http://popularhotrodding.com/tech/06...mshaft_basics/

If I have done my math right on the chart for my 383 ci with 2.08 valves I should be running a 106 LCA.

My actual solid roller cam is 236/244 112 LCA with 11.2 compression. according to article tightening the lobe center even though increasing overlap would increase low speed TQ??????????

If I have done my math right on the chart for my 427 ci with 2.08 valves I should be running a 104 LCA or so.

My actual solid roller cam is 242/248 110 LCA with 10.7 compression.

does this article really jive out there in the real world. ? David Visard has said other things that I wasn't sure of. I've only had one cam in my life with 104 centerline and it idled at near 2000 rpm and turned 9000 rpm every time down the 1/4 in a alki injected SBC dragster. I'm not sure how well my Vette would run with a 104 LCA?

 

George, i read the same article and based on Vizards math i should be running a 102-103 LSA for a 434 i'm building. When calling for input from the cam companies (Bullet, Comp, Crane, Crower) they all wanted to give me a 106-108 LSA along with huge lobes; 268-278 in duration. I talked them down to low 260s and a 110 LSA, but they ground the damn cam on a 106 anyway. So i guess we'll see how it does....

I posted a similar question on a drag racing site. Most concurred on the tighter LSA for drag racing big inch engines, but one guy had gone from 106 to 110 in a similar application to mine & no other changes and improved his performance. So maybe everyone read the same article, but the one real example told a different story.

 

Ralph that sounds like your going to have a monster in in that 434. 260 and 106 would have so much overlap that your DCR would be way down.

my cam in a 427 and 5 speed with 4.11 rear is really pretty mild. It's not drag car choppy and it idles at @1000 rpm

 

We're starting with 13:1 static CR. Probably will have to have it idle at 1200 or so. This is my nephew's car and not really a street car anymore. With a 110LSA i figured iut would eat the cam up ok, but with a 106.....it's probably going to lope like hell. I'm also sorry we're not at 13.5:1 now.

 

My guess is that this article has caused a lot of heated debates in the hot rodding community. It definitely didn't make sense to me but I am nowhere near his knowledge level.

This is the kind of thing that makes me want to get an engine dyno. I just wish it wasn't such a major undertaking to get one installed.

Steve

 

C'mon Steve...."Let's" get one!!

Notice how easy it is for me to spend your money? I'll come up and help you play with it.

I'm sure we can do something in that back area at the end of garage or in the last bay right?

JIM

 

I read this article too and decided it must be aimed at all out racing because it doesn't seem like numbers that would leave anything streetable. Also doesn't discuss how this changes with forced induction??

 

This article confused me as well. The LS1 and LS6 motors use LSA's of 119 and 116 respectively. According to Vizard, the engineers would have been better off shrinking the duration and tightening the LSA.

I don't know what duration they run in the new 427 CI LS7, but I'm guessing it's 112 or more.

 

Vizard has done good work in the past, but this one has me scratching my head. First, one must distinguish between a high performance street and racing engine. He talks both, but makes no real distinction.

For a high performance street engine I use as late a closing inlet valve as possible that will give me my low end torque objective, which is usually 80 percent at 2000. Then I juggle the exhaust valve opening point to achieve the bottom end torque spec without taking too much top end away, and E/I head flow ratio is a big factor. For the relatively restrictive OE SB exhaust port the exhaust valve needs to be opened fairly early, and you end up with more exhaust than inlet duration.

For a street engine I'll pick exhaust valve closing and inlet valve opening points to generate only a very modest amount of overlap - say about one sq-in-deg. and might juggle it a little as the last step after the inlet closing and exhaust opening points are nailed.

If the heads are massaged the E/I flow ratio goes from about 0.65 as OE machined to about 0.80, so you can open the exhaust valve later and end up with less exhaust than inlet duration.

Street engines with ANY amount of backpressure don't respond well to increasing overlap and exhaust backpressure tends to negate header wave dynamics, so low overlap and free flowing exhaust manifolds usually yield the broadest, street friendly, torque bandwidth. Exhibit A - LS7!

On a racing SB with similar head flow characteristics as above and headers and open exhaust I might end up with similar inlet valve closing and exhaust valve opening opening points, but open the inlet earlier and close the exhaust later, which stretches out duration, increases effective overlap substantially, and narrows the LSA.

On a tow or boat engine with similar head flow characterisitcs (and exhaust manifolds) the inlet valve closing would be much earlier, later opening exhaust valve and about the same overlap as the high performance street engine - maybe a little less, and the exhaust event would still be shorter than the inlet event.

The base vintage SB cam has about 0.8 sq-in-deg overlap with the 1.94/1.5" valves. With the 2.02/1.6" set the L-79 cam is about 3.6, 4.5 for the Duntov and LT-1 cams, 5.5 for the 30-30, and L-72 is 6.1. For a racing engine with high flow efficiency headers and open tailpipe, 10-12 sq-in-deg can be effective in maximizing the average power in the upper third of the rev range, but it just KILLS low end torque, which is why a street engine requires a completely different valve timing scheme than a racing engine.

LSA is not a primary design consideration, it is merely a byproduct of optimized valve timing and a vague high level represention of the cam. It's basically pretty meaningless, but subject to gross misinterpretation. The important overlap parameter is the actual overlap in sq-in-deg.

The LS7 was designed purposely with a restrictive exhaust port (E/I is about 0.65 even with the CNC machined ports) in order to get as much inlet valve and port area as possible. As a result, the exhaust valve is opened relatively early and the exhaust lobe has more duration than the inlet lobe. The inlet/exhaust duration at .050" lifter rise is 211/230 with points of maximum lift of 124/117 for an LSA of 120.5 and the effective overlap is a measly 2.1 sq-in-deg. The "short, retarded" inlet event means the inlet valve opens very late (and closes late) which minimizes overlap, and even though the exhaust event is fairly long, it is indexed fairly early, which, further reduces effective overlap.

A vintage 383 with massaged heads will make a very nice street friendly torque curve for a high performance engine and idle nearly butter smooth if you swap the LS7 durations and use the same points of maximum lift. Of course, the flank acceleration can't be as aggressive as LS7, and the valves are smaller so you end up with only about half the effective overlap, which is fine at the more modest mean piston speeds that a vintage engine can aspirate efficiently.

For a 3.48" stroke configuration advancing the above cam 4 degrees and 8 degrees on a 3.25" stroke configuration will be pretty close to optimum for those configurations, too.

Guys talk about "modern design" cams, but the fact of the matter is that all aftermarket cams are old school for high performance street engines - relatively narrow LSA for the duration class, which means they have too much overlap, and longer duration exhaust than inlet events as if you were running them on OE machined heads.

A "modern cam" for a vintage small block with heads massaged to an E/I flow ratio of about 0.8 should look more like the LS7 cam (with the durations swapped) as I described above, but you can't find a single one that is even close!

Duke

 

If you use the overlap estimator and his recommended LCA, I would be curious to see what the results would be with an LSX motor. If you went with today's agressive ramps lift to keep overlap down, could it be made to run on the street amd still pass emissions?

I am no expert but in my experience with SBC and carbs, the best running motors I had were with tight LCA's, 106-110 and moderate lift!

 

Vizard's overlap chart makes fairly drastic differentiations between street and race engines. but the LSA chart is almost all CID to valve size without significant consideration to the application. I think with todays better flowing heads, less overlap is required to make good power. I think Vizard is talking old school heads and theory.

 

One of the problems with the conclusions in this article is that most if not all of the testing appears to have been done on a lab dyno, and lab dyno tests are usually done with generous size piping and no mufflers so exhaust backpressure is essentially zero.

Then you install the engine in a car with a street exhaust system that generates even modest backpressure and you have a completely different configuration. During the overlap period average exhaust pressure is higher than inlet pressure, so exhaust gas flows into the inlet port and dilutes the fresh charge. The higher the backpressure, the more exhaust gas dilution and even with headers the best you might see for a brief portion of the overlap period in some portion of the rev range is zero pressure differential.

The OE Corvette exhaust system is pretty good and only generates about 3 psi on a good SHP SB, but backpressure goes to 5-6 psi on a SHP BB because of the higher exhaust flow. Backpressure increases with the square of exhaust flow.

High overlap with high backpressure will just KILL the torque curve throughout the range. When you start designing the valve events for exhaust backpresssure, the less overlap, the better. That's why a street cam should have much less overlap than a racing cam designed for open exhaust - completely different animals!

Duke

 

That's some really great info posted for free. I spent over $50 in Vizard books to get the same and spent many, many hrs to read it all though that article has provided even more insight.

I think that for 99% if us novice engine builders that info should find us a great cam. Sure there are exceptions and different applications in race only cars. Rod length has an effect on piston dwell time at TDC and BDC which inturn affects breathing also - but to a such tiny amount as not enough of a consideration for again 99% of us. Exh backpress is important but does anyone here know thiers? Anyone measure thier exh backpress? Heck most of don't know our hds E/I ratio other than from what some mag article is selling us on. But Vizard lays it all out for a combo's c.r., c.i., vlv size and vehicle use.

To say that LSA is "basically pretty meaningless" after all the good information presented to help a novice choosing an LSA is a fabricators crap. Who would care whether LSA determines vlv timing or vlv timing determines LSA - your saying the same thing but acting like you know something more than others. To order a cam u have to choose an LSA - many cams are sold with different LSAs and u have to choose one. If some has a better graph for LSA then post it. Vizard produced his from the dyno results he worked hard for.

cardo0

 

cardo0:

I am in your camp. I did have Vizards books and have read them. I just re-ordered them after reading the article.

What I do know is that Vizard is acknowledged to know his stuff and has done extensive research of the issues. Until such time as someone can prove that his research is bogus or no longer valid, why would it not be worth considering? A small LSA does not necesarily mean huge overlap. I think this is a normal assumption!

It seems like we are looking to disprove his research with assumptions.

 

I for one sure hope he's right. I'm installing a cam with a tighter LSA then i would like, but it is what they sent me and is closer to what Vizard would likely recommend. I'm also concerned that we don't have enough exhaust. We soon see how it runs.

 

If its really that important to u, take it to the dyno before installing in car. Swapping cams while out of car much easier and u have a mtr broken in and tuned up before installing rather than have to fight the gremlims to get it running in the car. Yes a few hundred bucks (well almost a grand with a new cam) but then u will really know what works and can share it here with us.

Just my 2 cents i'll take Vizard's recommendations before i try to prove him wrong or experiment on my dime.

cardo0

 

Yeah, i wish. I'm embarrassed to say how much we paid for this cam, and buying another to test the difference between LSAs aint in the budget. Like you said, I'll have to take Vizard's word on this one.

Actually, my engine simulator (EA pro version) shows the 106 to be better than the 110 in our operating range (4500-7000). I just hope the exhaust is up to the task and we can tune the sucker to idle 1/2 way decent.

 

I re-read that article last night and it seems to me that Vizard's definition of duration could be confusing. My interpretation is that he is quoting off-seat duration & overlap.......not advertized or at .050. For a solid roller this makes a huge difference. For instance, the cam i'm currently installing has a .050 duration of 260 deg and an advertized duration 293 deg, but the off seat duration (roughtly .012 tappet lift) is over 310 deg. Overlap at .050 is only 56 deg, but it is 108 off-seat. 108 puts me on the high side of the pro race category

And if my assumption on his definition is correct.....how the heck do you order a solid cam? No one that i know of quotes duration at .012 (which is the tappet lift required to close a lash of .020 with a 1.6 rocker). If i follow his logic correctly, i'd first pick an overlap. For my application (race 434) i would target something in the 90 to 100 range according to his table.......let's say 95. Next i'd pick an LSA and according to his graph it should be about 104 (103+1 for 13:1 CR). Then i'd pick the duration to fit those specs: 95/2=47.5 + 104=151.5 * 2=303 off-seat duration. Now what? How do i convert that to advertised or .050 duration? According to my measurements, the advertised duration is roughly 22 deg less. So i should be looking for an advertized duration of about 281? (303-22). This would put the .050 duration in the high 240s. Doesn't that sound small for a 434 race application? About 5 years ago i read an in depth article on a vary similar build (434 race engine). The article spent a lot of time on the cam selection process which included the use of some propriety software. They went with a cam that was 264/276 @.050 ground on a 104 LSA. The author was David Vizard. The cam i have is 260/272 @.050 ground on a 106. Yet following the logic above would have led me to a much smaller cam What am I missing?

 

You sure it's just not ground four degrees advanced? James

 

The flow characteristics of the heads and the cubic inches of the motor combined with the compression ratio and desired operating range dictate the cam needed. To simply apply a "chart" based on the engine size is ludicrous. There are too many variables to just use a chart. Hell, vehicle weight and type of tranny will also affect the proper cam design needed. James