and no definitive answers, guess that's why it's so interesting. Smokey Yunick said for a performance engine a 1.75 r/s ratio should be the minimum. He also said "use the longest damn rods you can fit in there" (or something to that effect). And he seemed to prove it many times over. Opinions vary on the ideal r/s ratio, but it is generally accepted that the closer this number is to 2:1 the better.
My personal opinion, on large displacement stroker motors, you don't necessarily need a long rod. In fact you may produce more low speed torque with shorter rods. But in small displcement short stroke engines I believe longer is better. I tend to agree with Xecute that somewhere around 1.80 to 1.90 is ideal.
I think engine building is an art and everyone has to find their own way, sometimes with a bit of help,(like from the great people on this forum).There is no best ratio. With every different application, the combination may benefit with a different number.

 

[Anonymous]

The real theory behind long rods producing more torque has its roots in cylinder pressure vs. crank angle. The simple fact is, a slower piston speed means that there is more pressure in the cylinder for more degrees of crankshaft revolution, not only making more torque, but also making it more smoothly. Let's say that on a stock 300, the expansion of combustion gasses is all over and done with by 20*ATDC. Beyond this, there is no notable increase in chamber pressures. Not to say it becomes a vacuum, just that beyond 20*, the expansion of the gasses keeps up with the piston travel. Now, what if we first dwell at the top a little longer, and then travel away a little more slowly? I think seeing 30* of push isn't being unrealistic. On this same note, I think this could also be friendlier to the rod bearings, as the force of combustion is spread out over more degrees of rotation.

Jared

 

[aussie7mains]

OK, been all over under around through this subject, but heres some first hand experience.
Built 351 windsor v8 with 302 cleveland rods and 2v cleveland heads 4v valves 235 degree cam, went 7000rpm, had widest power band of any 351 ive driven, had rod ratio of 6.05/3.5 seemed ideal.
However a 221 with 2v head and triple webers went 6000rpm easily with 1.48 rr.
Windsor 302 with 289 rods went better than windsor with 302 rods Slightly!
250 six with 1.5 rr will go 5500 in OHC form. The only reason Ford went to longer rods in au six was to improve NVH which is where I believe most is to be gained.
IMHO it isnt worth spending any money on for street strip type engine, ie under 6500rpm.
Examples of good engines with awfull rr, BBC, 400SBC, 250-2v, 221.
None of you have discussed the relationship between airflow and rod ratio, which I beleive is more important than anything to do with side loads etc. Ive done spreadsheets to calulate piston heights with any rr etc, and the difference is SO slight it wouldnt be worth worrying about.
So, on balance, give it away for SS type use.
A7M

 

[Anonymous]

Yes, airflow is one thing I neglected to mention. Since the piston is travelling slower, it gives the intake valve more time to open before the piston reaches it's peak speed. This means that more of the a/f charge is able to be drawn into the cylinder for the same displacement and cam profile. Also, with the slower piston speeds, this gives the exhaust valve more time to close before the piston reaches TDC, so, in theory at least, you could run a more radical cam with wider lobe seperation, with no fear of the pistons hitting the valves.

Jared

 

[Anonymous]

Yeah, that's true...

Ironically, the additional dwell time at BDC helps explain why l/r engines get a benefit only at higher RPMs. At all RPMs below max torque RPM, any piston engine will get a full charge of air on each intake stroke, and hence make as much power as that particular engine is capable of making. As the RPMs rise, however, the additional dwell time of a l/r engine allows for the cylinder to completely fill with air to a slightly higher RPM than an otherwise identical engine with a s/r combo. This is a virtuous circle, because now the engine builder can install a slightly bigger cam, allowing the engine to rev yet higher with no loss of power. And so on and so forth... :roll:

 

[xctasy]

I have to agree with aussie7mains

Examples of good engines with awfull rr, BBC, 400SBC, 250-2v, 221.
None of you have discussed the relationship between airflow and rod ratio, which I beleive is more important than anything to do with side loads etc. Ive done spreadsheets to calulate piston heights with any rr etc, and the difference is SO slight it wouldnt be worth worrying about.
So, on balance, give it away for SS type use.

Unless you can find a cheap shallow piston and an equally cheap rod, you'll waste $$$ and time chasing a theoretical petty distraction. The drag racers prove that to us with 500 plus cube donks with 1.5:1 rod length to stroke ratios. Its only in enduro settings or the top end that you'll see an improvement..Grumpy Jenkins 1.75:1 favour shines through. And thers nothing you guys have raised that doesn't agree with this. The 455 Poncho has an awfull l/r but the engineers found massive potential through designing a "fault" into a stump pulling tire burning asset.

L/R engines have a *real world* advantage over their more prosaic brethren only at the top end of the usable RPM band, say, the last 1000 RPM.

, said 54Ford.

After looking at dyno runs from l/r vs s/r engines, the frictional losses are greatest at high rpm. High rpm isn't used on the street!

NVH is greatly reduced though. An Aussie long block 200 cube XE Falcon our neigbour drove was so much smoother and "revy" than our 250 XE. Idle, open road cruising, everywhere. Just needed the extra 50 cubes with a 2:1 rod ratio.

Oh, and Aussie~, was the reason the 221 I6 could rev so hard is because it had a drop forged steel crank? Ooops, did I bring that up again? Still haven't posted the pictures yet, but they are "in the pipeline" Aussie!

 

[aussie7mains]

XECUTE, no it wasnt. LOL
The reason was probably the ported 250-2v head triple webers and 40/80 waggot stick.
In those days rod ratios werent discussed or thought of, if the thing had big cubes it had to be good!
Yes I had crossflow 200ci sixes in XDs when I worked for telecom, they went well, one van I drove went over 165km/h, on the old standard fuel.
A7M [/img]

 

[Hudson Nut]

The stock Hudson 308 cubic inch Hornet has a 4.5 inch stroke, and uses an 8.125 inch rod. This works out to about a 1.777 ratio. The stroker engine in my race car uses the stock rods with a 5 inch stroke, which is a 1.6 ratio. I'm not opposed to tryiing something new, but do you think I can find any rods longer than what I already have?

 

[xctasy]

Yes, the you Americans ran 1.8:1 ratios in practically all pre 1960 passenger and truck engines. Check any workshop manual, plot the figures, and you'll see the trend changed after then. It was only when the hot rod set started looking at stroked engines that the companies started dipping down into the low 1.5:1's.

Finding rods over 7 inches is a problem unless you look a diesel engines!

A Jag 4.2 ran 7 inch rods. Hudsons are like Texas...everything really is bigger!

 

[xctasy]

Further Plans:

Okay, back on track.

I said a while back

Someone needs to do a back to back comparison with the same head, carb, exhast, ignition, camshaft and gearing.

I'm looking at building three control engines, and now realise that I may be better placed than anyone in the world to do this. We have lots of junked Ford engines which have 7.808, 8.425, or 9.469 decks within only 6 years of release to the NZ maket.

I've got acess to:-
1.a 1966 200 Log 6 with 4.71" rods (1.51:1 ratio)
2.a 1972 200 Log 6 with 6.27" rods ( 2.01:1 ratio)*.
3. a ba$+ardised 1969 221 Log 6 with a 200 crank and, if I can get some, 5.45" rods from the 2.3 HSC. (1.74:1 ratio). This needs a CNC machined 125 thou plate epoxied to the block to ensure that there are three common engines with no changes bar the rod ratio.

*(Unfortunatly, the main bearing diameter is different, which changes the internal friction).


The premise is to use one non-exotic log head type ( a control item, cc'd and blueprinted), and prove or disprove the following:-

As L/R ratio increases:-

1.Rev range will be increased by an amount of rpm.
2.The horsepower loss due to friction is much lower. All the extra power expected comes from this.
3.On engines which show a 10% reduction in rod: stroke ratio (improving it from 1.62:1 to 1.8:1) there is a 4 hp per litre increase in peak power, with no loss in torque at the any point.
4.I infer that the 200 six would show 12 hp more with the stock control cam with a 2:1 L/R verses 1.5, and rev to 500 rpm more.
5.Wear will be reduced. (12 000 mile bore wear figures)
6.There is a slight reduction in effective compression which reduces engine pre-ignition (compression figures with control cam)
7. but a slight increase in residence time (the time the piston is subjected to heating). The result is that both factors cancel out any improvement in detonation resistance.
8. Actual road use fuel economy may improve at part throttle, but increase at wide open throttle. This means the economy the driver will most likely see if he choses to use the extra power while over taking.


Before I go too far, has anyone ever done such a test, and released the knowledge as public domain. All other case studies I've seen have messed up the sysmetry or controls by changing some other dynamics (bore/stroke, chamber volume, etc)

 

[Lazy JW]

Execute,
I do know that the late Gene Berg did extensive work of this kind on air-cooled Volkswagens, but I do not know about the availability of his data. Anyone unfamiliar with Gene Berg is most definitely NOT a hard-core VW enthusiast! At one time he claimed (and could prove) to have demonstrated more power per cubic inch from a normally-aspirated gasoline fueled four cycle engine than anyone else in the world. The guy was a pure VW fanatic, very outspoken and opinionated, and tended to rub people the wrong way. His engines have the race wins in the record books to back him up too. I have his book of technical articles, very interesting reading. He goes into the subject of stroking and rod length with his usual vigor and no-nonsense approach to a controversial subject. Basically, he says that a VW needs to rev VERY high to benefit from a longer rod, and it must have the headwork, cam, and carburetion to support that much rpm, and that low speed toque will suffer as a result.
Joe

 

[Anonymous]

X,

The engine changes I mention above meet your broad guidelines. When we jumped from 232 hp to the 240 range, that came from changing just the rods (to increase the rod ratio from 1.8 to 2.0) and pistons (same CR but higher pin height). All our gains came above 9000 RPMs, and there were no torque losses.

Since then we have switched to a 2 kg lighter crankshaft, re-ported the head, added 1 mm to the intake valves, added a few degrees of duration on the intake side, and gone to shorter header primary lengths. Hosspressure is up to 252 in the engine's latest iteration.

Still no changes to the engine's performance lower than about 9000 RPMs, but lots above that, and raised the red line to 10,500.

Hope that helps...

 

[addo]

The big question, Stan, is will your team win this year?

2kg off the crank? Wow, you wonder what it was doing there in the first place!

 

[Anonymous]

Those piston changes. What was the difference in weight between the original pistons and the lower CH pistons?


-=Whittey=-

 

[xctasy]

54Ford

You don't have to share all your secretes, racers can read too but...

Let me undertand Stan, the stock rod ratio for a 4AGE is 1.585:1. 81 X 77mm, 122 mm, yeah?

So your race engine started with rods longer than the stock 4.80" ones, say 5.45"?

Then you got some even longer ones, say 5.75".

And you got a 3.5% power boost for a 5.6% improvement in L/R at that 1.8 to 1.9 level.

Am I on the right track?

For our sixes, going from 1.5 to 2.0 should, be wild extrapolation, give a 21% boost with a 33% improvement in L/R, all at the rpm peak, and all optimised?


I think the block depth is 7.7", from memory. So a for the 4.8" rod, 1.38 tall piston, a for the 5.45" rod, 0.78" piston, and for the 5.75" rod, a 0.43" compression height doesn't sound possible?

Or do you use the taller 7AGE block?

I'm really excited, as I've got an engine like that that I could cheaply test this out on. One Toyota 4-cyl engine to play with is much cheaper than three I6's.

 

[Anonymous]

Wow..I go to bed for a few hours, and all sorts of stuff happens!

Addo, the proof is in the pudding. We'll see how much we can duplicate from last season. We are certainly going to give it our all.

Whittey, sorry I honestly don't recall the weight difference, but would guess at least 30 grams.

Xecute, I apologize for getting my numbers wrong last night. We were able to bump the rods from 122 mm to 140 mm (increased the rod ratio from 1.58 to 1.82). This was a three-way program with our engine builder for design, JE for the slugs, and Carillo for the rods. The block is a 4agze competition block direct from TRD. The weight from the crank came mainly from boring the journals (not permitted in CART Atlantic motors) and the lighter counterweights due to reduced net reciprocating weight. The old crank was a CART-spec model (that's good for 1000 hp according to my friends at TRD).

I can't really extrapolate to your I-6 engines, since I don't know anything about them. But my engine is an all-out racing grenade that is rebuilt every 800-1000 miles. This program might have limited utility for a street driven engine... :roll:

 

[xctasy]

Ah, good on you Stan!

That has answered a great deal buddy.

At least 3.5% extra power, from a 15% improvement in L/R if the engine is screaming.

On a Falcon six,
200 cube log, 4.71" rod, 3.126" stroke, 1.507,
221 ME or SP, 5.14" rod, 3.460" stroke, 1.486
US and Oz 250 Log and 2V all had 5.88" rods, 3.91 stroke, 1.504:1 L/R's.
as did 4.1 X-flows
and so too did 3.9 and 4.0 EA-EF OHC's

Infering from your data, shoving in a hypethetical rod 1.148 times longer (5.40,5.9, 6.75" replacing the 4.71, 5.14 and 5.88"),

then you'd get a 3.5% improvement in top end power.

Your 93 net hp at the flywheel 200 goes to 96 hp.
Your 135 net hp at the flywheel 221 goes to 140 hp.
Your 100 net hp at the flywheel 250 goes to 103.5 hp
Your 162 net hp at the flywheel 4.1 X-flow goes to 168 hp.
Your 221 net hp at the flywheel 4.0 XR6 SOHC goes to 229 hp.

In practice, it will be more than that as your engine is right on the limit, Stan. Mighty mouse it is!

 

[Anonymous]

[No message]

 

[StrangeRanger]

Given that long rod engines have more near-dwell time at TDC, doesn't it make sense that they would show their greatest gains only at those RPMs where the short rod dwell time was marginal to inadequate?

 

[Lazy JW]

If the short rod motor was already optimized, it's entirely possible you would lose power until you figured out the new cam, jetting, spark, plenum volume, etc.

That is pretty much what Gene Berg found with the VW engines. The long rod engines made more power, but the rest of the induction system had to be optimised for the extreme rpm range, and those big cams and carbs aren't conducive to low rpm torque in a VW.
Joe