Definition of seat to seat duration

[cratemodel014]

In one of david vizards books he mentions seat to seat duration. What exactly is seat to seat duration. Is it advertised duration?

[modok]

It should be advertised duration, but since there is no concrete definition of advertised duration is either, who knows.
Some companies it is, and others it isn't, and others just plain don't even know what they are selling.
IMO they both should be the duration minus lash ramps, but it can be hard to determine exactly when the lash ramps stop and the acceleration begins, especially on worse quality cams.

[pdq67]

This is why the old Big Duntov, 30-30, "fuelie", cam is listed by GM as 346!

It is really more like 290 or 313, take your pick. I probably would use 290 but I probably should go back and rethink this????

I do know that Isky's great old Z-30 is 290.

I want to say that the late, great, Pat Kelly got deep into this, but it's been years??

pdq67

[KOLI11]

In one of david vizards books he mentions seat to seat duration. What exactly is seat to seat duration. Is it advertised duration?

Yup, advertised duration.

[PackardV8]

As one who misspent another life in sales and advertising, "advertised" anything is a planned assault to bypass reason and take the short route to your wallet. That's why no one gives any credence to "advertised" and why the aftermarket cam industry had to be dragged kicking and screaming to @.050".

[pdq67]

Anybody here old enough to remember the old magazine's, advertisement, "camshaft wars"?

Isky, Crane, Crower, etc., etc..

Each one trying to out-dazzle the others back then...

If one came out with a 280, the next one would be a 282!
If one was .450" lift, the next would be .454"!!

And the names, 5-Cycle, 505-C, Super Lagara(Sp?), Ba-Ha(Sp?), Blazer, Sat. Night Special, Commander, etc., etc...

He, He!!

pdq67

[MadBill]

As one who misspent another life in sales and advertising, "advertised" anything is a planned assault to bypass reason and take the short route to your wallet. That's why no one gives any credence to "advertised" and why the aftermarket cam industry had to be dragged kicking and screaming to @.050".

'Advertised' is all over the map, but a least some mfgrs. define their own interpretation. By definition, 'seat to seat' should be measured from the crank angle at which the specified hot lash clearance reduces to zero until where it just becomes zero on the closing flank. The problem is the rocker ratio affects the resulting duration number and so needs to be part of the specification.

Hopefully camking will weigh in and give us the scoop, at least on his stuff...

0.050" seems like a good choice for assessing overall characteristics, but even with 'conventional' lobes, idle vacuum/roughness and low speed operation can be greatly affected by the length of the zero to 0.050" and return journeys . Throw in the more exotic shapes like inverse flanks and asymmetric profiles and the 0.050" yardstick (and LSA/LCA) becomes even less reliable.

Do I have a better idea? No, I sure don't.

[ProPower engines]

Seat to seat timing is just as it states. The duration from valve lift off the seat till it touches again.
May things have to be taken into consideration like spring pressure and push rod rigidity to get a definite.

I am currently working with Isky on a limited spec engine deal and the topic is as much as important as the @50 timing but seat to seat differs a lot compared to the @50 measurement. From when the valve lifts off the seat till it touches down again can be 10 deg. different

[Geoff2]

In DVs books, seat - to - seat referred to hyd cams. 0.006" tappet lift seems to be the most common lift point used for hyd cams. My understanding for using 0.006" was that with take up clearance/flex in the valve train, 006 gives the best ESTIMATE of actual duration for the valve leaving the seat, & the valve returning to the seat. Crane often uses 0.004" adv dur, Ultradyne used 0.0045"& 0.006", depending on the lobe series. Some companies give no figure, leave you to guess....

Aaaaaaaah yes, the duration 'wars' from the 50s/60s/70s. All the big players of the day were guilty [ this was before Comp Cams was born ]. Some of the car companies were the worst...HFT cams. Pontiac Ram Air 3, adv 301*, 276 @ 0.008*; Olds Cold air cam, adv 308, 289 @ 0.008".

Solid cams: Ford 427 Nascar adv 324, 283 @ 0.018"; Chev 30-30 Duntov, 346 adv, 331 @ 0.018"; Ford 289/271 Hi Po cam, adv 306, 266 @ 0.018"

[CamKing]

"Seat Duration" is the duration at the "Hot Lash" point. It's the duration the engine sees.
"Advertised Duration" is a meaningless advertising gimmick, and only shows you the duration the cam company wants you to see. It can be measured at any lift the cam company chooses.

[mk e]

In DVs books, seat - to - seat referred to hyd cams. 0.006" tappet lift seems to be the most common lift point used for hyd cams.

, there is an SAE standard that says .066 I'm prtty sure...noth that everyone follows it. 0.006 is the number dynomation software uses, but Mike's hot lash number it probably more accurate than a generic number form a standard.

[engineguyBill]

Problem with seat to seat duration is that it is very difficult to measure accurately. When the valve is just beginning to open, the crankshaft may go through several degrees of rotation with very little (or no) valve movement. Therefore, the person who is doing the measurement may take the liberty of "choosing" where the valve began to open (and close), which may result in inflated duration numbers.
.050" valve lift is easy to measure and this was accepted at the beginning and ending point of duration, in order to more accurately measure total duration of racing cams. The only problem with this measurement point is the fact that a lot of movement may be involved with the opening and closing ramps of any camshaft lobe.

[CamKing]

.050" duration is only used for degreeing the cam in, because the velocity is high enough at that point, that you can be fairly accurate finding it with a dial indicator, and degree wheel.
.050" duration tells you nothing about the cam, and has no significance in the design of the cam. You can't compare the .050" durations of 2 different cams, unless both cams have the same lash ramps, and come from the same family of lobe designs.

[wyrmrider]

SAE defines as .006 at the valve
which normalizes for lash and rocker ratio
You can find that number used by the big aftermarket grinders
Crane at .004 at the would be SAE .006 at the valve with a 1.5 rocker
BIG camgrinders do not want to use the SAE standard for obvious competitive BS reasons
and the duration for the same master does change with rocker ratio- as it should
PS MOPAR does not use SAE- they have been using the same method long before SAE
has to do from the blueprint where the lash ramp meets the acceleration ramp
sorta what CamKing says
and what he implies is that you need a CAM DOC and a computer to compare cams (or a lot of time with the degree wheel)
been there- done that

[CamKing]

The SAE .006" standard, is only for hydraulic cams