Minimum Cross Sectional Area: Question

[gmrocket]

Are the advertised ratings for the 274 and 283 taken at same lift?

You may better off with something like 275 and 235 or more...with 1.6 on intake only.They are out there.

The 274/224 w/.575" is right out of COMP's profile catalogue and is based on a 1.6 RR. That's an XFI intake lobe. The 283/231 was spec'd by Torque Master but there's no XFI lobe with that duration. 280/230 and 286/236 is as close as it gets with lifts around .575". My job will be to find a cam supplier that can deliver that duration with similar lifts of .575". There are two lobes in the Xtreme Energy category at 282/230. One with .544" and the other with .622". The .622" is too much IMO. The .544"....maybe.
Like you said, they are out there.
I have to get back into Jones' catalogue as I believe he has lobes that fit that bill.

Ok, but you didn't answer my question.. are you talking the same adv rating specs?

Is tq master giving you the 283 number at the same starting lift point?

[skinny z]

Ok, but you didn't answer my question.. are you talking the same adv rating specs?

Is tq master giving you the 283 number at the same starting lift point?

Oh. I see what you're asking. Good question. Well, there's no way I can tell if Torque Master is giving the advertised rating at the same lift (.006" or .004" or whatever) as COMP's which is specified as .006".
Plenty of folks use .050" values as the go to spec and the "advertised" spec means little to them. Seems to me though that's going to be lacking in some way information-wise.

[GARY C]

Ok, but you didn't answer my question.. are you talking the same adv rating specs?

Is tq master giving you the 283 number at the same starting lift point?

Oh. I see what you're asking. Good question. Well, there's no way I can tell if Torque Master is giving the advertised rating at the same lift (.006" or .004" or whatever) as COMP's which is specified as .006".
Plenty of folks use .050" values as the go to spec and the "advertised" spec means little to them. Seems to me though that's going to be lacking in some way information-wise.

I have not really messed with TM but most of DV's programs focus on what needs to happen at the valve as opposed to what the cam is doing unless he is specking a specific cam then his hyd cam stuff has always been at .006, you may want to PM Stan to get a clarification on this.

In order to compare lobes you probably need to look at advertised/seat numbers, .020 .050 and .0200 as well as total lobe lift.

[skinny z]

you may want to PM Stan to get a clarification on this.

That is an excellent idea.

[gmrocket]

Skinny: what do your computer sims recommend for duration at .050"...6000 RPM redline...3.75" stroke...? (intake specifically)
-i feel 240 degrees is excessive not many small-block tow-trucks have 240 intake degrees duration..

As far as the cam recommendations go, as well as the computer simulations, almost all peak at 6000 RPM. Makes perfect sense in that the CID, heads (in particular)and compression ratio are the same across the board. That said, I place my red line about 500 RPM beyond the peak HP RPM.
Professional picks ranged from 232 to 242 @ .050" on the intake. All peak at 6000 (so a 6500 red line shift). The lower duration tends to be flatter at peak. Essentially flat from 5500-6000. Then they all fall off the map after that as, I assume, the heads run out of steam.
My latest experiments use Torque Master as a guide and moving the inputs around to see the trends that develop. As an example, all else being equal, at 9.8:1, duration at .050" is 231. LSA 106, ICL 102. Push that compression up to 10.4 and suggested duration is 224 with 107/103.
There's one more iteration I want to study and that's the "too big a cam" in DV's write-up on a small headed 383. That particular combination used a small head (Edelbrock Performer RPM 170 cc) but the cam spec was 288/293, 236/242 (no spec on the LSA although given that is was described as Comps so I'd guess a 110 LSA). Plenty of low end torque which was attributed to the higher port velocity while the longer duration helped the top end.
I'm less enthusiastic than I was about the approach though.

Something's not right here

9.8 requires a 231 in at 102 with a 106 split

Yet with 10.4 it says you need a 224 in at 103 with a 107 split?

Don't make sense needing less .050 with more comp

[skinny z]

Something's not right here
9.8 requires a 231 in at 102 with a 106 split
Yet with 10.4 it says you need a 224 in at 103 with a 107 split?
Don't make sense needing less .050 with more comp

Keeping the minimum required flow at 250 CFM (as an input selection) regardless of the compression gets that result yes. Peak HP RPM drops by about 200 RPM with the higher CR but the DCR goes through the roof as you'd expect.
If I shift the target data point spec from max CFM available to a given HP RPM peak (5500 in this case) then the spread is less between the two CRs but the higher CR asks for more port flow than I can deliver. So what's being said here is the engine is certainly under-supplied and I have to limit my target peak HP RPM accordingly. At it looks like 5500-5600 is about the most I can expect. And that fits in the 9.8 CR as well.
Some screen shots of four different selection specs.

9.8 250 CFM.jpg

10.4 250 CFM.jpg

9.8 5500.jpg

10.4 5500.jpg

[gmrocket]

In the real world do you really think if you added more than half a point of compression, you would REDUCE the .050" numbers?

Effectively, put in a smaller cam? That's what I'm asking because you said the program suggested a much shorter cam

Sure don't sound right to me

[GARY C]

In the real world do you really think if you added more than half a point of compression, you would REDUCE the .050" numbers?

Effectively, put in a smaller cam? That's what I'm asking because you said the program suggested a much shorter cam

Sure don't sound right to me

Oddly enough back when NASCAR ran 9.5:1 Bush engines it was said that they ran a bigger cc head and a larger cam than their higher compression Cup engines and the claim was because they didn't have the compression to run the rpm that they needed to run?

I could never confirm the cam #'s but the heads listed showed the Bush head to be bigger than the Cup head in the few examples I could find.

[skinny z]

In the real world do you really think if you added more than half a point of compression, you would REDUCE the .050" numbers?

Effectively, put in a smaller cam? That's what I'm asking because you said the program suggested a much shorter cam

Sure don't sound right to me

I'd suppose that if the port flow was higher then the "real world" result would prevail.

[Orr89rocz]

In the real world do you really think if you added more than half a point of compression, you would REDUCE the .050" numbers?

Effectively, put in a smaller cam? That's what I'm asking because you said the program suggested a much shorter cam

Sure don't sound right to me

Doesnt make much sense to me either

[skinny z]

Is it an effective practice to over-cam an engine relative to the MCA? That's the question in the 1st post.
What happens with this higher CR and lower duration (call it under-cam) is that torque goes through the roof. There's only a 2 degree less difference on the intake but the DCR sits at 8.2 and estimated cranking compression is 211 PSI. 20 ft lbs more torque as an estimate over the 9.8:1 spec.
It's more like questioning why is 9.8 duration what it is and not the other way around.

[GARY C]

Another thought on duration, compression and rpm is that Pro Stock now uses 10 + less total degrees of duration but turns more rpm than what they were doing in the past.

[raynorshine]

Another thought on duration, compression and rpm is that Pro Stock now uses 10 + less total degrees of duration but turns more rpm than what they were doing in the past.

-my basic physics/ math sez it's hard to build compression when the valves are open, off the seats...

[GARY C]

Another thought on duration, compression and rpm is that Pro Stock now uses 10 + less total degrees of duration but turns more rpm than what they were doing in the past.

-my basic physics/ math sez it's hard to build compression when the valves are open, off the seats...

I just thought it was interesting that their reduction in duration has not effected their increasing rpm although most people equate duration with rpm.

[mt-engines]

Another thought on duration, compression and rpm is that Pro Stock now uses 10 + less total degrees of duration but turns more rpm than what they were doing in the past.

you want to hold onto as much torque as possible.. you want to run the least amount of cam to hit that target rpm, especially when nhra decides to rpm limit you.

with induction that exceeds what the engine is capable of consuming, valve events can change drastically and pick up.. look at modern ls engines. 200 degree cams making 450hp. 23 degree chevys need cams in the 227-230 duration to make that kind of steam