Super Charger Boost Control

[speedtripledan]

It's using a mini bypass valve to recirculate boost controlled by plenum vacuum. Trouble is when trying to cruise at 70ish the valve is opening and closing on the slightest increase on engine load thus boosting on a near closed throttle any ideas on how to control this ?? As it really awkward to drive with at legal speeds on a dual carrigeway. Ive been reading about boost controls and dump valves but not really sure how they would work with a charger as a pose to a turbo.

[Longboarder]

Without knowing the design of your system it's difficult to say. Position of components, position of bypass and where the bypass vacuum comes from. Can you draw up something like the pic below (fairly standard Rootes setup) to give an idea of yours.

 

Nigel

Edited April 23, 2017 by Longboarder

[richardm6994]

Nigel, set up is as per this diagram being the bypass is located on the supercharger intake and the vacccum feed for the BPV come from the engine plenum.

The only difference with Dan's set up is that the BPV recirculates the air before the intercooler and not after the intercooler as shown;

[richardm6994]

BPV is normally closed and only opens up under vacuum from the plenum

Edited April 23, 2017 by steamer

[Longboarder]

The two throttlebodies have surprised me. Haven't seen that before. Will have to have a think about the effect of the TB closest to the plenum.

Let me ramble a bit! I suspected the difficulty in light throttle was taking the vacuum for the BPV from the plenum which would tend to have more rapid variation into negative pressure due to low local volume and being close to the bit doing the sucking. The effect of having just the first inlet TB is to slow down pressure changes (including in the plenum) downstream of the supercharger giving a slower response time to throttle change, both on and off power. A loss in snap throttle and low revs pressure build which hits performance slightly. But perhaps gives a smoother drive?

Is the second TB to mitigate the slower pressure build allowing a partially pressurised system up to the second TB even when off the throttle. I don't think Rootes type chargers need that as their output is good even at low revs. I definitely think the problem of sensitive throttle at cruise 70 is due to the second TB making the plenum a small rapid changing pressure space. With a single TB I would put the vacuum takeoff for the BPV just after the supercharger. With your setup I don't know where to put it.

Experimentally, I wonder what would happen if the second TB was locked open and Dan just ran on TB number one?

 

Nigel

 

ps the off/on effect of this ticklish throttle may be putting a high/low alternating load on the belt

Edited April 23, 2017 by Longboarder

[richardm6994]

Ramble away....and allow me to join....Running 2 throttle bodies is something I've always done on my setups.

It all started with Terry's charger project (I think Terry had seen the idea somewhere). Anyway, when Terry's sr2 was rollingroaded, mike at roadrunner racing had got another supercharged sr2 in for rolling road as well, except this other one had only 1 TB.

Everyone at roadrunner made good comments about how smooth the power delivery was from Terry's sr2 in contrast to the other sr2 and they surmised that the 2 TB setup was to thank.

 

Fast forward a few more months and my next supercharger project (zetec) with 2 TBs. once again, the feedback from the rolling road (different venue - I think Baileyperformance) was that the power curve was very smooth.

 

I've never done a single TB charger setup and have just continued with the 2TB arrangement as they seem to work and the feedback from 2 different rolling roads (and customers) was that it seems to work very well - I'm not entirely sure why, I've just always ran with it.

 

You mentioned taking the vacuum from the engine plenum creating the sensitive BPV, this is strong in my mind as well. The set-up of going from engine plenum for BPV vac was nicked from the mx5 forums as they seem to do this a lot - and it was done on my last 2 chargers without causing any real headaches.

 

I think the problem with Dans is that at 70 cruising, due to gearing, throttle position, engine rpm etc....you hit an uncomfortable 'flappy' point of on/off boost. One idea I've pondered to help in Dan's scenario would be to put a restrictor (small valve) on the vac pipe so that it dampens the vacuum to the bypass.

[Longboarder]

Nice rambling. Always interested in new stuff and trying to understand why it works, how it works, pros and cons. As this is a tried and previously successful (x2) setup the essential design is proven. (I wonder where Terry got the original info from.) You have also pondered damping the BPV actuation. Restrictor and even a little plenum (the bigger, the slower) in the line would work. An easy mod!

MX5s will almost certainly be using single TB taking BPV vacuum from just downstream of the charger and will give a slower response than from the plenum. This might just suit their cars. I would still be looking at perhaps subtle differences in these three projects to see if an explanation is there but you're probably right in thinking Dan's car is just unluckily sensitive at 70.

Very interesting.

 

Nigel

[richardm6994]

A bit of googling of duel TB Supercharger's send you to the mx5 forums on most of the search results......seems they are big fans of the duel TBs when supercharging - id assume that's where Terry had seen set up from in the first place.

[Tazzzzman1]

You assume correctly Rich

[speedtripledan]

I have machined up a restricter that I have placed in the vac line, taking the pipe bore from 4.5mm to 2mm. On quick run out I found it has altered the 70mph problem it's definitely improved but not anywhere near enough, I also do wonder if the charger may be trying to stall under these conditions due to odd noises only present under these conditions. I.m also getting issues with all my boost pipes trying to separate, suggesting back pressure caused by the closed 2nd tb and the bpv closing. I.m going to try 2 things taking the vac line off the other side of the tb and also running with 2nd tb wide open. It's a minor problem but it's got to be irradiated as the car isn't nice to drive at normal pace, it actually behaves like my 45dcoes did when the chokes were to big wot it went like stink 70mph cruise was aweful

[Guest old_timbo]

How are the two throttles balanced? If the first opens before the second you could get the situation on light throttle openings where significant boost is being produced but the bypass is not open. So small throttle movements would cause large boost changes in the plenum. The bypass has quite a strong spring in it so a significant manifold depression is required before it starts opening and then it opens progressively as the depression increases. i.e. it does not snap fully open at the first hint of vacuum.

My supercharger setup uses one throttle on the plenum after the SC. I soon discovered that at 70mph i.e. barely open throttle, there was not enough vacuum to open the bypass so pressure could build up until a pipe would blow off (quite a bang!). My solution was to add a turbo blow-off valve in parallel to the mini bypass valve. This acts a pressure relief valve when required, but also opens before the mini valve. The mini valve is still required to give the large flow capacity.

[Tazzzzman1]

Dan I had mine adjusted so 1st TB was open 1/2" approx when 2nd one was closed ( no throttle)...

[richardm6994]

I.m going to try 2 things taking the vac line off the other side of the tb and also running with 2nd tb wide open.

 

These 2 things need to be done together for it to potentially work - in essence you'll be increasing the volume of throttled air thus delaying / dampening the BPV actions.

 

Just a couple of things to think about;

1) watch out for your boost pipes collapsing under vacuum - I seem to remember on Terry's set-up, when first set up, the 2nd TB open before the 1st and so you could see the boost pipes collapsing under idle vacuum - that's why Terry has mentioned the the 1st TB needed to be cracked open 1/2" on his set-up.

 

Based on what you're planning, I would just run the small BPV vac pipe back to it's original 'as-supplied' format (the diaphragm pipe connected to the little brass take-off just below it).

[Longboarder]

Had some very interesting reads on Miata dualTB.

Seems to be a solution for problems encountered by those who have a single intake TB setup like in my first diagram. The problems are mostly to do with getting a stable idle, avoiding snap close throttle stall and a stumble on snap opening of throttle. Not clear if the snap opening stumble is a lean stumble or just boost lag. The idea of dual TB seems to be to maintain some boost in the whole system except for the manifold even when the throttle is closed.

DTB have definite pros and cons., the cons having further potential solutions. All gets a bit more complicated. Most critical area seems to be the TB cable link and relationship between the TBs. Advice is to set the intake TB butterfly so it cannot close completely but the manifold TB so it can close completely. This relationship seems critical and takes some experiment to get the settings right.

Vacuum drawn from the plenum leads to rapid operation of the BPV and some have a restrictor in this vacuum line to slightly slow it down. However that can lead to too slow an operation of the BPV on snap throttle closure, high boost pressure between the charger and the second TB, high load on the charger and belt squeal and thus have to take the vacuum restrictor out. Solve one problem area and introduce a problem elsewhere. Some also talk of fitting an idle bypass valve round the manifold TB to improve idle.

No doubt those who have DTB fitted and problem free sing its praises. There are lots finding it difficult to set up. As I said the mechanical link and relationship between the two TBs seems to be absolutely critical.

Good luck Dan.

 

Nigel

[Snapperpaul]

Reading another supercharger build there was mention of spending time setting up the bov with different springs to achieve desired results.

If there is an adjustable bov on the market this would seem the next step