No, this has nothing to do with which bathroom to use...

I'm trying to come up to speed on how engine performance translates to actual driving performance. Caveat: I'm rebuilding my '88 L98 (auto trans, vert), starting with stock rotating assembly and then building in stages. I'm only looking to optimize my street performance, light to light and curves.

Another caveat, please forgive for not giving proper attribution for these graphs; if someone tells me who they belong to then I'll update their labels.

So I understand that the typical performance graph for the stock L98 TPI looks like this. I understand that it's a peaky torque, due to the stock TPI plenum(?) and that ideal range sits between about 3500-4300 RPM. I also think I understand that even if I replace the plenum (e.g. SuperRam, MiniRam, etc.), I will increase the baseline torque & hp and maybe slightly bump up that ideal range a few hundred RPMs (e.g. moving the center from 3800 to 4000) and a bit broader. And I also understand that I have to choose my cam to target that range. Again, I don't want to go down a rabbit hole with improvements intended for strip; this is street and sticking with TPI.

This graph is probably not an ideal one to make my point, but I'm trying to show the L98 curve and contrast against the LT1, showing that the centerline of peak torque only changes slightly.


So here's where my question comes in. In the next graph I see driving behavior and this peaked my curiosity. In first gear, it passes through the ideal engine torque range but in the remaining gears the RPM stays well above that ideal torque range. I don't see labels on the Y axix but it appears that in gears 2+ it operates well above 4500RPM. Also, it may be my imagination but it appears to my eyes that the angle of the acceleration rate is higher in first gear than than in the remaining gears. So here's my question: how do I take advantage of that ideal torque RPM range in all gears? What do I change to force those RPMs to drop down to something like 3800 at each shift so that the engine is operating at peak torque potential?

 

Here's an example where all the miseducation of the car world bites people in the ***. The RPM you want to focus on for best acceleration is peak power, not peak torque. Power is literally the rate (how fast velocity rises) at which work (accelerating a car's mass) gets done. Torque is irrelevant, and anybody that tells you otherwise simply doesn't understand what they're talking about. Ignore them. So you want to shift a little bit above peak power RPM and maximize the area under the power curve with each ****. For an L98 that peak is probably around 4500rpm; and yes, it's that lame because of the awful TPI intake. So you probably want to shift somewhere around 5000rpm and then it will drop to around 3800rpm. That's the fastest way to drive it in stock condition, and that's why Chevy set it up to shift at that RPM. If you try to lower the shift points, you'll go slower. You don't really believe that Chevy set up the transmission to shift at WOT a higher RPM than is fastest, do you?

If you change intakes to something that approximates that LT1 intake (like a Miniram), you won't likely increase peak torque* output but you will move that torque peak a lot higher in RPM, which translates to more power. That's why the L98 has around 245hp and the LT1 has 300. And the LT1-powered cars do indeed accelerate significantly faster.

*But as you see in your graph, you'll have more average torque over a much broader RPM range.

 

Thanks Matt, that does make sense. So is it possible to achieve that power curve with an intake (e.g. miniram) alone? I'm trying to form a reasonable plan of action, so bear with me... If I start with a miniram to determine my curve then will cam and headers achieve raising the baseline, or do I need to add a cam along with the intake to achieve that improved curve?

Also, if I went that route and changed the shape and position of the power curve, would that require me to change the rest of the power train as well? I guess I'm trying to understand the difference of sheer power increases (i.e. same shape, higher baseline) vs changed behavior... and holistic ramifications. To your earlier point, it seems the GM engineers knew what they were doing and in my ignorance I could set myself down an expensive (and potentially needless) path in the name of "improvements". Really a lot of options, all touted by people with different agendas that don't necessarily match my own...

 

I haven't done it personally, but you can certainly do just the intake swap and leave the stock cam in place. It might not get all the way to the LT1 power level and curve, but I bet it would get pretty close. There's been some other discussion on the forum about doing such things, and someone probably posted some good data on that. I can't actually recall for sure. If you do this, you will want to shift manually to hold each gear to a higher RPM when you're trying to go fast.

Long-tube headers generally add a little power over a very wide range, for at least a couple reasons. They are a nice addition, but they aren't cheap or easy because you have to either adapt the rest of the exhaust to them or install and all-new exhaust system downstream of the headers. The latter is probably preferable on an L98 car because I believe those exhausts were fairly restrictive. Cams can change the power characteristics of an engine in any way you desire, depending on the specs you order: duration, lift, overlap, advance/retard, etc. Typically, you'd install one that has more duration and lift, which will usually push the torque peak to a higher RPM and create more power, but that will also make the off-idle "punch" less dramatic. It's a tradeoff, and there are pretty significant limits to what the stock heads will flow, so there's no point going crazy with it. I think if you want the engine to run sort of close to how the 96 LT4 ran, the "hot cam" kit would be an easy recommendation. But this gets into looking at your rear-end gear ratio, torque converter stall speed, ECU tuning, etc. In short, it will quickly become a very deep rabbit hole. I think an intake and long-tube headers by themselves would be a lot easier, somewhat easier, and make a difference in power and performance.

 

Strategies for modifying an L98 can be daunting. You can make what's there better (meaning the intake), or you can trade for an intake that raises shift point(s) and power level. That doesn't mean you can't improve power levels for the limitations of a TPI. (For the record, all port injection engines can be categorized as "TPI". It's a matter of how you tune them (with runner length) that translates to bottom-line performance. For clarity, it's often good to refer to long-runner, mid-length runner, and short-runner intakes.

Being more pragmatic that most, I didn't focus totally on max power -- meaning that range Matthew talks about -- which tends to be between max torque and max HP. Because mechanical leverage (gearing) provides somewhat higher leverage (acceleration), shifting above max HP helps. That's because you stay in that mechanically better (leveraged) gear longer. The pragmatic part of me decided to look at how often I run to WOT, shift and repeat. That's what I consider racing strategy and what Shunyun may not want/need from light-to-light driving. That's where lower torque peak can help get the car moving with greater ease.

I would also add that automatic transmissions are already setup to downshift in a way typically designed to catch that max TQ through max HP range. Put another way, they "automatically" put you in that range Matthew rightfully points out as the best acceleration. One thing I didn't know until this year is (as another CF member told me) automatic torque converters add torque -- making the car accelerate better (especially in 1st gear) until you get into that maxTQ-maxHP range. From that knowledge, I figured out why manuals (such as a ZF6 are geared lower. Plus, they have an extra gear. It's their "equalizer".

From light-to-light, I don't typically run WOT and shift a few hundred rpms above maxHP -- which would [indeed] make the car accelerate as fast as possible. If anyone did that -- all the way through 1st and 2nd gear, you'd be going highway speeds -- in the city. Before building my L98, I made the observation that 3rd gear was disappointing (versus more satisying 1st and 2nd gears). That meant the need for more torque and/or HP to maintain the same impression of acceleration in 3rd -- especially when not running 4k-6k rpms. So, I focused on building torque -- which also raises potential HP. I think this is why most people who use [SuperRams, hogged out TPIs, FFI intakes, or any other DIY mid-to-] longtube intakes go that route. They want the car to pull as well in lower rpms. Sure, a V8 already HAS lots of torque, but torque is the reason V8's are often more fun to "tool around town" than a weak 4-cylinder high-rpm spinner.

As MM correctly points out, there are lots of ways to skin a cat. Headers, exhaust, and cam (with or w/o ported heads) would maximize torque for a light-to-light machine. Or...you could install higher gearing and a higher-revving (short-runner) intake. Moving more air using higher rpms probably means you'd also want those headers or (at least) a bigger exhaust.

In general, an LTx car is a better launching pad for building more power. The factory exhaust manifolds AND intake are already designed for higher rpms -- translating to best HP. L98s are just not very easy to accomplish the same thing. This means your concern of NOT needing to spend lots of money to reach your goal may be difficult to impossible. We could (and have) tried to quanitify each of the many, various mods (heads, cam, exhaust, headers, intake, etc) for people to pick-and-choose from a smorsgasboard of choices. As Shunyng correctly points out: The factory chose parts as a "unit" for an ultimate goal. From stoplight-to-stoplight, an L98 isn't going to feel much (if any) slower than an L98 car. On the highway the difference is more apparent. The same is true for any building strategy focused around short vs longrunner TPI intakes.

That's where pragmatism may provide your roadmap for any modifications.

I will finish with this: Revving it higher is always going to make the faster car. Automatics are setup to RUN the motor in those [designed] higher rpms. Building for higher rpms seems like the best option for automatic owners -- which are 90% of the C4s built. Having a stick provides more choice on where/how you shift. If you don't run WOT and shift at or near 5-6k rpms, you might want to stick with longer runners. On the other hand, having more gears (like a ZF6 has compared to an automatic) provides the ability to stay UP in the highest POWER (horsepower) rpm range. So...you could say HP is almost always the best possible target when looking to mod your engine. Put in the simplest terms, it's why EVERYONE always ignores torque and talks about horsepower. Torque only helps to pull you around town at lower rpms -- or pull tree stumps.