There's trade off between lower thermal efficiency/increased quench and piston speed decrease. Less heat dissipation leaves room for detonation. Piston speed decrease theoretically dispells detonation. And there's theorectically power loss from increased quench (remember our cast iron head bit?)

I don't really think it's a big deal. If you stay Fuel Injected, that's very very tunable. I have timing control for Cylinder Mass every .02 g/cyl or something crazy like that.

Length of Rod and Length of Crank matter for piston speed. But keep in mind, your highest piston velocity is going to be with the crank and rod are at a 90* angle.

You are correct in saying rod ratios correspond to piston speed.

None of it is terribly important. Good tuning can get rid of pretty much all detonation in Fuel Injection. Knock sensors pretty much take care of pitting pistons/breaking ring lands nowadays.

some good reading for you
http://www.contactmagazine.com/Issue...ineBasics.html

See, the dumb mechanics of it are actually what is important to me.

I understand that tuning can make up for deficiencies. I understand that tuning can make the torque hit early in a short stroke, big bore motor.

I want the parts involved, the ratios, the entire design and architecture to reflect what the engine us built for. You can look at a T-rex skeleton and know it belonged to a carnivorous critter. All it is is a set of jaws with some legs. Without cam cards or timing tables, I want you to see exactly what the motor is good at, just based on design.

I want what it is physically, all the ratios and geometry of its design, to reflect what it does or excels at. I don't want to tune an engine built one way to operate differently.

I want to build MY engine. Based on what makes sense to me. If it doesn't work, if what makes sense to me fails, then I will start over, with some new knowledge and experience to help improve the next one.

It's not a test of any particular principle or another, it's more just to see if what I think SHOULD work actually does. If it does, great, being right is awesome. If it doesn't work, great, learning to do something right is awesome.

It's more about the journey than the destination for this first engine build.

Regardless, it's all but impossible for me to screw this up. I am going to end up with an LS motor with the same, or better, displacement, redline, and head flow as the LS2. And nobody questions how potent that 364" small block is.

David,
I just meant to say that the detonation is a non-issue with a good tune. It's not like you're building a 14:1 engine. You should be able to run pump gas and not have detonation.. as long as the tune is set up.

I look forward to seeing your engine come together.

So, I'm pretty sold on the idea of running oil squirters and an LS9 (or similar) oil pump. But in researching oil squirters, I got to thinking, which can be dangerous for someone who doesn't know much.

Now, in relation to increased stroke causing an increase in heat/ friction...

Would the decreased circumference of a smaller bore translate into less friction (heat) being generated for the same stroke with a larger diameter bore/ piston?

We discussed the decreased surface area of the smaller bore being less efficient at transferring heat to the cooling system, and we talked about how increasing stroke will generate more heat. But it was never covered if increasing bore diameter has comparable increase in friction heat.

An undersquare motor would have smaller pistons, think smaller piston ring surface area, traveling a longer distance.

An oversquare engine would have more piston ring surface area traveling a shorter distance.

With both engines in question being of equal displacement, both spinning to the same redline, does one really generate more or less heat than the other?

Does less surface area traveling further generate more friction than more surface area traveling less?

More specifically, would MY proposed undersquare six liter, with its 3.78" bore and 4" stroke, generate LESS friction heat than, say, the LS2 with its 4.00" bore and a shorter 3.6" stroke? The 4.00" pistons have almost four more square inches of surface area interacting with the cylinder walls!

Using a relatively generic 1.2mm, 1.5mm, and 2.5mm ring set for both, the 3.78" piston has 61.78 square inches of surface interaction, per piston. The 4" bore piston, with the same ring set, has 65.36 square inches of piston ring surface, per piston.

Math:
3.78" x 25.4 = 96.01mm
Pi x 96.01mm = 301.63mm
301.63mm x (1.2mm + 1.5mm + 2.5mm) = 1568.48sq/mm
1568.48sq/mm / 25.4 = 61.78sq/in

I am no expert at all on this. But I do think you are overthinking this somewhat.

My thought is like this.

The piston rings I would think not making much more heat, modern low tension rings are not "springed" up to the cyl. wall. They are for the most part only under "tension" in the compression cycle.

And stroke, together with bearing size, I would think produce much more heat than anything else, because the piston speed is higher and the piston travels longer.

Since you say you would like a torquey engine, I would think your intake runner length, head size and camshaft choice has a much bigger impact on your engine than anything else.

Oldsmobiles and Buicks are known for their torque, I do not think that is (only) because of the stroke, but because of their wide intakes they got much longer intake runners than other brands. This is just my opinion though, not based on proof and facts. But to me it seems sensible.

All small block Olds` have an 3.385 stroke, they also produce good torque, because of the long intake runners.

And with a not so long stroke it is easier to have a better rod length without putting the pin up to high in the piston, I would think this creates a much smoother running engine, with a more stable piston. That runs smoother and quieter than an long stroke more "stressed" engine.

A lot of people who build stroker BMW engines, to fit the longer stroke they must put in a shorter Rod, that works fine, even though I think it is somewhat going the wrong way, they still make more power, because the engine is still bigger than it was before. There is no replacement for displacement any way how you choose to make it bigger.

Thank you for your response.

It has been brought up before that intake runner length and cam selection would have a more noticeable effect on torque production. And I am sure that they do.

But increasing mechanical leverage has to produce a noticeable difference. It has to. All other things being equal, if you had a LS2, a L99, and my proposed undersquare six liter... The intake and cam difference between the L99 and LS2 should demonstrate what everyone has brought up.

I am hoping that I can achieve that same difference with just the mechanics of how my proposed engine is assembled. Whatever makes the truck engine a L99 and not just another LS2, the change in the powerband that makes the L99 haul hay, while the LS2 hauls ass, that extra something down low that makes the L99 a truck motor... I'm hoping that I can achieve this same extra something down low through nothing but additional mechanical leverage.

For the sake of this discussion, we will say that I will put a LS2 cam, LS2 intake, LS2 throttle body, LS2 injectors, and a standard LS2 tune on my proposed undersquare six liter motor. It is essentially an undersquare LS2. At this point, the ONLY things different are the bore to stroke ratio, and head flow. Since I have been talking about using TEA's stage two ported heads, they outflow stock LS2 heads. So, please assume that the LS2 heads have been touched up to flow [email protected] as well.

Anyways, I build an undersquare LS2, right. All things being equal (tune, cam, intake, injectors, etc), what changes do you think the 4" crank would, or should, bring?

If the heads flow the same, using the same injectors and intake, same tune, same cam, same exhaust. All things being equal, except for the only difference being a 359ci undersquare longblock versus a stock 364ci LS2 longblock, do you think that there would be a noticeable difference between the two?

If it does give me something extra down low, awesome. I think it should.

If it doesn't, oh well, I have a forged six liter V8 that's a proven platform for boost... I think I will be ok.

But I do think that I can achieve this truck motor something extra down low, without sacrificing anything at all up top. If I choose parts that can withstand the abuse, and I am willing to spin the 4" crank as fast as GM spins the LS2, and my heads flow equal to or greater than the LS2, then I will be able to push the same cam/ intake/ injectors/ blah blah blah just as high as the LS2... But I will have the added mechanical leverage of the 4" stroke, if that proves to be beneficial or even measurable.

Thoughts?

My opinion:
IF it would fit I would think that you loose power everywhere, except maybe under 2K RPM maybe.

I checked those heads and they have 2.04 & 1.57 valves = 3.61" I dont think they would fit in a 3.78 bore, and IF they fit you would loose a lot of flow because of shrouding.

And they probably also have an combustion chamber that is bigger than 3.78 so they will get "overhang" and probably gasket problems.

And the .600 flow number does not matter since the cam only have .525 lift, and the mid range lift flow numbers are much more important than the peak lift/flow number anyway.

I see that all LS engines (and modern engines actually) have pretty big runners (compared to older engines SBC etc), but I would think that a 229cc runner does not promote a good low RPM torque engine. Even though it is modern tech. I dont know how small LS heads you can get, but I would think that you should not go over 200cc if you really want a torquey engine. You must match your heads to the rest of the engine.

EDIT: I think there is a page called Speedtalk? There there is a lot of knowledgable engine people I think, maybe they have some input.

Can you please explain why you think that my proposed undersquare 359" six liter motor would be worse than a 364" oversquare six liter motor, when all things are equal other than architecture?

Surely 5ci of displacement isn't going to be noticeable. And if everything else is the same, from the air filter to the rear tires... The ONLY difference is the LS2 achieves its 364 cubic inches with an oversquare bore/ stroke ratio, and "my" engine gets its 359 cubic inches via undersquare architecture.

Why do you feel that the undersquare architecture would be a detriment across the board? I'm pretty sure that the 5.3 and the LS2 both have heads based on the same casting. So if head flow is equal. Displacement is equal. And redlines are equal. Same accessories. Same cam. Why would "my" engine lose power everywhere?

If anything, worst case scenario, wouldn't it just be the same as a LS2? Why and how would I possibly make it worse?

What, exactly, are you referring to when you say "if it will fit"?

If you are referring to the intake valve size, yes, a 2.04" intake valve will fit. Yes, it will be shrouded in the 3.78" bore. Will ported 5.3 heads outflow, or AT LEAST flow as well as, stock LS2 heads even with shrouded valves? Yes.

I don't have to get the stage two porting. For the same price as TEA's porting services, I can get some trick flow heads. And they are meant for the 5.3L, or at least made for the 3.78" bore, and they have 2" intake valves.

No matter what, I can get at least as much air flow as stock LS2 heads. Given that the 5.3 is a truck motor, I'm guessing that GM already has done what needs to be done to make the 5.3 heads flow for best truck-like torquey performance.

Am I absolutely missing something? The way I see it is that it is fairly impossible for me to mess this up. "Failure" for this project is 400hp and 400ft/lb of torque. That's what LS2 parts on a six liter air pump produces.

Air flow determines redline. Redline determines cam. Cam determines how your engine uses the air flow up to the redline. Right?

So if I have a six liter motor with enough head flow to support the same redline as the LS2, and I am using the same cam, then at the very least I should expect to see the same result at redline... Regardless of whether the air pump is over or under square.

Now, I personally think that I will actually see an increase, everywhere. Let's say that the added mechanical leverage provided by the 4" stroke only adds 25ft/lb of torque, 500rpms sooner. Everything past that point will be higher than the same rpms on a LS2. Why?

Horsepower is a function of torque. Increasing mechanical leverage to increase torque will inherently increase horsepower with it.

Let's say that the LS2 cam is capable of climbing five stairs. In the stock application and configuration, the last step is at 400hp and 400ft/lb of torque.

But my 4" crank six liter motor gave us 25ft/lb extra, so the same cam climbing the same number of stairs reaches 425...

Assuming you can follow my analogy comparing useable powerband to a flight of stairs.

After all, an engine is nothing but an air pump. The fact that we choose to add fuel to and then burn the air, as we pump it, does not change the fact that an engine's primary function is to move air.

The amount of air the pump can move determines how much fuel can be added. Things like compression ratios and the timing of ignition events all determine how much power that amount of fuel, in that amount of air, can or will produce.

However, you can change how that power is transmitted from the pump to the ground. Things like mechanical leverage are dumb. Dumb in the sense that mechanical leverage exists without any air being pumped. Increasing the length of a lever increases torque. The end.

So, all other things being equal, increasing the stroke from 3.6" to 4" has to produce an increase in torque... Does it not?

And if you increase torque anywhere, any earlier, then all numbers after that point will be higher, because horsepower is a function of torque. So if you take it to the same redline, then theoretically, there's no way for the end result to be lower.

This is only speculation from my side. If you had 4" bore vs 4.3" bore it would be much more equal I think, over 300cid should not have less than 4" bore to get a big enough valve to feed all the cubes, if big power is goal!

If the valve is very close to the cyl wall, you not only get less flow, but it may restrict the amount of lift you can make before the valve hits the cyl wall.

But I dont know anything about that, you must try and test. But if you cant open the valve more than say .400 before it hits the wall. It would not exactly increase performance.

A shrouded big valve probably flows less than an small un shrouded valve, and the shrouded valve makes poor swirl since everything "chrashes". Unless maybe the wall for some reason increases swirl, who knows?

But it does matter what valve angles and center placement it has. And not forget about how the "TEA" combustion chamber diameter fits to gasket and bore.

With an head that fits well on both engines it may not loose as much power, but if you take advantage of the bigger bore on the big bore engine, that one will probably make best overall power, and more total.

How much power do you want? 350 or 450, 600 or 800?

I can try to make an graph in Engine Analyzer where I change nothing but Bore and Stroke and leave everything else equal. To make an comparison if you want.

Big bore = More Flow = More Power. Add a long stroke to that and it is win win!

Ok. I see where you are coming from. For most, the pursuit of large horsepower numbers trumps everything else.

I am not amongst this crowd.

I have no real need or overwhelming desire to necessarily best the performance of the LS2. A streetable 400ft/lb motor is more than adequate for my ambitions.

I would like for my particular design to outperform the LS2 based solely on pride, because I would forever convince myself that I am smarter than GM's engineering team. But I don't NEED more than what a bone stock LS2 has to offer.

I chose the smallest possible displacement using stock GM bore and stroke options that still allowed me to use the 4" crank. I chose the smallest bore diameter GM offers, on purpose, to test something that I think SHOULD work.

After choosing the small bore block and 4" stroke, I saw that made six liters of displacement. In finding out that my chosen combination was just five measly cubic inches from the LS2, I then decided to use it (the LS2) as my base for comparison.

I found that the LS7's redline is higher than the LS2, so I can still match or exceed the LS2's redline using a LS7 crank. No worries there.

I found that I can have the 5.3 heads ported to match or exceed the flow of the LS2 heads. So no worries there, either.

If my proposed undersquare configuration does prove to be better than the LS2, cool. Yay me.

If it doesn't. Then oh well, I am left with a stroked 5.3L, making it six liters, with a forged rotating assembly, and ported heads. That's not too bad for a "failed" project.

In fact, that is the beginnings of a wonderful boosted motor success story.

I know that I have been preaching about all things being equal, but they aren't going to be. I am going to use a more aggressive cam than the LS2 has. It is the most aggressive vvt cam from comp cams. Oh yeah, my undersquare six liter motor is going to have variable valve timing. My redline will actually be higher than the LS2. My heads will flow more than the stock LS2 heads. Given that displacement and compression are equal, but my engine having a better cam (with vvt), better head flow, AND a higher redline... It is literally impossible for me to not match or exceed the performance of the LS2.

I am not a high HP chaser.

Does the LS7 crank fit in a normal LS block?

It is more than an crankshaft that selects your redline.

Crankshafts have nothing to do with redline if we are only referring to the stroke of a crankshaft. With light enough, more specifically less massive, moving parts, stroke plays absolutely no limits on redline. The only limiting factor is heat through friction and vibration issues caused by large amounts of mass spinning really fast. Natural frequencies and the efficiency of the cooling system play a larger role than just the length of stroke.

I determine my redline based on cylinder head flow. I used online calculator(s) to see what the head flow numbers I had available could support. The Wallace racing online calculator said that the TEA stage two ported heads could flow enough to support six liters of displacement up to 8000rpms.

General Motors spun the LS7 to 8k during testing. So I knew the 4" crank (combined with lightweight parts) would not be an issue. Especially considering I will have smaller, less massive, pistons which is reducing weight/ mass out on the important end (so it doesn't matter as much if my non-titanium rods weigh more because that weight/ mass is closer to the axis of rotation).

Once I saw that the 5.3 heads could be ported enough to support six liters of displacement spinning at 8k, I honestly stopped worrying or even thinking about the redline.

Whatever the end of the powerband to the aggressive vvt comp cam is will be my redline. People on the interwebs talk about revving the vvt LS motors to over 6600rpms once the afm/dod is eliminated and new lifters are installed. And I'm sure +/- 6500rpms will be plenty for me.

But anyways, to answer your question, yes a 4" crank will fit in the standard block. I am not a pioneer in this effort. Wiseco makes forged pistons specifically for use with a 4" stroke and 6.125" rods in the 3.78" bore block. I believe K1 offers a complete forged rotating assembly using these pistons.

For reference:
LS2 cam is something like 204°/218° .551"/.547" 117° lsa

The aggressive vvt cam is 218°/222° .566"/.578" 114° lsa

Take whatever conclusions from that as you will. But, to me personally, it would appear that the aggressive vvt cam would favor a higher redline than the stock LS2 cam. And that's why I have stated that my redline would be higher than a stock LS2.

Simple question, seeking a common consensus...

Do you think that a bone stock LS2 would show a NOTICEABLE difference if you swapped in my proposed undersquare long block, assuming head flow and compression were the same?

Only change being bore and stroke, 4.00 x 3.60 vs. 3.78 x 4.00...

I hadn't originally started this thread to be about this particular engine, but I have used it to keep the conversation steady, and now I feel I have presented my argument enough that I should be able to get an honest opinion of what I am trying to accomplish.

Actually, forget the LS2 comparison. I know my proposed engine will best a bone stock LS2.

What is everyone's thoughts on the use of variable valve timing?

Note for future boost:
The aggressive vvt cam from comp cams is actually very comparable to the LSA cam, hmm, positive displacement blower on a stroker motor with vvt? Talk about torque.

LSA cam: 198°/216° .480"/.480"
Comp vvt: 218°/222° .566"/.578"

Thoughts?

Quick test from Dynomation simulation:

• Same Cam
• 11:1 CR
• Same Intake
• CNC LS2 Heads for 1st and 2nd test (did not adjust flow numbers based on bore size - will be lower with 3.78 bore)
• CNC LS3 Heads on 3rd test (note: will not fit 3.78 bore)
• 6.098 rod length on 3.60 stroke (stock 6.0L)
• 6.067 rod length on 4.00 stroke (stock 7.0L)

Typically, OEM’s don’t arbitrarily pick bore / stroke by accident. 6.0L was designed to be 4.0” bore x 3.6 stroke for a reason. As well, when 7.0L was designed (for a 4” stroke) bores were extended further into block for piston stability.

BTW, I did not spend any substantial time on this model - so although relative numbers are comparable from each test, outright numbers are not necessarily accurate.

Dave

David, the info that Dave (mikels) is posting is valuable. You can absolutely trust what he's telling you, as he does this for a living, and builds some of highest quality, time intensive, LS powerhouses in the country, with ultimate durability and big numbers. I'd be honored that he took time to run your numbers for you.

@ dave,

I very much appreciate your response, and data. I was just thinking about the rod length earlier today, actually. I found myself wondering why wiseco made their pistons for use with a rod length other than what GM produced. And I was thinking that I would probably get pistons made so I could use the LS7 titanium rods.

Regardless, I am trying to understand the data you provided. Why does the small bore fall off? What causes it to not at least be equal? If head flow is equal, then shouldn't the power be equal? It definitely shouldn't decrease, should it? Why would the 4" stroke reduce the power, if the heads flow the same?

Is it the piston speed leaving top dead center? What causes the decrease? The 3.78" bore cylinder heads can be made to flow AT least as much as bone stock LS2 heads... Even in the 3.78" bore. So what is causing the decrease? Heat through friction? If the heads flow the same, why is the 4" stroke showing a decrease?

Large bores take better advantage of head flow.

Stock rods are metric. Most aftermarket rods are standard - and pistons are made to match these rods.

Heads WILL flow less on smaller bore - valves are more shrouded by cylinder. So regardless of what you get heads to flow, they will flow more in a larger bore engine - but as I said, I did NOT change flow numbers for this comparison (which will make difference even greater than these numbers show).

So let's look at some additional differences:

While you are correct in assuming a longer lever arm will increase torque, this is only true if the cylinder pressure times surface area is the same. Say each LS2 head combination results in the same cylinder pressure. One is working on a total surface area of 12.566 in^2 (4" bore) and the other is working on a total surface area of 11.222 in^2 (3.78" bore) - or ~89% the surface area of larger bore. So to make same FORCE, will need 112% the cylinder pressure. Peak cylinder pressure occurs ~12 deg ATDC. The difference in lever arm at this crank angle for a stroke change of 0.4" is pretty damn small - certainly much less that the required 112% change in force.

Data below shows frictional losses for longer stroke are greater (no surprise). Pumping losses are slightly better for longer stroke (no surprise again). Mechanical efficiency favors the larger bore slightly. Both LS2 headed combinations flow nearly same airflow (again - I did not alter flowbench data for smaller bore). All of these are relatively small differences - up until you get to the aforementioned piston force. Now you see the primary difference between the combinations.

BTW: I do NOT believe for one second any of these combinations will make the high RPM power numbers that these simulations show. I would fully expect any of these to make peak power ~6500 rpm, and start falling rapidly after this point. If I took more time to detail the model, it would certainly reflect this.

Dave

Thank you for the kind words and compliments!

Dave

Thank you for the explanation. The reduction in force due to the smaller piston makes sense. I guess I was too caught up on air flow. It is really cool to see that the friction loss is negligible (below 1hp) all the way up to 4500rpms. And the mechanical efficiency is within 1% up to 7500.

And, to be honest, I was mistaken in my understanding of the physics. I was thinking that a set amount of air and fuel, with a set amount of compression and timing events, would make a set amount of force. Pressure is force divided by area, so the smaller piston with the smaller area, with the same force, would be more pressure on the crank. I guess I was approaching this from the wrong direction. So you thank you for setting me straight.

I was using a handloaders'/ ballistics approach. Same amount of powder used to push a big bullet, now pushing a smaller bullet, makes for more energy downrange. Velocity is squared in the equation for energy, so increases to the velocity trump increasing mass. I was comparing the air and fuel in the combustion chamber to the powder charge in a rifle cartridge. Pistons to bullets. And getting energy downrange to the crank.

Shows how little I know. It also shows that I am a redneck, not a physicist.

From the description of the type of performance you want, a conventional 5.3L or 6L with a custom cam would be a good bet. An engine can be cam'd for low end grunt, and peak hp at a desired redline.

A good guy to talk to is Chris Straub of Straub technologies. He does custom cam's for a lot of people.

While a run of the mill 5.3 or 6.0 may be plenty adequate for my ambitions, it is not what I am necessarily after.

The camshaft controls the timing events that move a mechanism. And while you can custom tailor the camshaft to achieve just about any result from any mechanism, I want the actual device, the mechanism itself, to be tailored for what I want.

You can cam a normally high revving short stroke engine to produce all its torque as early as possible. Is the motor itself built or designed for low end torque, no, but you can cam it that way.

You can cam a stroker motor for top end power, too. Is it designed for high rpms, no, but you can cam it that way, if you want.

I don't really know how to explain my thought process on this. Maybe I'm just wrong or confused. Maybe it's just ego or pride, or some petty desire to be a unique and special snowflake. I don't know.

I know I care about torque more than horsepower. I know I care about what is going on in normal rpms more than what is happening in the final few revs before you burn your **** up. And I know that I have not a single care about what my peak numbers end up being.

The aggressive vvt cam from comp cams says its operating range is 2000-7100rpms. That should be just fine, and it still gives me 20 degrees of cam phasing. Which will automatically make it more streetable than any non-vvt cam, and driveability is a huge part of all this. And I can't really rev any higher than 7k rpms without going dry sump, which I am not willing to do, so there's no reason to give up vvt.

I am still going to check out Straub, it's always good to know who can make a good custom bumpstick. So thank you for passing that on.