Carburetor C/O HP @ RPM % Gain Torque @ RPM % Gain Stock YF 7.5% 75/62 3950 Base 110/100 2500 Base Weber 6.0% 80/65 4100 6/5 116/105 2650 5/5 Autolite 6.5% 79/67 3750 5/8 118/107 2550 7/7 Holley 6.5% 80/68 3950 6/9 120/109 2600 9/9 NOTE: the peak numbers are posted first, followed by the average (example 75/62).

Part II
We swapped the stock large log cylinder head for a modified large log head, or what we commonly call a "2V Conversion" head. The conversion heads typically have larger valves (175/150), some basic port work, and the intake manifold modified and hogged out to accept a 2V carb, without using a 2-1 adaptor. The Conversion can be done to support a Weber carb, or a Holley/Autolite carb. In this case, the conversion was done to support Holley/Autolite carbs, which means we were unable to test the 32/36 Weber-DGV for comparison. However, for this round of dyno runs we brought along an Autolite 356 cfm, which is pretty close in size to the Holley 350. Again, the engine tested was a worn out stock 250ci with headers, a DUI ignition system, and the 2V Conversion head. For base numbers we used the results from Part I, using the stock YF carb (light yellow).

Large Log Head with the 2V Conversion, which is set up for a Weber DGV carb.

C/O (air/fuel ratio)
Unfortunately, we didn't have a good selection of jets on hand. Considering 6% C/O is ideal both the Autolite 287 and 356 carbs were tested in a very lean condition, at 4% and 3.5% (respectively). The Autolite 287, which already had 62 jets installed, netted 97 HP at 4%. The Autolite 356 was first tested with 54 jets, which netted 90hp at 2.0%. We then jumped up to 58 jets, which netted 94 HP at 2.5%. For the last run we installed the 62 jets, the largest jets we had on hand, which resulted in 99 HP at 3.5%. Even with the 62's installed, we were still running very lean at 3.5%. We estimate the Autolite 356cfm, with the correct jets installed, would have netted around 115-120 HP at 6%. On the other hand, the Holley 350 was dialed in perfectly at 6%. In the chart below, the numbers in the orange back-ground are the estimated results for the 287 and 356 carbs, with the proper jets installed. Hopefully we can verify these numbers with another dyno session sometime in the future.

I wanted to run out to a local carb shop and pick up a few jets, so we could retest the Autolite 287 and 356 using the proper sized jets. However by that time it was getting late in the day and everyone wanted to pack up and head for home. All in all, I feel like we accomplished a lot and I hope the information gained will be put to good use. Enjoy.....

Carburetor	C/O	HP	@ RPM	% Gain	Torque	@ RPM	% Gain
Stock YF	7.5%	75/62	3950	Base	110/100	2500	Base
Autolite 240	6.0%	93/78	3850	24/25	143/125	2550	30/25
Autolite 287	4.0%	97/80	3800	29/29	144/128	2500	31/28
Autolite 356	3.5%	99/80	3750	32/30	145/129	2550	32/29
Holley 350	6.0%	98/70	3850	30/14	140/112	2500	27/12
Autolite 287	6.0%	105/88	3800	40/42	155/140	2500	41/40
Autolite 356	6.0%	115/96	3800	53/55	160/144	2550	45/44
NOTE: the peak numbers are posted first, followed by the average (example 75/62).

Even tho the peak numbers are better with the bigger carbs, we have no idea how the larger carbs will effect the driveability and/or gas mileage. It's a proven fact that smaller carbs (or venturi's) increase velocity, which in-turn increases throttle response and bottom end torque. On the other hand, a larger carb offers more top end power, if it's available. You just don't want to oversize the carb, since that's the worse thing you can do. When you select a carb for street use, you want to pick one large enough to maximize the top end power, yet small enough to maintain throttle response, driveability, and low end torque. When sized properly, not only will the motor run better, the carburetor will also provide the best gas mileage. Unfortunately a dyno session won't answer these questions. The only way to find out which carburetor offers the best throttle response, driveability, and overall performance, is to take it out for a road test. It's best to try two or three different sizes, if the carbs are available. Ask your friends, neighbors, and family members if they have one you can borrow, or look for cheap carbs at swap meets and your local bone yards. Quite often you can pick up a good used carb for $25-50 bucks. Then when you've completed your testing and selected the carb you want to use, resell the ones you don't need. I have a couple used Autolite's (240cfm, 287cfm, and 356cfm), and a Holley 350, that I would lend out providing you send a cash deposit. If interested, contact me via email.

High Ratio Rockers
Before we packed up for the day, we decided to make one more run using the Holley carb. For the last pull, we removed the stock 1.5 rocker assembly and installed a new set of 1.6 high ratio rockers. The results were as follows.

Carburetor	C/O	HP	@ RPM	% Gain	Torque	@ RPM	% Gain
Holley 350	6.0%	98/70	3850	Base	140/112	2500	Base
Holley 350	6.0%	100/80	3950	2/15	143/129	2450	1.5/15
NOTE: the peak numbers are posted first, followed by the average (example 98/70).

When I first looked at the numbers I was somewhat disappointed. The high ratio rockers only resulted in a gain of 2 HP and 3 LBS of torque, at PEAK, which was far less than I expected. This made me question the value of high ratio rockers, as the gains didn't seem to off-set the high cost, at least in my opinion. However when I got back to the office I began looking at the graphs again, and I noticed something I didn't see previously. At the bottom of the page there was a little chart with two columns. The first column showed "Max Gains", which were 2 HP and 3 LBS. The second column showed the "Average Gains", which were 10 HP and 16 LBS. These numbers looked a lot better......

With the "Max" gains being so small, it's obvious the valves are not restricting factor at higher rpm's, at least not for this motor. My guess, the asthmatic log manifold simply can't supply an ample amount of air flow at higher rpm's, no matter how far you open the valves. However at lower rpm's, opening the valves higher and longer seems to make a huge difference, in both horse power and torque. The chart below shows the HP and TQ gains are much better in the lower rpm range, then taper off as maximum air flow is reach at 4000 rpm's. While the high ratio rockers may not improve top end power, due the the restrictive log manifold, they are great for increasing low end power and torque.

RPM	500	1000	1500	2000	2500	3000	3500	4000
Gain - HP	12	13	14	14	11	8	5	2
Gain - TQ	22	23	25	21	16	11	7	3

Part III
For Part III we test the same engine once again, only this time with a freshly rebuilt short block. (add comments and results......)

Dyno Test #3 - Back to Part 1

Dyno: #1 - #2 - #3 - #4 - #5 - #6 - #7 - #8 - #9 - #10 Track: #1 - #2 - #3 - #4 - #5 - #6 - #7 - #8 - #9 - #10