What is everyone's engine design preference?

[cjsgarage]

Originally Posted by DavidBoren

Also, I am getting conflicting information regarding possible knock issues.

Everything I read, up to this point, says that the long rods inherent to a long stroke will slow the piston near top dead center. This will provide the increased dwell time at TDC, as you stated.

But I read that decreases the risk of detonation, so I would appreciate some clarification, please.

Does the piston slowing down near TDC simply increase the amount of time it is possible for detonation to occur? Or does something about the piston slowing down actually cause or invite detonation to occur?

My understanding of it, from what I have read, and how it was explained to me, is that the reduced rate of compression decreases the likelihood of detonation. Am I wrong?

I can see detonation issues being inherent to the small bore size, as we discussed earlier... Less surface area to dissipate heat into the cooling system. More latent heat in the engine raises the risk of detonation.

It could be argued that I will see no benefits of what long conrods can provide because my 4" stroke gives me a 1.53:1 rod ratio, which isn't great. So my rod angles are still relatively severe, so I probably won't see any decrease in piston speed near top dead center. If anything, it may act like a short rod motor and increase in speed at TDC.

If it has more to do with the rod ratio than the actual length of the rod, then I probably won't see any increase in dwell at top dead center. And therefore, I probably won't have the detonation issues you warn of.

However, I am still in need of some clarification on this.

Does increasing dwell CAUSE detonation, or simply increase the amount of time it may occur?

When discussing piston speed, does the actual length of the rod matter? Or is it all about the ratio in comparison to the stroke? The change in rod angle determines the relative change in speed, and the ratio is constant in the sense that any length of rod with the same ratio will change the same relative angle, right? So any length of rod with a 1.53:1 ratio will give you the same relative change in speed near top dead center... Correct?

If that's the case, then my stroker is actually more of a short rod motor, because 1.53:1 is kind of low.

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

[DavidBoren]

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.

[cjsgarage]

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.

[DavidBoren]

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

[Twoblackmarks...]

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.

[DavidBoren]

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?

[Twoblackmarks...]

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.

[DavidBoren]

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.

[Twoblackmarks...]

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!

[DavidBoren]

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.