Hi Guys,

I have fitted an AEM AF/R gauge which also has a vacuum connection to log boost (inbuilt datalogger in the guage)

I have noticed that I am getting a strange boost curve, followed by a pretty big boost spike at 5500 exactly where my boost PSI jumps from 15psi at 5000rpm to 17psi at 5500rpm, and then back down to 15psi by 6000rpm.

Boost by RPM is as follows (based on 3 back to back runs), averaged out.


4000 = 14psi
4500 = 14.5psi
5000 = 15psi
5500 = 17.3psi
6000 = 14.9psi
6500 = 15.4psi
7000 = 16psi

The spike at 5500rpm is 100% repeatable, every single time I do a pull past 5500rpm, the datalog shows the spike of at least 2psi every single time.

I am not getting any belt slip or squeal (1353mm belt) and my tensioner is 1000miles old maximum

Interestingly, I swapped over my 17% pulley for a 13% along with a longer belt for a recent track day (and also to see if this cured the problem) and the exact same thing occured, but at 5100rpm (every time at the exact same point) - where the psi would jump up by 2psi and then drop back down a few hundred rpm later.

I have also replaced my BPV for a brand new OEM one, which made no difference to the data trend.

Modifications are
17% pulley
Newman PH2 cam (I think over in the US you guys call it the Dominator cam)
Scorpion 4-2-1 manifold with OEM catalyst
550cc injectors
Airtec TMIC

Any ideas?

The car drives fantastically, I dont think this is a serious problem - more of a curiosity thing... I have no idea why it could be happening?

 

 

Two things I would try, just as an experiment.

1. I know you've swapped BPV's but I'd still try cable tying the BPV closed and check again, this would not only take the mechanical operation of the BPV.out of the equation, but any worried vacuum or spring issue.

2. Find another way to measure your boost besides that gauge, this will take your gauge out of the equation. A full 2 psi should be noticeable when driving, not just visual on a log, so you really need to verify it's not something with the gauge or logging software itself.

Good Luck

 

The cam and VE of the engine may allow better flow above 5500 hence boost decreases above 5500 while airflow out of the supercharger is still increasing?

 

Almost looks like a resonance @ 5500, I wonder if there is a standing wave issue where you're getting a peak where that transducer hooks up. Can you correlate with OBD2 manifold pressure data? I'm curious if that shows the same peak.

 

Your assessment resonates with me.

 

OBD2 data is relatively slow, so its hard to keep an eye on it as the car accelerates past 5500 and hope that at the same point the OBD refreshes to catch the real data at that exact rpm.

I've been trying.. and OBD doesnt seem to show it - infact the OBD reads lower than the mechanical boost gauge overall - but there's no sign of the spike on ODB data although as above, it could be because 5500 comes and goes so quickly (even in high gears) that the OBD refresh rate may disguise it

 

Yea, an instant up and back down, 2psi would be very difficult to see by the driver.
Maybe if a second party with good eyes could watch the gauge, they "might" be able to spot the quick needle movement. 50% / 50% chance..!

Mike

 

Graph
Just a quick update, I’ve been doing more and more testing.. and my boost spike at 5500rpm is still happening.
Can confirm also that this is happening on the Scangauge – it is very difficult to see it but thanks to a slow-mo video and doing 10 pulls from 2000-7000rpm, I was able to get one video to show a big spike at 5600rpm exactly like in the graph.


A 17% pulley should take the pk1360 belt… I have now tried also the 1353 belt (7mm shorter) to try and prove that this is not belt slip..


The different belt made zero difference.

With the 1360 belt I can see ¾ of the tensioner tab top hole (pretty much perfect)

With the 1353 belt I can see 1 full hole and ¼ of the second hole on the tensioner tab (a bit too tight, as expected)

I don’t think I have any vacuum leaks.. my idle vacuum is identical to my friend (-8psi) and his car is making progressive boost like it should

To try to remove any boost leaks, I have

Removed and replaced my intercooler with OEM snoot boots and new clamps

Removed my inlet manifold – fitted a new gasket and also gasket sealant to ensure a good seal

Fitted a brand new Mini BPV and checked the butterfly closes / seals fully.. it does

Replaced the vacuum hoses around the throttle body with new rubber 4mm tubing just incase of any splits


The only thing I haven’t checked / changed is the supercharger inlet (green gasket) behind the radiator… but I don’t think a leak here would cause this issue? But I will replace this gasket too

Are there any other suggestions as to what could be occurring?

I have also added all my data into a big table in Excel and then done an average formula on 11 sets of data all 4000-7000rpm and plotted this below.

For reference, I have also changed my intercooler to the OEM one to see if this made any difference… which it did… but the overall trend is the same.
The pink line is my friends GP car fitted with a 15% pulley.

 

The stock intercooler also has a spike at 5500? I still think it’s a side effect of the cam, that the head flows slightly less at that rpm with your cam for whatever issue, causing the boost to spike. Mind you if you can’t feel it then it’s not a problem...

 

I will agree with you on this. I think it's a characteristic of a cam. I am running a Catcam and also have noticed not a boost spike like you, but my boost builds nicely until about 5100rpm or so and then it just flatlines pretty much all the way to rev limit. Never done that before and would have expected another 2 psi of boost before the cam was installed. So I definitely think there is a different flow characteristic with the altered cam lowering boost levels. Anyone else with a cam installed and logs data that can share their results?