Using my motor as an example.

I have a 418ci motor, 4.040 bore x 4.075 stroke, big 2.08/1.60 valves...

I run a 242/242//114/112ic cam.

How do big motors eat up camshaft.

Eric Koenig? <img src="gr_stretch.gif" border="0">

 

is that the theory that says, hypothetically, a T1 in a stock motor will thump like crazy, while in a 382 or 422 it will sound like stock?

 

bingo

Only thing I can think of is airflow requirements.

 

It is airflow requirements. A gas motor is nothing more than an air pump. The larger it is, the more valve timing it needs to injest the air and fuel it needs for a given power output. Thats why larger cams dont sound or perform as radical in larger motors. Its all in the cubes. It takes alot more camshaft to lower the engine vacuum in larger motors than smaller ones. A bigger motor can pull higher vacuum with a larger cam than a smaller motor can. Its that simple. The higher the vacuum at idle, the more docile the motor is.

 

Exactly, a cam must be matched to the airflow requirements of the engine. The two biggest players in airflow requirements are peak rpm and engine displacement. If you take a 346 and increase it to 422, you have just increased its volumetric displacement by 22%. So at the same rpm, the 422 requires 22% more air, by volume, in order to operate at the same volumetric efficiency as the 346 cid engine. The intake ports are the same size and the intake valve is only about (2.08/2.00) 4% larger, so the rest of the increase in airflow must be provided by increased intake duration by the cam.

If you move 22% more air through the same size intake port, the velocity will increase. So one indicator of how tame an engine will run would be a ratio of inlet port cross sectional area to the cylinder displacement. Using the same intake port, the larger engine will always have the smaller ratio and thus will be tamer with the same size cam because of the higher inlet port velocity for a given rpm. In order to keep volumetric efficiency constant when you increase cubes and use the same size intake port, you will need a larger cam.

[ December 11, 2001: Message edited by: 2quick4u ]</p>

 

How large Tullis? <img src="gr_stretch.gif" border="0">

Me...

590 lift
242 duration
114 lsa
112 ic
11.25:1

 

John,

I would think that after all this time you would at least know my last name. Hint: It is not Tullis. <img src="graemlins/gr_judge.gif" border="0" alt="[judgement]" />

A very rough way of approximating how much extra intake duration you will need to keep your Volumetric Efficiency constant that I use is:

[(displacement/346cid)^.5] x old cam inlet duration

In your case [(418cid/346cid)^.5] x 221(B1/T1) = 243 degrees.

So a cam with 243 degrees intake duration @.050" in a 418 cid engine might idle close to a cam with 221 degrees duration @ .050" in a 346 cid engine. Also the peak TQ, Hp, and VE numbers will be at roughly the same rpm. This is assuming that lift, LSA and ramp rates are kept nearly equal between the two cams.

[ December 11, 2001: Message edited by: 2quick4u ]</p>

 

Dont forget that the way you increase cubic inches affects cyl. head requirements. Why? Because the more stroke the motor has the less room for maintaining the same rod to stroke ratio.This affects the pistons dwell time at TDC,and its accelleration to and from TDC. Longer rods(higher rod to stroke ratio)have slower mixture motion thru port needing a more narrow port width to maintain good port velocity.And short rods need wider port width to maintain good volumetric efficency. Hope this makes sense.

 

[quote]
The intake ports are the same size and the intake valve is only about (2.08/2.00) 4% larger, so the rest of the increase in airflow must be provided by increased intake duration by the cam.
<hr></blockquote>

Nothing against your pseudo physics, but you are way over simplying some pretty complex concepts, to the point that I think you are misleading people.

Remember that the larger motor creates more vacuum, which in turn draws in more air on its own. You simply want to redesign your intake setup (heads/cam/manifold) to allow the motor to operate at as high VE as possible with the extra volume of air being drawn in. Generally speaking this means you will want a slightly wider camshaft and more intake runner volume. Other changes such as larger valves, more combustion chamber work, and more intake manifold volume will help conserve that great 346ci h/c VE when you step up to the big cube motor. Just remember you don't need to go overboard, we're talking small changes here.

FWIW I saw a 427CI motor at MMS the other day that puts out 530rwhp with a 229 229 .578 .578 114ls camshaft, and LS6 heads.

 

Terry,

This may be an oversimplification, but all we are doing is explaining why a bigger engine with the same heads needs more cam to achieve the same VE. Sure, if you change the valve sizes, head port sizes and volume, intake manifold runner size and plenum volume, you will require less intake duration increase to achieve the same VE.

But the simple truth of the matter is, that on LS1 heads, unlike big-block heads, you can't run intake valves much larger than stock. While heads can be ported and this helps, the inlet port shape is still not far from stock on cross sectional area and port volume. And for an intake manifold, you are pretty much stuck with the stock LS6 piece. So you will have to make up most of the airflow difference on a 422 LS1 with increased cam duration.

A larger engine will naturally want to pull in more air than a smaller one; but, a 422 engine trying to pull air through 346 sized valves, ports, and cam will have a big pressure drop due to inlet restriction and will make poor VE. This pressure drop becomes inlet manifold vacuum.

Pulling engine vacuum at WOT means that you have poor VE, a throttled engine, or a large restriction in the intake path. Afterall, engine vacuum is merely the differential pressure between your cylinders and the outside atmosphere.

BTW: On the MMS 427 engine you referenced, what rpm did peak Hp and peak Torque occur at? I would imagine that the 229 cam is a pretty tame cam for that engine, how does it idle?

If I had to guess, I'd say the Hp peak was near 5,800-6,000 rpm and the Tq peak was near 4,500 rpm and the car probably drives as tame as a 346 with an LS6 cam.

[ December 11, 2001: Message edited by: 2quick4u ]</p>

 

""Using my motor as an example.
I have a 418ci motor, 4.040 bore x 4.075 stroke, big 2.08/1.60 valves...

I run a 242/242//114/112ic cam.

How do big motors eat up camshaft.""


John, I think these guys have done a good job with this question. The readership of LS1tech is becoming more and more engine savvy every day!

With the bigger engine there is now a higher depression or pressure drop across the same heads since you have the volume under them increasing at a higher rate at any given rpm than with the smaller engine. This allows you to close the intake later because now the same head has more intake velocity than previously. If you try to run that same larger cam on the smaller engine the cylinder will fill sooner and then back back up the still open intake valve since it lacks the higher velocity and inertia that the bigger engine pulled on it. Since you now have more total intake in your bigger motor you also may need to open your exhaust a little sooner to start the blowdown process earlier so you can also run a wider LSA or keep the LSA the same and run more exhaust duration. It's all a compromise based on the heads I/E ratio and total cubic inches and head flow etc.

 

[quote] The intake ports are the same size and the intake valve is only about (2.08/2.00) 4% larger... <hr></blockquote>

Not that it really changes much, but wouldn't that be about 8% larger since we are talking the area of circle whose diameter is being refered to?

 

[quote]Originally posted by Ragtop 99:
<strong>

Not that it really changes much, but wouldn't that be about 8% larger since we are talking the area of circle whose diameter is being refered to?</strong><hr></blockquote>

You are correct Joel, you caught me napping <img src="graemlins/gr_zzz.gif" border="0" alt="[boring]" />
The area increase is what is important.

 

Ok, so say I take my T1 out of my stock block with ported heads and put it in a 382 block with the same heads. My power curves would not only rise because of the increased cubes, but should shift to the left as well, correct?

 

Whats bad about sounding stock with a 418 sitting under the hood??? <img src="graemlins/gr_devil.gif" border="0" alt="[devil]" />

 

[quote]Originally posted by Jake99SS:
<strong>Ok, so say I take my T1 out of my stock block with ported heads and put it in a 382 block with the same heads. My power curves would not only rise because of the increased cubes, but should shift to the left as well, correct?</strong><hr></blockquote>

Correct.

 

i was gonna throw in .02 but after ready all this ill just say "ya, what he said" <img src="gr_grin.gif" border="0"> <img src="graemlins/gr_hail.gif" border="0" alt="[hail]" />

 

[quote]Originally posted by Jake99SS:
<strong>Ok, so say I take my T1 out of my stock block with ported heads and put it in a 382 block with the same heads. My power curves would not only rise because of the increased cubes, but should shift to the left as well, correct?</strong><hr></blockquote>

Yes, your Volumetric efficiency would drop some at the upper rpm ranges so your peak hp and peak TQ would occur at less rpm (probably around 300-500 less rpm) than on the 346 with the same heads and cam. You would have killer, stump pulling torque.