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post #1 of 99 (permalink) Old 02-17-2012, 04:11 AM Thread Starter
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TwinSpark MegaSquirt conversion

Hi Folks,

I've shoe-horned a TS engine into my Super (here: Twin Spark conversion for '66 Giulia Super </end_shamless_plug> - and while it's puttering about on a mismatched Marelliplex single spark ignition and DCOEs temporarily (to get to work) - I'm at the stage where the pile of MegaSquirt bits and pieces I've been collecting and making are ready to go in to really get it humming.

I've been fossicking around the BB and net in general collecting a few different ways to implement it, and polluted an excellent Motronic thread along the way with incessant questions and thoughts here:
What the stock 75 TS Motronic can and cannot do.

... it was suggested I do so in my own sandbox - which might make it easier for others to find if it's helpful as well - so here it is.

I'll copy in the bits and pieces discussed on the other thread - if you have any tips / thoughts on it - contributions gratefully appreciated !

Scott Murray
'04 156 JTS Sportwagon -- brilliant car for the daily, kids taxi and parts hauling

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post #2 of 99 (permalink) Old 02-17-2012, 04:27 AM Thread Starter
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What initially drew me to the Motronic thread was the great discussion there on exactly how the ECU dealt with the VVT mechanism.

There were some differing opinions on the web about how it worked, and how it *should* work - for emissions, economy and power - but nothing concrete until the thread listed above.

According to what Festy was able to extract by reverse engineering the Motronic machine code - it's like this: (from here: What the stock 75 TS Motronic can and cannot do.)

Quote:
Originally Posted by festy View Post
VVT Operation
(numbers and addresses based on '191 code, YMMV)

* At WOT, VVT is immediately activated

* At part throttle (i.e. not closed, not WOT) VVT is activated when engine speed is above 1280rpm, and load is above a table value depending on RPM, car type and whether there's a lambda sensor fitted.

RPM threshold is determined by the value at address 0x4672 (20h, or 32) x 40, so 1280rpm for all configurations.

For those viewers playing at home, the load thresholds are found in the following table locations:
75, no lambda sensor: 0x5E49
75, with lambda: 0x5E71
164, no lambda: 0x5E5D
164, with lambda: 0x5E85

There's a couple of other scenarios that affect VVT, i.e. it's disabled if the ECU hasn't completed all it's 'engine start' functions, and two other cases which I haven't fully explored yet.

VVT is disabled over 5000 rpm, and won't be enabled over 4840 rpm. This is controlled by 2 values, the first is the max rpm value, and the second is a 'rpm below upper rpm to disable activation' if that makes sense...

Here's a screenshot of the RPM vs load tables for VVT activation, note the differences between car configurations. Calculated load has to be above this number at the rpm point for VVT to be acivated.

Scott Murray
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post #3 of 99 (permalink) Old 02-17-2012, 05:06 AM Thread Starter
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Post

Prior to this, myself and a few others had been taking a rough guess at how it might work - and some great info was gleaned from Nick's great thread about running the VVT with Carbs: here

Where David Munro, of the 200HP TwinSpark (boy have I pored over that thread )

suggested using an MSD RPM window based switch to activate / deactivate the VVT. It would need to have some hysteresis control as well to stop the VVT flickering on/off rapidly if RPM fluctuated around the window limits.

Precise cam info was provided by the learned Richard Jemison here with some alternative solutions around regrinds and centrifugal VVT for better than stock performance with carbs, and some RPM window contraints to try with the stock system

After a lot of noodle-baking, including pointers from Craig (Alfa 75/Milano Oracle) Craig's Place

it was realised that any control mechanism for the VVT really needed to factor engine load into the decision to actuate. This meant monitoring Manifold Air Pressure, or a Throttle Position Switch as well.

I found a cheap (like me) Jaycar solder-it-yerself kit for an RPM window switch like this, and a basic circuit was knocked up to control VVT just via RPM, with a manual override switch to experiment with it a little. After Nick made it legible it looked like this:



I hooked it up, and sure enough - powering the VVT under WOT engine load from 1200rpm up made a lot more power, and was much more fun to listen to - no surprise there. Under light throttle, gentle acceleration or cruising the engine still had quick throttle response and eager power, but I started to get some pre-ignition as well.. (more on this later) With the VVT enabled at idle the idle would become very lumpy and irregular.

VVT needed to be triggered more by load, and only within a given RPM window. I started thinking about modding the circuit to take basic TPS input, fit a TPS to the carbs and see how that went - thinking it might be useful for other carbed TS folks- until I came across Festy's work on the motronic and conversation headed in a different direction.

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post #4 of 99 (permalink) Old 02-17-2012, 05:14 AM Thread Starter
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Back on the Motronic thread, Grant posted some really useful info clarifying overlap in case we'd got it backwards. I had (again).... also some thoughts on why you might deactivate the VVT at very high RPM (apart from as a gentle rev limiter) As the Motronic does.

from here:

Quote:
Originally Posted by Grant View Post
Here is something I wrote about VVT theory on another forum....


"When VVT is off (the cam retards back to the normal position) the intake valve opens later, and CLOSES later too. There is less overlap when VVT is off. On the other hand, with VVT on, the extra overlap is good at mid-range to slightly high rpm because the previous exhaust pulses leaving the cylinder suck a waiting intake charge in, increasing the VE. If the intake valve opens sooner, (VVT on) more of the charge can be sucked in, increasing torque.

You cannot turn VVT on at low rpm though because the exhaust pulses don't leave the cylinder fast enough, and they get pulled back into the combustion chamber as the pistons is travelling down, diluting the intake charge and causing a terrible idle. If you've ever turned VVT on at idle you will see this. On my stock blacktop, the earliest I could use VVT was about 2400 rpm, below that there is more torque with it off.

At high rpm, you want to turn VVT off because the intake valve closing time is retarded as well. At high pistons speeds (high rpm) the intake charge "lags" a bit and can't follow the piston as well as lower rpm. Because of this it is beneficial for power to leave the intake valve open later, allowing more of the charge to keep coming in, even thought the piston is on its way back up at the start of the compression stroke.

The key thing I had to realize when studying all of this was that overlap is the time between the exhaust valve closing, and the intake valve opening (not intake valve closing, and exhaust valve opening)! This is key. I think from what you posted above you have this backwards, like I did in the past.

So more overlap means the exhaust valve is remaining open longer, and the intake valve opens sooner....with more vacuum being created by the exhaust heading down the header to suck in the intake charge. This effect is so strong, intake charge will be sucked in even before the piston is starting to come down at the beginning of the intake stroke. Said in other words, the intake charge can begin to enter the combustion chamber even while the engine is at the middle or end of the exhaust stroke. Add this to the intake charge that enters just due to the vacuum created by the piston during the intake stroke and you get more overall power. Some intake charge will make its way out the exhaust though, increasing emissions. This is why oem's can't run as much overlap as they'd like for performance.

My understanding is that overlap is good for high rpm as well, but if you only have so much overall duration to play with (i.e. stock cams), the 4ag prefers to have a late intake valve closing time over just having overlap at high rpm."

Scott Murray
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post #5 of 99 (permalink) Old 02-17-2012, 05:44 AM Thread Starter
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Grant also headed the discussion into an area I hadn't even thought of - when the VVT cuts in - the Volumetric Efficiency of the engine changes (more mixture is crammed into the cylinder = more compression) and as a result - the spark advance needs to be retarded slightly to prevent pre-ignition. Festy pointed out that with -twin- sparks, you might need to retard a little more due to the faster flame-front. (compared to a stock 4 plug 2L nord anyway)


With Carbs - they just deliver more fuel as the engine draws more air through the intake - but the ignition: with the standard curved MarelliPlex I was running on the engine - no wonder it was pinging with the VVT engaged under light loads.

I started to understand why I hear a few people have just welded the VVT engaged, accepted the lumpy idle and timed the ignition to suit - but this was noted:

Quote:
Originally Posted by Jim K. View Post
No matter where you weld the VVT it won't work well with carbs. Anyway you look at, it its a performance 300* cam profile. Weld it advanced and you lose significantly on top. Weld it retarded you have pathetic compression (and performance). Unless you regrind it to some more sensible 'fast road' spec, you really need an operative VVT.
Jim K.
and so back into the fray we happily went.

Scott Murray
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post #6 of 99 (permalink) Old 02-17-2012, 06:07 AM Thread Starter
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Some ideas went back and forth about ways to retard the ignition when the VVT cut in: Use a vacuum advance dizzy - and cut off the vacuum with a solenoid when the VVT engages. Use something like the 1-2-3 electronic dizzy - you can load it with multiple maps and switch between them on the fly via a trigger input.

Both modded of course to take the 8 pole nissan TS cap.

Around here I realised, for my project at least, it would be better to focus on getting my Megasquirt gear running than do any more hack work to get the carbs / marelli to behave.

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post #7 of 99 (permalink) Old 02-17-2012, 06:35 AM Thread Starter
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So started thinking about the Fuelling / Ignition / VVT as a whole. The VVT seems like a small add-on to make a hairy cam behave at idle / low load - but it is like an elephant in the room when it kicks in and out.

Grant mentioned here the issues with running a single spark / VE map, and using the known load / rpm values the VVT would be active to enrichen / retard as required:

Quote:
Originally Posted by Grant View Post
The issue I had with only having one table is:

If you are on the verge of VVT turning on, you have only so much fuel added, and pretty aggressive ignition advance. As soon as you bump into the next higher load cell, VVT turns on....you have a lot more fuel added, and a lot less timing (b/c VVT not only increases VE but it also closes the intake valve sooner, increasing the effective compression ratio).

The problem here is that the ECU interpolates between the two cells, and you get a result of "inbetween" ....some more fuel, and a bit less timing...which usually isn't good enough for me. I would get detonation right as VVT turned on...the only way around it is to make things extra rich and have a lot less timing until you can pull away from the load cells that are interpolated with the non VVT load cells nearby. I hope that makes sense.
It started to look like running two pairs of VE and Spark maps switched when the VVT did would be a better solution.

Madk4Speed posted some similar info, relating his own MS TwinSpark setup and some of the difficulties around the VVT he encountered - starting here.

Quote:
Originally Posted by madk4speed View Post
Hello to everyone. I have installed a Megasquirt 2 with DIS ignition in an 75 twin spark. I have to say the VVT adjustment gives me a headache. I first tried to use RPM/TPS based VVT, and it seemed to work pretty well. You felt a push around 1650 rpm and 1/3 of throttle pressed. That was my configuration.BUT......
Unfortunately this prevents the user from having an accurate VE map. Because you might be on the same cell on the fuel/ignition map but VVT both on and off randomly....So the VVT functionality has to be RPM/MAP based. I tried it a bit and the car was not doing as well as in the previous configuration.
Also BUT.....this does not give me the power to turn off the VVT in high RPM, like people say it should do. Megasquirt does not support enough parameters at the same time to control the VVT efficiently.
Then Festy suggested that keeping both the VE, Spark and VVT enable all a function of RPM/MAP could be a simpler way to use a single table here

Quote:
Originally Posted by festy View Post
See this post for the VVT params.
MAP pulses with ITBs are mainly an issue at idle, which is why idle is often handled by TPS/RPM rather than MAP with these setups. Once you get into the VVT range (~1250 rpm) the MAP data is more useable.
If you use a plenum setup for the MAP sensor and suitably restrictive restrictors (MIG tips work well), there shouldn't be a problem once engine speed is up a bit.
By using RPM/MAP to control VVT there's no need for table switching, because your part throttle VE and spark tables are also RPM/MAP. Your WOT and idle tables may not be, but VVT is disabled at idle and enabled at WOT, so no variables there except RPM.
Not being able to turn VVT off at the high RPM threshold (5000rpm from memory) is a problem, but could be handled by an external frequency switch (like the Jaycar kit mentioned previously) - just use it in-line with the MS's VVT control line but "backwards" - so it's always on (pass-through) until 5000rpm is hit, then it drops out. That way MS will control when it turns on, and the frequency box will control when it turns off

If you're looking for a MAP setting to enable VVT, start trying values somewhere around 60kpa as a very rough guess.
and Madk4Speed pointed out you could daisy-chain two or more of the configurable MS outputs together to add the High RPM threshold.

Quote:
Originally Posted by madk4speed View Post
ITBs always work in Alpha-n(Tps/Rpm), and not Speed-density(Map/Rpm).
Yes I agree the 60kpa threshold for the VVt is the value to start with.
I never tried to turn on VVT as low as 1250rpm, I thought it was a bit higher (1600 rpm).
Sharing a sensor (with Jaycar kit) could be a problem, since I use the stock VR crank sensor and not a Hall effect one.
Actually using a second output controlling a relay from the MS that will be always on until 5000rpm could be easier. So the Jaycar kit can be replaced by the Megasquirt! 2 Megasquirt "switches" in line would do the job more efficiently i think and provide a cleaner installation. But for sure I will check the solution you propose.
I'd been hunting through the options for the MSnS MS code variant - and wondered if some of the Nitrous control areas could be used to enable VVT, add fuel, retard advance as an algorithm function, rather than a map...

Or if running multiple maps - the MSnS can take an electrical input to trigger map change - this could be fed from the final VVT logical output to tie things together (if it couldn't be done internally with software)

So many options

Scott Murray
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post #8 of 99 (permalink) Old 02-17-2012, 06:52 AM
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Vvt

The VVT should not be turned off at high RPM. It should come on at an RPM that is really cam profile dependant. Typically about 3000 RPM and should have only RPM trigger, not throtle position dependant. All the hodge podge off and on crap from Bosch stock control is emission related. Loose it.
The engine becomes more efficient above 3K and additional fuel mix is cam profile dependant, not cam LC depandant.

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post #9 of 99 (permalink) Old 02-17-2012, 05:35 PM
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Richard,

I have no doubt that this may be true for Nords. High rpm for the engine I tinker with can be up to 9,000 rpm on stock internals. On a stock 4ag, VVT definitely should be disabled at about 6500 rpm. This is with 215 degree cams at .050". With a redline of 6000 rpm, you may indeed want to keep VVT on the whole time above 1500 rpm or w/e it is the TS likes it to be turned on. Luckily this can all be discovered in about 20 minutes at the dyno!

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post #10 of 99 (permalink) Old 02-17-2012, 05:39 PM
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Fantastic summary BTW Aikendrum105. I didn't even have it down that well as I was in and out of the thread in between work to be done.

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post #11 of 99 (permalink) Old 02-17-2012, 06:28 PM
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I think it my not be so bad with a carb or a flap/hotwire afm.
But MAP based systems it is a different animal different cam setups change the vacuum a lot and will make a MAP based system fuel way different. So a big map change or comp table is needed. but the carb is more like a AFM where air flow tells how much fuel to be there. so I think it will cope OK but the spark should be moved as the dynamic compression changes with VVT.

As a side note I do not think VVT rules have any thing to do with emission for the most part other then it not coming on at all in the test cycle aka low loads. the tests back then did not seem to test for hi loads at all just idle and cruse. That is way they can go pig rich on WOT. so as long as it did not kick on at low power that is used in the test. emission has not much to due with it.

I guess there might be a corner case where VVT might be OK to use at low loads and not effect MPG but cause emissions but for the most part what is good for MPG is good for emissions when it comes to VVT.
If you are only using around 10KW there is no need for the power and the MPG I think would suffer too if VVT is on at the low loads.
so a win win good MPG and good emission at low loads.
I think even today the tests are only start idle and low loads. I think almost all cars would go off the charts at WOT. I think most of the work is the start to warm up while keeping the emissions low.

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post #12 of 99 (permalink) Old 02-18-2012, 06:25 AM Thread Starter
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Quote:
Originally Posted by Alfar7 View Post
The VVT should not be turned off at high RPM.
Hi Richard, I'd completely forgotten - sincere apologies - you mentioned this, not once, but twice in Nicks VVT thread - and I still didn't get the message

I think once I headed off into that Motronic thread I assumed that was the way to go at least as a starting point. Do you think it just does it as a soft rev-limiter ? 5k seems a nasty place to lose some punch, when redline is up in the sixes... Is there possibly more chance of valve collision with VVT engaged at those speeds (betraying my ignorance with that one probably) it does seem an odd place for emissions testing to pick on back then - but wouldn't be the first time I'm sure ....

Quote:
Originally Posted by Alfar7 View Post
... Bosch stock control is emission related. Loose it.
I'm probably making more out of it that necessary - simply because I'm running that MarelliPlex which has too much advance for a STD 2L , let alone a TS with VVT engaged on part throttle, making it more of a problem in the back of my mind than it is with the right spark advance map.

At the risk of having my Alfa cred revoked by hinting at fuel economy, is it likely that turning off VVT at light throttle loads will improve fuel efficiency (much better wording) compared to that with it on ?

I know that coasting down a long hill above 2k rpms with VVT on gives the usual pop / burble you get from a carbed Nord - with the VVT off the burble / pop disappears - and engine braking seems to improve.

Quote:
Originally Posted by Alfar7 View Post
additional fuel mix is cam profile dependant, not cam LC depandant.
So to choose that path, we just kick the VVT in arbitrarily at 1200rpm or so, get the spark advance right for constant VVT over 1200rpm - we only need worry about one VE Fuelling table with a bump up at 1200rpm (or just use carbs) because the VVT never cuts out under light throttle loads. ?

Cheers Richard - appreciate it. Thanks for your patience

Scott Murray
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post #13 of 99 (permalink) Old 02-18-2012, 06:31 AM Thread Starter
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Quote:
Originally Posted by Grant View Post
Luckily this can all be discovered in about 20 minutes at the dyno!
I think once the MS is running - I need to find somewhere around Melb I can get some inexpensive and repeatable (hahaha no chance) dyno time to compare a few things. Or just do one test a year at Beninca's Dyno day

I guess a very rudimentary set of tests could be done using the MS logging / tuning software and a control strip of road to see where the acceleration times were most impressive - Reliability / Repeatability is always fun there - but you could get a bell curve going over a few sessions. Even Dyno sessions can be plagued with that Especially if you use a different Dyno every time


Quote:
Originally Posted by Grant View Post
Fantastic summary BTW Aikendrum105. I didn't even have it down that well as I was in and out of the thread in between work to be done.
Cheers mate - It's helping me straighten it out in my head - hope folks can glean a nugget here and there too.

Scott Murray
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post #14 of 99 (permalink) Old 02-18-2012, 06:45 AM Thread Starter
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Quote:
Originally Posted by slyalfa View Post
...MAP based systems it is a different animal different cam setups change the vacuum a lot.... carb is more like a AFM where air flow tells how much fuel to be there.
Ahh that's making more sense to me now - you're working off different sides of the throttle butterflies.... I'm slow but get there in the end...

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Originally Posted by slyalfa View Post
As a side note I do not think VVT rules have any thing to do with emission for the most part other then it not coming on at all in the test cycle aka low loads.
It is peculiar the way motronic cuts it out at 5k rpms.. Inconcievable (come on - The Princess Bride.. it was good)

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Originally Posted by slyalfa View Post
so a win win good MPG and good emission at low loads.
I must admit - I'm kinda hoping that's the case - just to make the VVT work for all the trouble we're going to to run it Some datalogging will tell I guess. That might involve a fuel flowmeter.. hrmmm Although the logging software can probably interpolate that from Injector type and duty during the test...

Cheers Sly !

Scott Murray
'04 156 JTS Sportwagon -- brilliant car for the daily, kids taxi and parts hauling

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post #15 of 99 (permalink) Old 02-18-2012, 09:09 AM
Richard Jemison
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Misinformation

Grants statements highlighted below are incorrect.

Cam events related to opening and closing are a function of cam lobe design. If a cam opens "early" it also closes "early"

Reducing overlap by opening the valve "early" closes the valve based on the duration of the cam at the lash setting .
Reducing overlap increases combustion pressures in the cylinder as less charge is wasted out the exhaust. At low to mid (idle to 3500 RPM0 this is a good thing as more torque is the result, meaning better drivability.)
When the intake charge begins to speed up and "charge" the intake passage combustion pressures might cause detonation if fuel octane is too low.
That point or 1000 RPM before is a reasonable time to advance the V V T.

If the VVT is switched off at high RPM you are killing the benefits of the "overlap" and back to potential detonation.

The benefits of using VVT with performance cams (cams with more lift and duration, stock TS cams are not performance cams) is the cam can be retarded allowing valve events (opening & closing) that produce less overlap that eliminates resulting backflow that ruins idle quality with both carbs and FI MAP sensing, but allows the same timing events through 2500-3500 RPM to give superior torque and low end power.

These devices were used as emission enhansing (idle gasses) and why they tried to monkey so much with their switching. For the motor to run at it`s best, (power & drivability at all RPMs) the V V T should be switched on as late as reasonable to prevent pinging, and remain on until engine RPMs drop below that threashold. The early centrifugal type were very effective & I use them on other applications for the very reasons above. This includes race motors that are driven to the track.

Quote:
Originally Posted by Grant
Here is something I wrote about VVT theory on another forum....


"When VVT is off (the cam retards back to the normal position) the intake valve opens later, and CLOSES later too. There is less overlap when VVT is off. On the other hand, with VVT on, the extra overlap is good at mid-range to slightly high rpm because the previous exhaust pulses leaving the cylinder suck a waiting intake charge in, increasing the VE. If the intake valve opens sooner, (VVT on) more of the charge can be sucked in, increasing torque.

You cannot turn VVT on at low rpm though because the exhaust pulses don't leave the cylinder fast enough, and they get pulled back into the combustion chamber as the pistons is travelling down, diluting the intake charge and causing a terrible idle. If you've ever turned VVT on at idle you will see this. On my stock blacktop, the earliest I could use VVT was about 2400 rpm, below that there is more torque with it off.

At high rpm, you want to turn VVT off because the intake valve closing time is retarded as well. At high pistons speeds (high rpm) the intake charge "lags" a bit and can't follow the piston as well as lower rpm. Because of this it is beneficial for power to leave the intake valve open later, allowing more of the charge to keep coming in, even thought the piston is on its way back up at the start of the compression stroke.

The key thing I had to realize when studying all of this was that overlap is the time between the exhaust valve closing, and the intake valve opening (not intake valve closing, and exhaust valve opening)! This is key. I think from what you posted above you have this backwards, like I did in the past.

So more overlap means the exhaust valve is remaining open longer, and the intake valve opens sooner....with more vacuum being created by the exhaust heading down the header to suck in the intake charge. This effect is so strong, intake charge will be sucked in even before the piston is starting to come down at the beginning of the intake stroke. Said in other words, the intake charge can begin to enter the combustion chamber even while the engine is at the middle or end of the exhaust stroke. Add this to the intake charge that enters just due to the vacuum created by the piston during the intake stroke and you get more overall power. Some intake charge will make its way out the exhaust though, increasing emissions. This is why oem's can't run as much overlap as they'd like for performance.

My understanding is that overlap is good for high rpm as well, but if you only have so much overall duration to play with (i.e. stock cams), the 4ag prefers to have a late intake valve closing time over just having overlap at high rpm."

Richard Jemison
RJR Racing

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