TWM 8 stack plenum too small?
 

I have a 427 FE Shelby aluminum block stroked to 482 and have not been able to get it to idle without popping and surging, and occasional popping with increased RPM, whether under load or not. The motor is in a Cobra with accel gen7 electronics and dual hi band O2 sensors with a custom IAC (Idle Air Control)plumbed into the rear of the manifold with 8 runners. The TWM stacks were synched using a synchrometer. It has a high lift Crane solid lifter roller cam with 10.5 pistons. We have tried everything and can't get it to run right. The MAP sensor is all over the scale and the latest diagnosis is that the stock plenum in the TWM manifold needs to be larger. This would mean installing an external plenum, as there is no room in the stock location. I don't know whether this is the right way to go, or another dead end. Has anyone had this problem?

Is your idle air control motor connected to the same plenium that you MAP sensor is connected into? If so, this is your problem. The IAC needs a seperately feed plenium from the MAP sensor. Otherwise, when you IAC opens at idle, you map sensor will give a false reading due to the vacumn leak created in the plenium to let idle air into the motor. If you think this is the problem, shut off (close) your IAC and see if you MAP readings stablize. I bet they will. If this is the problem, let me know and I can give you some suggestions to get you motor to idle well without the IAC.

- Fred

Idle stability
 

Although I do not have an I/R system I found that the only way to get idle stability with my cam (292 D.@.050) was to go Alpha N. I even set up a small accumulator with a restricted inlet for the MAP sensor. I finally gave up and went alpha N for the injection. It sounds like you have a fairly radical cam which may complicate the issue because of overlap. My engine has a very stable idle now and although I sometimes have to change the tune for altitude it is not a big deal because I can monitor the O2 sensors and adjust accordingly.

Fred is correct, the newer 302 and 351 manifolds are plumbed for the IAC, they don't use the main plenum. Al

There is a separate plenum for the IAC installed under the manifold that is considerably larger than the stock plenum that comes with the TWM. We had to locate it under the manifold as there is no space on top.

OK. What do your VE and spark tables look like in the 9 cells surrounding where the car idles when its warm? What do the MAP and RPM values do when your car tries to idle? What are your target and actual Air to Fuel Ration (AFR) at idle? Is the car in closed loop at idle - if so, how much O2 correction is being applied? Do you have MAP Acceleration enrichment turned on - if so what are those settings? Finally, can you check the setting for Min TPS for idle and let me know what you actual TPS value is at idle? Also, does your accell system have a timming trim feature for idle? If you can send this information to me, I will try to help you to get the car to idle.

- Fred

Fred,
After being totally frustrated, feeling like the Lone Ranger and not aware this forum had a section on fuel injection with a wealth of knowledge or I would have posted a long time ago. I flat bedded the car to a tuner with a dyno to set it up this week. The tuner claims the problem is that TWM's stock plenum is way too small, and is fabricating an external plenum as we speak, and is also fitting larger hoses for the IAC and stock plenum. He also contacted TWM to see whether they had a larger plenum and was told no, but they agreed with him that with the size of my engine and the cam, that it needed a larger plenum. I found this a little hard to swallow that no one else encountered this problem, and why, if TWM was aware of this, they haven't addressed it. I'm still a little skeptical that this is needed. I do feel a lot better knowing that you guys have my back and can get an intelligent response, as opposed to "put a carburetor on it..." Jim

The stock plenum works just fine, I ran the same setup on my 496" Shelby aluminum 427 for 5 years. I also used a FAST computer. Sounds like a tuning issue to me.

Al,
Were you running a solid lifter cam in your 496, and was your FAST batch or sequential, and did you have an IAC?

Jim

Something that is sometimes overlooked is the throttle butterflies MUST open up at the same time and RATE. This makes adjusting the throttle linkage critical in getting the engine running right. This can be a real pain to get right, but it is vital that the linkage be right.

Hint #1: with the sync tool and the idle air adjustment screws, it is possible to adjust each stack so they are pulling the same amount of air at idle but not at part throttle. This will make some cylinders lean or rich in relationship to the other cylinders...

Hint #2: lean cylinders will pop through the throttle bodies...

Hint #3 this is also true of weber carbs...

Well it will be interesting to see if the larger plenum helps. Let us know.

- Fred

Jim, I initailly had solid rollers and changed to hydraulic roller lifters because I kept eating up bearings in the solid rollers. It was set up sequential, no IAC. Here's a link to syncronize the bodies. http://www.twminduction.com/v8_kits/...tle_bodies.pdf If you have any problems feel free to call me 8am to 10:30pm MST 520-401-9142 Al

Al,
That's a very helpful link that I discovered a couple of months ago myself, and the Synchrometer is the key to getting the job done accurately. I initially tried a Unisyn, that I had used on a 911 with webers that I had owned back in another life, but I didn't like the way it sealed and is quite primitive compared to the Synchrometer. That said, Tom Kirkham made a great point about having the stacks adjusted so they are pulling the same vacuum at idle, but NOT at part throttle, making some cylinders lean or rich in relation to each other. I will have to recheck that... Jim

You close all the needle valves, and disconnect the linkage from the bellcrank, then get both sides to read as close as possible with the idle adjustment screws. Reconnect the linkage without changing the idle by ajusting the linkage bars. Then get both sides exactly the same with the needle valve adjustment. The end value is not important as long as they are all the same. Al

I am running TWM/FAST on a KC 427 FE (527 CI). My ECU came set up in speed density mode. It had plenty of power but had poor idle and lots of back fire. I've since moved to Alpha N.

The TWM manifold only has around 6lbs of vacuum at idle. Reading the fast documentation it says that you should use Alpha-N on motors that have below 10 lbs of vacuum.

"Alpha-N mode is often used on naturally aspirated racing engines with very long-duration camshafts that produce little or no engine vacuum at an idle. It is sometimes difficult to achieve decent drivability or idle quality with an engine of this type using Speed/Density mode because the amount of pressure in the intake manifold is almost the same at an idle as it is at wide open throttle. If your engine will not have more than 10 lbs. of vacuum at idle then choose Alpha-N mode."

I've been working on the program for several months and now have a VERY well running engine. Sold idle, power is back up where it was in speed density, no backfire, and it tolerates altitude very well. I live at a base of 4200 ft. I went on a cruse Friday up to 8000 with no negative effects. Previously that would have been a fouled set of plugs, and poor performance at altitude.

I'd still like to get on a dyno to see where the HP is and peak it. As for the A/F ratio I've "guessed" based on recommendations from this forum. It seems that I am pretty close. I pull dead even with another Kirkham with an equivalent setup, except the other car is carbureted. Once I set the A/F table up I made many logging runs and tweaked the fuel table to get the correction into an acceptable range.

Some of the other guys on the forum have done more elaborate things with the an IAC and a larger plenum. I figured that I'd give Alph-N a shot before I made things more complex, it's worked rather well for me.

len