LT-1 nokka

[tapio]

Kiinnostaisi tietää, millaiset ominaisuudet alkuperäinen, vuoden 70-72 mekaaninen LT-1 nokka antaa koneelle. Mainostettuja asteita nokassa on ihan sikana, mutta asiantuntijoiden mukaan sen intensiteetti on alhainen, joten sen ajettavuus kadulla pitäisi olla siedettävä. Edelleen kiinnostaisi tietää, paljonko koneessa pitää olla (todellisia) puristuksia, jotta se toimisi em. nokan kanssa. Vuoden 70 koneessa oli 11:1 puristukset, mutta jo seuraavana vuonna vain 9:1.

Kokemuksia?

t tapio

[coupe]

kertokaahan tietävät, asia kiinnostaa muitakin.

mitäs muuten tarkoittaa intensiteetti?

[tapio]

Intensiteetillä tarkoitetaan mainostettujen asteiden ja .050 tuuman nostolla olevan astemäärän erotusta (adv dur - dur @ .050). Nykyaikaiset nokat ovat huomattavasti aggressiivisempia, esim. Compin XE262 antaa luvut 262-218=44 imupuolelle ja 270-224=46. Toisaalta aggressiiviset nokat vaativat kovemmat jousipaineet ja nokan käyttöikä saattaa olla lyhyempi (?).

Tehdasnokkien, kuten LT-1, suunnittelussa lähdettiin liikkeelle siitä, että venttiilikoneisto pysyisi pitkään kasassa vakio-osilla ja siksi nokan rampit suunniteltiin loiviksi. Kuulemma LT-1:ssa on erikoisuutena se, että nokan laskuramppi on loivempi kuin nousuramppi, joka ehkäisee venttiilin hyppimistä istukassa (valve bounce at seat). Voi kuulemma antaa koneelle rundeja surutta tuonne 6500 rpm:n tienoille.

Jotkut vanhat parrat corvette forumin sivuilla vannovat tämän nokan nimeen, siksi asia kiinnostaa.

[tapio]

Jäi äskeisestä pois oleellinen eli "vanhoissa" nokissa tuo laskutoimituksen jälkeen saatava asteluku on huomattavasti suurempi ja intensiteetti taas pienempi. Eli loivat rampit, matala nosto sekä korkea mainostettu asteluku = pieni intensiteetti. Uusissa nokissa on taas tyypillistä jyrkät rampit, korkea nosto sekä alhaisempi mainostettu asteluku = korkea intensiteetti.

[tapio]

Tässä lisäinfoa aiheesta toisilla foorumeilla:

http://forums.corvetteforum.com/zerothread?id=690268
http://forums.corvetteforum.com/zerothread?id=691823

[tapio]

Päivitystä projektiin:

Oli pieniä vaikeuksia löytää kyseistä nokkaa mistään, mutta American Import paikansi sen Ruotsista alta aikayksikön! Tänään saapui postitse aito LT-1 nokka + nostajat yhteensä sopuhintaan 260 €. Mainittakoon, että US Parts hinnoitteli kyseisen nokan 300 €:n arvoiseksi + nostajat 11 € / kpl ja toimitusaikakin oli ollut jotain käsittämätöntä....

[Riku]

Testataanpas ymmärsinkö oikein, eli jos halutaan tehdä kierroskone niin on parempi käyttää pieni intensiteettistä nokkaa? Kuulostaa ainakin järkevältä... Saitko samalla jostain tietää nuo intensiteetti arvot LT-1 nokalle? Olisi mukava verrata niitä joihinkin nykyaikaisiin nokkiin!

---------------------------
http://www.finnshark.com
-Corvette Registry of Finland-

[tapio]

Ohessa speksejä:

This mechanical flat tappet is used on the 70-71 Corvette and Camaro LT-1. It is a good all around street mechanical cam (ID#3972182). The duration at lash point in degrees (intake/exhaust) is 300/312; duration at .050" tappet lift (intake/exhaust) is 242/254; and maximum lift with 1.5:1 rocker ratio (intake/exhaust) is 435/455. Valve lash is 024/030 and lobe centerline is 116 degrees.

GM:n numerolla varustettua nokka ei enää tehdä, sen sijaan Federal Mogulilla on tarkka kopio siitä. Ohessa sen osanumero: F-M Speed Pro CS1145R

Eli kaiken kaikkiaan tämä on varsin hellä venttilikoneistolle, ei tarvita sikajäykkiä venajousia vaan vakiojousilla pärjää aina 6.500 RPM paremmalle puolen. Jäykät jouset syö nokkaa nopeammin, mutta se taitaa kuitenkin harrasteautossa olla sivuseikka...

Eli näin. Jossain vaiheessa moottoriin heitetään korkeapuristemännät sisälle ja LT-1 nokka perään ja katsotaan, onko vanhasta tekniikasta mihinkään.

[tapio]

Vielä yksi detaljitieto liittyen mekaanisten nostajien välysten säätöön. Ohessa vanhojen Corvette-spesialistien John Hinckleyn ja Duke Williamsin kehittämä metodi nostajien säätöön. Tämä metodi toimii moottorin ollessa kylmä ja pysähtyneenä, ei siis tarvitse liottaa itseään ja autoaan kuumassa öljyssä. Artikkelin lopussa on viittaus LT-1 nokkaan ja sen suositelluihin välyksiin, jotka poikkeavat tehtaan ilmoittamista arvoista.


"30-30" AND OTHER OEM SB SOLID LIFTER CAM VALVE ADJUSTMENT

By John Hinckley and Duke Williams


The traditional method of adjusting valves one or more cylinders at a time with each cylinder at TDC is fine for hydraulics and for most solid-lifter cams, but NOT for the factory "30-30" solid-lifter cam used in '64-'65 L-76 365 HP and L-84 375 HP (FI) Corvette engines (and in '67-'69 Camaro 302/290 Z/28 engines); this cam has VERY long clearance ramps that are .020" high, and at TDC for any cylinder, both the intake and exhaust valve for that cylinder are still on their ramps, NOT on the cam's base circle, which is why the Service Manual for all cars so equipped says specifically to set them "hot and running".

There is, however, a better way to adjust the valves with a "30-30" - you can set them "cold and not running" by setting the intakes at 90 degrees ATDC and the exhausts at 90 degrees BTDC - so the lifters are on the base circle, not on the ramps. This has been confirmed with cam lift/crank-angle diagrams, and I've done mine this way - results in a nice mechanical "singing" sound, no "clacking", it runs better, sounds better, idle is more stable, and throttle response is improved. Several other Z/28 owners have followed this procedure as well since we developed it, and all of them have seen the same positive results.

Set them cold at .026"/.026". The actual measured (stamped rocker arm) ratio at the lash points is actually about 1.37:1 (not the design 1.5:1, which is a max lift measurement), so the clearance ramp, which is exactly .020" high on the lobe, is all taken up at .0274" clearance; .030" clearance with the valve closed is too loose - the ramp ends/begins before the .030" clearance is taken up, resulting in the valve being lifted off and returned to the seat at greater than ramp velocity. This will contribute to valve seat recession, and can cause valve bounce at the seats at high revs - it will also be noisy.

You can adjust two valves at each 90-degree rotation point, starting at #1 TDC, turning the crank 90 degrees at a time seven times (measure and mark your balancer first at 90-degree intervals from TDC). Removing the plugs simplifies rotating the crank, but you were going to change them anyway, right? Proceed as follows:

TDC #1 - 8E, 2I
90 deg. - 4E, 1I
180 deg. - 3E, 8I
270 deg. - 6E, 4I
0 - 5E, 3I
90 deg. - 7E, 6I
180 deg. - 2E, 5I
270 deg. - 1E, 7I

Start at TDC #1, then rotate 90 degrees at a time, setting at .026" cold. If you like, you can then go back after you're done to each cylinder's TDC position and check clearance on that cylinder's two valves, and you'll find that they've closed up to .024", indicating that both valves are still on the ramps at TDC, as I pointed out in the beginning.

Trivia - the point of max inlet lift on the "30-30" cam is at 112 degrees, with a lobe separation angle of 114 degrees (angle between points of max lift, not the geometric center of the lobe - the lobes on the "30-30" cam are asymmetrical).


Addendum (May 19, 2003)

This procedure should also be used for the LT-1 cam. The exhaust is “on the ramp” at TDC. The inlet is not, but just barely. With the Duntov cam this indexing procedure may be used, or both valves may be set with the cylinder at TDC of the compression stroke, or all 16 valves may be adjusted at TDC #1 and TDC #6 as outlined in the 1963 Corvette Shop Manual. The Duntov cam has shorter ramps than the 30-30 or LT-1 cam.

This indexing procedure may be used with ANY cam to assure that the lobe is on the base circle, and MUST be used for cams with very long ramps.

The following inlet/exhaust valve clearances are recommended with the engine cold and not running. The difference between “hot” (engine idle speed) and cold clearance on a cast iron pushrod engine is negligible, so clearances can be set cold, which is more convenient. These clearances are computed by “factoring” the OEM recommended clearances by the ratio 1.37/1.5 to compensate for the actual rocker ratio of 1.37 at the lash point. The computed number is then rounded down. The factory clearances are derived from multiplying the maximum height of the ramp above the base circle by 1.5. When running hard, such as sustained WOT, the exhaust valve head will heat up considerably. About 80 percent of exhaust valve cooling is through the seat, but the stem temperature will increase, which will cause the stem to grow and decrease running clearance. This is why exhaust ramps are typically higher than inlet ramps – to allow for more stem growth and maintain some running clearance to ensure the valves fully seat. Since the inlet valve is cooled with every fresh intake charge, it’s temperature and clearance will remain more consistent over the entire engine operating spectrum.

The rocker arm nut should be tightened until a light drag is felt on the feeler of the same thickness as the recommended clearance. Then the clearance may be verified by inserting a .001” larger gage, and if it does not go the clearance is between the two gages, which is just right. Note that the inlet clearance specification for the 1963 Corvette was tightened to .008” to give a bit more effective duration. This does not need to be “factored” anymore. We recommend this tighter clearance for all 327s, and it is optional for 283s for a little more top end power though the effect may not be noticeable. Normal engine service will usually result in slight loosening of the clearance, and Chevrolet service recommendations from the sixties recommend a lash check every 12,000 miles as part of a normal tuneup.

Duntov cam (283) .010”/.015”
Duntov cam (327) .008”/.015”
30-30 cam .026”/.026”
LT-1 cam .021/.026”

Note: Clearances are listed inlet/exhaust.

[tapio]

Jatketaanpa tieteen tekemistä. Yksi corvetteforumin arvostetuimpia guruja Duke Williams on kirjoittanut oheisen vertailuartikkelin LT-1 -nokan ja uudemman rullanokan eroista 327-moottorissa varustettuna normaaleilla pakoputkistoilla ja pakosarjoilla. Duke on hehkuttanut LT-1 -nokan erinomaisuudesta niin paljon, että on melkeinpä pakko asentaa sellainen moottoriin. Seuraava artikkeli perustuu Engine Analyzer -simulaatioihin, eikä siis mitattuihin dynonumeroihin. Jos jollain on alkuperäinen LT-1 -nokka koneessaan, niin olisi kiva kuulla miten todellisuus ja simulaatiot suhtautuvat toisiinsa.

Artikkelin taulukon tabuloinnit on menneet vähän sekaisin, mutta koittakaa kohdistaa oikeat numerot oikeaan kohtaan. Mielenkiintoista luettavaa joka tapauksessa!

--

Engine Analyzer (EA) Summary Results

Engine configuration: 30 over 327 with SHP windage tray and pan, OE type forged pistons at .003” clearance

Bore and Stroke: 4.030” x 3.25”
Compression ratio: 10.35:1
Cylinder head: Vizard modified 186 flow data, 2.02”/1.5” valves, EA computed port flow efficiencies 46.6/60.7 percent, inlet/exhaust
(Reference: unmodified 186 Vizard port flow efficiencies compute to 42.1/39.7 percent; unmodified 462 (2.02”/1.60” valves) flow efficiencies from the Engine Analyzer library are 44/41 percent)
Camshafts: LT-1, 34-80/89-40 seat timing, .306”/.323” lobe lift, .023”/.029” lash, mild solid lifter;
Crower 00466, 37-69/76.5/36.5 seat timing, .357”/.372” lobe lift, aggr. hyd. roller lifter
Rocker ratio: 1.44:1
Induction system: Street dual plane manifold, reduced manifold heat, 585 CFM carb.
Exhaust system: Streamlined manifolds or 1.625” OD, 34” prim. length headers with 60” collectors
SAE gross cond.: production water pump only, no mufflers, corrected to 29.92”/60F
SAE net cond.: production water pump and clutch fan, 500 CFM exhaust flow at 25” H2O (results in no more than 3.0 psi exhaust back pressure), corrected to 29.6”/77F


Tq @ 2000 Peak Torque Peak Power Pwr @ 6500

SAE Gross, LT-1 cam 224 379 @ 4-4500 382 @ 6000 373
SAE Gross, LT-1 cam, headers 229 408 @ 4000 395 @ 6000 381
SAE Gross, Crower 00466 cam 225 375 @ 4000 364 @ 6000 311
SAE Gross, Crower 00466 cam, headers 232 413 @ 4000 387 @ 5500 323

SAE Net, LT-1 cam 208 347 @ 4000 340 @ 6000 329
SAE Net, LT-1 cam, headers 213 359 @ 4000 337 @ 6000 327
SAE Net, Crower 00466 cam 208 341 @ 4000 323 @ 5500 269
SAE Net, Crower 00466 cam, headers 215 357 @ 4000 329 @ 5500 268


Comments and observations:

Engine Analyzer (EA) accepts port flow data, however, only one data point in the mid to high lift range is used to compute a flow coefficient that is the ratio of actual port flow to isentropic flow of a port with the same dimensions. I consider this to be a weakness. A complete flow curve such as is accepted by DD2000/Dyno Sim is more likely to provide accurate air flow simulation over the entire range of valve lift.

DD2000/Dyno Sim conditions are essentially SAE gross, however, if manifolds are used there is no choice of manifolds with open exhaust. EA allows the simulation of either SAE gross or SAE net conditions by allowing the user to specify front end accessories, exhaust flow restriction, and correct to SAE net atmospheric conditions. The later is more representative of power as installed in the vehicle. In particular, exhaust back pressure can have a significant effect on output, and for a given amount of back pressure, high overlap cams typically suffer more. This may be an advantage for the LT-1 cam as it appears to have been developed for a street vehicle with a muffled exhaust system. It seems that every time I get into a serious comparison of aftermarket and LT-1 cams, the LT-1 cam is always superior when taking into account net power as installed in a street vehicle with a modest amount of back pressure, a clutch fan, and SAE net atmospheric conditions.

The EA program requires valve timing data at the lash points and computes maximum valve lift from input lobe height, specified rocker ratio, and lash (for mechanical lifter cams). Cam acceleration is taken into account by specifying lifter type. In the case of the LT-1 I specify a “mild flat solid” lifter and “aggressive hydraulic roller” for the Crower 00466 cam. DD2000/Dyno Sim requires SAE J604d timing, which are the timing points at .006” valve lift. Thus, the programs require different data for the same cam, and to account for actual rocker ratio in DD2000/Dyno Sim, the actual valve lift specified at 1.5:1 rocker ratio must be factored and clearance for mechanical lifter cams taken into account unless specified lift is net instead of gross, which is total lobe height times rocker ratio. These programs would be more accurate and consistent if they allowed actual lobe lift-crank angle data to be input directly. As they exist now, the allowed cam lobe data only approximates cam action and is, in my opinion, their greatest weakness.

Equivalent configurations usually show reasonable SAE gross correlation between the two programs in the mid range and high end, but EA consistently yields lower torque at 2000.

Vizard’s rework of the OE 186 heads leads to an interesting configuration. The final valve sizes are 2.02”/1.5”, but despite the “small” exhaust valve the improvement in exhaust flow is much greater (quite dramatic, in fact) than the improvement in inlet flow when comparing both the raw data and EA’s computed port flow efficiency.

EA performs a number of useful calculations such as effective overlap and dynamic compression ratio. The LT-1 cam has 4.9 sq-in-deg of effective overlap and a dynamic compression ratio of 7.32. The Crower 00466 numbers are 7.8 and 8.05 respectively. My opinion is that the Crower cam has too much overlap for a street engine with mufflers, and this hurts output. Even though the LT-1 cam has greater lash point duration, the early phased exhaust lobe and late phased inlet lobe reduce overlap to a moderate amount compared to an aftermarket cam of similar duration. The late closing inlet valve detracts from low end torque, but improves top end power. The Crower cam’s high overlap detracts from low end torque, and its earlier closing inlet valve detracts from top end power. At least according to EA, the Crower cam’s more aggressive acceleration and higher total lift are not enough to make up the difference. Also the LT-1 cam’s lower dynamic compression will either allow a higher static ratio or a more aggressive centrifugal spark advance curve.

EA also computes a “Mach index”, which is a dimensionless coefficient that takes into account port flow and valve timing. Beyond .550 the heads become increasingly choked. The LT-1 Mach index is .559 versus the Crower cam’s .526 indicating that the LT-1 is getting the most out of the heads, which are the limiting factor. Again, the higher Mach index with the LT-1 cam is due to the lobe closing the inlet valve later allowing inlet inertia to continue filling the cylinder farther beyond BDC at high revs compared to the Crower cam.

These programs can sometimes generate so much data that the user can be overwhelmed and confused. This exercise appears to have reinforced my opinion gained from many other analyses that these sixties vintage Corvette SHP engines are jewels, especially with the LT-1 cam. I recommend upgrading the rods to Crower Sportsman or equivalent for a bullet proof bottom end. The windage tray reduces windage losses at high revs and the larger pan and baffling help ensure that the pump inlet will always be covered. A true compression ratio of 10-10.5:1 should allow operation on premium unleaded fuel without having to resort to less than optimum timing at lower revs via either reduced initial timing or an excessively slow centrifugal curve. The LT-1 cam and OE valvetrain produce a strong mid and upper range without undue low end torque loss and are dead-on reliable. The limitation of these engines is head port flow, so no expense should be spared on massaging the heads including a multi-angle valve job.

Comparing the SAE net files with and without headers is quite illuminating. They actually reduce LT-1 peak power marginally, but add a little to the Crower cam. It appears that the benefits of headers are negated by a reasonable street exhaust system, so considering the usual headaches they bring – corrosion, high heat radiation to the engine compartment, spark plug wire burning and radio noise from elimination of the OE radiation and RFI shields, I don’t think they’re worth the cost and hassle.

The 2.5 inch exhaust pipes are good and are best combined with a low restriction muffler – something comparable to the OE “off road” muffler.

Duke Williams
2/14/04