Holley carb questions for C2 327

[Timothy B.]

I would like some input from the experts on carburetor jets and power valve for my 67 327/300 car. My carb is a period correct Holley but is a list 3806 rather than a 3810. My understanding is it's the same carb (used on 67 327 Chevelle with L79) but is a 4150 rather than the 4160 as is the 3810. Carb is in great shape and runs well so no need to spend $$$ to get a 3810 on a driver car.

My car has (most likely) a standard cam for a 300hp car, and pulls 20 in. at idle. My car has L79 intake, 2.5 exhaust manifolds and pipes. Cruising at 50ish slightly uphill runs around 15" vacuum. My power valve is a stand 6.5" valve, but seems to me that for this engine setup and vacuum level, that a power valve of like 8.5 or 9.5 would be more appropriate? Right now, I have to go WOT to get vacuum down into the range where the power valve would activate. Normal aggressive driving doesn't get vacuum down to 6.5". I'm in Houston and at sea level. I know that the 6.5 is standard and typical, but seems like per Holley's own guidelines (such as the old idlevac/2 rule) would yield a larger power valve.... I do enjoy driving it aggressively :-) Thoughts and experience?

Secondly, this carb has #68 primary and #74 secondary jets in place. The primary on a list 3810 was #65 from what I have read and of course metering plate secondary. All other specs of the two carbs are the same (venturi, etc). Specs for the 3806 used on 327 L79 chevelle show that it had #65 primary and #72 secondary jets. Question is: Should I change my jets primary and secondary to #65 and #72 to match original on the list 3806 and original for primary on the 3810? Car runs OK, but could be running at bit rich and I don't know it. I do use allot of gas! ;-)

Thanks for your input!

[Duke W.]

Re: Holley carb questions for C2 327

For one thing the "Two-Inch Rule" has nothing to do with the power valve or the carburetor at all. It's use is to find the best fit vacuum advance.

A 6.5" power valve should be more than adequate. You say the engine pulls 20" manifold vacuum at idle, but that's meaningless unless you specify the idle speed that it was measured. I can get virtually any "idle vacuum" I want by changing the idle speed.

You should rig up a vacuum gage that you can read while you drive. You will find that vacuum is all over the place as you drive around, and if you open the throttle about half way at 1500 RPM it will immediately go to near zero.

If you're willing to spend some time experimenting, and you're certain that the installed 3806 was use on the Chevelle L-79, I recommend you get it back to OE configuration in terms of jetting and power valve

Keep in mind that the (non-K19) '67 Corvette base 300 HP engine and L-79 350 HP version used the exact same carb, 3810A. If equipped with K-19 it was a different carb that was set up for ported vacuum advance, and the same probably applies to Chevelle, so make sure the 3806 is also for non-K-19, or if it is, convert to full time advance. The '67 Chevelle AMA specs should tell you. The full time vacuum advance converstion can be done by modifying the base plate, but the easiest way is to tee the VAC signal line into the choke vacuum break.

If you have not already done so optimizing the spark advance map will noticeably improve performance especially in the low to mid rev range. The centrifugal advance has 30 degrees, but is very lazy coming in at about 5000. The black springs from the Mr. Gasket 938 spring kit should bring all in at about 3500 with most in at 3000. Then set initial advance at 8-10 or total at 38-40 a few hundred revs above the speed of max advance.

IIRC an OE VAC is stamped 355 16 (start at 6", 16 @ 12"). If a replacement is required install a B22 (NAPA VC-1802 or equivalent) if a manual trans, and B26 (NAPA VC-1765 or equivalent, start @ 6", 16 @ 12") These NAPA part number can be crossed to other brands, but the B-numbers should be the same. Most, if not all are made by Standard Motor Products.

I also recommend looking at the second from last thread in the sticky section and download and read the tuning seminar.

Duke

[Timothy B.]

Re: Holley carb questions for C2 327

Thanks Duke, nice to hear from you. Here's some more information:

> The Idlevac/2 rule I mention isn't about vac advance, its from many Holley guidebooks and from Holley tech info pages to use as a starting point (idle vacuum)/2 for Power Valve selection. It is mentioned in my old Holley book from the 70s and also mentioned on this Holley site: https://www.holley.com/blog/post/how...ey_carburetor/ There is another technique to use cruise vacuum under slight load less about 2" as a selector for PV. Either of those would point to a 9.5" or so (or more) for my car with 20" at idle. Using either of those techniques should show a standard 6.5" PV would come in a bit late for an engine with my vac level.... Seems like that would be true for almost all base cams except maybe high altitude....which I am not.

>My car pulls 18-20 at normal idle at about 550-600 rpm.

> I have rigged a vac gauge for road use and I get about 15" or more at cruise speed under slight load. Yes, wide open will get vac down below 6.5" but normal aggressive driving doesn't.

> List 3806 was used on L79 in 1967 on non-calf non-K19 Chevelle and has I believe same baseplate as 3810 and full time advance. That's why I have never bothered to replace the 3806 since its so similar to list 3810 with the added secondary metering block of a 4150 (so a step up from the 4160). Holley spec sheet for 3806 shows it came with a 65/72 set of jets but mine came from a re-builder back in the early 80s and has 68/74 larger jets. Sounds like Duke suggests I might switch them to 65/72 which is closer to factory and same as list 3810.

> My distributor is already setup for earlier advance (probably from your suggestions back about 15 years ago actually!). Should be good there.

> My vac advance is a B22 also from your recommendations in the past, on my manual tranny car.

I'll take a look at the tuning seminar... by the way the thermo topics are right up my alley as I am a NASA Thermal Systems Engineer/Manager and worked on Shuttle and and now ISS thermal control systems....;-)

[Mark E.]

Re: Holley carb questions for C2 327

To clarify, isn't your list 3806 a Holley 4160? The 4150 is a different animal (dual feed, changeable secondary jets, larger capacity).

Both the L79 Corvette and Chevelle originally use a 4160.

[Timothy B.]

Re: Holley carb questions for C2 327

Nope the list 3806 is a 4150. Still single feed inlet and same carb capacity, but has secondary metering blocks with changeable jets (no power valve on sec). My understanding is that its the same carb as the 3810 but just basically came from factory with the secondary metering block kit installed. For some reason GM saved a few bucks and went with the cheaper 4160 on some models, but apparently on the Chevelle went with the 4150. Seems like the Corvette would have warranted the slightly more expensive 4150 but who knows... Fits the car exactly like the 3810. There was also a 3807 for Chevelle with AIR.

Yes my 67 would have originally had the 3810, but when my family got the car it had lost its original carb. I ordered a rebuilt carb through "Auto Shack" in the early 80s (they used to be called that!) and what I got was an almost new 3806 and it cost like $85. Since its basically a 3810 with the secondary metering block I haven't ever opted to change it.

But just recently I learned that it may have larger jets in it than original for the 3810 or even for the 3806. I am wondering if I should switch them to 65/72 which is more like they would have been from factory for either car with 327.

[Mark E.]

Re: Holley carb questions for C2 327

Interesting.