Balancing Problems???

[scottj]

Something else, even a 6.25 rod with a 4" stroke would not be defined as a long rod motor. By definition, a ratio of 1.56:1 would be a short rod motor.

Brainsboy endorses short rod motors, thats fine, but the rod length(6.25") that Brainsboy argues against is actually SUPPORTED by his argument.

As the article points out, 1.8:1 is the "ideal" compromise between breathing and mechanical stress.

[Mr.Fixit]

Originally posted by brainsboy


My father neibour "Jeff Diamond" , satisfied now?

we all learn from somewhere, I suppose you learned everything on your own, with out reading? it sure is easier cut and paste then try and sit and type everything I learned from him.

Presenting somebody else's work as your own, without citing that it is somebody else's work, is plagurism. Try to avoid it in the future. Calling you on it this time sure seems to have struck a nerve however.

[flipper35]

brainsboy, maybe you should have posted the rest of that article. You know, so people can see both sides of the story.

Also, doesn't less rod angle improve the leverage since it is pushing more directly downward?

[brainsboy]

I will post quotes next time, didnt think it was a big deal but since fixit has a problem with it, not a probelm, thanks for bringing that part to my attention.

[Keithc8]

The problem with using anything shorter than a 6.200 rod in this combination is that the counter weight of most after market cranks will hit the bottem of the piston. The best way to build this combination for balancing, longevity and performance is to use the 6.200 or 6.250 rod. Like mentioned before they still do not have a great rod angle. When the piston is at bottem dead center and the crank starts back up you have more side load with the shorter rod and this is hard on the pistons, cyliders and the rings. If the longer rod caused some kind of problem with the piston then we might want to go another way. The piston design with the 6.200 and 6.250 rods is great and the pin does not get into the oil ring or even close with the top ring down .250 and 1/16,1/16,3/16 ring package. These are all of the reason for using the longer rod in this combination. It has worked for over ten years great and will work great from now on. I like the lighter piston and the ease of balancing if nothibg else. Hell you can make 600HP on pump gas with one of these combinations anyway. Good luck Keith Craft

[A Snake]

Interesting information. The racers had most of this figured out years ago. It just took a while for the masses to get the idea.

brainsboy, I'd like to suggest that a really knowledgeable person doesn't call an engine a motor. The starter is a motor.

Fixit,

you must be one hell of a mechanic. I envy anyone who has you work on their projects. You still do that don't you? If you ever come up to the rainy NW, let me know. I'll buy the coffee.

Al

[trularin]

V-8,

What are you trying to do?

If we know what your goal is, we may be able to suggest good paths to follow.

[V-8]

I want to built a 408w for more torque & hp and shot 200hp fogger Nos.
I just want to win the illegal race.
Can stock 351w later block okey for this power?
Please look at to "I must win the race" topic for more info.
Thanks...

[DAVID GAGNARD]

Since we are on the subject about rod length/stroke,I have a question....

What would be considered the "ideal" ratio (rod/stroke) for a street engine and the ideal ratio for a "race only" engine....

Will be building a 302 for road racing later this year and trying to gather as much info before starting as I can....
Specs on the 302 will roughtly be; 302 early 80's block,no more than .060 overbore allowed,you can "destroke" it but no strokes allowed greater than the factory specs,must use stock production iron heads made before 1972 (modifications allowed to the heads are generous),Edelbrock Performer RPM dual plane intake (can be ported),any single 4-V carb,any ignition,any compression,only solid lifter or hydraulic cam (no roller cams),roller rockers o-k.....

Plans right now are for the 80 block,69/70 351-W heads (ported/polished/gasket matched/1.90/1.60 valves/screw-in studs/guide plates),Edelbrock mainfold,650 Demon carb,MSD distributor and ignition box,compression around 12 to 1,cam specs around 550 to 575 lift,for the crank and rods I plan to use forged pieces on both....A few guys are running de-strokers,not sure of the stroke,but with .030 overbore they are at or around 290 cu.in. and most are running chevy rods...... This is a "race only" engine and for a new guy like myself,I'm starting out kinda conservative on the engine,need to make laps and learn the tracks and need a decent enigne that will last many laps between rebuilds....

David

[Keithc8]

I see no reason to destroke this combination unless they were giving a weight break. Install 5.400 rods and a 1.280 shelf piston, the same used in a lot of 408 stroker 351W engines. Do the 3.00 stroke and use the 5.4 rod which you can get in a Ford or Chevy journal. Have a lot of these out and they work great. Good luck

[cobrashoch]

I have something to add about stroker rod ratios, but it's about 385 big blocks and BB's in general though. When I was doing the dyno work on my present blown engine I first fired my 4.5 inch stroker with 6.6 Chev. Eagle rods and stock flat top pistons. After foolin' around a bit we switched to 6.8 inch rods and power- forged pistons (same flat tops and compression ratio) that weighed the same. The difference on the dyno was not subtle. The second piston rod combo. made over 125 more ponys@ 6500 and about 70 at idle. The second combo also rev'ed up much quicker. More about that later. Blowers tend to bring out such things between setups really quick.
In 1953 Chrysler did several studies on hemi heads that come to the conclusion that a 3300 feet per minute piston speed was optimal for the intake stroke of a naturally asperated engine. This report said that stroke/rod ratio's (stroke into rod length) of 1.6's to 1.9's was optimal. Generally speaking, below 1.6, a engine won't rev. (rod angle greater than 15.5 degrees varient) Above 1.9's due to higher bob-weights it won't rev. Remember too that the actual power numbers may be exactly the same in this range.
In 1966 G.M. released a famous white paper on it's new 396/427 that was a virtual mirror copy of the Chrysler paper from the 50's.
Same 3300 target piston speed number at 6500 RPM's. They showed the math for a 427 with stock stroke and stock rod length that optimized the then new square port heads to you guessed it, 3300 feet per sec. piston speed, for a 427's displacement, at 6500 rpms. When Chevy up'd the stroke of a 427 to 454 and left the rods alone savy drag guys knew where they messed up. Cube for cube the 427 ran better than the 454, it was the rod angle that changed. Power stayed the same. The measured voumetric effeciency for the 427 at the intake valve was over 100% on the head at 6500 rpms. It droped off to the high 80's for the 454 's larger intake charge. But the net intake airflow was the same. So Chevy rated the 454's power at 500 rpms below the 427. Hmmmmmmm.
People, it's not got any better since then. Yes, todays small block heads have caught up with thinking that Bill Grumpy Jenkins and Chevy was talking about way back in 67. (together with combustion chamber designs)
Todays thinking at the big three puts the rod ratio's right square at 1.75's because that's the best combo. for overall considerations. You guys can do the math on your favorite brands. I know I did that, one bored night. And when I plugged in the numbers on various famous winning engine combo's, certain things popped right out at me.
Remember too that we we have gone from stroke/bore ratio numbers of around .75's in the 60's and 70's, to the presant thinking in stroke/bore ratio's around .9's. And longer, lighter rods are in that equation, together with better head designs. At a 3300 ft. per minute piston speeds of course. JUST KIDDING.

For hot rodding in general the longest rod you can jam into the hole is the right one for a street car. There is NO disadvantages other than bob-weight and fitment issues. Even if your rod ratio is too long and the your heads are the latest maxed out super duper flow monsters, you can cut back to a smaller head. In the real world most of us don't use such heads. And we run our headers through sidepipes or mufflers. So the long rod ratio's work for most of us mortals using normal stuff. And that's a lesson the big three finally figured out.
Racing engines are the exception, but it's amazing to me that the 500 cube pro stock motors out there all run 7 inch rods w/ratio's around 1.8's. I know, I just got off the phone with a local pro stock engine builder that supplies engines to the pro stock crowd. And he concoures with what I just reported above. He also claims that his engines make more power per cube than a NASCAR cup engine too. Guess he doesn't worry about gas milage either, like the cup guys do.
You guys need to look a bit better at what the factorys are doing, and a little less at what the hot rod books are saying. Even the factorys have race cars, and they usially win.

cobrashock