Ultimate Diesel Builder's Guide - - Contents -

What you need to know for 500 hp at the touch of a but­ton

There's per­haps no eas­ier way to make a lit­tle ex­tra horse­power than with nitrous—but mak­ing big power with the bot­tle takes some do­ing. On the plus side, a well-planned nitrous sys­tem on an en­gine that's de­signed for it can nearly dou­ble the en­gine's out­put; we've seen an ex­tra 200, 300, or even 500 horse­power added with the use of nitrous ox­ide. While it may seem scary, nitrous ox­ide is one of the best ways to ex­tract ev­ery last bit of power out of a diesel en­gine.


When talk­ing to reg­u­lar gas-style hot-rod­ders, one of the first things they say is, "I didn't know you could put nitrous on a diesel." Many peo­ple also think nitrous is like propane (it's not), and that a cer­tain sized jet gives you a cer­tain amount of power (it doesn't). There are a lot of ba­sic dif­fer­ences when run­ning nitrous on a gas en­gine ver­sus a diesel, which we'll talk about here.

The rea­son that nitrous makes power is that it’s 33 per­cent oxy­gen (plus other gases), which is richer than the 21 per­cent oxy­gen con­tent of stan­dard, breath­able air. If we think of an en­gine as a gi­ant air pump (with fuel added), then it's clear why nitrous makes power: Each revo­lu­tion of the en­gine brings more oxy­gen into the equa­tion with nitrous, so, as long as the cor­rect quan­tity of fuel is added, the en­gine will make more power.

In a nor­mal gaso­line en­gine, ex­tra fuel must be in­jected along with the nitrous in or­der to main­tain an air-fuel ra­tio be­tween 11:1 and 12:1. This is crit­i­cal be­cause both rich and lean con­di­tions just a few points out­side of this range, say 15:1, can cause big-time en­gine dam­age like melted pis­tons, burned valves or even scarred cylin­der walls.

On the other hand, diesels can oper­ate at an in­cred­i­bly wide range of air-fuel ra­tios, although most com­monly they're func­tion­ing some- where within a range of 10:1 and 20:1. In a diesel, in­ject­ing nitrous just burns the ex­tra fuel that the en­gine al­ready has, lead­ing to a power in­crease. So un­like a gaso­line en­gine, if the en­gine goes too lean, say past 20:1, there will be no ill ef­fects.


Part of the rea­son that nitrous is such a pop­u­lar way to add power is be­cause it's so sim­ple. A ba­sic nitrous kit con­sists of a bot­tle, mount­ing brack­ets, a feed line, a so­le­noid, a so­le­noid-to-noz­zle line, a noz­zle and an arm­ing switch. That's re­ally all that's needed to add about 50-150 hp. The rea­son we give a horse­power range is be­cause how much power the nitrous adds de­pends on how much ex­tra fuel is on hand. On trucks that are nearly clean, a 0.080 jet might only add 50 hp. On trucks with a very rich air-fuel ra­tio (say 11:1), that same sized jet could add nearly 200 hp.


So now that you know the ba­sics, we can get into what it takes to make some real horse­power with the juice. Adding 300-500 horse­power gets a lit­tle more com­plex be­cause of the im­mense amount of nitrous that needs to go into the en­gine and what oc­curs within the en­gine it­self. For starters, kits with mul­ti­ple or over­sized bot­tles, so­le­noids, and noz­zles are al­most a must when mak­ing large amounts of power, and all the com­po­nents must be up-sized. A -6 feed line should be used in­stead of a -4, a 0.125 ori­fice so­le­noid might re­place a 0.080, and 15-pound bot­tles would take the place of 10-pound bot­tles.


One im­por­tant thing to re­mem­ber about nitrous jet­ting is that it's a sit­u­a­tion where 2 plus 2 does not equal 4. That is, two 0.080 jets don't equal a 0.160 jet. We're talk­ing about the ac­tual area of the ori­fice, which can be ex­pressed by the equa­tion pi x ra­dius2. This jet­ting dis­crep­ancy is one of the big­gest mis­takes that novice en­thu­si­asts make, as three so­le­noids with 0.080 jets—which is the largest jet most com­mon -3 lines can flow—is ac­tu­ally roughly equal to a sin­gle 0.136 jet.


Hit­ting a huge shot of spray right off idle is a good way to de­stroy parts; in­stead, nitrous must be ramped in as a diesel en­gine starts to pro­duce boost and power. While a small shot of nitrous can be used to spool a turbo, large hits should be best saved for later in the rpm band and when boost is up above 20 psi. The so­le­noids can be trig­gered in a va­ri­ety of ways, ei­ther man­u­ally, with pres­sure switches or with con­trollers like those made by Nitrous Ex­press or Nitrous Ox­ide Sys­tems. Nitrous can even be con­trolled by boost lev­els or set to ac­ti­vate in a cer­tain gear if more ad­vanced con­trollers are used.


While nitrous can be one of the most sat­is­fy­ing ways of pro­duc­ing power, it can also be one of the most danger­ous. From ex­cess or im­proper nitrous use we've seen melted pis­tons and tur­bocharger tur­bines, ex­ploded com­pres­sor wheels and in­take pip­ing—even bent con­nect­ing rods.

To pre­vent dam­age when run­ning a big shot the tur­bocharger must be heav­ily waste­gated in or­der to pre­vent over­speed­ing—run­ning an ex­ter­nal wa­te­gate is a good idea, but run­ning both in­ter­nal and ex­ter­nal gates is an even bet­ter one. Fur­ther­more, keep­ing EGT well in check with water-toair in­ter­cool­ing or water in­jec­tion is also rec­om­mended. Last, we sug­gest never trig­ger­ing a large shot of nitrous be­low 20-30 psi of boost, as a large back­fire can oc­cur.

1 How much nitrous can you put on a diesel? We counted seven so­le­noids on Cody Bea­man's 12-valve Cum­mins, which helped the Dodge put down nearly 1,200 hp at the tires.


Bot­tle pres­sure is cru­cial when look­ing to ex­tract ev­ery last bit of horse­power from a nitrous sys­tem: On a big kit, a drop in bot­tle pres­sure of just 100 psi can equal a 50hp loss. Most rac­ers try to keep pres­sure at about 950-1100 psi with bot­tle heaters when it's cold out.

2 Cole Dow's Dodge is a good ex­am­ple of a well-set-up, big-horse­power nitrous sys­tem. There's one large nitrous so­le­noid, two smaller ones (with a sep­a­rate feed line), as well as a purge so­le­noid. The in­take tract uses ei­ther v-bands or straps, and a large ex­ter­nal waste­gate keeps boost un­der con­trol. A Nitrous Ex­press pro­gres­sive con­troller op­er­ates the nitrous sys­tem.

5 The tre­men­dous ex­haust heat from a nitrous mo­tor can wreak havoc on the tur­bines of tur­bocharg­ers, so more and more peo­ple have started us­ing water in­jec­tion when run­ning a large dose of nitrous.

4 Nitrous is most com­monly in­jected af­ter the tur­bocharger into the in­take tract of a diesel. A pop­u­lar place to in­stall noz­zles is right by the in­take horn, just be­fore air en­ters the cylin­der head.

11 Con­trols are a big part of any nitrous sys­tem. Of­ten hit­ting a hu­mon­gous shot all at once is hard on the parts and will re­sult in trac­tion is­sues. Ei­ther dig­i­tal con­trollers or sim­ple boost-ref­er­enced Hobbs switches (pic­tured here) can be used to ramp in mul­ti­ple stages as boost and rpm rise.

V-bands, or boots with straps on the in­take pip­ing, are an im­por­tant part of a nitrous en­gine. A pipe that comes apart un­der boost can lead to just lots of fuel and nitrous with­out air in the cylin­ders, which will re­sult in a nasty back­fire. 10

7 The small so­le­noid off to the right in this pic­ture is known as a purge so­le­noid; it’s used to bleed off ex­cess bot­tle pres­sure or to get any air out of the sys­tem. Purge so­le­noids can also be used to in­ject nitrous into the en­gine, but those don't re­ally flow enough to make a big dif­fer­ence.

A big shot of nitrous cre­ates so much drive pres­sure that over­speed­ing a tur­bocharger can be a real dan­ger. A waste­gate that vents to the at­mos­phere or out the ex­haust should be in­cor­po­rated into any big horse­power nitrous build. 8

9 An­other pop­u­lar spot for in­ject­ing nitrous is into the in­ter­cooler, which not only adds horse­power, but also can re­ally drop the charge air tem­per­a­ture dra­mat­i­cally.

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