Engine is an F23A1, stock bottom end, milled head with mild port work, SRI, ebay header, cam gear, and tuned on eCtune.

In the past month I've added a Bisimoto level 2 custom cam, a full 2.5" mandrel bent exhaust, and modified the collector to 2.5". I just had it street tuned this week, but ran into a problem. When doing WOT pulls, the butt dyno felt good until ~5600rpm, at which point power quickly plateaued, and never recovered by redline at 6800rpm. I was describing the strange phenomenon to my tuner while we were tuning, and after viewing graphs for time vs MAP sensor, I realize I'm actually pulling a little bit of vacuum after 5600rpm.

My first inclination would be that the stock TB (58mm I think?) and stock IM have become a major restriction. Is this the obvious issue, or could it be something else?

I have an H23 IM in the garage that I intend to port out, and match the TB bore to a PBT 70mm TB, so would that fix this issue?

I'll do some datalogging this weekend and see exactly how much vacuum it's pulling. Thanks in advance.

-Jason


EDIT: Here's a graph of a second-gear pull I did, measured using an accelerometer. Notice the yellow line around 8.5 seconds. This is the issue I'm talking about...

 

i'm experiencing the opposite, my intake mani gauge shows a very slight amount of boost after i do a run , around .5psi.

 

Sounds like:

a. you're in the wrong forum
b. your MAP sensor needs to be calibrated

 

Maybe, but positive manifold pressure is possible with N/A setups.

If you're seeing vacuum at WOT, the throttle body could be a restriction. Is your filter clean?

 

Manifold pressure on the graph would be nice. How much vac? With a typical OEM-ish manifold and TB it's pretty hard to avoid it as RPMs go up.

 

I'll get that chart up tomorrow morning.

Filter is a cone style K&N, about 6 months old, on an 2.5" AEM SRI.

 

Jason, I would say that the 2.5" intake is also a restriction...for your power levels I would go with a 3". For reference, an AEM intake for my LS, stock, is 2.5", intended for a ~130 or less whp motor...but an AEM intake for the Integra Type R is 3"...and they only make ~165 or so stock. You should be well past that 165...
However, I don't pull vacuum even with my cams, and still have the smaller LS intake.

 

Hmm... I'm about to go make some runs with the datalogger and I'll save some graphs to post up. I might lose the intake arm just to see if that helps at all, but the TB @ 58mm is only 2.25" in diameter. I guess we'll see what happens...

 

It really depends on where you are measuring it at. An efficient 4 valve engine can show positive pressure behind the back of the valve when velocity is high and the valve closes. I doubt this would transfer out to the plenum as other valves are open at this time.

 

**** Jason. A B18B(LS), OBD1 or 2 B16, or Integra GSR TB is 60mm...even that would help you, and they're easy to find. No wonder you're pulling vacuum.
Do you already have a larger one?

 

What do you mean by that? I'm still using the stock MAP sensor mounted on the TB right now. If it is reading vacuum right there, my logic tells me that the engine is trying to pull more air in than what is available at that specific point. Which means somewhere along the way there is a bottleneck. I'm not certain whether the bottleneck could be something as simple as the TB/IM, or something more serious like the intake ports on the head not flowing enough.


I have another F23 TB on the H23 IM right now, both completely stock. I'm leaning toward the 70mm>68mm TB from Chris right now because of the great reviews and low price.

Here's a couple graphs of the MAP sensor doing WOT pulls from low RPM. I tried adjusting the graph to better display the MAP scale, but couldn't find a way to do it.

 

interested to see results with new IM, i'm gonna just try the S2 H22 mani on my F23 with an even more aggressive bisi cam 2.2/custom and high flow head i'm hoping i can make use of the S2. Maybe i'm way off base i dunno.......

 

I don't think it works. I'll fab up my Euro-R tomorrow and let you know what the issues are.

 

i am well aware that this is the all motor section, just wanted to report my findings
my defi map sensor is connected to the plenum, and i'm using a mugen intake and stock itr throttle body

 

Ah. What's the rest of your setup? Maybe your experiences are what DonF was speaking of.

 

Alright, took a video, but I just uploaded it from my phone so I don't know if this URL is correct.

After a few pulls, it looks like it's dipping down from .1 to .2inHg around 5krpm down to 1.8-1.9inHg (925-930mbar) above that point.

Vacuum Video

 

I can't see your vid, because I'm at work right now. For your MAP sensor to be picking up vacuum at wide open throttle & high RPM's would indicate to me that the restriction is before/in front of the MAP sensor. IE the throttle body, intake tube and air filter.

I wouldn't think that it would be caused buy anything in your head. The reason your seeing vacuum at WOT is because your plenum, runners, cylinder head and cylinders are sucking more air than your throttle opening can feed them....That's not from experience it's just my opinion/theory from how the air pump (engine) works.

 

have u tried taking away the filter?

 

I agree with you completely. I don't know the 'technical' aspects of this stuff, but we are thinking the same way.

The video I just posted was with only a short-ram arm installed on the TB with NO filter. So we're talking less than 2 feet of intake piping.

I measured the AEM intake this morning and found it has an inner diameter of 2.375". The stock TB has an inner diameter of 2.28" (if it is 58mm. Some sources say it's 60mm). I'm sure this is still too small.

I'll make another run tonight with an open TB and make another video, but in 3rd gear so the screen can keep up.

EDIT: Here's another video in 3rd gear where it actually goes to 2inHg.


http://s80.photobucket.com/albums/j1...0408091653.flv

 

Made some progress last night. Removed the entire intake pipe, leaving an open throttle body. Took it down the road for some pulls, and had a little bit of success.

Before with just the intake arm and no filter, it pulled 2.0inHg. Last night with nothing on the TB, it only pulled 1.2inHg. The car felt MUCH better in midrange, especially from 2.5-4k, but still pulls that 1.2inHg up top and flattens out a bit.

Looks like a larger TB is up next, along with a larger intake. I'm still working on my H23 IM swap, and that will be ported and bored over to fit the TB before I install it. Hopefully the TB and intake alone will resolve the biggest part of the problem, and the IM will just be icing on the cake.