Built LT1

iTrader: (3)

Which of these two cams are theoretically bigger?
The engine with cam 1 is 348 cubic inches.
The engine with cam 2 is 388 cubic inches.

Cam 1.

Lift 59X-59X
Duration at 0.050 242-248
LSA 113+4

Cam 2.

Lift 62X-63X
Duration at 0.050 248-254
LSA 110+4

I do not have the advertised lift numbers.

LilSlo1

iTrader: (3)

Duration at .050 is an industry standard for measuring duration. Many companys measure their advertised duration at different lifts (ex. .006 or .010). Having a duration rating at .050 lift makes it easier to compare cams from different companies.


Your cam #2 would be bigger. It has more duration and lift.

If you are comparing the sounds of these engines, the cams may idle very similiar.
If you were to have the same cam in each engine the bigger cubed engine will idle a little smother than the smaller cubed engine.

Built LT1

iTrader: (3)

I am aware that the larger the motor is, the smaller the camshaft will become. I am just wondering if the 40 additional cubic inches would make cam two smaller than cam one. I have heard before that 40 cubic inches is equal to six degrees of cam timing?

LilSlo1

iTrader: (3)

The only real way to compare the two engines with the two different camshafts you listed would be to see what kinda vacuum you have at idle. The "bigger" the cam (for the specific cubes) the less idle vacuum you will have.

Built LT1

iTrader: (3)

I understand what you are saying. I am just wondering how you know that cam #2 would be bigger? Is there a formula?

LilSlo1

iTrader: (3)

None that I know of.

When you say this "I am just wondering how you know that cam #2 would be bigger? " do you mean better ? as in peform better ?

Keep in mind that bigger is not always better. There is a point where a cam can be too big for your combo. If the cam is too big, you may be killing power rather than gaining it. Big cams will also sacrafise low rpm power, which many people say you need for street driving.

Also big cams are less streetable, but everyone has their own idea on streetable. My 10 second 3rd gen camaro I drove on the street, was it streetable ? Yes in my eyes. Although I did consider that having to hold the brake with one foot and give it a little throttle with the other foot to keep the engine running at a stop light was a pain in the ***.

Built LT1

iTrader: (3)

I guess I should have worded the question more clearly? I understand what your saying. Total performance and driveability aside, my question is what cam is larger when you factor in the cubic inches.

My question may be more understandable with the following example: A 220-220 cam on 350 inches would theoretically be bigger than a 224-224 cam with 700 cubic inches...follow the logic? This is my question.

LilSlo1

iTrader: (3)

Thats what I thought you meant. This is something that you may need to call a cam manufacturer on if you cannot test it yourself with 2 engines. I'd say that the best way you can compare it would be to compare idle vacuum on both engines. The engine with a rougher idle will have the bigger cam for that particular cubic inches.

Mabey someone will chime in here it some sort of fomula that you were asking for earlier.

SStrokerAce

iTrader: (2)

I can give you a dam good guess on the vacuum at idle if I had the compression of both motors.

IMHO the #1 cam would act much bigger in driving around. TQ curve will come on later and the motor will have a higher narrower power band.

Bret

Patrick G

iTrader: (14)

As a general rule, you add 8 degrees of duration for every 50 CID increase. 40 cubic inches of bigger motor will mean you need to add 6-7 degrees of duration to make the larger motor behave like the smaller one.

So your 242/248 cam would need to be around 248/254 in the larger motor for it to be the same. Now you've also monkeyed around with overlap on the bigger cam. The narower LSA will add more overlap than the increased duration will and therefore, the larger motor will chop harder with its idle. If the LSA was the same between the motors, they would theoretically act the same.

If the larger motor used the same valve sizes, going narrower in LSA is a good idea because the motor is more "under-valved" and would respond better to additional overlap.

edcmat-l1

iTrader: (10)

See, I would think it would be the opposite. Cam #2, with the longer duration, and tighter LSA (more overlap) would have the narrower, higher power band than the 113 LSA. IMHO
I would also think that regardless of the engine size, the 110 LSA will have less manifold vacuum than the 113 LSA.
Even adding duration per cubes, if you keep the overlap down the motor should produce more vacuum.
I dont really understand the "theoretically bigger". There are alot of dynamics that dictate cyl filling that cant be answered by just comparing cam #s in relation to CID. Head flow #s would be the main factor.
" Which one will 'act' bigger?" Is this a question about idle primarily?

SStrokerAce

iTrader: (2)

Ok first problem is... your changing the lobe familys here. Going from .590" to .625" lift within 6° of duration is going to change the ramp and nose of the lobes. So lets just stick with XE-R type lobes.

Both motors with the same 315cfm LS6 heads, 11:1 Compression

Idle Vacuum for the 346 242/248 113 20° Overlap = 11.5"
Idle Vacuum for the 388 248/252 110 30° Overlap = 10"

You increased the overlap of the cam 50% while cubes increased 12%, obviously you will have less idle vacuum there. If you cut that down to 25° of overlap in the 388 the motors will idle the same.

The larger cube motor would have more than 85% of it's peak TQ available for a longer RPM range in the 388 than in the 346. About 2000rpm in the 346 while the 388 would have it around 3000rpm. The larger motor even though it would "chop harder" would be easier to drive since it has more TQ available at lower RPM and low throttle openings.

You basically just made the cam for the larger cube motor have more overlap with the same IVC and EVO events. And with the addition of 42 cubes it will like that. Reguardless to what you read on here, 50 years of engines telling us what they want (because the engine IS smarter than any of us in that department) more cubes want more overlap.

Bret

edcmat-l1

iTrader: (10)

I understand (all of that).
When you said before that the first cam would "act" bigger driving around I thought you meant it would chop harder, be less drivable.
I think the second engine would have more low speed (rpm) issues, due to the lower manifold vacuum (tuning will help/fix, obviously).
It may make torque over a broader band, but isnt that gonna be just cause of the size increase?
PS thats pretty much a rhetorical question. Already know the answer.

onephatZ28

iTrader: (8)

SStrokerAce

iTrader: (2)

Yep basically what I ment the first and second time.

Yes it will make more TQ on a broader range but mostly because that cam and cube combo is better matched.

The lower vacuum can cause issues but it doesnt have too it's all the system.

Bret