Street Machine - - Stage Write - BOB KOTMEL

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.


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