TheSkunkWorks

To bring you up to speed on my application...

My C3 is currently heavy and on a diet with a target weight of 3400# (1st step). The primary purpose of the redux is for street use, but it may eventually see occasional road course duty (T1). Car has vacuum brakes, FDR is 3.55:1, and rebuilt Muncie will be getting a new M22 wide ratio gearset. Engine is 496 BB (4.310" x 4.250") w/~10.2:1 CR (coated pistons), Induction will be 995-1095 King Demon on Edlebrock C454. Heads are Edelbrock RPM-R's. Exhaust is Hooker sidemounts. Target performance is ~600 TQ/550+ HP pulling to 6500-7000 on pump gas, but genuinely streetable manners are a must.

While waiting to see if anything new was heading down the pipe at CompCams after the Lunati deal (btw, I'm told Lunati will continue to operate as a separate entity) I've decided to step up to 280/286 XSR lobes. For the advertised durations, they have relatively big figures: I = 242 @ .050", 164 @ .200", .646" w/1.7:1 / E = 248 @ .050", 170 @ .200", .653" w/1.7:1. What I haven't quite landed on yet is whether to put the LSA at 112* or 114*. My priorities are sufficient vacuum, a flattened torque curve and prolonged top end pull, but I feel 114* may be unnecessarily wide. I'm not past doing another conversion to manual brakes, but would really rather not go thru that PITA again over a cam.


As a sidebar to this main subject, my intention to run BG's Road King Demon RS w/vac sec has raised some questions (Jim), and I'm open to discussion on that topic as well, as I haven't committed the money just yet. I'll be trying both 995 and 1095 cfm sleeve sets. Vac or mech, I really like the King Demon's features, so I'll most likely be going with BG, either way.

Before anyone tries to convince me otherwise, please keep any arguments about ~1000 cfm carb being too much to yourself, as I don't intend on debating that issue, given my past experiences with large capacity carbs.

TIA for thoughts on either item.

R96IMP

I had a 468 at 11:1 and used Lunati's special purpose roller cam.722/722 lift,252/262 duration at .050.
Idled at900 with an 850 Holley and a Victor 454 intake,ported OVAL PORT heads.Made 680hp at 7150rpm,drove it around OCCASIONALLY,in my 78 Camaro.Had 3500 converter and 4.10 gears,small diameter(8") power brake booster for 2 reasons,1 was the valve cover on the drivers side and 2 was it was deeper than the factory booster.

ML67

I’d choose the 112 LSA – I think 114 is a little wide for the valve timing you’ve listed. You might consider stepping up to the next lobe in the XR series if you’re looking to regularly buzz it to 7000. I’m running a 266/272 on a 112 LSA in my 548 and have approx 10” Hg at 1000 rpm idle. My power brakes work fine.

Even though I’m only running a Holley HP 1000 4150 style carb on my engine, I think you’ll be better off with a “4500 Dominator” King Demon. I like the idea of the removable venturii sleeves. Will you be dynoing this engine? I think with careful tuning you’ll be fine w/ vacuum secondaries. In fact, it may be preferable in a big block road racer as the hit from the secondaries opening won’t be as violent, improving traction and control coming off a corner.

TheSkunkWorks

ML67, is that 266/272 @ .050"? I've been leaning (75/25) towards the 112*, and such comments as yours have me thinking that's probably best. Good to hear your power brakes are fine with that big stick you've got.

The carb thing is one of those issues where I keep reading/getting advise from both camps, and I do appreciate hearing your view. Of particular note is the opinion that mech 2nds will yield better throttle response for more accurate control when applying power off corners vs. the "cushion" effect you've mentioned of vac 2nds which could help maintain traction. With the torque I'll have, this is a major issue for consideration.

BTW, I do plan on having this one on the dyno before installing it.

gkull

My original 427 solid roller was 242/248 110 and I thought that it was too mild for my 5 speed.

I really studied the figures in dyno simulations of tight lobe centers like 106 through 114. The only time that 112 -114 really helps is when you are trying to limit overlap with Large roller cams. Like the 255@.050 and up intake duration.

I ended up with 248/250 110 .685/.714 and it drives really nice even with 60 less ci than yours

ML67

Yes, that's at .050". And also from the Comp XTREME ENERGY family of lobes. With the high intensity of the XR lobes, vacuum is very good for the advertized duration. I think you'd be fine going up one lobe size. Between the 112 LSA and 496, your streetability should be fine.

427Hotrod

Now we're getting serious!

I think a 242/248 will be one awful smooth running cam in a 496 with a dual plane intake...especially on a 112 or a 114 LSA.

Those cams DO have some impressive stats on the .200 numbers and as you can see move pretty quick.

But I really think it's way too small for carrying power at to the 6500-7000 rpm range. The old Comp 244/244 street roller..admittedly a lot milder peaked at 5850 in my old oval port 427. It DID have a great range of power and was still hanging on very well at 7000..but that's a lot less motor than you have. It had only had 158*@.200. You could easily move to the 255/260* range I think on a 112 LSA.

No matter what you do...power out of the corners will not be an issue!

JIM

TheSkunkWorks

OK. I've finally had (made) time to model some more with regard to the advice you guys have given, which I indeed value, and here's what EA Pro indicates after oodles of additional combinations...

Compared to the 242*-248* @ .050", 112* LSA specs, (given my intake, heads, pipes and constant 10.2:1 CR) stepping up to 254*/.660" on the exhaust side with 113* LSA really appears to wake things up from 5500 to 7000 without sacrificing idle vacuum very much at all. However, when stepping up both the intake to 248*/.653" and exhaust to 254*/.660" together, while there are slight potential gains between 4500-5500 as compared to increasing the exhaust side alone, there appears to be no improvement at all from 6000-7000 (even when a wider 114* LSA is plugged in) and significant losses below 4000. Advancing the bigger I/E cam helped the bottom end, but hurt top end. Increasing the cam beyond this point would require additional CR, as the point of diminishing returns has apparently been hit with 10.2:1 here (again, given my combo).

Looks like the exhaust side of my heads could use some work, no? But, since it's easier to up the cam, I won't be spending much time there.

Just to see where I was, I ran a hand full of large HR's (including PowerMax's and XFI's), but nothing comes close above 6000 to these solid rollers.

Thanks, again, for the help. Without your comments, I probably wouldn't have spent the additional time seriously looking at any larger lobes, on either side...

gkull

dd 2000 really rewards large exhaust durations also. I do not believe this to be true in the real world. Way too many variations in exhaust back pressure.

From the Comp Cams lobe lists is how I decided to use the Endurance road racing type lobe profiles. It makes a street solid roller lobe look like a wimp.

two billet solid roller cams. The one I pulled out of my motor and the new one going in

TheSkunkWorks

I appreciate what you're saying, but compared to some of the other heads out there the flow numbers on my RPM-R's definitely have something to be desired on the exhaust side. Some of the factory BB performance cams have significantly longer exhaust duration, which may be due to their experience with a number of heads produced for them by Edelbrock as well. So, I probably shouldn't be all that surprised at these findings. Comp's nitrous cams have a bit more exhaust duration too, and being I'm likely to spray this thing with a 200 shot at some point down the road...

Yes, I know it's still only a bunch of calculations, but EA Pro is vastly more complex than speed shop variety software, and it has gone thru a number of additional calibrations with subsequent upgrades since its introduction. Having thrown a few existing combos with known results thru both it and DD 2000, I've grown to put a good bit more faith in EA Pro, and only use DD 2000 anymore just to get a rough idea on engine combos for which I don't have most every minute detail. Really the only other engine SIM in or perhaps above this league is Dynomation with advanced pro tools, but since I don't run a shop I can't justify the expense of investing in another costly package, especially given that EA ties in with other Perf Trends programs I run and/or plan on adding.

Of course, the torture chamber will tell the tale, and I'm prepared to change cam specs once results are charted if things are very far off target. Fortunately, being right here in Memphis I can typically get a custom grind in a matter of a day or so.

gkull

Most people don't think about the reason behind increased duration.

As rpm goes up the time in milliseconds to fill or exhaust a cylinder is very small over 6000 rpm. Big duration gives more time to fill the cylinder with a loss of cylinder pressure in the lower rpm.

I'm a firm believer in very fast lift, very high lift, and keeping the valve open at max CFM for the longest period of time. So I get away with lower .050 duration numbers.

I really believe in split duration cams. But i would keep it split to about 6-10 degrees.

The only reason I have 248/250 is because my next lobe choice on the exhaust side was way out of bounds in the racing lobes. Instead of .714 lift they jumped right into these .760's or something in the 254 - 258 durations

69 N.O.X. RATT

Funny I have not talked about this cam in 5 years and now I make two posts about it in one evening. I ran that first Comp cam you talked about 242 / 248 .646 .653 on a 110 LSA in a 6 speed vette. The engine was a 454+.030 RPM intake, Holley 850 and out of the box Edelbrock Ovals. I would highly recommend that cam for your combination. It has excellent throttle response and due to you running a 4 speed rather than a 5 or 6 I think its outstanding tq will help you pull those 3.55 gears better than a bigger cam. In my combination it made 450 rwhp at around 5800 rpms but hung on well to 7000 and this was with out of the box oval heads.

Later on I had the heads ported, put a Victor Jr. on it with a larger cam 255 262 on a 112 .651 lift. The smaller cam had neck snapping response from any rpm much above idle. The bigger combination never really came alive until 4000 rpms, it would pull like crazy after that and made 500 rwhp, but the smaller cam was a better street cam for sure.

If the car is primarily intended for street use I would go with the smaller cam, and I would put it on a 110 or even a 108 LSA (due to your gears) With a 4 speed and 3.55's in a relatively heavy car it is a long pull for each gear and you want a lot of torque to make it happen.

This cam has a very choppy idle, but operated my power brakes and vaccum headlights fine.

MotorHead

If itls the XR280R we are talking about I love the cam

TheSkunkWorks

That's the starting point, except I'm looking to flatten/extend the power band of my 496 combo, particularly from 5500-7000. Thus, I'm looking at the wider LSA, increased exhaust duration, less/no advance... Don't want a tyre shredder, but it does have to pull ~3400# with that 3.55:1 and Muncie wide ratio until I can swing a TKO 600 or better(?).

Since EA Pro isn't the real world and I have the time to do so anyway, I'm open to all the advice and comments anyone is willing to share here, so keep it coming. Hard headed as I am I might actually learn sump'n in spite of myself if you guys keep at it...

MotorHead

Flat enough for ya ?

..and this is a small block. The dyno operator said the HP was still climbing when he stopped the pull at 6500 RPM