I know that the rod/stroke ratio will not geterate power by itself. However, if I am not mistaken, a R/S ratio close to 2:1(I can't remember where I heard this <img border="0" title="" alt="[Roll Eyes]" src="images/icons/rolleyes.gif" /> ) will allow the motor to rev higher---therefore (IF the motor is set-up properly) make more power. The stock ratio is @1.68:1. The common 4" strokers with slightly longer rods (6.225"?) will have a R/S ratio of @1.55:1. Is 6.225" the longest LS1 rod anyone makes?? What about a 6.225" rod with a shorter stroke crank? Let's see what the ultimate 346" motor might be built with:
Bore: 4.10
Stroke: 3.275
Rods: 6.225
That will get you a 345.9 cubic inch motor with a Rod Stroke ratio of 1.90:1. If someone could build a slightly longer rod---6.5" that would get you 1.98:1; real close to the supposed " magical " R/S ratio of 2:1. Think it would stay together up to 9,000 rpm? <img border="0" title="" alt="[Eek!]" src="gr_eek2.gif" />
What do some of you engine builders think?? <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />

 

Some folks think rod/stroke ratios up to 3:1 or even more help. There's a lot of good to be said about that, and not much bad. There is no magic upper limit to r/s ratio, IMO.

The limit is usually deck height. As you lengthen the rod (same stroke) the compression height of the piston (center of pin to top of piston) gets shorter. At about 1 inch you run out of room for the ring package.

If you can bore a 5.7 LS1 to 4.1 (don't think that is practical), why not increase the displacement? You'll get more torque and power from the cubes than from a larger r/s ratio.

Making power at 9000 in a 346 or 358 inch V8 takes heads that breath at least as well as Winston Cup heads. My guess is the 9000+ Cup engines have strokes near or shorter than your 3.275. They are reaching the limiting piston speed for current technology, at least for engines that need to run at speed for more than a few seconds. That's about 5000 ft/min. You have to do everything exactly correctly (and expensively) to live at those piston speeds.

Formula 1 engines turn about 18,000, but their strokes are just about 1/2 that of a Cup engine (1.65 in. or so...nobody will say exactly), so piston speed is just about the same as the Cup engine!

If I were building an engine for 9000, I'd have the stroke about 3.00 in. max. Unless the rules limited displacement to say 5L for a V8, I see no reason to run 9000, other than Winston Cup. You can do much more (for much less) with cubes.
C5R racing Vettes went from 6L to 7L (427 inches) to get power to win (and live)...which they do nicely now.

9000 rpm engines cost a ton more to build than 7000 rpm engines (assuming you count engine life in miles rather than feet). Remember, a 6000 hp Top Fuel engine gets rebuilt after every run, and the full run only spins the engine over about 700 times under full power...maybe less! A Cup engine turns over 1 million revs in a typical race...often more.

My (highly opinionated) $.02

 

For practical purposes R/S ratio is dictated by your displacement and minimum piston compression height.

I wouldn't put together an engine package shooting for a specific R/S ratia. Yes, to a certain extent you can crutch weaker heads with a longer r/s ratio and reduce peak piston velocity (though average is still the same) - but aside from definition of what a weak intake side is (I am thinking more like a 2 barrel rochester on stock old style GM production castings vs. the ls1 setups) you have to achieve a pretty decent difference in r/s ratio for there to be any difference. Same for cylinder side loading - yep, a longer r/s ratio will have less side loading, but realistically speaking at the differences in r/s ratio we can normally achieve the difference is nominal at best. Parts and build quality make a much larger difference - look at honda's, they typically run r/s ratios on the order of 1.4-1.5:1.

If you have a very free intake side depending on your cam timing a shorter r/s ratio may actually make more power even.

check out http://www.slowcar.net/shared/pistonmotion.xls
there you can play around with different r/s ratios and view the effects on the piston.

Realistically my suggestion for setting up an engine would be

1) Determine maximum displacement - as dictated by strength and budgetary concerns. This will determine your stroke size. Your Deck height is of course already fixed.

2) For your intended power level consult with your engine builder/piston manufacturer as to how much compression height you need/want.

3) You now have stroke, deck height, and piston compression height. Your rod length selection is now essentially fixed. If you have wiggle room I would go with the longest rod you can get now - but only within the window defined by (1) and (2).

Chris

 

Good words, Chris B.

 

So basically what Chris is saying is that it really doesn't matter about the rod length in an LS1, any of the popular 6.125", 6.200" etc lengths will not have any noticable difference on power output. Also, destroking an LS1 is a bad idea <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />

 

I've run 347 Ferds. They came in at 1.59, but at the expense of a 1" compression height. That makes for an unstable piston. But it ran like stink with NO reliability concerns.

R/S is good to reduce piston side-loading on the thrust side of the cylinders, and to reduce the proclivity of an engine to detonate, but is not necessarily the universal answer. R/S is usually confined withing the limitations of a piston compression height, and the minimum stroke of the crank you're willing to use. Remember, too small of a stroke greatly reduces the torque curve, and THAT is what moves vehicles. HP is just a theoretical calculation based on torque and engine speed.

Having said that, and if you're not dozing yet, my less-than-sage advice is to fit a piston with AT LEAST a 1.3" compression height for a good ring package (top ring down from crown > .300", the oil ring package NOT in pin boss), and the longest rod you can fit (in this case probably a 6.125"). Then find a good crank stroke to fill the space.

HOWEVER, you have to look at the valvetrain! A great R/S is only useful for insanely high RPM if the valvetrain will take it.

SC

<small>[ July 17, 2002, 10:29 AM: Message edited by: SS00Blue ]</small>

 

Yeah, like I said, IF the engine is properly built to run the mega rpms. Hence, Jesel rockers, Double Springs, solid cam and lifters, etc. Now, the cam would need to be real big to make power at say 7,500-8,500 rpms. This would mean a shitty bottom end torque number with only 346 inches. Plus the lack of torque because of the shorter stroke. That of course is why they make turbo chargers!! <img border="0" title="" alt="[Eek!]" src="gr_eek2.gif" />
I would just like to see the looks on folks faces when they ask how big the motor is and be able to tell them: "Stock 346 inches". <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />
I know the 382" all bore motors make good numbers. I would like to investigate how an all bore 346" motor would run.
Thanks for the tech info guys <img border="0" alt="[cheers]" title="" src="graemlins/gr_cheers.gif" />

 

More good words, SS00Blue.
Somehow I don't think the message is getting through.

"Torque rules...at any rpm."

 

You don't have to have a lack of TQ because it can spin to 8000. You can actually have more TQ.

The TQ peak is going to be alot higher 6000 maybe 6500.

With enough CR and properly picked out intake tract and exhaust you can set it up to pack the cyllinders alot more than a low RPM 346.

VE is a measure of how well you pack the cyllinders.

You'd think a well built motor with no limits and 13:1 CR, with an intake tuned for operation in 5500-8000 RPM powerband and purposely built headers would be able to pack the cyllinders better then a long runnered 346.

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by 1CAMWNDR:
<strong>I know the 382" all bore motors make good numbers. I would like to investigate how an all bore 346" motor would run.</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Slower than the bigger motor

<img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />

(seriously though)

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">You don't have to have a lack of TQ because it can spin to 8000. You can actually have more TQ.</font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">

Agree 100%. I think ther terminology is being slightly misused though - people aren't really talking about general torque, but low rpm torque.

Not to say you can't still have a good torque curve, but there are neccecarily going to be low rpm compromises on a high rpm motor.

Chris

Chris

 

Yes, the torque curve will deffinatly move up the rpm band. Don't know if the numbers would be the same only higher or not though. Very interesting info here.
Thanks
<img border="0" alt="[Firebird]" title="" src="graemlins/formula.gif" />

 

Chris B hit it all on the head. You also have to have enough cylinder length to accomodate the stroke and enough counterweight to balance it. This sometimes causes problems and having to add heavy metal which also adds expense. In the end cubic inches usually always wins!

 

Well, I guess I need to think about using a 4.1" bore and a 4.05" crank! That is 427.7 inches and use the common 6.225" rods. That is a R/S ratio of 1.53. Not great, but it will not matter too much in the long run. I bet that will still rev to 7,500 with the right cam and valve train.
This has been a great topic, now let's see what else I can throw around............Hmmmmmm Intake manifold plenum volume..........Hmmmmm. <img border="0" alt="[Burnout]" title="" src="graemlins/burnout.gif" />