L98 vs. LT1

[CTD4ME]

i need a rundown on the differenced between the two motors. major differences, strong points, weak points, aftermarket parts availability?

[corvette king]

here is some info on the lt1



Rebuilding the Chevrolet LT1 Engine, Doug Anderson, Automotive Rebuilder, September 1999

Thirty-seven years after the birth of the small block Chevy V8, the Generation II engine was introduced in the 1992 Corvette as the LT1. Although it shared many common dimensions, looked much the same and even had a few common parts, it was totally redesigned to provide more power with lower emissions and better fuel economy.

Compared to the 1991 Chevy 350 L98 with TPI, the LT1 made 20% more horsepower, got better fuel mileage, and had a much broader torque band with 90% of it’s peak torque available from just over 1,000 rpm all the way up to nearly 6,000 rpm.

GM Powertrain accomplished all of this by reverse cooling the engine so they could bump the compression ratio up to 10.5 to 1, tweaking the airflow in and out of the engine, and using sophisticated electronic controls for both fuel and ignition. This combination gave the LT1 300 hp in 1992 and ultimately led to the 1996 LT4 that used better heads, more cam timing, roller rockers and sequential fuel injection to make 330 hp .

Although the LT1 was only around for five years, there were two-bolt and four-bolt blocks, aluminum and cast iron heads, regular and H.O. cams that came with long and short dowels, and three different front covers. There was also the "Baby LT1," the 265 cid version that was the standard engine in the Caprice from 1994-‘96. With all that in mind, let’s take a look at this family of engines and see what goes where.

BLOCKS
350 - There are two blocks, one with two-bolt mains and one with four-bolt mains. They both have the same 10125327 casting number, so there’s no sure way to know which one you have until you get the pan off. However, if it came out of a Corvette, it should be a four-bolt block, and if it came out of anything else, it was supposed to be a two-bolt. GM used the two-bolt block for everything but the Corvette because it had plenty of strength and it weighed a little bit less.

265 - There was only one block used for the 265 cid version of the LT1. It’s a 10168588 casting that had the numbers "4.3" cast on the side, too. It’s real easy to spot if the heads are off because of the small 3.74" bore.
Getting the right cam in the right engine can be a little bit tricky because there were several variations over the years. There are essentially two different grinds used with two different snouts, depending on which distributor was used on the engine.

CRANKS
350 - The crank for the LT1 looks just like the one in the late 350 and has the same casting number 14088526, but it’s balanced for the lightweight pistons that were installed in the LT1. Be sure to keep these cranks separate so they don’t end up in a regular 350, and don’t ever use a regular 350 crank in a LT1. In fact, if you are short of LT1 cranks and don’t have a balancing machine in your shop, you would be better off using a crank from a 305 instead of a 350 because it’s actually closer to the balance specs for the LT1 crank.

265 - The 265 has it’s own unique crank with a 3.00" stroke. That’s the same stroke the original 265 had back in 1955; it’s funny how things go around and come back full circle. It’s a 10168568 casting.

RODS
350 - The original LT1 came with regular forged 350 rods, that were shot peened for localized hardness under the head of the bolt and nut. Powdered metal rods were phased in for the Corvette around 1994 and used in all of the LT1 engines by 1995. GM made the change because the powdered metal rods were cheaper to make and were much stronger than the GM high performance "pink" rods. In fact, they are supposed to be good for up to 450 hp. They are machined at the parting line so they can be reconditioned.

265 - The 265 rods are 0.240" longer than the ones in the 350. Both blocks are the same height, but the stroke for the 265 is 0.480" shorter, so the rods have to be longer to make up for half the difference. These rods can be identified by the single, raised dot on both sides of the shank.

CAMS
1992-’95 350 WITH ALUMINUM HEADS - The 1992 Corvette had a steel roller cam with a shallow hole in the snout that measured .450" in the front and tapered down to .240" at the bottom. It had a short dowel (.320") that was used to locate the timing gear and a hole with 16 splines in the center of the gear for the stub shaft that drove the early distributor. The 1993-‘94 H.O. cam had a few subtle changes, but all of the early H.O. cams are the same for all intents and purposes. They can be identified by the number "241" stamped on the barrel in front of the first lobe.

1994-’96 350 WITH IRON HEADS - The distributor drive was changed on the iron-headed motors only in 1994, so the front of the cam and the timing gear were changed, too. The cam had a pilot hole that was bigger and deeper (0.500" x 1.0625") and it had a longer (.685") dowel pin that stuck out beyond the timing gear to drive the new distributor. This iron-headed motor was used in the Chevy Caprice, Buick Roadmaster and Cadillac Fleetwood, so it came with a milder cam that improved low end torque and reduced valve train noise. These cams have the long dowel pin and either "600" or "779" stamped on the barrel of the cam in front of the first lobe.

1995-’97 350 WITH ALUMINUM HEADS - In 1995, the aluminum-headed motors got the late, pin-drive distributor, so there’s a second version of the H.O. cam with the big pilot hole (.500" x 1.0625" ) and the long (.685" ) dowel pin. Look for a cam with the long pin and either "242" or "705" stamped on the barrel in front of the first lobe.

1994-’96 265-INCH MOTORS - All of the 265 engines came with the later, pin-drive distributor, so they all had the later style cam with the big pilot hole and the long dowel pin. The 265 used the same mild cam that came in the iron-headed LT1. Look for the long dowel pin and either "600" or "779" stamped on the barrel of the cam in front of the first lobe.

CAM GEARS
The cam gear had to match the cam and the distributor drive, so there were two different gears used, depending on the year and the application.

The original cam had a small, tapered hole in the center and a short dowel pin. It was used with the cam gear that had the small hole in the center with 16 splines in it. It was connected to the distributor with a short drive shaft that was splined on both ends. The cam gear is a GM p/n 10128349. This combination was used from 1992-‘95 on the aluminum-headed motors.

GM had some problems with the early distributor due to both carbon tracking and moisture, so a new sealed distributor with a vacuum port was introduced on the iron-headed 265s and 350s in 1994 and used on all LT1s in 1995. The new distributor was located with a pilot shaft and driven by a pin, so both the cam and the gear were changed. The cam had a large, deep hole in the center for the pilot shaft and a longer dowel pin to drive the distributor.

The cam gear had a bigger hole and it didn’t have the splines that were found in the early gear. The pilot shaft for the distributor extends through the hole in the cam gear and seats in the hole in the cam; the distributor is driven by the long dowel pin that sticks up through the cam gear. The cam gear is a GM p/n 10206039.

350 ALUMINUM HEADS - There were two versions of the aluminum heads used on the Corvettes, Camaros and Firebirds. The later ones have less material around the top of the intake ports and weigh about 2- 1/2 lbs. less than the earlier ones, but they are identical otherwise. Look for a 10128374 and possibly a 649.

FRONT COVERS
The front covers have been changed three times, once because of the changes that were made to the distributor and once due to OBD II.

The original cover had three holes, one for the crank, a small hole (@ 0.70") for the water p

[corvette king]

ORIGINAL: CTD4ME

i need a rundown on the differenced between the two motors. major differences, strong points, weak points, aftermarket parts availability?

the l98At The Drag Strip
Back to carland, and some examples of how horsepower makes a major
difference in how fast a car can accelerate, in spite of what torque on your
backside tells you :-). A very good example would be to compare the current
LT1 Corvette with the last of the L98 Corvettes, built in 1991. Figures as
follows:

Engine Peak HP @ RPM Peak Torque @ RPM
L98 250 @ 4000 340 @ 3200
LT1 300 @ 5000 340 @ 3600

The cars are geared identically, and car weights are within a few pounds, so
it's a good comparison. First, each car will push you back in the seat (the
fun factor) with the same authority - at least at or near peak torque in
each gear. One will tend to feel about as fast as the other to the driver,
but the LT1 will actually be significantly faster than the L98, even though
it won't pull any harder. If we restate the horsepower formula, we can begin
to discover exactly why the LT1 is faster:

Horsepower x 5252
Torque = -------------------------
RPM

If we plug some numbers in, we can see that the L98 is making 328
foot-pounds of torque at its power peak (250 hp @ 4000), and we can infer
that it cannot be making any more than 263 pound feet of torque at 5000 rpm,
or it would be making more than 250 hp at that engine speed, and would be so
rated. In actuality, the L98 is probably making no more than around 210
pound feet or so at 5000 rpm, and anybody who owns one would shift it at
around 46-4700 rpm, because more torque is available at the drive wheels in
the next gear at that point.

On the other hand, the LT1 is fairly happy making 315 pound feet at 5000
rpm, and is happy right up to its mid 5s redline. So, in a drag race, the
cars would launch more or less together. The L98 might have a slight
advantage due to its peak torque occurring a little earlier in the rev
range, but that is debatable, since the LT1 has a wider, flatter curve
(again pretty much by definition, looking at the figures). From somewhere in
the mid range and up, however, the LT1 would begin to pull away. Where the
L98 has to shift to second (and throw away torque multiplication for speed),
the LT1 still has around another 1000 rpm to go in first, and thus begins to
widen its lead, more and more as the speeds climb. As long as the revs are
high, the LT1, by definition, has an advantage.

Another example would be the LT1 against the ZR-1. The ZR-1 actually pulls a
little harder than the LT1, although its torque advantage is softened
slightly by its extra weight. The real advantage, however, is that the ZR-1
pulls another 1500 rpm beyond the point where the LT1 has to shift.
There are numerous examples of this phenomenon. The Integra GS-R, for
instance, is faster than the garden variety Integra, not because it pulls
particularly harder (it doesn't), but because it pulls longer. It doesn't
feel particularly faster, but it is.

A final example of this requires your imagination. Suppose we can tweak an
LT1 engine so that it still makes peak torque of 340 foot-pounds at 3600
rpm, but, instead of the curve dropping off to 315 pound feet at 5000, we
extend the torque curve so much that it doesn't fall off to 315 pound feet
until 15000 rpm. (All of the moving parts would be made out of unobtanium).
:-)

If you raced a stock LT1 against this hypothetical car, they would launch
together, but, somewhere around the 60 foot point, the stocker would begin
to fade, and would have to grab second gear shortly thereafter. Not long
after that, you'd see in your mirror that the stocker has grabbed third, and
not too long after that, it would get fourth, but you'd wouldn't be able to
see that due to the distance between you as you crossed the line, still in
first gear, and pulling like crazy.

I've developed a computer simulation that models an LT1 Corvette in a
quarter mile pass, and it predicts a 13.38 second ET, at 104.5 mph. That's
close (actually a tiny bit conservative) to what a stock LT1 can do at
standard air density at a high traction drag strip, being powershifted.
However, our hypothetical modified car, pushing no harder than the stocker
(at peak torque) runs an 11.96, at 135.1 mph, all in first gear. It's also
making 900 hp, at 15,000 rpm.

Folks who are knowledgeable about drag racing know that any self-respecting
car that can get to 135 mph in a quarter mile will just naturally be doing
this in less than ten seconds. Of course that's true, but I remind these
same folks that any self-respecting engine that propels a Corvette into the
nines is also making a whole bunch more than 340 foot-pounds of torque.
That does bring up another point, though. Essentially, a more "real"
Corvette running 135 mph in a quarter mile (maybe a mega big block) might be
making 700-800 foot-pounds of torque, and thus it would pull a whole bunch
harder than my paper tiger would. It would need slicks and other
modifications in order to turn that torque into forward motion, but it would
also get from here to way over there a bunch quicker.

On the other hand, as long as we're making quarter mile passes with fantasy
engines, if we put a 10.35:1 final-drive gear (3.45 is stock) in our fantasy
LT1, with slicks and other chassis modifications, we'd be in the nines just
as easily as the big block would, and thus save face :-). The mechanical
advantage of such a nonsensical rear gear would allow our combination to
pull just as hard as the big block, plus we'd get to do all that gear
banging and such that real racers do, and finish in fourth gear.

The only modification to the preceding paragraph would be the polar moments
of inertia (flywheel effect) argument brought about by such a stiff rear
gear, but that is outside of the scope of this already massive document.

At The Bonneville Salt Flats
Looking at top speed, horsepower wins again, in the sense that making more
torque at high rpm means you can use a taller gear for any given car speed,
and thus have more effective torque at the drive wheels. Finally, operating
at the power peak means you are doing the absolute best you can at any given
car speed, measuring torque at the drive wheels. I know I said that
acceleration follows the torque curve in any given gear, but if you factor
in gearing versus car speed, the power peak is it. An example, yet again, of
the LT1 Corvette will illustrate this. If you take it up to its torque peak
(3600 rpm) in a gear, it will generate some level of torque (340 foot-pounds
times whatever overall gearing) at the drive wheels, which is the best it
will do in that gear (meaning, that's where it is pulling hardest in that
gear).

However, if you gear the car so it is operating at the power peak (5000 rpm)
at the same car speed, it will deliver more torque to the drive wheels,
because you'll need to gear it up by nearly 39% (5000/3600), while engine
torque has only dropped by a little over 7% (315/340). You'll net a 29% gain
in drive wheel torque at the power peak versus the torque peak, at a given
car speed.

Any other rpm (other than the power peak) at a given car speed will net you
a lower torque value at the drive wheels. This would be true of any car on
the planet, so, theoretical "best" top speed will always occur when a given
vehicle is operating at its power peak.

"Modernizing" The 18th Century
For the final-final point, what if we ditched that water wheel, and bolted
an LT1 in its place? Now, no LT1 is going to be making over 2600 foot-pounds
of torque (except possibly for a single, glorious instant, runnin

[corvette king]

with a squeaky-clean exhaust? A healthy dose of common sense, the right parts and an eye for details. In this particular case, the payoff is a car that, in full street trim (with a complete emissions-legal exhaust system in place and a full factory interior) has recorded quarter-mile times of 12.264 seconds at 111.95 miles per hour and exhaust emissions of 0.0% carbon monoxide (CO) and 34 parts per million hydrocarbon (HC). Those emissions levels are well below the typical allowable maximums of 1.2% CO and 220 ppm HC.

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configuration, the 350 cubic inch small block was rated at 230 SAE net horsepower. By 1991, the rating had risen to 245 (250 horsepower on coupes with 3.07 or 3.33 rear axle ratios). That’s a significant increase, considering the constraints imposed by full emissions controls. But with Fourth Generation Corvettes tipping the scales at over 3,200 pounds, 250 horsepower doesn't deliver drag strip heroics-- especially when owners of modified 5-liter Mustangs are a constant source of humiliation.
So what does it take to build an engine that can turn a 1985-91 Corvette into a quarter-mile terror
Is it stock or is it modified? It’s difficult to tell the difference unless you look very closely. The stock "Tuned Port Injection" ID plate on the throttle body would seem to indicate that this is a stock engine but ID plates can be easily changed.
CNC-ported 1988 and later Corvette aluminum heads with 190cc intake ports offer excellent power potential. In fact, they will flow more air than can be put through modified Tuned Port runners.
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[corvette king]

TPI Specialties cat-back exhaust system.
As is the case with any high performance engine, the foundation of one that's also emissions-legal is the block. Mike Osucha of MORE Performance began with a 1989 engine core and after disassembly and cleaning, machined it for zero deck clearance. He then bored and torque plate-honed the cylinders .030" oversize. The block was also align honed and detailed.
Osucha built one of his typical killer street engines, but since Tuned Port engines rarely see
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The engine responsible for this impressive performance was built by MORE Performance in Charlotte, NC. On the dyno, it cranked out 330 horsepower at 4600 rpm and 408 lbs./ft. of torque at 3900 rpm. Those readings aren't pie-in-the-sky dyno-only numbers. They fully relate to the real world because they were recorded with TPI Specialties emissions-compliant headers and dual Random Technology Super High Flow catalytic converters in place. The only difference between the dyno exhaust configuration and the one used in the car is the addition of a

[corvette king]

Heads, Inc. These castings are typically assembled with 2.00" intake and 1.56" stainless steel exhaust valves from REV, Inc. The springs, retainers and 1.6:1-ratio Pro Magnum roller rockers come from Comp Cams. After the heads were assembled, Osucha installed them using Fel-Pro gaskets (part number 1003).
Bolting a stock Tuned Port system to the intake side of the CNC-ported heads would have been analogous to installing a Geo Metro exhaust system on the outlet
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TPI Specialties offers both 52mm and 58mm throttle bodies for L98 and LT1 engines. Testing has shown that 350 cubic inch engines respond best to a 52mm throttle body. The airfoil up front is another flow-increasing component that maximizes the power potential of a Tuned Port System.
so if prodigious volumes of air and fuel don't flow into and out of the combustion chambers, power output will be restricted. Unless you've devoted your life to rotary, two-stroke or flathead engines, it shouldn't come as a shock that cylinder heads are crucial to any quest for serious power.
Osucha has found exceptional performance potential in original equipment 1988-1991 Corvette heads that have been modified by CNC Cylinder
Here’s something that’s rarely seen outside of a CNC-programming facility–a computer-generated profile of a port. After port modifications are flow tested and finalized, the port contour is translated into digital data. It’s then used to generate the cutting paths that CNC-machining equipment uses to make port modifications. CAD drawings like this allow the programmer to visually inspect the contour and make dimensional checks.
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[corvette king]

side. Something larger is required for adequate flow capacity and that something is a Big Mouth manifold, larger tube runners, ported plenum and 52-mm throttle body, all from TPI Specialties. This system was further enhanced with the addition of an air foil in the throttle body, and an adjustable fuel pressure regulator. (Both parts are available from TPI Specialties.)
One of the best all-around camshafts for L98 engines is the ZZ9 from TPI Specialties. It performs exceptionally well, yet is mild enough to keep exhaust emissions in line with inspection requirements in most states

[corvette king]

According to numerous sources who wish to remain anonymous, the Big Mouth manifold was named after TPI Specialties owner Myron Cottrell. Those same sources also noted that in the name of research and development, Cottrell has probably worn out more TPI-equipped engines than anyone else on the planet. Consequently, his maximum flow TPI system, which is 50-state legal, is tough to beat.
The final link in the air flow equation is the camshaft.
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[corvette king]

procedure, then rechecked timing and fuel pressure before making a banzaii pass.
As the dyno and drag strip figures indicate, the whole combination works extremely well. And the emissions test numbers demonstrate conclusively that you don't have to sacrifice high performance to achieve low exhaust emissions. For those Corvette owners looking to boost performance and having to comply with annual emissions testing, this may be the L98 buildup you’ve been looking for.
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A modified Mass Air Flow sensor is also part of the maximum air flow equation. Removal of the screens and fins can be a do-it-yourself modification if you have steady hands and machining experience. If you don’t, modifications are best left to companies like TPI Specialties.
In preparation for its trial by fire on the dyno and at the drag strip, Osucha filled the crankcase with five quarts of Red Line 10W-30 synthetic oil and installed a set of Bosch Platinum Plus4 spark plugs (part number 4419). After Osucha installed the engine on the dyno, he followed his usual break-in
A low restriction exhaust system is also an important ingredient in a recipe for maximum performance. This TPIS system in-corporates dual 3"-diameter pipes that merge into a single 3-1/2" pipe. Low restriction chambered mufflers complete the system.

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[corvette king]

1990 The ZR-1 (RPO ZR1) arrived as a 1990 model after much anticipation. At the heart of the ZR-1 was the 375hp LT5 engine. It was designed with the same V-S configuration and 4.4-inch bore spacing as the standard L98 Corvette engine, but was an otherwise new design with four overhead camshaft~ and 32 valves. LT5 engines were manufactured and assembled by Mercury Marine in Stillwater, Oklahoma, then shipped to the Corvette Bowling Green assembly plant for ZR-1 vehicle assembly.

For a limited time during 1990, dealers could order Corvettes destined for the new World Challenge race series. Merchandising code R9G triggered deviations from normal build, such as heavy-duty springs with FX 3. Owners could buy race engines from Chevrolet or build their own, and all race modifications were the owner's responsibility. Twenty-three 1990 R9G Corvettes were built.

An air intake speed density control system, camshaft revision, and compression ratio increase added 5hp to base-engines, up from 240hp to245hp (except coupes with 3.07:1 or 3.33:1 axle ratios which increased from 245hp to 250hp because of their less restrictive exhaust systems).

1990 Corvettes had improved ABS and improved yaw control.

An engine oil life monitor calculated useful oil life based on engine temperatures and revolutions. An instrument panel display alerted the driver when an oil change was recommended.

The RPO VOL radiator and B4P boost fan were not optional in 1990, both made unnecessary by 1990's more efficient, sloped-back radiator design.

Two premium 200-watt Delco-Bose stereo systems were available, the top unit featuring a compact disc player. To discourage theft, the CD required electronic security code input after battery disconnect.

The instrument panel for 1990 was redesigned as a "hybrid," combining a digital speedometer with analog tachometer and secondary gauges. A supplemental inflatable restraint system (SIR) with airbag was added to the driver side, a glove box to the passenger side.

The "ABS Active" light was removed from the driver information center.

Seat designs were the same for 1990 as the previous year, except the backs would latch in the forward position.

Chevrolet service departments returned LT5 engines to Mercury Marine for certain repairs. Customers had the choice of a replacement engine, or return of their original engine if repairable.