Why is this "Perfect squaring" of the motor a "good thing". I know you dont want more stroke than bore, and that the real power is made in bore, but why do you want bore to be the same as the stroke? Thanks in advance!

 

anyone else?

 

Bore and Stroke have individual effects on power production. To say that "real power is made in bore" is an incorrect statement. DISPLACEMENT has an effect on power, but that is true whether you make your gains in bore, or in stroke. The first thing you need to understand is that there is no "magic combination." Words like "best" don't really apply here any more than they do in "what's the best camshaft?" This, like most anything else you do when building an engine is completely dependent upon the engine's intended application, and desired output.

Increases in bore and/or stroke affect the engine's performance characteristics much in the same way that increases/decreases in camshaft duration do. You add a lot of duration, you lose torque, but gain peak hp. You pull duration out, you gain torque, but lose peak hp. This is the same with your Bore/Stroke ratio. When you have more bore than stroke, you have an "over-square" engine, more stroke than bore is an "under-square" engine, and equal bore and stroke is an engine that's considered to be "square." "Piston speed" is another term you will run into on this topic. Piston speed is basically how fast the piston actually travels and/or accelerates through the bore. Obviously, if you have two engines spinning at the same RPM, one with a 3.75" stroke, and one with a 4.125" stroke, in order to be spinning at the same speed, the piston itself MUST be travelling faster in the longer stroke engine. This, in turn, will create stronger inertial forces within the engine, greater stresses on the rotating assembly, and higher risk of catastrophic failure compared to a similar engine with lower piston speeds.

Most people consider a square engine to be the best balance of hp/torque, as well as rev characteristics. Over-square engines typically produce more hp at the sacrifice of some torque, but handle high-rpm's very well, making them well suited to racing applications. When you hear terms like "de-stroking" they are just talking about purposely taking stroke out to arrive at an over-square engine that can really eat the high RPM's. Under-square engines typically have more torque with some sacrifice to hp, and have less than desirable piston speeds at high rpms, making them better for street applications, or towing.

Lots of people have lots of opinions on this topic.. those are mine. Hopefully that disclaimer keeps me from catching too much ****.

 

Fact is, I have an opinion on this. But all I will say is that I have seen many different combinations of bore/stroke that flat out invalidate just about every opinion concerning RPM, piston speed, engine acceleration, etc.

eg. most people will tell you that an engine needs a shorter stroke to rev high, but I've seen engines that had more stroke than bore rev over 8k and do it well.(M3)

 

awesome, thanks for all the info. That makes sense about the piston speed being greater with more stroke. and for the record, i didnt ask for the "magic combination" for the ls1, i wanted to learn the mechanics of how an engine works, not cam specs and what motor to build

 

Good question Camaro-Zach,That was a good response 1999-SS-M6,I wondered about the same thing especially since the ls1 is bore limited.I"ve seen test where two sb Chevys were built one with 4.125 bore x 3.5 stroke and the other was 4.03 x 3.75 stroke heads, cam intake, carb.,everthing the same the power curves were real close but the 3.75 stroke made more torque to say 4500 then the larger bore caught up but the hp output was very close with both motor combos.This was in Chevy High Performance a few years ago.Also look at the 455 Olds small bore and long stroke but it has long rods,455 Pontiac same thing,they make tons of torque but only so much hp.Another thing with the longer stroke is your rod to stroke ratio,with the longer stroke you have to use a shorter rod to keep the piston from going too high in the block,this puts a higher side thrust on the piston and the higher angle of the rod don"t exhert as much downward force as a longer rod would so hp suffers,the high side trust causes more friction on the piston also makes the piston ROCK and this makes it hard for the rings to seal,losing power.Most engines are under square, but the ls1 with a 4 stroke makes great power,but so does the 408 ironblocks(4.030 x 4.0 stroke)almost square.I would like to see some 4.125 and 4.25 strokers built just to see how they would do!Anyway good question!!

 

Totally agree with this statement. A 706 ci nitrous promod motor will spin to around 8200 rpm. 4.605 bore X 5.300 stroke. $55000 motor, but still. Mountain motors up to 814 ci have even more stroke. Cubic inches make power.

 

sign me up for one...ok two, the trailblazer could use a pick me up too. haha

thanks guys for all the great info. i appreciate it.

 

It's human nature to instantly start searching for some caveat, or some example that invalidates someone's statement, or opinion. That's why I made that disclaimer at the bottom of my first post. It's that whole "opinions are like ********, everybody's got one, and they all stink" situation. There are exceptions to every rule, and I was painting with a very broad brush when giving my opinion on bore/stroke ratios, and at no time used words like "always" or "never." However, just because you "can" do something, or you've "seen it done" doesn't necessarily make it a good idea, or change the fact that there's a better way.

Also, Zach, no offense was intended... I wasn't implying that you were asking about the "ideal" set up, per se. I was simply offering that in these types of threads, the more Jr. members will eventually get around to asking "ok ok, so what is the best setup then?" I was just trying to get out in front of that.

Also, I hope that I didn't sound like I was speaking in absolutes about not being able to rev long stroke engines, or having to really spin the hell out of shorter stroke engines to make power. As Technical, and Lonslo mentioned, there are plenty of huge displacement and long stroke motors spinning sky high. What was left out though, is that those huge displacement engines intended are hardcore racing applications where you'll likely go through several engines per season. I didn't take your question to be applied to 700+ ci race motors, I understood it to be in regard to the kind of engines combinations that will live a long time, and perform well on the street and strip. Also, I really wouldn't call a DOHC inline 6 cylinder engine a good gauge for predicting your results with a 90 degree pushrod motor either, although I loved my M3 right up until I sold it. That thing loved the rpms, but inline 6's are smooth as baby's butt even if you're running on 5 cylinders.

I don't think anyone would argue that the combination of faster piston speeds, and/or slingining bigger, heavier parts to high rpm is going to shorten engine life. They could try, but they'd still be wrong. lol Do some research on "bearing load" and you'll see my point. It's just a matter of tensile strengths and psi.... physics physics physics.

 

Agreed. I was trying to get the point across that some on this board think that a motor needs to be square, or under square. That's simply not the case. It is definitely application specific. Those big cube race engines are short lived, but they make gobs of power too. I know if these LSX motors had spread pan rails, and a tall deck, I would be cramming big stroke cranks in them, but as it is...

 

Assuming no part breakage and adequate intake/exhaust/cam/etc, ring flutter is a primary limit to RPM. Ring flutter is controlled by stroke, rod length, ring mass, and ring tension (including gas ports for more tension etc). Bore size has nothing to do with this (other than the effect on the ring mass) so under/over square has nothing to do with how an engine PRMs. An under square engine with a 2" stroke may easily turn over 10K, but an under square engine with a 12" stroke won't. Be careful with generalities.

 

We're on the same page.

An engine is a system. Every part/spec plays a role. Therefore we need to talk in terms of a system. I usually hear people talking about one aspect of an engine being the reason it revs or makes power. eg. smaller stroke == higher revs; not true.

 

Sometimes, people say things that are just plain wrong.
This is the most ridiculous statement I've ever heard
You should've taken your own advice

Ring flutter deals ONLY with cylinder sealing.. that in no way has bearing on an engine's rev characteristics. Ring flutter is an afterthought to an engines intended RPM band. Also, it only applies OVER 8000 or 9000 RPM's.... well outside of the scope of 99% of the engine projects on this board. Besides that, there a several proven ways to prevent/deal with ring flutter if in fact you're intending to encounter it. Gas ports through the piston faces, manipulations in the top ring's mass, opening the top ring gap slightly to allow pressure to equalize back into the combustion chamber etc etc.

I think the conversation we're having here is in regard to over-square and under-square engines, and how that applies to combinations that build lazy revvers, or quick, responsive engines that blip up to 6500rpm, and how bore/stroke changes affect power production on a sliding scale.

 

In all fairness, I actually didn't see your post before I posted. That's a good thing because I would have blasted the crap outta this...besides, everyone knows synthetic oil is what makes an engine rpm- duh!!

 

How did we get on this subject? If it was because of my trailblazer comment, the trailblazer DOES come with the ls1 (5.3 ls1 as an option) and i was just making a joke because i think a trailblazer with 600 cubes would be a blast hahaha.

Anyways all this information is great. So in the end it doesnt really matter? I mean, i know not to put a 4.25" stroke crank in a 2.0" bore car. But if we are talking 4.0 vs 4.125, it doesnt matter which, bore or stroke, its still going to make power and move the car forward. So there is really NOTHING to be gained by a "squared" motor, just because bore and stroke are the same, you gain nothing at all. Thats what i was looking for. Thanks everyone!

 

Let's see .. I said we have no breakage issues and enough head/cam/etc so support any RPM we want. At some point in time, as the RPM goes up, ring flutter will set in, the rings will fail to seal, and the engine will not develop any more useful HP. Looks to me like it does have an impact.
Not true. Ring flutter is a function of the mass of the rings, the tension of the rings, and piston speed, (or perhaps more correctly, piston acceleration, which is a function of piston speed). Piston speed is primarily a function of stroke length and RPM, modified a bit by the stroke/rod length ratio. A stock LS1 exceeds the normally recommended piston velocity well before 8000 RPM.
Uh, I already said that.
And I responded to the post that said some under square engines can rev past 8000 and that is proof that conventional wisdom (that over square engines are revvers and under square engines are torquers) is wrong. In fact I agree with the conventional wisdom.

You can disagree, but you can't change the laws of physics

 

Agreed, you can't change the laws of physics, but you should understand how they are applied. If you understand physics, then you should understand what a system is.

 

Don't forget about valve size!

Obviously, a larger bore more can have larger valves, which help the engine breath, especially at higher RPM's. Where as a longer stroke motor can run out of breath.

When you give the example of larger stroke engines, as in race engines, they benefit from supercharging which can force air through the valve opening. Thus negating the "smaller" bore.

 

Ha ha, exactly. Just like the government's motto - "Why buy one when I can have two for twice the price..."

 

So the M3 engine which I mentioned is a race engine?

Volumetric efficiency is what allows an engine to rev. How you get better volumetric efficiency is dependant on many things...cam,valves,heads/combustion chamber,compression,pistons,intake,exhaust/headers. I could go on to name practically every part of an engine, but that's why it's a system. No one aspect makes it all happen.