Experience with 308/P6 jetting

Some of you know I have been playing with the jetting on my 77 following the intallation of P6 cams in the summer. I think I have it running reasonably well now with the larger (36mm) venturis, and, given someone else may go down this (lightly traveled, or at least lightly documented) path, a recap of the jetting may prove helpful.

Stock on my car, 77 w/4X series 40 DCNFs and 260 duration cams was 135 main, 55 idle, F36 emulsion tubes and 220 Air correction (135/55/F36/220). Stock (32mm) venturis. Car is equiped with K&N airfilter, uses the stock airbox and a Tubi exhaust. Timing is Electromotive HPX with a MAP sensor (allowing more advance at light throttle) and generally timed to provide more advance low down (3000 rpm) while sticking to the limit of 34 degrees of advance at 5000 and up.

When we put the P6 cams in the car earlier this year, I increased the venturi size to 34mm to take advantage of the higher-end breathing and upped the jets to 145/60/F36/210 (I asked for 185 ACs but they did not make it into the car right away). Ran well in this configuration, but was running lean at the top, seen after prolonged high revs at Road America (tail pipes were brick colored). I was surprized, given that I thought my intended jetting would cause the car to run rich. (This anomaly led to the discovery of the 210 ACs versus 185s...)

I briefly flirted with larger, 36mm venturis, as used on the Le Mans car and increased the idle jets to 65, mains to 150. In this configuration, it ran rich at idle, very strongly in the mid range (mid range acceleration was much improved) and (unsurprisingly once you know the AC jetting) fell on its face at 6000 rpm and up. Given the 185s were the richest AC jets I had at the time, I elected to put the 34mm venturis back in. My guess is a 145/60/F36/195 configuration would work well with 34mm venturis.

With some more jets in hand, and a need to remove the airbox for some repairs following Road America last month, I put the 36mm venturis back in with 155/60/F36 and 185 ACs this time. It is probably a bit rich in the mid range but until I have a Wide Band (WB) O2 sensor installed (see below), I don't want to run lean.

I had the chance to run the car up through the gears over about ten miles today and also to putter through some suburban side streets. It is a little "prickly" below 2500 rpm, roll on the throttle, anything more than a gentle stab causes some "chuffing", but beyond that it pulled very strongly through red line. Net, net, I think I am getting close, or at least closer than I've been to date with the correct jetting.

To aid tuning going forwards, I have ordered a wideband O2 sensor from Tech Edge, a specialist development shop in Australia, recommended to me by a well respected tuning house. The "kit" comes in various forms, pre-assembled and DIY along with a related display unit, wiring and so on. Along with the special WB sensor, the unit has been tested (Tech Edge reports) to measure AF ratios from 10:1 (or lower) all the way to 25:1. (Narrow band units, commonly found in most FI cars have a much narrower range of interpretation and correction around stoich, 14.7:1. Previously I was involved in a group setting up turbo cars. The readout from the narrow band sensor bore little relation [and therefore tuning guidance] to the WB unit).

I am planning to install bosses for the sensor in the two "B" pipes of the Tubi, just beyond the collectors and likely put the AF display on a long lead in the glove box so that it can be examined and put away when not in "tuning mode". When installed I'll post pics. While some of the benefits of the O2 readout (or logging) can be delivered with a dyno (as can EGTs and other useful data not available with the Tech Edge unit0, it should enable me to see how the AF ratio is varying under actual operating conditions. Until then, it is good to have the car running reasonably well by the SOTP tuning method.

 

Russ
The accels are stock. In fact I've not changed them as I've gone along. Mike at Pierce has opined once or twice that they might need changing, but I really need to see some data to understand what's going on to complement my SOTP experience.

I will update with some data when I have the O2 assembled and on the car - I'd guess about 2 weeks if the weather holds.

I will also try stopping down the throttle plates by 1/8 - 1/4 turn on F, R banks to help reduce the high idle.

I need more tuning and seat time to comment on the "roadworthiness" of the 36 mm venturis. So far I'd opine that it is better than I thought it would be and certainly better than last time - I think I got the carbs better sync'd. Certainly the cams have not been the horror story that others have suggested they would be. The 36s might be interesting on the 3.2 4V given the better breathing and bigger displacement. And, to you point, I would also get the timing stuff sorted out before you adjust jetting or any other carb parameters so that you've got one variable being tested at a time.

Finally, so far, the cams and jets do put a smile on your face as you stomp on it. It certainly feels like it has a lot more than stock, without giving up too much low down.

 

FYI, 1/8th turn of the F, R bank throttle stops seems to have done the trick on the idle speed.