5speedC4

I have a 1988 L98 corvette with a miniram and free flowing exhaust...

I am looking to purchase headers next.

Now that I have worked the bugs out of the miniram install, I think I am actually getting better fuel milage than with the TPI.

Now I wonder how far I can mod and still get good fuel milage.

Don't want to run premium octane. Can I continue with more performance mods and get decent fuel milage?

And what kind of hp do you figure I will be pushing with a stock L98 engine, miniram intake, headers and free flowing exhaust?

I want to keep my corvette easy to live with, but as powerful as I can get it, while being easy to live with.

hexane

You can mod L98s to the moon, literally. If I were you, I would bump the timing up 4 degrees and run premium. You will notice quite a difference in the butt dyno. Heads/cam/headers would yield about 330-360 rwhp. I suggest the CC503 cam as it is the best all around performance cam with both street manners and top end power. It sounds pretty mean on the 350 too.

5speedC4

1.
I wonder how far I can mod and still get good fuel milage.
Don't want to run premium octane. Can I continue with more performance mods and get decent fuel milage?

2.
What kind of hp do you figure I will be pushing with a stock L98 engine, miniram intake, headers and free flowing exhaust?

BADDUCK

1. What is good millage, (your question is too general). What is "decent" millage? Is it the same as "good". If you keep your compression around 9.5:1 and your base timing stock you should not have to run premium gas.

2. You will need to dyno your car. I'm sure you can get people to guess.
My guess is under 275 CHP. You will need performance heads and cam to really increase HP in a L98.

hexane

If you still want to run 87 octane, it will be much harder to modify the car to get good gas mileage (>16 mpg) and get good performance at the same time. Be conservative with spark timing or you will start pinging. With good heads, a nice cam (224/230 duration, at least .500/.500 lift), exhaust mods, headers, and the correct tuning, I would say 330 rwhp should be the maximum amount of power attainable on 87 octane gas, or what pops calls "cat **** gas".

hexane

In my accounting class, I heard that millage has to do with real estate taxes.

BADDUCK

Todays 93 octane is "cat **** gas" my Dodge 440 ran best on Amoco 102 octane. That was "in the day" you snot nosed kids don't know what hot iron really is. You think 350 HP is a big deal.

GREGGPENN

1) Why don't you want to use the gas that gets your current equipment in the best power range? For some cars (like my Grand Cherokee), chip makers can only bump timing and ask you to burn preminum to get better power/mileage. Often, the two go hand-in-hand.

2) Look at hp curves of a MR vs a TPI. Look at how much power is generated at different rpms. I'd bet your hp is LOWER until you get to 4200rpms. Then, it gets better. I've heard the stock cam is done anywhere from 5k to 5.5k rpms. That means you have more power for about 1k rpms. That could mean your car is slower than before. I'm sure it feels different because the power constantly builds until you need to shift. That's certainly different than what the TPI feels like.

I agree with others that suggest you install heads/cam. The intake you chose pretty much dictates that you do heads and a cam (or gears). That's really where it's advantage lies (in high rpm, top-end flow). With a stock motor, you'd want to make sure your heads are in good shape!

And, typically, the TPI would provide better mileage because it gets the engine to function better at lower rpms. I challenge you to measure your mileage before you say you think it's better. I also wonder why you installed a MR to get better mileage.

O.K., maybe that's not your goal. I think you should install the heads and cam. Your mileage will go done (some) because you'll hot-rod it more. Between the cost of the heads and cam (and abundance of extras), you be so busy agonizing how much that project cost, you won't notice the small change in gas expenses.

gp

5speedC4

I installed the miniram because I was tired of the TPI air leaks. Having problems with idle at stop lights, idles fine for awhile, then goes through phases of not wanting to idle. The poor idle at stop lights was such a problem that the wife didn't even want to drive the vette, because she didn't like having to deal with it.

Then I also wanted my extra hp.......LT1 has it.....I wanted it.

Does a LT1 have to run premium gas?

So, not such a bad thing, not to want to spend the money for expensive gas........

I want as much as I can get, for as little as I have to pay.

If I have to pay for expensive gas, then so be it.....I will.

That don't mean I won't try to find a cheaper way first.

I will run a couple of tanks of fuel through and check with my odometer.

I will let you know how the numbers go.

The vette runs great with the miniram.

hexane

If you think about it, it is only about $4 more to fill up with premium gas, at least in my area. More spark timing (to a point) equals more fun. It would feel like a TPI again, but with power on top.

5speedC4

I was under the impression that a stock L98 motor with a miniram, headers and free flow exhaust system was pretty close to a LT1.

vader86

You cant really do much more than you have without it drinking more gas, sorry.

Even if you leave the SCR alone, your chip tuner will probably bump timing and recommend you use midgrade or premium.

GREGGPENN

That's not too far off, but I've also read about some difference in head/cam. I started a Thread call "Does L98 + miniram = LT1" in mid-December. The differences were mentioned there.

I don't think most people realize there's a curve to hp either. (I didn't think about it much before last fall.) The TPI actually delivers more power below 4200rpms than a mini. The long runners (tune) the input air bursts so they have more momentum to fill the cylinders. The trade-off is it also chokes the car at higher rpm.

If you race (or run your motor to high rpms alot), the miniram is a good intake. It's a good intake for a car with high-ratio gears because the engine is spinning faster more often. The miniram definitely delivers more "power" at high rpms. At lower rpms it does not.

I've looked at the various intakes and came to this conclusion. A short runner intake is the best. But, it's only the best if you add a cam (preferably heads too). By installing a cam, you bring the low-end power back to (or above) TPI levels. Then, the extra power up top becomes a bonus vs. a necessity. (Obviously, that depends on your perception of power requirements, but I'm assuming most owners don't want to feel their car lost it's street sotp feel after a mod.)

After talking with a forum tuner, I'm sure he would say a tune is the best way to improve mileage and power -- especially since you swapped intakes). Without a tune, you're mileage might also have dropped simply because your computer isn't doing as good a job with your new intake.

Just for curiousity, what's your perception of power after having made the swap? When are you going to measure mileage to see what's really going on?

JackDidley

If properly tuned the car will make max power and get max mpg on premium gas. Premium is 20 cents a gallon more. You will find it costs less to run premium if you calculate $$ per mile.

hexane

If you think about it, premium gas is only $4-$5 more when you fill up from almost empty to completely full. Totally worth it, if you're looking to squeeze an extra 5-10 ft-lbs of tq from the lower revs with bumped timing.

Tuning an engine to run maximal amounts of power using 87 octane is so much more difficult. There's also much risk involved because it is much more sensitive to high cylinder pressures and therefore you have a much greater chance of detonation. From all the detonation you would be getting from 87 octane, the knock sensor would keep on adjusting (retarding) timing accordingly, which can lower performance even more. Good gas mileage and high performance really don't belong in the same sentence.

5speedC4

After the miniram swap I can tell I lost torque down low.
But It still has rubber burning torque down low, just not as much.
The power is there after 3,500 rpms and feels much better.

I really miss my low end torque, but the miniram runs so much smoother at all rpms.

The miniram makes a better driving machine over my TPI.

But that may be because the TPI may have had air leaks with so many various places that can develope air leaks.

I think when the TPI was on my corvette, I not only had air leaks, but my fuel injectors were no good.

When I installed the miniram, I installed 24 lb injectors that came with it. It ran really smooth. Then I installed the origional injectors and it did not run as smooth. It still didn't die at idle, and it still ran better than with TPI, but not as good as the miniram ran with the new 24 lb injectors. I put the 24 lb injectors back in the miniram setup, now it runs very good.

To answer your question:

Yes there is a noticeable power loss down low, but still seems to have plenty of power.

Would I be happier with a good set of injectors + TPI working correctly?...........No air leaks, with good idle........
MAYBE....

But the TPI is such a pain in the a$$ to work on, I will take the ease of living with the miniram.

For instance, when I decided to install the origional thermostat back into the miniram.........It only took about 30 min from start to finish.

How long would that have taken with stock TPI?

I don't know ............somewhere between 2 and 4 hours? "guessing"

When I changed the injectors with the mini ram........it only took about 1 hour, start to finish.
I wouldn't even want to guess how long that little job would have taken with the TPI.

Now for the milage...........it may take awhile to run a few tanks of fuel through it, and check the odometer......but

Now I have the origional thermostat in it..........it shows running 26 and 27 mpg at 70 mph on level ground. That is with the computer.

But the computer never showed better than 23 mpg on straight level ground with the TPI.

And Greg....I feel you gave some good advice on my other thread.......you said: Get a tune, get a tune......get a tune. LOL

I will pay for a tune after I get some headers installed. I want headers next.

I don't know, maybe headers won't change the tune.......maybe I should get the tune next, then headers.

GREGGPENN

If I were back up to the mileage you're getting, I'd probably wait for the headers. OTOH: I don't think that'll effect the tune that much, and running too lean can be a bad thing. (Sounds like that's not the case though).

I'm glad you like the new setup and I can relate to the ease of service for the new intake. I totally agree (and it's a something I've considered).

I wouldn't mind hearing why you think the new intake makes for a smoother machine. I assume you mean more the injector/leak problem, but I'm asking anyway. Do you think your car ever ran right with the TPI (as a basis for comparison)?

How does it feel above 4,500rpm??? For city driving, do you feel the power range is above where you'd drive -- if you didn't rev it to get to the better power band up top.

Glad to hear you like the new setup...

5speedC4

Smoother machine might be because of the injector/intake leak problem.

And no, I don't think my car ever ran right with the TPI, so I don't have a good "base" comparison.

Above 4,500 rpms it takes off like a rocket, you really need a rev limiter because it will go past 6,000 and seems to still be building steam, faster and faster.

And yes, for city driving the power band is above what I normally operate in. But bare in mind that it still has plenty of power down low. I haven't; yet, felt like I was to weak, while in the low rpm range.

I can always down shift if I need that power band.

I am sure it is just a matter of time before I stick a cam and a set of heads on it.........

Everyone tells me that I need to do heads, cam, exhaust to take advantage of what the miniram really has to offer.

All the same, I think I will be happy; enough, with just miniram, headers and free flowing exhaust. Pretty close to a LT1 I've been told.

hexane

I would recommend switching over to LT4 valve springs if you are always going over 5500 rpms, which I could only assume you are. Its a quick and easy job to change springs. Stock springs would cause valve float at higher revs. You could get better valve springs for really cheap online. Just google it or find it on ebay.

You may as well change over to the LT4 hot cam while you're at it. If you do your own work like me, it can be done in a day for less than $500 for everything.

Don't worry about the heads for awhile. Stockers are good to 400 chp max or about 330 rwhp.

GREGGPENN

I keep reading where the stock cam gives out at 5k - 5500rpms. How could going over 6k rpms be a big problem? Shouldn't if feel like it's giving out? How could it be pulling hard up to -- and past that?

Is it cause your TPI never let you go there... new feeling not worn off yet?