When the engine is cool, it will idle smoother with a leaner mixture than when it is hot. When cool, it will idle very smoothly with a mixture of 14-14.5:1. When it gets hot, 13.2 is about as lean as I can make it and get it to idle well.

What does this tell us about the slant?

1. Does the exhaust manifold/header make the intake too hot?
2. Or does the combustion chamber itself get too hot, and not atomize the fuel as reliably?

I think maybe this was true of when it was carbureted, but cannot prove it, as I had no AF ratio gauge. But, it always idled smoother when it was carbureted before the engine got really hot, such as on a long highway run. After getting off the highway, the idle would be rougher than when I had been driving around locally.

So now I am thinking I have my heat shields designed incorrectly. I have the aluminum above the entire manifold combo. The thinking was to keep the fuel rail cool. Now I am thinking I should try to figure out how to shield the Dutra duals so less radiant heat gets into the intake area. I have looked at many newer engine designs, and it is common for the exhaust manifold to be enshrouded in a stamped sheet metal heat shield. So far I have not been able to imagine how I would duplicate this on the slant. If memory serves well, the stock exhaust manifolds are actually further away from the intake runners.

Volare4life has a post on the "what have you done to your slant" thread of his exhaust header wrap. I think he is onto something as he took it all the way up to the flange.

Thoughts on any of this?

Sam

_________________

That's goofy. It should absolutely idle better with a lean mixture when it's hot.

Are you sure about your valve lash? My Barracuda idled poorly hot as someone lashed the exhaust valves at .010" and the valves weren't closing fully when the engine was hot. I lashed the valves properly and it was fine.

_________________
Joshua

I would not swear to anything, but I set them at .015 and .025 while hot. I tighten them until I can just slide the gauge in and out of the rocker/valve tip gap. It is worth considering though. Thanks.

Sam

_________________

Sam
do you have a cold air intake,,? when the engine is hot under hood temps go up,
As air expands, per a given volume, the oxygen content remains the same so it is less dense, basically there is more 'room' between each molecule of O2. The oxygen and gas molecules must be in proximity to each other in order to fire,, and with a hot air intake charge there are gaps in the O2 distribution,, extra gas molecules are needed to fill in those gaps, to make sure the available O2 molecules have a gas molecule close enough to them to fire.

Two thoughts:

1) Heat soak of some injectors more than others, and so variations in pulses are different across the 6 injectors when hot.

2) Valve lash is too tight. Forget the specs and adjust looser to see if idle will smooth out.

Lou

_________________
Home of Slant6-powered fun machines since 1988

Lou what would you consider loose? (just curious & trying to learn something!)

_________________
1964 Dart GT

"Loose" will depend on the particular cam. I run some of bigger cams at 0.026"/0.030".

IIRC, Sam's cam is smaller, but the lash will depend on the ramps on the lobes and such. I would start by opening them up 0.004" on I and E and see what happens. CJ and Doc posted a procedure for finding a good lash spec for a new cam, but I cannot find it right now.

Lou

_________________
Home of Slant6-powered fun machines since 1988

Sam I ended up with lash 0.023â€￾ to 0.028â€￾ on both intake and exhaust before engine smoothed out and vacuum gage needle settled down at idle in gear. I based the lash setting on compression tests to even out all cylinders’ pressures. In other words I was able to mess with lash to bring all six cylinders with in five psi or less of each other, where before with lash all set the same for intake and exhaust respectively resulted in up to 20 psi difference between high and low readings.

But, my cam is a big old lumpy thing, with long ramps. By adjusting lash by pressure, the result was to even up the ramp event which made overlap close to the same over all six cylinders.

Just for fun perform a compression test, see how close they are, and loosen lash for lower pressure, and tighten lash for higher compression until they all even out.

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Cool idea. I will certainly think about that. Loosening is first step.

Thanks.

Sam

_________________

Please repost all cam specs and installed centerline.

Thanks,

Lou

_________________
Home of Slant6-powered fun machines since 1988

Sorry, I lost the cam card some years back.It was in the top drawer of one of my files for years, and then vanished. We are kind of shooting in the dark now. I can tell you I installed it 4 degrees advanced with a bushing. That is about all.

Sam

_________________

OK, no worries.

I think my car has the same problem as yours, which I think might be differential heating of the injectors giving inconsistent pulse widths at low duty cycle. I have some ideas on how to fix this - lower T thermostat (195 now), heat shields, individual cylinder trimming with MSIII, to name the main ones.

Lou

_________________
Home of Slant6-powered fun machines since 1988

Smaller injectors would increase dry cycle at idle. I am interested in exploring he shield ideas with you.
Sam

_________________

I inspected things around the manifolds more closely and discovered that the top of the Dura Duals flanges both touch the bottom of the Offenhauser intake. This area is a dry chamber that can be used to heat the manifold with cooling system water, which I have not done.

It seems as if maybe this is transferring exhaust heat directly to the intake. At some point I will pull this all off, and do a much better,, more careful job of fitting all this. I will cut the flanges down, and design a shield of some kind.

Sam

_________________

I will be very interested in your explorations.

Thanks,
Lou

_________________
Home of Slant6-powered fun machines since 1988