Hi all,
I have heard the best bang for your buck on a 89 stock L98 is to do the intake and exhaust.How much hp gain will I get if I have the plenum ported, buy larger runners, and either port the intake or get a aftermarket intake.Right now I have a High flow cat and muffler elimintors on the car,will this allow it to breath enough or do I have to do headers also.. Thanks in advance...WW

 

Well that combo on mine with headers ran 13.22 at 102 this fall. Changing it out for a SR this winter.

 

10 RWHP !

Vic

 

Aardwolf, took measurements the other day on a base I did will have to look them up .It was over 1-3/4" if I remember right

To the OP

Dont know the exact# depends on what you use. I think Joe (anesthes)may know, sounds like he has done it all modding TPIs

On a build thats going together now:

Overported stock base
AS&M runners with some further work
Stock plenum siamesed, ported

IMO Siamesing the plenum by itself is a complete waste. Once the wall (3/4" thick) is gone you have the flat surface of the runner flange so IMO it does nothing or close to it. Took the wall down between the runners about an inch or so high and shaped/tapered it down such as an air foil (lol) might be.
You can also blend the lip of the runner where it meets the flange, work carefully there.

Real gains who knows but it makes sense to do it .

Have read about the plenum porter guy doing these and wondered about it so after doing one yesterday and fitting the runners up it was clear that more needed to be done to really be effective; . .Spend time on the entry way behind the TB. Should only take a few hours tops to do everything, not much else to do inside a TPI plenum.

Some call all of it a waste of time but being as TPIs arent the best to begin with everything helps. Some may disagree which is cool.

 

My big mouth base is about the same at 1 3/4".

A forum member dynoed just the intake change and got a gain of 16.5HP and 17.9LB/FT of torque.

http://members.***.net/effergyvette/...e_install.html

 

WoW! After reading that I am through considering those mods! For the money all that stuff costs and the small gains you get, it just doesn't seem worth it. Not to mention all the problems associated with the swap. I was looking at Stealth Rams earlier and they seem like a real winner! Except they need to be slightly modified to fit under the hood, and they don't allow for an EGR...

 

Seems nothing aftermarket is completely bolt-on without an issue somewhere, sure would be nice if the mfr would fix these issues or imform via install instructions ahead of time rather than finding out the hard way.

When you think about it 10-15+-hp isnt bad, swapping carbed intakes hardly ever gets you that.

 

I only paid $400 for my intake, runners, plenum, and TB so I thought it was worth it. I didn't have any trouble with the install, it was fun!

 

If you look at flow data, the stock TPI flows over 190cfm, but the heads flow about 180 @ max cam lift. Theoretically, you should gain nothing from porting. But, most people report anywhere from 10-15hp after porting/enlarging their intake. This can be attributing to lowering the resistance of the complete intake/head runner pathway.

The exhaust pathway on an L98 has more room to gain (flow) than the intake. Gains of up to 30hp are typically reported with complete upgrade -- including headers. There are several constrictions in the stock exhaust including both cats, the stock mufflers, and even the manifold.

Aardwolf's time of 13.22 could be misleading. I typically think of an L98 as a 14 second car. Some could think he gained 50-75hp because of the time. 16-20hp seemed too low as qualification. I would expect the gain to be between 30-40 with both exhaust, headers, and intake.

Ron,
I don't follow what you're saying about siamesing the plenum. It's usually done where the section between adjoining ports is completely removed. (This includes the rib on the backside). With both standard tubes (and especially siamesed SLPs), it's supposed to eliminate the turn of air flow into the rear of each paired port.

Less turns, better flow.

Edit: I re-read your post. I agree siamesing plenum has little value w/o mega-ported SLPs. (or if other runners can also be siamesed.)

 

Well if I found a deal like that I would snap it up real quick, but as far as paying full price which would likely be over $1000 for a little bit of horse power, I don't think it's worth it. There are plenty of intake options out there and tons of combinations, this is just one I have eliminated

I am still considering saving for the L98 STS turbo system which is in the works... It's a lot of money, but instant HP and Torque, and a sweet sound!

 

Mine ran 14.2 stock. There's a SR for sale I saw in the race section for $600. That's a nice deal and better then the LTR. I wouldn't pay $1000.

 

Here is how I see it.

Intake = $1,000 F.I.R.S.T or ported LTR system (Including new TB)
LT headers = $750
Heads (ported or new) = $1200
Cam = $250
Exhaust (hi flow cats) = $300
Rest of exjaust = $700
Tune = $350
Roller rockers = $200
Misc parts and fluids $100
Labor = Endless or 100 + hours $0

= $4850 = 315 Rwhp = Rough running, lopy, non-daily driver, leaky, constant attention = no fun = drop in an LS1 and forget about it and save $2000 and 80 hours labor. If I'm wrong please advise

 

LS1 conversion:

Typical junkyard score for motor &trans, $3500?

figure odds and ends, $1000, maybe 2k.

Project halfway done with 5k spent and lots more to go.


Spend $2 on for sale sign, toss a brick throught the windshield and get drunk

 

Should be able to do for $2,500 and still have 30 bucks left to get drunk and throw a brick.

 

So, you can buy a brand new LS1 for $2,800!

You exhaust would be needed for any motor too. Drop that from your difference.

A 315 rwhp motor doesn't have to be rough running or leaky either.

Do some more figuring.

 

Way off on price but I could see how some would buy new at retail prices.

 

Yep, about 375chp from a modded TPI L98: heads, cam, intake, exhaust...

 

Yeah, but how is your MPG compared to stock???

 

Mine has been slightly higher then stock, 30ish at cruise.

 

nice!