Racing Fuel Systems

Nothing on for the weekend so I thought I’d bolt the Mickey Thompson’s on and throw the dragy on the dash and do some part throttle to WOT testing. As I’m into street/strip tuning of carbs I thought this type of test had some relevance for the street aspect.

What I did was set some custom parameters on dragy - 60-100kp/h (37.3-62.1mp/h), 70-110kp/h (43.5-68.3mp/h), 100-120kp/h (62.1-74.5mp/h). I went on the same route in the same direction each time performing the same test at the same area in the same street....you get the idea. Four carbies were tested: 650 Race Demon, 750 Mighty Demon with Quickfuel 4 hole metering blocks, 850 annular Mighty Demon fitted with BLP 16 hole boosters and 975 Race Demon (1.500” gold sleeve venturi). For each of the three speed test I held speed steady 1kp/h below test speed ie 69 kp/h for the 70-100kp/h test, then mashed as quick as I could to WOT.

There was no fine tuning to find best possible results, just threw each carb on as they are tuned to date to get baseline. A couple need pvcr enlargement. The 750 I tried two different emulsion/mab combinations.

650 Race Demon:
60-100 kp/h - 2.10 sec
70-110kp/h - 2.03 sec
100-120kp/h - 1.23sec

750 Mighty (.028” mab, emulsion .028” x .028” x):
60-100kp/h - 2.34 sec*
70-110kp/h - 2.01 sec
100-120kp/h - 1.23 sec

750 Mighty# (.033” mab, emulsion.031” .028” x .025”):
60-100kp/h - 2.15 sec*
70-110kp/h - 2.00 sec
100-120kp/h - 1.26 sec

850 annular Mighty#:
60-100kp/h - 2.14 sec
70-110kp/h - 2.02 sec
100-120kp/h - 1.23sec

975 Race Demon:
60-100kp/h - 2.00 sec*
70-110kp/h - 2.00 sec
100-120kp/h - 1.20 sec

* had some wheel spin so not 100% accurate.
# lean from .91 to .94 lambda.

I was surprised how close the results were. Two of them I’ll open up the pvcr to get .87 lambda @ WOT or below and test again. The 750 has .055” and 850 has .059” pvcr’s. The remainder were already at that lambda area. And no, none of the carbs had any hesitation.

To come is the 825 Race Demon with several test including mini annulars and std downlegs. I’ll also mix up mab/emulsion on that one too.

Quarter mile test will have to wait until done on the track. Pointless with the dragy on public roads as I would not get traction and I do not want to be doing around 190kp/h on public roads.

For reference car is 3350lbs, 383 sbc, 26” tire, 3.89:1 gears.

Thanks for testing all these different carbs. Hope with some good weather you can do further tests and let us know what you did to the carbs and post the changes and results. Good work.

I’ve done some more testing but unfortunately a bit inconclusive as temperatures have dropped coming into winter here compromising traction. Previous test listed were on the same day with ambient temps ranging 21c to 24c. Today and last weekend cooler. Today as low as 13c. I think this is the main cause of traction issues with the same M/T tyres previously used. Despite traction issues what I can report is this;

825 Race Demon:
(downleg .160” boosters)
60-100kp/h - 2.01 sec (had wheelspin so promising combo. Ambient 19c)
70-110kp/h - 2.02 sec (as above)
100-120kp/h 1.23 sec

850 annular after opening up pvcr from previous testing:
70-110kp/h - 2.05 sec (wheelspin again as only 13c ambient. Only test saved on that drive. Other two are videos below that don’t capture custom parameter data).

As it’s panning out various cfm carbs from 650cfm (1.28” v X 1.688” t) to 975cfm (1.500” v X 1.75” t) downlegs and an 850cfm annular (1.56” v X 1.75” t) are all performing quite similar under street driving conditions when tuned for similar AFR windows. However, they do feel slightly different during the test from smallest to largest cfm making the data a little surprising. Most would choose the 650 for its instant response feel but it appears throttle response has two factors. One, the obvious of time it takes for the engine to respond to the rapid throttle angle change and two - acceleration rate after engine responds. These two factors are balancing out over the cfm spread to be in the same ballpark under the test I am performing.
Not sure how many more of these test I’ll do now temps have dropped making them inaccurate and price of fuel gone through the roof (paying upwards of $2.20 a litre in Aus at the moment) and M/T tyre price and availability like rocking horse poop as well. I may move on to quarter mile testing.

Below is 60-100kp/h test with the 850 annular Mighty Demon with 16 hole BLP annular boosters and modified from stock emulsion and mab. Took a while to steady at 59kp/h.

https://youtu.be/2Vc7jVFqqu8

Below is 99kp/h WOT (100 - 120kp/h test). Time under acceleration gets to 128kp/h as gps display lags.

https://youtu.be/6wrGBJM58Lg

*The two above test did not capture data

Something to consider when talking subjectively about the throttle response difference with different size carbs.
A carb with bigger primaries may feel like it has better response because it admits more air than the same throttle movement of a carb with smaller primaries. This would probably be more noticeable at less than WOT openings.
Slamming the throttle to WOT may feel more responsive with the smaller carb, depending on variables such as gearing, vehicle speed, weight etc.