[Old School Nut]

ok, well i know the topic of long vs short rods is like a monthly topic around here, but i figured maybe some1 could shead some light on this for all of us. my question is what is better for detonation resistance, i have always believed long rods were better for this. here are the reasons as taken from a June 1997 HOT ROD magazine

long rods cause the piston to dwell at TDC longer (as we all know) this keeps the combustion chamber smaller, longer as the expanding gasses push against the piston. therefore you get more HP from the combo by increasing combustion efficiency. and according to HOT ROD the increase in efficiency makes an engine less detonation sensitive.
--------------------------------- -------------------- ------------------------
however in the brand new popular hotrodding Jon Kasse (the new winner of the engine masters challenge) stated the opposite.

He said that short connecting rods and small cylinder bore diameters are more resistant to detonation than longer rods and big bores (opposite of the engine build in the 97 HR article) he states that the short rods don't dwell at tdc long so the pistons get away from the chambers as quickly as possible, then he states that more time at TDC increases the chance that non-homogenized portions of the mixture will ignite on their own. he says the smaller bores help reduce the distance the flame front has to travel and the smaller area offers less of a chance for a secondary flame front to develop...

sounds pretty good to me, and he must know what he is doing to be the winner, but has any1 here actually done a back to back test to detonation sensitivity and not hp? btw in the engine masters challenge most of them guys had audible detonation (including the winner) sounds to me like they know to push the limits!

 

[lluciano77]

Both have their benefits. You aren't going to get an ideal situation from long or short rods. You have to pick which one is more ideal in your application.

Life is full of compromises and trade offs.

 

[willowbilly3]

How that one factor plays into the mix is worthy of discussion but there are so many other things going on too. Combustion chamber shape, head design, whether you have aluminum or iron heads, type of fuel ect. And then the overall geometry of connecting rod angles, lobe seperation (which affects cylinder pressure). even spark plug indexing.
And, yes small cylinders theoretically fill better but they also have less displacement so that is another whole discussion. In the 70s it was widely accepted that an over square engine (bore larger than stroke) was the best for a quick revving engine. Nowdays all you hear is stroker engines and they seem to have forgotten that basic theory.
I'd like to see a 427 stroker Windsor engine up against a 60s side oiler on the dyno. OOPS, getting off track. And while I can neither prove or disprove John Kasse' theory. I would like to see actual research.
I did see some good research the Nascar builders do. They put knock sensors on every cylider and found the optimum horsepower gained from each cylinder before detonation occured could be had by varying the timing and compression from cylinder to cylinder.
These engines actually have as much as .7 difference in compression ratio and several degrees of advance from one cylinder to the next.

 

[Old School Nut]

compromises and trade offs being what??? both claim to do the same thing. of course combustion chamber shape, piston head shape, plug location and valve location and RPM/load are all variables, both sources seam to go in the opposite direction with the same material, and that is the problem, they are like opposite views. of course you have to pick whats best for the application, but whats better when you don't have a dyno to test crap for yourself? thus my question, maybe some1 out hear has something else to add theory wise as to why they think one is better than the other in this matter. i would sure like to hear it!

yeah willowbilly3, them nascar guys do some interesting stuff, always fascinates me. i should write to hotrod or some place like that to push em to do a test on rod lengths for detonation sensitivity.. be interesting... but like its been said, alot of variables out their...

 

[willowbilly3]

Well maybe this will help. After my last post I remembered an article I read in a magazine about a 350 sbc they put together with a small bore block and a much longer stroke. I don't remember the exact dismensions but they were able to run some hellacious compression on pump regular with out any detonation, and all with plenty of total timing. So put that into theory, you would have a longer dwell time a tdc and faster piston speeds.
Hope that helps a little.

 

[macx]

Long Rod SBC & SBF

That long rod 350 was a 400 block with a 350 crank, ran hi compression on 87.

Awhile before that, read a similar buildup on a 351 Windsor with stock stroke and the real long 400m rods (trouble is, put the piston pin up in the rings), also ran hi compression on 87 octane.

I was contemplating a similar SBC for an older Chevy and contacted AFR as they had the article in their website archives. They replied that they'd supplied parts for several similar buildups and recommended against it - said the results were disappointing although they didn't elaborate.

Would be fun to try, but I wonder if the extra expense going real radical with that would be justified versus doing an engine with more stock reciprocating parts and more cubes, just keeping the rod ratio within reason.

 

[willowbilly3]

A 400 block and 350 crank is a 377 with standard bore. The long stroke 350 I read about was a smaller bore, probably a 262 or 265 block. I think the crank was a custom grind. It was 350 cubic inches.

BTW the 400 block with 350 crank and 6 inch rods is supposed to be have a real long, flat torque curve.

 

[macx]

You're right!

the 400 block 350 crank is like a 377
or something? I've heard they can be
quite the hi power hi rpm race engine.

Now that I think of it, that long rod 350 I think was a 400 block and 327 crank maybe.

 

[engineman302]

:nono: long rods or short rods doesent affect detonation,compression does. because if you are going to have det.you will have it long before te pistion reaches tdc.because a normal engine has a 8 degree base timing with a total timing at around 33 deg. so taking that in account you should have total ignition of the fuel before the piston reaches tdc.anyway.the only way to get away from det. is to have lower intake temps. so the fuel takes longer to heat up.ie(fuel coolers ,air coolers,lower comp.higher octane levels) thats why alcohol wont detonate because it takes more heat to ignite it .

:smash: now to really put a burr in your sheets.how can a long rod hold the piston at tdc longer when you are using the same stroke crank?crank rotation is mesured in deg.correct?so just say that a crank is at dwell at tdc for 8 it dosent matter if the rod is 5.7 " or 10" long the degrees at which the crank dwells at tdc is what determins how long the piston stays at tdc.think about, it to take a 5.7 rod out and put a 6." rod you have to change the pistons or you would have .300" sticking out of the top of the block.so essentualy you still have the same lenght rod n piston assembly. the only thing accomplished by changing the rod lenght is to decrease the rod angle,which is less stressfull to the rotating assembly.
'

 

[Max Keith]

mixing it up

In reality, having longer rods doesnt keep the piston at TDC any longer than does a short rod. What happens with the long rod setup is that the pistons travel speed during the the combustion stroke isnt as rapid as it is with a shorter rod, due to the decreased rod angularity.
Supposedly, what the longer rod theory does is forces the combustion gasses to create more pressure against the piston, creating more force, IE more torque, prior to the exhaust valve opening.
Another advantage to longer rods is less side thrust against the cylinder wall by the piston, creating less wear in said cylinder wall.

From what Ive been able to read on the subject, long rods seem to inherently perform better at higher rpm as well.

 

[Old School Nut]

yeah guys, that engine buildup you guys are talking about is in the June issue i was talking about, its a 327 crank in a 400 block..

as for the TDC and sitting their longer, think of it like this, you have a crank throw with short rods attached to it, as the crank spins the rods reverse direction fast, almost snapping back, with a longer rod its slowed at TDC and BTC, however accelerates of both faster to make up for the time. you almost have to look at the extremes of both to see it. the reason is the rod angles.

if you still cant see it, think of it like this, with a longer rod the piston must travel further to turn the crank the same degree or rotation. likewise as the piston reaches tdc and is reversing it will have to stay their longer(near the top). this does not change the mean piston speed, rather it changes where the piston will be in relation to crank degrees. so the time spent around mid stroke will be shorter since the mean piston speed does not change. i dunno that helps, never was a good teacher..

 

[pmeisel]

OK, let me try my version

without disagreeing with anything said previously, consider this.

Imagine an engine with the shortest rods imaginable for the stroke, lets say 4.5" rods on a 3" stroke crank with a really short piston.

At mid-travel in the bore the angularity of the rod is huge, maybe 30 degrees (I'd calculate but Mr. Budweiser is visiting tonight).

One of Newton's suggestions has to do with equal and opposite reactions, so the side thrust from the angularity of the rod has to go somewhere. As Dana Carvey might say ... could it be.... friction?

Now imagine an insanely long rod, no limit on deck height, this is a stationary engine bolted to the floor.... the rod is 3 feet long.

The angularity is smaller, not insignificant but significantly reduced.

Back in the before my time days of Nascar, Smokey Yunick offset the crank bore in some straight sixes because the angularity of the rod on the power stroke caused more friction than more angularity on the exhaust and compression strokes.

I am considering all the time at TDC and piston speed arguments I here and they may be right but I haven't got my simple brain around them yet. I do understand trigonometry though....

Like most engine discussions there's probably more interaction to it than we have gotten to yet.

 

[Tech @ BG]

Long or short rods vs. detonation

There is not going to be a correct answer for this question, because you haven't set any parameters. Just stating which is going to reduce detonation isn't going to give you anything to go one, since you do not have criteria to work with. Also the bigger question would be why you have detonation in the first place, and what you're trying to prevent.

In the Engine Master's Challenge the engine builders are given criteria that the engine has to be built within. From there they are looking at the best torque, and horse power within given test parameters. Within that scenario, and the type of engine Jon was building the shorter rods worked better. Kaase was building an engine that he knew inherently was going to have detonation. The goal with that test for him was to see how he could suppress the detonation as much as possible to avoid loosing too much power, and to make sure the engine would live long enough to make the required number of dyno pulls.

There are many pros and cons to building a long or shot rod engine, (piston velocity, rod ratio, side loading, etc.) but unless you’re working within a specific parameter saying one is better than the other ALWAYS is like saying everybody should like blondes over red heads, or Ford vs. Chevy. It’s a matter of designing an engine to work within the given parameters you need.

 

[willowbilly3]

Blonds. Fords and long rods for me.

 

[Tech @ BG]

I hear ya....

I'm more of a Blonde, I mean Red Head... OH who my kidding... All women... as long as they're wearing a Bowtie...

 

[willowbilly3]

I hear ya....

I'm more of a Blonde, I mean Red Head... OH who my kidding... All women... as long as they're wearing a Bowtie...

You mean those women with bunny tails and ears with the bowtie? I like them too, well I don't actually knw any but they look nice.
I don't care much what women drive as long as it isn't some junk that I have to work on.

 

[Old School Nut]

Tech @ BG, yeah, your right, all engines are different, and what one likes another will not (like ignition advance) so i ask a slightly different, maybe a better question, what are the strongest variables that directly influence detonation resistance in relation to long or shorter rods? i assume, the biggest thing would be cylinder filling, (port/chamber design and cam). what do you all feel about their relationships with rod length and detonation?

my guess is that the better the heads the shorter of a rod you can use, while the longer dwell of a longer rod allows the use of heads that do not flow as well (maybe helping keep the air fuel well mixed?). being i know little in the area or air flow (why i am so vague), all ideas/comments are appreciated.

-Leo-

 

[Tech @ BG]

Leo,

I'm not sure I really understand the purpose of what you're trying to do? If you're working on a specific combination, or if you're just trying to get an idea. Still too many variables naturally aspirated, supercharged, nitrous type of block etc.

 

[Old School Nut]

humm well take a NA chevy, something i and many others are familiar with... and yeah i am just trying to get an idea, what are some things (in relation to detonation) that would lead you to use longer or short rods. what things do you take into consideration. it is a very general question. and its meant to be so i can get a better understanding. i want to hear the reasons why one would choose one over the other, you can use ford, mopar, whatever, but lets hear the theory as to why X part works best with part B, and why you get more resistance to detonation with the shorter rod, or the longer rod in a given example (again use whatever is easy)

thanks again, -Leo-

 

[Ohio-Dan]

Detonation is heard when two flame fronts collide. One flame front is started when the plug fires, and the other is a maverick, most likely started from a spike in cylinder pressure, or hot spot in the chamber.

Any given engine can only tolerate a given amount of cylinder pressure before detonation occurs. The shape of the combustion chamber, piston head, operating temps, and mixture quality are some of the things that dictate how much pressure an engine can handle.

In the Hot Rod world, we push for the highest amount of cylinder pressure, for the highest number of crank degrees as possible. The more pressure pushing on the piston, the more torque you make. Sometimes we go over board, and make too much pressure.

A typical Hot Rod motor will see peak cylinder pressure around 11* ATDC.

A long rod will allow the piston to dwell near the top of the stroke, allowing cylinder pressure to increase before the piston starts its rapid decent, increasing the volume, and lowering the pressure as it goes. If your pressures are below the detonation point, the increased dwell time, and increased pressures that come with it will net you more power.

If you are pushing the cylinder pressure envelope, using a short rod, and allowing the piston to start moving down the cylinder a little sooner, can help to cushion the pressure spike.

Unless you are pushing the envelope in a spec class, such as the Engine Masters, the long rod/ short rod vs. detonation resistance is a moot point. It's not going to make that big of a difference one way or the other. What you gain in pressure drop may well be lost in the frictional loss caused by the greater rod angularity. Your only talking a few HP one way or the other. It would take extensive dyno testing and big $$$$ to gain a couple horsepower.

I do know, that when Jon built the big block for the Engine Masters, he had a $20,000 cam budget............

Wonder how much was spent figuring the ideal rod length for that small block engine??????

To me, seems like a waste of money on a typical street rod motor. Dan