I have seen it on here that going to longer 6 inch rods can help with higher compression ratio engines. How? If someone is going with a higher compression ratio they should consider the longer rods?

 

The 6" rods send the piston higher up into the bore, and closer to the head, which results in a higher CR. I think the Rods will be too long and make too much compression, since they are .3" longer than stock.

 

I was referring more to detonation problems. The comment I read made it sound as if you can go with a little higher compression if you use longer rods. Maybe I just read it wrong.

 

That is not correct. CR is CR, regardless of how the deck height is split up betweeen the rod and the piston. The rod length has little to do with compression ratio in amy way.

The deck clearance, the distance from the top of the piston to the top of the deck, is more or less a constant regardless of rod length. Generally people shoot for anywhere from .025" to .000" depending on what the rest of the motor's specs are. Longer rods do not move the piston higher in the boer; instead, you have to run pistongs with their pin hole drilled higher up to compensate for the different rod length.

In any case, using non-stock rod length is an expensive proposition since it automatically puts you into the realm of "trick" pistons. CR has nothing to do with this issue. You cannot make up for some inappropriate head-piston choice in that way.

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"So many Mustangs, so little time..."
ICON Motorsports

 

I agree entirely with RB (Mr. SBC) about the rod length having nothing to do with compression ratio. As RB the compression ratio is determined by other factors: volume of cylinder and the volume of the combusion chamber (factoring in the deck height, gasket thickness, combustion chamber size and piston valve reliefs, dish or dome). Rod length is NOT a factor.

The primary reason to use a longer rod is increase "dwell time" of the piston at TDC. The theory is that the longer dwell time produces more pressure while the piston "dwells" and results in more HP at higher RPM. A shorter rod is suppose produces more torque at lower rpm due. In reality, I don't think there is much difference on a typical street engine.

A friend of mine who is an engine builder did say that with 383 SBCs, he likes to use longer rods as they are less prone to detonation. The 383 with its longer stroke becomes more prone to detonation than a 350. Going to a 6" rod helps.

I never thought about it, but what you may be confusing is that a higher compression engine should consider a longer rod to help with detonation.

These are the theories behind the longer 6" rod. But I will tell you something funny, if you ever look at a "dyno program" to estimate the power of an engine, you will seldom see "Rod Length" as a factor. I wonder why?

The only thing that does make sense with a longer rod is "angularity" of the piston as it travels up and down in the bore. The longer rod has a narrower angle which supposedly makes it wear better over time. But, then you have a tighter ring package which presents its own problems.

It's half a dozen of one and half a dozen of another.

 

I think I hear you saying it too.

Is it not correct to say that all engines have a compression limit and going beyond that limit creates detonation problems.

That limit can be changed by how we set up an engine. We can combat detonation by reducing heat buildup. Better cooling systems and aluminum heads for example. We also use higher octane fuels to control detonation in high compression engines.

"A friend of mine who is an engine builder did say that with 383 SBCs, he likes to use longer rods as they are less prone to detonation. "

If we have our engine right at its limit before detonation problems occur and we add rods that "are less prone to detonation" then we should be able to bump the compression by making other changes to the engine.

Would it not be the same as having an engine that is limited to 9.5:1 and adding aluminum heads so we can bump compression to a new 10.5:1 limit?

It sounds like there may be some truth to the longer rods allowing for higher compression. But how does it work? Reducing friction in the cylinder and consequently heat in the cylinder is about the best answer I can personally think of. But I am mostly ignorant yet on this subject.

In any case, whether there is a direct performance advantage from the longer rod dimensions or not, anytime you can reduce friction that has numerous benefits.

Maybe, the performance advantage is being able swap out your other components to bump up your compression. ???

 

I have a scan of an article that appeared in "Hot Rodding" magazine a few years back. Here's some excerpts and results, from their own tests of using longer rods.

" What does a longer rod do for you? Lengthening the connecting rod changes the rod-to-crankshaft geometry, allowing the piston to stay at the top of the cylinder bore(at TDC) for a longer period of time at the start of the combustion stroke. The piston "dwells" at TDC longer, keeping the combustion area smaller for a longer period of time, as the expanding gases push against it. The net effect is to extract more power from any given air/fuel mixture. Put another way, a longer rod will improve combustion efficiency.

As a result, longer rods will make more power than short rods with the same amount of fuel, particularly in the RPM ranges required for street driving. Further, because the combustion process becomes more efficient, long rods reduce the detonation sensitivity of any engine. You can take advantage of this characteristic by using a higher compression ratio in a long-rod engine without detonation, and higher compression results in more horsepower."

To prove the theory HR magazine built an engine with the following components:

Bore: 4.155(400 block bored 0.030" over)
Stroke: 3.25(327 crankshaft)
Rods: 6.209"(Ford 300 cid I6)
Rod/Stroke ratio: 1.91:1 - ratio on a stock 350(3.48" stroke/5.70" rods) is 1.64:1
Pistons: JE with raised pin location
Camshaft: CompCams 270HR10(hyd. roller retrofit)
Heads: AFR 190HST aluminum(milled for 56cc combustion chamber)
Compression Ratio: 11.00:1
Total Ignition Advance: 36°

The engine was dyno'ed at Duttweiller Performance, using electronic knock sensors.

The results? 412 hp @ 5700 & 435 lbs/ft @ 3800. At least 400 lbs/ft from 2800 to 5200 rpm. So you're thinking they must of used race gas, to avoid detonation, right? Nope, the test was done with "cat-pee" gasoline(87 octane). I'm thinking, with 92 or 93 octane, they might've been able to bump the CR another point upwards.

If anyone thinks this is bogus, I'll be happy to email you the article(the scan - not my own - is kind of poor quality, but it is legible).

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"The reasonable man adapts himself to the world.
The unreasonable one persists in trying to adapts the world to himself.
Therefore all progress depends on the unreasonable man"
--George Bernard Shaw.

[This message has been edited by 88IROCs (edited July 08, 2001).]

 

I would be interested in having the article sent if it is not too much trouble.

 

Does anyone recall an article a few years back that compared rod length to stroke ratio of the different stock small block combinations. I just remember seeing it while scanning through a magazine rack.

 

Breazlan,

You've got mail.

 

Just to add my 2 cents: I have seen several test with different rod lenghts. One test will say that a longer rod gives the piston more dwell at tdc, resulting in more power. I've seen the exact same test done by others that say there is no power gain. I've heard that low rpm torque motors benefit more from a shorter rod, and mid range motors benefit more from a longer rod. All I know for sure is that a longer stroke engine needs a longer rod to give it a better ratio so the piston isn't pushing so hard on the thrust side of the cylinder wall resulting in more frequent rebuilds. A 350 is supposed to have a nearly perfect rod to stroke ratio. A factory 400 has one of the worst, but was done for ease of assembly by the factory, as the factory never forsaw any hipo use for the motor

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50 year old kid in an 537hp 415 CID 84 Camaro
Real men aren't bottle fed

 

The rod length by itself shouldn't make much difference. But the additional resistance to detonate allows a higher compression ratio,... which does make a difference.

The internal combustion engine is completely inefficient in utilizing the heat generated by combustion. Around 60% of the heat that is released is wasted, conducted away by the cylinder walls, the cylinder head, the valves and out the exhaust. So it makes sense to make as much heat as possible as quickly as possible. That way expansion of the gases can have the most effect on the piston face. If you're going to lose the majority of the heat anyways, might as well use it while you've got it.

The longer rods increase the cylinder pressure, which means more force on the piston face. The decreased sensitivity to detonation means you can increase compression, which increases the heat. Sounds like a win/win situation.

I don't believe there is such a thing as the perfect rod/stroke ratio. Even if the longer rods caused a measurable loss in low-end torque, there are other means of recovering it(i.e.,... valve timing, intake and exhaust tuning). Smokey Yunnick always worked from the basis of using the longest practical rod possible. In his book he advocated rod/stroke ratio's as high as 2:1(or higher if possible).