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

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

  1. Put yourself in this situation:

    You have a 65 mutang and you don't like the idea of going to a 9",31 spline axle due to the additional unsprung weight. You don't want the added stress on the chasiss of a 400 cid engines torque, you happen to LIKE engines that rev to 8000 more than 4 or 5 times and that also don't break pistons at these speeds. If you can build 300 lb.ft. @ 7700 rpm like my 1970 bOSS 302 did you will make 440 hp.
    Put that through a 2.95 1st gear and a 4.30 axle ratio with a .68 od in 5th and I think I'd be able to keep up with a 5500 rpm short rod 400 on the street.

    BTDT and the BOSS only had a 2.78 in 1st
  2. i haven't read the article in a while so i can't remember that but one of the benefits is that is a long rod motor doesn't have the same octane requirements as a short rod motor. i'm going to see if i can find the article.
  3. Either the test was seriously flawed or the facts were mis-stated. Changing the rod length and not the cam timing as well as the size of the intake tract would be a flawed test. Long rods generally like more port velocity. Shorter rods like more flow.

    I have looked for any statement that long rods require less octane, but have not found one. I did see that long rods can help combustion in some cases, but cause detonation in others. I don't see how anyone can think that holding the mixture compressed longer does not increase the chances of end gases igniting. Flame speed of the fuel comes in too. There are far too many variables to assume that rod ratio is something that is a given.

  4. i haven't found the article yet and i probably won't for several days at least. my grandfather died today so i don't really feel much like looking for it. i will say that i personally am not making blanket statements, go back and re-read my posts and you'll see that i'm talking about one specific engine, the 351w and a stock stroke windsor at that. i've already stated that i'm not talking about stroker motors or even long stroke motors in general, especially ones with relatively short deck heights, i've also stated that for stroker motors with relatively short decks that the rod ratio is not going to be the primary concern, however i will say that i've seen tests and articles/studies that say that in short deck long stroke motors you do need to run the longest rod possible for the combination without sacrificing other aspects of the build such as piston compression height or ring packs.

    i'll also say that a long rod motor may not be the best motor for some applications but for the motor i'm building it's the best engine for my needs and goals. everyone should do their own research and decide for themselves what the best combo is for their goals, needs and budgets. i don't know for sure when i'm gonna be leaving town but i'll try to look for the article when i get home next week, if it was printed withinh the last couple of years i'll have the magazine.
  5. i do have one article here that i keep in my binder. it's hot rod's Engines annual from 1994 pg 120 the article is entitled Extra Strength. the engine was built AEM and the tests were done on their dyno. it was a 351w with 400 ford rods, and 9.5:1 compression and they ran it on 87 octane pump gas using cast iron crane 64cc fireball heads that flowed 237cfm at .500 intake and 172 at .600 exhaust (i believe these heads had a relocated spark plug similar to GT-40 P heads) and JE pistons with the dish pretty closely matched to the combustion chamber. the cam was a crane hydraulic roller retro-fit with 212/220 @.050 duration with 110 LSA the lift numbers aren't listed in the article but with that duration it couldn't have been a whole lot over .500 and probably less and they used a box stock 650 vac secondary holley and a weiand stealth intake. the engine made 380hp at 5250 RPM and 428tq at 4000 rpm and over 400lb/ft 3000-4750rpm, it made 17 in/hg at 850 rpm and idled so smoothly "you could balance a quarter on the air cleaner" also the BSFC numbers were way better than any other motor they'd ever tested with similar cubes and HP numbers. i understand these aren't earth shattering numbers but for such a mild cam they're fairly impressive, but they also said that if they'd played around more with the timing and rejetted the carb they could have made another 15-20hp. basically the cam was the limiting factor on this motor because power started dropping off after 5500rpm but they purposely used a small cam to show what the combo was capable of.

    this combo is right about where i want to be but i think i can make it even better today with better technology. if you build that same motor today with some AFR heads and 10-10.5:1 compression and maybe a slightly different cam it could easily make closer to or over 450hp. if you add some form of EFI to the equation you could make even more power with even better drivability and mileage (BSFC) numbers. i think that with the combo i mentioned in an earlier post this engine could very easily make 400-450hp and still get excellent mileage, idle and sound stock and look stock as well. could i make power with a stroker? probably but i don't think it would meet the rest of my requirements.

    i have other articles on long rod windsors somewhere but this was the only one i could find right now. i don't expect this to make long rod believers out of everyone because it won't be the right combo for everyone but it's the right one for me.

    EDIT: just for comparision sake at the front of the mag is a ford motorsport ad for the 351 HO SVO motor, part no. M-6007-a351 that made 385hp with 10:1 compression, SVO aluminum heads with 1.94/1.60 valves (same size as the crane heads) with a vic JR single plane, a fairly radical hydraulic flat tappet cam with more lift and duration than the crane cam in the long rod motor and a 750 holley. the m6007-b351 long block with iron heads and 9.0:1 compression tested with all the same cam and induction as the A351 motor(according the ad) made 340hp.

    so basically the cast iron head long rod motor made the same power as the aluminum head SVO motor with more compression a bigger cam, carb and single plane intake and 40 more horsepower than the same SVO motor with iron heads and a little less compression. i'd say that's pretty respectable for a much milder motor.
  6. here's an article on an 800hp 302 deck height block, Joe Sherman built LONG ROD stroker motor. it's built with a dart block 4.125 bore , 3.50 stroke with 5.4 inch rods. stock 302 stroke being 3.00 and rod length being 5.090. the stroker rod ratio works out to 1.55 and the stockers comes out to 1.7 but the stroker motor uses quite a bit longer rod than the stocker, if you were to use the stock length rod you'd end up with a 1.45 rod ratio. if rod ratio didn't matter i doubt Joe Sherman would have gone with the longer rod, but comparing a long rod stroker with a 1.54 rod ratio to long rod stocker with a 1.88 rod ratio is pretty much an apples to oranges comparison anyway.
  7. Bnickel, I am sorry to hear about your grandfather.
  8. thanks. i'll be leaving tomorrow and will be back probably on thursday or so. then i'll look for the article that Hack posted about.
  9. On the street a high rpm engine is anything over 6500 in my book.

    How often do they go through a pro-stock engine? Are they still running ported rings? If so they re-ring the engine about every 8 passes. Sorry, but I don't tear down my engines that often.

    I like 300 cid engines that rev safely to 8k+ for more than 6 sec. And the long rods are about high rpm reliability which directly relates to more power from the same cid. Dan Perrons 287 cid modified-eliminator Camaro (started life as a dual-quad, cross-ram, chambered exhaust 68 Z/28) reved to 13,000+ without breaking anything other than cams until he installed a belt drive. Rod-length-to-stroke ratio was 1.9/1. Dan believes that is the optimum ratio. Look at the 1000 and 600 cc *** sport bikes...all run 1.86 or better. Why? If the ratio didn't matter they could make the engines smaller and engine size in a bike is much more critical than in a car.

    If a long rod is not better PLEASE....PLEASE tell me why Ford used a longer rod for the T/A cars? And why did they install the rev-limiters in BOSS 302 cars? Wait.......
    before someone was because the blocks in early 69's were weak.....
    then it was because in late 69 and all 1970 the pistons were a defective design. Right? But the BOSS-302 was a better engine the the Z/28 that didn't have ANY of those problems. Right?

    Blind Faith, don't you just love it.....and wasen't that the name of Claptons band in 1969/1970? How ironic.
  10. Dude, you are funny. Does this sound familiar? Long rod motors are better at higher rpm and therefore will build more horsepower than a short rod motor of the same cid. PERIOD. When you make a strong statement like this you should stand by it. Not try to wiggle out of it by saying they have to re-ring them every 8 passes. It says "PERIOD". In my book that means there are no exception and nothing else to say about it, but clearly that universal rule does not apply to 500cid NHRA Pro Stock motors.
    One paragraph you act like prostocks are a bad example. The next you talk about a 13000 rpm small block. How many passes do you think those motors go w/o more than a valve setting. I would be surprised if it does not eat 1 or more valve springs every pass.

    Rod ratio does matter in some cases and some motors prefer a shorter rod. That IS a fact. But not many and surely not all american V8's NEED them to live. Do you care to comment on the C5R engine used in endurance racing? It has to be one of the more successful engines in endurance racing. My simple point is that not every motor needs the same thing. PERIOD.
  11. this is correct. the key is the overall combination. as i have said before, both have their advantages and disadvantages, and both designs have been successful. it depends on the engine builders philosophy, and what the engine is going to be used for. for a track with tight turns, and short straights, i would build a short rod engine. for long turns, and long straights, i would build a long rod engine.
  12. Yes, it has longer rods than a 302, but that's not a LONG ROD engine, but a short rod engine. The term LONG ROD is of course just short for "high rod ratio", it doesn't refer to the actual length of the rod.

    I doubt that 5.09 rods are long enough to provide clearance for a 3.50" stroke crank. At least, I've never seen a 347 stroker kit with stock rods. I'm sure there would be if it was possible (considering it's such a popular choice for many hotrodders).

    I think high rod ratios are a good thing (in general), but that article doesn't prove that they are important. Rather the opposite, if anything :D
  13. Sorry this is so late - I don't look at the forum over the weekend (only dial-up on my neighborhood). Since, for a particular RPM, the time to make one revolution is constant regardless of rod ratio, then, if the piston spends more time at TDC, it must travel faster elsewhere in order to make the revolution in the given amount of time. So, the piston on a long-rod motor moves faster at 90 degrees of crank angle than does the piston of a short-rod motor. This has long been though to "pull harder" at the intake and improve performance on restricted intake motors - like those in Smokey's time (intake meaning intake manifold and intake runner on the head).

    With all the frre-flowing intakes (manifold + head) around today, it's no surprise that long-rod motors aren't as fashionable.
  14. The long rod piston does indeed accelerate faster within the vicinity of 90 degrees (+/- some number of degrees), but there are folks who believe the shorter rod engine fills the cylinder better. I tend to agree with them for two reasons, the piston is moving faster at the beginning of the intake stroke and it is always further down in the bore over time than the longer rod engine throughout the downward stroke of the piston (the longer rod piston does not catch up until almost 180 degrees or BDC). My excel program shows this. The 5.155 rod 331 piston is .012" further down at 90 degrees than the 5.4 rod 331. That's 12.6 ccs of more increased volume at 90 degrees. By the time the crank gets to 180 degrees, the longer rod piston has caught up.

    The same is true for the exhaust stroke. The shorter rod piston has displaced the volume of the cylinder faster than the longer rod engine over time thus pushing out more exhaust gasses over the same amount of time. The longer rod piston doesn't catch up until the piston reaches almost TDC.

    I may have not realized it yet when I designed my engine 5 years ago, but I may have done my DOOE heads a service by using a shorter rod.
  15. I stand by my statement. Ford put that rev limiter in there for a reason. Andt hat reason was that when a BOSS 302 got 10 or 20 thousand miles on it and you reved it to 6500, 7500 or better you would eventually break either a block in the early 69's or pistons in the late 69/70. If the engine was fresh with good tight clearence you were pretty good-to-go. Just don't rev the damn thing with anything llike 5 digit miles on it. And just why did they score cylinder walls so badly? Remember the SS&DI BOSS 302 Maverick Super-Mod car? Even with 5.35 rods it scored walls faster than thay could imagine. And if you ever saw a piston that came out of a BOSS the tang was polished to as close to a shine as you would ever get a silicone impregnated piston to be. Now why is that? excessinve loading of the piston aganist the cylinder wall maybe? Once that happened and you then reved it...good-bye piston skirts.

    Ya'll build any kind of engine you want, but if I build a 6500+ rpm engine it will have a ratio as high as I can get it go without putting the rings into the wrist-pin bosses. And it will live longer than a short ratio engine at the same rpm. Period.:flag:
  16. blkfrd: I applaud you for the innovation of your early 331. However, with respect to how the long rod works, your reasoning is flawed for this reason: The faster piston speed of the long rod motor (at middle and end of down stroke) creates more momentum in the intake charge, which aids in late filling of the cylinder. For a restricted intake, this is especially helpful.

    That said, I built a 331, not a long rod 302. I fully subscribe to bigger is better, only deviating as to personal preference in how short a rod I am willing to use on the street. BTW, I don't think your 289 rod is too short, so don't get your panties in a wad :)>
  17. Mike, I am not sure that I have said that motors that have long rods don't help reliability at high RPM's. If I did, I should not have. My point is that there are other factors that influence rod length. Most people that build 8000 rpm V8 motors don't drive them 100,000 miles. It will be interesting to see how the new LS7 motor will fair. It has around a 1.5 ratio and turns 7000rpm's. Usually oem stuff is pretty reliable. I wonder how long the engine warranty is on the new Z06.

    I read an article in Circle Track magazine not long ago by Vizard. He talked of the importance of low lift flow. He said a port with poor low lift #'s will not be able to recover because it will have lower velocity once the valve is at higher lift. If it flows good at low lift, the velocity in the port will be higher through the complete intake cycle. It would go against his "use long rod" theory, but I would guess that it would also apply to lower rod ratio engines since they have a stronger draw early in the intake cycle. Most people agree that lower rod ratio motors have better low rpm numbers. I think this might be why. All this thinking is making my brain hurt!
  18. How do you figure that a faster piston speed in the middle of the stroke helps more than faster piston speed at the top of the stroke? Consider that the intake charge is accelerated by pressure differences. When the piston is near the top of its stroke, the volume of mix in the cylinder is small, so incremental movement of the piston has a much greater impact on the pressure in the chamber. Therefore there is a greater effect on the velocity of the intake charge as well. Also, there is more time remaining with the valve open for the intake charge's momentum to help fill the cylinder.
  19. Just having an intellectual conversation. I have no reason to defend my engine...this is not my motivation.

    I fully understand what you are trying to convey about the higher piston speed in the mid stroke part of the piston motion, yet there has to be an argument for the fact that the shorter rod piston has moved further down the bore at any given time between near 0 and near 180. The elapsed time is the same yet the amount of volume created by the shorter rod engine is higher. Would this not create a stronger overall vacuum? The instantaneous change in the vacuum signal may not be as much as the longer rod engine, but the total vacuum created would seem to be higher.

    Velocity does certainly play a role so I agree that there are two ways to look at this.

    P.S. Tell me about how you like your webers. What are your cam specs and how does your engine behave with webers? Where does your peak HP occur and how long are your stacks? I suspect that you can tune your power curve by simply putting longer or shorter stacks on the carbs (longer stacks...more mid range, shorter stacks...more high end). Speaking of velocity, the ram air effect of individual carbs is supposed to be excellent. Did you ever have a different induction system on the 331 and if so how did it compare to the webers? Thanks
  20. One argument says: Since air is elastic, you don't want to pull too hard as it won't recover. So, get the piston moving quickly and start filling the bore sooner - be more efficient. I think this must be where you're coming from. The other argument says: Maximize velocity of the charge in order to maximize the "supercharging" effect of charge momentum. This is, afterall, how you achieve a VE greater than 1, so go for all the velocity possible.

    I'm not advocating one over the other so much as I am trying to get all of the issues on the table.