I posted this in the scan and tune section, but I got few answers, so I am asking here:
1. What are the better ways to improve the low-end torque of a normally aspirated L-98 and still maintain emissions standards and stock drivability?
2. I know that the street legal performance PROMS that are sold can only alter the ignition timing and the fuel curve above 80% wide open throttle. Is it possible, however, to program your own PROM to take better advantage of modifications designed to improve low-end torque? Is it possible to do this AND maintain emissions standards?

David


[Modified by DSKRALL, 7:35 PM 9/12/2003]

 

I donno...

those things are gut busters already... the main reason they have sooo much torque in the first place, is caused by the intake runner design. that design causes the engine to stop producing power above about 4000 rpm!!!

 

Just changing the program is unlikely to really affect the torque output THAT much.
To improve low-end on a motor that is already designed for low-end, you would really need to enlarge the cubic inches. If you only want low-end.

Either that or a set of lower ratio gearing, but beware that you will lose top-end considerably with that mod.

Any kind of portwork, or exhaust work is going to raise the torque peak to a higher rpm, thus lowering the low-end 'feel' and giving more HP as a result. Although all this is possible and still pass emissions.

What we would do is install the SR and a good cam, that way you would have the HP you need up top, and the cam/intake together would improve torque across the entire band, not just low end. It is possible to pass emissions this way too, with a correct PROM.

So what exactly are you after

 

whats SR?

 

Superram.

 

Thanks to all who replied. Everyone is helping me learn more about PROMS and how they interact with modifications.

David

 

You don't mention the year of L98 but a good way to put more torque to the road with an '87-'91 L98 is to install a 2,000rpm TC. You will launch a little higher into the power/torque band (about 1,700-1,800rpm is nice) and yet keep that tight feel for street use.

If you port & work #113 heads, install a nice cam & hi-flow exhaust system as well as open up the intake and go with a 160 deg stat & fan switch you will indeed feel a stronger pull throught the power band. You just need to be careful to not "over-cam" it.

If an '85-'89 L98 use the method on my site to set the TPS base position for crisper throttle response; then advance base timing till just below ping with premium fuel.

Also, try the cut-back spark plugs (details on my site, link in my sig below) for a more complete charge burn and more HP.

Install underdrive crank pulley & overdrive alt pulley to reduce parasitic drag and put more torque to the drive train.

Since acceleration is a matter of mass as well as force applied, do a little weight reduction to get a harder launch:
-delete the spare & jack, worth 40-50lbs
-drive with around 1/2 tank of fuel, worth 60lbs
Don't use weight to get traction, it just slows acceleration; set optimum tire pressure for hookup and chose good tires as well as loose shocks.

Avoid a chip change, instead use an AFPR to add fuel at WOT as you increase air flow.



[Modified by 65Z01, 2:31 PM 9/14/2003]

 

1. Define low end. The strict answer to that question is better flow out of the stock parts and/or displacement increase.

2. I didn't know that. Where'd you hear the 80% rule? Custom tuning for a stock motor won't net too much. The more modifications, the more custom programming helps. Meeting emissions depends on your emissions standards, modifications, and tuning ability. It is probably possible.

The cam and heads make the motor a torquer, not just the TPI. A lower restriction, high velocity at low rpm exhaust will increase low end torque. Port work that removes restrictions but doesn't decrease velocity will increase low end torque.

 

Aaron,

1) "Low-end" would be around 3,000 RPM. I believe the L-98 has about 330 - 350 lbs-ft at 3200 RPM. I would want a torque increase to be concentrated between 2,800- 3,200 RPMs.
2) John Loughmiller, author of "The C4 Corvette Source Book," says that any chip that is sold as street legal must be authorized by the EPA. Consequently, it can only alter the timing and fuel curves above 80% wide open throttle (WOT). It makes sense to me because I would expect that most driving is done at less than 80% WOT, so the EPA is most concerned about emissions that are produced in that range. In addition, it may be hard or impossible to run a leaner, environmentally friendly mixture above 80% WOT without engine damage, so the EPA may relax its rules to compensate. This gives the chip tuners a chance to monkey around a bit.
By the way, according to many of the answers I have received, the modifications I am interested in can be handled by the stock PROM. I'm impressed.

David

 

Quote
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Avoid a chip change, instead use an AFPR to add fuel at WOT as you increase air flow.
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What is AFPR?

 

Quote
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"If you port & work #113 heads, install a nice cam & hi-flow exhaust system"
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Me again, I'm really looking foward in modifying my 88 vette next year, but I don't want to spend to much of a fortune in the car.
I though of a moderate cam ( I might already have one in the car since its a 383 and the engine was rebuilt), port work on the heads, SuperRam, and exaust headers

Reworking the 113 heads seems like a good option, but I'm honneslty wondering what kind of shop to go to have this done.
I'm also wondering what you meen exactly by hi-flow exaust system.
I have a flowmaster catback exaust system, I have the stock headers and pre-cats and cat on the exaust system.... is this considered high flow? probably not I'm guessing.
I want to keep the exaust system emunission legal.

Could I simply bolt up a set of shorty's headers and keep the pre cats or cat or should I have after market higher flowing market ones installed... or even long tube headers?
would the long tube headers eliminate the pre cats? if so, would the car still be emunission legal? if not, would there be an after market cat that would compensate for the pre cats being removed.

boy oh boy, so many questions.

Thanks for any help, but I've honnestly learned a lot because of you guys

 

Adjustable Fuel Pressure Regulator.

just that... allows you to adjust the fuel pressure.

 

The Superram will be emissions legal provided your cam isnt too big. And for that I would need to know the specs, but if it passes emissions now, then it will with the SR.

He means you will need to improve the stock exhaust by doing something to the front Y and the catback. Youve done the catback, now either get headers or a mandrel bent front Y. Headers would be better, since the stock exhaust manifolds are choking the 383. Yes they would eliminate precats, but so would a mandrel bent front Y. The stock Y is too small, just like the manifolds. It is possible, provided the local emissions standards arent excessive, to pass the sniffer test with headers so long as the main cat is good. For that i would get a Random Tech.

The same shop who did the 383 should be able to do the heads. Whomever in your area with a CNC machine could also, if they have the program.
Basically any engine builder should be able to.

 

Indeed for a 383 I would suggest 1 3/4" ceramic coated FL headers as the ticket for that hi-flow exhaust; it sounds like the cat-back should be ok.

If you can't find a good local shop (either CNC or a good rep. at hand porting) you might send the heads out to a shop with a good CNC program for #113 heads.

I would stay with the stock chip until it can't do the job because of mods; so long as the BLM cell values are within limits the ECM can provide a stock A/F ratio at part throttle; that's what the emissions sniffer is interested in.

 

ok, so a Front Mandrel Y pipe... is an aftermaket Y pipe right? with no pre cats? and bigger.

I was talking with one of my friends yesterday and he told me that the Front pre-cats are simply resonators and have nothing to do with the emunission, this was entirely the responsibility of the Cat.

so? is just having the pre-cats/resonator gutted out a good idea?

 

yes any new front Y w/o precats is going to be aftermarket, NO precats.

they are not resonators, they do affect emissions, they begin the catalyzation so the main cat has less to do
Cat tech wasnt what it is now, now we can do almost the same job with a higher flowing aftermarket main cat that we did back then with 3 cats.
Dont gut them, replace the entire Y with something bigger, or youre not gaining anything.

 

what does CNC stand for? and what about BLM cell value?

I'm guessing CNC is a certification of some type?

 

CNC is a machine, some guy ports a head by hand, then a computer reads/scans it and turns it into a program and then matches it for every head it does in a CNC machine. So they are all ported identically.

CNC=Computer Numerically Controlled

 

Indeed CNC porting relys on a machine set up properly to precisely port each cylinder in the same way to achieve the same flow in each cylinder.

Hand porting can achieve the same thing by a good shop and usually requires use of a flow bench to achieve flow matching between cylinders.

BLM (Block Learn Mode) cells are places in the computer's memory where the ECM stores injector pulse width correction data to achieve stochiometric A/F ratio under different driving conditions in closed loop mode. There are 15 such locations with a nominal value of 128, which means the ECM is neither adding nor subtracting fuel. As you add fuel for optimum WOT operation, with a stock chip, the ECM learns to change these BLM cell values to maintain stoch. A/F ratio. When the engine needs so much more fuel at WOT that the ECM can no longer maintain the stoch. A/F ratio by reducing injector pulse width in closed loop, you then need to have a custom EPROM burned.

This is why I use an AFPR for adding more fuel at WOT (tuning), until my ECM can no longer compensate in closed loop. A scan tool is needed to read these BLM cell values, which are an indication as to how well the ECM is managing with increased fuel pressure.

 

HOLY FRIG!

That was a bit of chinese for me hahaha :confused:

but I understand the big aspect of it at least, its when the ECM can't controle how much gas is required by the engine any longer and then this APRF unit kicks in to compensate.

Did I understand?


[Modified by TheStef, 6:27 AM 9/17/2003]