Assuming a solid roller with equal net lift and advertised "duration @ 0.02", but with different ramps and acceleration so that the duration is different @ .05, .100 and .200", which number has the biggest effect on idle. vacuum , and low RPM characteristics? I'm assuming it's .05", but do the other figures make a noticeable difference?

Thanks,

John

Originally posted by TD509EFI:
Assuming a solid roller with equal net lift and advertised "duration @ 0.02", but with different ramps and acceleration so that the duration is different @ .05, .100 and .200", which number has the biggest effect on idle. vacuum , and low RPM characteristics? I'm assuming it's .05", but do the other figures make a noticeable difference?

Thanks,

John Not really, biggest difference will be low lift & separation, separation being a biggie with the same low numbers.
Don't forget to check lash numbers vs. where the advertised numbers are rated,,, this can also make a bit of difference in overlap ;)

Thanks Mike, I guess I started "re-thinking" this when I saw a lot of others on the board using longer duration cams in their 496-509 combos and using them on the street. I noticed a 540 buildup in Feb. Car Craft ( FWIW) running 270-280@ 0.05 with 9.5 comp. I know it has an extra 31 cubes, but 20-30 degress more duration? That's a lot!

For my street use 509 with AFR 315cc heads, Holley wideband O2 EFI, I went with a custom Comp billet with RT series lobes; specs with 1.7 rockers: 278/281 dur @ .02, 248/250 @.05. Gross lift of 703/707, ground on 112, advanced 3. Lash is 0.20/0.26. I've upped the rocker ratio to 1.75's. Car is 79 Camaro with beefed T56, using 3:73 limited slip. AE Pro shows idle vac around 13", peak power at 6500.

I don't care if the ëngine idles as smmoth as glass as long as A) passes 1979 emissions ( no visual) B) beats diesel powered Rabbits at the stop lights smile.gif

Should I have went for a longer duration on this street vehicle? I felt I had researched this quite well before I started the build. I don't like to second guess myself; as in the case of exams, if you second guess your answer, you'll usually be wrong. Hope that's the case here!:0

John

I might have advanced it more than 3 degrees. How low do you expect (RPM) to go down to and "make power" and run well. What cruise RPM's. What shift RPM? You should make great power to 6500 with that cam, if not more.

From my experience it is better to go smaller than bigger.

For my 491 I went 254/264 (think that is off a degree or two, anyhow), but it will never see the low side of 4000. With good heads like you have, you should make great power with a milder cam. Poor heads need more duration to make higher RPM power.

With the same .050's, the rest of the duration at higher lift won't matter with idle. LSA has the biggest affect, given the same .050 number.

John,
I really think it would be worth a call to Harold or the cam designer of your choice as I am not a cam designer but I personally have a real problem with the acceleration rates that the RT lobes look like they have for endurance use which includes long term street use, especially for the exhaust lobe.
In Comp I like the Hi Tech & Hi Tech .420 for intake & exhaust & also NC for exhaust lobes for street stuff as they seem to be pretty easy on parts.
Again, I am not a cam designer & have never used the RT series & could be all wet but I would not use them without consulting someone like Harold, Ron Iskenderian, Chris Padgitt at Comp.,, Dema Elgin,, someone who really understands the effects of acceleration & velocity.

Thanks for the input everyone, great info. Guess I don't have to worry about being too small, perhaps the other way around.

Doug, at 60, cruise RPM's would be 3000 in fourth, 2250 in fifth, 1500 in 6th, with a tire just over 26" tall. I was hoping the low duration @ .02 would help low speed performance, especially with the EFI.

Mike, I hear ya. My original intent was to maximize the area under the curve, minimize the duration and make full use of the flow of the AFR's at high lift. Sorta have my cake and eat it too. Thats why I went with the Jesel SS rockers and Manton 5 series .120"wall 7/16" pushrods, in the hopes of minimizing deflection / harmonics, and with a presumed increase in valvetrain life. Though the car is for the street, it will be used mainly for cruises and such, perhaps a power tour or two. A trip down the strip isn't out of the question.

I'll give the people you mentioned a call. Thanks once again for your input! smile.gif

John

John, which lifters are you running? Comps?

Bob,

I'm running the Crower SD rollers with the HIPPO option. Will have to wait and see how long they last. Will do an inspection of them after every Summer, along with checking the lash from time to time, and listening for undue ticking. Next set will be Red Zones, fully rebuildable. Incidentally, when I received the Crowers, there was no oil on the rollers, all ran very rough. After a thorough cleaning and re-lube they were fine.

John

Does the 79 emissions include cats. I don't mean you have to run them, but if the level includes a cat equipped engine, that could be tight. What are the emissions levels you have to meet? Just CO and HC or NOx as well?

Hi Doug,

The maximum allowable, from the previous year:

DRIVING-25MPH (TYP) IDLE (TYP)

HC 350.0 (51.0) 400.0 (80)
CO 2.5 (0.05) 4.5 (0.17)
NOx 2500.0 (554.0)

(TYP) Considered a good reading for this vehicle taking into acount mileage.

As you can see, they're not very strict, though I do believe they have tightened it up a bit this year. I still think I can make the limits quite easily with O2 feedback and two high flow three way cats.

Seem reasonable?

John

Didn't know you were running cats. That should help a lot, although I haven't done emissions testing with a cam that big. Might ahve to run 15:1 A/F or so to squeek by possibly on CO. Just have to see how it goes. With the EFI and wideband you have the best chance you can ever get.

Thanks Doug, I'll just have to see how it goes. The cam has already been set up, so unless I find that I'm living dangerously with this cam, I'll leave it in and run it.
BTW, I've ordered the wideband controller, looking forward to setting it up. Thanks once again!

John