Rod lenght ratio

[V-8]

As you know stock 351w rod lenght 5.956.I want use 6.200 lenght Lunati sportsman h beam rods with Lunati special order pistons.There any advantages of using 6.200 lenght rods?

[Mr.Fixit]

yes, longer rods are better.

[V-8]

Can 6.200 lenght rods work properly with standard crank?

[Dwight]

MR FIX IT
WHAT ROD RATIO IS BETTER, 1.55 OR 1. 6?

[brainsboy]

6.2 rods with a stock crank, hmmm interesting question, not even sure it could be done. For one, your wrist pins would be up in the ring lands , the 351 has a fairly good ratio in its stock form anyhow.
Also using 6.2 rods is not exactly better. The purpose of using a longer rods is for better angularity with stoker cranks, to keep from serious wear issues due to a low rod angularity. In theory longer rods have less wear on a motor. However going from 1.55 to 1.60 is probably not going to make any difference because anything around 1.52 or better is considered ok. Keep in mind going from 1.45 to 1.55 is more of a difference then 1.55 to 1.65. becuase the ratio only helps you to a point.

[Mr.Fixit]

longer rods do more than decrease bore wear. A longer con rod makes the piston dwell at TDC and BDC longer, more cylinder pressure is generated before the piston starts to move away from TDC. Thus your motor will make more power. My basic rule of thumb, run the longest con-rod you can, without putting the wrist pin into the oil control ring on the piston. I hope you know your deck height (measured, not what the book says it should be)before you order your custom pistons.

[Junket]

The long rod will give more torque because of the longer dwell at TDC. the only thing it might not wind as fast as a short rod motor.
Ken

[brainsboy]

I know this gonna cause more chaos but here it go's....

A longer con rod makes the piston dwell at TDC and BDC longer, more cylinder pressure is generated before the piston starts to move away from TDC. Thus your motor will make more power.
__________________________________________________ _
In theory this is correct, in actuality its not always true. its kind of like compression ratio. The more compression is better.. Well not always, it is to a point. 9:1 to 10:1 creates more of a difference in HP then 13:1 to 14:1. This also applies to rod ratio and rod angle. Saying cramming a longer rod is always going to have more HP is not true. If this was true then new motor designs would have rod ratio's of 2.0+ . but they dont, why? Because for one, there are many things at play, weight of rods, piston weight, ring lands LOCATIONS, distance of the top ring to the combustion chamber, distance between 1st and 2nd ring.. All of this has a play in HP, it doesnt make sence to gain 3hp with a rod ratio then lose 5hp because all your rings are crammed up at the top of your piston with heat stress. Its not that people are trying to build with all the rod angularity they possibly can, its they are trying keep it with in reason or within spec. The normal standard is considered around 1.54 or more. This is seen in the 351w stroked to 4.17" off the top of my head I believe the ratio drops to 1.4's, here we have a serious need for a longer rod at the sacrified of moving the piston up, a shorter piston, less skirt area, ect ,ect.. In a stock 351 it is pointless, if anything its going to hurt you. It also gets back to the how much money per hp does it cost. 300$ rods custom pistons 400$, now you have spend 700$ for probably 3hp? Although my figuers may be a little extrem you can get the point. It just seems pointless from all aspects.

just my two cents, ok you guys are free to attack my opinion now

[Mr.Fixit]

"In a stock 351 it is pointless, if anything its going to hurt you."

Do you have any experience in building 351w's or is this mere conjecture on your part?

[Junket]

You won't get any flaxe from me the first 250 Hp. is cheap it's that next 250 that cost no matter how you break it down.
Ken

[brainsboy]

Mr Fixit, My specialty is with the 302 and 351, they are the reason I am on this planet, with out them I prefer death. I have been building them since I was 17, In the past I have had these motors in my dishwasher, oven, sink, bathtub, ect, I have had these motors everywhere. I eat, breath, and live by the ford small block, but Instead of rambling on about my qualifications and experience, lets use some logic. Lets assume I am 100% wrong for the time being. so lets go back to the original question.

"" As you know stock 351w rod lenght 5.956.I want use 6.200 lenght Lunati sportsman h beam rods with Lunati special order pistons.There any advantages of using 6.200 lenght rods?""

Are there any advantages.. sure #1, you just spent 233.33$ per 1HP #2 you have a nice set of pretty bullet proof rods, #3 your pistons are much shorter and will wear more due to the short skirts. #4 rings are now so close to the deck height you will have ring failure. #5 i almost forgot now you have a nice rod ratio to brag about

The logic point, why not spend this money on something more productive like head porting, or the difference between iron heads and going to aluminum. I have customers come to me all day long with the same problems. I had a customer come to me and say he bought a set of aluminum rods from BME and they claim he can run all day long under 6K rpm with out a problem in a street car, great, maybe so, but he has to buy a scat crank with a chevy 2.1 journal, then his rods are the old 327 2.0" so we need to turn the crank .010, now we have 9.27 rod pin, so new custom pistons, now we have aluminum rods that are thick as rat on steroids, so we have to clear the rod throws on the block which may not even clear with the 426ci.

MY point is keep it simple. Going through all the hassel of changing everying just for a 6.2 rod length is not worth it. Your wasting your money. If your writting a book, or just out to do test motors then great go for it. Or if you want a motor that no man has done before do it. The average person who wants to do a budget motor or have something that will last, stick to the basics, dont reinvent the wheel. I cant see doing all this work just to change to a 6.2 rod length.

[Sizzler]

Originally posted by brainsboy


...
MY point is keep it simple. Going through all the hassel of changing everying just for a 6.2 rod length is not worth it. Your wasting your money. If your writting a book, or just out to do test motors then great go for it. Or if you want a motor that no man has done before do it. The average person who wants to do a budget motor or have something that will last, stick to the basics, dont reinvent the wheel. I cant see doing all this work just to change to a 6.2 rod length.

I think everyone's point was:
1) Physically measure your deck height.
2) Within that deck height, given your particular stroke, try to maximize the length of the rods used.
a) within your budget
b) for your planned usage (drag racing engines to be regularly
torn down can run shorter ring packs and skirts).
c) with whatever baggage regarding parts you're carrying.

The point is, long rods are better. There is no magic number. I use 6.7" rods in an FE, but the FE is stroked so my rod/stroke ratio remains 1.7:1. Is 1.7:1 better than 1.6:1? Yes. Can you reasonably expect to be able to get a 1.7:1 ration in a Windsor block? If you destroke one. Do you want to destroke a motor just for a better rod/stroke ratio? No. But if you do destroke a motor, then you should look into lengthening the rods (3.5" stroke FE's with 7" rods are out there). Within the parameters above, get the highest ratio you can and don't worry about it.

[Perf. Eng. Eric]

On our 351W engines we use a 6.125 rod with the 3.500 stock stroke crankshaft. We get these from Scat. Then we can use a 302 style piston. We like to use the H- beam rods and forged pistons. Hope this helps.

[V-8]

Hey Perf. Eng. Eric ;
What are you thinking about stock 351w block?it is strong?