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

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


  1. bnickel

    bnickel Founding Member

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    so some of you guys aren't believers in the long rod theory, i am. i'm going to build my stock stroke long rod 351w and do it my way, if i'm wrong it's easy enough to switch back to parts.

    i am however going to take the best systems approach i can and try to match every part to the next as closely as possible. i want to end up with around 10-10.25:1 compression, smallish hydraulic roller cam with 93 cobra 1.7 rockers, 165-170cc almuminum heads with 64cc chambers to meet my compression goals with my piston choice, or 60cc heads and have a small dish milled into the heads to match the chamber shape of the head. if budget allows i'll do some coatings on the piston skirts and tops, combustion chamber faces, crankshaft journals and intake and exhaust coatings. if the budget allows i'll get a stock stroke forged crank as well. the idea for me is to make the engine as efficient as possible on my limited budget,with no really exotic or hard to come by parts to keep it simple.

    that's my plan and i've been planning it and thinking about it for a few years now, hopefully before next summer i'll be able to start on it. i figure if i shop around and by as many good used parts as possible i can build this motor for under $2500.
     
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  2. brianj5600

    brianj5600 Active Member

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    If you stick an off the shelf cam in with a long rod i don't think you will see any gain at all, maybe loose some.
     
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  3. bnickel

    bnickel Founding Member

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    i'm gonna play around with cam specs in my dyno sim software to see what effects different cams have on it. i'm sure i can find one that's close to what i'm looking for, if not comp will do custom grinds and you can choose which lobes and blanks you want as well.
     
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  4. Hack

    Hack Active Member

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    One thing I can add is that Hot Rod did an article (I think it was this year) where they built two different motors with different rod ratios. They dynoed both motors and they found virtually no difference. I wish I could remember exactly which issue of Hot Rod it was, but I give my old magazines away.
     
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  5. SoCalCruising

    SoCalCruising Founding Member

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    I believe that much of the long rod work was done on fixed displacement, restricted intake motors. If displacement is held constant (stroking or not is not a factor), then it is reasonable to play with rod ratio. Those of you who state that the piston spends more time at TDC should also acknowledge that the piston moves faster at mid stroke, for example, which provides more signal to the intake. Anyway, just a thought.
     
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  6. bnickel

    bnickel Founding Member

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    IIRC the article there wasn't much of a difference in peak hp/tq but the area under the curve was better and the BSFC numbers were better as well.
     
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  7. bnickel

    bnickel Founding Member

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    i have to admit that i'm kind of lost on that one but i think what you're saying is that basically the engine will have a little more vacuum. am i way off base there?

    i know better vaccum for the given application, assuming the same combo with and without the longer rod, is also one of the benefits of the longer rod.
     
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  8. mikethebike

    mikethebike Member

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    Long rod motors are better at higher rpm and therefore will build more horsepower than a short rod motor of the same cid. PERIOD.
     
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  9. blkfrd

    blkfrd Member

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    A magazine actually did something out of the box for once?!?!....wow! I may have to renew my subscriptions!

    I shouldn't use those words...all they do is "out of the box". Take the AFR185s out of the box and bolt them on, take the Edelbrock rpm air-gap out of the box and bolt it on, etc. (nothing against either one of these products, just all creativity has virtually vanished)

    I betcha if they revved those engines really high, the long rod engine would hold together longer than the shorter rod engine.

    Actually, what would happen is that the shorter rod engine would have the same rod length for a microsecond before kaboom! :D
     
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  10. blown65

    blown65 Founding Member

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    They did a dyno test of two engines, one long rod, one std. Nearly no difference between the two. So close that how would you actually know it wasn't from something being a tighter clearance in one vs the other.

    Personally I'd spend the money on things that will make a difference.
     
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  11. brianj5600

    brianj5600 Active Member

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    What do you call higher RPM's? Modern prostock engines don't worry about the rod ratio too much. I believe they turn around 10,000 rpm's. They run short decks because it shortens the push rods and gives a better angle for the intake runners. Shorter decks require shorter rods. They will not comprimise the ring package for more rod. The can make more power with a stable valve train and better intake ports than they can with long rods. One prostock engine builder said that if he put rod ratio in his top ten consideratios for engine design, it would be 15th. I bet every PS engine builder would caution you on making blanket statements.
     
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  12. brianj5600

    brianj5600 Active Member

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    Another interesting bit. Sonny Leonards 632" N2O motors have only 1.41 rod ratio. Speculation is that boosted motors build pressure so fast that the piston needs to start down quickly. It is way more complicated than Smokey Unick ever imagined. I think it was Darin Morgan at Reher&Morrison that said many people take one sentence out of Smokey's book and run with it. Can you guess which one it is?
     
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  13. Helmantel

    Helmantel New Member

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    Keep in mind that racing engines that are running 5 seconds at a time have different requirements than those that run 500 miles at a time. What works best for an endurance engine doesn't necessarily apply to a 2500+ drag racing engine.
     
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  14. brianj5600

    brianj5600 Active Member

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    How about the C5R? From what I have read it has about a 1.5 rod ratio. How many 24 hour races has it won. 12 hour? It seems to make plenty of power and great reliability. They to a systems approach and ended up with the best package.

    Edit: It has won over half the races it has entered. For a while it was up around 70%. I don't know where it is right now.
     
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  15. bnickel

    bnickel Founding Member

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    the rcae motors are prepped differently than street motors so it very well may have a longer rod than the road version. i'll have to check into that. you also have to keep in mind that race motors of any kind, be it endurance, short track, pro stock or whatever also have a lot of heat on the top of the piston and a bigger piston crown and longer skirts help to dissipate the heat, also the C5R is a long stroke engine with a short deck so it may be that in that particular engine the 1.5 rod ratio may be the longest they are able to get away without compromising the strength of the rotating assembly.


    there are more variable to consider than just running the absolute longest rod you can, but if you can run a longer rod without compromising the piston, the rings or anything else then longer is generally better for a street engine at least.
     
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  16. rbohm

    rbohm Founding Member

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    there actually was a big difference between the two engines. the original dyno test was done with 104 octane fuel to avoid detonation, but they then ran 87 octane pump gas in the long rod engine with NO DETONATION, and still made the same power it did with the race gas. the short rod engine detonated on pump gas.
     
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  17. ratio411

    ratio411 Founding Member

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    Is it: "The Holley 3310 is the runnenist carb on the planet!" ? ;)

    That is my favorite anyway...


    I am a diehard believer in the long rod theory.
    However, for what little it provides compared to other mods, especially stroking, I am not going out of my way for a long rod. I will go out of my way for a long stroke though. If that means shorter rod, then so be it. The long rod vs long stroke, loses every time IMO.
     
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  18. ratio411

    ratio411 Founding Member

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    One of the mags should do an article on long rod vs long stroke.

    They should build two TOTALLY identical engines, except for crank and rods.
    No other changes.

    They should find the shortest piston they can that doesn't have the rings in the pin, and use it on both.
    They build one that is max stroke in the test block, combined with that piston and an available rod.
    Then the second engine will use the shortest stroke stock in the test block, combined with a long rod suitable for said piston.

    Everything else the exact same, down to jetting and base timing.

    A series of 'base' pulls on the dyno, followed by a free for all tuning of jets/carb and timing.

    I know which will win...
     
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  19. bnickel

    bnickel Founding Member

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    i do too, but again i'm not building this motor for all out power. my goal is 375-400hp, smooth idle with 15 or MORE in/hg at idle and can get more than 25 mpg on the highway and to 15-20 in town with an AOD and 3.25 gears. it has to to start and idle in 30 degree weather with a remote starter and last forever all on no more than mid grade pump gas, eventually i'll convert it to E85 once our local E85 refinery (distillery?) gets up and running so it has to have at least 10:1-10.5:1 compression with aluminum heads and it has to be docile enough for my wife to drive it and not be scared to death of it. and it also has to be as "Green" as possible too. most importantly it has to look like 1969 when you pop the hood. eventually, it'll get a holley pro-jection with the ignition control feature or maybe a mass flo EFI system if i can afford it but the mass flo system may not fit with my "looks like 1969" criteria so i'll most likely go with the holley pro-jection since i can use a small cap TFI dizzy and tack a dummy vac. advance can on it and nobody will know at first glance.


    this is engine is going to be going in the 69 cougar project that i'm making into a "faux-totype" 69 XR7-GT or 69 GT-E neither of which ever made it into production but both were planned and pulled at the last minute, the GT option was even in some of the original option lists and brochures and the GT-E was production ready and even had a batch of emblems made up, they pop up for sale at swap meets and on ebay every now and then. the car will be totally modern and upgraded but will look bone stock. it's going to have a TCP G-Bar rear suspension, my TCP rack and pinion i already have, front suspension will most likely be a custom built Opentracker coilover front end, already talked a little about it with John on the board but i haven't emailed hime to work on any specifics yet since i haven't sold the stang yet. it'll have 15" GT wheels and some of Degins dropped spindles and a front disc brake setup as well. rear brakes will either be vintage venom rear CV type discs or some Ultrastang rears, undecided yet. interior is going to be pretty much bone stock 69 cougar except for the front seats which will be fox body buckets re-covered to match the original pattern but in leather which was standard on the 69 XR7 anyway.


    from outside and inside the car will look bone stock, for a car that never existed anyway, but it will look like it could have been that way from the factory. why am i saying all this in a thread about a long rod motor? just explaining my reasons for wanting it and the benefits to me. is it pertinent to the thread? not really but kinda. i know i could make a stroker look stock too but to get my goals and still look stock it won't work.
     
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  20. brianj5600

    brianj5600 Active Member

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    :bs: :bs: :bs: There is something seriously wrong with this post. That ain't going to happen.
     
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