Well I just helped a local buddy tune his 91 RS. It took 3 days of working on the car before it was ready for a tune. The reasons for the delay were broken rocker studs from having used flat tappet pushrods with the roller lifters . So in a rush he installed 416 heads, a performer RPM, my old TBI adaptor, barely used pushrods and we were on our way.
The only concern I have right now is the pushrods. Are the later self-aligning rocker arm pushrods OKAY with the narrow pushrod channel on the 416 heads? Are they hardened or is the steel going to be okay against the iron heads?
ANYWAYS, back on topic. The 305 is a fresh rebuild 30 over, stock 416 heads (305 TPI), a zz4 roller cam (upgraded valve springs), performer RPM intake, stock TBI, increased fuel pressure bolted to a T5. The thing feels really strong except for in 5th gear and below 2000rpm but I'm pretty sure the 2.73 gears in the back aren't helping. Yes they're 2.73's, the T5 was a swap from an auto.
Now here's the real kicker... it backfires through the TBI at 800rpm when the throttle gets flicked open while in gear. Any idea's? This doesn't happen but maybe once every 20 tries. With my car I had the same issue running the 8746 code and I'm pretty sure this is the code issue that was brought up over a year ago. It has to do with the AE fuel pulse being completely dropped all together when the sync and async pulse widths get calculated at the same time? The RPM intake, large adaptor plate, and open element really require a lot of AE but I just couldn't figure out the backfire. This is the first T5 TBI I've helped tune and he's kind of at a loss. I just can't for the life of me figure out why it's as bad as it is. I mean it's loud and the car will shut off as if the engine was WAY lean but we can't seem to duplicate it.
RBob, do you think this is the sync vs async issue if it's happening at 800rpm? I can't remember what RPM points they "overlapped" hence the reason for this post.
We tried taking out timing, adding timing, doing everything we could but the backfire was happening ~55kpa and 800rpm, only when going from a lower MAP value. I went up and down with the timing, then AE, then both, nothing would fix it. I told him what I thought and he seemed content for now. It's just discouraging to have such a serious backfire be "normal."

 

not really an answer to your question but more of a question arent 305 tbi heads 081 castings w/ center bolts....and dosent the 91 rs have a hydraulic roller block which would mean it would have pushrods for such same w/ the tpi isnt it a roller block...im asking cause i got some 081 casting heads on the way and i may need new pushrods if my ausumptions were wrong and please when u get the .bin worked out send me a copy of it if u dont mind since it will help as a rough tune and did u install a cam while u were in there cause i plan too. this would really help gettin this .bin for break in and driving to work till i data log and make changes for the long run..thanks

 

081's are what he HAD but he used the wrong length pushrods and destroyed all of the studs (rocker arms didn't have a long enough slot).
The 416's are perm bolt and weren't to my knowledge ever offered with a roller cam. They might have been in 86, the "weird" year.
Also, the bin won't work for just anybody. Start with a stock bin and make the changes you would from various tuning help offered here.

 

That's odd, I'm running a very similar combo as him just with 081's and zz4 cam, but they're basically the same heads. Mine idles very smooth at or around 800 RPM and it's never backfired on me. I am running a Holley 670 with GM injector pod but that shouldn't affect it much.

I started my tuning off of the AXKW cal, what stock .bin did you use as a starting point with it?

 

oh he didnt resuse his stock pushrods w/ the 081 heads is that what your saying or he did and they werent the right size for those heads..sorry im new to all of this. and i know i cant use your bin and expect it to run perfect but im sure it will be much closer to right than a stock one...i will still probably get help on here on some changes to make sure it will run okay for break in and to and from work but i just need some kinda rough tune to run till i can get the cam goin in it broke in and get me to work w/ out pingin and stumbling all crazy do u happen to have a stock bin too that is a true stock bin cause ive dl three and they r all different and one even has blank tables in it so i know its not right when its missing ve tables

 

The factory did it that way with the LO3. Not the best, though. Just going with non-SA rockers would be better so you avoid any alignment issues.

 

Jon,

At 0xD220 there is a TPS threshold to double the MAP AE. Above this, the AE term is doubled and it can make it appear as though you have enough AE at higher throttle but itll be lean if the TPS is below 50%. Set the threshold to 255 and it should keep it from skewing things. Might also try adding in some TPS AE. Ive found that the initial surge of air filling the intake needs some extra fuel from the %TPS AE.

The AE is basically calculated once per pass of the fuel loop, or 80 times a second, and outputted to the drivers. If the injectors are already open, then youll loose some or all of your AE that pass. The RPM that the sync and async pulses ocur at teh same rate is 2400 rpm.

I had a similar problem, and I had to add a truckload of fuel, and that was with sync AE pulses. With async you may need even more. Basically Id crank it up till it blows black smoke when AE is enabled, then take it down till it runs ok.

 

On one of the later truck pcm's the low RPM AE multiplier was like 50% of the rest of the RPMs. Look for an RPM multiplier and fix it if it's "broken".
Good Luck,
Jeremy

 

Jon, the stock '8746 or a '7747 ECM? As dimented posted the critical point is 2400 RPM. However, even at 1200 RPM 50% of the AE can overlap and be lost. And, at any RPM a lot of AE can be lost from simple overlap of large sync pulses.

Does your friends car have an upgraded fuel pump? It may be that there just isn't enough fuel volume. Many years ago I had this issue and found it once I installed and logged a fuel pressure transducer. Steady state fuel pressure was fine. But once AE came into play the pressure wold dip everytime. Short dips that didn't show up on a dampened mechanical gauge.

RBob.

P.S. check the fuel filter, if not replaced for a while, may be a good time.

 

hey all...this is my car and i have a walbro 255lph fuel pump and a brand new filter... the car is running around 14psi through 45# injectors... idk if this helps at all... the car laggs on the bottom end... from like idle to 2500 rpm then once it hits 2500 it pulls all the way up to 6000 rpm or so

i guess the cam moved the power band up a little and i dont really have a problem with that, but why the lack of power below that rpm and the backfiring is few and far inbetween and has only happened at ~800, 1200, 2400 rpm and only under about 60+% throttle

but once i get into the power band the car hauls ***. Is there anything in the tuning that can be done to get the power up under 2500 and what about that backfire... thats more important

 

Are there any knock counts being picked up in the area that its flat?

 

no knock counts, could the timing tables be moved to compensate? but jon would know better than i... im just getting into this stuff... also the VE tables have been moved almost DOUBLE stock so i dont think its VE

 

This issue may not be fixable with the '8746 ECM. The Performer RPM manifold has a lot of surface area and requires a huge pump shot. Even more so at low RPM as there is no air velocity inside of the runners. The '8746 is basically a MAP only AE setup. The TPS AE is based on delta TPS over 12.5 msec.

May be worth trying out a '7747 ECM. Will lose the speed limiter, but such is life

The '747 being a truck ECM has better AE routines.

With that cam and those rear gears, I don't think you'll be able to get much additional power below 2k RPM. As you mentioned the cam moved the power band up.

RBob.

 

We didn't double the VE tables, we doubled the AE.
RBob, this is what I was afraid of. I don't think there is a way of geting the AE correct with this ecm. I even tried going directly with the MAP AE and that isn't curing it. We'll try going crazy with the MAP AE and see if it helps, I hope it does.

 

See if you can push teh MAP, TPS AE, and temp correction till it blows smoke and neary pukes. If you can do that, then there is a shot that you can work backwards from there and get enough AE.

 

BTW, the tables for the AE are really sort of an added inj. duty cycle. Many of the def. files are way off with the conversion factors given. Dont be afraid to push regardless of the huge numbers they say.

 

IIRC, the conversion for converting teh raw table values to duty cycle in async fire is hex x .0012208. I think the max DC available was ~30% from the tables.

 

this sounds familiar to me.

any harm in running OL or CL (for that matter) and giving it a large value in the VE tables at that RPM where (stumble/backfire) occurs at high MAP (70-80-90-100)areas that you seldom see unless in AE ???

seems i have same issue of getting AE in fast enough to balance out the incoming volume of air.

i will assume my TPI FP does not fall on its face.

i confirm over 2500 rpms and all is very well with AE.

 

Maybe mine is a fluke but it does well down low in rpm's with an RPM intake. Less cam (LT4 stock with 1.6's) and slightly more gear (3:08) with a T5. Stock tb and injectors.

Maybe time to swap gears to something steeper.

DM

 

I ran an RPM on my G20s old 305 headed, mellings MTC1 cammed 355 with the factory tune for a while. Mine was running with the stock 1,200 rpm stall and ran well at low-rpms. No issues off-idle.

 

I'm going to try more AE. After looking things over between my last 8746 tune and his, then looking at the stock L03 auto vs manual it's appernt that the manuals need a LOT more AE.
I'm going to go ahead and disable the AE doubler by moving it up to 255 and then I'm going to pump up the AE tables some more. We'll hook-up a wideband once my Lockers board starts working again (btw, Bob, it's not working, lol). Then we'll see what the afr is doing. Right now it'll only backfire when in open loop mode so I might first try lowering the AFR in the open loop high MAP cells. I could also massage the AE CTS table... lots to do but very hard to do without Lockers .

 

Wow, we got it . Turns out the engine is ultra sensative to AE. The combination of the T5 1st gear and 2.73 rear gears = picky picky. The test went like so; richen everything up... woops, too rich. Okay, find a happy medium with some artistic skills. Go out again, much better. Rolling in 2nd gear at a low speed snap the throttle all the way open... yup, that feels MUCH better, let's try this, now that. okay, all done.
So yeah, these manual transmissions need a LOT of AE

 

The push rods are not hardened and will wear out pretty quick, use a hardened set for a TPI corvette with guide plates.

 

That's what I thought. I'll let him know to pick some up.
I think he'll be okay for a while seeing as the heads are cast iron and the pushrods are a forge or pressured steel. Are you sure they aren't all hardened? I think they might be since the pushrods wear on the rocker sockets.... and those aren't, hmm. We'll play it safe and thanks for confirming.

 

P/N 10046173 are heat treated for use with guide plates and P/N 12371041 are found on all other roller motors with out guild plates. Also make sure you don't use the self alining rockers.

 

But tell me this... are the flat tappet 85-86 pushrods hardened? If not, then we don't have a problem. Obviously guide plates are a hard steel so we need to use something harder but with iron heads I think the normal steel pushrods will be okay.

 

Sounds good to me.