MikeP84

I'm no engine builder but I installed 6" rods with flat top pistons, big heads and mild cam in my 84. Using the stock CFI I saw 205 and 290tq to the wheels. I did not intend on running cfi for much longer so was not worried about heads down "matching" intake. My point being I did not see much difference with longer rods but I wanted to decrease cylinder/piston wear and increase performance at higher rmp

cardo0

Years ago when I was a GTO owner and Pontiac fan the Pontiac 389 and 400 motors had what most thought an "ideal" rod/stroke ratio of 1.8. Don't know how that claim came about but for some reason the Pontiac 389/400 motors where designed with long rods. Cadillacs where even longer though I can't recall those lengths and would have to look them up. Maybe a little off topic but some more trivia is those long rods where cast iron. Yes cast iron - not forged. And what the major cause for rod failure in the Pontiac was the weak stock rod bolt - not the rod. Again I don't know the how they got that information on rods but Peterson and Johnny Angles wrote about it. Well forged Pontiac rods where pretty expensive in those days so the common practice was to recondition the cast rod but install better bolts. All that left me with the impression its not worth the expense in street motor to spend for fancy/expensive forged rods.
Well ya know the 327 sbc had a 1.8 rod/stroke ratio to. And a great reputation also. So I can agree with using longer 6.0" rods in a 350" or larger motor.

cv67

Olds used a 6.125 rod?
Will are you referring to K**** EM? Nice performing piece

rklessdriver

OF course it's him.

I reckon no matter how many times people get told they are wrong, proven on the race track that they are wrong..... that race engine technology has moved on from some book Smokey wrote 50yrs ago and Lingenfelter wrote 30yrs ago.... it's still right there in print on the Kindle so they keep on perpetuating these outdated concepts.

Will

JoBy

Wrong or not depends on your goals. Most everything in an engine is a compromise.

F1 uses rod stroke ratio of about 2.5.
Nascar uses rod stroke ratio of about 1.9.
Many sports-bikes have a rod stroke ratio of over 2.

I don't know about outdated ...

MatthewMiller

For F1, that was in the days of naturally-aspirated 19000rpm V10s and V8s. At those rpm reducing rod angles probably does make real differences, especially with the minimal piston skirts and ring packages they ran. Also, those engines had very high bore:stroke ratios, so very small stroke measurements. And physics of a large-bore V engine dictate that a minimum rod length is required just to keep the piston bores from intersecting at their bottoms. So there's probably no physical way to have low rod:stroke ratios in an engine like that, even if that were desirable. Some of those same dictates apply to sport-bike engines, too.

OTOH, since 2014 in F1 we've been in an http://era of highly turbocharged 1....nes by mandate. Fuel economy is a huge priority (they have limits on max peak consumption rate and max fuel allowed for the entire race). They are also allowed only three engines for the whole season, so these things have to last a long time. They are allowed by the rules to redline at 15000rpm, but nobody is going much above 11k due to fuel and longevity concerns. Finally, tight packaging is a major concern for aero reasons, and they try to keep the weight as low as possible. So they don't want taller deck heights or heavier engines than absolutely necessary, and they can't rev them all that high. And with lower speeds and an emphasis on economy and small physical size, the bore sizes are not going to be very big anymore. I don't think any data on the four engine types has been released to the public, but it's a safe bet the rod:stroke ratios are a whole lot smaller than in the days of the screamer engines.

cardo0

Your full of assumptions and we can read are good at that. We can take your word on it for sure. But what does any of that help?

MatthewMiller

Not assumptions: educated guesses. And very little of the above is a guess - most of it well documented fact.

Yeah, it helps because it addresses JoBy's post specifically. The point is that the F1 example of very high rod:stroke ratios is an outlier and was only made that way because there was no physical choice once the operating parameters of 20,000 rpm and 3.0L (later 2.4L iirc) were set. IOW, it's not evidence that there is some inherent advantage in big rod:stroke ratios. But whatever, maybe when you build your next SBC you should use 10" rods because if F1 was using 2.5:1 ratios 10 years ago, it must be the best way, right?

Tom400CFI

And you're NOT? Good lord, just look at post #22. You're "method" for determining rod length, some old Poncho....and that the "327 had a good reputation"?? (even though apples to apples 350s make more power with a "worse" rod ratio). After that kind of "diagnostic process"....you're going to bust Matt's ***** for "assumptions"? NONE of us know the "ideal"rod length for whatever combo, so ANYONE is making ASSumptions if they make any claims about "ideal" rod lengths. It's nothing more than a massive, WAG. Even more so when we're talking about some fictitious, mild 350 like the one the OP would be putting together. ..."Ideal". Come on, man.


I am of the camp that on a typical street motor, like the one described in post #1 rod length ain't gonna make a meaningful difference one way or another. There are other factors that are way, WAY more influential for power, torque longevity, etc. So, for most of us, fussing over rod length on a basic 350 is...."Worryin' about **** that ain't worth worryin' about". Put that worryin' and $$ into parts that make a real diff.

cardo0

Am I guilty of saying the "327 had a good reputation"? Ha, ha I'll take that hit. But I have owned and rebuilt a 327. Has MM ever driven or rebuilt an F1 motor?

BTW do know what WAG even means? Take a look: https://en.wikipedia.org/wiki/WAGs Talk about fictitious! More like fabricating on the fly. Are there other voices in your mind talking to you?

JoBy

Matthew, I agree with most of what you are saying. Also note my first line. I wanted to reduce friction by minimizing side loading on the cylinder walls. I also wanted the engine to handle high revs better.
Therefore I used a 3.48 crank with longer rods instead of maxing out the cubes with more stroke and short rods.

More cubes would prabably have made more power and tourqe, but that was not the goal for my street car. 1000+ hp and 1000+ lbs ft is enough for the streets.
According to my thinking I am making this power with less stress on the bottom end of the engine.

Tom400CFI

No Voices. WAG means
Wild
Assed
Guess


Most people on these forums "get it". I can understand how you would not. As you were, with your 327's.

MatthewMiller

Really? Is this the best you can come up with for "tech": the fact that you've "owned and rebuilt a 327?!" Wow. I've owned, driven, and rebuilt a Pontiac 400, SBC 305, and co-built a SBC 327 and multiple Ford 302s. And trust me when I tell you that I am very, very much a novice. I would never claim that this experience makes me an expert on rod:stroke ratios. But I guess you are since you've owned and rebuilt one 327, LOL! I'll give credit where credit is due: you are awesome. Own your expertise and your awesomeness.

For the rest of the folks reading this thread - you know, the sane ones - the specs on all the older F1 engine formulas are pretty well publicized at this point. The last formula mandated 2.4L V8s, and the max bore was limited to 98mm. Revs were limited to 18krpms. We know that all the teams were running max bore size for several reasons, such as to get max valve area and cylinder filling in the extremely short time window afforded by those stratospheric revs; and to keep the stroke short to rein in piston speeds at those revs. This meant that the bore:stroke ratio was 2.45:1 (98mm/40mm). Now simple geometry tells us that when you have 3.86" pistons and only a 1.6" stroke, you can't have a rod length of only 2.5" (which would give a rod:stroke ratio of 1.56) because the bores would run into each other on the insides of the V. We're talking about SBC-sized pistons here with a rod length less than half as long as typical factory SBC rods! It won't physically fit like that. A You don't need to own and rebuild F1 engines to know that, and you don't need voices in your head to fabricate anything either. For perspective, a 5.7" rod length on that engine (the ones that Smokey Unique thought were "too damn short" back in the 60s) would give you a rod:stroke ratio of 3.56! The point is that those F1 engines had big rod:stroke ratios because they geometrically had to, not because it was necessarily required or even the best way to hit their goals.

While we don't know the rod:stroke ratios of the current hybrid ICE units in F1, we do know their constraints and we know the rpms they are actually using (it's easy to measure that by the pitch of their exhaust notes). We also know that they are interested in efficiency above all else, so the shape of the chamber is now the single biggest area of R&D resources on F1 engines. So the super-wide and super-shallow chamber shapes are gone. We also know the teams have focused intensely on CG location (keeping it low) for minimizing weight transfer and very tight packaging for aerodynamic reasons. All of this gives us very good ideas of what typical bore:stroke ratios are among the four engine brands, and even more about the likely rod:stroke ratios. They just aren't going to be very big anymore. Anyone interested in all of this should subscribe to Racecar Engineering read it carefully each month. A lot of this info comes straight from the teams mouths, so it's primary source kind of stuff. Scarbs' blog probably has some of this stuff too, although he focuses more on the chassis/aero side of things more than power units. Although the exact internal dimensions of these engines isn't publicized (that I know of), there is plenty of info to make highly educated guesses about what they are.

I hear you. I wasn't intending to call you out, and I have no doubt that you understand the issue well. Just trying to point out some of the reasons that an F1 engine's rod:stroke ratio probably doesn't imply what some of us think it implies for a SBC build.

cardo0

Who claimed an "expert on 327"? I'll claim experience any day. Someone w/o experience but runs his mouth is just regurgitating information about F1 motors that seems to fit his imaginations. Who wants to hear that? Were here to talk about sbc. If you have any of that experience then lets hear it. Take your F1 expertise to an F1 forum!

C409

….. FWIW … the factory LS1 rods are 6.100" …..

cardo0

Yes listed as 1.098" and they have the same sbc rod journal size - same thickness also IIRC. I don't hear of anyone installing them in a sbc though. Maybe require expensive custom pistons IDK. If it was something the mfrs could make a buck on I expect they would sell pistons for this.

MatthewMiller

If your complaint is really that information about F1 rod lengths should be forbidden here, then you're replying to the wrong person. I'm not the one who brought that into this discussion, and in fact my entire point in my responses about that was to say that F1 specs basically have no analogy to a SBC build. So you and I actually agree on that, although you're too focused on asinine rebuttals to notice that.

The LS1 rods are for an engine that was designed at least 22 years ago, so not exactly up-to-date tech. However, it's notable that the LS7 rods (about nine years newer and revving a lot higher) are actually shorter at 6.067" on an engine with a 4" stroke. That's a 1.52 rod/stroke ratio, on a factory redline of 7000rpm and full production-car warranty. Hmmmm. Going much newer still, the Gen V LT1 is somewhere in between, with a rod:stroke ratio of 1.68. So basically, it looks like GM thinks the rod:stroke ratio is not very important one way or another, and going large with in particular isn't very important.

jim2527

Over on the other sites (ST/YB) they say its doesn't make a difference.....good enough for me.

mtwoolford

Say it ain't so

tpi 421 vette

Reher Morrison did extensive testing years ago on rod length and found, rod length didn't really affect power output. But rod length does have a big effect on piston wear and longevity. I always choose the longest connecting rod that is practical. Big stroke engine builds will usually have a less than ideal rod ratio, but will have the potential for alot of power. I built a 450 sbc 4.165X4.125 with a standard deck height block. It had a horrible 1.42 rod ratio. The only way to do it was with a 5.85 rod and a 1.0625 piston compression height. It wore pistons pretty good, and I would never do another one. I think a 4.00 stroke with a 6.00 rod is the most I would use in a standard deck height. I have a 468sbc tall deck with a 6.200 rod and a 4.25 stroke. The rod ratio is better, but isn't really great. Rod length is a compromise on alot of big stroke builds.