SWCDuke

...not a big deal. The 365/375 HP 327s centrifugal starts at 700, but they won't idle well below 900, and at 900 the centrifugal adds 2-3 degrees I've setup up several of these engines and they all idle stably. I set up Duntov and LT-1 cam engines the same way and have never had one detonate.

Your engine has the L-86/82 cam, but only 8.5:1 CR (actual is likely less), so it should run detonation free on 87 PON fuel with a fairly aggressive centrifugal. With the amount of valve overlap it needs 25-30 degrees total idle advance and like ALL Gen I small blocks 36-40 total WOT advance as far up in that range as the engine will tolerate without detonation. Total idle advance is the sum of full vacuum advance and initial advance. You should run a 12" B26 VAC connected to a full time vacuum source. Assuming a manual trans idle behavior of 750 @ 14-15" is right on, and it should be stable with a slight lope.

If you want to run mid-grade or premium fuel the centrifugal advance can be even more aggressive.

Search for threads stated by me and download my tuning seminar pdf.

Duke

alexandervdr

I did try that, in many combinations, I have a whole collection of tuning springs that I collected from different suppliers (Mr Gasket etc)

alexandervdr

Thanks Duke, forgot to mention your paper, which is included and referred to in one of Lars's write-ups. My set-up is basically the numbers you advise in there. I have a adjustable can in (Accel). Adjustable means it may not stay stable in time, so that's why I want to switch back to a fixed one. If I am correct the B26 is similar/identical to a B20, which is what I just ordered today. My manifold pulls 15" at idle, so I need a canister that pulls all in at 13" or earlier, right (the B26 does)?

My idle now runs at 31° total at 750 rpm. At the high side of of your recommendation. Other variable is that fuel in Europe is higher grade compared to the US (at least that is what I am told), so may burn faster, so may need/support less advance. I guess...

SWCDuke

Yes the 12" B26 is what you need. It's the least aggressive VAC that passes the Two-Inch Rule for your engine. I wrote an addendum to Lars' paper, but my tuning seminar is a different document. It does not conflict with what Lars and John Z have written, but adds additional info on how to set up a spark advance map.

The EU uses RON rather than the USA PON = (RON +MON)/2. I believe your highest octane is 98 RON, which is about equivalent to our 93 PON. My tuning seminar discusses this and how to covert from various octane ratings to others, RON, MON, PON, and Aviation Method. I don't know the lowest RON offered in Europe, but it's probably about equivalent to our 87 PON, which would be about 91-92 RON.

You should measure and report the total centrifugal advance in the distributor and I can make some recommendations on initial timing. Total idle advance is 31 is okay. The 25-30 number I quoted is a ballpark.

Total centrifugal of about 27 would be ideal in as low at 2500 with the 16 deg. VAC and initial timing in the range of 9-13 as high as the engine will tolerate without detonation, which will be a function of what fuel octane you use. Higher octane is obviously more expensive, but the more aggressive spark advance map it will allow will yield noticeably more low end torque.

Duke

alexandervdr

I'll definitely go look for that paper you wrote!
Initial timing is 10° at 550 rpm (vacuum disconnected, I assume no centrifugal)
I run 98 type fuel (the lower grade 95 has too much ethanol)
I'll report on my numbers by the end of the week, I am fully occupied in a training this week

SWCDuke

98 RON fuel should allow a very aggressive centrifugal curve. One of the best things to do as you are apparently going to do I replace the adjustable VAC with the B26 which starts at 6" and provide 16 crank degrees at 12" and above.

... best way to time the engine is at a few hundred revs above the point of max centrifugal (VAC disconnected) with a dial back light. Set it at 39-40, then check that total idle advance is in the ballpark. If it doesn't detonate you've got a well optimized spark advance map.

Duke

alexandervdr

will do! tnx! Mechanical advance is 37° (above 2900 or so).

I find that while driving during hard acceleration, there is a 'transition rpm zone' that feels 'harsh' (although still accelerating fine). Below and above if feels smooth. My best guess is that it is not detonation ( that I would hear) , but maybe close to (low CR). That 'harshness' rpm zone gets narrower when I dial back advance. But I am all but an expert, my analysis may be totally off. Any clues on that?

SWCDuke

...don't know what you mean by "harsh", but the L-46 cam may have a "torque surge" at about 3500. Make sure the carb secondaries are opening properly.

I recently set up a '65 L-79. which has identical overlap and idle behavior as the L-46/82 can. I installed a B26 VAC, shimmed up shaft end play, and used either the silver or gold springs from the Mr. Gasket 928G spring kit, which brought in the full 30 degrees centrifugal at 3500 with about 27-28 at 3000, and set total WOT advance at 39. It was a relatively low mileage car that had been stored for 20 years, but it had thick head gaskets, so the heads had been off and the CR was probably no more than about 10:1.

The engine ran great with a very linear pull from off-idle to 6000 and wanted to keep pulling beyond that. Shimming up the end play eliminated spark scatter and the engine felt smoother, and I couldn't get it to detonate lugging up a 15 percent grade in top gear at 1500.

The owner was very happy with the result.

Duke

alexandervdr

'harsh' is some resonance like a ball bearing is temporarily missing somewhere (which is not the case)

Jackfit

Ok, I can't wait to drive this car........


I will give an seat of the pants report......that we have all been waiting for....

Jack

SWCDuke

I still can't get my arms around what you mean. A "rattling" sound in response to throttle increases at low revs could be detonation.

Or maybe it's a carburetion issue. What engine is the manifold and carb from? Lean surge at low speed cruise or lack of crisp response to small throttle increases can often be exorcised by richening the idle mixture, but sometimes one or two sizes increase in primary jets is required.

Duke

pop23235

I'd recommend you carefully check out your harmonic balancer.

pop23235

Duke, did you mean B28?

SWCDuke

No, I meant B26. With 14-15" idle vacuum, the 12" B26 passes the Two-Inch Rule, and one should always use the LEAST aggressive VAC that passes The Rule.

Duke

pop23235

I just purchased an Airtex 4V1053 as recommended and it is marked B28??????
Here is what triggered my purchase: https://www.corvetteforum.com/forums...m-advance.html
I have the original GM but thought I'd put some on the shelf. Wrong one? What is the correct Airtex #?

alexandervdr

I know it's hard to describe in words the way a car feels. If I were closer and no ocean in between I would drive to your place to have your opinion.
Thanks for all the insights.

Frankie the Fink

A video - never hurts....some here are sharp enough to diagnose an issue if they can sorta see and hear it.. If its more of a 'seat of the pants' issue then that prob won't work...

SWCDuke

Yes, the Airtex 4V1053 is an 8" B28. It should be used on all small blocks with OE mechanical lifter cams or any engine that idles at less than about 14" Hg manifold vacuum.

Idle behavior is primarily a function of valve overlap. High overlap cams need a more aggressive VAC due to having less than 14" of manifold vacuum at the lowest acceptable idle speed.

The L-79 and L-46/82 cams are "medium overlap" and pull about 14-15" at 750, so a 12" B26 meets the Two-Inch Rule, and a "low overlap" base cam engine pulls about 18" @ 500 in neutral with a manual trans so the 15" B22 is the best functional fit, but a B26 might be required with an automatic transmission.

I have no idea what engine you have or the idle behavior, but use the Two-Inch Rule to select the proper VAC, idling in neutral with a manual trans and Drive with an automatic. If equipped with AC the test should be done with the compressor engaged.

These VACs are available in several different brands including "Delco", but they are all made by Standard Motor Products... just a different package and marketing channel.

Lars' paper has all the NAPA/Echlin part numbers/B-numbers, and any parts store that sells other brands should be able to cross reference, and of the dozen or more listed (some are not longer available due to parts consolidations) for single point/TI distributors either the B28, B26, or B22 is the best functional fit for all OE engines depending on the cam and just about any aftermarket cam when applying the Two-Inch Rule.

Duke

63 340HP

The symptom you are trying to describe may be from the points plate being loose (much like poorly shimmed distributor shaft end play), or excessive timing advance at mid-rpm.

The points breaker plate moves with the vacuum advance rod, and is held by a retaining ring. If the retaining ring breaks or is missing the breaker plate can move and produce the erratic timing SWCDuke describes with poor shaft end play.



High vacuum and vacuum advance added to the recommended centrifugal advance may be superimposed to result in excessive advance for the mid-rpm dynamic compression and fuel quality. Usually the superimposed advance is a benefit to cruise rpm fuel mileage, but it can be too much advance for fast burn heads and mild cams with high compression.

Once you address the correct vacuum can, you may need to address limiting the vacuum rod travel. Vacuum can rod travel limiters can be fabricated in a number of ways (IIRC Lars even made & sold a plate to address this concern).

Examples:





There are many ways to limit the end travel of the vacuum can rod, limiting the travel and establishing repeatable start and finish positions for the respective vacuum at the limit positions. Usually limiting only the pull end travel at full vacuum is enough (the small plate in the photo of the two cans). If you find erratic timing as vacuum starts to pull the rod, a larger plate that limits both rod travel ends like the second photo can eliminate the start, and end, vacuum advance minimum and maximum value and position inconsistency.

Getting that absolutely perfect ignition curve for a car's unique characteristics is a quest made easier by modern digital electronics, but doing the same with mechanical stops and analog springs with vintage components is much more satisfying. Good Luck.

pop23235

I should be OK then with the B28. It's a 64 365 w/ stock pistons, heads, cam etc.