Helping a buddy with a race car and we're a little puzzled over the large drop in RPM from neutral to first gear. The engine idles around 1800 in neutral but drops to 1300 or 1400 in gear. It is a big engine, close to 600 inches so it has plenty of torque. The converter stalls around 6300 to 6500 rpm going down the track. Transmission is an ATI TH400 full race setup. I told him to call ATI and see what they say but thought I'd ask here too. In my past experience the RPM drop going into gear was usually around 200 or 300 RPM, not 500+. Not sure if this is a transmission issue or something else.

What is the timing set at? Possible that its lower than you think?

fatten idle mixture would be my first move.

Vacuum leak, or conveter way to tight can cause it as well as timing and fuel. How far does it fall back on the shift?

Shifts at 7300, falls back to 6300 going down the track. Off the two step the RPM flashes to 6700 when leaving the line. Car seems to run fine going down the track so we don't suspect any issues with the trans but the big RPM drop is odd. It is an EFI car so I can easily tune for best idle quality.

Best idle quality...so a big engine like that will want AF at idle in the low 12's, maybe even mid 11's.

Open headers so no way to measure the AF ratio at idle. I just tune it manually for most RPM and most manifold vacuum. It is plenty rich at idle, I can see how much fuel it is using.

This is what I have ran into before. Big engines and big cams need the distributor locked out to help with the idle quality.

I would try to get the idle much lower. Being efi should make that pretty easy.

Does whatever EFI system you're using adjust timing to maintain idle speed? All modern stuff does it but I'm not sure about aftermarket EFI. When it drops into gear it should add timing to compensate for the additional load.

There would be much less load to compensate for with a lower idle speed. Any decent modern efi system has no problem idling any engine with any cam at 1000 rpm or lower.
Now there may be mechanical reasons to have such a high idle speed, like oiling the lifters or whatever. If that’s the case, ignore my advice.

EFI engines typically have an IAC which is used to control the idle speed. They will also vary the timing to hold a steady idle but this is a race engine with a special throttle body that doesn't have an IAC. The high idle speed is required due to the big cam. It barely runs at 1300 RPM, doesn't smooth out until around 2000 RPM. The EFI goes into closed loop at 2500 RPM since it can't control the engine below that.

Aren’t you using tps based idle? You can blend tps/ map strategies so there is no map fluctuations in the pw calculation. What system is this using?

Is there enough difference in map that you can have a large step in base timing between in gear and out of gear? Have something like 15* at neutral idle and 30* at drive idle at the same engine speed?

600 cubes and a large drop assume its NA. IMO 1000rpm is alot on the shift, certainly leaving ET on the table. I know nothing about EFI other than OE applications but I would check vacuum. But like to see the converter be a bit closer too.

Are you running in Alpha-N or Speed density ? I would assume Alpha-N since it sounds like it doesn't make much of a vacuum signal.

Have you checked the timing map to see if it transitions through a lot of timing changes between RPM in gear and out of gear ?

Could be that you need more idle fuel down low, A/F sensor will be pretty much useless with a lot of overlap and open headers so won't be able to use that for reference.

May just need a lot of idle timing also...

Maybe recheck the fuel calc. parameters and maybe the injector opening time offset, possibly something there.

AG.

I think I would ask this question on a GM or EFI forum.
I'm getting ready to start using a EFI system with a turbo charger on LA small stroker 360 motor E85 street car only car so I may start perusing one of the EFI forums also

I agree, the converter could be a bit looser to keep the engine in a tighter powerband going down the track.

Andy, I don’t know if you got anywhere with this. Maybe I’m on ignore or you discount my experience. Anyway, this situation is totally fixable if you would disclose the ecu brand/ model.
I assure you that the engine can idle much lower by using the correct strategies.
I’ve been tuning engines with these idle characteristics for longer than most people in the industry.
Once you learn how to make a large cam, low compression, odd fire v twin to idle low and even, an 8 cylinder is a piece of cake.
Even if the converter is to tight, you could program a staged dump valve for the converter in whatever conditions you desire.