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Discussion Starter #1
Hi,

I've read through numerous threads regarding the VVT function on Twin Spark engines, but can't find a definitive answer to a particular question that I have.

When converting a Twin Spark to run on twin carburettors and a modified 8-pole distributor (ie removing the Bosch Motronic ECU, etc), what needs to be done with the Twin Spark's VVT on the intake cam side?

For example...

(i) Do you you just leave the VVT assembly in place, but not connected? If so, will the Twin Spark engine still run/idle correctly, and as well as it can?

(ii) Do you remove the VVT assembly and blank off the opening in the cam cover? If so, does any modification need to be made to the intake cam assembly if the VVT is removed?

(iii) Is there some kind of mechanism which can be made up to allow the VVT to operate as if it is still being controlled by the Bosch Motronic ECU?

Any assistance/advice would be very much appreciated.

Cheers,


Nick
 

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Nick, check Brians build on this sub forum - probably the easiest way to do it.
 

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Discussion Starter #3
Nick, check Brians build on this sub forum - probably the easiest way to do it.
Hi Pancho,

Would you happen to have a link, or Brian's user name?

Cheers,


Nick
 

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Its under the Building a 200hp twinspark thread on this sub forum. By the way I saw a TS with FI in a stepnose at Monza in Melbourne this weekend. It fits with mods to the intake manifold and no inner fender body mods. Perhaps speak to Hugh if you're still going down that path.
 

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When I ran my TS on Webers I got myself an RPM window switch . I never got around to hooking it up.MSD Ignition 8969 MSD Digital RPM Window Switch It is the back of a drawer somewhere.

I connected a manual switch in the cabin and tried switching the VVT on and off the only difference I could find was the engine sound.

When I put in the computer I set the VVT up but again it never seemed to do much. It did not make the motor idle better it may have made it burn cleaner .

Now I have remover all of that VVT stuff as the cam I using now will not work with the extra advance .

You can see how I got rid of it here http://www.alfabb.com/bb/forums/engine-conversions/172068-building-200hp-twin-spark-5.html#post955530

It's not fancy but it works . I may make a cover if I get a milling machine on day.
 

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I dont know why I keep calling David 'Brian '. Sorry David.
 

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Discussion Starter #7
Its under the Building a 200hp twinspark thread on this sub forum. By the way I saw a TS with FI in a stepnose at Monza in Melbourne this weekend. It fits with mods to the intake manifold and no inner fender body mods. Perhaps speak to Hugh if you're still going down that path.
Hi Pancho,

Thanks for that... I'd pretty much arrived at the realisation that an unmodified Twin Spark with EFI won't fit into the unmodified engine bay of a stepnose 105... either the EFI intake or the engine bay needs to be tweaked.

However, as I've also installed a hanging pedal box from an Alfetta into my stepnose engine bay, that completely nobbles the use of the Twin Spark EFI intake... nevermind, I really like the idea of twin carbies in any case.

Cheers,


Nick
 

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Discussion Starter #8

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after working with intake VVT on a Toyoda.
it seem you want it off for idle and then turn it on after 2000-3000 (depending cams etc the best point cam move around a lot) the it switches off again when you start getting close to top end.

The only way to find the points to turn it on and back off is to do back to back dyno runs with in on and with it off and find the cross over points.
but they do make a big difference in mid/low torque.

but in the past I thought is was just switch over at x RPM but it turns out it should only be switched on in the middle of the RPM range.
And it hurts at the very bottom and very top of the RPM range.
 

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Discussion Starter #11
Hi slyalfa,

Thanks for the feedback. I did some reading on the MSD Digital RPM Window Switch, and it seems that you can set it so it would switch "on" the VVT solenoid at one RPM point, and then switch it "off" at another RPM point.

David, does that agree with your understanding of/experience with the MSD switch that you have?

If it does, then the MSD switch could be set to switch "on" the VVT solenoid at (for example) 2,000 RPM and then switch it "off" at (for example) 5,000 RPM. In other words, the VVT would be inactive below 2,000 RPM and above 5,000 RPM.

The main question I now have is (ie without access to a dyno) what high/low RPM trigger points I should use to define the VVT active rev range? Does anyone happen to know what these might be... perhaps based on the VVT active rev range on a stock Twin Spark engine???

Regards,


Nick
 

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The OEM's also can/do use load too this might be more of a smog thing.
but I think they keep it off at low loads.
but with out a dyno it might be hard. maybe a accelerometer thing might work. like the g-tech one that maps RPM and can download. do a few runs on the same spot of road say 5 runs with it on the whole way and 5 runs with it off the whole way and see if you can see where it helps and where it hurts. and then map it based on that.
 

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Discussion Starter #13
but with out a dyno it might be hard. maybe a accelerometer thing might work. like the g-tech one that maps RPM and can download. do a few runs on the same spot of road say 5 runs with it on the whole way and 5 runs with it off the whole way and see if you can see where it helps and where it hurts. and then map it based on that.
I'm assuming an accelermoter would be plugged into an ECU in order to collect the data I need?? However, as I have removed the stock Bosch ECU (ie switching to twin carbs) from my Twin Spark, this wouldn't work for me. I may just need to book some dyno time.

From what I've read elsewhere, the VVT should be activated somewhere between 1,000 and 2,000 RPM... but I can't find any reference to what RPM level (ie approaching the redline) the VVT should be deactivated.
 

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Things like the g-tech are standalone but some models do not log RPM and some do not let you download to a pc. I would say you need one that logs RPM and lets you download.

BTW it is common to get one on ebay use it to tune and then put it back on ebay so you get your money back.
 

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G'Day Nick,

It seems it gets more difficult, from the old Alfa75 Yahoo group I get this:

If you connect a multimeter to the vvt with a long extention lead so
that you can view the voltage in the cabin whilst you are driving you
will find that the vvt is activated whilst you have load on the engine
but is de-activated when traveling at constant speed and low demand.
When accelerating it will re-activate. So the vvt is switching on and
off all the time to meet the (load) requirements.
 

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Discussion Starter #16
G'Day Nick,

So the vvt is switching on and
off all the time to meet the (load) requirements.
G'day Craig,

Yeah, I read that the stock Alfa 75 Motronic ECU controls the VVT using both load (ie TPS) and RPMs.

After a twin carb conversion, I suspect the RPM window switch option is the only really viable option available. As slyalfa originally suggested, I think I'm going to have to "get me to a dyno" for final tuning and adjustment of the RPM window switch.

Another question someone may know the answer to... if the VVT comes on when the Twin Spark engine is under load and around the middle of the rev range (ie not high revs and not low revs), then I assume that advancing the cam via the VVT will improve performance... possibly at the cost of fuel economy and higher emmissions??

If I were to use only RPMs to activate the VVT (ie using an RPM window switch), then the cam would always be advanced around the middle rev range... and therefore always at the cost of fuel economy and higher emmissions.

However, I guess I could also install a dash switch to override/bypass the RPM window switch so that the VVT solenoid stays off when I'm just cruising along... I assume that this would be similar to the "power/economy" buttons present in many later model Japanese cars.

In other words, with the override switch "on", the RPM window switch would activate the VVT solenoid inside the defined rev range... obviously this is still a compromise compared to the stock Motronic ECU control of the VVT, but possibly the best compromise available.

Regards,


Nick
 

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I would guess that if you can't use the multitude of computer controlled gas saving turn off points, you should just turn it on off idle and use the cam everywhere but idle. 1500 - 1700 would be my choice.

Your manual switch idea can be added on top of that.
 

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Very handy info here folks - many thanks :) I'll be fittng my twin spark with carbs initially while I complete the megasquirt build and was wondering what to do with the VVT while on carbs.

Perhaps an electro-vacuum switch might be a better way to detect engine load / actuate the VVT, rather than straight revs ?

Cheers,
 

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Discussion Starter #19
Very handy info here folks - many thanks :) I'll be fittng my twin spark with carbs initially while I complete the megasquirt build and was wondering what to do with the VVT while on carbs.

Perhaps an electro-vacuum switch might be a better way to detect engine load / actuate the VVT, rather than straight revs ?

Cheers,
Hi Scott,

If you come up with a viable alternative to the RPM window switch for controlling the Twin Spark VVT, please let me know.

Regards,


Nick
 

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perhaps Richard Jemison (Alfar7 on here) has a solution to this Nick
 
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