NITROUS OXIDE ( nos / n2o ) advice forum

Ok, basic thoughts based on only Normally aspirated experience.

If you take a well designed NA engine and add double the cylinder filling, (Double MAP FI) OR (double oxidant content NITROUS) You will have much more exhaust gas to get rid of.

Ways to get rid of the extra exhaust.
Longer exhaust cam,
Larger exhaust valves,
More exhaust valve lift,
Better flowing exhaust port,
Better flowing exhaust pipe.
Exhaust cam timing?

All bar the longer cam and timing will most probably need the better flowing port and pipe to go with them.

The biggest difference I see here between Nitrous and FI in this is the INLET MANIFOLD PRESSURE during cam overlap.

FI this will be positive so a longer cam and timing may work nicely.( with sequential injection it can be used to purge the cylinder of any remaining exhaust gas with clear air ready for next stroke)

Nitrous the inlet will be atmospheric at best so ANY positive exhaust manifold pressure will force exhaust gas back into the cylinder and possibly through it and into the inlet tract on overlap.

My guess is that for Nitrous the priorities would be Large, High flowing exhaust valve, Port and pipe combined with a High lift, Short(relatively) cam timed with as little overlap as is practical.

This is my "work in progress" based on NA Experience only.

OK now lets assume that we've done all we can to get the exhaust out except play with the timing, what would you propose for that?

Well my thinking is unless you've done something radical like your reversed port V8 idea the exhaust will still be restrictive to some extent.

I would be looking to keep overlap as short as possible above all else.

Inlet timing would be in the Top end area as bigger nitrous gains should be had at lower rpm before the exhaust becomes so restrictive.

So advance the exhaust cam and retard the inlet?

Exhaust closing earlier,inlet opening later, = less overlap ??

That's my thinking.

I wouldn't change my inlet timing just to reduce overlap.

I would time the inlet more top endy than for a NA engine though.

Im my picture inlet timing will work the same NA or Nitrous as long as you can keep positive exhaust pressure out of the picture.

I like the idea of a huge exhaust and port with a big valve on a short high lift cam.

Again based only on NA experience.

Can you puts some 'relative' figures to that so everyone can understand your think please.

Here's a couple of other thoughts for you;

1) There may be more than one spec. that suits different motors (so you could well be right for some if not all)
2) There is often more than one way to skin a cat (so again you could be right, even on all motors)
3) Since cam timing is currently what I would consider the final frontier for most people (excluding all the EXTREME stuff we've been discussing that could be limiting nitrous performance), I'm sure very little work has been done in this area (except possibly some intensive stuff on one or two specific engines), so I doubt there are very few people who could claim to know any better than anyone else, especially on a range of applications and only a few that could do so on a specific engines.

With all that in mind, I'd let rip with whatever further thoughts you have, as there's nobody here who could prove you wrong even if you were and at least it gives others food for thought.

The issue is really is its a small difference that will be goobled up by bottle pressure variations or boost accuracy on back to back passes(2bhp)so unless it makes a difference of say a clear 10bhp its not going to add much to the party.When at the track these guys can move the timing by significant ammounts whilst tuning the bike and the dyno doesn't always give the same load as the track so some fine tuning will take place.
When the bike goes to the line theres tolerence on how much power its making a nitrous or turbo bike is quite possibly +/-10 or 20bhp.
On the streetbike stuff its favoured to make too much and have the tyre or clutch just slipping or both.

It's a very thought provoking thread (to the point that Amazon have just delivered two books on the subject to my doorstep).

I need someone to help me with an example - I think it was gsf1371 (hope I got that right) who said he tried 114/114 timing - can someone explain that to me? - is that a long overlap or vice versa.

What do we think the plusses and minuses of that timing would be for a nitrous motor.

Why was he told to narrow the timing? - accepted wisdom, or fundamentally wrong timing?

Hope someone has the patience to explain, I would appreciate the input and don't mind displaying my ignorance.

I think that at 2bhp it will be gobbled up by the variation in bottle pressure or accuracy of the Pulsed electronics or boost controller.Bottle pressure variance alone will probably easily exceed 2 bhp.I think that a Superstreet bike will go to the line with a potential variation of +/-15-20bhp.These bikes slip the clutch and tyre a fair ammount of the run having a balance of the two for min ET.
A dead hooked tyre and locked clutch is not necessarily the way to go quick.

Regarding a measure change measure of cam timing in order to get the 'best numbers' ,other than other variances some mentioned above,dyno testing a nitrous bike is far from pleasurable especially at 350-400bhp bracket.To be quite honest its fookin scary!!!
A 500 plus bhp turbo bike is far easier,I think it must have some to do with the noise but a nitrous bike is more like testing/riding a bull dripping snot and a turbo bike more like a donkey ride a long the sand they're that different.

stephen, it was 111/114 fyi

a reply i got on oss to the same Q
GOODAY ENZO ,i'm also on gixxer.com and ASF so i'll check out the cam specs for you. on nitrous they usually open the ex cam earlier as it has heaps more cylinder pressure and it uses pressure still in the cylinder to help
blow the ex out iff you have the ex open too long after TDC more inlet will go out the ex than normal -overlap
so they normally run the ex 112-114 lobe centres with the extra duration itt would be pretty close to stock overlap -i'll get back with the cam specs
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lorenzo

I'd deliberately kept the figures out of it so as to avoid misleading anyone.
Different engines have different sweet spots depending on their character.
The changes either way always seem to make the same difference though.

I'll try with the timing explanation, it's easily misunderstood though sometimes.

Inlet max lift @105 ATDC and Exhaust max lift @ 105 BTDC would be a basic setting
Inlet max lift @115 ATDC and Exhaust max lift @ 105 BTDC would have less overlap
Inlet max lift @105 ATDC and Exhaust max lift @ 115 BTDC would have less overlap
Inlet 115 would be more top endy than Inlet 105

Ok, based an engine with a sweet spot of 105 105 (biggest area under the curve or best compromise).

I would move the inlet to 115, this will increase top end at the expense of lower rpm power.
It will also lower the overlap period.

Move the exhaust to 115 or in that direction given valve to piston clearance.
Again to lower the overlap period.

My thinking is this will give an engine that will benefit most from the nitrous in the lower rpm and, at the higher rpm, where the exhaust is becoming restrictive with nitrous the engine is stronger without.

This is all based on my opinion that a Nitrous engine lacks Inlet manifold pressure relative to the exhaust.

I have NO experience of BIG power nitrous use.

Trev I'd like to hear your thoughs given your experience is opposite to mine, you have lots of Nitrous experience and less cam timing experience.

I'd also like to hear the thoughts of someone who has both.

What would be the effect of doing both?
Take my figures Inlet open 5 BTDC
Inlet close 27 ABDC
Exh open 28 BBDC
Exh close 5 BTDC

These figures -according to the manual- are at 5/100 preload(do not know what that means) and 3mm of lift ,given that the cams look pretty symetrical peak about 101 degrees? (not given in the manual)

Those figures probably help explain why Ian didn't want to make too big a statement.Different manufacturers quote their figures in different ways,as well as all of the various engines having different tunes.

My rev limit is only 8700 so at what sort of rpm's might retarding the inlet have an effect.
I have already advanced the exhaust cam,after a backfire blew my plenum apart Probably got very little overlap now,if any at all.

Well explained Ian,I've only ever been able to get my head round this recently and that was by looking at pictures.

I think your assumption of 101 is reasonable.

Max lift is the most universal timing figure,

as you say different manufacturers measure duration differently.
The Japanese seem to use the break from valve to seat, Kent cams use 0.5mm from seat (big difference in figures)

Your yellow comments would be correct.

I'd imagine at 8700 rpm 105 inlet would be top endy, I'd guess 98-100 for sweet spot which would fall in line with your 101 assumption.

I still have to look at circles sometimes

IDP 'dyno'd out' thats a headache all weekend then

Heres some rpm guides from the Busa
Launch rpm 8000rpm shift at 12000rpm
2nd drops to approx 8800rpm shift at 12000rpm
3rd drops to approx 9700 rpm shift at 11200rpm
4th drops to approx 9800 rpm shift at 11000rpm
5th drops to approx 9900 rpm shift at 10600rpm
6th drops to approx 10200rpm

On a better run you may rev highers say 12500rpm it depends on how the bike 'feels' if for example its wheeling I will short shift and let the clutch slip a bit.

I'd imagine a motor with peak torque at 9500-9800 would be best? and let the nitrous do the rest.

On the subject of rpm I remember now that I did nitrous a GSXR600 a few years back that made 145bhp and revved to 14000rpm dunno how that fits in with the time aspect and theories? 110bhp aspirated.

Cheers Ian,looks like another job added to the (very long)list,make adjustable cam wheels luckily they're car type,centre bolt and a dowel to locate,so just some new dowel holes
Best check valve to piston clearance before I do too much to the exhaust

Here's an example from some old dyno runs.

I've done a 2006 600 Honda Supersport engine that was making 125 BHP NA

We ran it up to just over 150 BHP at 16000 RPM with Nitrous using the Std injectors for fuel with no issues whatsoever.

Relatively small amounts of Nitrous though where I would leave the engine spec as normal.

I think Trevs thinking more radical power increases.

Sorry for not chipping in, had a long day in the workshop, but at least the bike ran on the first hit of the button.

Two questions, and I'll tell you why once I see your response.

My understanding is that some quick nitrous bike guys are running 112/114 - why does that timing work?

And if you were running that and then you went 114/112 what difference would you expect and why?

Cheers
Stephen

112-114 is probably near where i'd start on a bike engine.

I think it would work for all the reasons i've pointed to so far.

I think based on Brad's take on Nitrous engines (which i agree with now he's pointed it out) 114-112 would make little to no difference.

I'd expect a theoretical difference if you went 114-114 because that would give an intake change as described earlier but also lower the overlap period.

Also worth noting, retarding the Inlet cam also reduces the effective compression ratio.