Like Richard said, we went back to the dyno shop yesterday.....
If you recall, after our dyno session last Saturday, we came to the conclusion that the current 1V carb was way to small for this motor (200ci with a warmed up cylinder head, mild cam, headers, and electronic ignition). At WOT (wide open throttle) the motor was pulling 3" of manifold vacuum with the stock rocker assembly, and 5" with a set of high ratio rockers, which isn't good. Basically the motor is trying to suck air (and fuel) into the cylinders, but the carb is restricting airflow, thereby creating manifold vacuum. Ideally, manifold vacuum should be zero at WOT, or very close to it. As such our goal yesterday was to see if we could reduce manifold vacuum to zero, by installing a bigger carb, on a 2-1 adaptor. However we didn't know if this would be possible, as the 1.75" carb bore may become the choke point. We also wanted to make sure we didn't over-size the carb, as this reduces throttle response, low-end torque, gas mileage, and overall driveability. An over-sized carb can also cause hesitations and/or flat spots during acceleration. A good rule of thumb is to slightly under-size the carb for street use, slightly over-size the carb for drag racing.
We started off the day using a Carter YF carb (1V) that Richard installed earlier in the week. The YF carb is 220cfm, as compared to the Vaporizer at 200cfm, so it should make a bit more horsepower. Surprisingly, the results were pretty impressive and more than I expected. After a couple pulls we yanked the YF carb and installed the 2-1 adaptor. The first two carbs we tried were a 32/36 Weber, and a 38 Weber. However we didn't do a full pull with either one, as both carbs were running very lean. Unfortunately I didn't have time to pick up a variety of Weber jets during the week, so we pulled the carbs and set them aside.
Next up was a 350 Holley. However while they were installing the carb, it gave me a little time to think. It just didn't seem right that both Weber’s were running lean, as they both came off running engines. Therefore I thought it would be prudent to check the seal between the manifold and the adaptor before we did the next pull. As it turned out, I was right, it only took one squirt of juice to find a massive leak.
Once we got the leak fixed, we reinstalled the Holley and did two full pulls. Finally, some results…. The horsepower was up, and manifold vacuum was down, to just around one inch. It read zero until we hit 3000 rpm, then it quickly went up to one inch and stayed there. Still not to zero through the entire rpm range, but it was close. Since the carb is considerably larger than our calculated CFM (255-280), I’m assuming the carb is over-sized, and that the restriction is now the carb bore, rather than the carb. We'll try to verify this when we go back, by installing an even larger carb. If we get the same results, then we'll know.
The last carb for the day was a 1.02 Autolite (245cfm). Peak power was a little less than the Holley, however it made a little more low-end torque, and it seemed to idle a little smoother. However manifold vacuum went up to two inches, which would indicate that the carb was a bit on the small side. When we go back in two weeks we’ll try the 1.08 (287cfm) and see how it does. I'm pretty sure it will work fine with the stock rockers, however I think we'll need a bit more carb when we reinstall the high ratio rockers. Fortunately I have a 1.14 Autolite (300cfm), a 1.21 Autolite (351cfm), and the 350 Holley that we can try. I'm not sure if I have a 500 Holley, but if I can find one we'll take it along too. And we still have the Weber's to test.
Even though we only tested two carbs, I was very pleased with the results. In a previous dyno session we only gained 4-5 hp using a 2-1 adaptor, but the gains were much better this time. Unfortunately we didn't have time to reinstall the Weber's, so we'll test them when we go back in two weeks. I just need to make sure we have a good assortment of jets on hand. Once we get the carbs figured out, we'll install the HR roller tipped rockers and see what happens.
Here's the results so far.
The calculated CFM requirements are 255-280cfm.
Air Temp – Carb – CFM – HP/TQ – A/F – Vac
80* - Vaporizer – 200cfm – 101/164 – 12.5 – 3”
100* - Vaporizer – 200cfm – 95/150 – 12.5 – 3”
100* - Carter YF – 220cfm – 114/160 – 14.1 – 3”
105* - Autolite – 245cfm – 123/169 – 14.0 – 2”
105* - Holley – 350cfm – 127/167 – 12.3 – 1”
We still need to test these carbs (in 2 weeks)
32/36 Weber - 325cfm
38/38 Weber - 390cfm
1.08 Autolite - 287 cfm
1.14 Autolite - 300cfm
1.23 Autolite - 356cfm
500 Holley - 500cfm (if I have one)