I am running a small Holley 1848, 465 cfm, on a 307 ci engine. It is a 4160 that I have added a metering block to the secondary side. It runs great, but I'm at about 13.5 to 1 A/F/ratio when cruising at 60 mph. I'd like to see that a tad leaner, maybe 15.5 to 1. Would enlarging an air bleed orifice be the way to go? Thoughts? Article links?

 

At cruise you should be running on your primaries , so you simply jet down if needed.

Your plugs are the only real way to check your mixture. just like ignition timing you have to go by what works .

 

14.7:1 is the max you want, don't go to 15.5:1 that's too lean.

 

440's right, changing afr at cruise is simply a primary jet change. With a low to medium compression engine, you can get away with 15 or 15.5 at cruise, just make sure you've got a power valve that kicks in at the slightest vacuum drop. And be prepared to re-jet as soon as temps drop in fall, cause you're gonna go really lean.

 

First jet up or down till you reach your target cruise AFR. 15 to 16 is not "too lean" for an OHV carbureted engine at steady cruise, even Flatheads run 15.5 to 16 AFR very well I'm told. The whole point of jetting in a street driven engine is to go as lean as possible without surge or misfire. Racing engines is a different deal and doesn't apply. I'm not sure to what extent compression limits this. Seems to me it should work well. Modern computer controlled fuel injection runs considerably more, around 18-1! No carbed engine will be able to do this. Anyway 14.7 is a theoretical thing, it needs to be way richer than that under load, though can be leaned out under no load conditions. The ignition needs a good hot spark and plenty of ignition advance including vacuum advance. If the ignition and/or timing is defective it won't tune worth a damn. Still need to look at the plugs, maybe step down in heat range, check for surge or engine running hot. Every engine is slightly different.

Then, and this is important, once you've settled on the cruise jet AFR now need to check the AFR at wide open throttle. Droppin down or increasing the power valve # won't change the wide open throttle AFR, at least not very much, though you can experiment. The problem is, underneath the power valve are the channel restrictors, this is what determines the wide open throttle AFR, which are basically fixed jets.

If the wide open throttle AFR is now too lean they can be carefully enlarged slightly with a pin vise and numbered drill bits. This works slick as hell. I run around 16-1 on the highway at level cruise and 12.5 to 1 at full throttle. The key thing is to never run lean under load, under acceleration. Think of what an acetylene torch does when you crack open the O2.

Here's a great link on general calibration:

Holley Tuning

"If you want a very economical cruise, and still have the correct A/F ratio at WOT, increasing the size of the PVCR's, and dropping the size of the primary jets a few sizes is a good option."

 

the plugs don't lie.

 

Absolutely True, but ya gotta know the right questions to ask. It's really tough to read plugs and modern gasoline without some experience.

 

Excellent information! Thank you, Tedster9. That article is just what I was looking for.

 

A point I failed to mention: Cruise is not where you're at highest AFR. With a manual transmission at least, you'll peak AFR at transition from cruise to part throttle acceleration, going up slight grades at cruise, and while accelerating from stop to normal driving speed. Having extra load in the bed, or pulling a trailer will cause that peak to get worse.

That said, it's probably best to tune for those peaks. I do. If you tune for highest AFR at cruise, you'll have to have a power valve that opens at the slightest hint of vacuum drop, or you'll lean out badly. And if you're going to split hairs like this, you should have a vacuum gauge right next to your AFR gauge. I do.

An example: Lets say you cruise at 14" vacuum, and you're at 15/1 AFR. On the slightest up grade, you'll need a power valve that opens up between 14"-13" vacuum. That power valve is going to open and close A LOT, and will surely result in poor fuel mileage. Other things to consider...If you develop the slightest vacuum leak anywhere, you're power valve will stay open much more than it should, causing a really rich condition. And sooner or later we all get a vacuum leak. At your tune, I doubt you could even limp the truck home.

 

I am perplexed with my lack of performance. I recently installed a new engine with a ported head, a cam with more duration, and larger exhaust. At the same time I went from a super-tricked out c6, to a slow shifting zf5. Now, after a shift, I have a flat spot where I have to wait for the rpms to climb before I have power. After so many changes I am confused about where to begin with eliminating the flat spot. I have checked timing and advance, and all is good, 32* all in at 2k rpm. I did change the acc. squirter nozzle size from .034 to .031 with the manual, so I will begin there, and try to bump it back up to the .034.

 

I don't know with today's gas, but maybe you're running too small of a cfm carb. I assume you have a afr gauge installed?

 

That's a lot of unknowns for us. With only that, I'm thinking differential ratio, camshaft, and cylinder heads. If you built this engine with a bunch of race stuff, it may be making peak torque at much higher rpm than a typical street engine. Typically, you give up some low rpm torque with a strip build. That would probably give you sluggish acceleration at normal street driving shift points.

Can you give us your camshaft specs, and gear ratio. Also, do you know specs on intake ports, before and after the porting job? Opening up the intake runners actually slows air down at lower rpm's and doesn't start showing dividends until engine reaches high rpm.

 

Thanks for the help. I hesitate, and please don't take this badly, but my engine is an inline six with different characteristics than a v8, longer stroke, 4.030 bore. Typically 300 inlines behave like a tractor engine, a ton of torque down down, and not much up top. 300's do well with 600 cfm carbs. Mine has over-size valves, and did great with a 600 cfm Summit carb, but the vapor lock caused by the header right below it, made me go to a smaller 1848 holley, 465 cfm, which certainly was a step down, performance wise. I do run it with an AFR gauge. At freeway speeds I'm showing what I have read should be normal afr, 13.5 to low 14's, depending on speed. On every cfm calculator, this engine should do well with a small cfm carb. Experience, however, has shown that it loves carbs with around 300 cfm primary side.

My rear ratio is 3.31:1, with oe tire size. The cam is 280* adv., 220* @ .050, .475" lift, with 112* LSA.

 

No insult taken. Yep, I caught that it's an inline 6 by your stated CID and "head" instead of "heads". I did think it was a 302 at first though. But principles are the same for air flow on V8 or I6.

I'm pointing my finger at the camshaft. Comparing the specs to a Comp RV type cam, yours has a lot more lift. Typical of a high rpm camshaft. That, and the addition of larger intake ports in the cylinder head, is probably bumping peaks quit a bit to the right. If I was building this engine, I'd estimate it wouldn't start making good power and throttle response until 2300 rpm.

 

If you still have the 600 carb, look at a phenolic spacer and maybe make an aluminum heat sheild out of a piece of aluminum. With the lift you have the spacer will help (I think) a little on the velocity, but it does help on the linkage clearance.