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Intake Port Dynamic Pressure - High Speed Data on Kiggly Racing Drag car

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kiggly

Supporting Vendor
318
189
Feb 17, 2003
Ann Arbor, Michigan
Over last winter I worked out a high speed data acquisition on my drag car, here are some combustion pressure and intake port pressure plots. Each channel is taken at 60,000Hz, which is fast enough to have a data point at every crank angle degree at 10,000rpm. I had a pile of questions I was looking to answer by generating this data and also a big pile of new questions have popped up since taking data at a couple dyno sessions. It has been a ton of programming and learning Octave to make some reasonable routines, but I have what I feel is a pretty good home-brew combustion system now and I can process the data quick enough to make decisions between dyno pulls. Ten seconds of data is about 25megs.

One early question I wanted to answer was what pressure do the intake valves really see during high rpm operation. My scenario is a bit unique as I'm running my own sheetmetal intake manifold and a 2g cylinder head, but it still gives a pretty good data point. The data below was gathered up at Force Engineering, Tyler and I had a blast putting this through the paces and learning a ton of new details. The pull below made approximately 740whp on a mustang dyno through the auto trans. It was on a #7 precision converter, which stalls at about 8300rpm at this power level.

Notes:
The green trace is cylinder pressure, this is a 5000psi sensor
The blue trace is intake port pressure, it is a 150psi sensor located about 50mm up the port
The red vertical lines are exhaust valve opening and closing events
The blue vertical lines are intake valve opening and closing events

This is 18psi and 6000rpm, dynamic pressure bounces around about 10psi and peaks at near 25psi at the back of the valve (40psi absolute). This rpm phases nicely with the intake cam advanced all the way and the pulse activity lines up nicely, but it is a little due to low port velocity.
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This is 7500rpm and 40psi, dynamic pressure bounces around about 30psi and peaks at near 55psi at the back of the valve (70psi absolute). This RPM is about the best VE, at just over 110%. I can probably gain some by running a little more cam advance.
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This is 8100rpm and 46psi, dynamic pressure bounces around about 40psi and peaks at near 70psi at the back of the valve (85psi absolute). This is peak torque and about 108% VE.
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This is 9700rpm and 45psi, dynamic pressure bounces around about 50psi and peaks at near 75psi at the back of the valve (90psi absolute). It appears the header is tuned wrong for this rpm and not clearing the cylinder out well at exhaust valve close. Peak power is already past at about 8900rpm.
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The point I was initially trying to get a handle on was how hard the port pulses were trying to pop the valves open. This proves it is WAY above boost pressure. Make sure you have good seat pressure as you start cranking things up. I'm going after exhaust port pressure next trip to the dyno, which I expect to be a good bit worse.

I'll post more on the dyno session after I have some of it sorted out better. I had a pile of fun stuff I was experimenting with including methanol, a new programmable version of the ARC2 ignition, etc. I'm excited to get some new parts in-hand from Fuel Inejctor Clinic so I can crank the boost up on methanol with 8 injectors. I'm also excited to get a Haltech Elite 2500 on it for my next trip back to the rollers.

Thanks,
Kevin
 

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I love this kind of information, I'm a bit of nerd so keep it coming please. I'm interested in your intake manifold specs ( if you want to share) and how the relationship between runner length, diameter and taper (if any) change the resonant frequency of the runner. It's interesting to see that pressure at the back of the intake valve can be THAT much higher. I've only read, no first hand knowledge, that on a n/a engine a tuned intake manifold can add as much as 10 psi positive pressure at the intake valve. Your highest pressure had an additional 30 psi on top of boost pressure! You think that higher than atmospheric pressure compounds the strength of the pressure pulse or, is your intake manifold that good!
 
I have long been interested in the ability to monitor combustion pressures on live data. I am sure that having access to this information is a lot of fun to work with. I am curious as to what type of sensor you are using, and its location. What seemed to be the best solution that I have come across seems to be the psiPlug, which seems to be the only solution that doesn't require any machining. Have you tuned the timing tables to take best advantage of the peak cylinder pressures? If so, is there a certain ATDC angle that you aim for to optimize these pressures? I have read a few different optimum angles, and was curious what you have come up with since the information is scarce on this subject in the "tuner" world. This information was what interested me the most about it, having precise timing tables down to the finest degree.
 
Kevin, Thank you so very much for posting this. I can't wait to see your information on the exhaust side. Are those valve opening lines correct? EVO at ~90bbdc and EVC at 20 ATDC, IVO at 20btdc, IVC 90 Btdc.

Looking the the differential pressure it looks like it would be hard for the pressure wave to pop the intake valves open in your case, unless the pulses get way out of time with other cams/intakes.

I'm expecting to see the exhuast side be worse, as EVC is what I have heard and seen from other pressure plots. My setup has always been higher backpressure stuff, and It always seems to hit a hard limit where it just will not make anymore power. The first time was with stock springs, and switching to your beehives netted 100whp. LOL.
 
Sorry, I'm not going to share exact specs on the intake manifold. It is a regular 2g port size and the runners are probably in a "normal" range for a lot of the aftermarket stuff.

The pressure pulses appear to be prettio ratiometric to absolute pressure, so they scale up with boost. The longer octave length seems to work better na, but the shorter tuned pulse better in turbo applications.

The sensor required machining the head. It is a 5000psi optrand unit. Mbt timing (which stands for maximum brake torque regardless of what internet lore suggests) puts peak pressure somehwere in the 10-15deg atdc range. Once you're in the ballpark here, a change makes almost no difference. We cannot run anywhere near that timing in boost due to both knock and cylinder pressures that it just can't survive. We get stuck more in the 30-40deg atdc range. This puts cylinder pressures down into the 2500psi avg range with peaks closer to 3000psi. I don't know what the oem design point was, but I haven't heard of a gas engine made for over 130bar or about 1900psi peaks. Modern diesel design points are in this 3000psi range. It is not about finest degree of precision except during a brake torque where pressures are low and it isn't knock limited. After that it is knowing knock and trying to keep it together.

The cam timing is shown correctly, intake cl is about 130deg up top and exhaust is static 124deg. I'm also very interested to see exhaust, particularly because I think I have that tuning wrong for the top part of my rpm band, causing the power peak at just under 9k and hang more flat out to 10k rather than rise up. Intake fill doesn't appear to be an issue based on the pressure behavior, just too much exhaust still trapped in there.
 
Thank you for the info on runner length and a general idea, it was out of pure "scientific" curiosity! Didn't you get an engineering degree? Anyways in regards to advancing the intake cam, do you have enough room in the mivec mechanism? I've heard of machining the vtc mechanism to allow advance of up to 45 degrees from 25 ( it was a Honda k 24). Just throwing it out there . This stuff is gold, thanks again!
 
I have opened up the mivec gear for about 45-46deg movement. I do use the whole range and I'm not sure how much some additional angle would help really. At the top end it wasn't super sensitive to center lines after 120deg, but still picked up a little to 130deg so maybe later yet would make more power. I know I have gains just playing with the advance tables in the 6-8k range though, so I will focus a bit on that next dyno session.

I am a mechanical engineer, my day job is an engineering department manager.
 
Would love to see any additional information or, if you got around to logging the exhaust side pressure trace?
 
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