M20 B3x Naturally Aspirated Stroker build past, present and future

[bradnic]

incredible. where did you source the Alpina crank?

[suruba]

I want tony to do a head package for my 3.1 soooooo bad. this thing looks great. I can't wait for you to get it into a vehicle.

Dave

[digger]

Yeah most was done a few years back though lol, while he waited for me to send the final parts etc , just did finishing touches and assembly a few weeks back

[LJ851]

Hot Damn ! That head looks A+. You had Knight do the headwork, right?

[digger]

I think it’s time for an update.

So my head is 99% assembled minus some studs, sprayer and the shaft plugs

RHD rockers and 292/292 cam installed. 110 lobe separation with just under 13mm lift. Think schrick 288 but extra 1.5mm lift and a few degrees duration more so nothing too wild (yet).

Valve lift seems to be what a big stroker needs to get VE at any sort of rpm approaching 7000rpm, the roller followers enable a higher valve velocity than you can get with stock type rocker pad giving a nice fat lift curve.

i was going to go for the 306 cam but i thought id start mild to begin with as i want to still drive it on the street without too much compromise, maybe im getting old or just soft ;) however everything is being designed around being able to fit the 306 cam with 106 LSA at a later date.

port work is mint 45/36 valves lots of chamber reshaping (remember that the chamber is an extension of the port...), the RHD intake was fully port matched to.

you can feel the shapes are right with your finger the short side profile is miles better than any head ive had ported before.















Time for the bottom end to get serious. Lightweight 93.8mm stroke crank from an Alpina B3 3.3 / B3S should do the trick for some midrange torque. I wouldn’t expect anymore topend hp compared to 89.6 crank all else being equal but shouldn’t have to rev it as hard.

[digger]

i certainly agree that with intake length the filling just before BDC (quite alot before BDC) to IVC is the key to better output

[The Dark Side of Will]

Are you saying the additional mass that enters between IVO and piston “suction” starting proper (say 50cc volume worth possible more ) is less then the benefit from just starting the flow early and getting better filling due to better “ramming” from BDC to before IVC and less reversion at that point? My main discussion point was you want the flow to start early as possible there are benefits to this. My guess it might not be clear cut. you could probably do a few what ifs with the sim to check but its hard to isolate one thing without affecting something else

Put numbers to it...

At 11:1 compression, the clearance volume is 1/10 of the displaced volume. For a 500 cc cylinder, that's 50 cc's. The max additional charge you can get from scavenging the overlap is therefore 50cc's. That's certainly nothing to sneeze at and should absolutely be an objective.

However, the volume in the cylinder between BDC and IVC is... What? 100 cc's? 150 cc's? I don't know your cam specs.
They're both important, but the air below IVC is most likely a bigger chunk.

[digger]

on the pressure traces i posted you can see that between IVC to IVO the inlet port pressure waves (black) waves do oscillate within the volume formed by the runner and closed valve and airbox. the airbox does link closed runner with other working runners but the same effects are present when you run a true IR manifold

[318aye]

The same mechanism operates, but the mechanism you describe doesn't exist.

When the exhaust valve opens, the blow down creates a high pressure pulse which travels down the header primary. The pulse reaches the collector where the large and sudden change in cross sectional area causes the pulse to undergo an open pipe reflection (google "open pipe reflection"). The open pipe reflection reverses the sign of the pulse. The low pressure reflection travels back up the primary and arrives back at the exhaust as a low pressure pulse. If your header primaries are 12 feet long and you're operating on the first reflection, this low pressure pulse arrives during the overlap period, enters the cylinder just before the exhaust valve closes and travels up the intake pipe as a low pressure pulse to kick start the intake pulse tuning. The piston is high in the bore, so the change in cross sectional area is minimal and the wave doesn't reflect, it just passes through.

The *low* pressure pulse travels up the intake primary until it reaches the plenum. It undergoes another open pipe reflection and comes back down the pipe as a high pressure pulse. If your intake primaries are 7 feet long and you're using the first reflection, that high pressure pulse arrives at the intake valve just before it closes and kicks the last little bit of charge air through the valve and into the cylinder.

The idea that intake tuning results from a high pressure wave bouncing off the back of a closing intake valve is nonsense.

Right for the wrong reason was partial credit back in school

The fact that open tubes reflect an inverse wave (regardless of which harmonic you look at) was the part I was missing, and your explanation was easily followed, thanks!

For anyone else, here are some links that helped me visualize this:

https://www.quora.com/How-does-a-wave-gets-reflected-in-an-open-organ-pipe

http://www.burnsstainless.com/Theory.aspx

http://www.studyphysics.ca/newnotes/20/unit03_mechanicalwaves/chp141516_waves/lesson51.htm

http://www.eng.buffalo.edu/shaw/student/m3_digimusic/02_documents/sound_wave/MusicInstruments_OpenEndAirColumns.htm

Follow up question(s): Exhaust length is dictated by pulse timing which is partially due to the harmonic the design is targeting. Are there harmonics that produce inherently "stronger" pulse waves? Is there such a thing as a reflected pulse being too large in a scenario like this? The burns link states that pulse strength corresponds to the change in acoustic impedance, but I'm wondering if specific harmonics contribute to this as well?

[digger]

the timing of the reflection relative to the overlap period is something that VANOS can allow you to tune. Move the IVO relative to the EVC and the relationship of the reflection to the overlap period will change.

This is what I was referring to. One reason why advancing a cam with fixed LSA can hurt you, what you might gain by advancing the inlet can sometimes be lost by the ECL being on an excessively advanced position. One of the possible downsides is it can cause the suction wave to be less effective as it happens at a less productive time particularly if the wave is not a broad wave and doesn’t span the full overlap period which appears to be very common especially at low rpm. On my current engine I advanced the cam until I stopped getting more power on the dyno. The bottom end basically showed less gains to and this might be one reason for it along with the early EVO reducing energy harnessed from the firing pulse.

During the overlap period, the piston is at the top of the cylinder, so there's very little exhaust left to remove, especially in a high compression engine with very small clearance volume. It needs to be removed, of course, and getting it out allows more room for intake charge. I'm not sure the % effect of the final push just before IVC vs. the % effect of scavenging during the overlap on VE. At least overlap scavenging can be calculated.

Are you saying the additional mass that enters between IVO and piston “suction” starting proper (say 50cc volume worth possible more ) is less then the benefit from just starting the flow early and getting better filling due to better “ramming” from BDC to before IVC and less reversion at that point? My main discussion point was you want the flow to start early as possible there are benefits to this. My guess it might not be clear cut. you could probably do a few what ifs with the sim to check but its hard to isolate one thing without affecting something else

[The Dark Side of Will]

Note that moving cam centerlines change when blow down starts and when waves return.

Well... yes and no. The timing of the reflection relative to the valve opening is a function of duration and tube length. VANOS doesn't change duration, so the timing of EVO vs. reflection return doesn't change. However, the timing of the reflection relative to the overlap period is something that VANOS can allow you to tune. Move the IVO relative to the EVC and the relationship of the reflection to the overlap period will change.

If we just look at overlap for the moment which starts with intake valve cracking open. In order to move exhaust gas out of the cylinder into the exhaust pipe we want a low exhaust port pressure, to move air into the chamber we want high pressure in intake port so the pressure differentials/ pressure ratios are all working in harmony so the flow moves in the right direction.

During the overlap period, the piston is at the top of the cylinder, so there's very little exhaust left to remove, especially in a high compression engine with very small clearance volume. It needs to be removed, of course, and getting it out allows more room for intake charge. I'm not sure the % effect of the final push just before IVC vs. the % effect of scavenging during the overlap on VE. At least overlap scavenging can be calculated.

[The Dark Side of Will]

I've always understood the theory behind tuned length intake manifolds to rely on the wave from the closing intake valve to bounce back off the boundary at the inlet and draw in the next charge. Is mechanism present in the exhaust merge simulation here similar?

The same mechanism operates, but the mechanism you describe doesn't exist.

When the exhaust valve opens, the blow down creates a high pressure pulse which travels down the header primary. The pulse reaches the collector where the large and sudden change in cross sectional area causes the pulse to undergo an open pipe reflection (google "open pipe reflection"). The open pipe reflection reverses the sign of the pulse. The low pressure reflection travels back up the primary and arrives back at the exhaust as a low pressure pulse. If your header primaries are 12 feet long and you're operating on the first reflection, this low pressure pulse arrives during the overlap period, enters the cylinder just before the exhaust valve closes and travels up the intake pipe as a low pressure pulse to kick start the intake pulse tuning. The piston is high in the bore, so the change in cross sectional area is minimal and the wave doesn't reflect, it just passes through.

The *low* pressure pulse travels up the intake primary until it reaches the plenum. It undergoes another open pipe reflection and comes back down the pipe as a high pressure pulse. If your intake primaries are 7 feet long and you're using the first reflection, that high pressure pulse arrives at the intake valve just before it closes and kicks the last little bit of charge air through the valve and into the cylinder.

The idea that intake tuning results from a high pressure wave bouncing off the back of a closing intake valve is nonsense.

[digger]

I've known this for a while, but my merge is way too close to the collectors. I think it's about 18", maybe closer - probably not even good for a 4th harmonic. When I swap engines I'm going to ditch the cat and move it all the way back where it should be.

18" is not terrible at all from a 4th harmonic perspective but you could probably get more bottom end with twice the length which does fit in much easier than doing the 2nd harmonic, its something you could test.

You need to look at what your goals are. With a mild cam and fancy 24V trickery in modern BMW the balance / compromise point might be a little different to a restricted 12V.

with mine i think the compression is low for the cam im using so the exhaust becomes critical at the bottom end to get things moving. Some might say the longer collector is a bandaid for a poorly spec'd engine and i wouldnt disagree. Some people dont even car about the bottom end at all

However ive seen more than enough data on far superior engines to be able to say with confidence there are some fundamental trends with this.

[digger]

What other parameters/unknowns did you have to give ENGMOD4T to fully simulate this? Any base assumptions?

You have to tell it everything for the bottom end spec, including friction, cylinder head data, cam lift curves need to be generated. Inlet and exhaust need to be modeled every change in section needs to be input. Merge angles, air box volumes. There are tools to help you do it.
The big assumptions i made were the combustion and temperature models, i used the same as the Porsche example that came with it as it’s a head from same era slightly different chamber (hemi vs semi hemi) but i don’t believe it would have a big effect on the basic trends.

How much am I hurting myself with a 2.5" single after the merge?

2.5” single pipe is plenty, back pressure due to tailpipe size are pretty small more importantly would be cat efficiency and muffler style (straight through, baffled etc) as these can give more backpressure and that is a bad thing if they are not a good design.

Do you have an explanation for why/how the merge is physically affecting pressure levels behind the port?

i believe it’s mostly the speed of pressure wave and length of pipe dictating when in the cycle the pulse arrives back to the port/valve based on the time vs crank angle relationship. This is one reason why the pipes come in and out of tune with rpm. Note that moving cam centerlines change when blow down starts and when waves return.

I've always understood the theory behind tuned length intake manifolds to rely on the wave from the closing intake valve to bounce back off the boundary at the inlet and draw in the next charge. Is mechanism present in the exhaust merge simulation here similar?

Yes the mechanism is present in that waves are acting and moving.

If we just look at overlap for the moment which starts with intake valve cracking open. In order to move exhaust gas out of the cylinder into the exhaust pipe we want a low exhaust port pressure, to move air into the chamber we want high pressure in intake port so the pressure differentials/ pressure ratios are all working in harmony so the flow moves in the right direction.

So I would say you are generating boost in the inlet ports rather than drawing things in more if that makes sense.

You see with the RHD intake when it’s in tune at 5500-6500rpm there is a positive pressure in the intake port at inlet valve open so this helps airflow into the chamber, the rpm where its in tune is a function of the length of runner. If you use too short runner length they are in tune at an rpm that the engine never sees and the engine cam timing and flow capabilities probably aren’t matched for. If you use longer runners it’s in tune at lower rpm and you get more torque but at the top end the runners come out of tune and you lose hp with longer runners so It’s a compromise.

It’s the same compromise with header primary length use longer headers to work better at bottom end and you lose out at the top, use ones that are too short and you end up with nothing but a free flowing manifold that don’t tune well to the engines needs.

My engine primary length and the RHD inlet length work in harmony together at 6000rpm so the thing limiting power is the size of the valve aperture and flow efficiency (cam profile/timing, head flow, valve size, throat size etc) to take advantage of it. Also there is an efficiency thing with compression and combustion quality.

One thing you start to learn that tuning an engine is its nothing more than learning how to control pressure differentials to move mass through the engine. The key is understanding how to shape the waves based on the configuration, volume, diameter, taper, length of intake and exhaust system etc.

[nando]

I've known this for a while, but my merge is way too close to the collectors. I think it's about 18", maybe closer - probably not even good for a 4th harmonic. When I swap engines I'm going to ditch the cat and move it all the way back where it should be.