'65 FI Mixture question

[George J.]

Re: '65 FI Mixture question

Duke,
I forgot to mention that the idle is definitely more lumpy and was idling lower than before the adjustment. I only needed to adjust the idle speed and mixture screws to bring it to where it was, about 1200 rpm's, but it is still lumpier.

George

[Duke W.]

Re: '65 FI Mixture question

The idle is a bit lumpier because of the increase in effective overlap due to the tighter valve lash, but that's the price you pay (and some loss of low end torque) to make life easier on the valvetrain and make more top end power.

At idle when inlet pressure is very low compared to exhaust presssure, even a little more overlap increases exhaust gas dilution and lowers manifold vacuum at a given idle speed and low speed/low load cruise, so you need a little richer mixture to get consistent combustion.

It's less a factor at WOT when manifold pressure is only slightly lower than exhaust pressure, which is why the WOT mixture should not be affected to any significant degree.

You should find a shop with a Dynojet if you want to really get the car dialed in. An hour of time should allow you to optimize the idle, cruise, and WOT mixture, and find the best initial timing to maximize torque/power across the full rev range. Best idle mixture is probably about 12.5:1.

If the guy won't start the pulls at 1000 RPM, FIND ANOTHER SHOP! You want the entire torque/power curves from off-idle to peak revs. Find out what kind of output device the computer has and have him copy the test files for you. If the computer has email, he can email them to you. The digital test files are small.

Then download the WinPrep software from the Dynojet test site, which will allow you to view and manipulate the files on your computer.

Look at the Tale of Two Camshafts article in last fall's Corvette Restorer, to see what we did, but don't let the guy get away with starting the pulls at 3000 after agreeing to 1000 as John suffered.

If you want the written test plan guidelines, I can send it to you via email. With a good test plan you can get a lot done in an hour on the dyno to maximize output fuel economy.

Duke

[Michael H.]

Re: '65 FI Mixture question

Duke,
I forgot to mention that the idle is definitely more lumpy and was idling lower than before the adjustment. I only needed to adjust the idle speed and mixture screws to bring it to where it was, about 1200 rpm's, but it is still lumpier.

George

I hope you recorded the original starting positions of the lean/power stops on the FI unit. You may want that info in the future.

[George J.]

Re: '65 FI Mixture question

Duke,
Yes, the dyno plan would be welcomed.

Michael,
Yes, I know it is between two and three full turns richer, now. I have had this car and unit for over thirteen years, now, so I'm fairly familiar with it. Just a little lazy in the valve adjustment department. I do have a starting point that I used, previously, related to vacuum readings.

George

[Duke W.]

Re: '65 FI Mixture question

Send me an email through the TDB, and I will send you back the test plan guidelines. You can use them to develop your test plan.

As far as peak torque and power is concerned, IIRC you had some head work done, but I don't recall the extent and don't think you had flow numbers, but your lab dyno test corrected to STP conditions was very good.

You should see, SAE corrected on a chassis dyno near 280 lb-ft torque, and top end power should approach 300.

Once you have all the mixtures set up and determine the best initial timing for best broad range torque and power, it should hold the tune for a long time.

Duke

[George J.]

Re: '65 FI Mixture question

Ha!, it appears I don't know jack! I seem to have had a small vacuum leak that was causing my trouble. I have put the economy stop back where it was to begin with and will start from there. I'm still getting a small amount of surging when cruising.

George