Mod Advice for L98 and LT1s

This page is something I'm writing only to save myself the trouble of writing an essay in every post on the forum. Its my usual response to threads such as "what cam for my 19xx?" or "what heads for my 383?" and the like. If you've been here for awhile then you know that threads like these come up once per week and everyone expects to get all their answers without having to look things up in the Search or Techtips.

So I'll start with several modification outlines that may work for each car, but moreso the L98.

The first questions you need to ask yourself:
What do I want to spend?
What do I want out of the engine when I'm done? (as in HP/TQ numbers or ET)
How much of the work can I do myself?

Once you've answered each of those questions, you can move forward with a plan. If you dont have a clear plan from the start, you'll end up with mismatched parts that don't fit well together. For example, you dont want to put a LT4 hotcam in an L98, and then add 227cc heads and the TPI intake on it. Youre choking the hell out of the cam with TPI and the heads are way too big for it. So, form your complete strategy first, it doesnt need to be in stone, but give it alot of thought.

Nitrous/Forced Induction

I'm not one to recommend using this stuff, but if your engine is built for it, go ahead. 1984 and 85 C4s come with forged TRW pistons (464644) from the factory, as holdover parts from the late C3 and these cars will be able to handle more generous amounts of juice. I would start low, say a 50 shot, and go from there. The forged pistons 'may' allow you up to a 150 shot before entering danger zone. Stock compression is low already because of the large 76cc chamber. Be sure to use higher octane gasoline when you are going to be running that much juice however. There is no way to definitely know what to use without a scanner to detect knock counts, and you want to use the lowest octane gasoline you can and not have them.

86-91 C4s use a cast piston, which is not good for juice, so I would only stay in the 50-75 shot area. 92-96's have a hypereutectic piston, which is not much better than cast. These engines also have much higher compression, so you better keep an eye on knock.

As far as building an engine for extended use with Nitrous, you must use forged pistons and rods, thats pretty much set in stone. These two are going to be taking the brunt of the punishment in the bottom-end. Cranks are very hard to destroy unless you lose oil pressure, and unless you want to spin it above 7000rpm regularly you dont need a forged one.
You would ideally want the engine to be 4-bolt main splayed (thats where a 2-bolt main block has 2 extra bolts drilled into the meat of the block, which is stronger than a regular 4-bolt), and have heads with larger combustion chambers and to have the final compression ratio in the 9.x:1 range.

Everything above also pertains to things like Superchargers, where boost psi is analogous to the shot of nitrous you want to run.

Nitrous write-up contributed by forum member 'neat'


Emissions equipment Overview:

The L98 is severely choked from the factory in terms of exhaust flow, so this is the #1 place to begin your modifications.
The main catalytic converter is huge monolith, largely 1970's technology, and is a major restriction on the engine's power.
The precat Y-pipe that started in 86 was added to help meet tougher standards in California and do not do very much work in cleaning up your exhaust fumes. 84-85 do not have precats in their front Y pipe.
The AIR system's primary function is to inject O2 into these cats on startup, which helps them to light up quicker and clean emissions soon after startup. Once the engine is hot, the AIR really does nothing useful. AIR pumps to the manifolds on startup, then switches to the main cat in closed loop.
EGR introduces exhaust gases back into the engine, which lowers combustion chamber temperatures, which itself prevents high NOx formation. I have another page that details EGR that you can read.

Adding longtube headers to an L98 is proven to give roughly 15hp back to the engine. This requires deletion of the exhaust manifolds and front Y-pipe. To pass emissions however, its best to replace the main cat with a new one. The new cats have improved flow design and will not be a huge restriction on the engine. I suggest Catco, Random Tech, or Car-Sound.

Deleting AIR: Alot of the manufacturers of headers make a set with the AIR pipes deleted. As I said above, AIR only functions on startup until the engine is warm, so you can safely remove it with no ill effects to emissions testing (provided you get the sniffer done when the engine is hot). You can cap off the lines on the headers or not, as the valves are supposed to be one-way only, likewise for the main cat. The AIR eliminator kit sold by many magazines does NOT give you any power, as the AIR pump is a fairly freely-spinning unit, but it does clean up the engine a bit and so is mostly a cosmetic mod. Any connections can be left alone as the L98 does not throw codes when AIR is removed.

One caveat: the AIR pump has a bracket which attaches to your alternator, this bracket is useful in maintaining the geometry and support of the alternator when the engine starts twisting at throttle. It is possible that you will crack and destroy your alternator without the upper support, though its uncommon with people who run stock manifolds with the rear support.

Header brands I would use:
Hooker 2149 (with O2 bung, AIR pipes, and EGR, ceramic coated)
Hooker 2151 (no emissions, no bung, uncoated)
Hooker 16720 Y-pipe can be used with either.
These are all 1.75" primary pipes.

Lingenfelter made a set at 1 5/8", but these are no longer in production.

I wouldnt mess with any other brands, especially Hedman.

TPiS allows use of the stock rear alternator support, Hooker does not. This will eventually destroy the alternator with the AIR removed.
Only difference in these brands is fit & finish otherwise.

Any of them should give the same 10-12hp gain, and the only difference is sound between them. I like Power Effects, Borla, and B&B which all have deep bass sound. Corsa is best for those who do not like resonance (loud droning sound at around 2000rpm usually).

Note: if you have a 91, the angle of the Y-pipe/muffler connection is different. You must purchase a complete catback system, unless you find mufflers made specifically for 91. Everything bolts to the cat the same way so any catback for 85-91 will work.

Intake system:

The TPI intake is limited in HP due to its long-runner design, which prevents it from feeding the motor at high rpm. The first step here is to decide exactly what kind of engine you want to you want a LT1-type, high-rpm screamer? you want something that looks fairly stock in design? you want something that does what the stock system does and adds alot more midrange power? ...who is going to retune the engine computer chip? You'll need to answer those questions before going further.

You cannot have the best of both worlds.

Any cam change will require a custom computer chip made for your engine.

Documented write-up of some problems with an install and the expected power gain

High rpm screamer:

Use the TPiS Miniram or the converted LT1 intake ( This engine will lose alot of the low-end power you have now and may only begin to wake at 3000-4000rpm. The intake should flow to over 7000rpm without a problem.

The bottom end: Needs to have lighter components, the best parts available for bearings, rods, crank, and pistons. Forged is best for everything. Better have a new oil pump put in too. A larger, deeper pan may or may not be necessary.

Top-end: You'll need to get heads with larger ports as they are required to feed the engine at high rpm. 185-200cc would do for a 350. Also, lighter components are best here, with titanium retainers and 10* locks. I think its best to pick springs that are slightly stronger than what is recommended for the camshaft, just so you dont run into float.

You will need a rev-limiter for this engine, L98s fuel cutoff is set at 10,000rpm. MSD Digital 6 ignition has one built in, and it will boost the HEI system where it begins to fail at high rpm.

Rear axle: A high-spinning motor will need more rear gear to get you to the power zone, so something in the way of 3.54/3.73/4.10 would be best. This, combined with gas mileage, is why I would not use this strategy for an automatic. If you do have an automatic, you will need a higher-rpm stall converter to go along with all this.

Camshaft: Many selections here, each with their own rpm ranges. The only thing I really pay attention to is duration @050", so something in the 220/220 range is where you want to be for a driven car (slightly smaller or larger should be ok). This leads to TPiS' many cams, ZZ9, ZZ9X, ZZ409..CC305, CC306..along with the usual LT4 hotcam. Just remember, idle gets worse with larger duration and smaller LSA. Larger engines can handle this better though.

Modified TPI-stock looking:
This may only get you to the 350hp range, and not further than 400hp.

Bottom-end: requires nothing special. Just be in good working order.
Top-end: Likewise, though that depends on year. 85-86E have iron heads, which are terrible. Too heavy, too small a port, and the chamber is too big. (161cc intake, 62cc exhaust, 76cc chamber, 1.94/1.50" valves) 86L-87 have '128 aluminum heads, which are better and can support to the 300hp range unported. 88-91 has '113 heads which should get you to the 350hp range unported. Read my page on these heads. If you have the earlier models, and want to replace your heads, you might want to look for a head in the 170cc-180cc range. If you have stock '113s, just get them ported and a 3-angle valve job done along with new springs and better valvetrain components aluded to previously.

Rear axle: needs nothing. For automatics the best bet is to go to 3.07 in a D44 axle. Lower gear would not be good with TPI.

Tntake: You will need to port the plenum, add larger runners (AS&M, Edelbrock, Lingenfelter), and a larger base manifold (add TPiS to those mentioned). Port matching the manifold to the heads is a good idea.

Camshaft: to get past ~300, this is needed. I'd recommend the TPiS ZZ9 or Accel 211 cam for this strategy, even if you leave more stock than what i've suggested to change above, these are suited to the TPI's general design.

Automatic transmission: its best to give it a stall in the 2200-2400rpm range. 2000rpm comes stock on 85-86, so you dont need to change anything.

Results from Vic89s car:
Track Times
Bone Stock -> 13.87@98.60 MPH (60 degs , 2.0 60')
With Crank Pulley -> 13.74@99.56 MPH (60 degs , 2.0 60')
With DR's, TB Bypass , K&N , Air Foil , Cut Lid
-> 13.34@102.20 MPH (40 degs,1.85 60' )
With Air Pump Elim , 12" GM Convertor , Ported MAF ,160 stat
-> 13.16@102.04 (65 degs , 1.76 60')
With TPIS catback ->12.85 @104.90 (45 degs , 1.74 60')
With Accel lower Manifold ->12.96 @105.18 (45 degs, 1.79 60')
With AS&M runners ->12.81 @107.09 (65 degs, 1.73 60')
With Hooker headers he made 271rwhp and 345rwtq

Finally, the Superram intake:

This does what the TPI does well, and boosts the midrange and top-end well above what TPI can do, which is why I like it best. I have a separate page on installing it but I'll give an overview here.

Bottom-end: nothing special
Top-end: will need at least the '113 heads, as this is what Lingenfelter uses (in ported form) on their engines. Preferably you'd pick 180-190cc heads for the 350. Better valvetrain components would be nice, but can just slap this on just like the TPI-looking mods. It would want a chip retune however.
Rear axle: see above.
Automatic transmission: slightly higher stall than above, 2400rpm would be nice.
Camshaft: designed for use with the Accel 219. Though you can go with something in that ballpark.

Holley Stealthram

This particular HSR has the custom plenum already.

I wont write up recommendations for using this intake because it does NOT fit under a C4's hood. You'll have to get a custom plenum box made, or go to a high-rise hood. In addition to these links, you can try running forum archive searches for the user "grumpyvette", he used to champion the intake years ago and has disappeared since.
Installation of the HSR into a 3rd Gen TPI Camaro.
A thread on the HSR from
A thread on the correct MSD distributor for L98s.
Another thread on the same subject
HSR installation notes

A comparison between ZZ9 and the 219 camshaft from a forum member:

This is with a STOCK chip program.

"few months ago I posted my results of my dyno runs with my Superram and ZZ-9 cam. I then mentioned that I would change to the 219 cam and rerun the dyno tests to see what the diferences were. From dyno test to dyno test I made two changes to the car that may be effecting the HP changes from the cam, I changed to a dana 44 rear end with 3.45 gears from my dyna 36 with 3.07 gears and I changed the cam from the ZZ-9 cam to the 219 cam. Other then that the car is exactly the same.
The ZZ-9 cam made 311 RWHP at 4700 RPM and 376 RWTQ at 3700 RPM and had run a best ET of 12.36 at 111 MPH with the 3.07 gears. With the 3.45 gears and dana 44 this combination has run a best ET of 12.30 at 112.5 MPH. The power curve with this combination was very broad and the car pulled hard to 5800 RPM.
The 219 Superram combination made 316 RWHP at 5400 RPM and 405 RWTQ at 2700 RPM??
Some interesting comparisons, the 219 cam made more low end HP and TQ until 3500 RPM where the ZZ-9 cam passed it and pulled away with a max difference of 15 RWTQ at 3700 RPM.
The ZZ-9 cam continued to make more power then the 219 cam until 5000 RPM where the 219 again overtook the ZZ-9 and pulled away to a new higher HP peak of 316 RWHP at 5400 RPM.
Another interesting item was that the 219 combination had two HP peaks on my car. It peaked at 316 RWHP at 5400 RPM and then dropped backed slightly to 312 RWHP and then repeaked at 316 HP at 6000 RPM before falling gradually to 295 RWHP by 6600 RPM.
Overall I was disapointed by the cam change results, I was expecting to see a greater gain than 5 RWHP at the peaks. I am not sure if this 219 combination will be any faster then my ZZ-9 combination. The 219 combination has more low end torque but less mid range power and then more top end power. The mid range drop in power is right where the engine RPM drop to when it shifts gears, I do not know if this drop in power will be made up by the higher peak power and increased RPM range of the 219 combination.
When I dynoed the car I had a vacuum gauge on the intake and a pressure gauge in the exhaust system to check for any possible restriction in the intake and exhaust system. The intake vacuum never went over 0 inches of water at full throttle, and the exhaust would reach a max reading of 3 PSI at 6000 RPM. Maybe the exhaust is costing me some power but I have been told that exhaust readings of less then 5 PSI are not really costing much power. "
1986 Coupe, auto, 3.07 gears, 1987 355, 10.2 to 1 compression, SLP cold air, gutted MAS, 52 MM throttle body, ported plenium, Superram intake, TPIS BASE, 23 degree Trick Flow Heads, ZZ-9 cam, TPIS headers, SLP 2400 Stall, Cheap and easy free mods, stock chip, daily driver


Now if you have an LT1/4, you must effectively copy the same thing said above about the Miniram intake. The intake design is basically equivalent, short runners have the ability to feed a motor very well at high rpm, since the air doesnt have to travel far to get to the chamber.

Nathan Plemons' dyno results show most of the popular modifications to make and their power gains, so you should read that just after this.

Most effective mods:
Gears-it doesnt matter if its automatic or 6-speed, an LT1/4 needs gears. Gears enable the car to get to the higher rpms where the LT1/4 makes its power, much quicker. This results in much lower 1/4mi times and improved 0-60.
Yes it hurts top speed, but who drives at 160mph?

For the manual, 4.10s would be good, possibly lower (higher numerically) depending on your exact cam choice if its not stock. For stock and most street applications, 4.10 would be best. It'll give you a good use for 6th gear without costing mileage, and it'll absolutely roast the tires off the line.

For the automatic, 3.54 or 3.73 is best. The deciding factor between the two, for a street-driven car, is fuel mileage. A highway driven car is going to lose at least 2-3mpg on average from 3.73s in 4th. (if you want to get an idea of how it could be, drive at 70mph on the highway in 3rd) But, 3.73s will be faster 0-60, and in the quarter mile times, so they are optimum for a city car or drag-only car. 3.54s lose less mileage, and you'll still get the jump off the line that the LT1 needs. Combined with a higher stall torque converter, you may be looking at a cut of half-a-second in the quarter mile.

Torque Converter-The 'Ideal' lockup rpm is 500rpm below the torque peak of the engine, so for a drag-only car you could easily run 3500+rpm TCs and really smoke the tires or cut even more time off the 1/4mi. But, from personal experience, anything that high is going to really drive you nuts, because the transmission will feel like its slipping (which it IS). So, for a street car, something like 2400-2600 would be best. 2800 for a lightly drag raced car.

Those are the very first mods a LT1/4 driver should do; long before delving into the engine. The camshaft choices all really need the gearing to have been fixed beforehand.

Now, the choices can range greatly depending on power goals. I'm going to restrict my recommendations to street-driven cars that might be raced on weekends or from stoplights.

Camshaft-the ever-popular LT4 Hotcam is the one recommended most often, for good reason. The kit from GM includes about everything you'll need to install it. I always give one caveat though, the springs in an LT4 are made for the lighter valves it carries, so they will not be as stout with the LT1 valves....this would force me to buy 'slightly' stiffer springs than are recommended or come with the kit. Hotcam CAN pass emissions testing with the proper tuning. Its about as big as you can go and still pass though. It has a slight lope that can also be reduced with tuning. You dont have to worry about heads/intake choking this one so much, but it is advisable to put headers on and do the heads.

Do you really need LT4 heads and manifold?
No, the LT1 heads can be ported to outflow the LT4 out-of-the-box heads and the manifold can be port matched.

My personal choice for cams would be the CC306. Its downright nasty and can have a good bit of lope. It's
powerband comes in a bit higher than the hotcam. This particular cam will need the porting to have been done, and the exhaust system.