could you do that to my dax next
Righty-ho!
I'm looking into putting a positive displacement supercharger onto a vauxhall XE16v engine.
I am looking to run around 20psi boost, where I can without mechanical limitations.....
That's where I'm a little unsure of how best to do things!
if I gear the blower to give me 20psi of inlet pressure, I'll then get that throughout the rev range, which is going to cause me problems at low
revs I would assume!
How do I deal with this, can I use a wastegate and operate this to give me the boost I need (I can drive the wastegate via the dta ecu)
Okay so....
could you do that to my dax next
A bypass valve would work, it would need to be quite big, maybee 30 - 50mm as it will need to flow a fair volume to reduce the boost. I would say a
throttle body fron a small car will work well.
Darren
would a 60mm wastegate do it?
Don't really want to get into actuators for a butterfly!
quote:
Originally posted by cd.thomson
Okay so....
could you do that to my dax next
quote:
Originally posted by NS Dev
quote:
Originally posted by cd.thomson
Okay so....
could you do that to my dax next
Its not for the locost! I's even madder than that!
surely it wont be running max boost all through the rev range?
It wont be spinning as fast 1k RPM as it will at 7k?
unless the pulleys on the engine and supercharger are the same diameter, wouldn't the boost rise in a sort of exponential graph way?
ie, if the supercharger pulley is half the diameter of the crank pulley, for example, if the engine spins at 1k, the charger goes at 2k,
engine--------charger
2k----------------4k
3k----------------6k
4k----------------8k
5k----------------10k
6k----------------12k
7k----------------14k
i think thats how it would work anyway, so the boost increases the faster the engine goes? so 20psi is your maximum boost, but only goes to that at
full speed?
flak monkey would know more about it i think. he's supercharging a duratec isn't he?
quote:
Originally posted by blakep82
unless the pulleys on the engine and supercharger are the same diameter, wouldn't the boost rise in a sort of exponential graph way?
the belt will effectively give a gear ratio, so charger speeds will be as you posted blake rising at a greater rate then revs.
You will only make peak boost at max revs which is why you get away with keeping a high CR.
EDIT, positive displacement, Doh! Mike is right, if the engine can consume say 3l of air for each cycle (150% VE jammed into 2l) and the charger
compresses 1.5l per rev and runs twice as fast, ignoring odd losses this relationship will stay the same throughout the rev range.....
I dont see why an external wastegate couldnt be made to work with the appropriate springing but I dont know how well they seal when closed (potential
boost leak)
What charger are you using? 20psi is a fair old whack!
IMO, work out your desired power, select a charger which will flow that amount of air below its peak safe rpm, gear it to run at max safe rpm at the
rev limit and leave boost control entirely until you see if you have a problem with knock or excessive ignition retard when mapping.
Edit again: as above for the power you want but as you noted and I misunderstood, run a wastgate or a throttle butterfly and a wastegate actuator from
an internally gated turbo to control the boost.
[Edited on 1/4/10 by alistairolsen]
quote:
Originally posted by Miks15
surely it wont be running max boost all through the rev range?
It wont be spinning as fast 1k RPM as it will at 7k?
quote:
Originally posted by alistairolsen
the belt will effectively give a gear ratio, so it wont be exponential, but will be as you posted blake rising at a greater rate then revs.
You will need a low pressure spring, as this is your lower boost limit. I.e. a wastegate set to 8 psi will not go lower that 8 psi as the gate will
close(and boost will go back up).
You may need to lap the valve to get a good seal, although leakage will be small in any case.
Darren
quote:
Originally posted by MikeRJ
quote:
Originally posted by alistairolsen
the belt will effectively give a gear ratio, so it wont be exponential, but will be as you posted blake rising at a greater rate then revs.
The supercharger will be moving a volume of air proportional to the speed of the engine, the engine is consuming air proportional to it's speed. e.g. imagine the engine consuming 500cc of air per revolution and the supercharger moving 1 litre of air, this ratio remains fixed.
The only simple way I can think of reducing boost is some kind of bypass valve. A young lab in a recent copy of PPC magazine had used an external turbo wastegate to do exactly this on a supercharged K series in his Imp. It was only running 4psi though I think.
quote:
Originally posted by turbodisplay
You will need a low pressure spring, as this is your lower boost limit. I.e. a wastegate set to 8 psi will not go lower that 8 psi as the gate will close(and boost will go back up).
You may need to lap the valve to get a good seal, although leakage will be small in any case.
Darren
quote:
Originally posted by NS Dev
(though 4 psi sounds a bit limp to me!! )
I dont really know owt, but isn't it not quite that simple? I mean at idle/low speeds there'll be leakage round the rotors etc. I.e. you
dont really get max boost at idle/low speeds anyway. I guess it would build to max pretty quick though, so with 20psi peak I can see why you might
want to reduce boost at low speeds!
Sort of sounds like you'd rather have the characteristics of a centrifugal blower or turbo, so why not use that in the first place? Again I dont
know owt, or know owt about your application either - I'm sure there's a reason you want the positive displacement blower or you
wouldn't be asking.
So that probably doesn't really help then
Liam
With an eaton sc on my old mx6 the boost I wanted was only 7psi, it was probably around 4psi at 900rpm (idle) with 7psi arriving by about 1000rpm and
was solid all the way to 7k, the later eatons had a recirc valve integrated into them so no need for external ways to get rid of unwanted pressure,
mine was an earlier one so I had to plumb my own in, basically a normal recirc valve although I seem to remember slightly different as it had to be
open while cruising as well as decelerating, all in all they are very easy to set up (must be if I managed ).
You can see the acuator for the new chargers on the bottom right of the pic, same arrangement as turbo wastegates:
Edit: From Eatons site:
Do the Superchargers provide boost at all times?
No. Under cruising conditions, the compressed air from the supercharger is bypassed, and is recirculated in the supercharger, improving fuel
efficiency. Under acceleration, the bypass is closed, and the “boosted” air is sent into the engine to provide increased response and power
[Edited on 1-4-10 by flibble]
Eaton/rootes blowers are massively inefficient at high boost levels (over 10psi) and you are far, far better with a centrifugal one.
The reason for this is the amount of 'mashing' you get from the rotors which heats the inlet charge much, much more than a centrifugal
one.
You can run constant boost and limit it with an external wastegate, which can have electronic boost control controlling the wastegate to dump any
boost over a certain level. Then its all electronic and doesnt rely on any vac controls or anything.
I would seriously think about running a C38 rotrex or similar though for that sort of boost or you'll end up with more detontation issues than
anything else I would suggest.
Have a read of Supercharged! by Corky Bell, its the only book you needs. Coveres both types of supercharger and gives their merits and weakenesses and
a whole host of other useful info.
ETA yes you should run a bypass valve, or preferably a BOV which will dump the boost at idle and light cruise which keeps close to NA MPG figures on
the road.
David
[Edited on 1/4/10 by flak monkey]
Agreed with the inefficiency and heat at high boost, also with corkys book being excellent although i'd have to disagree with a BOV over bypass
valve, I tried running with it ventining to atmosphere and the noise is horrific, even at idle
Edit: Having a brain fart about the high boost inefficiency now, wouldn't a big charger with a relatively low rpm be ok? If it's deigned to
supply say a 4.0L engine at 10psi wouldn't it be just as happy pumping the same volume into a 2.0L but resulting in higher pressure?
[Edited on 1-4-10 by flibble]
................exackerly!
The "plan" and I use that term VERY loosely, is to use an eaton M90 supercharger on a 2 ltr 16v vauxhall engine, with 8.5:1 compression
ratio.
I DON'T want a centrifugal blower, we already have a 500hp turbo 16v vauxhall and it would be no good at all for what I need it for!
It won't be used on the road so economy etc is of no issue whatsoever.
The idea is to use a big eaton blower, spun nice and slowly (around 1:1 with the engine) so it stays reasonably efficient (ok pushing things at 20psi
but not ridiculous)
As I say, I know nothing about supercharging though, and I already realise that one of the issues I overlooked is that I planned to throttle the inlet
on the blower, doing it the "old fashioned way" which should resolve the idle issue anyway.
On those "internal bypass" eaton blowers, do they throttle the inlets or outlets in a typical OEM setup?
I think on the jags (where I plan to liberate the blower from! ) they throttle the inlet don't they?
I now don't know what the issues will be when the throttle is cracked open from idle, I assume due to high boost/low rpm/low piston speed, there
will be a big det. risk, so lots of ignition retard and excess fuel, but I have no idea!
Everything I have read/heard talks about high boost only at high rpm and piston speeds..............
Pretty obviously (though will be blue sky thinking I'm sure! ) I'm trying to "simulate" a big engine's torque delivery with a
smallish and lightish unit.
I know from the vauxhall turbo that the std pistons and rods are quite happy at 425hp and 380nm of torque, but its what happens at lower rpm when the
boost is far higher than it would be in a similar turbo setup............
[Edited on 2/4/10 by NS Dev]
quote:
On those "internal bypass" eaton blowers, do they throttle the inlets or outlets in a typical OEM setup
no I mean the actual throttle, as in operated by the driver's foot.
Ahhh, doh! Not sure on that count then I had throttle on the outlet, I'd guess they normally throttle the inlet but that's based on
bugger all an guesswork
If you are running without a bypass you will need to put the throttle on the intake side of the supercharger.
If you run with a valve you can throttle on the outlet side if you wish.
The main problem with having lots of boost at low RPM is controlling knock/detonation as it much harder to overcome at low RPM than at high.
Certainly sounds like an interesting project anyway! 
Yea, was worried about the detonation issues!
I am still not clear on the facts there though....
one of my big questions!
Why, on a supercharged engine, running the same boost throughout the rpm range, will it detonate worst at lowest rpm?
Indeed does that happen, or not?
My gut feel is that it will, and that its because the piston speed will obviously be low, yet the flame front travel speed will be much the same as at
higher rpm, therefore peak pressure will build early and cause det....................
Is that anything like right?
If so, presumably 8.5:1 CR, plus a gobload of excess fuel "should" help?
On the plus side, just talked to my mate who has the big turbo xe engine, and I don't need 20psi, he made 425hp on 15psi last time on the rolling
road, using the same cams and pistons, valves etc etc that I will be using.
Yes, that thinking is correct Nat. Essentially at higher rpm you have less chance of having time for detonation to happen. Hence why most detonation
is experienced at cruising rpm.
Low compression, lots of fuel (you'll want to run around 12-12.3:1 AFR under boost anyway - any more is just wasting fuel.) You'll also need
to back the timing off, you can run negative advance at idle if you need to. If none of that solve it then you may need to look into water injection
as well.
David
OK sounds like a plan!
Will seek to avoid water injection as I'm trying to keep the installation as simple as possible. Being for a grasstrack car its going to get
covered in mud and in a total mess so the less to go wrong the better!
The 15psi news is good in that at lower revs the boost should be more sensible anyway.
quote:
Originally posted by NS Dev
Why, on a supercharged engine, running the same boost throughout the rpm range, will it detonate worst at lowest rpm?
Indeed does that happen, or not?
I do need lots of torque low down, 75mph 1st gear and huge startline grip on autograss tyres!
Turbos on autograss cars never seem to work, even wrc spec engines with anti-lag always get beaten by the decent v8's and the twin bike cars.
quote:
Originally posted by alistairolsen
quote:
Originally posted by NS Dev
Why, on a supercharged engine, running the same boost throughout the rpm range, will it detonate worst at lowest rpm?
Indeed does that happen, or not?
Becuase the relatively low speeds involved mean that VE is very high. At high rpm you are using boost to make up for low VE and to overcome it. At low rpm you get a much greater cylinder fill for a given pressure.
Also at low rpm the piston speed is low, so it hangs around at the top of the bore for much longer. Det is related to PCP(peak cylinder pressure( and this is higher at low speeds for a given fill.
try to imagine in your head the relationship to both time and angle and how this is affected by ignition advance.
Certainly be an interesting project, its not done a great deal in the vaux world. IMO however unless you need lots of towrque at 1500rpm Id still be going for a turbo.
I dont mean the VE of the FI unit, I mean in terms of an N/A engine.
if you take an xe and try to feed in 1 bar, you will get twice as much air into the cylinder as you would do at atmospheric pressure....
but:
at 2000rpm the n/a engine will have a VE of say 90% so now you have 180% of the cylinder volume under boost.
If youre doing 7k, the VE is waaay off, probably as low as 40% and you now have 80% or the basic cylinder volume under boost.
This is why the honda boys can run 20psi on standard compression, because they only run high boost at high rpm to make up for the massive drop in VE,
theyre not overfilling cylinders at very low RPM.
Big boost at low rpm has always been difficult and always will be. for cheap bottom end strength Im inclined to suggest the saab unit as its
significantly better than the vauxhall one!
quote:
Originally posted by NS Dev
Agreed on the piston speed, but not sure about VE, the usual rules go out of the window when you have forced induction/scavenging
Nat,
For the pressures you're talking, an intercooler would be an essential.
If it's for grasstrack, can you run on methanol?
This can cool the charge to the point of icing, and solves a lot of the detonation problems. Add a little nitro to stabilise things, and you'll
have a winner.
Otherwise, a mister spraying the intercooler will help a lot.
Cheers,
Nev.
quote:
Originally posted by alistairolsen
Im inclined to suggest the saab unit as its significantly better than the vauxhall one!
Saab T5 - 9000 engine...
Good for 450bhp+ on stock internals.
http://www.ericsaabsite.com/
If working on a low budget, big bhp can be had for not a lot of bucks.
As you say Nat, lag may be an issue for a grass car though....
You will see my T7 Saab soon....
