I have a 87 GTA with a lt1 inake and 24 lbs injectors. the rest of the mods are in my sig. Anyway I have a problem with whenever I step on the throttle aroung 1/2 or further I get a loud backfire and it spits and sputters for a second then takes off. It is a maf system. Any ideas?? If anybody wants to TAKE A LOOK at my bin to see if I have something totaly wrong pm me.

 

Does it run alright otherwise? Idles good? Have you tried moving the timing?

 

Yeah it idels fine and otherwise runs fine but just when you step into it it just backfires. i have removed timeing and added timing but that had no effect, it still does it.

 

I also noticed that when the engine is cold and I rev it up the engine seems lazy, like is being choked out or something.

 

Definitely sounds like something a little PROM tuning can clear up.

Likely needs some pump shot. With the short runners, air accelerates into the heads much quicker when you dig into it. The MAF can't see all of it instantly, thus your bog.

 

You are going to need to change the MAF scalar tables and the AE. The majority of the effort needs to be with the scalar tables.

RBob.

 

is a backfire same as a stumble? mine coughed/stumble on hard acceleration in low MAP area. i increased pump shot 150% over stock everywhere and it helped to a point it went away for the most part. with cold weather came back. will add more at low TPS % and see how it likes that. speed density TBI...

 

How cold blooded is this engine? Which intake manifold and what does it have for heat (water/exhaust?). Are you still running the preheat on the aircleaner? What size throttle body?

RBob.

 

Ok it is the stock throttlebody and it is a deffinate (sp) pop through the exhaust. Now for the pump shot I dont see that table in my tunner cat program. I know it is not good in the engine to get on it when it is cold but it feels like runs strong. It is the lt1 intake and it has no preheat to the aircleaner. Is there a specific way to adjust the maf tables, I was told by kvu a long time ago how to do it but everybody said it was wrong so I put it back to stock. But it always has stalled or tried to stall when put into gear, if that helps.

 

Look at the sticky's at the top of the thread list. You'll find in one of them the way to adjust the MAF tables. You'll also find out what tables relate to the pump shot (AE Acceleration Enrichment)

 

Sorry about confusing you with the cold-blooded questions. They were for Ronny. His is wet flow and without some controlled heat the tune will never be on (either that or an IAT sensor and code changes).

Yes, changing the MAF scalars is correct. It can be tricky as to how the table scalar term works along with the tables going from one to the next to form a continuous curve. But once understood it is straight forward.

As JP84Z430HP mentioned there are a couple of writeups on adjusting the tables.

RBob.

 

single plane weiand xcellerator for small chevy. low profile. flows 1000 cfms in total. no egr, no heat riser. has custom plenum on top for two single bore TB at 2.00 inch. former 84 crossfire. i dont believe it can use IAT with 7747? or can it? i assume that is a sensor in air cleaner? had a sensor in air cleaner for the doors from heat riser/manifold. headers now so that is gone. the cold start/choke is good. idles nicely and comes up on temp. first five miles has the stumble issue unless you take it real easy on throttle. goes closed loop at about 155F with 180 therm. seems to run better cold in open loop. blms are pretty good. PE is 12/1 on dyno but since then i enrichened a bit so maybe 11.7/1 ? pump shot everywhere is 150% of stock. I know i can enrichen open loop air fuel vs coolant and vs map. i may do that and run a second "cold weather " chip. will add pump shot now over 150% but only in lower map areas. dyno was good in PE in lower map at 14/1 to 13/1. in summer stumble almost gone unless you wack the pedal in low map. i can force a stumble. 3.07 gears dont help.

 

This all sounds too familiar. A cold blooded wet flow setup. Does the manifold x-over run under the TBIs? If so then running coolant through the x-over will help.

Nope, no IAT support with the '747. Even though the '8746 supports an IAT unless the code is changed it still won't do much good.

Two cal items can help with the AE: Decrease the MAP & TPS filter table values. This will increase the AE and (more importantly) increase the duration of the AE. Do this for the rows that match up with the coolant temperature when it stumbles.

Increase the last row of the delta MAP & delta TPS AE tables. This will help when the pedal is rapidly depressed (wacked) to the floor.

There is another table accel enrich vs coolant that can be increased at the higher coolant temperature to increase the AE.

RBob.

 

Can anybody tell me how the frequency-based MAF on the LT1s (8051 PCM) can be used for pump shot? I don't believe there is any hard-coded pump shot for that PCM, the MAF is supposed to catch it all. Is it just a faster/better responding MAF?

 

RBob,
not sure of the coolant flow? it is just a common single plane used on SBC. the plenum bottom is custom machine made. it resembles a pie pan with center cut out to match the square bore carb flange. that attaches to the manifold. on top of plenum is a flat plate with two holes for the TB's that are staggeredright/left. on that the linkage attaches. with the cold weather issue is the lack of atomization within cold plenum on start up as injectors are too close to bottom of plenum, hence the plenum floor may have a layer of gasoline on it that needs to be brought into the flow within runners and atomized. the path of fuel molecules is zig zag into intake port of head

 

The manifold is collecting too much fuel film. Adding a little heat helps prevent it from building up. Higher air flows also assist in this. It has been known to run coolant through the manifold cross-over (instead of exhaust heat).

RBob.

 

RRob:
manifold cross over? i thought i read my edel alum rpm heads do not have that inlet outlet to intake manifold. maybe the edel performer do???