What is everyone's engine design preference?

[Twoblackmarks...]

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.

[DavidBoren]

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.

[DavidBoren]

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?

[mikels]

Originally Posted by DavidBoren

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...

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

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

[Che70velle]

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.

[DavidBoren]

@ 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?

[Vegas69]

Large bores take better advantage of head flow.

[mikels]

Originally Posted by DavidBoren

@ 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?

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

[mikels]

Originally Posted by Che70velle

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.

Thank you for the kind words and compliments!

Dave

[DavidBoren]

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.