Most Ford V8's have RR of around 1.7. The optimal rod ratio of 1.84 was identified by Enzio Ferrari in the 1950's. It's no mistake that the 289, the 255 Indy Motor, the 426 Hemi, the Boss 429, and many other high performance motors have a RR of exactly 1.84. There's a compromize to be meant according to the engines use though.

With longer rod motors the piston comes down the bore less by 90* crank shaft rotation. This means that cylinder isn't filled as well during the intake stroke at low RPM. This is the primary reason that shorter rod motors seem to have more low end punch.

On the other hand, with a long rod motor, the piston approaches TDC more quickly on the compression or exaust stroke. A long rod motor has better dynamic compression at higher RPM. With a long rod motor the piston dwells longer at TDC and it holds compression longer too, extracting more mechnical energy from the available chemical energy. This can be a potential diaster with a supercharged engine though. It's no mistake that the 4.6 mod motor has a rod ratio of 1.68, as this is optimal for a supercharged motor. Ford expected that the new engine would be supercharged in many applications.

When I was a kid, my dad's best friend ordered a heavy duty F250 4x4 with a 428. With heavy pulling it had plenty of power, but it kept blowing out the bottom end. It couldn't handle the sustained high rpms needed to climb and decend mountains in low gears towing large heavy livestock trailers. The solution was a 390 with a 1.73 rod ratio instead of the 428's 1.62 rod ratio. It's no mistake that Ford settled into a rod ratio of around 1.7 with most of it's V8 motors. This the limit were the rod angle, at 90* rotation, remains less than 17*, for better rod angles and optimal bottom end durability and superior thermal efficency, but you still get decent cylinder filling, and not too much sustained compression at TDC, compared to say 1.8 or higher.

When the Tunnel Port 302's kept blowing up at high rpm in 68, the superceding Boss 302 went to a 1.73 rod ratio (same as the 427) from 1.699, and this along with a 4 bolt block, allowed reliable sustained engine speeds exceeding 8000 rpm. Just a few tenths of a ratio can have a profound influance.

Another factor is the reciprocating weights. A longer rod weighs more and you can reach the point were it's just too heavy. As with everything in engineering it's matter of juggling the various trade offs.

It's the ratio of rod length to stroke that effects the reciprocating geometry, not the absolute distance of the rod, or the crank throw by themselves.

 

In the above post I meant to say hundrth of a ratio instead of tenth.

Just a few comments on why rod length may matter a great deal: Those articles that seem to prove that rod length doesn't matter are only relavant to the big block chevy. Of course there's hardly any difference, because the big block chevy has such a poor rod ratio to start with. They go from an awfull rod ratio, to just a bad one. If they were to compare to motors with trully high rod ratios, there would be a difference. They are simply comparing a shorter rod motor to another short rod motor. I grow tired of the chevy-centric high performance world, thinking that what happens on a particular chevy is all that matters, or is unmutable law for everybody and everything else, but that's another topic.

Furthermore, the fact that short rod and long rod motors, ofton make the same peak HP per cubic inch, doesn't deal with the fact that higher rod ratio motors typically have longer, flatter power curves.

 

I agree 110%. Everytime I pick up a classic car/hot rod/muscle car magazine all the stuff I see in them is for chevies. All the half-wits out there see this kind of attitude and decide chevy engines are the holy grail of high performance, the start talkin smack about every other engine like they are some kind of seasoned engine builder.

 

My experance with chevys is the parts are much cheeper and parts are much more interchangeable..example..why does ford use so many differant bell housings..lol..when the bowtie boys start to brag I just remind them ford has won lamans many times gm cant even seem to finish...I LOVE MY FE...bill

 

ford uses 5 that I know of for the RWD engines. 1 hasnt been used since 65/66(5 bolt 289), another hasnt been used since 76 (FE). They still use the 6 bolt windsor/cleveland bell (also alot of v6s, I4's and I6s) and the 385 series bell, and the diesel pattern. Thats at least 3 for sure.
Im sure chevy has about the same amount, theres definately the SBC/BBC/Diesel bells. Im not sure what their inlines and v6s used/use. Thats also at least 3 for sure.

There are many ford parts that interchange...most people just arent as in tune with it as others. If you want to know more, I suggest the "High Performance Ford Parts Interchange" book by George reid. I think the reason so many more chev parts swap out is because they had many more small blocks...283/302/305/307/327/350/400, thats 7 off the top of my head, and they also shared alot of measurements, IMO to save on R&D costs. Where Ford had the 289/302/351's, thats 4 (two 351s W/C)..the M/400 is arguabley a small block, Im leaving it out however, since it only shares the same bore spacing, and a couple other crank measurements.

 

EXACTLY. The whole time I was reading that article, I was adding "on a big-block chevy" to the end of every sentence, in the first paragraph. On the second paragraph, change it to "on a small-block chevy". Then it starts to make sense.

The whole "chevy" mindset really annoys me. Especially when they cant make a fuel pump (or any number of other things) that works... But that's another topic entirely!

(maybe that's why their parts are so cheap, more of a demand!)

 

so to revive this, why do shorter rod ratios lend themselves to forced induction better than longer ones? Since the dwell times are increased, and piston speeds are slower I would imagine those 2 points alone would really help in getting the cylinders stuffed full of air/fuel. The way Im picturing it, longer rods would also help ease the effort it takes to compress the mix, since there is significantly more volume being forced into the cylinder compared to a n/a engine.

 

With forced induction you have extremely high dynamic compression, and much higher charge tempatures. A long rod engine aguments dynamic compression even more. From around 240* crankshaft rotation on the compression stroke, when the intake valve has typically closed, back to TDC, the longer rod motor moves the piston a greater distance with all the valves closed. Once up near near TDC the longer dwell time means that the compressed gasses are held at high pressure longer (it's also filling the cylinder longer at BDC dwell). Increased pressure equals increased tempature according to physical laws. It can all be a bit too much, especially on gasoline.

A faster rod motor may see a greater increase in volumetric effciency, with the great pressure differential of forced induction on the first portion of the intake stroke.

Historically, gasoline engines with medium rod ratios, have really done very well under forced induction. They can make good use of supercharging without blowing up.

 

Well I buy your theory, but the longer rod motor will make the same power with a slightly reduced boost pressure.

The longer rod motor does a better job of expelling exhaust.

 

P51D, I see what youre getting at....it just seems to me that with the longer rod motor moving the piston a greater distance with all the valves closed, says it needs more cam duration, which IIRC is what you want for a supercharged engine since incoming air velocity isnt a concern with the positive displacement blower handling that task. Again, to me, slower piston speeds say better valve timing. Another thing, if I understood right, is that higher combustion temps result in more power output (not talking intake air temps), which is why fuel with a higher BTU makes more power/better effeciency.
Bdox, I agree also, the leverage the longer rod will help expell the exhaust better...along with the power and compression strokes...the only thing I see it hurting is the intake stroke, where the piston speeds slow down and in turn reduce the air intake velocity. Which if you factor in the point that everything else the same, the longer rod engine will rev higher making more intake air speed.

one last thing, it makes sense to me that the longer rod engine would make a different exhuast note than its opposite. Slower, smoother exhuast pulses when compared to the short rod.

 

Or the shorter rod motor can make as much with increased boost pressure. Perhaps even more, as the increased boost may result in more total airflow. I think it comes down to balancing the trade offs and reaching the best compromize. A compromize would be a more medium rod ratio.

One of the most successful forced induction combonations is the EFI 302, with 9:1 static compression, and using the E303 cam. The 302's rod ratio is 1.70 (rounded from 1.699). I don't know if we should call the 302 a long rod, or medium rod motor, but it's less than a 289, and more than many. The 4.6 is proving to be very forced induction friendly as well.

The 4.6's RR is 1.68. I'd call that a medium RR, but someone else may not? Some turbo 4V 4.6's are exceeding 1000HP on as little as 16 psi of boost at 7000 rpm. The 3V 4.6 in the Saleen is getting 390 Hp to the rear wheels on as little as 3 pounds of boost from a roots type blower. The lesser RR 5.4 4V(RR=1.60) doesn't make as much HP per liter on equal amounts of boost as the 4.6. This would indicate that the longer rod combonation is more efficient under boost. Will increaseing the RR further result in even better effciency, or have we reached the point of diminishing returns, with an medium RR?

 

Clearly there are a lot of variables to consider. Type of aspiration, cam grinds, intake and exhaust system efficiency. But I maintain that it is always better to use the longest rod that the block can accomodate and build everything else to suit it. In the interest of saving weight and space the manufacturers have been making blocks as small as possible especially with the advent of overhead cams and the associated bulk. But those considerations aside, if you are going to go with forced induction, you can do better to use a somewhat shorter stroke in favor of a longer rod and adjust the boost accordingly.

 

well Im tired and only made it half way through this http://www.stahlheaders.com/Lit_Rod%20Length.htm but it is definately a very good read.

 

Im going to have to read that a few times to make the most sense of it. Theres alot of info to take in.
Like the longer rod has slower piston speeds during the upper 180* crankshaft rotation (90* BTDC -> 90*ATDC) and higher piston speeds in the lower bottom half. The short rod engine is the opposite. (might have read that in another article too)
If I read right, the long rod engine will draw air in better during higher rpms due to the slower piston speeds at the beggining of the intake stroke. Something about back eddies being created at higher intake velocities causing a restriction of airflow into the chamber...kinda like the same thing as why you get better mileage with your tailgate up, the "air bubble". Now Im talking all things equal except rod length.
It actually said something about the shorter rod being able to exhuast gasses also, but I think that has to do with the ability to run more exhuast lobe, as they stated would be possible.

 

I have a copy of a Chrysler engineering reasearch paper from the 50's that examines many of the tenants in the linked online article, and it also seems to have been influanced by Ferrari's ideas. It confirms many of the points above in the linked article.

The Chrylser engineers found that the rod angle should not exceed about 17* at 90 * crankshaft rotation if the engine is to survive extended high rpm running. Even such solutions as a stronger rod, rodbolts, piston pins, and greater cylinder wall strength don't solve the problem of short rod engines breaking the big end of the rod near broaching areas. Increasing weights with more heavy duty componants is counter productive as well. 17* rod angles or less, limit the rod ratio to about 1.68 or higher. They also point out that stroke is limited by max allowable piston speed as high piston speeds will increase stress on reciprocating parts exponetially. Stress =speed squared multiplied by mass.

Too ofton in quest for more stroke people end screwing up these dynamic geometric relationships, and it's displacement that determines tourqe anyway. A larger bore, shorter stroke engine of equal displacement will produce the same tourqe.

They mentioned that long rod engines gain dynamic compression with greater rpm, and short rod motors actually decrease. They even have a graph showing this relationship.

This might be why they found that cam selection should more conservative with a long rod motor. This would seem counter intuitive, but the theories about valve timing in the linked article do help me understand their caution better. Most longer duration cams add duration by opening the valves sooner (moreso than closing them later) and this may increase pumping losses on a long rod motor. It would seem that a long rod motor would benifit from higher overlap on the other end though, creating more blow down, and more scavanging, with increased intake charge draw, with the piston near TDC.