Menu toggle

Supercharger project

Sounds fun !

Does the BOV vent to air ? Often causes fuelling to go off when lifting off at higher RPM's as the metered air is lost from the system. Common for Subaru owners to fit a VTA instead of the factory recirc valve only to find that the car becomes hesitant & 'jerky'

Also, BOV's can often leak air as the boost rises, this could be limiting your boost ?.
 
ORIGINAL: Peter Empson

Hi Gary, my setup uses at a BoV, although I read somewhere that this isn't a necessity with my SC.
Click to expand...

I guess that makes sense as the purpose of a BOV or recirc valve is to prevent the back pressure from the manifold stopping the turbo compressor from free-wheeling in between gearchanges. With a supercharger the compressor will contiue to be driven irrespective of the direction of the airflow through it and as soon as you got back on throttle the airflow would re-establish itself. In fact i'd imagine you'd be better off not running a BOV as you loose that volume of air from the manifold which would take a half a second or so to pump back into the manifold after the gearchage.
 
Certainly with a next to useless VTA bypass valve. A re-circulating type may help, but youre never going to stall a supercharger...
 
My BoV is the recirculating type, I can't hear it working from inside the car but it seems to do the job. Coming on and off the throttle is almost as instantaneous as before.

I couldn't resist the thought of more boost (yes it is contagious) so in the morning I tightened the belt as much as I dared. Sadly it hasn't made any difference, it's not I'm unhappy with what I've got but I would just like to make sure it's running at its optimum first. I'll have a checkover for boost leaks as a matter of course. I could always be tempted to fit a smaller pulley to up the boost if I feel the need [;)]

The first track session went better than expected, the car was definately quicker everywhere than before (and I was able to drive it home too [:D]). A few issues arose but nothing major:

-The dipstick is being forced out, the first time it did it it made a right mess of the engine bay [:mad:] the subsequent times I was lucky that it had just risen a small amount when I checked. I seem to remember Rick runs his car with the oil filler cap loose, I wonder if it's for this reason?

-Something odd is happening to the boost pressure, Paul noticed that in 5th the boost is not reaching the same peak as in 4th, I'd noticed how the power dropped off but had never been able to watch the boost gauge whilst going at these sorts of speeds. I really can't understand this, any pointers would be a great help.

-Engine temps are getting a bit too high (I was warned this would be a problem!), I think it's the coolant and not the oil that's causing it. I think it's the warm air coming off the intercooler before going onto the radiator that is causing it.

I suspect this will be the most difficult problem to solve, but thankfully I was able to get quite a few laps in before having to back off (and the temps drop almost immediately, which I think confirms this theory). I do only have a single fan fitted and not the twin setup of the S2, so I might consider changing it. I'd rather not replace the IC (I'm rather doubtful that a 951 one with fit under my badge panel anyway). I already have a larger radiator fitted, so I might look at the general airflow in and out of the radiator to see if there's any gains that can be made there.
 
Water wetter may make a worthwhile difference. I cant promise that you wont ultimately get a build-up of polymer with continued use though. Its certainly upset the water cooled CPU geeks.....

The real solution wont come in a bottle though. Id suggest measuring your inlet temperature, but Id guess that a SC will run cooler per PSI than a TC does, which means that cooling the charge isnt going to be as critical. If this is the case then the best solution will be to utilise a 951 intercooler. Ive seen a number of 931s fitted with such a part, so Im confident that itll fit a bolt on badge panel car. You should also look at the radiator and intercooler and try and establish where the air is going. Just because your car has a gaping snout doesnt mean that air is going in, or that its going where you need it to go. Flies or dirt can be a good indicator here. Even if the whole surface is covered, you may still be able to increase the volume/benefit by carefully masking and directing the available flow.

With regards to your crank pressure: if you dont want to drastically alter your standard breather/seperator, then Id suggest lock-wiring the dipstic in place...

Im still thinking about the drop in boost pressure in fifth gear...[:D]
 
It was nice to see it up and running Peter and even better to get a ride in it round Silverstone [:D]

My thoughts on the problemettes are that you definatley need some sort of splitter and undertray arrangement. I think a lot of the air is hitting the front of the car and then going underneath rather than through the intercooler and then radiator, maybe even some simple ducting around the edges of the radiator and IC. This would stop air leaking out from behind the IC and not even reaching the radiator.

I'd been thinking about the boost dropoff too. Bearing in mind that the speed of the supercharger is controlled by the speed of the crankshaft, could it be that when you change to 5th gear the engine speed obviously drops and therefore the speed on the charger does too. The main straight is not long enough to see the red line in 5th but obviously you were hitting high revs in 4th hence the greater amount of boost? I admit I wasn't paying attention to the amount of boost when lower down in the rev range.

I can't think how the crankcase is being pressurised (unless there is leakage through the piston rings [:eek:], but that would have to be pretty bad to blow out the dipstick )
 
Thanks for all the suggestions, I was wondering if there would be any benefit in fitting fans behind the intercooler, there's a large gap to the radiator so this might help to maintain the air speed.

I'm sure I can make the current setup work more effeciently, I'll definately try sealing off the underside between the two and see what happens.

Paul, that's a good point about the boost pressure! It does feel flat as I drop into 5th but there is quite a large rpm drop so it could well explain it. I'll have to have to do another run and note the boost at each rpm. I wonder if the conservative timing is being masked to some extent in the lower gears and only when into fifth does this become more obvious.
 
Could it be the high pressure in the intake manifold blowing the dipstick out. Don't know about the s2 engine but the lux has a breather pipe from crank case to intake manifold. With your supercharged setup you probably have high pressure in your intake manifold at certain points which might result in high pressure in the crank case if you have a breather pipe connecting them. If you do have a breather pipe you could disconnect it from the intake and feed it into a catch tank.

Edit: I guess this is what 944man is talking about.
 
ORIGINAL: sawood12

the purpose of a BOV or recirc valve is to prevent the back pressure from the manifold stopping the turbo compressor from free-wheeling in between gearchanges.
Click to expand...

Not quite true. The main reason is to maintain flow through the compressor above the surge line. When a compressor is spinning and the flow reduces below the surge line (ie closing in the throttle), the discharge pressure of the compressor reduces, this reduces the flow, which reduces the pressure and flow / pressure collapses. The compressor then exits the stall, flow re-establishes but below the surge line and the cycle repeats itself.

This oscilating flow can cause damage to the compressor.

Some competion cars allow the compressor to enter surge/stall and its this noise you can hear in cars like the Audi Sport Quattro, not a BOV's so beloved of Subaru owners :)

In fact, a centrifugal compressor does less work the lower the flow (ie its not shifting a mass of air), so in theory a turbo will spin for longer against a closed throttle with no BOV than with one.

ORIGINAL: 944 man

Certainly with a next to useless VTA bypass valve. A re-circulating type may help, but youre never going to stall a supercharger...
Click to expand...

It's not that kind of stalling. It stalls aerodynamically and does no useful work but remains spinning.
 
Peter

For your intercooler installation there are three rules:

1. Airflow and ducting
2. Airflow and ducting
3. Airflow and ducting

From my experience it is critical that the intercooler has a direct air feed to its face and that the air cannot take the easy route around the IC rather than through it. It will always take the easy route. This means the IC will not be working efficiently and if it has heat soaked, may even be acting as an inter heater! The second thing to remember is that as soon as you stick an IC in front of the rad, you have taken the direct airflow away from the rad. The IC needs to either be right up close to the rad so the air is forced to go through both, and that the combined thickness is not more than 4 - 5 inches, or the rad and the IC need to have their own air supply. You can use fans but they are more of a band aid to a poorly designed setup. Getting the IC and the rad to work well together is one of the biggest challenges to any forced induction setup. Once you have this right you have a reliable setup but without it you will never make sustained and reliable power at the track or under repeated hard use. Take note that huge openings at the front are not necessary, it is preferable that the entry size of the duct is as small as 1/3 the size of the face of the IC (just look at the 951 duct opening size), this causes the air to be forced to go through the IC as it is harder to turn round and go back with a small opening, kind of like a ram air effect. If you have a big opening just split it into 2 wide but thin ducts, one for the rad and one for the IC, if you can make this work.
 
Thanks all, very helpful replies.
I think I'll try a breather tank and drill out the AOS, the dipstick had popped out on the way home last night too, so I was greeted with rather more to clean up than I was expecting [:mad:].

Nick, that's a great help and confirms what I was thinking, there's a large gap between the IC and rad and I'm sure the air is stalling between the two (or escaping under the rad after it comes out of the intercooler). I'll have a good look to see if I can seal off any air routes as a temporary solution and will investigate the various intercooler options over the next few weeks. I should add that I think the temps were below a dangerous level, my car normally runs at the first marker and yesterday ran at just over half way, I must have stayed out for a good few laps by that point, but it's clear indication that something should be done, especially if/when I go for higher levels of boost.
 
Just to echo what the others have said, it sounds like your intercooler cannot cope with sustained boost and that air is possibly going around it too. My Supra never used to pull that well along the second half of Bruntingthorpe's runway until upon investigation it turned out the oem intercooler was in very poor condition on the rear face and that the airtight joins in the ducting on both faces of the intercooler had perished over the years. A new direct replacement intercooler and a refurb of the ducting transformed the car in the long gears and it pulled solidly to the speed traps.

What I noticed from my ricer days is that many people completely overlooked what happens to the air after it has passed through the intercooler, as Simon suggests you can look for evidence of this with dust/dirt (the M25 between j16-17 is very dusty in lane 1 at the moment if it helps [:D])

I assume your supercharger vents surplass boost via a wastegate once it reaches a disired pressure?

Excuse my spelling, my spell check is on the fritz.
 
Peter, I have just been reading through my old emails from John at Vittesse when he was supporting me setting up the map for my Piggyback. Here is a direct quote that may well help you in determining your Air Fuel Ratio:

When you install the PB, it's normal that it runs a bit leaner as the MAF signal goes through the PB.
Your goal is 12.5:1 up to 7psi boost, the 11.8:1AFR 7+psi..

As you are only hitting a max of 6psi then it looks like 12.5:1 could be the one to aim for as you previously said [:)]
 
Superb work Peter, congratulations.
How much psi do you think you can run safely with the original S2 pistons?
 
Thanks all [:D]

Gareth, my IC is a snug fit with the opening, so I dont think there's much chance of the air going around it (although it might be going under the car as the opening is only just over the bottom of the bumper, the opening is massive (as is the IC) so Nick's suggesting of dividing it is a possibility.

There's no wastegate as the engine sees all the boost it can generate (which is only at the highest rpm's).

Paul, it did smell a bit rich, didn't it [:D] I'm glad I made the mixture a bit richer for its first run, I'll lean it down a notch now and get it back into the 12's. Might be some more power to be had as well.

Thom, I'm really not sure how much is safe, there are quite a few 7psi setups out there, so based on that I'd say it's a fairly safe level. 9M ran a lot more on the 968 (14psi I believe without changing the compression), but I think this was very borderline given some of the tales I've heard.

The bottom end I will have built will almost certainly be designed to be safe with a bit more boost, but to be honest I'm happy with the current setup, it has good torque and the power is manageable so who knows. The lure of running a serious amount of boost is tempting, but I fear the lower compression ratio will kill performance at low rpm's.
 
You need to drive my 3.2 when I get it back before you make your decision. It has the lower compression pistons but I think you'll be very happy with the low rpm response. Ok I know its got slightly more cc's then your setup but with your supercharger providing even a few psi at low rpm it should more than make up for it (in theory [;)] )
 

ORIGINAL: Diver944

You need to drive my 3.2 when I get it back
Click to expand...
:p Now that's an offer I can't refuse (although I'm not sure I would want the responsibility to even park it let alone drive it!).

I have to admit I'm really excited by all the possibilities at the moment, I can see several routes to financial ruin and I can't decide how many I'm going to take [;)]
 
John at Vitesse has been a tremendous help with this whole project, his after sales service is peerless.

I mentioned in passing about the dipstick problem and he suggested "I would drill out the AOS opening, have a catch can that is routed back to in front of the Supercharger. When on boost, the SC will siphon out the pressure from the Crankcase. "

I hadn't thought of routing it to in front of the SC but it now makes perfect sense, so that's one problem potentially solved!
 
You must log in or register to reply here.

Posts made and opinions expressed are those of the individual forum members

Use of the Forum is subject to the Terms and Conditions

Disclaimer

The opinions expressed on this site are not necessarily those of the Club, who shall have no liability in respect of them or the accuracy of the content. The Club assumes no responsibility for any effects arising from errors or omissions.

Porsche Club Great Britain gives no warranties, guarantees or assurances and makes no representations or recommendations regarding any goods or services advertised on this site. It is the responsibility of visitors to satisfy themselves that goods and/or services supplied by any advertiser are bona fide and in no instance can the Porsche Club Great Britain be held responsible.

When responding to advertisements please ensure that you satisfy yourself of any applicable call charges on numbers not prefixed by usual "landline" STD Codes. Information can be obtained from the operator or the white pages. Before giving out ANY information regarding cars, or any other items for sale, please satisfy yourself that any potential purchaser is bona fide.

Directors of the Board of Porsche Club GB, Club Office Staff, Register Secretaries and Regional Organisers are often requested by Club members to provide information on matters connected with their cars and other matters referred to in the Club Rules. Such information, advice and assistance provided by such persons is given in good faith and is based on the personal experience and knowledge of the individual concerned.

Neither Porsche Club GB, nor any of the aforementioned, shall be under any liability in respect of any such information, advice or assistance given to members. Members are advised to consult qualified specialists for information, advice and assistance on matters connected with their cars at all times.

Porsche
Lockton
Pirelli
Officially approved
Oracle Finance
WEX Esso
Collecting Cars
Back
Top