'69 L71 Cam Recommendation

**Mike T.** · July 14, 2020, 11:20 AM

Re: '69 L71 Cam Recommendation

Chris - The factory compression ratio listings were not exact, maybe even 'optimistic'. Now that you have your engine torn down, it'll be easy to determine what the true compression ratio is. Also, since you'll most likely use a composition gasket up around .040 in thickness, that alone will help drop the compression ratio. On any of compression ratio formulas you come across, they'll be asking for that measurement. So, you may not have to worry as much about finding a smaller cam.

**Duke W.** · July 14, 2020, 05:40 PM

Re: '69 L71 Cam Recommendation

I realized after we talked on phone that the reason I didn't see your thread on the CF is that I ONLY look at the C2 discussion and rarely look at the C3 section.

I can't believe what Comp Cams told you and that they would recommend a cam that only pulls 6-9" at idle... actually I do. IMO that company has very poor products and even worse advice. I would not buy anything from them unless they were the only source, and I would certainly never ask them for any advice on designing a cam.

If you go with a roller cam and want to maintain the same general operating characteristics as the OE cam you need to consider the following.

First, the .050" lifter rise of the OE cam is 242 degrees, however, since it's mechanical lifter cam part of that, namely the first .012" of lifter rise is clearance ramp. So to compare a hydraulic cam you have to compute the .062" duration which is 231 degrees that I computed right off the engineering drawing. So, including the following caveat a comparable hydraulic roller cam, would be somewhat less than 230* .050" lifter rise duration of a hydraulic flat tappet

Roller cams have more aggressive dynamics than flat tappet types so for a given duration flat tappet type, a roller can be shorter duration for the same area under the curve. Also for a given LSA and duration the roller will have less effective overlap. If a roller spec gives you the SAE duration which is at .006" VALVE lift it will be significantly shorter than the SAE duration for a flat tappet hydraulic of the same .050" duration.

Rollers also have higher lift than flat tappet types. So you have to consider springs/damper/seal geometry and may have to shorten the valve guide to achieve the specified lift without damage. Also, since hydraulic rollers generally require greater valve spring seat force and rate, the rockers/balls are seeing much more load. There's a reason why modern roller cam Corvette engines have roller bearing trunnion rockers. They also have plain tips that are curved so they ROLL rather than slide over the valve stem, just like vintage stamped rockers. This is why "roller tip" rocker arms on a vintage engine are basically worthless other than increasing valve train inertia (which lowers limiting speed) and lightening your wallet.

As far as compression is concerned I recommend no more than about 10.25 with your 93 PON fuel, and you should be able to run an aggressive spark advance map, which will pump up low end torque. Since you have the CR article and calculator link, my recommendation is to have the piston domes cut down, and assuming the block and head surfaces are not warped (not likely) stay with a thin gasket to minimize quench clearance. BTW I finally did find the dome spec in my 1995 F-M performance parts catalog - 36.3 cc for standard bore pistons, and I recall you measured 36.

A common misconception is that increasing overlap reduces dynamic CR. This is absolutely false. Overlap has NOTHING to do with DCR, which is a function of SCR and the point the inlet valve CLOSES. For a given SCR the later the valve closes, the lower the DCR. The problem is that there is no "industry standard" valve closing point. How about the SAE .006" valve closing point! I recall seeing one that said add 15 degrees to the .050" closing point, which is totally arbitrary. Also, the calculators my not even tell you how they determine the inlet closing point. Try three different ones and you will likely get three different answers. DCR in NO WAY is a go/no go criterion... just a subjective guideline.

Overlap and conventional exhaust manifolds and mufflers don't like each other, so keep overlap to a minimum, certainly no more effective overlap that the OE cam. As I skimmed through the CF thread I saw header discussion, but highly recommend Against adding headers.

Read the Tale of Two Camshafts article, again. Carefully study how modern cams differ from vintage cams. Given the much better head flow of modern LS/LT engines they don't need a lot of duration, but the inlet event is phased much later. This is how the McCagh special inlet lobe ended up being nearly the same duration and indexing as the LS2/3 lobe, but I didn't realize that until after I completed the design. Of course, the engineers didn't have simulation programs on PCs back then where they could run dozens of simulations in a relatively short period of time until they maximized the torque bandwidth from off idle to design speed.

So based on my experience and intuition I'd say pick a duration of about 220 degrees on the inlet side. Vintage cams inlet IPOMLs are usually in the range of 108 to 110 degrees with the exception of the L-46/82 cam, which is 114, and that's what I'd go with. (The McCagh Special in a 3.75" stroke SB retarded four degrees to an IPOML of 120* ATC is a real torque monster with useable power to 5500-6000 depending on other configuration components... more like a 427/390 than any OE small block) Exhaust duration is a function of inlet duration and E/I head flow. IF the ratio is about 0.75 equal duration is about right. If less add a few degrees exhaust duration and put it on the front end to open the valve earlier. If more you can use less duration by opening the exhaust valve a little later.

That's pretty much how I designed the McCagh Special cam. I didn't have much time. Mike's massaged head flow data yielded about 0.8 E/I flow ratio, so purely on intuition I said how about just swapping the '67-up 300 HP lobes and keeping the same 112* LSA because the engine had to have the smooth 450 RPM idle in Drive to pass a PV. Then I just played around with the phasing until I got maximum average torque/power from 1500 to 5500 RPM and that ended up being 116 degrees ATC, versus 108 for the 300 HP cam and given that duration is 8 degrees longer, the inlet closing points was retarded about 12 degrees to pretty close to the L-79 cam making DCR very modest for the 10:1 SCR.

So back to the 220/xxx 114 LSA hydraulic roller cam, I'd start with the IPOML at 114 ATC. If you like to play games I advise you buy the Engine Analyser simulation program. Back when I bought 3.0 it was about a hundred bucks. I don't know what's current now. 3.0 eventually became 3.2 and then 3.5 came out with more functions, but more expensive. I don't use 3.0 much anymore, but for what it cost it met my needs. Mike's engine made 364 SAE corrected lb-ft of torque on a lab dyno versus 370 predicted, and SAE gross measured max power was ONE LESS than the predicted 316. As usual the dyno test started at 3000 instead of the 1500 I wanted, but I have no doubt it made the 90 percent peak at 2000 bogey because it was so flat from 3000-4000.

Take a look at what GMPP has to offer, then look at Crane's lobe library and see if they have some suitable lobes. And don't go with something that has super aggressive dynamics, considering that your valve train is not as stiff a modern LT engine stay with something no higher than mid range dynamics for a hydraulic roller lobe.

Then it's just a matter of playing with the overlap and phasing in a simulation program, looking to maximize torque/power from 1500 to 6000 or more.

One thing about roller cams is that they add mass to the valve train, so some aren't designed to rev to 6000+, but with massaged heads and some proper system engineering it should be noticeably stronger than OE from off idle to 6000.

Duke

**Thomas N.** · July 14, 2020, 06:49 PM

Re: '69 L71 Cam Recommendation

Chris:
Have you considered using another L71 cam and the solid lifters with the .012 EDM'd hole to improve oiling at the lifter cam lobe interface?

'69 L71 Cam Recommendation

'69 L71 Cam Recommendation

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