25 replies to this topic

[Ray Bell]

In our Dodge project it's come nigh on to the point where ordering the rods and pistons will have to take place... and there's a quandary about what rod length to choose...

Does anyone have a url for a decent article on the subject? Bearing in mind that this isn't a drag motor and it isn't a F1 engine, and that Ben's talking about only revving it (consistently) to 6,500rpm, what figures do the 'experts' come up with?

Just for the record, the stroke is 3.31" and the original rod length is 6.125"... 1.85:1.

Ben, thinking principally about compression height and piston weight, wanted to go for a 6.3" rod, which would put the ratio up to 1.9:1.

I wonder what extra weight there is in an extra .175" of piston? Seeing as it's a custom, I'd think the only extra material is there for the longer pin bosses... any thoughts on this?

[Greg Locock]

from the point of view of L/r you are well safe, the Falcon is around 1.45, and small changes don't seem to make much difference.

[DOHCPower]

There are pro's and cons to long rod/short rod motors. with the longer rod motor the piston has more dwell time at TDC, a shorter rod will "run" the piston away faster from tdc, running away from the flame, so to say. With the kind of revs modern formula cars turn id imagine the ratio's are very overly sqaure, probably in the 2.5:1 range.

Heres a good link to a r/s ratio read.

http://victorylibrar.../rod-tech-c.htm

[Ray Bell]

I'm finding that there's an abundance of different views on the subject...

Obviously the most important thing is to have porting, valve sizes and cam timing that fits with the ratio chosen.

[McGuire]

IMO there will be no measurable difference between a 6.125" and 6.300" rod in this application, in performance or service life. Piston dwell will be very similar, and at 6500 rpm the velocities and accelerations will be virtually identical. Also, it is unlikely there will be any significant advantage in piston weight with the longer rod, as both pistons would probably come from the same forging blank.

Below are graphic depictions (approximations but accurate) of the two connecting rod lengths in question, the first for piston velocity and position, the second for acceleration and position. As you can see, the traces overlay each other almost perfectly. That is pretty much how your engine is likely to see it too -- no real difference.





It is true that some engines in some applications may respond to connecting rod length. That is, a given port volume/valve timing/rpm range combination may like a longer or shorter rod, depending. However, this is an extremely subtle property that requires intensive dyno time and development to sort out and nail down and is only worth a handful of hp anyway, and then only in a very narrow rpm range. (In fact, many engine developers are convinced it's largely mythical.) But certainly in this case, a road racing engine that will never see more than 6500 rpm, it's not worth the trip. In my view engine tuning via connecting rod length is a drag racing/oval track wrinkle that has gotten far too much attention in the greater world.

Actually, a 1.85 rod/stroke ratio is very nice geometry for an engine of this type and usage. (Better than a SB Chevy, for sure.) Personally, if I were building this engine I would adhere to the KISS principle and stay with the stock rod length, especially if the longer rod requires any compromises whatsoever in piston or ring package.

[DOHCPower]

Small block chevy is around 1.6:1 r/s ratio, 1.75:1 is ideal, anything more can potentially hurt performance.

[Fat Boy]

I'm not an engine guy, but here's my take anyhow.

Go with whatever components that you can get that are 'off the shelf' and are of good quality. It will be much more important to have a reliable and consistent engine package rather than one that is 'perfect', but too expensive to maintain.

If you're looking for places to spend money, put it in the chassis (probably the shocks). If you absolutely, positively have to spend money on the engine package, then spend more time on the dyno and on the track getting the fuel and spark mapping (carb and distributor if it's that type of engine) dead nuts on.

That's my take

[275 GTB-4]

I am also no engine guy....however...I do know that there are dangers in mixing and matching rods and pistons....

The example, the one I know, is Morris Mini 1100...

Here they used a long, heavy conrod...connected to a short light piston....the result was not really sympathetic to well balanced operation as the piston tended to slap around on top of the heavy rod....some people swear by this engine others criticise it

RIDER: All the above could be complete BS as I have only heard it and not actually experienced it....but it sure sounds feasible to me.....mismatched weights in a reciprocating environment can't be healthy....

0.05c worth

[Engineguy]

I'm solidly with McGuire on this. And his explaination is thorough. I've written software in the past to study the issue, and I just don't see anything concrete within the range of rod lengths that are physically possible, on the short end, or reasonable (from a mass standpoint), on the long end. The effects are a magnatude smaller than those of minute cam changes, and are recoverable through those different, not better or worse, cams. Given the freedom (block deck height not forced on me) I'll choose a fairly short rod to reduce reciprocating mass and block weight.

A shorter compression height piston, even if from the same forging, may be able to be machined for less mass... it gets down to the details of what is available. The increase in rod weight will likely be very close to what you can take off the piston... again it depends on the particular rods in question. Go with the stock dimensions if it's less costly or less hassle... there's nothing to be gained from such a small change.

Paint your engine yellow to optimize horsepower, or blue to optimize torque. Green is a good compromise. If your R/S ratio exceeds 2:1 you need to paint red 17mm poka-dots on the engine (regardless of its base color) and drill a 6mm hole in each header pipe 32mm from the flange.

[Ray Bell]

Thanks guys, you're really nailing it down here...

Having discussed it further with Ben, and recognising that the piston weight difference would only be the difference in the gudgeon pin boss (skirts the same, crown the same), he then came to the realisation that he'd settled on the 6.3" rod because it combined with an off the shelf piston to make the overall length he wanted.

He'll go with the stock rod length now, but with the Chevy big end size to cut bearing speed and take some weight off the crank. Campbell Enterprises came up with a super price on custom pistons, better than the ones he'd zeroed in on from an off the shelf range.

In a week or so he'll have all his dimensions finalised... see how we go.

[indigoid]

Originally posted by Engineguy
Paint your engine yellow to optimize horsepower, or blue to optimize torque. Green is a good compromise. If your R/S ratio exceeds 2:1 you need to paint red 17mm poka-dots on the engine (regardless of its base color) and drill a 6mm hole in each header pipe 32mm from the flange.

edit: i wanted to immortalise this comment in my sig, but it won't fit

[McGuire]

Originally posted by Engineguy
I'm solidly with McGuire on this. And his explaination is thorough. I've written software in the past to study the issue, and I just don't see anything concrete within the range of rod lengths that are physically possible, on the short end, or reasonable (from a mass standpoint), on the long end. The effects are a magnatude smaller than those of minute cam changes, and are recoverable through those different, not better or worse, cams.

Right. I have been down this road and all I can say is I wouldn't want to go there on my time and money.

Originally posted by Engineguy
drill a 6mm hole in each header pipe 32mm from the flange.

So you know why Smokey did that...

[Patrice L'Rodent]

There has to be a story here :-)
Why did Smokey do that?
PDR

[NTSOS]

So you know why Smokey did that...

I actually remember reading the explanation years ago and it wasn't obvious like a temperature probe hole.....seems to me it was something like an 1/8" hole to observe the combustion flame for proper cylinder firing.....but don't quote me.

I love old age!

John

[McGuire]

The holes simply give one a very fast way to identify which cylinder is misfiring, say on a pit stop or in the heat of a practice/qualifying session. (If you can't see fire inside, that's the one that's not hitting.) The traditional way was to wet a finger and touch the header pipes one at a time, which is slow and hurts.

That was clever enough, but if you can get everyone else wondering what the holes are for and concocting all kinds of weird theories to explain their function, you have scored another victory...every minute the competition spends worrying about your stuff is one minute they are not worrying about their stuff eh.

The secret of magic is misdirection. Smokey carefully cultivated his image as wizard & crackpot, but it was no act. A moment of silence please:

[Hoop-98]

The rod length greatly effects piston velocity at TDC and BTDC so the camshaft phasing, port velocity, low lift flow, exhaust tuning, etc.. all need to fit the L/R ratio.

An exaggerated example was the 302-351 "Boss" Ford head. This engine liked Short Rods to increase the port velocities around TDC/BTDC.

Long rods in theory are more efficient as they increase piston dwell at TDC, which allows more expansion before EVO and blowdown.

rh

[Ray Bell]

Thanks for your input, Hoop... and welcome to our happy forum...

I think the decision's made to go with the original length.

[NTSOS]

Nothing new here, but around 1971, I made a flat cardboard "engine". I could easily adjust crankshaft stroke, rod length and piston compression height. I measured piston movement from the deck with calipers vs crank angle.

I ran two plots, 5.7 and 6.0 rod lengths with a 3.0" stroke.

Generally speaking, the short rod was indeed quicker around TDC than the long rod, but was slower at approximately mid stroke and slower around BDC. The long rod was obviously slower around TDC, but quicker at mid stroke and around BDC.

I then increased the stroke first to 3.25 and then 3.48 and measured using a fixed rod length. It is interesting to note that each incremental increase in stroke length obviously not only increases displacement that helps generate kinetic energy in the operating cylinder with a fixed port geometry, the increase in stroke also helps in this regard by increasing piston acceleration rates with each increase in stroke length. Kind of a double whammy to attack the fixed port geometry and get the air moving.

John

[McGuire]

In piston dwell we typically focus on its effects on intake airflow, when we also have the compression and power strokes to consider.

For example, Smokey always believed that the real benefit of a long rod in the SB Chevy was on the power stroke. Peak pressure and torque occur in the 20 degrees or so ATDC; a longer rod produces longer piston dwell at/near TDC, which delays the cylinder's expansion in volume early in the power phase, maximizing peak pressure and torque, was his view.

[NTSOS]

Yes, reading about Smokey's ideas about long rods and the effect of increased dwell to help maximizing peak pressure kind of got me interested in rod ratios all of those years ago......along with Jenkin's book on rod ratios as it relates to port volumes.

It is kind of interesting to note that at this year's Jeg's Engine Masters Challenge:

http://popularhotrod...004/0502em_emc/

The winning and most of the top engines were all short rod motors.....I think for the most part the reason for this was the 92 octane rule and needing to get the pistons in and out of the combustion chamber as quickly as possible to help limit detonation.....kind of Smokey's deal in reverse.

John

[Ray Bell]

I think we're limited to 98RON fuel, so that could be something that applies to our engine as well...

[Hoop-98]

From Kaase:
"Our entry's dimensions and chamber design were pretty much finalized after a consult with "W.J." My Pro Stock mentor from nearby Sugar Hill confirmed what I believed to be true: a small chamber with lots of quench and turbulence was the way to go. 'Detonation Prevention 101' soon began, as the whole engine was built on that premise. A long stroke combined with a short rod would produce very fast piston action at top dead center. This should liven things up. I like short rods anyway. Two inches longer than the stroke--it works on a 5.0 Ford as well as an 815-inch IHRA Pro Stocker.

"When planning our Engine Masters Challenge entry, I knew that there would be no going back once I committed to rod length, bore and stroke. Because of the low test rpm range, I felt I needed the longest stroke and smallest bore with which the heads would work. With pump gas, detonation was a huge factor in the design of the short-block and heads. A smaller bore has less chance of detonation because it doesn't have some far-off place for a secondary flame front to start. The 460 Ford block starts out at 4.360, which I felt was too big. The smallest bore the heads would flow reasonably well on was 4.250-inch. The intake valve still had ample room between the valve head and the bore, but the exhaust wouldn't even open until I ground a huge notch in the top of the cylinder. The stroke needed for 468-inch would be 4.125-inch."

Rules make for strange bedfellows.

The following BMEP's show

IRL 3.0 281
05 F1 205
69 F1 205
05 NASCAR 186
67 NASCAR 191

The lower 3 have been the same for 35 years, they hold the BMEP and go up the RPM scale. The top number shows where you go when yoy can't use RPM to make horsepower.

So, I guess my summary is, horse's for courses, any design paramater generality doesn't hold up under rules like rpm limiting, or octane limiting.


jm2c
rh

[275 GTB-4]

Originally posted by Ray Bell
I think we're limited to 98RON fuel, so that could be something that applies to our engine as well...

Quite a few Historic sedans have Av-Gas log books Ray....

[Ray Bell]

Okay... so does that mean they will run out at some future time and we'll be stuck with the need to get new heads with bigger chambers?

[275 GTB-4]

Originally posted by Ray Bell
Okay... so does that mean they will run out at some future time and we'll be stuck with the need to get new heads with bigger chambers?

dunno....but that sounds a bit drastic...I would have thought a small retune would have done the trick....maybe some fiddling with compression ratio at worst....

[malbear]

http://img260.echo.c...age=fig38rz.png

I hope that this is of some use to all
Malbeare
http://www.jack-brabham-engines.com/