Whats the downsides for making a 4.0" stroke from a 3.622"? (cylinder bore being the same). You gain 37 cubes but is it worth it?

 

piston speed increases

 

haha i'm not even going to comment on this

 

Like full force said, your peak instantaneous piston speed will be higher.

For a street motor that should be manageable. I am not the one to ask but it is not clear to me that even H-beam rods will be required if you build for torque with a 5500-6000 redline.

If you are plannign to spin it fast for peak power you are going to be spending more money on your rotating assembly.

The other one is about the pistons scraping up the bores from the longer rods. There is a name for it that I don't recall, but it doesn't _seem_ to be a problem for meticulously assembled engines that are not over-revved.

Try advanced search, use "Tony Mamo @ AFR" as the user id with '383' and/or 'corvette' as the search terms. At least two threads should come up, both quite illuminating.

 

Wouldn't piston speed stay the same, as only the stroke is changing?

 

Nope. Let me dig around my hard drive. I promise not to knowingly post any viruses, but once the .xls file leaves my hard drive I am not responsible for it anymore.

I see I can't post a .xls as an attachment, so I will have to go stick it in my freaking yahoo space and link you to it.

At say 3500 rpm the peak instantaneous piston speed on a 3.65" stroke is lower than the peak instant piston speed on a 4" stroke. Makes more sense when you can see the graph.

I'll be half an hour easy.

 

Distance covered increases over the same period of time for a given RPM. The piston moves 4" in the same amount of time as a 3.622" moves, and is therefore going faster.

This can translate into heat and additional wear, but proper lubrication and cooling will help offset that.

Its also somewhat pricy, as you usually have to replace the entire rotating assembly. The extra cubes provide a much better baseline for the overall engine.

 

So i guess someone thats on a budget and is rebuilding the engine would just use the stock crank, forged rods & pistons, and spent the money on better heads...

 

Downside will be your et's will drop, your tires will smoke easily, and everyone in your car will have a smile on there face that can't be wiped off after you go wot

 

http://briefcase.yahoo.com/bc/joeuse...r=/GTO&.view=l

Hopefully this link works. I'll edit as needed. As Mr. Miracle pointed out with a 4" stroke your pistons will move 4 inches per half revolution, and so on.

In the public folder linked above is an eXcel worksheet you are welcome to. You can input your choice of crank throw and connecting rod length in the top left corner of either sheet one or sheet two. Youcan check the math on sheet one, or look at the graph on sheet two.

Scott

EDIT: The link worked for me.

 

BAD things about STROKE

RPM limitation
As stated, a longer stroke means faster acceleration for your pistons (they have to go further in the same amount of time). This means more stress on your rotating assembly. This will decrease the max RPM your rotating assembly can handle. Right now, that max RPM is probably 7 or 8k, but your valvetrain is much lower. If you never intend to up your redline, this doesn't matter.

Hard to breath
You will have a deeper cylinder to fill with the same size heads. If you don't have really good flowing heads, you may be dissapointed by your results. It doesn't matter how big your cylinder is if you can't fill it with air.

Expensive
There are a lot of things you could do cheaper than increasing your stroke. Intake, Exhaust, CAM, Heads... If you've done them all, maybe it is time for you to get a stroker. Even then, I would go for bigger bore first.

GOOD things about STROKE
If your stroke was 0, your engine would be really boring.

My 2 cents
F1 cars have a stroke of about 2", while their bore is about 4". They spin at 18k RPM and could smoke any of us. An oversquare (more bore than stroke) engine will be more efficient, easier to breath, and can handle more RPMs, which all mean more power. Don't get me wrong. More stroke WILL get you more power. I just like bore better.

 

For just about anyone building a gen 3 motor and almost any combination out there there is just no reason not to put the biggest stroke you can safely obtain (no super retard 1.2 r/s ratios, no crazy pin locations in the piston, weird compression heights, etc.) in the motor unless you are running a size limiting class. There are some valid points here but for god sakes the fastest n/a six speed car here right now has a 4.125" stroke and turns his motor 8000rpm. Yes, you do need more cylinder head, intake, and even valve train to fill the bigger motor but even that being said even if you take the same breathing components and put a larger stroke underneath of them the motor will still likely pick something up and when you add the larger breathing potential look out!

 

Our scene has over eight years of building 4.000 stroke LSx engines, and I have rarely seen folks complain about excessive cylinder wear, or any kind of wear from stroking their combo.

Might be different for the 4.125 stroke folks, but they are a much smaller segment.

 

if you are REALLY that worried about piston speed and cylinder wear...go with a longer rod and a piston with a pin thats located hgher in the piston...

 

F1 engines have a 98 mm bore limit (3.858 in). Most are pushing right close to that which gives a stroke of just about 1.565. Their 20,000 rpm is a mean piston speed of about 5200 ft/min. That's equivalent to about 7800 rpm with a 4 inch stroke. FWIW, with those F1 bore/stroke dimensions, rod ratios are over 2.0:1 just to get a rod in there.

No comments on the rest of your points doesn't necessarily imply agreement.

 

If I am not mistaking, there are larger engine displacments out there from the past and still being built. I do not remember what the 454 Chevy bore or strock is, but I think its bore is over 4". I know that that old tech engine makes great power and can turn 7000+ rpm when built right with the right components. Also, the beloved Chevy 350 of old had a 4" bore and a stroke around 3.48". If stroke is bad then why would chevy make the new 346 LS1 with a smaller bore and longer stroke then the 350 chevy it replaced? Now the LS2 has a 4" bore with the longer LS1 stroke. I also remember that the 400 small block chevy and the pontiac 400 both had a 3.75" stroke. Both great engines that stood their ground well in thier day, heck, they realy still do.

I guess what I am saying, 4" stoke is nothing for and engine built right with the right components, for turning less then 8000 rpm. I never plan to go that high in the rpm's. Extra stroke will do lots to make your car move. Personaly I wish I could figure out how to stick a 4.5" stroke in one of these LSx blocks. If you could, I bet you could still spin it to 7000 rpm, if built right.

4" stroker LSx's are well worth the expense. It will live just as long as the stock stroke engines. It will be stronger then the factory bottom end engine due to the better material used in the crankshaft, rods, and pistons.

 

454 has a rod to stroke ratio very close to that of a 4" stroke ls1 with 6.125 rods. I just wouldn't worry about these common 4" strokes in a ls1 or even a 4.125. If I could've got the compression height needed with my small 38 cc combustion chambered heads and pump gas friendly pistons (38 cc for my 4" stroke) I would be running a 4.125 stroke instead of my current 4" stroke.

 

If you want to mathematically compare the piston speeds of various strokes and rod lengths,
use the following formulas with math/graphical software (search web, I use DPlot):

position
= x
= r.cos(A) + sqrt(l˛ - r˛.sin˛(A))

velocity
= v
= dx/dt
= dx/dA.dA/dt
= -r.sin(A).(1 + r.cos(A)/sqrt(l˛ - r˛.sin˛(A))).w

acceleration
= a
= d˛x/dt˛
= d˛x/dA˛.(dA/dt)˛
= (-r.cos(A).(1 + r.cos(A)/sqrt(l˛ - r˛.sin˛(A))) + r˛.sin˛(A)/sqrt(l˛ - r˛.sin˛(A)).(1 - r˛.cos˛(A)/(l˛ - r˛.sin˛(A)))).w˛

w = dA/dt = 2.pi.RPM/60 = constant

where:
l = rod length (distance between piston pin and crank pin)
r = crank radius (distance between crank pin and crank center, 1/2 x stroke)
A = crank throw angle (from cylinder bore centerline at TDC)
x = piston pin position (from crank center along cylinder bore centerline)
v = piston pin velocity (upward from crank center along cylinder bore centerline)
a = piston pin acceleration (upward from crank center along cylinder bore centerline)
w = crank angular velocity in rad/s

You can put in the formula for v without multiplying by w
(w just scales the velocity for different RPMs),
and plot for 0° to 360° or 0 to 2.pi radians.

Keep all units consistent (convert in/s to ft/min at the end).

 

The poster's questions were:
1. What are the down sides of extra stroke?

Don't misunderstand me. I didn't say stroke was bad. I was just listing some downsides he asked for. Everything has them.

2. Is it worth it?

This depends on his car and his goals. If he has a stock exhaust and intake, he should worry about those before messing with the rotating assembly. If he wants a really high RPM, too much stroke can be a problem (though few poeple rev that high).

If he is going to replace the rods anyway, this probably would be a good time to get extra stroke. If he is replacing the pistons anyway, now would be a good time for extra bore too, which will allow more breathing plus even more displacement.

He won't see the full potential from his extra stroke until he gets better heads. That doesn't mean don't do it, just that heads should be his next buy.

 

Pay attention to your rod/stroke ratio. A general rule I follow is a 1.6-1.8:1, with 1.7:1 being the best of both worlds number. Higher ratios will put the torque curve higher in the rpms, combining it with hp(people argue about "torque gets you going, hp wins races"... why not have both at the same time?), along with applying the power stroke in a more downward manner which saves wear and tear on your cylinder walls. Lower ratios have more leverage on the crank, giving more low-end torque at the expense of increased wear and tear on the cylinder walls.

While max piston speed is the same given the same stroke, a longer rod accelerates the piston more slowly, effectively allowing higher rpm operation. A longer rod also "squishes" the intake charge on compression, rather than "slamming" it, which can lead to detonation, or even pre-ignition. Thus, the longer rod allows for more compression and spark advance... it's even possible with lower-octane gas.

At 1.6836002208724461623412479293208:1, the factory 3.622" stroke is close to optimum rod/stroke ratio already. A 4" stroker with 6.125" rods will have a ratio of 1.53125:1... too low for my liking, but to each his own. To be in that happy median I mentioned, you'd need a rod length of 6.8".

Personally, I'd suggest a big-bore setup... possibly a 4.125" bore LS1 with stock dimension crank. You'd wind up with a 387ci Gen III that'd be happier at 6500+ rpm, and would have torque and hp in areas where you'd be more likely to use them.