I have been looking at all the posts on flathead distribs and I wanted to summarize what I plan to do and see if it makes sence for my set up. New flathead, 265 ci 2 94s with vacuum ports Edelbrock heads, 8.14:1 compression Isky Max 1 (3/4 race) lakes headers converted chev old style distrib I checked TDC on #1 and marked it on the pulley About 4° initial advance from my mark. Mechanical advance 16° at 1200 rpm Vacuum advance, 6° additional at 2500 RPM Hook vacuum advance to manifold vacuum, not ported vacuum on carbs. Total advance at 2500 RPM is 26° Is this a good starting point for the best economy, driveability and power? I know every engine is different, but is this a good place to start? -Dave
Ron Holleran is currently flogging flathead timing and is giving out these numbers as his current generic starting place. I trust him entirely on this stuff! Obviously, individual engines may like a bit of tuning from this starting point. You MAY want to very slightly slow centrifugal if it pings and speed vac so its all in during most slow speed driving. You have the advantage here of probably the easiest to tune advance system there is.
Thanks Bruce, you are one of my sources for these numbers. I forgot to mention that I have an accel adjustable vacuum advance as well. Are you thinking more like mechanical 16° at 1500 and vacuum additional 6° at 2200? or should I start with my original numbers and see what happens. I just do not want any damage on a new engine.
There is, of course, the on-going debate about manifold vacuum vs ported vacuum for vacuum advance... Without a dyno, it's hard to know for sure, but in general, all engines want the least amount of advance to effecitvely burn the mixture. I watched a dyno pull where torque/hp went up as timing was pulled out from "standard" numbers. This engine was making best power at 18* total... ...I'd always considered 24* total to be the right number, but I think there's far more to it.
and...ANY generic curve is just a starting point for flogging the individual combination. Vac curve obviously is left out while tuning the full throttle curve, later it is probably the part where you have to almost go on faith because its conditions of use are essentially infinitely variable, unlike the full throttle. Volume and shape of chambers and valve area, plug location and depth, any relief used, and so on are obvious variables in flatheads. I think one of the critical areas (very well demonstrated in the good--bad--good flow in the history of SBC heads and pistons!) is the squish area, specifically getting it down to no more than .050 over piston. More turbulence, less sensitivity to knock as well as less need for excess advance. Chevy started realizing this and later SBC needed several degrees fewer in rodded engines... Another part of this, beyond much control unless you have plenty of time and money, is actual fit of piston dome to squish area dome...available pistons vary quite a bit here, and your .050 clearance might necessarily vary all over the surface if the curvatures are not same as head! Without a dyno, I suspect there is a new good way to tune the full throttle curve, those accelerometers that attach to the windshield and give readouts of several factors...cannot think of name. You could flog it in say 2nd gear over the same speed range repeatedly while staying within legal limits...
Also...going from a poor curve to a good one is one of the very best and most noticeable hopups possible! Probably more felt change than anything else you could do except adding a blower...
26FR, when you say you are using 2 x 94's with vac ports, I ***ume you mean the Loadamatic type carbs. I believe if you want to use those with the GM dizzy and vac advance, you want to plug the vac drilling inside the carb you tap into, that goes to the venturi boosters. That p***age is to provide the Loadamatic's version of mechanical advance. I'm using a single 94-style 1-1/16" carb (2110 Holley) with both drillings and an MSD dizzy. It works OK, but mine is a stocker and not very sensitive. I'd like to hear Bruce's take on this. Mine also has the "economizer" valve on the lower drilling, I've pluggged that.
From what I have seen and read so far there are 2 "total" numbers . The total advance under full power (like on a dyno or a drag strip) I believe this comes directly from the mechanical advance curve. When the engine is working like this, the throttles are wide open and there is very little (if any) manifold vacuum. The other "total" number is when you are cruising down the road in top gear under very light load. In this case, the throttles are mostly closed and there is much higher manifold vacuum. The mechanical advance would be all "in" but then there would be an additional advance from the vacuum advance module. This would be better for a lean mixture and it would get better mileage as well. If this is true, then I would want to use a vacuum source that was not exposed to manifold vacuum at idle, but it would be at low throttle openings. All this being said, should the mechanical and vacuum advance numbers I have listed above work with this engine and this distributor? I know each engine is different, but I just want to start with reasonable numbers and adjust from there. -Dave
I have attached pics of my carbs. I do not know what type of ports these are or if they are suitable for vacuum advance on the Chevy dist. Comments?
Those ports connect to two p***ages, one reading venturi signal, the other ported vac. As is they are totally inappropriate for use with a real distributor, but if you plug the sraight-in p***age in each you will have ported vac. Mallory actually supplied a plug for that purpose with its old line of performance distributors for '49-53. Incidentally, the OTHER p***age probably offers an interesting source of tuning information on your venturi activity...something to think about!
Heres my two cents worth on the subject... After a few days on the dyno with a flathead we determined that 28 degrees was THE number where we made the most horsepower. After using many chevrolet distributors on the test bench i also know that the average chevrolet distributor is a 12 degree max advance distributor. (***uming the rubber limit bushing is intact). So if we curve a chevrolet at 12 degrees to be all in at 2500 it is almost perfect for the flathead engine. Set the intial timing on the balancer at 4 degrees add in the 24 mechanical advance and ya got the 28 degrees. After shipiing 65 of these units i think most will agree on the numbers. Most of my conversions have gone to hambers. I offer a special hamb deal of $135 to hamb guys and i pay shipping. Actual build info is at www.bubbasgarage.com. Now the use of vacuum brings another issue into light. The vacuum range of a small ( by most standards) cubic inch engine, camshaft and carb choices makes vac a different animal. Vac advance ( on a small block chevy) has proven to be a 2-3 mpg at cruise , however it would need to be adjusted ( accell adjustable like mentioned). I got one going out to flaternie to run with vac , maybe he can post his thoughts etc. Many have claimed the use of the mechanical advance chevrolet conversion have asctully made the engine run much cooler and lots better...
Plus you get some really good hot sauce in the deal as well. Haven't put the one you did for me on the engine yet, but will report back in the spring.
Yeah, retarded is REALLY bad news on a flathead, since all the excess heat dumped into exhaust is inside the water jackets. If you have a stock '49-53, you are running retarded almost certainly...and measuring your actual on the road advance is a real challenge as well.
Thanks to Albu and Bruce on the info on the ports. How do you go about plugging the straight thru port?
Thanks for all the input, but I am a little confused. I realize that all engines are different, but I have seen here that 18° advanced and 28° advance (at 2500) makes the best power in a flathead. Is this the potential range I am looking at for my specific engine? or is one number for bone stock and one for full race. I just ***umed the starting point for my engine would be more defined than that. Vacuum advance: Is this always in addition to the above numbers? If so how much? If that cannot be answerd, that's ok, I am just wondering if I should shoot for 3° of additional advance or should it be 12°. If this is added to the numbers above I might be looking at 31-40° of total advance at part throttle cruising speed. Also, I really appreciate Bubbas input on this especially since you have made so many conversions. If I did not already have a bunch of time and money in my set up, I would buy one from you. I may still do so, if my experiment does not work.
Yo Bubba, Are you sure about your website? I went there and it's a place in Georgia, nothing about Chevy dizzies. I was hoping you had a way to order some hot sauce on line. By the way, ain't got your dist. on the street yet, but the thing sure runs good in the back yard.
I'm using the MSD "ready-to-run" (which it isn't, it comes with SBC springs in the mech advance) and a single 94. I have the vac advance limited to around 10 deg. (accel unit) with the springs MSD supplies to give fastest mech adv (all in by 1800), the 18 deg advance limit bushing, and static on the factory ****on (4 deg). Total is low in the range stated (w/o vac). I intend to try incremental increases in the mech advance limit, but it runs great with this setup. Caution -- Ernie and I are both at high al***udes (5,000+) so take our results accordingly. Thin air is OK with more advance.
Thanks to all of you for your input. 4° sounds like a good number for base timing. I think what I will do is to start with about 16-18° mechanical at about 2000 and then add another 8° vacuum at 2500 for cruising. I want to get the engine started conservatively and then add timing when I get the car on the road and can test things out. Maybe I can make various sized stop bushings so I can easily adjust the timing. -Dave
I would be more than happy to check your unit and offer complete test report to the [post and board for no charge , you pay the shipping one way and i will pay to send it back.....Make a good case study for the board...
Collating replies...all in crank degrees Bubba 4+24+vac Ron H 4+16+6-7 vac full throttle curve to correlate with power valve Albuq 4+18+10 Fords finest, the 3 '41-48 distributors with 11A advance, are difficult to categorize because Ford, as normal, used Mallory ideas mixed with Ford ideas in the parapsychology lab. These had their LONG curve, essentially the part throttle high vac curve, in the centrifugal. For full throttle, this curve's RATE was limited by a spring loaded disc brake that placed a drag on the advance mechanism. Total was not actually limited, it simply came in so slowly (3450 for total with heavy spring load) that some was not normally used. And, as with a modern vac can system, the vac operation was not a definable second curve, it was a big range of infinite gradations tuned by exact sprng vs. vacuum math. Cl***ic Mallory... The disc brake was APPLIED by a coil spring, PULLED OFF by vacuum acting against the springloaded brake puck. Spring load was adjustable by a moveable seat, and procedure was to accelrate car, loosening load on spring til car started to ping under full throttle. Got that? Simpler than Federal Tax Code! Now...in the past, when the cars were new, spring load was apt to be heavy due to poor octane gas, so you had a slow WOT curve (ending as high as 3450) and a quick cruise curve, reaching full advance as early as 1200 with lots of vac keeping brake released. In modern times, our unleaded **** gas is actually a magnificent feast for an engine born during the depression, and most find they can fully back off the adjuster, leaving it in maybe a turn to stabilize the advance (You can't totally eliminate drag on brake disc as that leaves it free to float a bit and timing drifts a few degrees), and the quick curve or a close approach to it is available at all times. This is a curve that the engines seem to like, but has 2 deficits...no way remains to advance more for low throttle use, and actually curve is very difficult to alter. Nobody makes variant hotrod leaf springs for the dual leaf setup, which has a light spring to get timing moving fast and a heavier spring for upper end of curve. Total available with 4 static is listed as 25--28 degrees, reaching full advance as low as 1200 and as high as 3450...so you can see why the vac adjust is set to pretty much cancel the brake. So there's the scorecard so far for starting places. It is interesting that the stock 4 degrees is so popular as the static setting, even though curves normally start coming in rapidly in the first 100 RPM off idle.
Thanks for the offer Bubba, but I am working with a local guy who has built engines for me and everyone else for years and has a sun distributor machine. I know he knows all the ins and out of checking and adjusting distributors, I just have to decide what the thing should be set at. I had it on his machine the other day and he showed it maxed out at 22 crank degrees (mechanical only) in the distributor itself. As far as the terminology, I agree we have to talk the same language. I think that any time someone talks about advance, they should always be talking in terms of crank degrees and from TDC (not from some predetermined advance point) That way you can talk about any engine with anyone and they will know what is going on. Most people do not have a distributor machine and I for one did not think about the 2:1 ratio before you mentioned it. Like I say, I think I am going to start conservative (less advance) to get the engine going and increase it later when I have a chance to drive it or dyno test it. all this info will be extremely valuable when I get to that point. I will let you know more when I get closer to starting the engine.
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