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« on: September 03, 2017, 01:33:51 PM »
Update on collector headers vs. zoomies.
Took my roadster to the local 1/8 mile track yesterday and tested headers & zoomies. In an earlier post on this thread I took a guess that in certain low horse power circumstances, both exhaust systems would create similar if not close to the same results. This will be the first of two or three tests with future engine combinations, God willing:) The engine used in this first test is a Chevy 350 street machine engine that produces about 260hp in the car, idles at 700rpm, and pulls 14” of vacuum at idle. The cam has 1 degree of overlap at 0.050” lift while the compression ratio is 8.3-1 with the stock 86,000 mile dished pistons, cylinder walls, and stock heads.
The car is foot braked and uses a 2100rpm stall convertor along with 4:10 gears and 31 tall slicks. Since the power band of this engine is relative low compared to an all-out race engine, it’s shifted at 5300rpm and goes through the traps at 4450rpm. While it would probably go quicker if I shifted it at a higher rpm, the Stahl Headers Newsletter comments earlier in the thread that zoomies have little effect at lower rpms compared to collector headers was the reason for this test. My header tubes have a 1 5/8” dia, are roughly 39” long, and have a 3” collector. Just about right for this engine’s rpm operating range. The zoomies are 10” long and have a 1 ¾” dia. The zoomies weigh 10# per pair while the headers weigh 24#.
The first 2 runs were with headers and the air temperature was 80 degrees. The last 3 runs were with zoomies and the temperature was 85-90-85 degrees. Everything else about the car stayed the same.
1st run with headers: 7.008et at 100.45mph
2nd run with headers: 6.975et at 100.22mph
3rd run with zoomies: 6.924et at 101.58mph
4th run with zoomies: 7.014et at 100.22mph
5th run with zoomies: 6.985et at 101.12mph
Average with the headers is a 6.991 at 100.33mph, and with zoomies is a 6.980 at 100.97mph. That’s an average difference of only 0.011 elapsed time between the two and a difference of 0.64 miles per hour, with the zoomies having a tiny advantage. If you apply these numbers to a performance calculator the headers make 254hp at 1620# race weight while the zoomies make 257hp at 1606# race weight My take-away from this test is that it makes little, if any, difference between the two types of exhausts if you operate at less than 5300rpm and your cam has little or no overlap. The lack of overlap will negate some of the wave scavenging effect of the headers. And the zoomies are staying on this engine as long as it’s in the car. When the race engine is done, we’ll do this all again with larger & shorter collector headers along with the same zoomies. Testing ideas start somewhere and this first one is the ground floor. The car may be slow right now but I’m supporting my local race track and enjoying life!
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