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  1. #1
    Member Rogue's Avatar
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    Default 300-350 HP L98 w/Bolt Ons?

    Hello everyone. It's been a while since I posted here. Life has been up and down for me lately and haven't had much time to enjoy my Corvette. However, things have settled down a bit and now I am enjoying driving the 'Vette more and enjoying it with the wife.

    My ride is a 1991 Corvette, L98, Automatic, 52k miles on it. The car is mildly modified with the usual stuff enthusiasts do to their rides. It has a K&N open top airbox and filter, TPIS TB Airfoil, and a Corsa catback exhaust. I have not dyno'd the car but based on known information, the car is most likely putting out 230-240 rwhp.

    My plan is to go to the next level of modification. What I am after is to get more top end horsepower without resorting to a full on stroker kit or supercharger, while still being able to pass the stringent CA bi-annual emissions inspections/tests. I'll be the first to admit, I am not that knowledgeable when it comes to camshafts, cylinder heads, ignitions, etc., All I can do is read up on what others have done. I have done some research, reading articles, posts here in CAC, and spending a lot of time with one of the definitive TPI experts: TPIS website. Armed with this information, I would like to upgrade the usual suspects, intake and exhaust.

    Intake: TPIS Big Mouth Lower Plenum, TPIS Large Tube Runners, TPIS 52mm throttle body.

    Exhaust: TPIS Long Tube headers (complete with emissions hookups), HFS high flow catalytic coverter

    I plan to tie this all together with a TPIS ZZ9 Hydraulic Roller Lifter camshaft with a Duration of 212/226 at .50, Gross lift of .483/.520, Lobe Seperation of 112, Re-programmed ECM, and ignition upgrade.

    My questions are:
    1) Should I upgrade to an aftermarket cylinder head? If so, what are your recommendations?
    2) I know I need to upgrade the ignition. But which one?
    3) Will all this take me to the 300-350 rwhp level?

    I would greately appreciate any help you guys can give. Thanks.
    Rogue
    1991 L98 Coupe


  2. #2
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    Default

    Exhaust mods wake up a L98 real good.
    Forget the 52mm TB; not needed
    Stock ignition fine

    What is your budget?

  3. #3
    Technical Writer for Internet & Print Media
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    The best approach to performance is a practical one and, in that light, I submit that your existing engine set-up is probably somewhat short of the about 235 at the wheels that you suggest. The only mod you have, so far, that typically results in a noticeable improvement is the Corsa exhaust and, on a stock or near stock L98, that's probably worth 5hp.

    In your post you state that you're going after top end horsepower. The L98 is the wrong engine for that because it's decidedly a low rpm, high-torque engine and to convert it to one which makes a high rpm power, while possible, requires a lot of work and a fair amount of money.

    L98's big problems with respect to high rpm power are 1) restrictive intake tract and 2) excessively long port runner length. To get top end power, most of what you do must be to eliminate those two problems.

    So, any intake modifications (and you list several in your post) that significantly increases port/plenum volume and decreases runner length would be good.

    On the exhaust side the mods you list..."real" headers and a low restriction cat are good choices. Your cam profile while a great choice for air flow may be a problem with the California smog check. You may need more lobe sep and perhaps less duration.

    Lastly, don't forget calibration. It's not going to be easy to get a good tune which will be compatible with the modifications you need and will have the engine passing the Smog Check but the right tuner can do it.

    Your questions:
    1) Should I upgrade to an aftermarket cylinder head? If so, what are your recommendations?
    I think the OE 91 L98 head is a good choice if you properly prepare it with a street port/polish and 3-angle seats and faces. I don't think the gain in an aftermarket head will be great and you'll need that money to spend on intake hardware.
    2) I know I need to upgrade the ignition. But which one?
    The OE HEI ign. is pretty good to 6000 rpm as long as it's got a hotter coil (MSD makes good ones) or good wires (I prefer MSD Superconductors)
    3) Will all this take me to the 300-350 rwhp level?
    Unfortunately, not.
    325 at the wheels is, basically, 400 at the flywheel. To get that and pass the smog check is going to require a larger engine or spinning the 350 to 6800. Think 383. Better yet, think crate motor and one which does not use the L98-type induction system.

  4. #4
    Member Rogue's Avatar
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    Default

    Thanks for the input. I do appreciate it.

    Answers to questions:

    My budget is flexible but I had about $4k in mind for the engine alone. I did a spreadsheet for the initial estimate of parts only and it's already closer to $5k. So I'm preparing for the inevitable.

    Hib, do you really lose that much on parasitic on the dyno? I was thinking 25-30 hp loss to the rear wheels!

    On cylinder heads, I was thinking of just working with my existing late L98 heads as you suggested. It surely would cost less. I did think about putting Vortech cylinder heads in it since I read in an article that these do flow great on the L98. But I like keeping the engine very stealthy for emissions and cost reasons. I also thought about the Mini Ram from TPIS to get the high RPM power I'm looking for but it would likely not pass emissions and my car is not really geared to take advantage of anything passed 5k RPM. I do understand the L98's TPI limitations so when I said "high RPM power", I meant that in relative terms! Meaning, a bit more power and better breathing from 4,400 to 5,000 RPM, which is about the upper limits of my mods I think. I am very happy with the L98's torque right now, but I would like it to NOT run out of steam above 4k RPM. At the same time, I'm not looking for a 6,000 RPM screamer. Maybe when I go on to the next level of mods at a later date....

    I agree that I need to get a proper tune of my ECM and that's going to be more of a challenge than the parts alone. I have no clue who is capable in SoCal in doing this kind of work. Does anyone know of a trustworthy shop that can do light engine mods and ECM tuning for California emissions located in Orange County, California?

  5. #5
    Technical Writer for Internet & Print Media
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    Quote Originally Posted by Rogue View Post
    Thanks for the input. I do appreciate it.

    Answers to questions:

    (snip)
    Hib, do you really lose that much on parasitic on the dyno? I was thinking 25-30 hp loss to the rear wheels!
    Parasitic loss for C4 manuals is .18 and automatics is .22. So, based on an OE rating of 250hp, figure 205 at the wheels with a manual and 195 for an automatic. If we were to be really liberal with our guess and figured a 10hp increase for the cat-back, you'd be at 205-215 at the wheels, SAE corrected.

    On cylinder heads, I was thinking of just working with my existing late L98 heads as you suggested. It surely would cost less. I did think about putting Vortech cylinder heads in it since I read in an article that these do flow great on the L98.
    Only problem I have with that idea is that, as far as I know, the Vortec head is cast iron. I'd take the modified L98 aluminum head before I'd switch to an iron head.

    But I like keeping the engine very stealthy for emissions and cost reasons.
    There's no freakin' way you're going to build an L98 to make 400 at the wheels and have it be "stealthy". 250-275 at the wheels and stealthy might be reasonable.

    I agree that I need to get a proper tune of my ECM and that's going to be more of a challenge than the parts alone. I have no clue who is capable in SoCal in doing this kind of work. Does anyone know of a trustworthy shop that can do light engine mods and ECM tuning for California emissions located in Orange County, California?
    Contact my pals at Westech out in Mira Loma. They tune on a Superflow chassis dyno. I do all my chassis dyno work there. Call 951.685.4767 and ask for Steve. I'm not sure if they tune the "chip" stuff any more, but if they do, they're the ones to try.

  6. #6
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    since obama passed a law to where each state can make there own emmisions laws I personally would hold off on any mods till you know what california has in mind.

  7. #7
    Member Rogue's Avatar
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    I'm not holding my breath as California is one of the strictest in terms of emissions control. In fact, CA pioneered it. If it was up the EPA and CARB, they would ban all high performance upgrades on all cars.

    At this point, I would be happy with the headers and camshaft upgrade if it means passing the bi-annual exams. If this doesn't work out, I may go the Pro-Charger supercharger upgrade, which is CARB approved and will get me to the power level I want.

  8. #8
    Member mseven's Avatar
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    I thought long tubes were out of the question on the left coast. Regarding cylinder heads, using an aftermarket head can be benificilal when the costs of re-working/porting, re-surface, valve job (and possibly bigger valves), springs, retainers, seals etc. etc. then are equal or more than buying a new cylinder head. At that point (depending on choice mfg. and budget), can provide some good gains over the 113.
    Other things I am aware of is that you will be restricted to being able to use only a couple of intakes (carb), SR. worked over TPI. My first set-up was w/a zz9 and is a good cam. It is not a difficult cam to tune (no overlap/02 issues), should have no problem passing.....can't help w/tuners out there though

  9. #9
    Moderator KANE's Avatar
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    Default

    Quote Originally Posted by Hib Halverson View Post
    ...325 at the wheels is, basically, 400 at the flywheel. To get that and pass the smog check is going to require a larger engine or spinning the 350 to 6800. Think 383. Better yet, think crate motor and one which does not use the L98-type induction system.
    To Hib's point, 400HP at the flywheel is a pretty stout request from an L98. I have a crate (cam lift is .450 & .460 before the 1.6 rockers, approx .480 and .490 with compensation for the rockers), modded TPI intake (SLP parts), custom chip, etc with perhaps 300 even at the flywheel.

    If you want 400... you could go with a supercharger. They are pricey but there are many kits that will bolt on as a true kit. Assembly time is around 8-12 hours. You'll have great throttle response and keep that great L98 low end torque.

    Powerdyne has one that is a sealed unit... so no need to tap into the oil lines.
    1982 Dark Blue Corvette
    CFI | Dynamic EFI EBL | .465" / .488" cam


    1995 Trans Am
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  10. #10
    Member Rogue's Avatar
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    Quote Originally Posted by mseven View Post
    I thought long tubes were out of the question on the left coast.
    I haven't looked into whether the TPIS long tube headers are 50-state legal or not. I assumed they were because they kept all the smog related hookups and I had no plans to eliminate the cat, only upgrade it to freer flowing one. I will need to call TPIS for more info.

  11. #11
    Member Rogue's Avatar
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    Quote Originally Posted by DarkShark View Post
    If you want 400... you could go with a supercharger. They are pricey but there are many kits that will bolt on as a true kit. Assembly time is around 8-12 hours. You'll have great throttle response and keep that great L98 low end torque.

    Powerdyne has one that is a sealed unit... so no need to tap into the oil lines.
    It appears that my goal is too lofty with the given mods I had planned for.

    A complete engine replacement or stroker kit rebuild is not in the works. With only 53k miles on it, I hate to just tear that engine apart and waste the life that is still left in it. I would like to save that job after 2011 when the car no longer requires the bi-annual smog inspection in CA and I can go all the way!

    Likewise, the Supercharger option is out due to the added complexity. Although ProCharger makes a complete kit that is 50-state legal, I want to save that after 2011 so I can build the engine to use a Supercharger and take full advantage of it.

    I want to stay with simpler bolt-ons for the next couple of years. I guess 300 rwhp is more realistic.

  12. #12
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    Hi Roque, How is it that you wont need to smog after 2011 ? I must have mine done bi- annually at a "Smog only" station. Roger.

  13. #13
    Member Rogue's Avatar
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    Roger, I apologize as I misspoke about the smog inspection. I read somewhere that cars over 20 years old no longer require smog inspections in CA. Turns out, it only applies to vehicles made before 1975. Our Corvettes will continue to get bi-annual inspections if we wish to continue driving it in the streets.

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