well here is the deal i have a 91 and my previous things were.cut air lid , k&n , tb bypass,cat removal ,12* timing, and borla exhaust.i took it to the strip , and made an 8.84 1/8 mile pass.i was ok with that.i bought a new set of as&m runners ,bb&k adjustable fuel pressure regulator, accel big mouth base , ported my plenum , and installed 1.6 roller rockers. i just drove it and i am so dissapointed in the way it feels, it just disgusts me.i guess those things really hurt my torque.or maybe i was just expecting to feel too much.i will know the whole tale when i go back to the strip.

 

adjust the FP and timing when you go back to the track

12* seems like way too much to me

 

An intake isn't going to give you much. I got around 7hp from mine.
The first 2 mods I would do are pulleys and headers. Make them your next ones and you'll notice it.

 

with both
6-8 degrees is all you need.

 

I dynoed my old car a few years back and I gained ZERO changing the timing between 6 and 12. I left it at 6.

 

I;ve run that on street tread in my 85 with iron heads and the free mods with a cold air intake.

You'll need more than one pass. What was your 60' time? What was your trap MPH? Launch is everything. You won't always feel the gains, but they will show up in your MPH.

 

Cut lid with K&N is worth about 10chp, the exhaust work might have been worth say 15-20chp.

The RRs maybe 10-15chp and the Accle base w/ AS&M runners another 10chp (I blieve).

Remember that you won't feel a 10-15chp gain.

The best "free" dyno is your trap speed but that needs to be normalized for weather and launch weight.

Since you do not list these mods, give em a try:
-160deg T-stat & fan switch
-toss the frisbee from the front of your water pump pulley, story on my site
-cut back the electrodes of a std set of AC Delco spark plugs, details on my site too.
These should give you another 25-30chp.

Also, lighten up:
-toss the spare & jack (~40-50lbs)
-run with <1/2 tank of fuel (~60lbs)
This 100lb weight loss is worth 10chp in trap speed and acceleration.

 

So you think he'll be making 45-60 more HP with his mods, and has another 35-40 on the table with free mods?

He's not pulling 300 hp at the crank now, and he sure as hell won't gain 70-100 hp with all those mods.



sweet, post your 1/8th ET, MPH and 60' time, and I'll be able to tell you how your car is doing.

Also if you have a timeslip before the mods, that helps too. And how was the weather?

 

I think his mods may have made ~30-40chp gain, which should be "felt".

The "frisbee" gave me 10chp, cut-back electrodes 8chp and 160deg T-stat (in <80deg air) another 10chp.

The RRs are advertised at 10-15chp though I saw hardly any gain from mine.

I believe Vic'89 saw 10 (or was it 15??)chp gain when he installed the AS&Ms with his Accel base.

Both Vic & I have seen around 15chp gain, with stock headers and hi-flow cat but also with mandrel bent front Y. Perhaps I was too optomistic on his exhaust mods, so I downgraded my est above.

I believe that the "simple (relatively) boltons" can bring him up to the 300-310chp mark.

Now toss in heads & cam, along with a tune for the SD intake, and we will see ~380chp. That was proven by TPIS years ago on an '87 Vette.

BTW, thanks for tapping me on the shoulder on those power estimates...

 

With an appropriate tune, the changes should be worth a solid 25-30 HP.

15-20 for the intake mods and another 10-15 for the rockers is reasonable. An inadequate tune can easily negate gains of this magnitude.

If a custom tune is not planned, I agree with the others that some time should be spent on dialing in the fuel pressure and timing to get the most out of the present mods.

12 degrees initial timing can work which puts the total advance at around 42 degrees, but this may put you into knock retard which can subtract 8-12 degrees at wot depending upon rpm.

The end result may be less advance than running with the standard 6 degrees initial advance when knock retard is considered.

Unless you're scanning and have a handle on the knock retard, I would suggest that you back off the timing and start working with fuel pressure.

You will most likely need to raise the fuel pressure over stock settings since your mods will tend to increase the airflow.

If you post your current fuel pressure setting with vacuum off, you will probably receive some guidance from the forum on which way to go and how much pressure to try as a starting point.

 

i was so ticked yesterday that i left out some stuff, sorry.also the runners i used are slp , not as&m. i had the frisbee remove before, i got that off of vaders site i believe. and i already have a 160 stat. my 1/8 was 8.84 @ 80.790 mph my 60 foot was 2.01. this was before the intake , runners , ported plenum(lots of work there) and roller rockers. the temp was around 60 to 65 degrees out , it was a nice day.

 

what do you guys recommend on fuel pressure? i still have the stock injectors.oh yeah i was running with the spare still in , and my sound system,(my speaker box is close to 100lbs) like i said the initial 8.84 i was ok with , i just thought i would feel the new stuff.

 

I suggest you bump it up a couple psi for now, and wait until you get back to the track to test things, unless you get it on a dyno.

 

I've made essentially all the same changes to my '91 Vette.
Except I used the AS&M runners on it.
Went to Vaders site a couple years ago and went down the list of his reccomendations.
I already had installed one of those 'Raptor' transmissions with a 2800 stall converter.

After making ALL those changes I felt absolutely NO SOTP change in it EXCEPT..... at Wide Open Throttle...
At WOT it easily breaks the tires loose on the 1-2 shift now and before it didn't do that.
'Regular' driving it just feels like it always did.
The additional power is there you just have to dig down 'deep' to get it.

Don't know HOW much more it's got as I've never had it on a dyno.
I'd take a guess at maybe 20 HP or 25 HP max more.
My engine has 110,000 miles on it so I figure it doesn't have the whole 250 HP it came with. I'm discounting that down to maybe 235 HP.
So with my old engine and all the changes I've now got maybe 260 HP at best.

Setting the base timing to +12* on 'my' '91 made it 'feel' much better and didn't cause additional knock counts.
+12* base timing made affected the SOTP feeling quite a bit.
Additional timing made it feel more like it wants to 'go'.
I think each of these engines is a little different and that accounts for some running great at 6* and others running great at 12*.
Timing chain stretch is a little different, ring wear is a little different, cam lobe wear is a little different, tuning is different, etc, etc.
MHO is the 'proper' value is somewhere between 6~12* btdc but you need to monitor the knock counts and knock retard to know for sure.

My '91 WAS detecting some "false knock" around 2200 RPM... could never find a cause for it. There was no discernable 'pinging' or abnormal engine noises.
I retarded the timing in the PROM at that speed and it didn't help. Lot's of toluene in the fuel didn't help it either so I retuned the knock detection in the PROM to eliminate the knock retard at that speed. That took a BIG soft spot at 2,200 RPM, that could be easily felt, out of the engine.

I think the old L98 is going to need a cam and heads before it has a chance to get close to 300 CHP.
My 305 HP LS1 will easily eat the L98's lunch from ANY speed on ANY day. The LS1 is in an entirely different league than the L98, torque and whatever included.

 

With hypereutectic pistons, my preference is to be conservative with
timing.

This style of piston is quiet, long lasting and inexpensive - but fragile
when it comes to holding up under conditions of pre-ignition and
detonation.

.

 

It's much better to 'sneak up on it' rather than 'sneak away from it' when the subject is detonation or preignition either of which will run the cylinder pressure off the 'Richter' scale.

 

If you think about all the modifications you just made, they all add upper RPM horsepower and kill low end torque.
Maybe even beyond were you like to shift. often times mods like these have to go along with a gear change like 4:10's so you can feel the difference.

I have a drag car that I have experience with , that's how I know what works and doesn't.

Desert

 

What we know so far

1991 L98 Auto w/ undetermined rearend ratio
Of these, my vote is that the two modifications that would
impact torque and driveability are the SLP runners and the
ported base. If you wanted to go back towards your previous
performance, swapping from the SLP runners back to the OEM
runners would help restore the lost intake velocity.

Have there been any other changes?

When the runners and base were 'ported', did the porter remove
portions of the wall separating the inlets in the runners or base?

You mentioned in the past that bigger valves were being
installed. Did this proceed? If so, what size are they now?

You also mentioned 'thinking along the lines of .550 lift with
238 @ .050. and a 112 to 114 LSA.' Has the cam changed?

Has there been any tuning since completion of the modifications?
If not, see tequilaboy's remarks above and consider contacting
Alvin at PCMforless for assistance with dialing in your present
combination.

Are you willing to swap to a higher stall torque converter and
numerically higher gears? Or is this farther than you want to
go with the mods?

.

 

Isn't a EPROM update required to change the timing? I thought the static timing had no effect once the ECM takes over?

Joe

 

The base or static timing is a mechanical adjustment and the ECM has no control over it.
That darn pesky ECM just can't MAKE me set the base timing at 6*.
Matter of fact that ol ECM isn't very smart at all.

In the ECM 'constants' there is a data entry for 'Initial Timing Advance' and that is where you have to tell the ECM what YOU set the base timing at.
There is another data element for 'Maximum Spark Advance' and this one is the (my term) 'Spark Advance Limiter'.
It's like the speed limiter...... The ECM uses the Initial Spark Advance and the Spark Advance from the timing tables to arrive at a 'Total Advance' number.
If THAT number exceeds the 'Maximum Spark Advance', yep, you guessed it... the ECM will subutute the 'Naximum Spark Advance' number in the code and that's all the advance you'll get.

IIRC (and oftentimes, I don't) the EPROM timing data is a timing 'additive'.
Whatever is in the tables for a specified RPM & MAP (on my car) adds to the base timing.

If the base timing is set MORE than the number set in the 'Initial Timing Advance' constant in the ECM constants, then that's cheating the ECM outta knowing what's going on......
It will then use the base timing that it knows about and the timing from the spark tables to arrive at the 'Total Spark Advance'.
But since there is MORE advance than what it knew about, the complete timing curve will be shifted up by the degrees of advance that was set into the BASE timing BEYOND which was set in the Initial Spark Advance.

Example: Initial Spark Advance (set in the ECM constants) = 6*
Base timing is REALLY set to 10*..............
Now.... the complete spark curve at all RPM & power settings is advanced by 4* 'more' than 'computed' because the ECM has NO WAY to MEASURE the actual physical static timing advance.

I'ts been awhile since I've driven around with a laptop in the front seat so I hope I ain't getting that 'oldtimerz' disease on this.

A crankshaft or camshaft 'Position Sensor' would allow the ECM to know where the static ignition timing is set.. but, alas, we haven't got them on our L98's.

Did that confuse everyone else as much as it did me?????????


That statement is another way of saying what I posted above.
The additional power is there you just have to dig down 'deep' to get it. 'Down deep' means at greater throttle opening.
I'd like to have my power in the 30% to 70% throttle opening range for driving on the street but after my 'improvements' there is little more THERE than before. But from 70% to 100% throttle there is definitely more power than before.


You need some data collection on your car and an EPROM to make the most out of your 'improvements'.
Where the heck are you? I'm around Columbus, Ga. If you're near here, PM me. We'll get together and I'll scan it for ya and burn you a chip or two.
Heck, I've got the first 32 I burned for my '91 around here somewhere. One of those would probably be about right for your '91.

What IS IT with folks wanting to be 'hiding' all the time now? Are they afraid or terrorriwsssst?