Whats the difference between these two:

LS7 block re-sleeved with 5.800" sleeves.

Engine #1: 4.155 bore x 4.125 stroke...447ci

Engine #2: 4.155 bore x 4.000 stroke...434ci

Not the cubic inches, thats obvious, but is there any advantage of the shorter stroke. More power through rpm's, more reliable, torque, etc

I've heard that piston speed with long strokes is bad..............why?





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ok, now i may be mistaken but i think in an earlier thread i read that piston speed has a direct relation to power.

 

Well, if you're going to spin two engine's to the same RPM, but one has a longer stroke, that one will have a higher piston speed. This is what I heard is not a good thing, I just don't know why. I can only assume its just more stress on the rod, rod bearings and crank

And I'd like to know when a faster piston is good, for what application?

Roll races, top end, drag racing?????

Does a longer stroke allow for higher rpm's? Or does a shorter stroke allow for it?

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I'll try my best to help with this. With a longer stroke engine (shorter rod), at any given rpm, vs. a shorter stroke engine (longer rod), the longer stroke motor's piston has to travel a longer stroke in the same amount of time. With the piston moving a furter distance in the same time frame of a shorter stroke motor, this is more wear on the rings and cylinder walls. Also longer cranks (shorter rods) will have more lateral forces on the side walls because of the increased angle that the rod is facing towards the cylinder wall during the stroke. This also is more wear on pistons, rings, and cylinder walls.

Shorter stroke (longer rod) is higher rpms. Since the rod isn't going at such an angle towards the cylinder wall with a longer rod, it can rev higher. With a longer stroke motor, you get more piston speed, which is good for creating a "vacuum" in the cylinder so you get more air in the cylinder within the same amount of time and the velocity of the air is coming in much faster since the piston is going down the bore faster. Any stroker has that effect. That's why, say, a 572 BBC can get away with such a bigger cam, bigger port heads, and bigger carbs for street applications than smaller motors can. Faster piston speed, more air in and out in the same time, valves can open further and longer. Also with higher piston speed, a big motor like a 572 can run more radical parts on the street and still pull a decent vacuum for accessories.

This whole theory is why stroker motors are so good on the street. And why short strokes are good for things like 20,000 rpms F1 cars.

Hope this helps.

 

I would suggest searching on topics like VE, displacement, pulse tuning,
and valve timing.

Piston speed as it varies by stroke/rod ratio has little to do with making
power.

 

So what would be best suited for a roll racing ONLY full-wieght street car. I'm having an engine built as soon as I decide which stroke and bore to go with. I'm using an LS7 block re-sleeved. Darton 5.800" long sleeves. LS7 heads, Hogan or Harrop intake, 11.3:1 cr. 250 DP progressive shot. Cam ???

I'm thinking 4.155 bore and 4.125 stroke..447ci
or
4.155 bore and a 4.000 stroke...434ci.
OR
If the long stroke won't be a problem and it'll be reliable and last a couple years, 4.155 bore and 4.250 stroke...461ci.

I'll give up the cubes and go with the 434 if the shorter stroke is an advantage for what I want. If high rpm's is what a roll racer needs I'll go with a shorter stroke and spin it higher, if it'll make more power that way.

 

A givin stroke is not going to determine a set rpm. You can rev a 4" and a 4.125" to the same rpm if set-up properly. In this instance, it should be a givin that cubes will make more power. I'm not clear on the facts as to how much piston speed is to much, but I don't think were going to find the limitations with these cranks. The longer strokes may add a little to the premature wear catagory, but I don't think it will be anything drastic. Hell, look at the ERL motor. It has a 4.500" crank. Maybe you should specify a pre-determined RPM range instead of just a "roll race". If you don't want to turn anymore than 7k, I think you would be fine up to 4.250".

 

I just wonder why some builders say they won't, or would rather not build a big cube engine with more than a 4.00" stroke. Maybe I'm worrying about it too much because I still don't see what the problem is. Or maybe its because of the sleeves not being long enough, now that they're available I guess it doesn't matter.

I just want whats best for a roll racer, say 40-50mph to 180mph.


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That's easy. Most large cube motors produce hell spawn upon start up and there a bitch to get rid of. My 4" 383 produced a suckubus upon start up and I still can't get rid of her. I hate to know what's gunna come from my 454.


I think you are worrying a little bit, but that's only natural when your around something new. I believe the longer sleeve is gunna help alot.

Get the same block, but with max 4.185" bore, put in a 5.3" crank, get big heads, big solid roller, and run it to 10k.

 

max the displacement and keep the same redline.
that'll make the most power.

problem solved.

 

If F1 could run longer strokes they would, and they would continue increasing in power as the stroke increased.

Racer7088 is saying pretty much for the most part, that if a 5.65" in a mountain motor of 800 plus cubic inches can turn 8000 + RPM, that your 4" or 4.25" stroke for that motor turning 7000 RPM is not really a big dieal

Of course you going to need the correct parts for the venture.

 

I am assuming that you would use the same length rod in both motors.

If this is true, then the rod/stroke ratio effects how the motor revs too.

A good example is '67 camaro 327 vs SS 350. Both engines used the same heads, carb, intake, camshaft, and 5.700" rods. 327 was 275hp and the 350 was 295hp. both had a 4.00" bore. testing in '67 showed both cars equal in performance. though the 327 made less power, I think the engine acceleration speed was faster in the 327. and that made the cars equal.

A shorter stroke with the same length rod changes the accelleration of the piston away from top dead center. It slows down the speed in that area of travel, and in theory promotes cylinder fill and volumetric efficiency.

I think that if you built either motor the best that it could be with the money you have to spend, the bigger cube motor would be slightly faster. Just my opinion. car weight, and gearing etc. all change how to build the motor too.

 

Sleeve length is a major consideration in my opinion. Depending on piston skirt barrel design a small amount of the piston skirt (.100-.200) may not be all too harmful protruding out of the bore at B.D.C., but much more than that and I am looking at a way to fix it (less stroke, shorter piston). Best case is to design an engine package that keeps the piston up in the bore in the first place, I won't build one any other way.

 

You have that wrong, slower piston speed does not promote increased cylinder filling. If displacement per degree of piston travel is less, there is less negative pressure in the cylinder because less volume in the cylinder has been created. What it does do by slowing the acceleration away from TDC is provide greater combustion efficiency during the power stroke by keeping the effective size of the combustion chamber smaller and not wasting the energy created by the air fuel mixture explosion filling newly created space.

 

Thanks for setting me straight 96 comp t/a. I guess my memory is not as good as i think it is. I read about it 15 or more years ago.

Which way do you think quickin should go?

 

A stroker brings more torque because the stroke on the crank is increased and so the piston moves easier to the bottom and up. This is right or? What I mean for e.g. a with a big ratched you need less power to tighten a bolt then with a small one, because it is longer.

 

That's a pretty common myth. It's actually the increase in displacement
that increases torque.

Additional capacity of the cylinder allows more energy to be exerted on the
piston per power cycle.

 

To speak VERY generally, the top end of the motor (including cam) determine the power. The displacement you put under that top end determines where the power is made. The better the heads/manifold become, the more displacement you have to put under it to place the rpm range down to a point where the valvetrain is stable, reliable, and the valves aren't floating. When you have a displacement rule as in Cup, Pro Stock, or F1-you basically up the rpm and horsepower as you make cylinder head and valvetrain advancements. Compared to horsepower, torque actually remains pretty close to the same year to year..it just moves up the rpm scale. Back to LS land, the newest heads these days can enable even the 500ci to pull strongly through 7000rpm with medium sized hydraulic cams and a factory intake, so bigger is better for most of us. If you're gear limited or have a self imposed cam duration for driveability, even more so.

Lets' create a average target piston speed of 5100..no big deal. The 3.622 stroke has to turn 8700 to achieve it, the 4" turns 7900rpm and the 4.5" is about 7000. This is a bit of a generalization, but let's say we've got a killer drag race top end on a 3.622 stroke that makes peak power at that 8700rpm. the 4 and the 4.5" engines will make about the same peak power at the lower rpm that coincides with the equal piston speed. This is why you would want a lot of displacement with a valvetrain limited engine.

For theoretics and little else, the peak compression load on the rod occurs at about 30 deg. abdc and peak tension remains at tdc through any of the common rod/stroke combinations for these 3 engines. In reality, with a given piston speed and rod ratio, the SHORTER the stroke-the higher the PEAK tension and compression loads will be. Using the same #'s from above, a 3.622 stroke at 8700rpm, a 4" stroke at 8300 rpm, and a 4.5" stroke at 7900rpm have about the same PEAK COMPRESSION loads. The 3.622 stroke at 8700rom, the 4" stroke at 8150, and the 4.5" stroke at 7700rpm, all have the about the same PEAK TENSION loads. In my opinion, these peak loads are more important than average piston speed (and certainly more than rpm) to reliability.

My basic point is you don't hear of pins getting yanked out of the bottoms of pin bosses or rods snapping in 1/2 very often with decent aftermarket parts, so increasing stroke and even piston speed can actually help the overall reliablity of the engine. One of the above posts made a very good point about the sleeve length being the limitation. A call to a knowledable piston manufacturer can help determine how much stroke can be put in your block. The taper on the piston has a break point. The break point must be above the bottom of the sleeve at bdc to keep the piston from cocking to avoid obscene amounts of skirt wear. When you see a piston that is severely worn in a long stroke engine, Bubba usually say's "that durn bad rod/stroke ratio ate up my piston". In reality, the piston manufacturer didn't do a good job working with the engine builder to determine proper skirt taper and maximum stroke.

-Brian Nutter-Wiseco Piston

 

Listen carefully to Brian, guys, and not just because he sells pistons. it's the best post here.

His target 5100 ft/min P/S is just about spot on for a 9500 Cup engine and 5200 works for a 20,000 rpm F1 engine. Only a few are turning over 19,500 in a race and that's almost exactly 5100 ft/min. Good "target" number.

As he said, it's rpm that gets you the big inertia loads assuming a given P/S. Cup engines see over 5000g piston acceleration @ TDC while F1 engines see over 10,000g. Of course the 3.858 dia F1 pistons only weight about 1/2 what a Cup piston weighs.

 

Brian's post is pure gold for anyone bulding a motor!!
Thanks Brian for the info.