Can you please help me understand rod stroke ratio and how it affects performance? WHat is a good ratio ? THanks

Divide rod length by stroke to get the number. Usually a longer rod will have better higher-rpm performance according to some, as it has more piston time near TDC and BDC . I liked using shorter rods in my smaller 331/341 SBC race motors. They give quicker piston acceleration thus I ran more intake port volume and camshaft, and a bit more timing. The theory is short-rod motors have more low-end torque. On the other hand, Reher-Morrison did a study for GM using a BBC and they stated all the rods did was link the crank and pistons, and length made no real differernce...who knows?

A shorter rod/stroke ratio will have a slightly higher volumetric efficiency because of instantaneous piston speed. The engine will pump slightly more air basically, making a little more torque. However, the increased piston speed will also cause more stress on the rods. Likewise the greater angularity of the rod in relation to the cylinder wall can result in faster wear. Using shorter rods also usually means that you will have a taller piston, which generally weighs more, increasing the weight of the rotating assembly. These factors combine to make a slower revving engine that will be put under more stress at higher rpm.

A long rod/stroke ratio engine does lose a little volumetric efficiency, but benefits where the short r/s ratio lacks. Generally, this can be a lighter rotating assembly, will rev more quickly, and will go to higher rpms without as much risk. Also, the slower instantaneous piston speed means that the piston dwells at TDC longer, which promotes quench and resistance to detonation. Therefore, with a higher rod/stroke ratio you have the ability to run a slightly higher compression ratio on the same gas. There is an interesting article called "the small block chevy should have built" that can be found in the article archives at www.airflowresearch.com. (http://www.airflowresearch.com.) Here, they built a very long r/s ratio engine that could run at 11:1 compression on 87 octane without a huge duration camshaft.

All in all, the differences aren't that pronounced and probably wouldn't be very noticable in a street engine.

I read once in a special "engines" edition magazine by Peterson Publishing Co a few years back where they claimed that a 1.75:1 rod/stroke ratio was about a perfect balance for an "accelleration engine" for the quarter mile. They said that as a balance between VE and wear characteristics a 1.6:1 ratio was good.

Here's an article that puts the relationship in a physical perspective:

http://www.rustpuppy.org/rodstudy.htm

Onovakind, that is a GREAT summary, good stuff bud. The comment on the end stating that he would like to see an objective dyno comparison is a good one, I had considered doing a long rod upgrade to my 350. Problem is though, I would have made other changes that would have killed the comparison(porting, more cam, etc). I have since decided to go with a stroker kit for more cubes.

One item I am tossing around a bit is compression height. For a 6" rod 383 chevy, the compression height is 1.125". I heard from someone that the minimum CH they recommend is 1.25", which would suggest that a 5.7" rod is a better bet with a larger compression height. A counter to that is that the longer rod piston will weigh less, which is an obvious benefit. Compromises..... I will probably go with the 6" rod version.

And the ultimate short rod engine is one where the rod is the same length as the stroke!!

Probably can't be built but it will show you if you draw it out that the piston will stay at BDC for 180 degrees of crank revolution!!

This is finally how I figured out what they are talking about with respect to this stuff..

The other extreme is a rod that is infinately long, impossible to build but good theory.

This allows the piston to spend the same time at both TDC and BDC!!! Think about it for a bit...

pdq67

Unless you are really buzzing the motor, piston weight is not that critical. If you intend to run 8000+ all the time, that is different. But, it is possible to get some real light pistons even with a larger compression height. I have run some for a 4.030" bore that were just under 400g with a 1.750" and 1.780" CH and massive domes. They were BME pistons with dome profiling and lightening work done by Patterson Racing in Augusta, Kansas. The only thing is you are looking at just shy of $1K for a set like this, and they are 2-ring pistons also.

pdq,

OUCH!!! My brain hurts now. To quote Curly from the 3 stooges "I'm tryin to think, but nothins happening". It's too late for me to think that abstract. Seriously though some good exchange here on comp heights, rod lengths and dwell time. A lot of people still argue the extra dwell time doesn't buy you anything, but I don't agree. I think it will make a difference especially if you tune properly for it and run the correct camshaft to take advantage of it.

I think one of the problems that people are pointing out to Mean 69 about the 1.125 pin heights is that pretty soon the rings start getting stacked pretty closely and you end up with the pins located up in the oil ring grooves.

I noticed that there are 327 single ring pistons for sale right now on ebay by the way.

Dave

By 2-ring pistons, I meant a single compression ring and an oil ring. Usually that is how they refer to the type of piston. By a single-ring piston, I would guess it actually should be called a 2-ring piston. Most 2-ring pistons are set up with gas ports and an .043" top ring with a very snug clearance so it will not flutter at high rpms. I had the best luck on these pistons using a standard-tension oil ring too. smile.gif

That pretty much is what the deal is with the short compression height. I had a stroked 5.0 Ford (be nice...) from a company that had moved the piston pin slightly away from the centerline of the piston, it lowered the effective height and reduced the rod angle a bit. It also prevented the oil ring lands from intersecting the oil ring lands. They clained that this gave them superior "oil control" than the pistons that did intersect the ring lands. (The company is called Coast High Performance, by the way). I think that Chevy is doing this with the new 383 stroker small blocks too, but I am not sure.

To be honest, I am not sure what they meant by oil control. I don't see any benefit to oil blow-by, or consumption, They must be referring to lubing the cylinder walls effectively? Not sure. My stroked rat motor piston pin DOES intersect the oil ring land, they supply thin supports for the rings, and it seemed to be fine.

I have also heard that if you pack the compression rings too closely that the ring lands become really weak, and this can cause problems.

For a race engine, who cares? I think the debate is for a routinely driven street car, longevity is the claim. I guess the counter argument to that is that a guy who builds a 500 HP street motor probably isn't going to leave the thing alone for five years and 100k miles anyway!

I'd be interested in comments regarding the rings/compression heights though. I saw a cool 3.875" stroker small block, 396 CI. Hmm??????

Steve take a look at this set of Manley pistons, they're the ones on ebay right now.

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=33623&item=2414421893

Dave

yikes... what kind of stock heads worked with a 22cc dome piston? something like 85cc chambers?

Those ebay pistons would work real nice with 64 cc heads to give 14:1 comp ratio, and LIGHT to boot !! I can see a alky-SBC 331 w/5.7" rods, large cam, idling at 2500-3000, wicked !!

Why not go with a Little M block and build a 440 vice a 396?

Doubt it would have to idle that high :D My 4.030" x 3.340" 341" idled at well under 2K with a cam that was 288/296 @ .050", with over .700" lift on the intake side. :eek:

He, He!!, Dave,

That's what I did until I actually drew the sucker out on paper to see what they meant!!!

And Mean, they've been doing stuff with moving the wristpin to the side for years.

I flopped the pistons in my junk 301 side to side so that my wristpin would be to the side that gave it a very slightly better angle to help the piston push..

My old Machinist had told me to do it b/c he had done it for years and years.. The only thing is that you get a very slight amount of extra skirt slap until she warms up. He told me not to rpm it until she was warm b/c he said you CAN break a a skirt right under the oil ring if your motor is set up on the loose side which my junk 301 was b/c of my dingle-berry hone "bore" job......

pdq67

Thought about the Little M block, or the other aftermarket blocks too, but the price scared me. I already have the engine, so a stroker kit and bore job would be about the same costs as a aftermarket block alone.

So guys, from a compression height standpoint, how small is too small?

Isn't it about 1.125" or so??

Heck, do yourself a favor and check with who you want to get the pistons from. Ask them point blank what their recommended min. compression height is. That will set your rod length along with stroke and deck height...

Please note that you may have to move your compression height some to be able to use a cheaper "off the shelf" rod length. But if you have the money go for what you want!

pdq67

383 is 1.125", a 396 would be 1.063", both with a 6" rod. 1/16" of an inch difference in compression height doesn't seem like much difference, might be important though.