1965 L75 327/300 Vacuum Advance and Dist date question

[Mark P.]

I saw a very detailed post on this from last summer. I'm still confused.

The distributor has a newly installed B26 vacuum advance. This was installed when the previous owner took the distributor into be recurved 3 months ago.

Should I swap this for a B20 vacuum advance instead ? This tuning site mentions that the B26 and B20 are the same and have part number VC1765. Duke Williams is referenced in the article.

http://www.lbfun.com/warehouse/tech_...ance_Specs.pdf

Are the specs of 0 at 6lbs and 24 at 13 lbs what I should target ?

The 1111076 distributor has a date of 8/24/64 (4 H 24). This is 5 months and 11 days before my VIN assembly date. Could this distributor be original ? Would it pass judging ?

[Duke W.]

Re: 1965 L75 327/300 Vacuum Advance and Dist date question

The '65 300 HP VAC has been discussed before. It is a one year oddball, and IMO 24 degrees is too much. I'm not sure if a duplicate spec VAC is currently available and don't care.

If it has a Powerglide, keep the B26. If it's a manual consider swapping to the B22, which is 0 @ 8", 16 @ 15" if you have any part throttle acceleration detonation.

The B26 and B22 specs are 0 @ 6", 8 @ 12", and this matches the specs of the VAC used on the 300 HP base engine in '66 and '67. Check with a Mighty Vac and timing light to be sure what you have meets spec.

If the dist. is within six months of the body assembly date, it should be considered as "original" as long af it is the correct part number.

The VAC is worth a total of 3 points - IIRC two for originality and one for condition, so if the JG calls out a specific VAC number and you have an aftermarket B-number, you should only lose one point, and it's definitely worth one point to have a VAC that matches the manifold vacuum characteristics of the engine/trans combination.

Duke

[Mark P.]

Re: 1965 L75 327/300 Vacuum Advance and Dist date question

Duke - thanks for your response. I spoke with the guy that did the distributor curving and he says he installed the B26 with a custom steel bushing he made to limit advance to 10 degrees. He will run my car on an exhaust gas analyzer and other equipment and will consider the B22 if he is unhappy with the specs with the B26.

He rebuilt my AFB recently and will tune that along with the distributor.

Thanks,

Mark

[Duke W.]

Re: 1965 L75 327/300 Vacuum Advance and Dist date question

Get rid of the bushing to allow the full 16 degrees vacuum advance. I believe the max. centrifugal is 26 deg., but have the guy verify. Select an initial timing so the sum of initial and full centrifugal is about 36 degrees. Road test it with different springs and use the lightest set that does not create any detonation or allows full centrifugal as low as 2000 without detonation.

If the paper you are talking about is from Lars, it has a supplement that I wrote to explain how to select a vacuum advance and what it does. Most "mechanics" haven't got a clue as to the underlying engineering attributes of vacuum advance.

Duke

[Wayne M.]

Re: 1965 L75 327/300 Vacuum Advance and Dist date question

The '65 300 HP VAC has been discussed before. It is a one year oddball, and IMO 24 degrees is too much. I'm not sure if a duplicate spec VAC is currently available and don't care.

Duke --- I've been looking at my spec books and am now wondering if the 1116238 vac can that you mention above (pic of the culprit below), and called out for the base and 300 hp motors in 1965, is in error.

The 5th ed. of the 1965 TIM&JG says 1116236 for ALL small blocks, and the 1965 Corvette Shop Manual seems to back that up by quoting zero advance @ 4" Hg, and max. adv. of 16.5 degrees (crank) @ 8.2" Hg [essentially the Delco spec's on the '236' can].

The only place that I see the '238' can specified for the low-horse 1965 distributor 1111076 is in the Delco Remy Test Specifications DR-324S-2 supplement(s); I have 3 of these ('64-66; '64-68; and '64-73, and they all quote the same. I figure if it was a typo, it would have been corrected over this time span.

So much for the books . What about what really was installed back then ? I would ask that owners of unrestored '65 base engine or L75 cars report what they have.

In spite of all this, I agree with your suggestions above; I've downloaded Lars' paper with your supplement, and will get around to checking my cars when the weather warms up.

[Duke W.]

Re: 1965 L75 327/300 Vacuum Advance and Dist date question

Parts books and specs are full of errors, and, unfortunately, they never get corrected. ...same seems to apply to the JGs. It's like pulling teeth. But I think the Delco spec books were better maintained and more accurate than the GMPD catalogs.

Corvette News back in that era had a table of tune-up specs for each model year, but did not list part numbers (except for the distributor). They were usually in No. 2 of the Volume, with No. 1 being published in the fall with new model year data, so any very early production changes were probably picked up. The service manuals were probably finalized at start of production, if not before, so the CN specs are later and may be more accurate.

For 1965 the CN-listed VAC specs for 250/300 HP (1111076 distributor) are 0 @ 6", 24 @ 13"; 350/365/375 HP are 0 @ 4", 16 @ 8", which is the Delco ...236 VAC.

IIRC the scoring sheets show 2 points for the VAC (originality) and one point for condition. Some JGs list a VAC number, some don't. If the JG has a Delco number and the distributor has a B-number, then I deduct one point for originality, and since they usually are in good condition, they don't get the condition point deducted.

So as far as judging is concerned the VAC is not a big deal, and I recommend installing one that is properly matched to the vacuum characteristics of the engine even if it isn't the "correct" part number. GM didn't always get this right, and the JGs don't have then all right, either.

Duke

[Mark P.]

Re: 1965 L75 327/300 Vacuum Advance and Dist date question

Thanks Duke for that advice.

If my vacuum unit is within a 1/16 of an inch from touching the stock intake manifold and timing (with vacuum line plugged) is 7 degrees BTDC at idle does this mean my distributor is a 1 gear tooth off ? I can't advance it anymore the way it is installed.

I was planning to turn the engine until TDC and the rotor is pointing at the #1 cylinder on the cap then pull the distributor and adjust the oil pump shaft slot with a large screwdriver slightly clockwise (one gear tooth amount of travel) then drop the dristributor back in and reset the timing.

I have verified the wires in the distributor cap are in the correct position per the shop manual diagram.

Does this make sense ?

[Duke W.]

Re: 1965 L75 327/300 Vacuum Advance and Dist date question

The dist. drive gear is probably on 180 degrees out. Set the balancer at 10 deg. BTDC #1. Pull the dist. and verify that the dimple on the side of the gear is pointing the same direction as the rotor tip, not 180 out. Correct as necessary.

Reinstall the dist. starting with the rotor about 45 deg. to the right of engine centerline. As the gear engages the angle will back down to about 20-25 deg.

Rotate the housing until the points just open. The VAC should now be about midway between the interference points.

Start engine and set timing to your normal value.

Duke

[Ted K.]

Re: 1965 L75 327/300 Vacuum Advance and Dist date question

The '65 300 HP VAC has been discussed before. It is a one year oddball, and IMO 24 degrees is too much. I'm not sure if a duplicate spec VAC is currently available and don't care.

If it has a Powerglide, keep the B26. If it's a manual consider swapping to the B22, which is 0 @ 8", 16 @ 15" if you have any part throttle acceleration detonation.

The B26 and B22 specs are 0 @ 6", 8 @ 12", and this matches the specs of the VAC used on the 300 HP base engine in '66 and '67. Check with a Mighty Vac and timing light to be sure what you have meets spec.

If the dist. is within six months of the body assembly date, it should be considered as "original" as long af it is the correct part number.

The VAC is worth a total of 3 points - IIRC two for originality and one for condition, so if the JG calls out a specific VAC number and you have an aftermarket B-number, you should only lose one point, and it's definitely worth one point to have a VAC that matches the manifold vacuum characteristics of the engine/trans combination.

Duke

Duke, What is the difference with powerglide? What VAC should one use with Powerglide?
Ted

[Duke W.]

Re: 1965 L75 327/300 Vacuum Advance and Dist date question

P/G idling in drive at 450-475 pulls less manifold vacuum than with a manual trans idling in neutral at 500.

Thus I recommend a 12" VAC with P/G, which is what GM used on the '66-'67 300 HP, but a 15" VAC (B22) is okay with a manual trans.

There's no need to install a VAC that is more aggressive than necessary because it can increase the propensity to detonate at low revs high throttle. You can't load an auto below torque converter speed, but you can load up a manual at idle speed.

Duke