that deal where it keeps advancing forever as RPM goes up is a typical deal for a factory distributor. The "right way" to fix this is to tear it down and weld up the slots that limit the travel of the advance mechanism. This is best done in a shop that has a heli-arc machine and a distributor machine. you chuck up the dist in the machine and check the amount of advance it will produce, then tear it apart, weld up the slots, file them out with a chain saw or other small round file, check it again. Usually takes a couple go rounds. both pins need to hit the ends of the slots at the same time more or less. once the amount of advance is established at the desireable number the distributor can be re-assembled and the weights contoured and springs changed to give the curve wanted. Then the vacuum advance is tested and limited or changed out to get the right amount. While it's apart the old dried up grease is changed out and a new top seal is installed. This is why a real distributor guy gets good money to properly set up a factory distributor.

The carb: that carb is a 585CFM unit if it's the L-79 unit. 585 is more than enough to feed a 327 to 6500. A 327 at 6500 at 85%VE needs about 525CFM. If it built 3" of manifold vacuum I'd guess that secondarys are not opening fully for some as yet unknown reason. Was the air filter clean? It has a dual snorkel L-79 air filter housing, right?

Forget that stuff about the vacuum sucking the secondaries shut. That's similar to claiming that the oil pressure in a hydraulic lifter is causing the valves to float. Any chance you could rig a video camera or stand near enough to see if they're opening all the way? Do you know for sure that the diaphragm is good, the vacuum feed hole to it is open and whether or not the check ball is installed? I know it sounds stupid, but have you looked down the carb while somebody else sits in the car and floors the pedal? A large percentage of rods out there have less than full throttle opening. While somebody is holding it WOT and you're looking down the carb reach down and get a finger under the link where the secondary diaphragm rod hooks to the peg on the end of the throttle shaft and lift it up, make sure the secs will go to WOT, vertical throttle plates fron and rear.

The A/F numbers are good, the jets and metering stuff are OK.

Don't feel too bad about this stuff, this is what usually happens when an engine is dyno'd for the first time unless the guy who built it and tuned it up does this for a living, all day every day.

 

Forget that stuff about the vacuum sucking the secondaries shut. That's similar to claiming that the oil pressure in a hydraulic lifter is causing the valves to float. QUOTE]


Tom, after reading that I just had to ask, if hydraulic lifter pump up isn't what causes hydraulic cams to float valves than what does? I've always run performance hydraulic cams with only 1/8 turn past zero lash and seen noticable increases in rpm before valvefloat compared to the normal 1/4-1/2 turn adjustment.

 

any chance you could hook up a good vacuum gauge inside the car, do a few WOT runs to establish a baseline and try it without the A/F element in place? Did you already do that on the dyno? I personally am not a big fan of K&N stuff.

 

I can hook up a vacuum gauge and do just that and I plan to do so soon. I have a new paper element air filter handy and the original vacuum secondary spring is also easy to locate.

I also think I need to find a shop with a distributor machine around here so I can investigate in more detail what the distributor is actually doing, get it running right and then just concern myself with where to set the initial.

 

Rich I can only give you a couple of thoughts. First would be if your engine ever saw 52* while loaded against a dyno I would expect the engine to be in pieces. I have never seen a mechanical advance that had 40* of travel. I would definately check the balancer for slippage and try another timing light. I also don't buy into the less than 2" theory the dyno operator put forward. That might be considered optimum for max HP but not necessary for a street car. I think the numbers you saw might be accurate for a small carb.

Tom I have never seen a distributor that had two pins for the advance limit.
Mike

 

We tried it once with the air cleaner lid off so the filter didn't even come into play that time. The filter is a fairly new K&N.

I'll check the WOT on both sets of throttle plates and I'll double check the vacuum can. The check ball is in there. I installed a quick change kit for the spring, maybe I nicked the diaphram when I installed the quick change kit.

But can too much advance explain too much vacuum? Or do I have two independent problems?

 

Rich the check ball would only affect the rate that the secondaries are pulled open. Some people throw the ball away. I would completly forget about the vacuum reading at WOT and focus totally on the distributor and the timing and I wouldn't run the car hard until I had it figured out. Also the vacuum that opens the secondaries has nothing to do with manifold vacuum it is directly related to airflow thru the venturi and I don't believe manifold vacuum would have any way to close the throttle plates.
Mike

 

Rich the check ball would only affect the rate that the secondaries are pulled open. Some people throw the ball away. I would completly forget about the vacuum reading at WOT and focus totally on the distributor and the timing and I wouldn't run the car hard until I had it figured out. Also the vacuum that opens the secondaries has nothing to do with manifold vacuum it is directly related to airflow thru the venturi.
Mike

I was wondering about that.

My first plan of action is to address the distributor.

I do have to stress, the thing run great on the street. One thing I did learn about the dyno is that it puts much more load on the engine than it will see on the street pulling the weight of the car itself (and I could certainly feel the difference). Perhaps they had too much load factored into the dyno system that is appropriate for my little stock small block and 3.31 gears.

Know of any good books on tuning that may cover vacuum and distributor tuning?

One additional thing occurred to me. The vacuum reading the dyno was getting was taken from the vacuum line that feeds the distributor vacuum advance. This line is attached to the side of the front metering block on this particular Holley. Where does that vacuum line get vacuum from, manifold, venturi or elsewhere? The dyno guy was thinking he was getting manifold vacuum and maybe that's not what we were reading.

 

I will add this much, the HP figures provided by the dyno match up almost perfectly with the figure provided by the HP-at-the-rear-wheels calculator factored from vehicle weight and 1/4 mile ET.

 

Rich the vacuum would have been manifold. It might have come from a ported source but at WOT it's all the same.
Mike

Bill I completly confused the dyno runs with the driveway tuning session. I am now completly sure I've lost it.

 

Rich did you have the vacuum advance hooked up when you checked the timing?
I think I will just stick to asking questions maybe I won't screw that up. LOL
Mike

 

Get your distributor guy to set it up for 22 or 24 degrees mechanical advance all in by 3000rpm, set your initial timing at 16-18 degrees and use an adjustable vacume advance with a limit set at 10- you'll see a noticeable increase in throttle response and top end. Also move the vacume advance hose to full manifold vacume on the carb baseplate.

 

so what kinda tq/hp numbers did the 'ole L79 pull?

 

Ditto. Can you post curves?

 

On the dyno run sheet it shows 0.8 inches of vacuum at 3000, climbs to 1.3 inches at 4000, by 5000 it's at 2.3 and when we shut down at 5800 it was at 2.9 inches with a peak of 3.0 inches at 5700. The dyno operator says the vacuum should be below 2.0 for the whole run and that it would be better to have it around 1.5 for the whole run. What he thinks may be happening is that the secondaries are opening as they should, then as manifold vacuum builds, it is pulling the secondaries closed part way or all of the way thus killing top end power.

Not possible for vacuum to close the secondaries. Not likely that the vacuum is caused by a restricted air filter or air cleaner housing. Restriction above the carb would have the same effect as closing the choke--your mixture would go pig rich and you'd billow black smoke.

By my calculation using the formula shamelessly stolen from the Edelbrock site, you need a larger carb. This is a rare time that I'll disagree with Tom Mobley
CFM needed = CID X RPM / 3456 X efficiency X manifold factor
327 X 6500 = 2125500
2125500 / 3456 = 615
615 X .85 = 522.75 where .85 relates to the volumetric efficiency
522.75 X 1.3 = 679 cfm where 1.3 relates to the efficiency of your intake manifold. 1.1--1.3 is good for a single plane manifold; where 1.3--1.5 is used on a dual-plane. Lower numbers represent higher-performance manifolds. If you're running an iron stocker, you may want to multiply by more than 1.3.

A 650 or a well-tuned 700 would be just fine. I'd be looking for a 750 cfm Q-jet, but that's me. This assumes that you actually rev to 6500; and I'm not certain that's the case.

After my dyno run we hooked up his light and confirmed I have about 36 degrees of mechanical in at about 3500 BUT, it doesn't stop there! The mechanical continues to climb and we saw as much as 52 degrees mechanical at around 5000! ...I never thought to check if the mechanical advance went beyond that.

Common, I see this all the time with HEIs. I don't deal with point-type distributors much. Either you need distributor work--and fast--or you have some artifact of the dial-back timing light playing games with you. 50+ degrees at part throttle is perfectly normal. 50+ degrees and WFO does make me wonder why the engine is still running...

In driving the car on the street, which I did as soon as I got off the dyno, it runs great, pulls like a freight train (well as well as a 327 can pull) from 5000 and revs like a wild thing until I take my foot out of it.

What else changed between the dyno run and the street driving? Ambient temperature humidity, or barometric pressure? Did you use the gas in your tank or the dyno's tank? Is there something in the dyno test that could have screwed up the engine so that it wouldn't have run at it's best?


Ignition first, carb second.

 

What else changed between the dyno run and the street driving? Ambient temperature humidity, or barometric pressure? Did you use the gas in your tank or the dyno's tank? Is there something in the dyno test that could have screwed up the engine so that it wouldn't have run at it's best?


Ignition first, carb second.

I drove off the dyno and went for a spin. Absolutely no changes to anything other than the dyno puts a heavier load on the drivetrain than just pushing the car around will do on the street.

Rich did you have the vacuum advance hooked up when you checked the timing?
I think I will just stick to asking questions maybe I won't screw that up. LOL
Mike

Of course, the vacuum advance was disconnected and plugged when checking the timing.

 

valve float causes pumped up lifters, not the other way around.

when a valve floats it creates clearance or lash in the valvetrain which is immediately taken up by the lifter. that keeps the valve from seating next cycle which lets compression leak out into the manifold or exhaust, whichever is floating. the lifter bleeds off in a few cycles and things are back to normal. running very little preload on the rocker arm adjustment does not affect the RPM at which valve float occurs, but if there's only a few thou preload on the plunger in the lifter it bleeds off quicker, giving back your power.

 

I'm not up on that manifold correction factor Schurkey is using.

It appears that it's an attempt to compensate for manifold restriction. But, if the engine is at 6500 at .85VE and needs 522CFM why would it help to put a bigger carb on the manifold? The carb is upstream of the choke point, the manifold.

Maybe I'll learn something from Schurkey.....

 

I'm not up on that manifold correction factor Schurkey is using...

Like I said, I stole that from Edelbrock. It does seem to explain why engines tend to like larger carbs than what would be selected based on VE alone. Perhaps this correction is just a cover-up of something else that hasn't been explained yet. All I know is that using that correction factor seems to work.

 

if the vacuum advance was hooked up when you checked the total timing, it's possible that there is a vacuum signal pulling in more timing when you hold it at steady rpm's to check the advance.
unhook the hose and check it again. i've never seen a distributor that pulls more timing after it stops advancing. i checked 3 different stock HEI distributors in my Nova looking for one with a good curve, and all of them stayed at the same advance from about 3000 rpm all the way past 7000.

 

if the vacuum advance was hooked up when you checked the total timing, it's possible that there is a vacuum signal pulling in more timing when you hold it at steady rpm's to check the advance.
unhook the hose and check it again.

2 inches of vacuum shouldn't pull in the vacuum advance--but--you're right, unhooking the hose is the sure way to tell.

i've never seen a distributor that pulls more timing after it stops advancing. i checked 3 different stock HEI distributors in my Nova looking for one with a good curve, and all of them stayed at the same advance from about 3000 rpm all the way past 7000.

I see it in HEIs when two conditions are met: (1) light springs; and (2) the weights are acting on the gently-rounded part of the center plate. If you flip the plate upside down so the weights act on the dogleg angle of the center plate, the extra advance goes away--but of course you've also changed the timing curve. If the new curve is acceptable--you're fine. Another solution is to weld/braze the slots to positively restrict the amount of advance.

This extra advance is spooky, it'll look like the advance stops at some reasonable point. And it'll hold there for ~1000 rpm, before starting to advance again, maybe providing 10--15 degrees of unwanted advance! If you don't REALLY WIND the engine (or distributor machine) when looking for total advance, YOU'LL NEVER SEE IT.

HEIs have two slots for limiting the advance, I suppose you could limit one side and be OK--but it wouldn't be "symmetrical". The slots are so huge that I don't think GM acutally intended them to be a limiting method. Point style distributors have one slot--and--new limit bushings and the installation tool are available at any NAPA or other "real" parts store. NO promises that the bushings will give you the proper amount of limitation.

Limit bushing NAPA number DP101
Installation tool NAPA number DP100

 

and M. Maner is right about the single pin and slot, I had a brainf@rt on my part. First mistake I've ever made.

That's what I get for posting faster than thinking. Working outside in 112*, come in for a cool off.

 

Schurkey, I've had a 700 dblpmpr on a 327 +.060 and it was not good, too much carb. dropped back to a #6619 600VS and all was well.

In fact I was digging around in the garage today and found the nice #1850 600CFM I built for the 327 in my 57 Chev Pickup. Is has a #3310-1 on it now, but the 600 will go on it tomorrow. It's doggy with the larger carb, snappy with the 600. Even better with a Q, but I don't have one built laying around now.

 

The distributor I went out and looked at after seeing M. Maners' reply was a points type. I can't remember where I got that thing, it has the shortest limiter slot I've ever seen. Probably why I picked it up in the first place.

Lot's o' info in this thread.

 

Even though this is not my thread. Thanks to all involved for the great information.
Mike

 

I'm also not convinced the engine is over carbed. Kind of circumstantial evidence, but go here: http://www.youtube.com/watch?v=nHOjF_CoKY0

Once there hit PAUSE, let the video load, skip ahead to about 4:15 in the video and ride with me while I go through the gears. I was shifting at about 6200 in second and third and it has no problems getting there and had more RPMs available to it but the power curve drops off after 5800 so I only went as high as I did for the sound and the video.

And if anyone is watching the clock, by the time I stopped pushing it in 4th gear and downshift to slow it down, the engine is a better brake than those tiny drums, I had already hit nearly 100mph.

 

Rich,i know you touched on the timing issues but didnt address the 12 deg initial your running which can be some of your issue of being down on power.

If the cam in your motor isnt a stock gm grind and is an aftermarket perf cam or copy cam then the 12 deg initial your running is retarded by approx 6+ deg which is substantial esp with only 327 cubes.

I know you said your already at like 36-38 total with only 12 deg initial so you need to get the mech adv in the dist recurved for 20 deg max all in by 2800-3k rpm.

Then you can run 16-18 initial + 20 deg mech (in by 2,800-3krpms) = 36-38 total right on or much closer to where your sbc's ign curve should be.

But backing off the total to get to get it down to 36-38 deg makes the initial end up retarded like yours is by 6+ deg. This results in lost power/trq(eps with only 327 cubes),wasted fuel,little hotter running motor,soggy throttle responce/feel to motor in low-mid rpm range.

Try adv the initial to 18 and take it for an easy drive and bet it will feel stronger but dont get into it hard during the test run. This is because the total will be over 40 deg untill you get the mech adv in the dist recurved for 20 deg max and you dont want it to detonate when your on it hard and possibly damage it .

I am sure getting the ign curve dialed in better will make the motor feel crisper with more low-mid range umph along with a few more HP/TRQ too.

Scott

 

I've tried initial settings as high as 20 with no appreciable change and things get worse above 14 degrees. With the compression I've got I keep the initial at 12 to account for older fuel (I only fill the tank once or twice a year but I do use Sta-Bil) and for really hot days.

This is a completely bone stock L79 with a GM L79 cam. Nice lope, some overlap but nothing outrageous, not a lot of lift at .447. The only modifications are Pertronix ignition module in the distributor, a Pertronix stock looking coil, recurved with softer springs to bring mechanical in faster and a softer spring in the vacuum secondaries to compensate for the K&N filter's easier flow which lowers the vacuum signal at the top of the carb (apparently). The carb has the stock jets and power valve (can't recall what numbers they are though). Still running exhaust manifolds and 2.5 inch head pipes through stock style mufflers. The cam was installed straight up per GM specs.

 

These are rear wheel figures:

max HP: 215 @ 5600 rpm
max TQ: 252 @ 3100 rpm

According to their documents you can multiply the above HP figure by 1.25 to arrive at an at-the-flywheel number.

Factory gross specs show 350hp @ 5800 rpm and 360tq @3600 rpm.

 

Rich,if you motor truely has a the stock gm 151/327-350hp cam in it then like you said more initial actually hurt your perf because it was too much,sorry that suggestion wouldnt work out in your case.

But your chassis dyno results of 215hp x 1.25 = approx 268 hp which is way down on power.

I would not expect it to be right on 350hp after doing the conversion due to stock ex manifolds,alt,pws,etc but i would not expect it to be approx 80hp less either. I think some good tuning with the ign curve & carb could wake it up some more.

Is is possible the cam isnt the 327/350hp grind and its the 300hp cam ,now that would make sence esp with the RWHP #s you got that would be realistic for a 300hp cam.

Scott

 

I can't speak for Rich, but after seeing his videos, I'd say the idle is a little lumpier than the dead-smooth 300hp cam.

 

That's what I was thinking too. his idle sounds more like a 30-30 Duntov. I've had a 327 with a GM 350 HP cam, it idles smoother than what Rich's sounded like.