With an otherwise stock new L31 longblock, would it be feasible to port the exhaust side to a point where a split duration cam would not be required due to the weaker flow on the exhaust side compared to the intake?
Clean up the bowl below the valve seats, or anything more?

Street use, possible Lunati custom single pattern 219 @ .050” hydraulic roller.
Never heard of a Vortec’s exhaust being improved to that point, JUST asking.
Thanks.

 

I don't think you can get them to flow to the point that they won't benefit from (as in, make more power with) a split-pattern cam.

That said, most off-the-shelf cams like the Comp XE, Lunati Voodoo, etc. use a series of lobes that are the same on both int & exh; and they use one on the int, and the next larger on the exh. By far NOT the best way to deal with those heads. The int & exh flow rates of change are similar past .250" lift or so (that is, if the flow increases by say 15% for some added lift on the one side, it'll increase about the same on the other), up to the point where they stall. Which for both sides isn't much past .500". IOW addl lift past that point accomplishes little or nothing except run up your valve spring and machine work bill, except to the extent that lifts at durations of less than peak lift are also increased.

You can jam any cam you want into it and it'll "work". Butt, the probably best all-around approach would be to do a mild bowl and port cleanup on the intake, and the usual guide trim and mild roof raising on the exh side in addition; then use a cam with near .500" lift on both, with about 6 - 8° of split toward the exh. Esp since you're already looking at custom cams anyway. For example, look at Bullet; check their HR273/340 exh and HR276/3175 int lobes, on 110 - 112° LSA. Use good springs, like the PAC1218 and Comp 787 retainers.

 

Sofa,
Thank you for the reply & also for your advice on the Bullet separate design intake/exhaust cam lobe combination. 218/222, .476”/.510”

As a last question, because I neglected to mention this earlier, I will not be changing the 3:08 posi to anything numerically higher. The loafing RPMs @ 80 mph with the T5 are getting to be enjoyable at this point.

Can you, or would you recommend discrete int & exh lobes with a few degrees less duration @ .050” to retain low end response.

Also, full disclosure, if it’s not excessive or goofy, I kind of would like to do the Vizard thing with 108’ LSA if there wasn’t too much overlap from duration @ .050”.

Thanks again

 

Yes you could use lobes with less duration. Note that the ones I specified are their "dwell" type (steep ramps, and many degrees held open near full lift) on the exh, and normal on the int; both are their "torque" design, and are asymmetrical (faster opening, gentler closing). Using the "dwell" on the exh reduces the need for really long duration a little bit. I just used the Bullet lobes as an example because I'm slightly acquainted with them (or was at one time), and they publish them on the Internet which makes them easy to pick out. Other cam grinders might have something comparable.

With too small of a cam and too tight LSA for the compression and operating RPMs, you can run into pinging and detonation problems. That's probably one of the reasons that factory cams tend to have very wide LSA. Yes it will make more power and all that but will be more picky about the fuel, air intake temp, ignition timing, etc. Without knowing EXACTLY what the compression and "quench" of your motor, NOT the "specs" but rather the ACTUAL MEASURED, it's hard to guess at what point you might start to encounter trouble. You can also advance the cam slightly (have it ground that way so you don't have to mess with adjustable timing sets), like maybe have the intake CL at 106 - 108° ATDC instead of the usual 110ish, which will generally lower the peak torque RPM a little bit. Keep in mind when installing a cam with "asymmetrical" lobes, the design "centerline" (peak opening) is NOT the same as the midpoint between the .050" points.

 

Thank you Sofa,
More serious food for thought. Seems like you were able to work alongside Harold Brookshire or Billy Godbold or something!.

Even though this is just a street, after-work L31, really appreciate your help making the most of a cam/spring/Performer/carb’d L03 6210 Holley.

 

As you may know, Vizard's approach goes something like this:
​​​​​​1) LSA. This value of 108 has been predetermined to be effective for a typical 10.5:1 350 SBC.
2) Overlap. This is were you have to determine the engine application. Measurements are at seat to seat in this case. 55-65 degrees (suggested for street/strip builds) is a considerable spread however, if low end speed torque is what you're after, then the smaller number in that range the better.
3) Duration. This will be the result of LSA and overlap. You can decide on split duration depending on the health of your exhaust system.
Note: I've not come across a Vizard Vortec build that uses split duration. That said, these are dyno engine results with headers and collectors.
This approach has resulted in a lot a arguments from both professional and amateur builders alike. Many go for duration as a determining factor and let overlap fall where it may. His approach has the duration as result. You may want to keep that in mind going forward because folks will bite your head off in their defense.
As for the Vortec exhaust port, the shop I had my aftermarket RHS Vortecs done at concentrated on cleaning up the bowl. In particular the abrupt transitions for bowl to seat.
If you have an interest to this approach, do a search for Vizard 128 Formula. Interesting insights there. If you can't find it, I have a copy buried in my PC at home.

​​

 

I don't use that approach; not because it's "wrong", but rather, it's too much of a one-size-fits-all.

Not everybody is looking for the same outcome.

The idea that there is exactly ONE LSA that's perfect for all applications is, to me, absurd in its extremity.

My own personal approach goes something more like this:

• Choose intake duration to put the engine's optimum RPM range where it agrees with the car's gearing, intended use, etc. ... for a street 350 in a daily-drivable car it will probably be between 212° and 232°, more or less, in most cases
• Choose exhaust duration to support the heads' characteristics according to the intake duration
• Choose lift on both that accords with head properties as well as mechanical ones, such as valve train acceleration, springs required, etc. ... a typical modern "shelf" lobe from a major mfr is usually reasonably well designed in this aspect. (math will explain why: has to do with higher-order derivatives of the curve) Peak lift is not so much of a factor flow-wise, since the valve spends so little of its time there; "area under the curve" is more important. High peak lift of course requires careful and robust valve train setup to support it and is therefore wise to use sparingly. If you imagine the lift curve as looking like a "hump", the steeper you can get the sides of the "hump", and the longer you can get the valve to stay open up at the top of the "hump" without necessarily making the "hump" taller overall, the better it is for flow; but that can be EXTREMELY hard on valve train parts in its own way, from violent opening and closing events. A compromise is usually necessary.
• Choose LSA to get the desired "spread" between peak torque RPM and peak HP RPM; the wider the LSA, the wider the RPM range, but the lower the 2 peaks will be... narrow (like 108°) is GREAT for a strip motor with lots of converter such that it only runs at one RPM all the time, but wider is better for a motor that has to run over a wide RPM range and has to have good quality idle.
• Fine-tune with intake centerline: advance to lower the RPM range and favor the torque peak, retard to raise the RPM range and favor the HP peak
• "Overlap" in and of itself is irrelevant. All it is, is the time at TDC while the exh is still open as the piston passes through TDC, and the int is just beginning to open. It has no effect on engine running whatsoever; it is ONLY a proxy, a stand-in convenience, for characterizing SOMETHING ELSE, namely reversion BACK into the intake at the instant that the int valve opens, which is what causes "lope", overall poor low-RPM performance, etc. It does not CAUSE reversion, in fact it REDUCES it; but it provides a picture of how much of that tendency there will be, according to how early the int begins to open.

Again, not saying Vizard is "wrong"; I'm arrogant and full of myself, but I'm not that much of an idiot. He's obviously WAY more experienced than I'll ever be. Just, overly dogmatic, and too focused on one specific set of operating circumstances.

 

Just to clarify, the formula is very specific in that it applies to 10.5:1, 350 CID SBCs with an intake discharge coefficient typical found in a 2.02" valve and seat that's performance orientated and not overly "trick" or "pedestrian". That's the 128 approach. Up the compression, put in a larger intake valve or change the CID and it no long applies. There are amendments to the formula as learned through his engine building seminars.
​​​​​And I'll be the first to admit that there's more than one proven way to go about it. His is just an example as brought up by the OP.

 

Skinny & Sofa,

Thank you for your very thoughtful replies.

Optimum camshaft application almost seems as much a black art as much as it is pure science, with a little subjective taste(taste meaning ONLY referring to the “drive-in sound” aspect of cam choice)

A lot to further decide here. If I’m not mistaken, the L31 long block is not 10.5 to 1, so Vizard’s 128 formula for 108 LSA might not perfectly apply?

Thank you.

 

Cam recommendations are as varied as the people offering to provide one it seems. And for the cam grinder spec'ing a cam for you, the results also vary with the application. Case in point: I contacted Mike Jones (Jones Cams) for a cam profile. He offered "X". That result was based on the form filled out and his interpretation of the application. It was well outside the Vizard 128 spec with the typical 110 LSA. I asked him to revise his choice and this time let's call the application a dyno experiment where peak torque (and consequently HP) is the objective. The spec came back to me with values much closer to what the 128 suggests. Keep in mind, the 128 rule is based on his empirical data from tens of thousands of dyno runs which at the end produced his 108 for a 350, 106 for a 383, etc.
Widening the LSA results from increasing the CR, improving the CD of the intake valve or decreasing the CID. Conversely for narrowing the LSA. The difference in LSA with respect to CR is marginal when going from 10.5 to 9.5:1.

 

Skinny,
If I am not overtly concerned with idle vacuum and gas mileage, and have a reasonably free-flowing exhaust, a 108 LSA would be feasible for that original cam spec stated at the top?

 

Do you the advertised duration for that lobe?
I would use that and your 108 LSA to calculate the overlap. Idle vacuum, mileage and just about everything else is tied to that value.

 

These are all too true... which no doubt is why people would LOVE IT if one could wave one's magic wand and confidently declare "this" is The One For You. It's kinda the holy grail.

Having been around cam designers and grinders for many years especially from the late 70s up to the turn of the century, I've seen ALOT of cam fads come and go. Many of them are reflections of the marketplace of the moment - the junkyard motors people are working on, the new cars coming out they want to run with, the memories they *think* they have of Times Gone By, the supporting parts around them like valve springs and EFI tuning, and so on. Unfortunately that idea that cam design can somehow be reduced to sausage grinding, where you just put the ingredients in one end, crank the handle, and heavenly wonderfulness squirts out the other, seems to consistently elude even the best of them. A new fad always emerges to take its place.

I really still feel like, if someone doesn't know enough to spec their own cam themselves, they are TOO EASILY led astray by "Cam of the Month" syndrome. A logical, measured rational approach is needed; one that takes into account what the form-filler-outer might be "inferred" to have "meant". People GROSSLY overestimate their tolerance for "big cam" issues (poor gas mileage, no leave, gutless stoplight-to-stoplight, "slap a turbo on it someday", "intend" to put something more appropriate than a stock converter in it, etc.) in favor of big HP #s on the dyno. The cam that "makes the most power" on the dyno is NOT always the one that satisfies the customer. The single biggest mistake people make is over-camming a weeeeeeek short block: the old adage about "there's no such thing as too much cam, there's only not enough motor" is just as true for the LS3 crowd in 2020 as it was for the L48 crowd in 1978.

AFAIK the L31 short block "specs" at something like 10.3:1 CR, but that's more of a MAXIMUM than anything else. Given typical factory block machine work, I'd be willing to bet that if you ACTUALLY P&Ged all 8 cylinders of 20 of em, you'd come out with #s anywhere from 9.8 all the way up to almost 10.3, with some of the widest variations within the same engine. Hardly the stuff a well-matched cam recommendation is made of. Now if you're building the motor yourself, and have control of the machining such that YOU KNOW what the CR is and the CID and all the other things that vary RADICALLY on a "stock" motor, it's a different story... kinda like Edelbrock building a 350 with their Performer RPM package and getting 435 HP, which I absolutely DO NOT dispute the truth of, and Billy Bob next door getting his uncle Cletus to build a 350 and putting that "kit" on it and only getting 275 HP on its best day, and they're pissed because Edelbrock lied. Well lah-de-dah...

And THOSE are some of the problems with a one-size-fits-all cam recommendation. Again, not that Vizard is "wrong"; just, take what he says with a gravel-sized piece of rock salt on this one.

 

Skinny, Sofa’s recommended lobes (at top of posts)with adv dur of 276 int & 273 exhaust overlap works out to be 58.5’ when using a 108’ LSA....if my math is correct.

I’m not trying to squeeze blood out of a turnip or do a moonshot here, just a slightly more serious improvement on the factory cam & springs.

You gentlemen man are some smart cookies here, & I appreciate it.

 

I suppose this is where you and Vizard would disagree as he places great emphasis on overlap.
I'm not about to enter into the debate other than to say I know enough to say that there's more than one approach. There are plenty of championship winning camshaft people out there and they don't all follow the same path to success.
That said, attached is Vizard's chart for overlap and it's application. I'll say it again, he goes for the throat when it comes to cam specs and torque. That, as it appears to me, is the objective with any of his projects that I've followed.
As for the OP, he can take whatever approach that promises the kind of results he's after.
Personally, after having built and raced a Vortec 350 (10:1) with a 276/224, 282/230, 110 LSA, 106 ICL, it would have made for interesting experiment to tighten up the LSA while keeping the overlap equal. With 59 degrees of overlap and tightening up the angle means the duration values drop to 272/278 and the intake valve closing point is now 60 ABDC as opposed to 64. This will have an effect on compression pressure and reinforces the need to know exactly what the static compression ratio is. Something Mortorq will have to pay attention to as mentioned.

 

I think that's a typo. 283 perhaps (although rereading the post that is what's written: 273.)

And before getting too much further, do you have a way of determining exactly what the static compression ratio is? All this theory and conjecture doesn't mean much without that piece of data.

 

Skinny,
I took Sofa’s numbers at the top of this thread as gospel when I went to Bullet to look up the lobe number specs.
The exhaust side lobe numbers looked a little low, but what do I know?

Have you seen that Vizard PHR article from, I think, it’s November 2007, where he took of Vortec 10:1 short block and EQ Vortec lightning heads and with his custom 224 @ .050” single pattern, 108 LSA hyd roller cam & made 447 supposedly streetable hp?

 

Perhaps Sofakingdom made a typo. In his post, he did say 273 but he comments on an exhaust spread of 6-8 degrees. So I'm guessing an input error.
As for the article in question, yes I'm very familiar with it. It's somewhat along the lines of my 2nd Vortec build with intake duration at .050" and CR the same.
I'll have to dig out some of my tech papers by him and post them. I'm guessing you'll find them interesting. The one on how he arrives at 108 in particular.
Back to your cam for a second, I'd look for clarification on the Bullet spec. I'm suggesting you pay attention to where your dynamic compression ratio might end up with a tight LSA and relatively short duration cam. Your static CR is important to know (and not guessing).
EDIT: I crunched the magazine article engine cam specs using 276 adv with 224@.050". 108 LCA, 104 ICL. That gets an intake valve closing of 62 degrees. Certainly workable with 10:1. This is assuming that the cam has 4.degrees of advance as installed.

 

And this advice isn't to be taken lightly. I went down this rabbit hole and it became a messy engine to work with. I don't like the notion of restraining the timing curve to compensate for too high an operating compression ratio. It's a trade off that reduces power and economy.

 

“Without knowing EXACTLY what the compression and "quench" of your motor, NOT the "specs" but rather the ACTUAL MEASURED, it's hard to guess at what point you might start to encounter trouble.”


“And this advice isn't to be taken lightly. I went down this rabbit hole and it became a messy engine to work with. I don't like the notion of restraining the timing curve to compensate for too high an operating compression ratio. It's a trade off that reduces power and economy.”

Skinny
I wouldn’t want a pingy engine either, for sure.
The L31 would be a box stock new crate engine from SD or so.
Not sure how one could find the exact CR measurements etc before the cam purchase & install.
Maybe I should just go with this on a 108 LSA instead of the supplied 112 LSA & be done with it.

262/270, 211/219 @ .050”, .507”/.515”
Performer, 6210 Holley, 3:08 & T5, Hooker 2055’s, 3” Hooker Aerochamber exhaust.

https://www.lunatipower.com/voodoo-r...k-262-270.html

 

That cam (262/270) on a 108+4 (104 ICL) would be almost unworkable at 10:1. The dynamic compression is 8.6:1 which for all intents and purposes is approaching the practical limit of pump gas. My opinion of course but that's based on having gone too far for my application.
I've got a Lunati/Vizard cam catalogue at home. I'll post up the chart and you can see where he's going with it. FWIW, I wouldn't hesitate to pick a cam from there. And I may do just that with the next engine.

 

“That cam (262/270) on a 108+4 (104 ICL) would be almost unworkable at 10:1. The dynamic compression is 8.6:1 which for all intents and purposes is approaching the practical limit of pump gas. My opinion of course but that's based on having gone too far for my application.”

You see, THAT’S why I love this site. You guys are practically scientists, not to mention the down to earth truly helpful responses.

Could I cheat with a slightly larger Voodoo cam, to “bleed off” some compression so the timing wouldn’t have to be backed off to prevent detonation, but still keep the 108’ LSA?

https://www.lunatipower.com/voodoo-r...k-270-278.html


I understand of course you’re supposed to buy the right cam for the combination, but that can be a matter of opinion too I think.

If I want the snappiness of the extra torque of 108 LSA, & gas mileage, idle quality, & high rpm’s are not a concern, as the saying goes, “Is it wrong?”

 

I think you'll like the layout of the Vizard/Lunati cam charts. Compression ratio range is specified as is application (to a degree). And of course they all follow his formula. So if that's what you're after, that's what you can get.
I'll post up later today.

 

Disclaimer: The values posted with respect to the cams in question were derived from basic online calculators. This is to say, by example, what may be an intake valve closing of 54 degrees, may actually a degree or two one way or another. This is significant in that when it comes to an actual choice, these values have been positively determined.

As promised here are Lunati's Power-Profile Endurance Grinds and their Super-Profile Hi-Po Street & Strip Grinds. All hydraulic rollers. Note the LSA is indicated by the suffix at the end of the part number. Eg: DV282-08HRH where the 08 indicates a 108 lobe separation angle.

I think you owe it to yourself to contact Scoggin-Dickey and inquire about the CR.

 

SkinnyZ,

Thank you for posting up the Vizard/Lunati cams.

Looks like the 9.3CR of a new factory L31 easily meets DV’s required listed MINIMUM for using a VD262-08HRH (211’ @ .050” & 46’ overlap), or the DV 270-08HRH(219’ @ .050” & 54’ overlap).

No mention of not using shorter cams, or excessive static compression issues, seems like there is a caution to use ENOUGH compression to operate these single pattern cams, or am I mistaken?

What cam would you use, the DV270-08HRH?
Performer, #6210 Holley, T5, 3:08 posi, 2055’s, Aero Chamber 3” exhaust

 

If it's simply a matter of choosing between two cams, the 262 or the 270, conventional wisdom says select the smaller of the two. That doesn't mean that conventional wisdom is right in this instance but I can make a case for it based on one key thing you mentioned. That would be cruising with the 3.08 gear and T5. The 262 will build a little more compression pressure and that should translate into greater torque below peak RPM. It could make that cruising experience somewhat more satisfying.
Between the two, the 270 would net a dynamic compression of 7.75:1. The 262 result is 7.96. Not a lot but still better. It may be that you could run on regular grade fuel but personally, I spend the extra few dollars per tank for that safety margin. As it is, you'll have to pay close attention to detonation especially when lugging it at lower engine RPMs.
Now, I wouldn't necessarily choose either cam as I wouldn't be particularly happy with the weak CR. But that's my approach as I like to build towards performance as I'm still a racer at heart.
All of that said, if you're focused on giving the 108 LSA a go and a single pattern, I'd say the 262 would be a good fit for a stock L31 longblock.
Don't forget to address the valve spring issue. That 262 IIRC has a net lift of .507" with a 1.5 rocker. Well beyond the capacity of the stock Vortec.

 

Within reason, I would focus more on quench than on the actual compression number. The stock Vortec quench is terrible, and that's even more insulting because of the chamber's having a really good table top for using quench to your advantage. I went for hitting exactly 0.040" quench with my L31 build and the engine loves it. I have no issues with detonation at 9.8

I gave the heads larger exhaust valves, and used a split duration cam for exhaust flow "crutches". I'm very near to 400 crank HP on an iron head 350....... I'm not sure how much more there is to be had or if it would even be worth it to try without going to an Aluminium head like an AFR 195.

What's the theoretical max on as-cast iron Vortec's? I feel like a 108 LSA would be unreasonably rough for little gain over the 113 cam I'm running. I can tell you my cam is plenty rough for me using my car as a daily driver. It's mild enough to idle at 800.

GD

 

Hi GD. While it hasn't been part of the discussion, taking the engine apart for machine work I don't believe is part of the build. A cam upgrade and otherwise stock although I had touched upon a thinner head gasket to up the compression and consequently tighten the quench. Stock piston to deck is supposed to be .025". It certainly wasn't on my 70's vintage 350 but the newer blocks are supposed to have tighter tolerances so the quench could be reasonable.
As for cam on a 108, with only 262 adv duration, overlap is 46 degrees. This thing should idle like a watch. 276/288 on a 113 is 56 degrees and you've demonstrated that a smooth idle is possible. I've managed a reasonable idle with 70 degrees (288/292/110). Drove it everyday too and still knocked out 20 U.S. MPG on the highway.
Not entirely sure what the OPs objective is but I don't believe maximum output is part of it. More of a cruiser with a 3.08 gear and T5.

 

skinny z & General Disorder,
Thanks so much for the replies & the dynamic CR calculations.

“Now, I wouldn't necessarily choose either cam as I wouldn't be particularly happy with the weak CR.”

skinny, what of those other listed cams do you like, if the L31’s 9.3 CR is weak, should the duration @ .050” be even smaller than the 262?

 

The 9.3 CR isn't so much of an indicator as the resulting dynamic compression is. Some don't put a lot of stock in the DCR but I've always understood the cam and compression ratio are tied together in a significant way. That being said, the 262 and 9.3 CR with the intake valve closing at 57 degrees gets you to about 8:1 DCR. That, I've found, is a workable number with the iron heads. Any shorter on the cam and your performance goes away and the risk of too much compression pressure goes up.

General Disorder makes an interesting point about the quench. He's targeted .040" and that's where my current engine (RIP) was built to. It allows for more compression than you might otherwise be able to run. The unknown in your new L31 is what the quench actually is. Something else GD alluded to was the change to a .015" shim gasket. If the GM gasket is .035" with the piston .025" down and the resulting CR is 9 and quench is a little wide at .060" (but workable in a million other SBCs). A 015" gasket pushes that CR up to 9.7:1 and the quench to that sweet spot of .040. With that CR another range of cam choices comes into play. Keep in mind though as the duration goes up, so typically does the operating range. You don't want to build something that won't play nice with your highway gear and overdrive. Something to think about.

In typical hot rodder fashion, you could always build to what you have now and then make improvements down the road.

And another disclaimer here. Where not talking about significant differences. A full point in compression it's said might be worth 4% of the peak. But that 4% might hurt driveabilty more the performance gain helps. If that makes sense.

All in all, I'd say the Lunati/Vizard 262 would be a decent fit. There are other similar cams to that as well. GD sent me some intel a little earlier about a couple from Isky. One of them on a 108 LSA. Both single pattern.

 

Since a new swapped-in L31’s with a slight cam & spring change, shorty headers, 3” exh, 6210 mech sec sprd bore Holley on a standard Performer might almost double the HP of the present LO3, I’d be satisfied for now.
Thank you for your help.