The engine builders "black art" (AKA Rod ratio)

Discussion in 'Classic Mustang Specific Tech' started by Rusty67, Nov 7, 2007.


  1. Rusty67

    Rusty67 Well-Known Member

    Joined:
    Dec 3, 2002
    Messages:
    3,586
    Likes Received:
    7
    Trophy Points:
    59
    Location:
    SB, CA
    Project.65's thread about his 351w build got TOTALLY hi-jacked by this discussion (sorry dude) but there was a lot of good info and debate in there. I'm going to post quotes from that thread in here and hopefully we can get some great engine tech info into this thread.

    Be prepaired for a long initial read, we have a debate raging already. (I'm not trying to start a fight, I think this is important info for all of us. Lets keep this tech related and civil please.)
  2. Rusty67

    Rusty67 Well-Known Member

    Joined:
    Dec 3, 2002
    Messages:
    3,586
    Likes Received:
    7
    Trophy Points:
    59
    Location:
    SB, CA
    Related stuff from the first page:



  3. Rusty67

    Rusty67 Well-Known Member

    Joined:
    Dec 3, 2002
    Messages:
    3,586
    Likes Received:
    7
    Trophy Points:
    59
    Location:
    SB, CA
    Related stuff from the second page:





  4. 302 coupe

    302 coupe Founding Member

    Joined:
    Mar 2, 2000
    Messages:
    1,996
    Likes Received:
    0
    Trophy Points:
    36
    Location:
    Macon, Ga.
    some of it looks appealing on paper, but the fact is, it doesn't add up in real life, especially on a street engine. Rod ratio has negligible effect on wear, or torque, or fuel economy. OEM rod ratios are all over the place, and just about any OEM engine will last 150k, 200k, maybe more miles without any danger of popping a piston through the block due to some mythical side loading issue that exists on paper. Anybody can regurgitate stuff from a magazine article, an article which is paid for by someone selling "long rod" parts/kits....so take it for what it's worth. Work out the comression you need, stoke and bore, then let rod ratio fall where it may.
  5. blkfrd

    blkfrd Member

    Joined:
    Feb 1, 2003
    Messages:
    426
    Likes Received:
    0
    Trophy Points:
    17
    Location:
    Anaheim Hills, CA
    Ahhh...a subject I can relate to!

    I designed my 331 back in about 2002 or so using 289 length H beam rods and a 383 Windsor piston. I did this for reasons that are somewhat obsolete now...because the oil ring intersected the wrist pin bore even for 331 kits back then, and I did not like the small skirts on the pistons. Soon after building this and talking with the supplier of my crank, speedomotive, about turning down the counterweights to clear the bottom of the piston, they started selling this design as their "budget" 331 stroker kit. The timing was too coincidental...doesn't matter though.

    My 331 runs very well. 345 RWHP@6400 with the old build including fully ported '70 351w heads...more now since I made a few changes including a single plane, beehive springs, 1.7 shaft rockers, tighter lash, a bit more head work.

    The rod ratio of this engine is virtually the same as a 5.400 rod 347. The rod angle is less than 1 degree more than a 5.4 rod 331, but the piston skirt and surface area to spread the side load is larger.

    Before going forward with my engine design, I wrote a computer program to look at the effect of rod ratio on piston motion (I have this program on Excel if anyone wants to get it). Let me tell ya something...unless you make the rod considerably longer the difference in piston motion is very little. For example, a 5.155 rod 331 vs a 5.4 rod 331 results in a MAXIMUM difference of about .012 at 90 and 270 degrees and LESS at all other rotating angles. The extra amount of "dwell" is really small between these designs. Therefore, I went forward with the 5.155 rod 331.

    What does longer rod ratio buy you:

    1. Less G force applied to the rod when the piston changes direction...due to the geometry and more deceleration at TDC and BDC
    2. Smaller/lighter piston resulting in less force applied to the rod when the piston changes direction
    3. Less friction due to a smaller skirt face on the piston
    4. More dwell at TDC at ignition, but again unless you make the rod a lot longer...forget about it! Unless your engine is going to be a very high revving engine, nothing gained.

    What does it cost you:

    1. Cylinder filling and excavation is not as good as a shorter rod design, because the piston is dwelling more at TDC...probably a small issue unless a big difference in rod length
    2. More side loading per square inch due to smaller skirt...the significantly smaller skirt probably outweighs the difference in rod angle
    3. More piston rocking
    4. Can't think of any more...gotta go back to work now.
  6. bnickel

    bnickel Founding Member

    Joined:
    Aug 21, 2002
    Messages:
    5,642
    Likes Received:
    0
    Trophy Points:
    77
    Location:
    lubbock, texas
    again, i state that i was talking about a long rod stock stroke 351w not a long rod, long stroke stroker motor and in that application the rod is more than .500 longer than the stock rod, this combo using a 400 ford rod, so there are big advantages to this combo. in a longer stroke engine ther is not signifacant gain. most long rod 351 stroker motors use a 6.2 rod vs the stock 5.956 rod so, yeah, not much of a gain.
  7. Hack

    Hack Active Member

    Joined:
    Mar 24, 2004
    Messages:
    1,905
    Likes Received:
    6
    Trophy Points:
    39
    Location:
    Minneapolis
    Well said. :nice:
  8. ratio411

    ratio411 Founding Member

    Joined:
    Apr 22, 2002
    Messages:
    3,872
    Likes Received:
    69
    Trophy Points:
    69
    Location:
    Pensacola FL
    I was all gung ho about rod ratio for a while...
    However, after going through it in my head over and over for months, I came to the conclusion that no matter how 'optimum' your rod ratio is for power, it can't come close to designing in more displacement.

    So I agree with the previous post about building your displacement for what works and rod ratio be dammed.

    I get in on the discussion from time to time because I know there are benefits, but they are in opposition to stroking an engine... therefore stroking becomes the lesser of two evils when seeking streetable power.
    If you could stroke an engine a great deal, AND plan in a generous rod ratio, that would be awesome. But we all know what that means: A taller block, more weight, fitment issues... so it is a losing battle.
  9. rbohm

    rbohm Founding Member

    Joined:
    Apr 13, 2002
    Messages:
    5,923
    Likes Received:
    177
    Trophy Points:
    104
    Location:
    tucson,az
    you cant pick one area to concentrate on when building an engine. you have to use a systems approach, and you have to match the parts to each other, and to your own theory on engine building. this is one of the things offenhauser found out in the 60's when long rod engines first came into vogue on the race track. they went to the long rods, but they lost power. why? because they did not rework the rest of the combination to suit the longer rods. head ports, cam timing, combustion chamber shape, header tube size, valve size, intake runner size, shape and length, piston dome shape, all have an effect on engine performance. sometimes the effect is small on the dyno, but huge on the track. sometimes it is the other way around. why is this? you now need to get into the transmisison gear ratios, and the rear gear ratios you are running, the type of racing you are doing, even the tires you are running.
  10. blkfrd

    blkfrd Member

    Joined:
    Feb 1, 2003
    Messages:
    426
    Likes Received:
    0
    Trophy Points:
    17
    Location:
    Anaheim Hills, CA
    I would definitely do it given what is available today as long as your piston compression height isn't really short. Your going to lighten up the piston and reciprocating mass, reduce stress on the rod and reduce friction. What is the compression height of the piston? What is the application, street or race?

    I used a 6.5" rod and a 5.956 rod in my program for a 351w and the difference in piston motion is about .023 max at 90 and 270 degrees and less everywhere else (longer rod design has the piston .023 higher in the bore at 90 and 270 degrees)...not a big difference, it amounts to about 1.1%
  11. bnickel

    bnickel Founding Member

    Joined:
    Aug 21, 2002
    Messages:
    5,642
    Likes Received:
    0
    Trophy Points:
    77
    Location:
    lubbock, texas


    exactly. that's what i was trying to say in the other thread. i'm all for someone doing a stroker motor but i'm not building a high hp engine and don't need a stroker. i want to stay with the stock cube motor just make it more efficient, i'm doing it more for mileage, emissions and the added benefit of a little more hp and torque under the curve. it will not have a radical cam more a mild hyd. roller, but it will have some nice aluminum heads, probably either E'brock, ford racing x303 or AFR 165's still undecided. it'll have cobra roller rockers, stock exhaust manifolds or shorty headers and my old c90x aluminum intake and a holley commander 950 pro-jection system and if the budget allows i'll do some coatings throughout the engine, probably the home applied variety but i'll send 'em out if i can afford it.

    i'll use stock 400 rods(ill have them modded locally) with KB pistons and again if the budget allows i'll either use a Scat cast stock stroke crank or an eagle forged one. i wish i could find an aftermarket 400 rod but there appears to be no such critter. my plans have changed a little since i'd originally planned to use F4TE roller block because KB didn't have a piston for the shorter deck 69/70 block but they do now so i'll be able to use the original block from the 69 cougar that this motor will be going in. i'll just have to use a retro-fit cam or the crane link bar lifters, probably the link bar lifters so i can use any roller grind i want. the heads and lifters will be the most expensive part of the build. hopefully i'll be able to keep the standard bore on the 69 block too since it's a low mileage original, KB supposedly has these pistons for standard bore motors now too. that'll save me a bit more money too.
  12. ratio411

    ratio411 Founding Member

    Joined:
    Apr 22, 2002
    Messages:
    3,872
    Likes Received:
    69
    Trophy Points:
    69
    Location:
    Pensacola FL
    If I was to build a long rod 351w, I would use the 400 rod.
    Nice and long. Like 6.58" IIRC...
    Built that engine in my head a few times.
    I imagined all sorts of exotic parts, like titanium pins (floating of course!).

    There is a few years where the engine block was a few thousandths taller.
    Would help slightly with piston selection.
  13. ratio411

    ratio411 Founding Member

    Joined:
    Apr 22, 2002
    Messages:
    3,872
    Likes Received:
    69
    Trophy Points:
    69
    Location:
    Pensacola FL
    What needs modded on 400 rods?
    They should be bolt in...

    Or are you refering to general modding, like bore reconditioning, beam polishing, ARP bolts, and the like?

    Btw: You stole my thunder by posting the 400 rods while I was typing!:D

    GMTA!
  14. bnickel

    bnickel Founding Member

    Joined:
    Aug 21, 2002
    Messages:
    5,642
    Likes Received:
    0
    Trophy Points:
    77
    Location:
    lubbock, texas
    you have to narrow the big end by like .050" (.025 on each side) and remove the protrusion on the top. nothing major. KB is now making the long rod pistons for the shorter deck too, which i like. BTW, what does GMTA mean?
  15. ratio411

    ratio411 Founding Member

    Joined:
    Apr 22, 2002
    Messages:
    3,872
    Likes Received:
    69
    Trophy Points:
    69
    Location:
    Pensacola FL
    Nothing really.
    I just made it up at that moment.
    Great minds think alike...
  16. ratio411

    ratio411 Founding Member

    Joined:
    Apr 22, 2002
    Messages:
    3,872
    Likes Received:
    69
    Trophy Points:
    69
    Location:
    Pensacola FL
    This surprises me.
    I am not saying it isn't so, but back in the day, the 400 crank was machined slightly and used as a stroker crank in the 351w block, often with W rods.
    Seems like the rods would not fit the crank if the stock rods were that much wider.

    I knew about the big chunk of metal on top of the rod, but had forgotten about that.
  17. mikethebike

    mikethebike Member

    Joined:
    Jun 12, 2007
    Messages:
    358
    Likes Received:
    0
    Trophy Points:
    17
    Location:
    Greenville, SC
    Must disagree with you here..from my personal experience with a BOSS-302 that used a beefed-up hi-po 289 rod at 5.15 and it broke 7 pistons at 7500 rpm but my 271/289 with a 1/8" shorter stroke never broke the first one and my 302 Z/28 with a 1.9-1 ratio would rev 8500 every day of its life and never broke a piston or scored a cylinder wall says otherwise.
    And those engines you talk about are rev limited either by electronics or should they be old enough to not have those the hydraulic cams stopped them around 6000 rpm.
    All the engine builders I spoke with back then agreed the short rod in the Ford limited the max rpm of the engine. To quote Dan Perrin:

    "10,000 rpm heads, 10,000 rpm crank, 6500 rpm rods, 6500 rpm cam.....sounds like a good combination to me....if you want to come in 2nd place."
  18. bnickel

    bnickel Founding Member

    Joined:
    Aug 21, 2002
    Messages:
    5,642
    Likes Received:
    0
    Trophy Points:
    77
    Location:
    lubbock, texas


    ok, yeah we do don't we.

    well if the 400 crank in the windsor used 351w rods then it would probably work just fine since the 400 crank is slightly wider than the 351 crank, you could run a larger fillet on the rod journals or even just using it as is they would probably work just fine just with a little larger rod side clearance.

    speedomotive modifies the rods this way and i found the exact amount on a post somewhere on the mustangs and more site. when i get ready to do this i'll mock up the rods unmodified and check the clearances to see if it's really that much or not. better safe than sorry, i'd rather check them unmodified than realize after they've been modded that they shouldn't have been modified in the first place.

    i did find a place called wheeler racing (i think) that sells a stage II 400 rod that is shot peened with polished beams and ARP already installed.
  19. Helmantel

    Helmantel New Member

    Joined:
    Aug 9, 2006
    Messages:
    206
    Likes Received:
    0
    Trophy Points:
    0
    To all of you who make all kinds of claims about how the rod lenght affects the combustion process, have you ever calculated the differences in piston position and combustion chamber volume, like blkfrd (and me) did? I think you will be surprised.
  20. jasonn

    jasonn Founding Member

    Joined:
    Sep 26, 2002
    Messages:
    312
    Likes Received:
    0
    Trophy Points:
    16
    Location:
    By Rochester MN
    If you are building an all out, no expense, high rpm, race motor?? Then I would definatley look into it. But otherwise??


Share This Page