Dose anyone have a list of vac. can specs? My L36 is using a 1810 can. Motor is bone stock. Timing is set at 4 deg. base timing. I have not maped out the timing yet. I need to go over to my other garage to get a dial back timing light. The car runs perfect. But the temp will rise slowly when driveing. Highway or around town. It already has a new Dwittits rad. I tried removing the clutch fan and put a flex fan on . It made it better but it will still creep up to 210 and sometimes over, it just takes longer for it to happen. Now my L88 has no vac. can at all same fan and will not run past 180. It can idle all day and never go over 180. I checked the temp on the L36 with a ir gun and the gauge is right on. What am I missing. I can make a gear head motor run nice and cool. but an all stock L36 is getting on my nerves. I have three big block cars this is the only one that runs hot. I have to be over looking something.

 

Since you are trying to keep this car bone stock, are you running ported vacuum to the can or full time manifold vacuum?

 

aworks
here is a tech papaer by Lars on the various vacuum cans
just click on the file called "Vacuum Advance.pdf"
if you right click it you should also be able to download it if you need

http://69.240.140.212/page1/page65/page65.html

 

The .pdf file posted at that web site is a really outdated version of my paper and does not contain all the correct specs and info. For a correct, clean copy of the paper, you can e-mail me a request at:
lars.grimsrud@lmco.com
and I'll send you an MSWord paper as an attachment to an e-mail.

Lars

 

Lars, I sent you a PM

Barry

 

A more suitable vacuum can for L-36 is VC1765, which has max advance of 16@12" (versus 16@8" for VC1810) since L-36 should pull the minimum 14" vacuum to keep VC1765 pulled to the limit at idle, however, if there is no part throttle or transient detonation the VC1810 is okay.

Is the new DeWitts radiator an OE equivalent to the original brass radiator?

The initial timing spec for '66 L-36 is "4* nominal, 4*-10* range". (max centrifugal is 30@5000 and starts at 900). You might see lower temps with more initial timing, so advance it to the detonation limit. Then install a VC 1765 and see if you can bump it some more.

Also, make sure the vacuum can is signaled by a full manifold vacuum source. Total idle timing @500-600 should be initial plus full vacuum advance, which is only 20* with your current setup. Mid twenties should lower coolant temperatures, but 210 is not outside the acceptable range in hot weather, especially in urban traffic.

Duke

 

Thanks everyone for the quick response. And yes it is full vac. not ported. I'll check it out tonight. The rad is a direct replacement but alum.

 

aworks
Lars sent me an updated vacuum can spec article so here is the link again.
http://69.240.140.212/page1/page65/page65.html

the file is called "Vac Adv Spec.pdf"

 

possibly stupid question that nags at me since I found that I am pulling 18 " of vacuum (full manifold vacuum) at 800 rpm idle: is the VC 1810 still the correct can for this engine (a 65 L76 with possibly an L79 cam in it, timed/setup with L76 specs)?

 

For the umpteenth time in the last five years:

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.

One other issue. Some think that ported vacuum advance is "correct", but it is NOT on pre-emission controlled engines.

Ported vacuum advance is an emission control technique to increase EGT, which promotes oxidation reaction in the exhaust, but it also increases operating temperatures, increases the tendency to detonate and run-on at shutdown, and increases fuel consumption. With a handful of exceptions, all GM pre-emission engines equipped with vacuum advance used full time manifold vacuum.

For some inexplicable reason, the '63 FI engine used ported vacuum advance- the first year vacuum advance was used on Duntov-cammed engines. Maybe GM thought that idle quality (always a problem on FI engines) would be better with ported vacuum advance, but it wasn't, and the '64-'64 FI engines got full manifold vacuum advance.

L-72/71 have ported vacuum advance to meet CA emissions since there was only one version of this engine for all 50 states.

If your experience with Corvette engines does not go back to pre-emission engines, then all you've ever seen is ported vacuum advance on emission controlled engines.

I am never going to talk about vacuum cans again. EVER!

Duke

 

I know you are trying to bring me up to a higher level of knowledge and understanding, much appreciated of course, but my previously-disclosed stupidity forces me to ask this:

does that advice mean the VC1810 is correct/best for this application?

It would appear to satisfy your "basic rule": "SHOULD PROVIDE FULL ADVANCE AT NOT LESS THAN 2" LESS THAN PREVAILING IDLE VACUUM AT NORMAL IDLE SPEED WITH APPROXOMATELY 24-32 DEGREES TOTAL IDLE TIMING."

 

Jack, I believe this is the answer.