Stroker Cranks

[Guest Crucial Conflict]

Just purchased a Eagle cast 3.75 stroke crank to

drop in a SBC. It calls for a 5.7 rod to be used.

Does that mean you can't use 6 or 6.125 rods on the assembly??? Is there any mods to can do to the counter weights or anything to run the longer rods???

graemlins/bsflag.gif

[20G TSi]

Rod length coresponds to the size (height) of the piston. Shorter piston = longer possible rod length.

[slow4now]

If you don't match the rods and pistons accordingly, you will have piston to head contact. Sounds like a whole lot of no fun to me!

[Dave1647545494]

it calls for stock rod length because the crank increases the stroke on a stock bottom end. you can use a 6 or a 6.125 rod but you'll have to replace the pistons. watch your piston to valve clearance too, you may have to notch the pistons to get it to work properly. HTH

[Guest Pats16V]

You can use a longer rod with that crank. The only things you will have to watch for are the weight of the rods which is what corresponds to the counter weights on the crank and piston skirt clearence. If you have any more questions call me . tomorrow night at Jeg's EXT. 650 after 8pm.

[Guest 614Streets]

All you need for the 6" rods is matching 6" 383 Pistons , then have it balanced at Fowlers or the machine shop of your preference. 383 is externally balanced at the balencer and the Flywheel/Flexplate.

[Trouble Maker]

Making your pistons shorter and your rods longer would decrease the weight of the pisons and increase the weight of the rods.

 

Do you all think this would help to decrease the side load on the piston walls, the pisons being lighter?

 

The only reason that I don't necessairly think this would be true is because of the increased weight of the rods, but the weight is in a different spot. It just isn't intutively clicking, and I really don't feel like doing that much research (on the weights and everything) to make an 'ideal' less than realistic model, nor do I have the time.

 

That would be pretty trick becasue that is one of the concerns when you stroke an engine (increased side wall load).

 

Anyone have any thougts on this?

 

EDIT: a longer rod would increase (or decrease, whichever way you look at it, what's the standard nomenclature? ) the angle too, further decreasing side wall load.

 

Would a smaller (shroter) piston be a weaker piston?

 

[ 07 November 2002, 07:08 PM: Message edited by: Mad Jack Bonney The Crazy Pirate ]

[20G TSi]

piston side loading has alot to do with the rod/stroke ratio. a long rod with a mediocre stroke will not cause much side loading. a large stroke with a short rod WILL cause excessive side loading/cylinder wall scaring. As far as power is concerned, the shorter rod will make the more high end power, however, when longevity is a factor the shorter stroke/longer rod will last longer w/o scaring the cyl. walls and compromising cyl. pressure.

 

A good median is 1.61 rod/stroke ratio. My 408w has a 6.25" rod/4" stroke, making a 1.56 rod ratio. A stock 454 chevy has a 1.54 ratio. anything less than 1.48 is considered a race only combo (regardless of what someone is trying to sell you).

 

The combo this guy has is a good one, 3.75" stroke/6" rod = 1.6 rod ratio, which is ideal.

 

[ 07 November 2002, 08:02 PM: Message edited by: Jasons TSi ]

[20G TSi]

to add, a shorter piston is not necessarily a weaker piston. the biggest drawback to shorter pistons is that sometimes the piston wristpin is into the oil rings. this obviously results in poor oil control.

[Guest 614Streets]

Correct , most strokers drink oil because of the oil ring land intersecting with the piston pin land. Some Piston manufactures make pistons to conteract this. An example would be Probe Piston 347 ci.

[Boost-n-Juice]

So the actual weight of the rod doesn't make a difference? The clearance is a given, but what about weight?

[20G TSi]

Originally posted by BC:

So the actual weight of the rod doesn't make a difference? The clearance is a given, but what about weight?

sure it does, thats why they make aluminum rods...

 

It just becomes less of an issue with the increased torque that cubic inches bring. Especially when you're only talking about a few grams (a good h-beam steel rod should weight roughly the same as an iron I-beam, or just a tad more). I had Bischoff (tony bischoff/gary rohe) balance my stroker, so I dont know exact values going from a stock 351 rod to a 6.25" H-beam... but since you typically use a shorter piston, you'll gain some weight differential back there.

[Trouble Maker]

Originally posted by Jasons TSi:

</font><blockquote>quote:</font><hr />Originally posted by BC:

So the actual weight of the rod doesn't make a difference? The clearance is a given, but what about weight?

sure it does, thats why they make aluminum rods...

 

It just becomes less of an issue with the increased torque that cubic inches bring. Especially when you're only talking about a few grams (a good h-beam steel rod should weight roughly the same as an iron I-beam, or just a tad more). I had Bischoff (tony bischoff/gary rohe) balance my stroker, so I dont know exact values going from a stock 351 rod to a 6.25" H-beam... but since you typically use a shorter piston, you'll gain some weight differential back there.</font>

I would think thoes few grams on each rod, rotating fast, would help alot in how well an engine can rev.

 

But besides that do you think the decreased weight of the piston itself would help decrease the sidewall loading.

 

Pressure=Force/Area

Force=Mass*Acceleration

 

So Pressur=Mass*Acceleration/Area

 

A smaller piston would be lighter and have less area. If the porportion of the Mass/Area of the pison that hits the sidewall were drcreased it should cause a decrease in sidewall loading. This may not be that important when biulding a street engine, but then again there are some people on here that have engines that are far from 'street' engines.

 

Smaller piston would also mean less rotating mass, and less mass that the combustion process is directly moving. Better reving, more efficient, engine. I think I'm onto something here, but I've never actually biult an engine before. So opinions and criticisms are very welcome?

[20G TSi]

The angle (the degree of angle actually) and speed in which the piston is in is what causes the side loading, more so than any other factor.

 

With a big stroke/short rod, the piston speed is greater than with a big stroke/big rod. Because the piston is moving slower, it spends more time at TDC allowing a more complete burn.

 

so, say a motor has a 1.46 rod ratio... at 4000RPM, the piston speed will be the same as in a 1.6 rod ratio motor at 6000rpm. Since the piston speed is slower:

 

1. the weight of the piston is not as critical

2. the rings do not wear as fast

 

[ 08 November 2002, 08:59 AM: Message edited by: Jasons TSi ]

[Guest Crucial Conflict]

Finding compression height of a piston...

 

~Stroke divided by 2, then added to rod length, then take that figure and subtract from deck height.. You must use 9, and not 9.025.. 9" deck height is whats blue printed...

 

3.75 / 2 = 1.875

 

6 + 1.875 = 7.875

 

9 - 7.875 = 1.125 Comp. height needed

[Guest 614Streets]

Originally posted by Mad Jack Bonney The Crazy Pirate:

I would think thoes few grams on each rod, rotating fast, would help alot in how well an engine can rev.

 

But besides that do you think the decreased weight of the piston itself would help decrease the sidewall loading.

 

Pressure=Force/Area

Force=Mass*Acceleration

 

So Pressur=Mass*Acceleration/Area

 

A smaller piston would be lighter and have less area. If the porportion of the Mass/Area of the pison that hits the sidewall were drcreased it should cause a decrease in sidewall loading. This may not be that important when biulding a street engine, but then again there are some people on here that have engines that are far from 'street' engines.

 

Smaller piston would also mean less rotating mass, and less mass that the combustion process is directly moving. Better reving, more efficient, engine. I think I'm onto something here, but I've never actually biult an engine before. So opinions and criticisms are very welcome?

Less weight certainly offers the advantage of less stress loads imposed on the entire rotating assembly and the block. There is comprimise. Making parts lighter makes them weaker , which is why lightened parts are manufactured with higher tensile strenth. Lightweight parts are engineered with higher quality materials such as billet steel

and SAE grade 1-15 4340 steel. Lets take a look at grades of Steel from strongest to weakest listed in SAE Tensile strenth.</font>

• 6415 Billet Steel 160,000 lbs tensile strenth</font>
• 4340 Forged Steel 140,000 15 grades available</font>
• 4130 Forged Steel 120,000 ? grades</font>
• 5140 Forged Steel 115,000 Legendary L67 Chevelle used this material</font>
• 1045 Forged Steel 105,000 Typical Factory Forging</font>
• SCAT 9000 Cast 105,000</font>
• Cast Iron 95,000
  </font>

Tensile strenth and Fatigue strenth are linear one goes up with the other.

Most crankshaft failures occur from the use of bad dampners. Although companies such as fluidamper and TCI have no elastomer dampners , most professionals stick with a rubber/elastomer dampner, and the lightest available. ATI and factory are elastomer type. These recomendations are based on the fact that elastomer is stationary and fluid dampner and rattler balancers have a inertial moving force inside. An Ill affect in its own right.

The less weight the less mass the easier to turn , but there are other variables affecting ability to have high revolutions. One of these variables is crankshaft stroke. A crankshaft with a shorter stroke can reach higher revolutions because the piston speed. A bigger stroke imposes more load and the piston travels faster to cover the same revolution. This is because the piston moves over a greater area of the bore than a shorter stroke yet doing so at the fixed time of the central crankshaft revolution.

 

The number one and most overlooked cause of rod failure is contributed to the camshaft. Revolutions that exceed the camshafts ability imposes Rod shock. Under normal engine operation the piston encounters a "buffer" or cushion of traped air within the combustion chamber. This is the normal compression that the pistons creates when rising in the bore. When exceeding a camshafts revolution capacity and exceeding other valve train capacity such as springs and lifters , valve float occours. When valve float occours the valve is uncontrollably thrown down to open and a valve springs inability to control the action leaves the valve open and cylinder bleed off occours. Compression is lost and the buffer/ cushion for the piston is lost as the rod tries to exit the block towards the cylinder head.

 

Your understanding of less weight affecting revolutions can be seen and related to the affects of changing steel flywheels to a lighweight aluminum unit.

[Trouble Maker]

Originally posted by 614streets Performance:

Your understanding of less weight affecting revolutions can be seen and related to the affects of changing steel flywheels to a lighweight aluminum unit.

Yeah, I have this ideal daily driver in my mind I guess. I would like to do an awd dsm, stroked, all lightweight, but strong internals, blah blah, something like a big 16g, fmic. Something that will be very fun to drive, well reving, widepowerband car. A low 12, high 11, nothign insanely fast, just able to beat ANY stock car . Handels like it's on rails, nice stereo, dubs, hehe, j/k on that last one. I just want it to be a very 'fun' to drive car.

 

[ 08 November 2002, 01:27 PM: Message edited by: Mad Jack Bonney The Crazy Pirate ]

[Guest Pats16V]

I think the original question to the post was not answered here. When most crank mfgrs. builds cranks, Forged or Cast they set up the counter weights for and approx. rod piston combo weight. When you purchase the the crank the company does not know what pistons and rods you are going to use, so that is why you still need to have the assembly balanced at a shop. Then they can weigh your pistons, rings, rod bearings, and rod. Then they add bob weights to the rod journals and spin test it. From there weight is either added or subtracted form the counter weights on the crank. This is how a crank assembly is balanced. Alot of shops (including Eagle) will sell a balanced rotating assembly with a Balancer and Felxplate and will take weight off of the flexplate also to get it to figure right. This is usually done with a torch by blowing several holes through it. Or they add a weight to the flexplate by welding to it. This is a cheap way and personally I don't like it. Some Mopars have the weight added to the Torque Converter. When you balance an assebly w/o a flexplate/flywheel and harmonic balancer this is considered neutral, zero, or internally balanced. This can get to be exspensive due to the use af alot of mallory metal. But it allows you to run a variety of balancers and flexplates/flywheels. Sorry kinda long but just my 2 cents.

[20G TSi]

I would have to totally disagree with you. the counter weights are to counteract the crankshafts rod journal "throw" and has nothing to do with maching rod/piston weight.

[Guest Pats16V]

Sorry about double post. To answer the second question. You can run a longer rod just make sure you buy a piston w/the correct comp. height. Make sure you take into account if the block is going to be line honed and the deck is milled. Most Mfgrs. on the market leave the pistons about 20-30 thou. in the hole so if you mill the deck and line hone that amount you will leave the piston at zero deck height. which means that the piston top will be level with the deck. While this gives you excellent quench between the head and piston you will really need to watch out for your vale clearences. Please call me tonight if you have any more questions. The deck height for a Small Block Chevy is 9.025. to figure you piston comp hgt. use this formula. dh-strke rad-rod lenght= comp hgt. Or

9.025-1.875-6.125=1.025"comp hgt.

 

To figure stroke Rad. take stroke of crank 3.75"and divide it by 2= 1.875

[Guest Pats16V]

Originally posted by Jasons TSi:

I would have to totally disagree with you. the counter weights are to counteract the crankshafts rod journal "throw" and has nothing to do with maching rod/piston weight.

Oh, then why would a machine shop need to know the weight of the pistons and rods? Or have to add or subtract weight from the counter weights on the crank? Maybe that's just their way of getting more money out of us?

[Guest 614Streets]

Patte is correct. Bobweights are set up to the balanced Piston/ring /rod weight after the piston and rods are balanced , truly balancing rings is questionable. First you weight match all the Pistons by grams, then the rods on a special fixture again by grams. The total gram is set up on a special fixture called a bobweight which is shown below

http://www.centuryperformance.com/images/tech/v8boblrg.jpg

The Crankshaft is spun balanced with the bobweights and metal is removed or added (like Patte mentioned Mallory Metal).

The balancer and the flywheel are individually spun balanced like a wheel/tire.

 

I do disagree that the original question wasnt anwsered , Balancing was my first post. I just didnt walk the post because I assumed he knew about Balencing. But nobody is wrong here so lets not have a great post go sour with sarcasm.

[20G TSi]

I agree, this is a great post.

 

Does that mean you can't use 6 or 6.125 rods on the assembly??? Is there any mods to can do to the counter weights or anything to run the longer rods???

The first was definately answered, and I assumed the second was implied. Any time you put together a rotating assembly and you expect to spin it over 5000, it should ALWAYS be balanced....

 

NEVER NEVER NEVER NEVER assume a crank/rod/piston manufacturer has machined/balanced ANYTHING correctly. I've seen "ready to assemble" 'kits' need severe machining and balancing. Everything should be checked and double checked, IMO.

 

[ 08 November 2002, 05:17 PM: Message edited by: Jasons TSi ]

[Guest 614Streets]

Originally posted by Jasons TSi:

I agree, this is a great post.

 

</font><blockquote>quote:</font><hr />When most crank mfgrs. builds cranks, Forged or Cast they set up the counter weights for and approx. rod piston combo weight. When you purchase the the crank the company does not know what pistons and rods you are going to use, so that is why you still need to have the assembly balanced at a shop.

Thats the only part I disagree with. Also, I do believe the original question HAD been answered within the first 3-4 posts.

 

Does that mean you can't use 6 or 6.125 rods on the assembly??? Is there any mods to can do to the counter weights or anything to run the longer rods???

The first was definately answered, and I assumed the second was implied. Any time you put together a rotating assembly and you expect to spin it over 5000, it should ALWAYS be balanced....

 

NEVER NEVER NEVER NEVER assume a crank/rod/piston manufacturer has machined/balanced ANYTHING correctly. I've seen "ready to assemble" 'kits' need severe machining and balancing. Everything should be checked and double checked, IMO.</font>

I guess you disagree with Patee but from a 3rd party perspective Id say you guys actually dont disagree. Both of you state that a rotating assembly should be balanced and to never assume that throwing in a off the shelf crank is the right thing to do.

 

I'll admit I was thrown off by what Patte said about the crankshafts being set up from the factory for a certain piston / rod. I guess two things. It would be better said that Patte works at Jegs and deals with manufactures on a daily basis and that he understands that the manufacturer provides the closest to bolt in possible. It could be said that his statement could easily be picked up as "EAGLE SETS UP THE CRANKS AT THE FACTORY FOR .......ETC ETC..."

I know what he is saying though. The manufacturer sets it up in a general fashion so its as close to universal as possible.

 

When my freind talked to Jeff Fowler about buying a crankshaft for his engine Jeff mentioned COLA as a preference and said EAGLE was a bit on the unperfect side and needed more attention. But hey we all know we get what we pay for when we take it straight up without devoted background reasearch.

 

[ 08 November 2002, 05:30 PM: Message edited by: 614streets Performance ]

[20G TSi]

Originally posted by Patte:

</font><blockquote>quote:</font><hr />Originally posted by Jasons TSi:

I would have to totally disagree with you. the counter weights are to counteract the crankshafts rod journal "throw" and has nothing to do with maching rod/piston weight.

Oh, then why would a machine shop need to know the weight of the pistons and rods? Or have to add or subtract weight from the counter weights on the crank? Maybe that's just their way of getting more money out of us?</font>Re-read... I think I read too far into what he said. I was thinking, as a whole, a mfg does NOT say, "well, we'll make these counterweights 35oz cuz the customer will probably use aluminum rods, and these counter weights 45 for use with IRON rods". Its a given that during the balancing process the crank and usually the flexplate have material added/removed. I guess it was just the way I read it.

 

Regardless, this thread contains some very good info!