SWCDuke

This may not be for everyone, but if you are interested in the low level details of cam design, you might find this comparison of the design features of the Cosworth BD-3 cam with the SB LT-1 cam interesting The BD-3 cam is an early design for the DFV F1 engine and has been cut on Cosworth Vega blanks. The lobe is very similar to the production CV cam though about 10 degrees longer in duration. The narrative is from an e-mail I sent to some members of the CV community who are collecting data on the CV cams in an attempt to optimize valve timing.

As you will see, the difference in detail design is so great that any comparison of "high level" design parameters such as duration and overlap between a pushrod cam and a DOHC cam are essentially meaningless. The narrative also points out some of the features that make the LT-1 cam such a well thought out design - the best design ever conceived for a street high perf. SB IMO where the design objective is maximum torque bandwidth with an absolutely bulletproof valvetrain using off the shelf production components.

....................

I have some cam pro curves of the LT-1 SB cam, and they make an
interesting comparison with the BD-3 cam data taken by Carl. The LT-1
data is a curve of displacement and acceleration. Unfortunately it
doesn't include velocity, but acceleration is the best indicator of a
lobe's "aggressiveness". Data is in inches/deg. squared times 10**-5
and is estimated from the data scatter. (The first number "32" would be .00032 inches per degree per degree and so on for the other data.)

........................................ ................LT-1 cam.......................BD-3 cam

Max accel. on opening.........................32...... ...........................70
Accel over the top................................-17................................-20
Max accel on closing............................25... ..............................68


Note that the LT-1 cam has much lower max acceleration on opening and
closing. This is where the pushrod is under compression, and it can't
take anywhere near the acceleration of the direct acting CV valve
train. Acceleration over the top is not that much different as this is
where valve motion is controlled by the spring. Also note that the LT-1
acceleration on closing is milder than the opening acceleration. This
is to unload the pushrod compression force as mildly as possible and is
why the LT-1 lobe has noticeable asymmetry. If the pushrod is unloaded
too quickly on seating the valve can bounce. Such "soft action" of a
pushrod cam is necessary due to the flexibility of the pushrod, so a
DOHC lobe with its more aggressive acceleration can have more area under
the lobe for a given duration, and long duration is not necessary.
Also, this is why I believe the most aggressive action - roller lifters
- should be chosen when simulating on DD2000. My cam files for the OEM
CV cams are now 258 degrees duration with a roller lifter.

Another noteworthy observation is that the BD-3 lobe reaches max velocity
at about .050" above the base circle on opening and doesn't slow from
maximum negative velocity until about the same point on closing. The
LT-1 peak velocity occurs at about .150" off the base circle due to the
modest acceleration. These major differences in acceleration make
"overlap" comparisons meaningless. Due to the quick acceleration of the
CV lobe, EFFECTIVE overlap in square-inch degrees would be much greater
at the same overlap in degrees compared to a pushrod engine cam -
another reason to use a conservative overlap figure (degrees) on CV
street engines.

Pushrod flexibility is a limiting factor on a pushrod design. This and
poorer head flow means that a pushrod engine must have much more
duration to achieve anywhere near the specific power output of a DOHC engine, but the longer duration kills low end torque because of the excess
overlap that results from long duration cams. Bottom line is that a
comparison of cam specs for a pushrod engine and DOHC engine is apples
and oranges.

I couldn't make any sense out of the CV production cam data for velocity
and acceleration because the scales are offset and don't show a proper
origin (zero). Mark - maybe you can get these corrected.

BTW, Carl's BD-3 data has some incorrect signs on the acceleration values.

Duke





[Modified by SWCDuke, 1:10 PM 8/27/2003]

Tyler Townsley

Would also like to add some more food for thought taken from another list. The M/S refers to Mega Squirt a DIY EFI setup found here.
http://www.bgsoflex.com/megasquirt.htm


This M/S thread is something that I know something about - especially
since I have the technology to look at the valve lift curves in a way
that 99.9% of the motorsport enthusiasts CAN'T look at them.

"Once you are at the point where you have to use solid rather than
hydraulic followers then plenums can start to cause issues....."

This statement is inappropriate in the impression it gives and here
is why.

Effectively, the only difference between a hydraulic and mechanical cam
is the method by which lash is assimilated before the valve moves.
Realistically, once the valves start to move, the plenum has no
conception whatsover as to whether the lifters are solid or hydraulic.

The issue with respect to how "big" a cam is does relate to idle
quality and performance and there are a lot of preconceived
misconceptions out there relating to hydraulic and mechanical cams -
here is the straight story.

Until/unless one references the duration of ANY cam to another at a
common NET VALVE lift point, cam timing comparisons are totally
meaningless.

This is why it helps to look at the "cam running duration" defined in
SAE J604d and also to cam lift area (degree-inches or degree-mm) at the
same reference lift points.

Trouble is, you CAN'T do so because most cam companies won't publish
specs in a fashion that facilitates this standardized process.

However, after looking at every cam sold by TRW (and many other private
label parts while I worked at TRW for ~10 years) plus literally HUNDREDS
of cams sold by other folks (Crane, Comp, Isky, GM, Ford, DCX, etc)
since then, I can assure you that the following relationship pretty much
holds true.

The NET RUNNING duration of a mechanical cam with the SAME 0.050" CAM
duration ends up roughly 10 degrees SMALLER than a hydraulic cam of the
same 0.050" cam timing once you take up the lash and get above the
clearance ramps. This is what J604D does.

Thus, a 230 @ 0.050 deg hydraulic cam will idle worse than a 230 @
0.050 mechanical cam because the damn thing is about 10 degrees bigger
at the point where the valves are moving in a reasonable fashion. Thus,
there should be no surprise that a 230H would have more idle/low speed
issues than a 230M.

If you then compare the 230 hyd to a 240 mech, they act pretty much the
same. This shouldn't be surprising because as far as valve motion goes,
they pretty much are the same as far as the engine "sees it".

The things that guys forget about carbs and big cams is that air
reversion pushes some of the air/fuel mix back out the carb venturi.

Keep in mind that a venturi mixes fuel as a function of "fluid flow" in
either direction and the amount mixed is a function of fluid density.

Thus, air gets drawn thru carb and fuel is mixed. As an a/f mix (which
is denser due to the fuel already mixed) gets shoved back thru the carb
- more fuel is mixed, again.

Depending on the plenum/air box, the fluid could get sucked back thru
again, mixing even more fuel and so on and so forth if the "stand off"
mixture gets recycled over and over.

A plenum provides a damper or collection chamber that can contain a
"reversed/stand-off" a/f mix from getting shoved back and forth thru
carb venturi.

With injection, you can shove the a/f back and forth till you're blue
in face cause you only get the 1 shot of fuel and no more unless you
rotate 720 degress.

But, by then, you already ingested the air so you have fresh, dry air to
mix fuel with next time around. No wonder M/S's idle better than carbs
- they only mix fuel once.

Few people can really see what's going on with cams and the cam
companies WANT/NEED to keep it that way. However, if you can get access
to see what's really going on like I can, you'll see that it all ends up
being a lot of bunk and a ruse to sell unwitting customers something
they really may not need.

This is a prime reason why I TOTALLY steer clear of the retail/DIY cam
business. It is just too hard, too time consuming and too unprofitable
to provide good, sound engineering logic on how to REALLY use the right
cam to folks who've learned a bit too much bogus car magazine
technolgoy. Simply put, you can't unring a bell when it comes to
straightening out the preconceived misconceptions about mechanical and
hydraulic cams.

SWCDuke

I might add that SAE J604d calls for measuring duration at .006" VALVE lift as there is little effective flow at less lift. Some aftermarket grinders use SAE J604d to specify "advertised" duration, and this is the value that will usually give most accurate/realistic results as "seat to seat" duration on DD2000.

BUT, the other wild card is rocker ratio. On a SB it starts out at about 1.37:1 and peaks at 1.44:1. It is never 1.5:1! This is why OEM lash settings on mechanical lifter cams must be "factored" to compensate for actual rocker ratio. Even GM did not take rocker ratio into account, so if you lash to GM spec your valves are too loose. Also, even if you use compensated lash the actual duration at .006" valve lift will be a little less than advertised duration.

Your point about comparing duration at .050" between mechanical lifter and hydraulic lifter cams is well taken. Because mechanical lifter cams have much higher ramps, much of this "lift" is clearance ramp, so you can't compare the duration at .050" of a mechanical lifter with a hydraulic lifter cam, and your 10 less degrees EFFECTIVE duration for a mechanical lifter relative to the raw spec is probably a good rule of thumb, but very few understand this issue. Also, hydraulic cams typically have less aggressive accleration than mechical lifter cams, so comparisons based on a few simple numbers can lead to erroneous conclusions.

The real important part of a cam is the first quarter and the last quarter of the lift curve. This is where acceleration is a big determining factor, and it is also where all the overlap is, so comparing "degrees of overlap" is meaningless unless you KNOW that the cams have similar acceleration profiles.

The LT-1 cam has 242/254 duration at .050" tappet lift versus 222/222 for the L-79 (151). The LT-1 sounds bigger, right? Maybe too big in fact, but consider that the top of the ramps are .016"/.020" above the base circle. If you compared actual VALVE lift at .050" you'd find the inlet lobes very similar. The exhaust lobe is a good deal longer on the LT-1 cam. It is also phased earlier, which is one of the secrets of the LT-1 cam. The early phased long duration exhaust event helps compensate for the SBs relatively restricted exhaust port. Actual EFFECTIVE overlap for the two cams in sq.-in.-degrees is very similar with a slight edge to the LT-1.

It's too bad the aftermarket cam and hot rod publication industries don't do more to educate users, but I think both are more interested in hyping simple (but meaningless) numbers rather than educating their customers and readers. I've never seen a hot rod mag. or book that ever discussed actual rocker ratio in any meaningful way, but all you need is two dial indicators and a degree wheel to measure it youself. With all their talk of "degreeing in the cam" you'd think that some discussion on rocker ratio would also be appropriate, but I guess they've never figured it out in the nearly 50 years that the SB had been around. Go figure!

To accurately characterize a lobe you need to specify the .006" valve lift timing points (preferably rocker ratio compensated) and peak acceleration and velocity on both the opening and closing flanks. Only then can you reasonably make even a subjective comparison. Two cams with the same basic timing numbers can have significantly different torque bandwidth and idle quality due to differences in acceleration and velcoity profiles.

BTW, that Cosworth BD-3 cam at 268 degrees (.006" valve lift) duration appears pretty mild, but because of the aggressive acceleration the duration at .050" lifter lift is 240 and 236 at .050" actual valve lift! (The ramps are only .010" high.) The OEM CV cam is 258 at .006" valve lift, 228 at .050" tappet lift and 224 at .050" valve lift. Ramp height is .016". A 258 degree cam for a pushrod engine would only be about 205 at .050".

Duke




[Modified by SWCDuke, 8:14 PM 8/27/2003]

Matt Gruber

Anybody have any recent dyno tests comparing the LT-1 with the XE262 , XE268 etc?
.
LT-1 cam sounds like a good budget cam for use with stock Vortec heads.
.
Does the LT-1 cam have enough torque for a 308 rear with a 2100 stall TH400?
.
many run aftermarket heads, like AFR, which have amazing exhaust flow. LT-1 would be a mismatch?

SWCDuke

Real dyno tests are hard to come by. Your best bet is to spend fifty bucks for DD2000, but you have to be careful about how you input cam data. There is no way to spec the LT-1 cam properly in DD2000 using any published specs. I have LT-1 cam files that are based an actual lift-crank angle diagram that I measured. If anyone with DD2000 wants these cam files just send me an e-mail.

The LT-1 cam, with it's early phased, long duration exhaust lobe works particularly well on stock or modified vintage heads since the exhaust flow is only about 65 percent of inlet flow. Modern heads that are more balanced - say 75 percent - will work best with equal duration on both sides and more conventional exhaust phasing.

The LT-1 cam bottom of the 80 percent torque bandwidth is about 2000. If you value strong mid range and top end more than stump pulling off idle performance it will probably work okay with a 2100 stall converter, but this is a very subjective question so there is no single answer for all tastes.

Duke


[Modified by SWCDuke, 12:31 PM 8/28/2003]

Tyler Townsley

Slight correction on my post. The information presented was by a individual who grinds cams for some NASCAR teams and did not want to be identified. His thoughts seemed to fit well with yours so I posted them, I only wish I had the knowledge and insight he has.

Tyler

Matt Gruber

i did find a Hot Rod test in the 12-87 issue.
"the LT-1 cam kept pace with the L82 cam from 3000-4500-and then it left it far behind"
+23hp at 5500

SWCDuke

I'm not surprised. The LT-1 is a great cam! The difference with the L-79 cam would be even greater at the top end with about the same low and mid range.

What was the power/torque difference below 3000 or did they just test the mid and upper range? My simulations indicate ?Duntov, L-79, L-82 and LT-1 cams are about the same at low revs. The 30-30 losses big time below 3000.

Duke

Matt Gruber

No. The test starts at 3000. LT-1 was slightly + at 3000.
they used a TargetMaster with 8.5:1 Lt-1 was 297 HP.
They swapped to 87 c4 mildly ported heads and made 327hp@6000 w/9.5:1.
With modern heads i would guess 350hp, about 40-50hp less than CC XE268. But with 116 Lobe Seperation Angle i would expect great MPG compared to the 110 on CompCams. My 61 went from 20 to 14 when i went to 110 LSA from 114 :rolleyes:
The magazine articles never tell you this, and very rarely go below 2500 rpm. I'd like to see their dyno tests start at 1500. :cheers:

66427-450

But which cam produces the most power up where you need it when you're running thru the gears? say, 4500-6500+?

I would bet that the 30-30 cam designer(s) didn't care much about efficiency at 3500 and below, as the cam was intended to be used for High Performance applications.

:seeya

SWCDuke

My simulations characterize absolute peak horsepower of both cams as very close, but the 30-30 has a little less rolloff beyond the peak.

Over the rev range the engine will see going through the gears with shifts, at or near the redline, which you correctly characterize as 4500 to 6500 with a CR four-speed, the LT-1 cam makes more average power in this rev band, and all other things being equal, would win the race.

The 30-30 actually wants to rev higher, but your valvetrain might be in jeopardy if you go to 7000 all the time.

Duke

JohnZ

Most of the hi-po magazines do their readers a great disservice with their buildup/dyno articles that trumpet stuff like "600HP FROM A STREET 350!" splashed across the cover; they build stuff with solid rollers with rectangular lobes at 104-106 LSA and ignore everything below 5000 rpm, then guys run out and buy those cams and Dominators, and then you see their posts in places like the Camaro boards with "how come my car runs like crap", "why do I keep wiping cam lobes", "runs rough", "won't idle", "broken valve springs", "bent pushrods", "power brakes don't work any more", etc.

If they'd show dyno curves for TORQUE from 1500 rpm all the way up and calculate the "area under the curve" for comparisons, it would be a far better method for a "streetable" engine article - but then they'd have to explain "torque bandwidth" as a concept, and they'd rather not do that. Having "600HP" on the cover sells more magazines. :rolleyes:

SWCDuke

I don't thing magazine guys (who at best might have a degree in journalism) have the foggiest notion of the concept of torque bandwidth or why it is important.

Their view of engines continues to be one dimensional - peak power!

Duke

SWCDuke

Any attempt to compare the LT-1 cam with almost any other cam using traditional benchmarks such as lobe center angle is meaningless. The LT-1 lobes are highly asymmertrical and the actual centerlines between the lash points is offset several degrees from the points of maximum lift. The effective overlap of the LT-1 cam in square inch-degrees is about the same as the Duntov cam, which has a symmetrical lobe and a 110 degree LCA.

My LT-1 cam model for DD2000 is based on measurements from an actual lift-crank angle diagram I took years ago. In DD2000 the LT-1 cam beats the XE268 in peak torque and power with about the same bottom end. My XE 268 model is based on the specs from the Comp Cams catalog.

That's why I'm always ranting about the LT-1 cam. I've never seen anything remotely like it. It is absolutely unique in its lobe design and lobe phasing and it shows up as the best torque bandwidth on a vintage small block that I've ever seen, and it should be equally superior with higher flowing heads as long as the exhaust/inlet flow ratio is about the same as all the vintage SB big port heads, which is about 0.65.

Duke

Matt Gruber

what magazines are you guys reading?
.
Duke,
looks like the LT-1 cam sucks gas too! or maybe it doesn't? :confused:
.
somebody needs to re-testthe LT-1 cam. The problem may be LIFT. newer heads flow MUCH better at higher lifts, and have MUCH better exhaust flow.
gkull likes the Crane Powermax 278 solid cam.
that got 17.4 mpg. 114LSA, 238 @.050

SWCDuke

The LT-1 cam, all other things being equal, will produce about the same fuel consumption as the Duntov cam.

Modern heads with more balanced flow would probably have different optimal valve timing, but if you use vintage heads, modified or unmodified, the LT-1 cam can't be beat in terms of torque bandwidth for an SHP-like engine - high specific output with reasonably acceptable torque, drivability, and idle quality.

Lifting the valve more than about a quarter of its diameter does not materially improve flow, but it's best to lift the vavle at least 0.5" and keep it at or above this value for as long as possible. NO OEM cam produces this much valve lift. The only way to achieve more lift is to increase acceleration or increase duration (so peak accleration can be maintained longer), or both, so you either end up with either valve train reliability problems or excess duration with kills bottom end torque. With a pushrod, the acceleration cannot be as high as with a direct acting DOHC valve train. That's why I compared the acceleration characteristics of the Cosworth lobe with the LT-1 lobe in my original post. They are very different.

If anything, designing a good cam for a pushrod engine is a lot tougher than for a DOHC engine.

Duke

mrg

The latest GM Performance Parts catalog lists replacement factory cams as being manufactured by Crane Cam Co. .. On the camshaft page is the sentence - "These new generation of camshaft kits are designed and manufactured by Crane Cam Co." .. Yet the listing for the LT-1 cam (as well as the "151" and "346") is stated as being a "blueprinted replacement" for factory P/N 3972178 .. Does the term "blueprinted" mean that this is a direct copy of the factory LT-1 design. ? .. It's a bit confusing using the terms, "new generation" and "blueprinted",on the same page. The Crane cam spes show the LT-1 cam as being 242/254 duration @ .050 - .459/.485 lift - LCA 116 ..

Did Crane take over making GM performance cams along the same lines as Speed Pro .. ?

SWCDuke

I'm not sure what to make of the term "blueprinted". Certainly the original parts from 30 years ago were built to print. I think it's marketing fluff. You can also buy most of the vintage OE SHP cams from Federal Mogul Speed Pro and that would be my choice. The specs you quoted are what is listed in the Speed Pro catalog and matches some old Chevrolet specs.

The 116 degree "lobe center angle" is actually the distance between the points of maximum lift which are not centered between the lash points because of the asymmetrical lobes. The actual geometric centers of the lobes between the lash points are spread at about 114 degrees. The points of maximum lift are 110/122 inlet exhaust. The early phased and longer duration of the exhaust result sin a very early "centerline", which makes the "lobe center" seem high relative to the actual EFFECTIVE overlap. This is one reason why the LT-1 specs cannot be compared to the specs for a conventional symmetrical lobe. The early phased and long duration exhaust is very effective as squeezing more torque bandwidth out of vintage big port heads, which have a rather restrictive exhaust port. The timing was also developed to get the most out of the engine when equipped with the OE cast iron manifolds.

The duration at .050" is not as aggressive at it seems because of the tall ramps. A roughly equivalent hydraulic cam with a symmetrical lobe would be about 230/240 at a 114 degree lobe center.

The L-79 also has similar lobe phasing - 110/118, but the LT-1 cam goes a step further by having a longer lobe for the exhaust . The L-79 lobes are the same for both sides and are symmetrical as far as I know and the .050" duration is 222/222 at a 114 lobe center. The longer duration of the LT-1 cam at what is essentially the same lobe center means the LT-1 cam has a later closing inlet valve, earlier opening exhaust valve and more effective overlap.

I believe the GM package is a kit that includes lifters, but I don't know if they are the edge orifice type or piddle valve. The edge orifice type are best. The F-M number for the cam is CS1145R and the F-M edge orifice lifters are AT840R.

Duke

Vetterodder

I also think advertisers use of the term "blueprinted" is confusing. In my useage of the term, a "blueprinted" cam means a cam that was ground with the stock published specifications for lift and duration but anything else that could improve performance was. A racing class might require the use of a stock cam but, if inspected, only certain measurements are checked. As you've pointed out, lobe profile can have a tremendous effect on the performance of the cam. These "blueprinted" cams opened the valves at the same time they were supposed to, opened them only as far as they were supposed to, and closed them when they were supposed to. In between though, the valves spent more time opened farther than stock. The difference isn't nearly as dramatic as your comparison of OHC profiles vs OHV profiles but, when the goal is to win, these "cheater" cams can make a big difference. They're one of the reasons that the pro's can run 12's in a car that couldn't get into the 14's unless it'd been rear-ended by a 747.

The downside to the blueprinted (as per my definition) cams is that, besides being a compromise in order to be "in spec", they usally beat the crap out of the rest of the valve train. No big deal in a race car where all those abused pieces are replaced more often than most people change underwear but definitely not something you'd want in a street car.

SWCDuke

Good analysis - jogs my memory about what NHRA "stock" meant way back when - maybe another reason to go with the Speed Pro LT-1 cam.

Duke