steve40th

I am getting ready to cure the split BLM issue in my LT-1 (142/128 hot idle) and was curious if my HP/TQ and ilde will be better. Steve

STL94LT1

I am getting a split BLM idle, but not nearly as far off as yours. Are you sure you don't have an exhaust leak?

Have you considered using VE Master, it may get your BLM's closer without alot of work.

steve40th

I ahve no exhaust leak, and I have a 93 so VE Master doesnt work, I believe. I have a PROM, not a flsh computer like yours. The BLM issue is very eazsy to fix, at least with a BBK throttle body. When I get mine done, I will do a back to back run to prove it, as I saw it first hand this weekend that it works.

96GS#007

VE Master is for non-MAF cars. If I remember, '93s are speed density so VE Master will work.

That's a pretty hefty split you have with a HOT Cam. I modified my BBK to correct split LTFTs (BLMs) [see write-up in "Tech Tips" at hashmarks.com], but mine were pretty close to start with...within about 4%

To answer your original questions...Your idle quality should improve (better fuel distribution), and hp & torque may improve. With such a large split, it's likely your WOT fueling is way rich.

After drilling the throttle body, something else that'll help split BLMs is adjusting the throttle blades. By closing the throttle blades as much as possible, more air will go through the idle circuit vs the main plenum. The ECM will correct the idle rpm via IAC positioning. With the car running, you can adjust the throttle blades and watch via scanning software as your BLMs move closer together.

:cheers:


[Modified by 96GS#007, 9:59 PM 12/31/2002]

steve40th

I am no tunrer, thats for sure. What do youhave your IAC set at for idle?
MY WOT is130left 120 right. But I was also informed that the BLMs are going to be close to whatever they were right before WOT.
I checked my Diacom data and this is right, my WOT BLMs were always close to what they were before I floored it.

96GS#007

My IAC count is 27. Keep in mind it's a fallout of adjusting the throttle blades for the optimum fuel trim balance vs adjusting for a specific IAC count.

WOT BLMs are meaningless for tuning. First thing you should do is get your idle rpm set and idle BLMs trimmed correctly (i.e. minimal split). After that, adjust for part throttle driveability. With a non-MAF car, you'll do this via the VE tables (LT1 Edit) or their TunerCat equivalent. Once that is done, then you need a Wide Band o2 sensor or WB equipped dyno to tune WOT. Lastly you can go in and tweak timing (changing 2* at a time while watching for knock counts seems to be the rule of thumb).

You'll want both your BLMs to be slightly below 128 during cruise. When you go WOT, the ECM will Always use 128 as its base and then adjust via whatever is programmed in the Power Enrichment (PE) vs RPM table (LT1 Edit speak again). If you have BLMs above 128 during cruise, the ECM will use whatever the BLM was at the time you went WOT in addition to the PEvsRPM factor. Since the BLM may be 129 one time, 135 the next time, 132 after that, etc, you can see how the WOT fueling is inconsistent with part throttle BLMs above 128.

How in the heck are you getting 400+ at the rear wheels with a HOT Cam? Spray? LT4s with ported heads are lucky to see 380.

:cheers:

steve40th

I have a 396 LT-1 with CNC stage 3 heads and shorty headers . The cubes make allot of the power.
SO after getting my idle BLMS fixed, get my part throttle fixed, then WOT just play with the timing?

96GS#007

After you get idle and part throttle BLMs squared away, tune WOT via the Power Enrichment (PE) vs RPM table. If you're using TunerCat, they may have a different name for the table than LT1 Edit does.

After you're done with WOT, then you go back in and fine tune the timing across the rpm range. I suspect most of your timing changes will be in high load/high rpm areas of the map if you need them at all.

Tuning is definitely an iterative process

As for having a 396 :thumbs: :thumbs:

:cheers:

steve40th

Hey thanks allot. I will have it returned when I get the BLMs fixed here soon. Steve

qkvette

Would this TB fix also apply to a 58mm. I have this problem alos, and have been trying fix it. my IAC count is at 120 when cruising and shoot up to 160 when at idle. should i open the blades more ?

thanks,

joel

96GS#007

Try to get the IAC counts to be 20-40 at idle. 1st thing to do would be to set the minimum idle air. Warm the engine to operating temperature and then turn the engine off. Next, turn the key to "run", but don't start the engine. Disconnect the IAC electrical connector. Start the engine and adjust the throttle blades via the set-screw to the minimum idle rpm at which the engine will run. Turn the engine off and reconnect the IAC connector. Start the engine. The idle may hunt for a few moments and then should settle down. Check your IAC counts. They should be pretty darn close. If desired, adjust the throttle blades until the IAC settles exactly at your target IAC count. If this last step doesn't get you to the correct counts, you have mechanical problems somewhere.

:cheers:

qkvette

thanks.

i will try it this weekend and give you an update.

joel

ANTI VENOM

I too had split BLM's and did the hole in the BBK TB trick. I just want to warn you to start small with the hole. I started small and it seemed to take things in the right direction, so I went a little bigger and so on. I thought I had everything in pretty good order. The car was warm and the IAC counts were about 25 and the BLM's could actually be shifted. If the left was high, you could actually change the throttle plates until the right was high. I thought this was the ticket, until the next morning. It was a cool 40 degree morn and I started the car. It initially roared to life and then it started to chug and then it died. I tried to start the car again and it would not start without a little throttle. As soon as it warmed a little it ran fine. I noticed that the IAC was at 160 while it tried to idle cold. My hole is now too big, as far as I know. I am going to work on it tomorrow. I hope that all goes well with your modifications!

CorvetteZ51Racer

What exactly affects BLM values? I have an '87 with the stock 165 ECM, using TunerCat.

ANTI VENOM

I believe that the splits are caused by uneven air flow through the plenum at idle. The uneven airflow is caused by "reversion" (I think that's what it is called). Some air get's past the intake valve while it is still open and is forced into the plenum. Something like that. I presume that cams with tight LSA would be worse. Hopefully someone else will clear this up. This only my take on the situation and is probably not 100% accurate.

CorvetteZ51Racer

Reversion can be caused by a few things, one of the most common of which is actually exhaust gas being pushed by up in the intake runner at the very beginning of valve overlap. When a car has a really hard lope to it, it's not a "healthy cam", it's choking on EGR...the valve overlap is effectively introducing a lot of EGR into the motor and it's trying to catch itself, while the cylinders fire sporadically at or near idle. If you notice, most all engines that have a hard lope don't have very good low speed throttle response...the EGR effect is why. There's another cause of airflow distribution problems in intake manifolds, but it happens at higher rpm and has to do with resonance frequencies and the inertia of the inducted air. Basically, the air moving down an intake port has speed and mass....momentum. Ideally, the intake valve would close when the speed of the air at the intake valve is 0. However, once above a certain engine speed (which is determined by the cam size), the valve closes while air is still trying to get in. When the air hits the back of the valve, it bounces back up in the intake runner in a pressure pulse, and enters the plenum. Depending on the design of the plenum, the wave will either almost vanish upon hitting the plenum (as in Formula car style manifolds) or it will cross the plenum and shoot down one or more runners. As one could imagine, this causes not only airflow distribution problems, but AFR problems for whatever cylinder gets a "piggyback pulse". On that cycle, that cylinder will burn lean. Try to compensate for that lean cycle in an effort to prevent burning a hole in the piston (assuming a tightly wound motor), and whenever there is a cycle in that cylinder that doesn't have that piggyback pulse (remember, engine airflow is truly chaotic...there is a discernable patternt to a lot of it, but these pulses are truly chaotic in nature), the cylinder will run rich.

Probably a lot more than you wanted to know about airflow distribution problems in intake manifolds (which is my area of research), but I hope that helped.

ANTI VENOM

I don't believe that L98's have a problem with split BLM's. The Lt-1 intake has a passage for idle intake air seperate from the main plenum. If you open the throttle plates a little, then less air goes through the idle passages and vice versa. There seems to be a compromise between the amount of air going through the plenum and the air going through the idle passage on Lt-1's with regards to how much reversion affects the BLM count. That is what I've seen anyway.

96GS#007

If I recall correctly, L98s use one o2 sensor (which means the ECM generates only one BLM number). With one sensor, it'd take a custom computer setup or EGT monitors in each exhaust port to know about split BLMs :)