62Jeff

Two weeks ago I posted about my 65 with a 350 Horse engine and a wandering idle.
http://forums.corvetteforum.com/show...ing&forum_id=4


After many helpful suggestions, the idle problem actually went away on its own after a 10 mile drive and has not returned. However since people asked about vacuum advance data, here's what I've found out about my car.

The distributor has a tag of 1111076 which is for the 250/300 horse engine. Who knows if the distributor weights and springs are still 250/300 horse components. Did I hear that Lars has a distributor kit? Where would I get one of those and what does it consist of?

Vacuum advance in the car now has the following stamped on it:
MC
238 24

Do those numbers mean anything to anyone?

The vacuum advance begins to move at 6" Hg, and stops moving at 14.5" Hg. The Mighty Vac vacuum pump gauge actually reads 10" and 18.5" respectively but the gauge also reads 4" Hg without any vacuum at all on it, so I assume the REAL numbers are 6" and 14.5" Hg. I bring this up in case that matters.

Clearly this is not the correct vacuum advance for my alleged 350 Horse engine, as my 1965 Corvette Specifications Guide lists the vacuum advance range being 4" and 8.2" Hg.

My question then is this: The spec guide lists the same vacuum ranges for the 350/365/375 horse engines, so can I just buy the 365 Horse vacuum advance JohnZ listed in another thread (NAPA #VC-1810, stamped "B28") and be in good shape or do I need a different advance?

Other data in response to some of the original questions you asked of me - After adjusting the warm idle down to 850 RPMs with the vacuum advance disconnected, the engine vacuum is a steady 17".

Jeff

65air_coupe

The VC 1810 is correct for your motor (and mine!).

62Jeff

Thanks!

7 minutes to get the answer, I really need to pony up some $$ and support this fine forum. Think I'll do that tonight.

C2BOB

What I found to be a good vacuum can for these motors is the one for a 400 smallblock in a station wagon. High springforce so it backs off immediately and better yet it has only 8 degrees of advance. So you can taylor the advance to the whatever you need. Good for stopping the rattle mid throttle caused by too much compression and low octane gas.

MikeM

Is the correct number 1801 or 1810?

6T5RUSH

62Jeff,

MikeM's right. You want the NAPA VC1810. There's quite a bit of threads/topics in the archives searching under NAPA VC1810.

Here's a link to my thread on this:

http://forums.corvetteforum.com/show...um_id=4&arch=1

Good Luck!

Regards,

Jim

62Jeff

Sorry, I'm getting typographically dyslexic in my advancing age and incorrectly transcribed the number from a JohnZ post.

I've corrected my original post.

K2

Jeff;
I don't mean to be contrary here but if you have an idle vacuum of 17" Hg at 850 rpm idle you don't want a VC1810 vacuum can. The VC 1810 can is specifically for engines with low vacuum (in the neighborhood of 10-12" at idle. When selecting a vacuum can consideration to the amount of vacuum the engine produces must be made. You want to select a can that is fully deployed at about 2"Hg less than the idle vacuum your engine produces. If you have 17" of idle vacuum a VC 1765 or VC 1805 would be more appropriate. The VC 1765 is fully deployed at about 13" and the VC 1805 is fully deployed at about 15" HG. You wouldn't want a can such as the VC1810 because it is fully deployed at 8" HG and it would be much slower to react when rapidly opening the throttle, with likely resultant spark knock as you transition from cruise to wide open throttle

62Jeff

That's very interesting.

If the VC1810 is the correct vacuum advance for a 350 Horse engine, yet I have 17" Hg of idle vacuum and should not get that advance, does that suggest that my cam is not a 350 Horse cam?

My engine details:
327 NOM block
350 Horse pistons (don't ask how we know)
Double hump heads but unknown valve sizes
350 Horse intake
350 Horse carb - Holley 4150

So it looks like a 350 horse engine but I don't really know for sure what the heads and cam are.

JohnZ

For the application quoted (17" Hg. idle vacuum), I agree that the VC1765 is the correct can. However, reaction of the can when rapidly opening the throttle is essentially instantaneous, as it's driven by the spring in the diaphragm, and the advance drops out (retards) on throttle opening, so it won't cause detonation, which results from excessive advance.

65air_coupe

Don't hold me to this, but I seem to remember my vacuum was in the neighborhood of 15-16 at idle (<1000rpm) and I know for certain mine is the correct cam.

FWIW, I don't have any detonation problems with the 1810 can.

K2

The vacuum can does not know which engine or cam is in the car. It only reacts to actual vacuum conditions. At full deployment you would normally expect upto around 52 deg total advance at light cruise or overrun condition. As you get on the accelerator you want the vacuum can to immediately begin retarding the ignition in relationship to the amount of throttle applied. If you have a can that is fully deployed at 8" Hg then manifold vacuum has to drop to below 8" before it begins retarding ignition. This is not the ideal situation and is exactly why there are so many different vacuum can choices available. Who knows what cam you have unless you installed it yourself. The bottom line is you should match your vacuum can to measured actual manifold vacuum conditions. The VC1810 is not the ideal choice if you have 17" of manifold vacuum at 850 idle RPM.

62Jeff

Thanks for the data. Certainly I realize that a vacuum reading won't tell me what cam is in my car, but I thought it might tell me what cam is not in the car. I'm grasping to understand why this thing is such a dog for an alleged 350 HP car, and thought we'd hit on something when a certain advance is supposed to be the right one for a 350 hp engine, yet by your recommendation isn't at all correct because I've got too much idle vacuum. Seemed like it might indicate that my cam isn't right.

Vacuum advances are cheap, think I'll buy several and see which one works best with the car.

Jeff

babbah

Jeff,

Here is an excellent response I had saved from Duke on the subject of vacuum advance cans which helped me with a similiar problem of overheating at idle on my BBC L88 configured engine, I use the 1810 can on my engine that has a large overlap cam and at idle I have only 8" of vacuum...This can reduced my coolant temp 40 degress and all the power is back, no more bogging off the line.....

Lars G. also has a technical paper vacuum cans which is outstanding, search the forum for it....
--------------------------------------------------------

The basic rule for vacuum can selection (henceforth referred to as THE RULE):

THE VACUUM ADVANCE CONTROL SHOULD PROVIDE FULL ADVANCE AT NOT LESS THAN 2" LESS THAN PREVAILING IDLE VACUUM AT NORMAL IDLE SPEED WITH APPROXIMATELY 24-32 DEGREES TOTAL IDLE TIMING.

This is a system engineering rule of thumb, and total idle timing should be in the upper half of the range for "big" (high overlap) cams and the lower half for "mild" (low overlap) cams. With a 16 degree vacuum can this is achieved with 8-12 degrees of initial timing for mild to medium cams and 12-16 degrees for medium to big cams. Based on overlap, the "300HP cam" is "mild", 327/350 and all BB cams, except L-88/ZL-1, are "medium", and all SB mechanical lifter cams are "big". L-88/ZL-1 cams are "REAL big".

Idle vacuum in neutral is an inverse function of effective overlap, and the range on C1/C2 engines is the very high overlap 30-30 cam, which only pulls 10"@900 to the low overlap base engine SB cams (which were also used on some optional engines) that pull about 18"@500. All others are in between, except L-88/ZL-1, which are pure racing engines that were never intended for street use so they were not equipped with vacuum cans. In all cases, typical idle vacuum is effected by both idle speed and total idle timing. Higher idle speed increases vacuum and, up to a point, so does increasing total idle timing, which is why high overlap cams need both higher idle speed and higher total idle timing.

Initial timing should also be established to keep maximum WOT timing in the 34-40 degree range for SBs and 36-42 degree range for BBs, and WOT detonation may dictate the lower end of these ranges depending on compression ratio, cam, and operating conditions such as ambient air temperature and altitude. Higher ambient temperatures promote detonation as do low altitudes where average air density is higher.

Higher overlap increases exhaust gas dilution at idle and cruise, which slows flame propagation speed, which increases the timing requirement. Insufficient total timing at idle and low speed cruise increases EGT, which will cause more heat to be absorbed by the cooling system, which can result in high operating temperatures and, in extreme cases, overheating to the boilover point, even if all cooling system components are within their original performance range.

Using THE RULE, one of the following three NAPA/Echlin vacuum cans should be appropriate for all C1/C2 OE engines, including those converted from ported to full time vacuum advance and C1 engines that are converted from the non-vacuum advance dual-point distributor to a single point vacuum advance distributor.

Vacuum cans for modified engines (such as cams that alter OE idle vacuum characteristics) should be selected using THE RULE.

My system engineering "best fit" for all OE engines is also listed including those not originally equipped with vacuum cans, but a few "best fit" vacuum cans (396/425, 427/435 and '63 327/340,360) are significantly different than OE, due to either a poor match to engine idle vacuum characteristics i.e. don't meet THE RULE ('63 327/340) or will not meet THE RULE when converted from ported to full time vacuum advance (396/425, 427/435, '63 327/360). My "best fit" for 327/350 is also different than OE, which I discuss below.

VC680 (stamped "B1") 0@8". 16@16" (283/220, 230, 245, 250, 275; 327/250, 300)
VC1765 (stamped "B20") 0@6", 16@12" (327/350, 396/425, 427/390, 400, 425, 435)
VC1810 (stamped "B28") 0@4", 16@8" (283/270, 290, 315; 327/340, 360, 365, 375, L-88/ZL-1)

These same Dana Controls manufactured vacuum cans are also marketed under other brand names/part numbers such as Delco, Borg Warner, and others, but I don't have a complete part number cross reference list. The alphanumeric code stamped on the mounting bracket (B1, B20, or B28) is the code that denotes the specifications regardless of the brand name/part number.

If you're at the ragged edge of THE RULE a small increase in idle speed - on the order of 50-100 revs - will usually achieve the 2" difference since vacuum increases with increasing idle speed, and IMO some OEM recommended idle speeds, especially on SHP/FI engines, are unrealistically low - mechanical lifter cam engines should be idled in the 800-1000 range, and add at least 100 revs with FI. OE vacuum cans with Powerglide may not achieve the required 2" margin idling in Drive, in which case the next more aggressive can should be installed.

The correct total vacuum advance for most pre-emission Corvette engines is about 16 degrees. Any ported vacuum signal lines (such as SHP big blocks and '63 327/360) should be converted to full vacuum advance, and on some of these applications, a new can (B28 for 327/360 and B20 for 427/435) must be selected to comply with THE RULE. The OE '63 327/340 has full time vacuum advance, but the OE vacuum can does not comply with THE RULE, so it should be replaced with B28.

A "more aggressive" than necessary vacuum can (significantly more than 2" difference between idle vacuum and full vacuum advance) is okay (but not necessarily "ideal") as long as there is no detonation. Too aggressive vacuum advance may cause transient detonation, such as on upshifts or part throttle acceleration. One choice is to reduce initial timing, which may reduce total WOT timing below optimum. Another is to install a less aggressive vacuum can as long as it meets THE RULE. The "best choice" is to install the "best fit" vacuum can.

Using THE RULE, one of the three above mentioned vacuum cans should provide full advance in the range to at 2"-4" less than typical idle vacuum, which is the "best fit" range.

For example, 327/350 was originally equipped with a Delco can (stamped "236-16" - last three digits of the "long" GMPD number and maximum crank advance as are all other OE Delco cans) equivalent to the current B28 replacement can, which is more aggressive than necessary. Since 327/350 pulls enough idle vacuum (14"-15"@750-800) to keep B20 pulled to the stop at idle with 2"-3" margin, it is the "best fit".

A "not sufficiently aggressive" vacuum can - one that does not keep the plunger pulled to the limit at idle to "lock-in" maximum vacuum advance can cause high coolant temperatures due to insufficient total idle timing, and variation in total idle timing due to an "unlocked, dithering diaphragm" can lead to idle instability, poor idle quality, and even stalling! Engine run-on at shutdown is also a symptom of too little total idle timing, which heats up combustion chamber surfaces and causes preigntion that can also lead to detonation during normal operation. My '63 327/340 suffered from these problems for several years until I realized that the OE 15.5" can (essentially equivalent to the B1 can) was not suitable to an engine that only pulled 12" at idle, so I replaced it with a Delco 236-16 8" can (equivalent to current B28), which solved my idle quality/stability/run-on/detonation problems, and THE RULE for vacuum can selection fell out as sure as E=mc**2 fell out of Einstein's Special Theory of Relativity.

ctjackster

kinda went through this exercise with my <ahem> L76 with what appears to be an L79 valve train set-up. First came to learn of it when I saw waaay more vacuum at idle than a true L76 should show, then went ahead and measured lifter movement with a fixed dial gauge - seemed to confim the L79 cam profile.

And then I went and thought about the vac can I was using, as I had a nagging off-idle stumble that I had never been able to completely tune out with timing and carb adjustments - had gotten close though.

In the end, these vacumm cans are really not all that expensive and they are easy to swap in and out, if it seems your car's vacuum @ idle might call for something different, you can purchase the next in line can and simply try it out and decide if your permance has improved or the issue has otherwise been solved - if not then stick with the correct can, or if it is solved keep the old can on the shelf, you are not out any real $$. That's what I did.

StrayDog

The VC 1810 is 8 bucks at Nappa , installed on my 427 >> W/ extreme energy cam hyd cam ( idle 14.5 vac) cured the same problem you are having & great throttle response / no nocking, good advice from other forum members,yes could be other things ... but its a realitivly easy to do ..... worked for me, the Dog woof

babbah

Straydog, IMO, If your motor produces 14.5" of vacuum at idle, you should probably be running the VC1765 can instead of the VC1810, which is designed for lower vacuum readings at idle. Your idle vacuum is high compared to more radical cams that produce much less vacuum at idle. just my $0.02

StrayDog

Babbah ,good info,your post on timeing indicates you have great understanding of timeing understood , I have been doing some timeing & vacuume advance experimenting ..... my engine has alum heads & approx 9.8 compression ratio, found it likes a lot of initial timeing ,& less total.... was running 38/39 total with 24 in the distrib, at hwy speed 2800 RPM , was getting a surg, not pinging...as stock vac can B1, was wandering on & off at cruise.....So backed timeing from 38 to 36 still continued, tried the 1810 vac can & cruise is now great no chugging & throttle response great..........when the summer hot weather hits may have to look at things again , as she, MY 427, may not want as much advance as I have dialed in & will try the VC 1765 , whats working for me now................timeing 21 in dist,15/16 at crank full advance 2,500 rpm......tolerating 1810 can ..no pinging .... The dog