Cam intensity, overlap, ICA issues in selection

[Jim V.]

Below is a table of possible cams for my in-process 1965 L75 (300hp) rebuild. The table is sorted by ascending overlap. Given an OE street motor (max TQ) with static CR of 9.8:1 and 461 stock iron heads, I am concerned with detonation issues. Originally, I was going with the an OE 929 grind but see that given the early intake closing my dynamic or effective CR would be very high ...8.5:1.

So...given a 9.8:1 static how best to select the cam?

Please vote and vote often....I do live in Chicago...

Notice, the large disparity between cam intensity (diff between advertised duration and .050 tappet duration). Seems the high intensity cams yield higher under the curve durations and lowered overlaps (ie improved low-end driveability) at the cost of valve train noise and durability. Is this trade-off worth it? My last L76 had a very noisy, very annoying valvetrain tickety-tack with a cam intensity of 50 (Comp 12-212-2). Interestingly, the OE929 cam has a aggressive 56 intensity while the current Speed-pros have a a gently 74.

[Paul B.]

Re: Cam intensity, overlap, ICA issues in selectio

Just curious; since drivability seems to be a big concern, and it's a 300HP hydraulic, any reason why not a roller cam? Would it not help you achieve your drivability performance goals?
Paul

[Duke W.]

Re: Cam intensity, overlap, ICA issues in selectio

DCR is not an absoluite go/no-go criterion. It's a GUIDELINE. Guys put WAAAAAAY to much stock into it, often using incorrect valve timing data, and there is no consistent "industry standard" for computing DCR. The Keith Black calculator says to add 15 deg. to the .050" closing point, which will give a higher number than using the .004" lifter rise point, and unless you have the detailed lobe data for each cam, you can't verify this point, especially on OE cams that have asymmertrical lobes.

For the 929 cam I recommend a maximum of 9.75:1 SCR, which is about what 300 HP engines were at as built, and owners of OE built 327/300s don't report detonation with premium unleaded gasoline. This is partly a result of the "lazy" centrifugal advance curve on 327/300s. GM's "advertised CR" was about as realistic as the advertised gross HP ratings.

The actual .004" lobe rise closing point of the 300 HP engine is 65 ABDC or 69 with 4 deg. retard. This yields a DCR of 7.76:1 or 7.49:1 with 4 deg. retard assuming the bore is 4.030".

As with any engine restoration where you are looking for maximum power without altering the engine's operating characteristics including idle quality, massaging the heads is the first order of business. If you want more top end power and usable revs to 5500+ on a 327/300, retard the 929 cam four degrees, but I don't recommend this with Powerglide.

ANY increase in overlap will degrade idle quality and low end torque.

So-called "intensity" - the difference in crankshaft degrees between the .004" and .050" lifter rise points is also just a guideline, and on OE cams it's higher on the closing side due to lobe asymmetry. Actual "intensity" on the opening side of the 929 inlet lobe is 26, and 38 on the closing side. The exhaust lobe is essentially the same, but the .004" lifter rise durations are 258/270, not 250/258 as listed in the chart of unknown origin that you posted. Most calculations just assume the lobe is asymmetrical and add the two. To make an apples to apples comparison with the data you presented the "intensity" on the 929 inlet lobe is 52 on the opening side and 76 on the closing side. There is no way to tell if the other cams listed have symmetrical or asymmetrical lobes. Thus any attempt to "analyze the lobe" without the detailed lobe dimension data from the engineering drawings is fruitless and leads to incorrect conclusions. The flaky data for the 929 cam probably assumes the closing side is a mirror image of the opening side. IT IS NOT!

And "centerlines" are not geometric center of the lobe on asymmetical lobes. In the case of the OE cams, the "centerline" is the "point of maximum lift" (POML).

You can assemble your engine based on solid engine system engineering and reliable empirical data or you can take "opinion polls." Your choice!

Duke

[Jim R.]

Re: Cam intensity, overlap, ICA issues in selectio

Jim go with the gm 151 l79 cam you wont be sorry, this cam works great with flat top pistons and your stock 461 heads, you should be at 10-10 1/4 to one comp, i know this from one i built 327 ,with 461 heads otherwise stock ,and as far as overlap goes look for a cam that is ground on 112 or 114 lobe center, as the 151 l79 cam is ground on 114 ,when you get to 110 108 106 driving quality gets jerky and is not much fun to drive at slower speeds, oh ya one more thing make shure you buy that cam directly from gm parts counter there about 140.

[Mark #28455]

Agree - stock works well

The stock 300 HP engines with flat top pistons ran fine with the stock cams. We used to buy Impala 300 HP 350's in the junkyards all the time. They ran GREAT on pump premium even with a tweaked advance curve and had decent torque - enough to move an Impala or station wagon just fine. Also agree with Duke that reasonable street pocket porting is a benefit. Before looking to all the racing cams, remember if you're using a stock exhaust the racing cams sound nice but frequently are SLOWER!

I have played around a bit with various BB combinations and I have tried all the "usual" racing tricks. You can frequently get a better torque curve up to 5000 RPM with a SHORTER duration cam (like the factory stock cams used in the 300 HP 350 - or in my case the 390 HP 427) and decently pocket ported heads. If you're going to make a mistake on the cam, going with a little cam will always yield a better street motor than a too large cam. If you run the dyno tests, using the SAME long block engine and just swapping the cam, for a street RPM range and compression ratio, the small cam will outshine the big cam in the low to midrange, and for all the obnoxious idle, etc, the big cam will be lucky to get a few more top end HP - do you honestly run 5000+ RPM frequently?

It's up to you, but I blew a LOT of $$$ trusting the speed shop "experts" until I finally wised up and started matching my cylinder head flow and cams to my "REAL" intended usage of the engine - ie: 1200 to 3500 RPM 95% of the time with occasional blasts to 6000+.

Mark

[Jim V.]

Re: Cam intensity, overlap, ICA issues in selectio

Hey Duke... Table data on the 839 is per your supplied lobe data at .006 tappet vs your recently posted .004 values. This is indicated in the column titles.

Given your detailed knowledge, are "gross" intensity numbers significant between 56 and let say the 74 of the current Speed-pro cams (options 4-7)? I had a comp cams 12-212-2 (gross intensity=50) in my last L76 which was a real PIA with tickety-tack valve train noise, not to mention questionable long-term durability issues. Frankly, I am surprised by the rather intense lobes of the GM 839...compared to speed-pros current offerings with gross intensities of up around 74. I thought harsh quick ramps were the stock-in-trade of the after-market hotrod stuff.

Given their reasonable overlaps, what is the downside of options 2,3 or maybe 4? if they can safely lower DCR a touch? Note too sure about option 4 however, given its strange relationship of ICA at .050 vs .006 tappet.

Yep...I know you recommend lowering the static by upping the quench... but , might there be another way to bring down the dynamic without performance loss?

Thanks as always for sharing your passion and knowledge on these things....!

[Jim V.]

Agree....

Yep...right on. KISS applies. I am not trying to out engineer the general but rather just trying to go down the road using todays crappy gas....

Thanks

[Jim V.]

Re: Cam intensity, overlap, ICA issues in selectio

Jim...shooting for a LCA of 112. However, the L39 is a bit too long for my iron topped small valve L75 given my max TQ criteria. The table is sequenced by overlap with the L79 being quite large!

thanks

[Terry F.]

Re: Agree - stock works well

Mark, do you like the factory grind 390 hp cam in your 427. Do you have it set up any particular way that seems to work best?

Thanks, Terry

[Duke W.]

Re: Cam intensity, overlap, ICA issues in selectio

SAE defines "seat timing" as .006" valve lift, which is .004" at the lobe, which it really isn't because the low lift rocker ratio is 1.37:1 not 1.5. Most vendors use .006" valve lift at 1.5:1 rocker ratio as "advertised duration", but some don't. Here we go again with specsmanship and trying to split hairs with data whose context is often not known.

Forget about this "lobe intensity" nonsense. So you have the 839 lobe data and you can see it is different on the opening and closing side. Draw your own conclusion.

There are several hundred cam grinds available for SB chevys. For a good street SB Corvette engine, I've boiled it down to three - the 929/839, the "special 300HP cam" that I designed, and the LT-1 cam. I have no use for any others, including the L-79 cam, but I did use the L-79 lobe on the inlet side of the "special 300HP cam" though it is indexed a few degrees later.

My recommedation for L-79 restorations is to use the LT-1 cam. With proper clearance no one will be able to tell for sure that it has mechanical lifters. If you think the hour or so it takes to adjust lifters every 10-15K is too big a challenge, use the special 300 HP cam, set the idle to 750 and lean out the idle mixture to get a little lope.

Duke

[Duke W.]

Re: Cam intensity, overlap, ICA issues in selectio

Lobe separation angle comparisons are MEANINGLESS unless the lobes are IDENTICAL. The real measure of overlap is square-inch degrees, which you can calculate by hand, but Engine Analyzer gives a pretty good estimate.

For example, the base cam/L-79/LT-1 LSAs are 112/114/116, but the effective overlap in sq-in-degs is 0.9/3.5/4.5, respectively. So the cam with narrowest LSA has the least effective overlap and the cam with the greatest LSA has the most.

How can this be?

Because of the increasing durations - so even as the LSA is increased, increased duration more than makes up for it to yield greater effective overlap.

So much for LSA being a meaningful measure of anything.

Most cam comparison discussions are about as mindless as motor oil discussions.

Duke

[Duke W.]

PS

Like I said, 9.75 SCR is okay DCR wise with the 929/839 cam. Why are you obsessing over this?

If you want to drop the SCR to 9.5 use a thicker head gasket or grind out the chambers another cc, and don't obsess over quench clearance. Most of these engines were built by GM at about .050-.060". Taylor says the benefit of increased detonation resistance is lost once quench clearance exceeds .005 times bore, which is .020" on a SB, which is waaaaay below Chevrolet's minimum
recommended spec of .035".

Some internet bloggers obsess over quench, but have never read Taylor or any other engineering texts or technical papers. They obsess over motor oil, too, but have no engineering level understanding of either issue.

Shut down your your computer and put the engine together!

Duke

[Mark #28455]

Re: 390 HP 427 cam

To select a cam, I look at the true intended usage of the engine. The stock 390 HP cam is a really good combo for off idle to 5000 RPM and the price is decent too. Right now, I'm messing with a few short duration hydraulic roller cams and a few custom ground solid lifter cams that are similar to the stock 435 HP cam just with a little more lift.

After deciding upon the intended RPM range/powerband, I explore cylinder head options. For the 1500 to 5600 RPM range, you can't beat the mid 1970's 049 and 781 open chamber oval port heads. Most people don't notice that they're not the 1969 vintage heads but there are subtle differences in the castings - AC and PS help to hide that. Give them a good pocket porting and they're ready to run. I have even run them with up to 2.25 intakes and 1.88 exhausts to improve the port short side radius, but it's only a few HP for a lot of trouble.

Pick a piston to end up with a real compression ratio of between 9:1 and 10:1 hopefully about 9.75:1 with the 120 cc open chambers.

Given the low cost of stroker cranks and pistons, I am currently running 4.25 stroker cranks in all my new engines - they even make the rectangular port heads have better low RPM torque especially with a short duration cam.

About 5 years ago, I was toying with the Gen 6 Vortec heads on a 502 short block. The stock 502 HO was making about 440 HP and 550 ftlbs with the HO cam (about 214/230 duration) and rect port heads. We saw 430 HP and 560 ftlbs with the vortec (small oval ports) and a stock pickup truck cam (204/209 degrees duration). I'm currently playing with a 532 short block and hyd roller cams with 224/230 and 230/230 duration and 114 lobe centers - so always sticking with a LOT shorter duration than most speed shops would even consider and keeping the port velocity as high as possible.

Is it worth it? You won't believe the difference until you run the cars side by side! I ran my 489 stroker L89 with 10:1 and a 245 duration solid lifter cam - similar to a 435 cam (power doesn't kick in 'till 3000 RPM) up against my wife in her 1970 454 with 9.5 compression and 781 casting open chamber oval port heads with decent porting and a 220 duration hydraulic cam. As expected, she got into the powerband quicker and I spent all of 2nd and part of 3rd gear just trying to catch up! Just goes to show you, it's low RPM grunt and getting into your powerband first that shows the taillights to the other car!

Mark

[Jim V.]

Obsessing....

Duke, if we weren't obsessive...there would be no NCRS. BTW, my SCR is more like 9.84:1 and prior to rebuild the motor cracked 7 out of 8 top compression rings with 100% OE specs...probably due to detonation. I don't want to do this twice.

Not sure who you consider "expert", but according to David Vizard, "if you are building from scratch, make maximizing the quench your number one priority toward achieving compression and avoiding detonaton". He goes on to state, ".065 quench is way to wide". Suppose, I read too much.

Thanks

[Duke W.]

Re: Obsessing....

Vizard does some good work, but I disagree with him on this issue. The #1 priority should be achieving a SCR that is within the correct range for the cam, and there is a good body of anecdotal evidence to back this up. Like I said, most OE built SBs had quench clearance in the range of .050-.060". On many blocks it is impossible to get down to .040" quench even with the thinnest shim gasket without decking the block because many OE block decks are high.

In mid 1962 GM began double gasketing SHP/FI engines to address customer detonation complaints. The second gasket dropped SCR about 0.5 point, but increased quench clearance by the gasket thickness which was .018" to .026" depending on what gasket thickness spec you want to believe, so they could be as high as .080".

It stopped the detonation complaints.

Grinding the chambers to add one cc volume or increasing the head gasket thickness about .005" will drop the CR about 0.10, which will get you down to about 9.75. Or add more to drop the SCR below 9.75, and don't be concerned with adding .010" quench clearance. It's not a big deal.

Add 4-5 to current PON to get RON. Depending on the premium fuel PON in your area, the RON could be as high as 99, which is within the 99-101 range recommended by GM back in the 60s for the 300 HP engine, and most engines had conservative spark advance maps to deal with worst case situations. Today's gas insn't as "bad" as many might think.

Here's another look at DCR. The LT-1 cam inlet valve closing point (.006" valve lift assuming 1.5:1 rocker ratio, which is .004" lobe rise above the top of the .012" clearance ramp or .016" lobe rise above the base circle) is 85 ABDC. With a 10.5 SCR the DCR is less than 7:1. In order to achieve 8.0 DCR the SCR has to be raised to about 12.5:1. Will this work on pump premium? No way! So much for the "magic" 8:1 DCR that internet bloggers worship!

Tests on a recent 327 LT-1 showed that 10.5:1 is a practical limit with the aggressive ignition advance curve of the 327/365 HP engine and the initial timing could not be advanced beyond 10 degrees in hot summer weather, which is the bottom of the OE recommended range and yields 34 total WOT advance at 2350. Any higher SCR would require slowing the centrifugal (or increasing total centrifugal and decreasing initial), which would hurt low end torque with only a marginal improvement in top end power.

The owner has easy access to avgas, but didn't want to be dependent on it, however, he wanted to push the limit, and it was found.

Duke