EA81 supercharger revamp by Tweety

[Tweety]

Thanks again.

Bennie that option is most desirable. We'll have to get our heads together sometime in the future.

Gannon there are 3 vacuum lines in the manifold you mention for auto, and one not used for the recalibrated dizzy. so can use that. easy I've been reading up a lot on those bypass valves lately.

[Subydoug]

Yeah, but you could also hook the vac line up to the other side of the supercharger if you liked. Just means under boost it will hold the BOV closed. Operation should be the same though.

[El_Freddo]

Yeah, but you could also hook the vac line up to the other side of the supercharger if you liked. Just means under boost it will hold the BOV closed. Operation should be the same though.

My mind is trying to work out how you get boost if the BOV is open... I know I'm not a part of the boost "fraternity" but this doesn't seem right!

Sounds good Tony. I'm looking forward to a project like this now - it's also got me thinking of the day I get a brumby - that's still factory right??

Cheers

Bennie

[Subydoug]

Ok, Il try and talk thru it. Start at idle. The engine is choked by the throttle butterfly, so its pulling a vacuum inside everything after the throttle butterfly (manifold, supercharger, bov etc. Because of this large vacuum the bypass valve is open. (for those who don't know, inside a BOV there is a spring forcing a piston shut. When the vacuum is applied to the spring side of the BOV it pulls the piston up and opens up the port to let air flow back to the low pressure side of the SC). When Tweety feeds it the berries, the throttle butterfly opens up, the pressure rises dramatically inside the manifold, so the pressure of the spring overcomes the suction of the manifold and the bypass valve shuts. Now with the BOV shut, the butterfly wide open and the rpm increasing, manifold pressure starts boosting. When tweety lets off to look around at all the holdens he just left in the dust, the throttle butterfly shuts choking the engine's air supply. The engine then uses whats left of the air in the manifold until its drawing a vacuum again. This vacuum then pulls the Bypass valve open and equalises the pressures on both sides of the supercharger.
During Cruise? When a steady speed is established the engines air demand drops off dramatically as load comes off, so the manifold pressure drops again, allowing the bypass valve to re-open and equalise the pressure again. Only when Tweety encounters a hill or a gooner in a holden and the engine load goes back up does the bypass shut and allow all the fury to the wheels.

Make sence?

In truth if you hook up the vac signal for the bypass valve after the supercharger, it may be a little bit too much like an on off switch.

Regards

Doug

[Tweety]

ok, think I got that. Currently I have a turbo BOV mounted just after the SC towards the original intake manifold. It's vacuum line is not hooked up. I believe that vac line is for a blow thru system turbo (?). Anyway it was installed for backfire reasons and has proven to work very well indeed when that occured during retuning or when I got my spark leads wrong (oops...should have put those coloured zippers on eh Jonno)

I assume this bypass valve is a smaller version of a BOV with a very light spring in comparison but working in reverse. eg on vacuum it opens and with boost it closes.

So I assume this bypass valve (the $54 one) is sufficiently large enough in diameter to supply enough bypass air for this purpose, to equalise the air pressure at cruise? Wouldn't it be the case of the bigger the diameter of the bypass and hose connection the better?. I had water injection recently that I've taken off. I am only using 5pai and it was more trouble than its worth for my application with no economy improvement and there wasnt any detonation anyway. I could use the same 12mm hold in the carbie to SC manifold for the hose. I can tap into the aft manifold near the BOV for the bypass. wouldn't be a big job.

The result, I presume will be that at cruise say 100kph on light throttle the air/fuel mixture will go through the bypass valve as well as feed the engine. The mixture goes back through the SC in a cycle. Wont the pressure build up though because the sC is continuing to compress the air? And could one make this bypass setup adjustable so for example it operates at a throttle position equal at cruise at 105kph? After that it closes and economy is sacrificed?

This is very interesting.

this from Wiki. http://en.wikipedia.org/wiki/Blowoff_valve

Now thats the BOV like what I have. Thats a good diagram on wiki. . mine is (when on backfire) vents to the air filter box. As for bypass valve for draw thru set up I dont read on many google searches it use in these set ups.

[Subydoug]

you could try the 12mm hole but Id say your going to need at least a 35-40mm hole or your still going to get restriction thru the bypass which will allow the pressure in the manifold to rise slightly. Not a huge deal but something to be aware of.

Think of it like a fuel pump with a pressure regulator and return line. Essentially the same thing but with air. Remember, air flows where there is a pressure difference. There is always going to be a low pressure between the supercharger and the carby, caused by both the butterfly choking the engine slightly (no load) and because the supercharger is drawing air as well. This pressure will always be lower then the manifold pressure because of the supercharger. This is because it supplies more air then the engine can use, regardless of the butterfly position. Because its higher pressure (relative) in the manifold then between the SC->carby manifold the air flows thru the bypass, stopping the sc from pressurising the manifold.

So how does the bypass work then? Why doesn't it perpetually stay open? Because of the spring pushing it shut! When the throttle is depressed, the engine has a response time. During this time the engine is supplied with all the air it needs, so the pressure behind the throttle butterfly rises, usually to around about ~ -3psi. The bypass valve shuts because of the spring pressure and then your boosting .

Tweety, and anyone else who is interested in this sort of thing. An invaluable tool for learning about this sort of thing is a vacuum gauge. A simple manifold pressure reading can tell you so much about how an engine is running, what sort of load is on it, performance limitations of the engine. If its a good diagnostics gauge it can even indicate faults like blown HG's, worn valve guides, rich, lean, bad seats, poor seals ect ect.

Unfortunately nowadays its getting rare to find a mechanic who actually owns a vac gauge. I take mine everywhere I go in my car .

As far as controlling boost with speed, it works more on engine load, because air flow thru the engine is directly proportional to engine load. What will happen though is as you approach the higher speeds, maybe around 120-130km's in your trike, you will find that the engine load is sufficient to kick in the SC.


Regards

Doug

[dfoyl]

1. I have an EA81 long engine suitable for rebuilt sitting in the garage you can have for free
2. I have a complete EA82 SPFI kit partially rewired to suit an EA81 (from a USMB member, not generaldisorder) if you're interested at cost (which was mostly freight cost!).

[Tweety]

gone! will take up that offer and send you a message dfoyl thanks.

[tambox]

I used to run those bypass valves on my Turbo EA82's.
They reduce the load on the Turbo/SC bearings by letting the them spin freely when the butterfly is shut. ie gear changes or deceleration.
Allowing the turbo to spin freely, gives boost instantly on gear changes while accelerating, rather than having to spin it up again.
Without this, when the butterfly shuts, the turbo/sc is trying to ram air down a blocked pipe, which slows the turbo/sc down.

[Tweety]

This thread has evolved a bit. If I was to retain my engine and suck thru SC set up then the bypass valve would be installed on this set up. But now I'm looking at a fully recoing Deans ea81, purchasing a SC14 (not only is it a little larger than the SC12 but doesnt have a dip stick and can be mounted sideways) and utilising the ea82 spfi system.
The new engine will have new SC manifolds and I'll install a bypass valve on that. All this work to be done out of the trike on an engine stand.

There is a chance I'll obtain a spare auto and transaxle too but dont know yet. I'm tired of oil leaks lol

In the interim I might as well stay with what I've got and not worry about a bypass valve and put up with the extra fuel costs. Due to these circumstances it isnt worth my while to modify the manifolds again.

Been a good learning curve however.

[TOONGA]

If you are going EFI you may want to talk to RSR555 as he has a set of EA81 EFI heads.


TOONGA

[Tweety]

will do Toonga

[Subydoug]

The gear from Dfoyl is SPFI so standard heads should be fine .

Tweety, I only referred to the bypass idea as a Temporary fix/improvement. EFI with boost is the way to go, though that being said, I'd be Very surprised if it matches an ej22 for driveability.

Regards

Doug

[Tweety]

yes mate, I dont expect EJ22 performance. But as I've said before the trike doesnt have a place for engine mounts and I'm all set up for the EA81 so am happy to keep it all ea81 and improve it.

I'm not looking forward to installing the SPFI. there is a good thread in ultimate Subaru but like most threads it would cover the Brumby/Brat vehicle not a vehicle unassociated with Subaru eg the wiring loom install. A switch on the neutral position etc. Also a fuel return. I'll get that done when a fuel leak in the alloy tank is fixed by a welder soon.

So its going to be a challenge.

[steptoe]

Gosh, thinking a throttle postion sensor does not have to be on your throttle either - could be set up on its own spindle and extra cable somewhere convenient, 'just' hooked up and adjusted by its own cable - like a pushbike adjust on the outer casing to reflect true throttle condition. The EA81 and three plugger EA82 used a basic throttle off, throttle open and flat out positions ?? rather than the variable potentiometer style of later systems

[Tweety]

Jonno....this is where my mechanical/electronic knowledge runs out...you lost me after "Gosh" lol

[steptoe]

Glad I didn't call it a TPS then ! It is a black box mounted to the side of the throttle body of ancient Subie efi/mpfi/spfi. The throttle shaft extends into the unit and switches the 3? wires contacts inside depending on where the throttle is in relation to closed or wide open , almost telling ECU a bit more about what to do.

I was thinking if you need to stick to a basic throttle body that does not cater for TPS mounting and calibration - thinking along lines of say, a kickdown cable or cruise control cable that is along for the ride in throttle movements - a cable could be used to attach to a remotely fitted TPS switch

help ??

[Gannon]

Id stick with your throttle sensor on the throttle. Having a cable is asking for unreliable readings.

If you could grab some EA81 heads with the injector bosses and the electronic distributor to suit, then an EA82 3 plug ECU and loom would work on your EA81

[Tweety]

ok, bit clearer now but would be if I saw these items in the flesh. Like the SC12 draw thru set up, it was all a bit daunting until I really got into it...now its a piece of cake.

When this projects starts it will be full of pics taken like my VW-ea81 conversion then the SC install. A SC14 is a bit of an overkill but it can be mounted sideways directly above the original intake.

[El_Freddo]

SubyDoug, I know the principle of a BOV, just the vacuum pick up being beside it confused me - brain couldn't work out how that would work.

Nothing wrong with an "on/off" switch effect

Jonno....this is where my mechanical/electronic knowledge runs out...you lost me after "Gosh" lol

I'll pick up the slack next time we catch up You need a TPS with EFI.

Id stick with your throttle sensor on the throttle. Having a cable is asking for unreliable readings.

Agreed!

If you could grab some EA81 heads with the injector bosses and the electronic distributor to suit, then an EA82 3 plug ECU and loom would work on your EA81

I don't see why we couldn't go with the 4 plug EA82 ECU with the hotwire AFM - fit the dizzy and the injectors and you'd be pretty much set I reckon - it should handle a bit of boost without issue. Even better would be getting a turbo EFI hotwire system.

I just see the 4 plug being a little better than the 3 plug system, why else would subaru go with it?

Cheers

Bennie