hi all,

I am new to the lt1 engine and was wondering who is lt1/4 guru here now. i believe tjwong was one time i maybe wrong. drop if this me a line i would like to chat.

 

There are some who know more than others, but no one knows everything. So, actually it's sort of a collective effort where everyone contributes.

Of course, as with most things about our engines, there will be areas of disagreement, differing opinions and even differing experiences. So sometimes you'll get different and conflicting responses on the same topic.

Give us an idea of what areas you're concerned about and we'll kick it around.

Jake

West Point ROCKS!

 

thanks jake,

just the general differences between the l98 and lt1. At this point not knowing much about them its hard to know what to ask.lol

 

Short version.
The L98 is a conventional SBC with a long runner intake (TPI)
The LT1 has superior heads and a reverse flow cooling system that puts the coolant to the heads first as well as a distributer mounted at the front behind the waterpump; both driven off the cam.Also has short runner intake better suited to higher revs.
Obviously the LT1 parts only fit an LT1 but the blocks are the same physical dimensions, use the same internals , crank , rods , etc ( LT1 cam has a longer pin to drive the dist and waterpump )

 

L98:
The compression ratio in 84 into 86 was 9.0:1 and during the 86 MY was boosted to 9.5:1 for the duration of the L98's production.

The intake runners on the L98 were a carry-over from the (anemic) 305 cid motor. In the L98, as in the 305 cid V8, the runners characteristically boosted torque early, peaking at around 3800. However, the torque curve was rather "peaky"; rising rapidly from off idle to peak at 3800 before falling off precipitously after 4000 rpm.

However, max torque was a respectable 370(ish) ft# which give a good "seat of the pants" rush in normal street driving, and certainly proved itself in SCCA Challenge racing!. But, due to the rapid decline in torque above 4000 rpm, it forces the shift point to around 4500 rpm for max acceleration (i.e., drag racing). (To spread this torque out a bit, the L98s typically were connected to rather "tall" rear gears; e.g., a 3.07:1 was considered the "performance" ratio, back in those days)

Fuel injection on the L98s was PFI or Port Fuel Injection, i.e. "batch fired" or "ganged" together so they all fired simultaneously. In contrast, the LTx motors use SFI or "Sequential Fuel Injection", where the injectors are fired only as the intake valve is opened.

The LT1 was significantly different in many ways, hence it (and the LT4) is referred to as the Generation II SBC. As Rodj pointed out, the intake runners were shortened, fueling changed, coolant flow was routed to the heads first - allowing the compression to rise to 10.25 in the '91 MY, and then to 10.5 on all later LT1s. The heads were improved, a true dual exhaust was implemented, and a cam profile change resulted in an exceptionally flat torque curve, which went to 90% at about 1500 rpm and was relatively flat out to about a peak of 330 ft# (and increased to about 340# in the last 3 years of production) at 4800 before gently dissipating. This, IMO, resulted in one of the best general driving/performance engines of it's time. (You can "stick your foot in it" anywhere on the tac and the LT1 leaps into action...no "ifs, ands or buts!") And, because of the higher rpm potential, the M6s were typically driving a 3.54 (or was it a 3.45?) rear gear, increasing the torque at the rear wheels by about 15% over L98 car's 3.07 gears - effectively negating the slight peak torque advantage the L98s had over the LT1s at the flywheel (making the LT1 equipped cars a significant overall performance improvement over stock L98 cars in most cases).

As for the LT4s, "more of the same" of what made the LT1 perform was used to "tweak" the LT4. As for torque, it remained essentially on the same track as the LT1s, except with the higher compression, warmer cam, bigger intake runners, improved heads and increasing the rocker ratio from 1.5 to 1.6:1, the torque curve extends that of the LT1 by about 300 rpm, resulting in the greater HP rating (conservatively pegged at 330 fwhp).

And, the LT5 is a whoooole different BEAST altogether...But, I digress...

P.

 

As with most of GM's builds, it can be a little confusing to generalize because the LT1 was used in other models, not just Corvettes. So, when installed in, say a 'F' body (Camaro/Pontiac, etc) some differences in mounting, etc. can exist.

But generally:

You'll read about "Reverse cooling" when referring to the LT1/LT4. What the Reverse part is that GM designed the heads, block and cooling system so that coolant flows to the cylinder heads first then to the block. Since the coolant would have just left the radiator, it's cooler.

So with cooler coolant flowing to the heads, the CR could be raised a bit without running the same risk of detonation. The downside is that change made the L98 style blocks and heads not compatible/interchangable with the LT1s. The coolant passages are different, so L98 heads can't be used on LT1s and vice versa. The LTx engines also have a coolant steam tube running from one head to the other, mounted on the back of the heads, near the firewall.

The blocks, although dimensionally the same, are different too. The distributor was moved from the rear of the engine near the firewall to the front of the engine under with water pump. It's referred to it as Opti-Spark. The Opti is nothing more than a distributor mounted horizonally instead of vertically and mounted at the front of the engine instead of the rear. However, that move has become a sore spot with many.

Another change to the distributor is that the coil and ignition module were removed and mounted on the front of the passenger side cylinder head, at least on Vettes. At short coil wire is used to connect the coil to the Opti. There's a short electrical harness connecting the other.

Placing the Opti below the water pump has caused many problems over the years especially if the water pump springs a leak. The Opti is susceptible to moisture and washing the engine, driving in high water, etc., can cause it to fail. The later designed Opti's attempted to address this by modifying it with two small diameter rubber hoses, one connected to an intake vaccum source and the other to the bellows (between the MAF and throttle body.) The idea was to allow engine vacuum to suck out any moisture that may get inside the Opti.

Someone can correct me if I'm wrong, but I believe the first LT1s is a batch fire system and later years are sequential fire. Sequential systems only fire one injector at a time; fired in the order of the firing sequence. The sequence is the same as with other SBCs 1-8-4-3-6-5-7-2.
Another thing I've never seen discussed is the later LT1's use of both MAF (Mass Air Flow) and MAP (Manifold Air Pressure) sensors. On L98 type engines, it was either one or the other. Why GM went that route, with both sensors, is a mystery to me.

Then you have the variant, the LT4. There are significant differences between the LT1 and the LT4. GM lists the differences account for a 30HP increase for the LT4. 300 Vs 330. Some question that, stating that the LT4 actually makes more. I'll edit this in a minute to include a link to a site that lists all the major differences between the two engines. The LT4 no longer has EGR. Supposedly the additional over-lap of the cam made the EGR unnecessary.

http://www.grandsportregistry.com/lt1vslt4.htm

Balancing remains the same, with the damper being neutral and the flex-plate/fly-wheel having a weight.

LT1/LT4 water pumps are different and don't use the pulley/serpentine belt to drive it. It's driven off the camshaft - hence the longer dowel pin in the cam.

Other than the two sensors already mentioned, the other sensors on the LT1 perform the same purposes, although in different locatons and are LTx specific. For example, the coolant temperature sensor used by the engine's computer (now called a PCM instead of ECM) is mounted on the water pump instead of the intake manifold. Later LT1s have two knock sensors, one on each side of the block and (I don't know about all the different model years) have two catalytic convertors.

The PCM is now under the hood, driver's side near the firewall, instead of under the passenger side dash board. Later year LT1s no longer have the removable PROM, so re-programming is done by directly plugging into the PCM instead of just removing the PROM and re-programming it.

Some parts are interchangable with L98s but most aren't. The 96 LT1/LT4 are 0BD II and many, if not all of those engines don't have to be tail piped for emissions testing. CA and others may be different, but here in TX the tester just plugs into the link under the dash.

The cam specs are different, even between the LT1 and LT4, but I'm pretty sure the rest of the valve train parts are interchangable with factory roller cammed L98s.

I may have missed some differences and any mistakes. I'm sure, will be corrected by others, LOL.

Jake

West Point ROCKS!

 

Early LT1's were batch fired(92-93), 94-97 were sequential fired, 94-97 idle better and from my experience they were smoother accellerating.
LT1's cats are much improved, so they don't limit hp as much as L98's.
The Optispark on LT1/4's provide better spark control and with the LT1/4's better ECM/PCM you get better timing and fuel control, which improves MPG and HP.
The opti has it's downside, it sits right under the waterpump's weep hole. But you can fix this easily by attaching a hose to the hole and flowing the water down past the opti. Some people say the opti is not worth the cost, but w/preventive maintenance and care it will a very long time.
Sorry, don't know enough about the L98 to tell you much about it.

PS: learn to diagnose problems, don't throw parts at the problem and hope it fixes the problem. Suncr is the a/c guru, petek is trannies and toptech6 knows alot about engines in general (ex mechanic).

 

Thanks for all the info guys. There seems to be alot of pro's with the engine and not alot of cons.From what i gather with some minor mechanical upgrades and some bolt ons the
lt1 can be running right up there with an lt4. Wish i would have known that before i dumped a bunch of money in the my 85 to stroke it and get here breathing properly.

once again thanks all for the info.

Chris