My question is in regards to the induction system on the TPI
What is it exactly that maxes out the powerband at 4500 rpm.
Is it the induction system?
Is it mainly the cam and heads?
Is it the combination of these components?
If a person was to swap in a miniram setup for instance on a stock TPI, would a higher rpm band result?
Does anyone know the powerband of the L98 cam?
What about iron L98 heads? Are they a 163cc intake runner size like the vette aluminum L98 heads? Can the cast heads support higher rpm like 5500 or 6000?

Thanks....

 

To start with the engine's powerband is mostly dictated by the cam. The limiting factor of TPI engines, in terms of rpm, is the restriction caused by the TPI induction. If you were to switch to a Miniram you could get additional rev potential. YOu may or may not make any additional power and you could hurt your low end torque. If you are thinking about a Miniram or HSR, then to get the most out of it a cam and head change needs to be done as well. The powerband of the L98 cam is designed to work with the TPI intake. So to answer your question, it is the combination of the heads, cam and TPI that limit your RPM potential to about 4500. However, it is mostly the TPI. If you are looking for more rev, you can siamese port the base and get large tube runner as well as port the plenum. Understand though that to really wake up your engine, the heads will need work and a cam change is in the equation as well. As to whether the vette heads are the same, I do not know, someone else here will need to help you with that. So

 

i put a stealth ram on an otherwise stock L98 and it changed the powerband completely. it pulls past 5000rpm easily. the intake is what chokes the L98. the cam and heads arent optimal for that high of rpm but it will still make better power with a higher flowing intake

 

Damn why did you have to get me started. I hate the stock TPI but my sig shows what it can do.

In short, all the stock parts on the L98 work together. If you're going to pull the TPI off for a Stealth or miniram, don't expect to benefit from it without numerous other mods and especially tuning. With just good parts, what you gain in airflow you may lose from a now crappy fuel curve.

Even though slapping on a miniram or Stealth with no other changes will give a 20?+ higher peak power number and hold it to 5300 rpm, I've never seen anyone actually prove a faster ET with before and after timeslips by just ditching the long tube TPI, especially with the stock gear.

People say "My car pulls hard past 5k now", which is true, but now you lost midrange torque that makes the car accelerate. Now you have to do all the other stuff to take advantage of the changed power band like steeper gear and higher stall if you have an auto.

 

In my opinion, two identical f-bodies, bone stock, L98, one with an HSR and one with stock TPI... the HSR car is going to garner a considerably better ET. The HSR altho not absolutely necessary for a stock 350 is a great induction system for it. You'll make more power in the long run (sacrifice some lowend, gain lots of highend).

 

In my lowly opinion ( and TPI GURU's( cause I am not a TPI expert by any means) who've written books for info sake, not to sell parts ) the TPI is FALSELY accused for being the reason ET's are low.


The STOCK TPI system does stop pulling efficiently at 4500 RPM or so...but the BENEFIT of the TPI system is it's massive low end torque and the relatively FLAT torque band in the mid range.

WAY too many people get hooked on the "more HP" bandwagon, not realizing that when aftermarket part companies and magazines give "HP GAINS" they are talking PEAK HP gain. In other words, a certain part may gain you 40HP , but it's at say 5800 RPM, LONG past where everything else is designed to work most efficiently. That's the problem...like putting a Hot Cam into a stock TPI 350...sure it'll pull farther, but at the sacrifice of the MAIN BENEFIT of the LTR TPI setup..which is brutal low end and mid range torque.

Consider this...

Mike Crews ( do a search for the HUGE thread on his combo, lots of good info ) runs a fairly mild 383 w/ Vortec heads, an UNTOUCHED Vortec TPI base, MILD cam ( he just changed ), headers, aftermarket runners, and he's REALLY close to hitting the 11's in a pump gas, STREET car, at FULL WEIGHT.

Anyone who's ever driven a big block powered "musclecar" will know what I'm talking about...

If you want to build for ultimate HP, go for it...but be prepared to change the whole induction system, cam, heads, ETC to get the most out of ANY HP driven combo.

Anybody remember the Super Duty Trans Am's from '73-74 ? They were born during the REALLY CRAPPY smog control days....they had a Q-Jet, PUMP GAS ( 87 octane ) compression, manifolds, etc etc , but with 455 cubic inches, the whole system was tuned to make GOBS of low end...hell they only made something like 290 HP, but almost 400 ft/lbs of torque ( NET figures ).

With 3.08 gears they ran the 1/4 mile in 13.60 or so at 104 MPH!!!

And that's in 73 with CRAPPY tires of the day.

TORQUE, TORQUE, TORQUE.


Same reason alot of guys with $5-8k wrapped up in a high-winding, high HP combo start to get pissed when several guys I now bring out their cars with 500cube Caddy motors dumped in 'em.

One of 'em is a '79 Malibu, with 3.08 gears, 500 Cad motor that is BONE STOCK except for all new gaskets, oil pump, new valve springs, and an Edelbrock Performer intake. With the stock Q-Jet rebuilt, and air bags in the NON posi 3.08 rear, letting the Turbo 400 shift itself at 4200 RPM, he runs high 12's.

He's got less than $3k in the WHOLE car. And he's adding headers, and a better cam this year, which will add a dyno proven 30HP and 60 ft/lbs of torque...

I'm not knocking HP...I've built a few screamers in my time, and while loads of fun at the track, they were dogs on the street.


If you make your mods to your TPI car sensible mods, that WORK WITH THE EXISTING induction system, you WILL see ALOT of gain in performance. It ain't all about HP.

My car WILL be in the 12's by year's end, time willing, and it won't cost a ton.

And this is my EVERYDAY driver.

Headers and a good exhaust are building blocks for any other mods.

A ZZ4 or similar type cam will help HP some, but will make MORE low to mid range torque than the L98 already does.

Getting it to hook....that's the rub.

That's why alot of people run faster with stock chasis cars, built for higher HP...they aren't as quick out of the hole ( loss of torque ) but they pull on the top end. This loss of low end helps them hook better.

What you rather have? A 350 HP small block or a 350 HP big block? All other things being equal, the big block will STOMP the small block.....and it has to work ALOT less to pull the car around in everyday situations.

Don't get hung up on the more HP game...it will leave you with a lighter wallet.

Now don't get me wrong...the Holley Stealth Ram is an AWESOME induction system...IF you have the cam / heads / engine to put it to use....and the finally cost is the same or cheaper than building a TPI system to smoke the 1/4 mile...

And also don't forget almost all dyno claims of HP gains are just that...on a FLYWHEEL dyno, no accessories, big 1 7/8 dyno headers ( help upper RPM and HP peak, but KILL low end torque on smaller cube motors ) with a carb that's tuned to push out that last few HP on the TOP END.


It's simple really....

You can easily build a car to do ONE THING very well ( dragstrip, autocross, mileage etc ) but it's careful component selection / matching that lets you build a car that will do everything.....cause you're always going to have compromise on a street driven car.


Just my 2 cents...take it or leave it.

 

There's lots of ways to go fast. Mike Davis runs ONLY 350c.i. with AFR heads, mild cam and a HSR. And he HAS run 11s. On pump gas. STREET car, at FULL WEIGHT.

What you said about TPI having a value of it's own is true. Just thought i'd point out an example of what a non-TPI EFI can do. Going fast is as simple as having a balanced combo and a good tune.

 

Yup.......Mike D's setup and tuning is AWESOME......


But remember, he DID break into the 11's with a modified LTR setup


Chris
85 IROC
Will work for posi

 

Abso-freakin-lutlely!! Everything needs to work together as a system!! THere is no way around it!!

 

All this stuff is true, and another thing to consider is that
TORQUE, TORQUE, TORQUE equals SPIN, SPIN, SPIN
and LOSE, LOSE, LOSE the damn race if you can't hook it up out of the hole.

I love my L98's torque, it's more than my LS1 C5 put out even with a 3500 Yank converter. But I can't tell you how many times my vette got wasted out of the hole by TPI cars and then at 60 mph I drove past them like it was my job.

Damn I miss my vette I try to lie to myself and justify my lack of horsepower but it's not working.

 

I had the pleasure of driving a brand spankin new ZO6 when they first came out. My neighbor traded up his 99 SS camaro for one. He let me drive it, dumb ***!! That car could move. Sorry to be off topic, but I do like the manners of a C5!!

Any way my TPI 383 has no trouble frying the tires from a stand still, I don't even have to power brake it!! Thats with a built suspension on street tyres, though.

 

Wow! It's pretty obvious you are not going to have much of a problem frying them off the line with the combo you are running.

Thanks for all the replies. I appreciate all your comments and suggestions. I think that I have a compression problem or my rings or heads are bad. I'm not getting enough umph off the line. In fact, my GTA feels like a sedan or cruise mobile.

 

You can test your compression easily enough. If you don't have a compression gauge you can get on from the auto parts store or a nearby Sears. They are not too expensive. I would want to see at least 120 psi per cylinder. Also watch the guage to see how fast the pressure drops off. I would let it sit for about 5 min. then check it again. A more accurate way to test is a leak-down test but the guage for that is more expensive. Good luck with your GTA.

 

Over here!!!!!

My setup right now Pulls HARD well into the 5000 RPM range!!! I think that the right combo is essential to an engine producing power high into RPM band. My L98 pulled hard, but fell off BIG TIME after about 4000. This VORTECH headed beast I have now is UNBELEIVEABLE!!!! The best part is that the TPI system is basically stock (except for the SCDP intake base) and pulls to 120 with just as much power as it does to 60!!!

Just a little info there

Later

 

i hear about the massive low end and mid range the TPI makes in stock form. let me list my mods to make sense

ported plenum
bottoms cut off air boxes
headers
hollowed cat
dynomax exhaust

i was wonderin why my stock head and camd 305s power band really kicks in at 3K rpms and i shift out at 5200. that seems more of a midrange partial top end power band doesnt it?

 

ttt

 

If I could do a screen shot of my dyno run it would be easy to show but I can't. (sorry)

The TPI motor has an interesting torque curve. It "spikes" instead of stays flat. Peak torque in your motor is at around 3000 rpms. That's why you feel the almost dramatic "kick in" at that rpm. It's more dramatic in the 305 than the 350 because of the less cubes.

After you pass above 3000 rpm, your torque takes a nose dive and your horsepower climbs to 4500 rpm. Then your horsepower takes a nose dive too and the motor is done. 5200 is an optimum shift point even though it's past the motor's peak is because it places the rpm at the best spot for the next gear.

The cardinal rule behind all that is:

Torque MAKES speed, horsepower KEEPS it.

 

Here is a dyno chart from my '87 Vette when it was stock and had well over 100k miles on it. Shows what the TPI powerband pretty much is.

At the wheels....
211.2 hp
315.1 ft/lb



With my transmission I had the best ET & MPH shifting at 4600 from 1st to 2nd and then 4600 for all shifts after that. I was able to get it to 13.89 @ 98.17 on street tires before I started modifying it.

Some people will do some modifications and say "it pulls hard to 5000 now." Well "hard" is all relative to what you know. Its great if you can get a few more usable RPM out of it before it noses over, but when you compare "hard" to a TPI with an aftermarket intake & cam its rediculousy different.

This chart has my car's stock curve and current curve on it. It needs tuning horribly right now as below 3700 is rich as a pig. SO much left to be had in her down low and a little up top I think. I can't want for better weather.

The "Delta Time * 200" curve is acceleration. I took the time between each datapoint and multiplied it by 200 to make it show up visually easier. It shows me where the car accelerates the fastest. Nice to know so you can set yourself up in the sweet spot for a race with someone.

Current at the wheels..
322 hp
369 ft/lb



Horrible current A/F as you can see below....



As of that dyno pull the modifications were these minus the Nitrous and MSD sytems in that list that it has now.