kelobro

I'm a scientist by profession. My 2004 Chevy with a 5.3L has started burning oil and blowing blue smoke when it's cold. I don't want to buy a new truck and I've always preferred engines that I've built over factory engines anyway so I am going to build a new engine for the truck.

Goals:

• More torque
• Reliability
• Towing capacity
• Fuel economy
• Acceleration

I'm planning to use a LSX454 iron block with a 4.125-4.200 bore. The 4.8L crankshaft with 3.26 stroke would put me over my desired displacement of roughly 5.7L. Some older, maybe more experienced builders believe over-all displacement, no matter how it is achieved is key to producing torque. The more common claim is that torque is achieved through a longer stroke. In the 80s and 90s, it was the chevy 383 stroker vs the chevy 377. the 383s dominated because they were so easy and cheap to acquire but the few people I knew who had a 377 absolutely loved them.

So... Any links to articles where actual tests were done to answer the stroke vs. bore vs displacement question would be greatly appreciated. Additionally, if anyone has built a short stroke LSX block, I'd like to hear from them.

Thanks...

BattleSausage

iTrader: (3)

Just google piston speed vs. torque. By increasing stroke, at any given RPM the piston speed is increased, thereby creating more force being exerted on the crankshaft and voila, more torque. Pretty simple concept, but there are consequences such as increased side loading, higher stresses on parts at a given rpm, and will lower the theoretical max RPM potential...which can somewhat be negated by the use of higher quality parts.

Since HP is just a calculation of (torque x rpm)/5252, you are shifting the parameters around. A larger stroke will limit theoretical max RPM, but will increase low output torque and hp. Whereas a shorter stroke will increase theoretical max RPM, in most cases low end torque will suffer, but a higher yield HP will be attainable. That's the reason all dyno graphs have the two crossing at ~5250 as the 1:1 ratio has both HP and torque equal at that point.

That is not taking into account other variables such as compression & valve events that could negate losses due to using a shorter stroke, but all things being equal, the longer stroke will make more power. That's a basic explanation but there are far more things to consider.

usdmholden

If you are really a scientist then you should know some stuff about physics.

Torque is a vector, cross product of radius and force

Force comes from cylinder pressure which is dependent on valve timing and ignition timing and air/fuel mixtures and many many other variables. Since nobody on this website has any high end combustion FEA software, we should just assume pressure is a constant at any given engine speed and engine load and leave it at that.

Radius is simple, it's half the stroke length.

If pressure is constant and radius increases torque has to increase.

Crankshaft stroke does not determine piston speed. Piston speed is determined by the pressure in the cylinder, which comes from the cylinder head, combustion timing, air fuel, and lots of variables which we on this website won't be able to quantify. Piston speed determines crankshaft speed, not the other way around. A shorter stroke crankshaft will allow a higher engine speed before the piston and piston rings exceed the physical limits of the system. With modern engines and material this physical limit is so high it's mostly now irrelevant, so no need to worry about it in a street engine.

The powerband is mostly related to valve timing. A short stroke engine with valve timing that produces a 2000-6000 RPM powerband will make less torque than a long stroke engine of the same displacement with different valve timing that produces the same 2000-6000 RPM powerband.

Has anyone ever empirically quantified all this? I'm sure the big OEM auto manufacturers have, but they don't post on this website.

edit - one can make the argument that an increase in cylinder bore increases piston area which the constant pressure acts upon which will also increase torque. Which one increases torque faster? I don't know, people normally do both at the same time (bore and stroke).

Your idea of a large oversquare engine sounds appealing for a race car, but not so much to me for a truck. A 6.0 iron shortblock with 243s or 799s and a good cam for the powerband may not be ideologically appealing, but I bet it will do 99% of what you want from your list.

garygnu

go to Texas Speed and check out the price of a rotating assemblies .I say use your current block and do a ls 383 stroker short block .get some ported heads or have yours ported .if you budget is unlimited ,contact Tony Mamo at Air flow research and get some heads from him .check out Tony M threads on engine builds if you want to make crazy power .

BattleSausage

iTrader: (3)

You're mixing things here. Yes, other variables such as valve events, timing, compression, etc. have a direct effect on piston speed as they can increase or decrease the effective rate at which the engine is able to accelerate through a given RPM range but they in and of themselves can't change piston speed. Piston speed is a direct calculation of RPM & Stroke...the piston will have to travel at (x) speed to cover (y) distance for any given RPM and stroke, the only thing changing is piston speed. As I said, it makes no difference with the other variables as they don't change the speed needed to cover said distance, they just affect the engine's ability to move from say, 3500 rpm to 3501 and so on. Two completely different setups with the same stroke/deck height will have the same average piston speed at any given RPM, the one optimized the best WILL however have better acceleration and thereby create more power by use of rotational mass.

In other words, the stroke WILL determine the piston speed.

garygnu

how much hp do you want to make ?what rods and pistons do you plan on using ?maybe rebuild the current heads on you truck engine .you are not going to gain much fuel economy with a heavy block .put a different cam in when you rebuild the heads .talk to Martin at tick performance ,he has been making great power with 5.3s. or get a baby EPS cam.I seem troll .

726.0chevelle

iTrader: (17)

I don't believe there is much upside in building a destroker unless you have a lot of money and just wanna be different. Not trying to bash you but when these motors are built mild they can put a smile on your face and still get good mileage. I have a 1999 Silverado swapped from a 5.3 to 6.0 lq4 small cam and full bolt ons and lost 0 mpgs. Way more power and torque and inexpensive. Check out performancetrucks.net link at the top of the screen for more ideas.

joecar

This would be true if there was no friction/pumping losses (these being proportional to piston speed)... it has been found that the increase in torque due to couple-arm increased length is offset by the increased friction/pumping losses.

kelobro

Not nearly as concerned with HP as I am with low end torque. I want the motor to have over 400 ft/lbs of torque as early in the RPM range as possible. The engine will rarely see over 6000 RPM.

If I were to just do a head/valve/header/cam job, how much longer will those thin little LS rings last? I already hear piston slap until the engine gets up to operating temps. Sounds like a 6.2L diesel!

kelobro

Additionally, the majority of torque comes from roughly the first inch of downward motion on the power stroke. A shorter stroke engine means the piston is moving slower at the same RPM, keeping the piston in the "power zone" (for lack of a better term) longer.

IMHO there are too many variables to consider to just stand on theory. That's why I was asking if anyone has built the engines and made the comparisons with LS engines. Gen I comparisons between the 383s and 377s don't necessarily translate.

More generally, consider this... If stroke is more important than bore, why did the GM 307 (3.875×3.25) and 305 (3.73x3.48) perform so poorly compared to it's short stroke brothers, the 302 (4.00×3.00) and even the 283 (3.875×3.00)?

The 4.3L L99 (3.73x3.00) in my old 95 caprice performs surprisingly well pushing around a 4000+ lb car. This was very unexpected but it would seem that all those nifty LT1 parts on the little 265ci mouse motor made a significant difference despite the small valve, small stroke and even small bore of the engine.

JakeFusion

iTrader: (7)

What is your budget? You were looking at the LSX block... I'd just grab a 6L block and stuff a 408 assembly in there. Then just go with some 215cc heads and a 22x duration cam and watch it make ungodly torque with good mileage.

I've seen folks on here put AFR 205s and TFS 215s on 402/408 strokers and make 500/500 at the wheel with mid 230s duration cams (which are very mild in those cars). Drop down into the 220s and you'd probably be in the 470/520 range with one of those motors with peak in the upper 5000s with lots of low-end torque. Another option would be AI 5.3L heads with 2.00" valves and 218cc port - they would make a lot of torque on a 4" bore combination with that valve and port size.

kelobro

I'm not trying to make a "destroker" per se. If I were, I'd be looking for a custom crankshaft with an even shorter stroke. When I was building gen 1 350s, bore tended to limit valve size and efficiency. A 4.125-4.200 block has better valve options and reduces valve shrouding. I'm not really interested in any engine that has a bore smaller than 4.00. Crank choice is merely a function of desired displacement (5.7-6.0L).

Thanks for the link reference. I'll look around there.

kelobro

I'd hate to spend more than 10k on it. Budget is a function of time. I can spend roughly 5k now which should be enough to put together the short block. My other option is replacing the truck for 30k-50k

Exidous

iTrader: (11)

A shorter stroke is 100% the wrong way to get as much off idle torque as possible. A longer stroke has more leverage in that "magical first inch" as you describe it. The crank isn't rotating as fast for a given inch and velocity but the force the torque a crank is producing is higher.

If you were to assume a certain cylinder pressure, increasing bore will increase torque. 180psi on 3" /= to 4".

F1 motors have crazy short strokes and rev to the moon to make power. What don't they have? Low end torque. A longer stroke is better for torque. Period. It limits RPM because it increases piston velocity at a given RPM.

Not an apples to apples but close enough

Look at the ratio of HP to TQ on the 346 and the HP to TQ on the 383. EVERY time you will see a 383 closer to 1:1 than any 346 could ever hope for. (unless stock bleh)

346 460/419
383 488/475

An over square motor will ALWAYS produce more torque than an under square motor assuming all other variables are the same. If you are fixated on a specific displacement lower the bore and increase the stroke. A stroked 5.3L will get you in the 5.7-6.0L range and make great torque.

The LARGER port heads on a 346 would kill low end torque. On a 383 it just eats it up. In a truck there is no better mod for torque than a stoker motor except maybe a PD blower. :-)

Look at my motor as an example. Pretty good power and bleh torque down low. If everything stayed the same except the stroke I'd gain probably around 10-15rwhp and about 40-50rwtq.

kelobro

Not a bad idea to use the existing block, but this is the wife's daily driver. I can't have it down for more than a week. Worse, I don't know of a trustworthy machine shop within 250 miles of here.

Thanks for the info!

Busted Knuckles

Increasing bore OR stroke increases displacement which increases the amount of fuel/air charge that, once compressed and "lit off", increases pressure on the top of the piston which increases downward force on the piston which is transferred to the crank pin which increases torque.
One of the rags did a comparison several years ago with a 496 (.060 over = 4.310" bore 454 block with a 4.250 = .250 longer stroke crank), a stock 502 (4.494" stock bore, 4.0" stroke) and an oddball 4.600 bore with old school 3.766 stroke crank (427). All made with a few hp and ft lbs of each other. I'll grant you that none of these was optimized by tweaking cam specs, but they did make a point. Increased displacement, whether by stroke or bore, increases torque.
The most efficient is short stroke/big bore due to decreased ring and bearing drag. It also slows piston speed at any given RPM's which makes life easier on the rods, soooo...it likes to rev which IMHO is where engine music is made.

lemons12

iTrader: (71)

Look at dyno results.

I made 508/484 with my 408 before the last few changes... That is about normal for torque and hp to be that close with a stroker.
That would be very odd for a stock motor to make that close of power.. There is usually 50 or so difference.

ViaBellator

iTrader: (2)

I have an LQ4 based 408 with TFS 220 as-cast heads and a 239/247 cam and make 475/490 through a 3600 stall in an auto.

Can't say much for fuel mileage, I get roughly 15-16 MPG avg in a 3800 lb GTO, but it makes enough torque to put a smile on my face any time I bump the throttle.

gagliano7

http://www.ebay.com/sch/i.html?_nkw=...on+motorsports

kelobro

Nice prices for good stuff! Thanks. TSP looks interesting too as it sounds like I can have them tailor a complete long block to my needs. I WISH I had the time to assemble it my self, but I don't.