Considering longer itb trumpets 172

[Biker303uk]

Hi anybody looked to match there intake perfectly to harmonic? Am currently running 120mm trumpets giving me a total runner length of 31cm where with my spec of build and with 197 cams on group N alignment my peak point of harmonic alignment sits at 36.84cm so I need a trumpet of 178.4mm really to be perfectly on point but means to be then modding the bonnet. Is it worth it above what i have here with 31cm?

 

[prodriv3r84]

Hi, what diameter are your ITBs?

 

[Biker303uk]

46mm itb's off zx12r with custom made manifold. Have taken measurement from back off the valve through too end of trumpet.

Can see to why the standard system was an up and over intake manifold in order to get length on it.

 

[prodriv3r84]

The reference that I have in my Dyno tests, are from head face to the end of trumpet.

I run 3 trumpet sizes on 45mm Jenvey with both, Cat 421 and Clio 197

25cm: best option for track power up to 7600 very stable AFR. 229Nm 213hp at 7200
28cm: best option rally, more 5nm at mid range up to 7000, 234Nm 207hp at 6900 but no engine after 7100/7200
31cm: strange power curves at all range. AFR instable (worst setup)

Tested also different exhaust and intake diagram.

 

[Biker303uk]

Hmm fits too where most people go for short and stubby to try for peak hp and sacrifise that mid to just have it be screamer of one harmonic at top..

Yeah this was where 28 comes in of that one weak harmonic before lining up in the mid instead but not at the top which makes sense to what you observed as was my fall back consideration was that length.

Haven't tried in testing or running up on the 31's yet.. when you say odd curve can you give more detail? And to alittle more info on the afr behaviour 🤔 what cam timing you running your 197's at??

This was where in the maths to have a harmonic alignment happen of perfect of one to happen mid range and one to happen at top was right out at 36.8cm 🤔.

May give it a run up just to observe for myself on the 31s but will prepare to compromise to drop it back to 28's if needs and just have the one harmonic in the mid.

Odd it dropping right off in the high thou on 28s what timing advance on the cam you running. i mean even thou aint a harmonic there at that point id have thought the cam range should still carry the power through above 6900rpm 🤔. Sounds like almost that of flow restriction starting to occur?? Or back pressure of the exhaust coming into play?

 

[Biker303uk]

I'm running of 8 degree advance on my intake of my 197 set so more mid orientated anyways. But yeah I'll go for hitting just the first harmonic at 28 in the mid to support that if I'm not going to get to the full 36.8 to integrate that second harmonic proply. Be intrested to more info on your observations on running 31s just for my own info to save on trialling it to observe it thou.

 

[prodriv3r84]

About the timing on 197s l, its a matter of piston valve distance on intake, i didnt remember the axact advance on intake, I measured the and 8° After TDC Valve to Piston distance and I use that as Limit.
On exhaust You can r****d a little.

 

[Biker303uk]

Right so you have gone with maximum intake advance to draw the power down the map then added a little r****d on the exhaust to draw it back up abit to have maximum valve overlap but still be on peak. That would make sense on being equivalent of a 9° advance over standard position then on the 197 cams then running a touch of r****d to pull the peak torque back up being on same point really as mine at 8° but with running alittle more overlap.

Intresting way to go and shows to where is little more favorable to the hp in the shorter intake length setups with that extra overlap high up so gains to be had in someway.

Would thou exsplain to the odd afr effects and instable power curve seen with the longer runners of back pulsing and blow through starting to occur.

possibly also the drop at peak end in the 28s setup of back pressure on the overlap causing reduction in draw through of the overlap. What exhaust diameter and exhaust manifold was you running on the 28s at time of testing?

Being can't realistically get right out to the 36.8 to have the twin harmonics, I might go for the 28s and have the one harmonic in the mid then being mine is alright 1 degree less then yours on intake tweek the exhaust cam alignment r****d only a smidge to keep that peak power going up at 7200 still as can then still have my hard limiter set at 7500 with a 300 soft, which I found gave nice trasition of rpm when changing up to come back right on the power range still of next gear above so would like to maintain. Giving best of both worlds.

For you then that would mean simply knocking 1 degree off your intake advance and then tweeking you exhaust in alittle of advance to match but I'll let you know how I get on first to if its worth it for ya. 👌 see what I get..