HATS off to all those who competed in Hot Rod Drag Week, and to all the spectators who flew over to the US to support the Aussie contingent. I followed Drag Week on the internet, but I was disappointed at some of the comments I read on social media. It takes a massive effort to get the cars over there, and a huge commitment to race. It’s not like our Aussie racers have massive corporate sponsorship; these guys are self-funded, diehard drag racers. I’ve got a lot of respect for the Down Under Drag Week competitors, who did Australia proud.
In fact, one of the entrants, Jamie Farmer, is the inspiration for this column. Customs stopped him getting his turbo Falcon to the track, but he didn’t give up. Undeterred, he bought a Fox-body Mustang, gave it a shot of nitrous, and had a ball.
And I love nitrous! The first time I helped fit a Marvin Miller nitrous kit to a motor was back in August 1978. It was a mild 351-powered Ford panel van, and I couldn’t believe the transformation. The black 2V Cleveland was almost uncontrollable. Then in 1989 I crewed on some of the world’s quickest nitrous cars, and spent a couple of weeks with Mike Thermos, founder of Nitrous Oxide Systems. Nitrous is cheap horsepower. Kits sell from $400 second-hand to $700 new on ebay for a single-plate 100-250hp kit and 10lb bottle, and they’re easy to install. There’s a lot of information on the internet about how nitrous works. There are good downloadable instructions on how to fit a kit, retarding ignition for each incremental horsepower step, jetting recommendations and plug reading. One of the best things about racing with nitrous is the consistency. Back in 1989, the 200mph nitrous doorslammers ran within thousandths every pass. One of the key ingredients to these clockwork-like passes was getting the bottle pressure exactly the same every pass. My role as a crew member for Bill Kuhlmann was to either cool or heat bottles in the staging lanes so the car left with, say, 900lb pressure every pass. Bill’s Beretta was fitted with two bottles, one for each nitrous system. There was a plate under each carb and a fogger in the tunnel-ram on the SAR 632ci mountain motor. Every pass, we’d fit two fresh bottles. Nitrous has come a long way since 1989. Sonny Leonard’s 959-cube nitrous motor makes 1800hp naturally aspirated and over 3000hp with a fourstage kit. That’s an insane 1200hp from four stages of nitrous, and there’s an optional fifth and sixth stage too. Here are some of my thoughts on the exhaust side of a dedicated nitrous motor. I’d normally talk in terms of cfm for airflow, but I think for a wider understanding I’m going to refer to airflow in terms of horsepower. I’m also going to reduce the scale of the engine down to, say, a 355ci small-block in a 3600lb street car.
To build a good naturally aspirated race motor, an intake valve as big as possible is fitted; the general rule of thumb is 0.52 multiplied by the bore diameter. So for a 4.030in-bore small-block, this would be a 2.09in intake valve. With a 1.60in exhaust valve, the intake-to-exhaust ratio will be around 68 per cent. For a naturally aspirated race motor, getting the air in is a key ingredient. Theoretical horsepower by intake airflow can be achieved and sometimes exceeded even with only 68 per cent intake-to-exhaust ratio with the right cam, compression and pipes.
So here’s some nitrous philosophy for a 355-cube small-block. A 2.08in valve will flow around 610hp in a wedge head. On a well-built naturally aspirated 355ci smallblock that makes 610hp, adding a 100hp shot will often show 100hp at the track, weight-shifted. In a 3600lb car, theoretically that’s going from a 10.20@130mph to a 9.75@137mph. But add a 200hp shot of nitrous, and the extra 100 neddies don’t materialise. In one case the car hardly went any quicker with more nitrous over the 100shot. The engine had a really good ignition, the plug gaps were reduced, high-octane fuel was used; everything was tried.
Here’s one of the reasons why I think the engine stonewalled. If you think about the 610hp intake with a 68 per cent ratio, the exhaust is flowing a max of 415hp. With the 200hp nitrous shot you’ve got 810hp of burnt gas trying to get out a 415hp exhaust system. That’s like having a 51 per cent intake-toexhaust ratio.
By rights, if you want to build a really good dedicated nitrous race motor, you need to get the gas out, and here are some options. Reduce the intake valve and increase the exhaust valve diameter. Port the exhaust to increase exhaust flow. Increase exhaust lobe duration. Increase exhaust rocker arm ratio, and increase exhaust pipe size. It might sound crazy, but for a 355-cube nitrous bracket motor I’d use a 2.02in intake and a 1.69in exhaust with a 200-shot of spray. The intake would flow around 580hp and the exhaust would flow 465hp. With the 200-shot it would give around 60 per cent intake-to-exhaust ratio, and go pretty close to making 780hp on the track.
I LOVE NITROUS! THE FIRST TIME I HELPED FIT A NITROUS KIT TO A MOTOR WAS BACK IN AUGUST 1978. IT WAS A MILD 351-POWERED FORD PANEL VAN, AND I COULDN’T BELIEVE THE TRANSFORMATION