Stroker kits

[achalmersman]

So what is the disadvantages to running a slightly longer rod than stock length? Why do all the stroker kits I see use 6.350 rods if the general consensus is to use shorter rods?

 

[charlief1]

So what is the disadvantages to running a slightly longer rod than stock length? Why do all the stroker kits I see use 6.350 rods if the general consensus is to use shorter rods?

Most of the kits usually use a stock length rod because the piston will be much shorter than stock and the ones that use the longer than stock rod increase dwell time even more. If you want a hoot, I've got a set of 6.625" rods that require a really short pin height and the piston is quite small compared to the stock ones. Theses would increase dwell time quite a bit over stock and the cylinder charging is even better, but the pin height is about 1.12" so it's more for racing than street use.

One of the issues with a stroker is that you have to clearance the bock, the rods, and sometimes other things to make a stroker set up work.

 

[bison]

With a longer rod, in an NA or a turbo engine, the piston has a longer amount of time to stay at the top and bottom of the stroke. This is dwell time. The longer that piston stays in that position means the static flow of the air/fuel mixture, or exhaust, will come closer to a stationary flow rate. In other words, more will move due to the fact the piston has stopped moving for a fraction of a second longer. This increases the cylinder charge as well as turbulence, which reduces the chance of detonation to some degree. Is that clear as mud or should I throw some more dirt in the water.

What effect does this have when the exhaust pressure is double that of the intake pressure and both valves are open?


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[charlief1]

What effect does this have when the exhaust pressure is double that of the intake pressure and both valves are open?

The same effect as EGR Brian. You will get a dirty mix of sorts and if you're running that big a cam then hypothetically, it's a race car, not a street car. What type of ratio are you wanting to plug in there?

 

[bison]

The same effect as EGR Brian. You will get a dirty mix of sorts and if you're running that big a cam then hypothetically, it's a race car, not a street car. What type of ratio are you wanting to plug in there?

I know what happens I'm wondering if a longer rod will further exasperate the problem because of added dwell. Every cam we've analyzed had some overlap near tdc. There's no way around it. Close the ex valve sooner. Does a longer rod hurt the engines efficiency with higher exhaust pressure and longer dwell at tdc? Theoretically it should because the cylinder can't begin to fill till the pressure in the intake is higher than the pressure in the clearance volume. But it may also help if the CR is high enough and the ex valve closes nearer to tdc The lowest pressure you can have at tdc is the lower of the two pressures. High pressure flows toward low. Most everyone is running higher ex pressure and usually at radios of 1.3:1 or higher. Speeding the piston up near tdc faster theoretically would help because the pressure in the clearance volume would equalize at less crankshaft rotation. We will probably never know for sure but theoretically speaking a shorter rod with a higher CR should increase cylinder fill with higher drive pressures because of the cylinder pressure relative to crankshaft position will equalize sooner. The ex closing relative to crank degrees is very important to look at in a turbo engine. We've definitely seen positive effects by closing the ex valve at the proper time.


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[achalmersman]

Wow. Some really cool technical discussion going here. Keep it coming!

PS: here's that technical discussion you were looking for a few weeks about Brian! Lol

 

[Mike E]

If a long rod was important, they would not have created short deck blocks for comp eliminator motors. For those that don't know, comp eliminator guys spend comparable money on motors and r&d as Pro-stock. They look for every last .1 horsepower.

The rod needs to be long enough that the piston skirt clears the counterweight on the crank and that's about it. The longer you make the rod the easier it is to buckle.

 

[cruzn57]

in a NA motor ,I'd agree, but in a forced fed (turbo) motor, I cannot see the advantage of a longer rod,
yes ,I'm aware of dwell time with a long rod, but with a turbo application, you are no longer relying on vacuum to fill the combustion chamber. I've had some lively conversations with Kenny Duttweiler about this, and he can back his reasoning with dyno test results, me, I just do what I'm told (sometimes)

 

[20psiofevil]

Most go with longer rod because of geometry. Longer rod you typically see less side loading in the bore due to rod angularity. Also theory is the engine will live longer. I've never read up on the dwell effect but I'm going to look into it. We went from 5.915" to 6.2" on my motor.

 

[Jerryl]

There are "theory", "real world measurable results", and cost. For 99.9% there is probably more power in tuning and nailing the total combo than "any measurable increase in power" due to rod length. Even if you were able to measure an increase, I highly doubt it is cost effective to compare combo to combo on the same car with the same tune and same test conditions.
Lots of people want the last 0.1 hp out of the motor but save on other parts and things that would far exceed rod length benefit . . . . if there is one. lol

 

[Nick Micale]

There are "theory", "real world measurable results", and cost. For 99.9% there is probably more power in tuning and nailing the total combo than "any measurable increase in power" due to rod length. Even if you were able to measure an increase, I highly doubt it is cost effective to compare combo to combo on the same car with the same tune and same test conditions.
Lots of people want the last 0.1 hp out of the motor but save on other parts and things that would far exceed rod length benefit . . . . if there is one. lol

Your comments are spot on Jerrl!

What most turbo Buicks owners do not understand is the Stage II technology and methods developed and used by Buick Motorsports back in the 1980's was for Regals running all out for 250/500 mile races which required durability and reliability for hours, not seconds!

Most of the engines were running a large carb and the normal rod was 6.50".

When the turbo application and use is WOT for only a few seconds street or strip, the rod length becomes as insignificant as any gain in power or reliability.

I understand theory, but I operate and build in the "real world" based upon my experience, as well that of others who also have proven records, not opinions!

 

[V6sleeper]

Ok this thread is starting to get informative....Sub'd...

 

[charlief1]

in a NA motor ,I'd agree, but in a forced fed (turbo) motor, I cannot see the advantage of a longer rod,
yes ,I'm aware of dwell time with a long rod, but with a turbo application, you are no longer relying on vacuum to fill the combustion chamber. I've had some lively conversations with Kenny Duttweiler about this, and he can back his reasoning with dyno test results, me, I just do what I'm told (sometimes)

This also has to do with power level Lee. Just like the dual pattern cam, at lower levels the longer rod helps, but at higher performance levels it starts to hurt. If you built 3 identical engines, except for longer or shorter rods and the right pistons, then do the testing, while changing only the boost level, and any external parts, you'd see the differences between how they behave. If I had the funds and place to do it I think others might understand a little better.

Most have read how much you can get out of a stock engine, and Brian as well as others, have proven it many times over. Now if we only changed the rods and pistons for a longer rod and shorter piston, keeping the same stock compression ratio, cam, and using a slightly larger than stock turbo, then you might see a faster reving engine (not much but a little) that produces some extra power. If you change to a 204/214 cam the increase will be a little more. Now if you went with a TE60 (example here) then the stock engine would most likely be the winner. It is about the combo and how much you want to get out of the set up. If you only want to go high 10's then you should be fine, but if you want to go low 10's or faster then you're better off with stock length rods and pistons.

 

[turbolou]

We especially use a stock length rod in most all our stroker builds as the longer pods locate the pin higher in the piston. Unless the customer want a longer rod, we stay with stock length in all our builds. Simple logic is a shorter rod is lighter than a longer rod.

From our personal experience, and the only other "opinion" which I value in our use of stock length rods is from the most successful and experienced engine builder in the turbo Buick community who stated "rod length makes no difference in performance"!

Nick I agree & have never seen any power increase or other benefits from using a longer rod . But that's just my opinion & would love to hear any proof to the contrary from any of the respected engine builders here.