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Improving the L98

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Old 05-03-2018, 03:13 PM
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Space387
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Default Improving the L98

I'm working with a 1990 L98 and would like a little more out of it. I cant help but compare it to the firebird I daily. It has a 94 LT1 with an LT4 hot cam, mild ported heads (removed the casting flaws and evened out the sizes) and a thinner head gasket(0.026) not looking for big numbers mainly a bit more punch low end and less fall on your face over 5k. Ideally I'd like to keep under 2k for a price and I will do the work my self. I do not want to have to bore or stroke the engine.

if I gather the general consensus about the L98 the issue is in the intake. I played with the thought of an LT1 I take but I'm up for any other suggestions.
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Old 05-03-2018, 03:23 PM
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Tom400CFI
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Originally Posted by Space387 View Post
I'm working with a 1990 L98 and would like a bit more punch low end and less fall on your face over 5k.
I don't think you're going to meet both of those goals. More over 5 k is pretty easy; short(er) length runner intake.

"More low end punch" is going to take more compression or more cubes.
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Old 05-03-2018, 03:32 PM
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Originally Posted by Tom400CFI View Post
I don't think you're going to meet both of those goals. More over 5 k is pretty easy; short(er) length runner intake.

"More low end punch" is going to take more compression or more cubes.
This engine has 107k on it right now. As much as I dont want to I may concede to having it bored during a rebuild and build it for higher compression. Do we know what the factory ratio is? I have heard a slew of different values.
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Old 05-03-2018, 03:39 PM
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Originally Posted by Space387 View Post
This engine has 107k on it right now. As much as I dont want to I may concede to having it bored during a rebuild and build it for higher compression. Do we know what the factory ratio is? I have heard a slew of different values.
If you're going to refresh, put a stroker in it. That will me a a REAL difference in low end tq and general drivability and punch. You can't do much with compression b/c the engine is already on he verge of detonation with the stock compression, chamber design and operating temp. I think stock compression for a '90 is ~10.25:1.
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Old 05-03-2018, 03:51 PM
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Thanks Tom I'll keep that in mind. Are there any suggestions for intakes?
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Old 05-03-2018, 04:46 PM
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Tom400CFI
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I have all kinds of suggestions for intakes....but how handy are you?

I think and LT1 intake is great, cheap, pretty easy.

Miniram is the same effect but way easier, but more money

HSR has proven to be a winner, but won't fit 'Vette hood w/o mods.

My favorite "C4" intake in the world is the SLP T-Ram intake....direct bolt on, but $$$$....IF you can even find one.

I like different, so I'm always looking for T-ram's for sale. Also, there are some really cool Marine intakes that would work great, look different/unique, that I look at occasionally.

All depends on what suits you, your ideas and your abilities, the best.
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Old 05-03-2018, 05:00 PM
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Tom you have been a world of help. The idea of the LT1 intake does sound interesting as I too like to use a different approach. Do you know if there is significant difference from the design (aside from mounting bolts) of L98 vs LT1 heads? I'm wondering if it would be worth grabbing a full top end and using the vent ports as coolant out to a remote thermostat.
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Old 05-03-2018, 05:34 PM
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Vortec heads, vortec tpi lower intake (baseplate), and a mild cam.

Add some long tube headers and it will certainly make more power than your LT1.

Last edited by Krusty84; 05-04-2018 at 02:08 AM.
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Old 05-03-2018, 05:44 PM
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It's probably not worth doing LT1 heads. You'd need to weld the water passage shut, the runs from the block to head, then mill flat. You could likely get about the same results from a ported 113 head as you could from a ported LT1 head.
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Old 05-03-2018, 05:48 PM
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LT1 heads are pretty similar to the D-port on the 90 already, aside from the cooling setup. Just P&P the D-ports unless you want to build a high horse motor.
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Old 05-03-2018, 06:15 PM
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..... FIRST fuel injection intake manifold and a nice cam ... the FIRST can be bought new for around $1000 ... used much less (6-700) ... and it comes with a healthy breathing mono blade throttle body ... that plus some long tube headers and you get what you seek in your 1st post .....
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Old 05-03-2018, 06:38 PM
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If you want more low end punch from an l98 then you don't really understand the l98. Like Tom said, the only way for more low end is to stroke the motor.

What no one has mentioned that I can tell is when you change your intake manifold you are going to LOSE your low end torque, a lot of it. What you lose you'll gain on the top end in horsepower. An lt1 manifold doesn't have provisions for a distributor or a water outlet so you'll have to drill and weld the manifold in the correct spots or buy one already modified.

What it really sounds like you want is an enlarged tpi setup. It will let you breathe a little more up top without killing your low end like an lt1/miniram. Parts are not as easy to find but look for an aftermarket tpi base and larger tpi runners. Have the base and your plenum ported and roll with that.
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Old 05-03-2018, 06:43 PM
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Originally Posted by C409 View Post
..... FIRST fuel injection intake manifold and a nice cam ... the FIRST can be bought new for around $1000 ... used much less (6-700) ... and it comes with a healthy breathing mono blade throttle body ... that plus some long tube headers and you get what you seek in your 1st post .....
There you go, look into this. I have not seen these before.
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Old 05-03-2018, 09:29 PM
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Just to add another option here is a fast intake which is really a single plane with injector mounts: http://www.fuelairspark.com/fas/smal...uel-railshtml/

Add a intake elbow from Nitrous Outlet : http://www.nitrousoutlet.com/perform...ke-elbows.html

Now you have a low restriction intake for higher RPM power. If you want to lower the powerband some then have a good dual p!and intake like an Edelbrock RPM converted to EFI.

Ya know I recall the L98 motor as a 350 with a bunch of potential. Forged crankshaft, 4 bolt mains with forged (or maybe hyper) pistons and aluminum heads. Having those heads ported with bigger valves and install a real performance cam should easily get you north of 400hp. What I'm saying is you can go a whole lot faster w/o touching the bottom end.

Good luck.
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Old 05-03-2018, 11:50 PM
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How about using an LT1 engine with L98 intakes with some Iso-Butanol fuel? Should wake things up a little?

I'd avoid the HSR unless you are willing to do body work. You'd have to cut a hole for the intake and maybe have a hood scoop on it. BTDT. Hood scoop will not match the hood so you are going to have to do both. Whether the hood system will match the rest of the car depends on how nit picky you are.

The other way is to somehow bring down the height that someone has also done BUT not sure if it affects characteristics.
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Old 05-03-2018, 11:52 PM
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Originally Posted by thurman_merman View Post
What no one has mentioned that I can tell is when you change your intake manifold you are going to LOSE your low end torque, a lot of it. What you lose you'll gain on the top end in horsepower.
True but OTOH, if you have massive low end torque that is roasting the tires every time you start from a stop light, maybe trading it in for something higher up might not be a bad thought?
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Old 05-03-2018, 11:59 PM
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Originally Posted by cardo0 View Post
Just to add another option here is a fast intake which is really a single plane with injector mounts: http://www.fuelairspark.com/fas/smal...uel-railshtml/

Add a intake elbow from Nitrous Outlet : http://www.nitrousoutlet.com/perform...ke-elbows.html

Now you have a low restriction intake for higher RPM power. If you want to lower the powerband some then have a good dual p!and intake like an Edelbrock RPM converted to EFI.
The first option isn't going to help low end at all.

Neither is the second. "Dual planes" don't work the same with EFI, that they do with carbs. Since you have EFI, you don't need draw and velocity through the venturies....'cause there are none. Don't put a dual plane on an EFI engine, please.



Originally Posted by cardo0 View Post
Ya know I recall the L98 motor as a 350 with a bunch of potential. Forged crankshaft, 4 bolt mains with forged (or maybe hyper) pistons and aluminum heads. Having those heads ported with bigger valves and install a real performance cam should easily get you north of 400hp
While I agree he can go way further w/o getting into the bottom end, there are no forged cranks in L98's and only '85's had forged pistons. The rest had cast. As said earlier, ported head would help.
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Old 05-04-2018, 01:10 AM
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Originally Posted by Tom400CFI View Post
The first option isn't going to help low end at all.

Neither is the second. "Dual planes" don't work the same with EFI, that they do with carbs. Since you have EFI, you don't need draw and velocity through the venturies....'cause there are none. Don't put a dual plane on an EFI engine, please.




While I agree he can go way further w/o getting into the bottom end, there are no forged cranks in L98's and only '85's had forged pistons. The rest had cast. As said earlier, ported head would help.
I wasn't trying to help his bottom end with a single plane intake but giving him a bolt on alternative that "it won't fall on its face over 5k rpm". But on the other hand how much better will the engine perform with a single plane, better heads and a bigger cam none us know until its tried. The increase in power over all may overshadow what a restricted TPI can make even at low rpm. Low end power won't disappear if the correct cam and compression are used.

And I disagree a dual plane with longer runners won't help low rpm torque. If that were true then dual plane intakes would never have been so vastly popular while more difficult/complicated/costly to manufacture. It seems your confusing carburetor sizing or venturi sizing with runner length. While the vacuum "signal" is greater from a duel plane intake that still has nothing to do with pulse wave reflection of the intake valve back and forth through the runner. But since reversion is reduced by the separation of the "banks" inside the plenum area it can only help the intake air charge whether it's carrying fuel or not. If you have ever measured the shorter runner on the inside/center runners of a Victor Junior intake which are only 5 and small fraction inches long. While a common dual plane are twice that length. And all the runners of a single plane converge directly at each other inside the plenum. The single plane advantage begins once airflow increases and the less restricted flow path combines with the faster intake pulses generated by increased rpm.

Good night
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Old 05-04-2018, 01:35 AM
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Originally Posted by aklim View Post
True but OTOH, if you have massive low end torque that is roasting the tires every time you start from a stop light, maybe trading it in for something higher up might not be a bad thought?
Oh I agree, that's why I went with a miniram. OP wants more punch on the low end AND more top end, I wanted to point out that you trade low end torque for top end power.
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Old 05-04-2018, 04:07 AM
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I do need to clear a little bit up. My goal is to create a torque profile similar to the LT1, by this widen the power band. Here is the exact differences I feel with the 2 cars, the Firebird ( LT1) takes little to no throttle to get rolling and cruise, where the L98 in my corvette seems to take a bit more pedal and finesse to not stall around town. The other is the Firebird when spooled out has a much more gradual decrease in torque as you near redline vs the L98 that has a much more profound reaction to high RPM's. I keep seeing claims you cant get both but ask how is it possible with the same geometry below the deck on an LT1 and the same stock compression? To be fair I'm not looking for more grunt per say just a more user friendly launch in that case. The top end is pretty obvious how to fix, intake and cam.

With the response I am seeing it sounds like the best option will be to go for an LT style intake and a good healthy cam. Thanks for the help guys, and Aklim... please dont bring him back, I'm glad he who shall not be named finally moved on to another forum.
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