You seem to have an impressive “understanding” of cam design/application, and the effects of ignition timing (as well as the hardware involved). I’d appreciate you opinion if you please………


I’ve decided to go with a single point Delco Vette distributor in my BB replacement engine……… I’ve had enough of the TI setup (so it’s going to be oiled up and stored). I’ve used these std. distributors before in SB applications, with success, to 7000 RPM’s (that’s with Accel point, etc.).

I’ve obtained what looks to be a fairly fresh distributor. The side-to-side shaft clearance can’t be felt by hand, but up-and-down clearance is .012”. The point plate also seems fairly tight, but you can feel a very slight “wobble”. When rocked/measured, I recorded .008”, as measured at the outside diameter of the plate (.004 up, and .004 down, from the static position). The plate does slide up and down on the shaft (about .040”, or so), I assume that’s typical and acceptable. The vac advance canister is marked “B21”.


My questions are:

1) I’d like to add a shim to reduce the up-down shaft clearance to .002” (I assume you would place the shim just above the gear, correct?). Can these shims be purchased, or do I need to fabricate one?

2) Point Plate fit: Could I expect a new one to fit any tighter, or is the fit I describe above about as good as it gets?

3) What grease do you recommend for inside the distributor, wheel bearing, chassis grease, or?

4) The Vacuum Canister: I’ve always went full mechanical with these in the past, with all the advance (36-38 degrees) in by ~2700 RPMs. I’d like to use the vacuum advance feature this time, if possible. What does the “B21” mean? My engine is still on the stand, and while I’ll be fine tuning it after it’s installed/running, I’d like to set it up as close as I can before I drop it in, which “can” would it be a good starting point? Basic info on engine below (I’m expecting very low vacuum at idle):


468 CID, 454 +. 060
10.7 static compression
Rect. port heads, pocket ported, etc
Stiff solid/flat cam. 256/266 @ 50, 110 LSA (105/115)
890 CFM DP Holley, 4 corner idle
Headers

I had someone do a desktop dyno mock up and he sent me the following:

RPM / HP / Torque
2000 149 390
2500 206 430
3000 250 441
3500 320 480
4000 394 519
4500 469 548
5000 533 560
5500 579 551
6000 600 525
6500 607 491
7000 591 445

Any other comments concerning timing/advance curve, would also be appreciated.

Thanks in advance - Mike



[Modified by 66427-450, 9:54 AM 8/31/2003]

 

i am not DUKE but i would use your TI distributor to fire a aftermarket MSD igniton system as you will have a lot better system than points.

 

I just put in a lengthly response, then had a glitch and lost the whole thing. I'm too POed right now to type it all in again. I'll do it later in the day.

Duke


[Modified by SWCDuke, 11:52 AM 8/31/2003]

 

:nonod: I know the feeling........... I type/save in "Word", and then paste it in here

By the way, I just got back from the office and looked at that distributor again, the up-and-down movement of the Point Plate (that I estimated as ~ .040") is actually .024".


[Modified by 66427-450, 7:22 PM 8/31/2003]

 

Okay, it's 1930, I just started dinner, opened a beer, and am ready to attack this thead again.

Clem's advice is fine, but I admire your goal to make a single point work in this appliation, and will do what I can to help out. I'm certainly no stranger to going the opposite way of conventional thought!

The OEM end play spec for the dist. is two to seven thou, but only the FI versions were shimmed up this snug in production. There has been lots of discussion about this on the NCRS board. My '63 Corvette shop manual says two to seven thou and DOES NOT distinguish between FI and non-FI distributors, and I found that tight endplay helps the single point function properly at high revs.

GM used to sell shims in six and thirty thou sizes, but I'm not sure if they're still available. You can use one six thou shim to bring the endplay into spec. Since it usually takes at least two thirty thou shims to meet spec, I place the thin shims in the middle, but the shims are hardened and I have never seen one wear. If you can't find one from GM or the aftermarket one of the distributor specialists should be able to fix you up. If one shim gets you to six or seven thou, I don't think it's worth the time to grind down a thirty thou shim .003" and add another six to get to two to three thou. The need is to keep the shaft from bouncing up and down the typical 60+ thou that most distributors were built to.

Your measurements of breaker plate wobble are admirable. I have to admit I have never made those measurements. I just go by feel that they are "snug". They are cheap and readily available, so you might want to buy a new one and see it it's snugger, then use the best one.

GM used to sell a special siliconized grease that was recommended for the distributor bushing well and starter motor bushings. It's no longer available, so if you have a good synthetic general purpose grease laying around use it. Otherwise, I suppose good old white lithium is okay. The felt washer that lays on the plastic grease well seal should be soaked in SAE 30 oil, then allowed to drain for 24 hours. The oil will lubricate the breaker plate bearing surface that bears against the bronze bushing on the housing in response to the vacuum can. I recall that the plate is secured to the housing with a wavy snap ring, which should keep downward pressure on the plate and keep it from jumping up and down, but I don't recall feeling for end play. Again, a new breaker plate would be a good reference point.

The "B21" vacuum can specs are 0@8", 20@17. The can is from an early seventies emission controlled engine, and it is not suitable for the vacuum characteristics of your cam. It will probably be doing good to pull 10" at idle. Use a NAPA/Echlin VC1810 can (stamped "B28") which has specs of 0@4" and 16@8", which will, hopefully keep it pulled to the stop at your idle vacuum. You want constant and high total idle advance. This will help keep the engine cool and improve idle qualilty. Total idle advance should be in the range of 30 degrees and will be the combination of vacuum and initial, plus maybe a little centrifugal if you have a quick curve.

Your initial timing will be dictated by the amount of full centrifugal advance. The combination of full centrifugal and initial is the total WOT advance you are looking for. On a BB it depends on the chamber design, but I think 36-38 is a little short. Clem can give you a better idea, but I though BBs with open chamber heads like about 40-42 degrees. Whatever the total, you want to get it in as quickly as possible without detonation.

The sixties vintage SHP single point engines used a low resistance (0.3 ohm) ballast in conjunction with the 091 coil. This combination generated more primary current for more spark energy, but could be tough on points. These parts are hard to come by nowadays, so you should just go ahead and use a generic 1.8 ohm ballast and coil and see what happens.

If you plan on revving over 5500 you want to use the high breaker arm tension (28-32oz.) points equivalent to the out-of-production Delco D112P. I have the following part numbers, but there is some controversy (or mispackaging) as to whether these part numbers have the high breaker arm tension. They should be easy to verify compared to a set of standard 19-23 oz. points merely by opening the breaker with your fingers and comparing the resisting force. They were used on many late sixties/early seventies BOP muscle car engines, but GM never called them out on the point igntion Corvette BBs.

NAPA/Echlin CS7860
Borg Warner A112HP
Standard DR-2371XP

The key to making the single point work at high revs is having all the parts fit snug with minimum clearance and play and use the high breaker arm tension points. As built they are typicallly sloppy. If "blueprinted" with the 28-32 oz. points, it should work with a high performance engine up to 7000 revs. It has less per spark energy than a modern electronic ignition, so if will be more sensitive to spark plug fouling. I notice that the bottom end of your 80 percent torque bandwidth doesn't start until a lofty 3000 revs. This is going to make for a soggy feeling engine in normal street driving and it may be difficult to tune the carb to provide a suitable and cylinder balanced mixture at idle and low load. My recommendation would be to "decam" it and get the bottom of the 80 percent torque bandwidth down to 2000 revs.

Also, a 4" stroke engine at 7000 revs is running at 4667 ft/sec mean piston speed which is approaching Formula 1 levels. For sure nothing but the best bottom end parts and careful prep and assembly will be absolutely necessary for reliability and longevity.

All the overlap is going to result in a lot of exhaust gas dilution at idle and low revs/light load. Exhaust gas dilution slows flame propagation speed, which is why lots of idle advance is required. Also, exhaust gas dilution makes the mixture harder to ignite. The ideal ignition for this would be inductive, since they have long duration sparks. CD igntions work very well with rich, dense mixtures that don't have excess exhaust gas dilution, which is the case with a racing engine at high revs. The reason the MSD has mutiple sparks at low revs is to insure that its short duration spark lights the fire, but the second and third sparks are retarded from optimum timing.

I think we'll all be interested how the single point works out and the engine in general. Keep us informed.

Duke

 

just to add to what duke posted,if you use a aluminum bodied distributor i would not go under .010 end play because of the growth of the aluminum housing vs the steel shaft. you only need over 40 degrees total advance if you are running high dome pistons,12.5:1 or higher. i never had much luck using a point distributor in high RPM BBC because the only way i got them to rev and not fall down on the top end after a couple of passes on the drag strip was to use a "dwell extender" which was a electronic device i built using a silicon controlled rectifier to electronically close the points as soon as they opened to get enough dwell to prevent a misfire at the higher RPMs. i never had much luck when using high tension points because they seemed to bend the plate as the points opened so on these types of setups i machined the housing so the plate rested on the housing outside edge and bolted it down which did not allow for the use of a vacuum advance. i even filled the cavity with epoxy to get a better place for the plate to set on some distributors. please lets us know how things work out


[Modified by clem zahrobsky, 8:15 AM 9/1/2003]


[Modified by clem zahrobsky, 8:17 AM 9/1/2003]


[Modified by clem zahrobsky, 8:23 AM 9/1/2003]

 

And it’s 0530 here, on Labor Day, and I’m in the office (the life of an Engineer/Manager with “weekend duty” at a nuclear power facility) lol

I’ll pick a new one up and compare them..... Knowing me, I’ll probably end up out in the shop personally hand fitting a plate, and making a shim for the bottom to obtain .002” +-0 axial clearance.

lol, just trying to communicate the amount of existing clearance

So: fit up all moving parts for minimal clearance; synthetic general purpose grease; felt washer gets SAE 30 oil w/24 hr drain; NAPA/Echlin VC1810 (B28 can); high tension points (I have 5 or 6 set of NIB Accel point from the early 70’s, I think they came with 2 springs?, I’ll check them out tonight); look for a total idle advance of ~ 30 degrees; play with the weights/springs to get a total of 38 degrees in ASAP w/o detonation (I’ll start with 38).

Yes, 80% of peak is at 3000, but look at the amount of torque remaining at 2000 revs, 390#, plenty, wouldn’t you say? Besides, I'm thinking that killing a little bottom end may not be a bad idea (when trying to launch this thing... my car looks stock, and I really can't fit very much tire on the rear). Duke, I understand and truly do appreciate your advice in this regard, and I may end up advancing the cam 2-4 degrees (and/or opening up the lash a touch) during the tuning process to take the edge off it, but really, this car will be seeing very little “normal street driving”, and the gains this cam offers 4000-7000 make it an appropriate choice, “for my application/intended use”. That’s what it really boils down to I guess, “intended use”. I think the biggest error made by “Hot Rodders” is not really being honest with themselves concerning how the car will be used, and the failure to design/put together, a proper "combination". By combination, I mean not only the combination of power producing parts, but the appropriate bottom end strength to provide durability with the new found power, as well as the gearing to be able to apply what they’ve added to achieve their performance goal.

Yes I understand. The bottom end has the best of parts, and has been meticisually clearanced and assembled; 4 bolt ARP studded mains, Scat 4340 crank/H-beam rods with upgraded bolts, JE-SRP pistons/floating pins, CL77 hard bearing, etc., all hand fit. I did go with rods ¼” longer than stock, I’ve never done that before, but it does make sense to me that it would lessen the “side loading” of the piston by moving the pivot point closer to the piston face.

Sounds like a high tech MSD setup would be a good fit for this application, but I’m going to give the points a try. I just like the idea of using 60’s technology on a 60’s car, besides, the simplicity, ease of adjustment/maintenance, and the inherent reliability of the Delco point distributor makes it attractive to me. As far as I’m concerned, part of the enjoyment of owning one of these cars is tuning it periodically; lashing the valves, changing points/cap/rotor, screwing in fresh plugs..... What was it that Steve McQueen said in “The Hunter”?.....”New things are no good!!” :-)

As usual, many thanks for the info/advice Duke...... I refer to your posts in the archives often.




[Modified by 66427-450, 1:45 PM 9/1/2003]

 

I understand, the one I picked up is factory unit (not alum).

thanks for the feedback, i was planing on keeping it around 36-38max

interesting, are you talking about operation over 7000? My old SB's (with "Accel kits") were ok to 7000, maybe I just got lucky and got tight distributors. My stock TI was also good to 7000, when it worked, that is :-).

Again, thanks!

 

we were running our 427+ BBC to 8000+ rpm. we used single point with a double spring distributors,never check the tension, with the D.E. i mentioned before and if the D.E. went south we could not pull more than 7000 RPM without a misfire. BBC eat sparkplugs and even the stock street ones with the TI would need plug changes quite offer to keep them up on the "pipe". i have found that street driven BBC like R-45 XLS extended tip plugs for a reasonable plug life. the hotter plug keeps cleaner longer and at speed the fuel wash cools the plug tip. :chevy