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Got my car back from the tuner, and took it out for a quick spin round the block. However, the car was back firing and stuttering all over the place! I was so disappointed as I was so looking forward to finally driving it. I called the tuner straight away and told him what was happening. He told me to check the plugs and sure enough they were heavily sooted up despite the fact these were brand new plugs I bought for him to tune the car. To be more precise the 5,6,7,8 were very sooty and 1,2,3,4 were less so and I am guessing that this is because the flow of air into the front 4 cylinders is better.
Anyway, once the plugs were clean it was a different car, it revs like crazy and sounds like a race car. But this car only wants to race, as soon as it spends any time at low RPM the plugs are going to soot up and that's because of the cam. I have the AF ratio that the tuner derived and he said that he couldn't lean the mixture any more at low rpms. So his recommendation is to go to an MSD ignition system to improve the spark quality across the range. What are you guys using and does what the tuner says make sense?
Currently I have a DUI coil in distributor upgrade.
Check spark map and also air fuel ratio. Check idle rpm, that thing should have 1000 rpm idle and alot of spark advance at idle to work well. No reason it cant go lower in fuel at idle when it has proper advance and rpm. Closed loop may be an issue, try open loop for idle and have it turn back on at suitable cruise rpm. I actually just turn open loop on all the time anymore
I noticed an HEI distributor on what appears to be a LT1 style intake; I'm assuming that it is aftermarket. If you look at the bottom of an LT1 intake, they had small ports at the intake ports for all of the cylinders. These small ports connected directly to the idle air control. They would try to distribute the small amount of air molecules evenly at idle to all the cylinders. Without those, the few air molecules coming into the motor through the throttle blades go to the first vacuum source the find, the front four cylinders. On LT1 motors, if the throttle blades aren't closed, you end up with the same issue of very rich mixtures on the rear cylinders.
If you think back to the 58-65 fuel injection systems, the air came in the side of the intake, not the front. It was the best location to get even air distribution for all the cylinders. Pretty smart!
If your intake has those little LT1 idle ports, then I would look at the throttle body and idle air circuit.
It does not really make sense to me. If he cannot get the closed loop idle to be leaner he can run the idle in open loop. It sounds to me like he needs to work on the low RPM VE table until it runs where it is happy.
242/248 @113.
I would not expect this to be trouble.
If at lower rpms cruising you see it adding fuel(high BLM/reversion) then OL is the answer.
dynamicefi-ECU allows you to run OL at lower MPH which in my case equates to lower rpms(<1700) being OL. Not sure if this ECU can run OL RPM based. Maybe? You are 2200-6200 So I would expect BLM to behave over 2000 rpms.
I noticed an HEI distributor on what appears to be a LT1 style intake; I'm assuming that it is aftermarket. If you look at the bottom of an LT1 intake, they had small ports at the intake ports for all of the cylinders. These small ports connected directly to the idle air control. They would try to distribute the small amount of air molecules evenly at idle to all the cylinders. Without those, the few air molecules coming into the motor through the throttle blades go to the first vacuum source the find, the front four cylinders. On LT1 motors, if the throttle blades aren't closed, you end up with the same issue of very rich mixtures on the rear cylinders.
If you think back to the 58-65 fuel injection systems, the air came in the side of the intake, not the front. It was the best location to get even air distribution for all the cylinders. Pretty smart!
If your intake has those little LT1 idle ports, then I would look at the throttle body and idle air circuit.
Thanks and sorry for my late reply. Life getting in the way of corvettes at the moment, but in a good way ;-)
The intake is a MK1 Miniram that I had to heavily modify. I had plates welded on the bottom so there was enough meat to port, then a friend gutted it so it flowed. I have not noticed any un used ports on it but I will check out what you say.
I think my problems lie elsewhere. My tune worked, I have dyno evidence and now it doesn't so something has changed.
Interestingly my tune will no longer idle in Alpha N in which it was tuned - it's literally like a kill switch at Idle. It idles at least in speed density which leads me to think that something has gone wrong either in the fuel system or most probably the spark.
With Alpha N injector pulse width is used so if the injector circuit is not getting the voltage then the motor will back fire and stumble.
When I get a chance I will investigate the ignition system and see where I get.
I think the first 4 cylinders robbing the air is definitely a concern though.
Check spark map and also air fuel ratio. Check idle rpm, that thing should have 1000 rpm idle and alot of spark advance at idle to work well. No reason it cant go lower in fuel at idle when it has proper advance and rpm. Closed loop may be an issue, try open loop for idle and have it turn back on at suitable cruise rpm. I actually just turn open loop on all the time anymore
The tune runs open loop up to 1800rpm. I can get a reasonable idle in speed density but not in alpha N. I have a tune from the builder (Jim Barth) which the car ran reasonably well on, but now even that doesn't work. So I think the tune isn't the issue, I've screwed something up somewhere or something has now broken.
It sounds to me like the injectors are too big and the ECM can't reduce the pulse width enough to get the correct AFR at low RPM.
Hi, I sized the injectors using a few calculators online and then crossed referenced them. The target was for around 600hp so that was the number I calculated them on. I will have to have a chat with the FAST guys to see if the old Classic system would work at low RPMs,
In Alpha-N mode the only sensor that is used to determine fueling is the Throttle Position Sensor. Are you using the stock ECM to accomplish this? I would test and verify that your TPS sensor sweeps ok from closed to fully open position as this is a critical sensor in Alpha-N. Is there anyway to post your BIN , I would like to analyze it and see if I could find something wrong.
In Alpha-N mode the only sensor that is used to determine fueling is the Throttle Position Sensor. Are you using the stock ECM to accomplish this? I would test and verify that your TPS sensor sweeps ok from closed to fully open position as this is a critical sensor in Alpha-N. Is there anyway to post your BIN , I would like to analyze it and see if I could find something wrong.
Hi, I am using a FAST Classic and a new TPS. I have not checked the TPS sweep, but it starts around 20 in the closed position.
I am sorry what is BIN? If I can send it, it wont be for a week or so, I am out of the country.
Thanks
Here is a video of it running which is why its so frustrating that I suddenly have these issues! (if I have done the link correctly!)
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Slideshow: A Jaguar designer's personal project imagines what a modern front-engined Corvette might look like if Chevrolet revisited the golden age of the Stingray.
Issues after tuning 427 with XFI292
