Recipe for a 300HP L98

• '90 convertible
• Auto
• Pulley set (Was 3-pulley, but...)
• modified air filter lid
• 160 thermostat
• high-flow filter element
• air foil
• 52mm throttle body
• cleaned up plenum entry and interior and matched to runners
• 1.6 full-roller rockers on both intake and exhaust, with guide plates
• no pre-cats
• no cats
• Flowmasters
• 3.31 or 3.54 rear (never did check which one it was. It's out of an early six-speed)
• 285 GCSs
• custom prom
• matched 24# injectors (out of a Ford. Cheaper than GM stockers, even for he matched set)
• GM ignition and GM "black" wires (as good as Accel in my use, and cheaper)
• Adjustable fuel pressure regulator, at 45lbs with vacuum line attached
• Oh, and most important, a vanity license plate. Gotta have one, ya know!
• In other words, the usual bolt-ons, except for the rear end.

Last time I actually ran it on a drag strip was at Beech Bend during the 94 reunion. 13.80 @ 99 mph. 60ft was 2.08. I won't tell you how slow my reaction time was--but since it was only time trials it didn't make any difference. The car is a little quicker now; maybe 13.6-13.7? I estimate the horsepower to be right at 300, given that et and MPH.

And it hasn't idled correctly since I put the rockers on. Cooincidence? Don't know. Cottrell says in his book that 1.6's on the exhaust "show no real improvement and reduce idle quality". He was talking about an 87 (I think), but my experience backs that up. I can't speak for any other year, but on my 90, it does make the idle rougher.

I put the matched injectors on it because I could get them at a good price, and I needed to replace the GM 22s anyway. (It's amazing to how a car with 20K miles on it can need new injectors, but there they were, some leaking, some with high resistance...).

The throttle body and some of the other stuff is there because I'm sneaking up on the price of a "big mouth" manifold base, cam, larger runners, headers, and heads. (And a 383? Or maybe I'll just say screw it, and get a supercharger). I can only afford to do a little at a time--and besides, I like to see how much difference each of these changes makes.

I've been through four chips. One of the downsides of going at it a step at a time is that you need to re-do the chip fairly often. I started with a Hypertech. I've also had TPIS. Seems as if Hypertech put a lot of advance into their chips, as well as the mixture change. Both also change the manner in which the converter locks up. Anyway, with either, there was some change in the upper mid-range that you could feel, but in retrospect, I'm not sure it was worth $400. Now, of course, I need the chip. It won't run correctly with the stock one anymore

I feel essentially the same way about the damn mufflers. Not worth the money or noise on an otherwise stock car, but necessary after you do a couple of other changes.

Air foil? Who knows? I don't think that anyone's butt is sensitive enough to tell whether it's there or not--but it *seems* as if it should help, and it's cheap.

Easily, the most noticeable change I've made is the rear gear. I think a ratio in the mid-3's is nearly perfect for the street. Even back in my hot rodding days, I was running 3.55's and 3.70's when others were thinking that 4.10's and 4.56's were the hot setup. I can't see it, unless your power peak is way up there--and it ain't on TPI motors, unless you've done a bunch to 'em, like getting rid of the GM TPI runners and manifold, a la Accel Super Ram, or TPIS mini-ram. I haven't, and won't. My "thing" is for it to always appear totally stock at first glance, and to be completely driveable with very little compromise necessary.

Second most effective? I'd say it was the exhaust setup. But if you do that, you *will* need a chip made for you.

Why the high fuel pressure? Don't know. It just runs best there, so that's where it stays. Your mileage may vary, as they say. I've heard of guys running them as high as mine or more, but not too many do.

It used to like to have it's initial advance set at 8-10, but since the last chip, I set it at 6. We put a faster rate into the chip, and it seems to like it. I think we could even use a little more, since on cool days my Diacom doesn't show me too much retarding going on. Maybe I should push it a little higher? My engine does seem to run a little cooler now that I use Red Line Water Wetter (Thanks Hib), so maybe I can handle a little more initial? I always did like to run a lot of initial advance whenever I could get away with it.

Diacom also says I've got a few fat spots in the fuel curve at WOT. In a couple of places mid-range it goes to 11.2:1. I like to stay in the 12:1 range at WOT. I'll fix it one of these days. I could back off on the fuel pressure, but then it'd lean out in other places, so we'll change the curve in the chip.

Diacom is a great piece of software. If you've got a laptop PC, you ought to have Diacom. They (used to?) sell two versions. Only major difference was one would record several minutes of engine data, and would print graphs, while the other one would only record 20 seconds or so of data, and did not print. Difference in prices was substantial. I just print the graphs out of a spreadsheet program. Does a better job anyway. And I don't need "minutes" of engine data. A hint: Diacom doesn't like to run on really fast cpus. I run it on an old 386-16 laptop that I had around, and it works fine. You could pick up one of those for $100, if you looked in the paper hard. It is really neat to drive down the road and be able to actually *see* all of those read-outs changing in real time. After you get used to it, you can see wheelspin, converter flash, slippage, etc. Now if I just had a Vericom...

Throttle response is "right there". No bog, no problems. The 60-foot times indicate that I'm not having any real traction problems. (I use the "stab it from idle" launch technique). I might get it down to 1.9 or so, but it seems to me that that's about as good as it's likely to get.

Supposed to be that the 90-up TPS is self-setting so it doesn't make any real difference that it's .57-.64 volts when the throttle's closed, but by golly mine seems to "know the difference", and it likes to be in the .65-.67 range. I run more Initial Air than the book calls for, to smooth the idle some and avoid stalling, and with the non-adjusting TPS I just set it as low as the slot will allow. That works out to be about right.

Oh! The pulleys. The damn thing idled too slowly to keep the battery charged with the full set of three, and you can't really set the idle up too much with an auto car, so I replaced the alternator pulley out of the set with the stock one. Works fine now. Does it make it faster? Nah, not really, but you know, all of those little changes might just be worth something some day when I finally work my way up to the 383..

I've not invested any money in the trans. I'd like to maybe get a mild shift kit, mostly for the adjustable shift points. Right now, I pull the cable out a click or two at the throttle linkage. (What the hell's the name of that cable? I've forgotten.TCC? Anyway, it goes from the throttle linkage to the valve body on the trans). Pulling it out a little works to lower the shift point to 4900-5000, but I don't feel it's good for the trans to be driving it around like that all the time. It seems to slip a little more than it should at part-throttle shifts. If I adjust the cable the way it's supposed to be, the things shifts at 5300-5400 RPM. I think that's too high for my motor. Anyway, I pull it out when I want to to go fast, and put it back when I don't.

This has been a little long. (I do ramble). Sorry. Hope it helps someone, somewhere.

WOT = Wide Open Throttle
TPS = Throttle Position Sensor
TCC = Transmission Control Cable(???)
Cottrell = Myron Cottrell; owner of TPIS (TPI Specialities)
Beech Bend = Beech Bend Raceway Park, near Bowling Green Kentucky, and the Corvette plant. The location of the National Corvette Reunion in the past, (and again in the future, thankfully).