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!
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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
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