The subject is my 1969 Mach 1 with a 1968 302 engine. It was bored .03 during the rebuild, and I topped it off with an Edelborck F4B intake and Performer heads with 1.9/1.6 valves, 1.6 roller rockers, and a fairly generic Holley 600 cfm carb with vacuum secondaries. The cam is a Comp 31-442-8 retrofit hydraulic roller: .224/.224 @ 0.05, .533/.533 valve lift, 110 LSA.

I'm seeing acceleration that I think may need improvement. Accelerating hard from a dead stop, I can feel the engine rev up at its characteristic rate, but a surge at around 4000 rpm where it jumps to 5500 very quickly. I am guessing that's where the secondaries finally decided to open up. I would think that if there was a steady increase of airflow, the secondaries would slowly open up as they're needed, and I shouldn't feel a distinct surge. In any case, I'm thinking the acceleration could be better if the secondaries opened up sooner. Should I look into lighter springs for the secondary vacuum motor? Or, is there something else I should be looking at?

 

That would be a good place to start I think. Those springs are really easy to change and maybe $10 or $12 for the whole set.

 

Well this is a broad area of tuning for any engine, are you tuning with a vacuum gauge? You should get a Holley quick change secondary kit if you want to tailor your vacuum secondaries, However your timing curve can slow your acceleration also ex: a stock dura-spark II will usually have all the timing in by 4000rpm or with lighter spring ramp it in by 2500/3000 rpm. A heavier spring will slow the opening of the secondary not lighter. How many inches of vacuum are you seeing at Idle? a Mach1 being a larger Mustang would likely need a slightly stiffer spring. last the ported manifold vacuum instead of full manifold vacuum can give IMO a more stock ride feel..

 

Thanks Clinton. The idle vacuum is around 12" at 700 rpm (rough) idle, and if I bump the idle up to 900, the idle smooths out, and vacuum actually goes up to 15"

I am using an aftermarket Duraspark distributor with ported vacuum, so I assume it has a stock centrifugal advance curve. I've never "mapped out" its advance to 4000 rpm; too scary to be under the hood with the engine spinning that fast. Perhaps I need to.

Yes, I was thinking of getting the Holley quick change kit. It's probably easier to test that first than to re-curve a distributor.

 

OK, 12" seems low to me . When you time and tune do you use the vacuum gauge on manifold vacuum and plug the distributor? Then tune to the highest achievable vacuum and advance only enough so it will still hot start (meaning you may have to back off some timing) so there's probably quite a few suggestions on how to do this and a few good vids on youtube as well, anything from intake leaks or bad plug wires can interfere with an ideal tune if you can get 15" at 900 you should be able to see that at 700 i feel..

 

Thanks Clinton, I've gone through all those procedures to optimize timing; maximizing by the vacuum gauge, and backing off enough to hot start.

It's a fairly new assembly (about a year old), so there should not be any vacuum leaks. I did spray carb cleaner around all the interfaces around the carb and intake, but did not get any reaction. But I also tried forcing a leak and spraying that, and got the expected results.

And the plugs, plug wires, distributor, DS2 module, are all new.

I'm just wondering if the low vacuum is normal for a cam of these dimensions. Even Comp's CamQuest (when it worked) claims a "noticeable" idle.

 

So with 12" of vacuum you could try a 5.5 power valve 6.5 is ideal for about 13" vacuum, could be squrter size, pump cam and even the secondary spring could use tailoring to your application. also adjusting timing curve to get hard acceleration your looking for (earlier advance possibly have an adjustable vacuum secondary on there 3/32 allen key and screwing in pull in sooner) . Depending on what you want performance wise adding an wideband o2 is a nice tuning tool as well as getting a long vacuum hose and bring the gauge in the cab for info on your test drives..

 

In fact, I have a long hose going to the vacuum gauge hanging from my dash; that's how I was monitoring the vacuum before.

I can tell from smelling the exhaust that it's too rich at idle, even at the optimal timing. I blame that on the cam overlap. A wideband O2 sensor may be a useful monitoring tool, but I will also have to be able to change the vacuum secondary. An adjustable device sounds intriguing; it would be easier than changing springs.

I can try the different power valve as well.

Then last thing would be the distributor.

 

Carburetor Tuning the Scientific Way ....this has some useful info. Power valves "The lower the number, the later the valve opens, the higher the number, the earlier the valve opens"

 

Excellent site; I will study it thoroughly. Thanks.

 

Thats a pretty hefty cam for a street 302. Probably not much power below 2500rpm or maybe higher.

 

I got it on the advice of a local speed shop "...you'll love how this will rev...". I later found out from the Comp Cams website that its range is 2500-6000 rpm, a little too high for street use. Not going to change it again unless I have to tear down the engine for some reason.

I figure that must account for the low idle vacuum.

 

Received the spring kit yesterday, installed today, and did some test runs. Most of the instructions I read online say to use the lightest spring that won't cause a stumble on tip-in. To provide extra load on the engine, do this in 4th gear.

I started with the lightest spring, fully expecting a stumble when flooring it at low speeds, but did not feel it. I tried flooring it at different speeds, and never felt the stumble.

So then I tried the original condition where I noticed the surge, and there was quite a difference. Again, starting at different speed in first gear, the engine feels like it was generating a lot more torque from about 3000 rpms, and at around 4000, it would consistently break the rear end loose. Even when I tried it on an uphill on-ramp, it would break the rear end loose around that speed. This is through a helical type limited slip rear end, so both tires were loose, and the car yawed to the left. If it was still surging, I couldn't feel it on account of the tires letting go. So I don't have enough tires right now to continue this part of the experiment.

I also noted that actually watched the gas gauge drop by about 1/8 of a tank during these very short runs.

 

I laughed at the 1/8 tank drop.. sounds like you'll get it..