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406 lsx vs 408 lsx

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Old 08-09-2017, 10:49 AM   #1
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Default 406 lsx vs 408 lsx

We all know that the diff is two ci. But a l92 headed 4.030 x 4.00 408 vs a 4.155 x3.75 l92 headed 406. Is there a clear winner if both can breathe and properly cammed. There are lots of 408s out there but i can nearly no info on a ls 406. I know the 406 will be more due to having to either sleeve or destroke something but how will it stack up to a 408 with the above dimensions. Thanks

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Old 08-09-2017, 11:35 AM   #2
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I think the 408 would make better torque and peak at a lower RPM, whereas the 406 would make better power and peak at a higher RPM.
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Old 08-09-2017, 12:53 PM   #3
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406 is something i've toyed with building - old school 400 small block, but with LS heads. I think KCS nailed it, but I think the gains in power on the 406 will be far greater than the lost torque vs the 408. Because you could use LS7 heads
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Old 08-09-2017, 01:15 PM   #4
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So what would be the difference if both were rev"ed to 8500 please show how much better the bigger bore would be. Using the same compression 12:0 and lift .800 lift. Let's see this. One of you guys have the simulator. Ls7 heads for both. KCS your on point but I just wanna see the results.
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Old 08-09-2017, 01:19 PM   #5
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Pantera EFI your needed for a simple test using the software you have.
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Old 08-09-2017, 01:47 PM   #6
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The ls7 wont fit a 4.030 bore. Ive seen some pretty nasty 406 ci motors. I fig on the ls platform they should really shine.
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Old 08-09-2017, 01:51 PM   #7
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Yes it will small bore Mast or TSP (4.065) Ls7 heads. I'm not arguing about the bigger bore just the fact that it will be some Huge increase in power.
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Old 08-09-2017, 02:04 PM   #8
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Understood. Like u i wanna see winner of this one.
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Old 08-09-2017, 02:18 PM   #9
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Quote:
Originally Posted by my406cid View Post
We all know that the diff is two ci. But a l92 headed 4.030 x 4.00 408 vs a 4.155 x3.75 l92 headed 406. Is there a clear winner if both can breathe and properly cammed. There are lots of 408s out there but i can nearly no info on a ls 406. I know the 406 will be more due to having to either sleeve or destroke something but how will it stack up to a 408 with the above dimensions. Thanks
Depends on where you want the power and what you are using the engine for. For a pure torque monster, 408 with the best cathedral heads you can get and a long runner intake like the FAST 102. For big horsepower and RPM, the big bore engine with some top of the line LS3 or LS7 heads and a short runner intake.
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Old 08-09-2017, 02:20 PM   #10
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408 all day. Now when you say 4.155+ bore size and starting with at min. 3.9+ crank size. That's Hp
I understand the thought of someone wanting to rev a engine. Why have a nice bore size with what I call non existent Tq. What 4 inch cranks don't spin anymore? That one little thread about a destroked Ls7 now 4 inch cranks don't rev to 8k+.


I understand being different. But cutting your self is not good. Bigger is better and easier on parts to make the same hp. The same 406 starting with a 4 inch crank does what in comparison to the 406. Destroke for what? I see guys saying how well they rev with smaller crank sizes... Didn't Tony Mamo just do something nice with a stroker build.

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Old 08-09-2017, 02:47 PM   #11
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For 7000 RPM and above I will ALWAYS Take a Bigger Bore with equal
CID. Bigger Bore = Bigger Valves and Bigger Airflow. Torque is Primarily
CID + Compression. Valve Events determine where the peaks are and
Shape of curve/plateau as long as cammed for intended purpose/RPM
Range.

Example:

Boss 429 VS 426 Hemi in NASCAR 69-71. Boss 429 made consistently
50 HP more and only Super Bird & Daytona bodies kept it close!
4.36" X 3.59" VS 4.25" X 3.75" 8000-8500 RPM.

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Old 08-09-2017, 03:01 PM   #12
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For me it not about destroking or a 4" crank cant spin. 406 and 408 are 2 ci apart yet vastly different. Never seen this camparo before and just lookin for comment on what motor is the alpha by input. Please carry on!!

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Old 08-09-2017, 03:17 PM   #13
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Why would anyone buy a sleeved or aftermarket block and not build bigger? Waste of good money. We all know the bigger bore is better. But the smaller crank does not and will not be a thought.

And for examples listed: how do you like SAM"s engine or this one not with some tiny crank.
And not some big block limited on displacement. Yes bore is better but why carry a 22 when you can have a 44 magnum in the same size. I want to see the comparison. I know of 2 408's spinning past 8k and both were past 800. ER with a bigger bore would make 800 but a drop on TQ and more RPM to make the same 800+. I'm with you all on bigger bore Not stroke. Also as I said once before go look at Speeds build and look at Navys build. It's there for you to see simple as that 406 vs 408 basically the same as Speed and Navy.

http://www.superchevy.com/how-to/ght...-440ci-na-lsx/
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Old 08-09-2017, 03:40 PM   #14
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Not every chassis or application wants to maximize torque bellow peak.

The longer the stroke the more demand you place on your top end which far and away is the most expensive component - valvetrain, heads, and intake can either be $2000 or $10,000...

3.75" vs 4.00" is only a 6.67% increase in piston speed which is not outrageous. That's 6000 rpm vs 6400 rpm.

A stock ls3 stroke vs 4.00" stroke is 10.5% difference in piston speed which is getting to be significant.

Looking at the 4.125" racing builds gets a little nutso.. almost 14% increase in piston speed compared to a stock stroke ls3.

Yes - if you can feed the air to it build the biggest engine possible. Otherwise you'd better size your engine around your heads and intake or you're going to have a bad time.
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Old 08-09-2017, 03:46 PM   #15
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Quote:
Originally Posted by Patron
Yes it will small bore Mast or TSP (4.065) Ls7 heads. I'm not arguing about the bigger bore just the fact that it will be some Huge increase in power.
My post might have been poorly worded. I think that you'll gain way more power compared to any lost torque. Like if you lose 5 tq I think a gain of 15-20 hp

Didn't mean for that to sound like hundreds of horsepowers.
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Old 08-09-2017, 03:49 PM   #16
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I forgot to write also that the frictional forces increase dramatically with piston speed and will show a greater disparity the higher the revs get as it is an exponential increase in drag.

The shorter stroke engine will be smoother running, last longer, have a quicker throttle response, and will make more power at the same RPM with all other variables held constant.

The reduced friction in my opinion will allow you to generate more average power where it matters (torque peak to past peak power) than you can with a longer stroke engine as the power will begin to fall off no matter what you do. (again - with all other variables held constant like cylinder head size and airflow potential)
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Old 08-09-2017, 04:10 PM   #17
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Quote:
Originally Posted by spanks13 View Post
The shorter stroke engine will be smoother running, last longer, have a quicker throttle response, and will make more power at the same RPM with all other variables held constant.
My experience says what is in bold is not true as a generalization. First, I offer that this depends on the RPM. With "all other variables held constant" the longer stroke engine is going to make peak torque a little earlier. So, at that earlier RPM, the longer stroke will make more power. With that said, once the engine RPMs get above peak torque, the short stroke and better road ratio engine will perform better for sure.

Also, I am assuming you meant that if both engines have equal displacement. If they both have the same bore, the longer stroke engine will have more displacement and make more power. It would also allow you to increase the duration and in some cases lift of the camshaft to allow you to make even more power than the smaller engine in the same RPM range.
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Old 08-09-2017, 04:11 PM   #18
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Man just keep it simple if one were to go out and spend the amount of $ needed to run a 4.125 + bore size and run a 3.6 crank. I have no comment. Also for the car not wanting all that Tq depends on stall speed as most guys here are running 3800 to 4K stalls with 3.73/4.11 gears. Also who does the work and dials in the suspension. Some guys I know are running 6000-6800 stalls with 4:30's/4:56. It depends on the application but bigger is better. 4:155 bore 406 ls vs 433 Ls same engine with a larger crank. It's That easy. You already have the block and you need a crank So which crank do you choose simple as that. Because starting with a 4.030 bore only means your not spending $ for the start up= smaller bore size. You bought a block only to limit it?? And can't any engine Rev with Correct camming? You make a 406 with the $ & I know what I'd do 427+. Your $'s spent when you bought the 4.125+ bore size and you need a crank.

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Old 08-09-2017, 04:30 PM   #19
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Quote:
Originally Posted by spanks13 View Post
I forgot to write also that the frictional forces increase dramatically with piston speed and will show a greater disparity the higher the revs get as it is an exponential increase in drag.

The shorter stroke engine will be smoother running, last longer, have a quicker throttle response, and will make more power at the same RPM with all other variables held constant.

The reduced friction in my opinion will allow you to generate more average power where it matters (torque peak to past peak power) than you can with a longer stroke engine as the power will begin to fall off no matter what you do. (again - with all other variables held constant like cylinder head size and airflow potential)
You can call all this irrelevant, but at work, I have to deal with an oscillator that is limited by the tensile strength of its materials. So, this means I am required to know the peak acceleration and forces on the oscillator for any oscillating program. The stroke of the oscillator greatly changes how fast I can oscillate within the confines of maximum peak acceleration.

I can oscillate at 360+ strokes per minute at 6mm stroke, but at 210, I can oscillate 14mm. I know compared to an engine, those seem small, but the mass that is oscillating is several tons.

Based on this - how I apply it to a motor is -- at a given RPM, the longer stroke has higher acceleration forces on the pistons and rods (necessary to reverse direction). As RPM increases, so does the increased stroke increase the effect on acceleration force. Also, as RPM increases, the acceleration on the pistons increases faster than RPM increases. Y=x vs Y=x-squared. BUT, the longer stroke due to more air and fuel should be making more torque and power. UNTIL the increase in internal acceleration of the engine increases faster than the airflow in can increase.

I think this is why shorter stroke engines tend to peak later in RPM and carry better, all else being equal. Conversely, I'd expect the longer stroke to bring the peak RPM down and fall off faster past peak. So, I think I'm seeing what Spanks is seeing.

But I also get what Patron is saying - why build a $15K 406, when for basically the same money you could build a 441.

What i'd like to see is dyno curves of both and the optimal shift points calculated and then calculate average power between upshift and shift point on each motor.
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Old 08-09-2017, 04:36 PM   #20
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IMO, this is pretty similar discussion to a 3.9" bore x 4" stroke 383 vs a 4.125" bore x 3.622" stroke 388.

I'm really curious to see how 98_ws6_m6's 383 he's building with a mamo top end will compare with Big Hammer's 388. Both of those are very well built motors - not like its a bad 383 and a tremendous 388
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