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David Allison
United Kingdom
665 Posts |
 Posted - 02/07/2008 : 12:05:23
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Peter - my sister has changed the blower to allow her to run on petrol - the Chinn blower fitted originally was a 2000 cc capacity giving around 25-30 psi boost at engine speed.
The Volumex only gives around 10-15 psi boost at engine speed and will run on petrol without damage.
The horsepower drop is significant though - my NA can now keep up with its younger sister (it never could before)!
Richard - you are quite right - it was more an anorak type observation! |
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Peter Green
United Kingdom
1682 Posts |
Posted - 02/07/2008 : 14:01:27
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David,
I thought that was the reason the Chinn blower was changed for a Volumex. Do you know if Derek Chinn makes a blower with a similar swept volume as a Volumex which would be suitable for use on a 1087cc or 1286cc engine running on petrol?
Peter.
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David Allison
United Kingdom
665 Posts |
Posted - 02/07/2008 : 14:06:41
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Dereck does indeed make a blower with a 750 - 1100 - 1500 - 2000 cc capacity (indeed I think all the way up to 10,000 cc although the larger ones come in from the USA).
It is also worth mentioning about the Chinn type blower that the internal workings are quite different.
Instead of the figure 8 style rotors - you have two 4 bladed crosses - these are able to rotate faster with less pressure loss than the two blade rotors in the conventional Roots type blowers.
The design of this type of blower is a cross between the earlier type Roots and the later Saurer type cabin blowers.
Saurers are similar internally to a screw compressor.
Later again we get the Sprintex (this is really a screw compressor) and these later blowers are able to run higher shaft speeds and boost pressures with minimal pressure losses.
The "establised" rule that a Roots blower can not run at more than 15 psi boost is actually not the case - although it is true that a 2 bladed Roots blower will become inefficient if run beyond a shaft speed of 7500 rpm.
The modern thought is to use bigger blower and keep the shaft speed down and you not only get the higher boost but dont lose as much efficiency.
Dereck Chinn's expertise is in modern Drag Bikes.
Running 50-60 psi (and more boost) at shaft speeds of around 8000 rpm on engines revving to 18,000 rpm.
His advice was to increase the size of the blower and keep the shaft speed to a minimum to get the boost up and produce more power.
The Chinn blower worked very well for around 14 years on the car running very reliably - untill worn out shaft seals inside the inner casing spelled the end for now of a high boosted 1100 cc MG racing in the UK.
My sister is a very keen driver (if lacking in confidence) but the patience required to run a "race only" car is extreme.
The seal failure only manifested itself on full throttle and to do that you have to be on the race circuit - finding a problem is an expensive and embarassing weekend away.
When it ran well though the car was a monster - the power delivery used to be quite savage and at Silverstone in 2007 in the wet (not having driven the car for a couple of years either) gave Jane the most awful fright.
This is why we took the decision at the end of last year to convert the car back to road use and allow Jane some valuable experience of road driving to add to her confidence.
The VSCC 1 hour race at Donnington and around 200 miles of road driving too have helped.
At Silverstone the car with less horsepower (it now has less than half) than its last full season in 2003 - Jane was a lot more confident and was pretty close to her previous lap times.
This in spite of now running 18 inch wheels (instead of donut 16 inch racers) and full road equipment.
At the end of the meeting she then drove the car home.
Racing cars are a complete pain in the sit upon when they dont go right and unlike a road car you cant take them up the road to test them before a meeting!
The kit we used to install the blower was one of Richard's cast aluminium inlet and carb mounting assemblies and a front "MG" cast aluminium plate for the front of the blower - the installation is very neat.
The blower was one that Tim happened to have in his garage (as you do).
Hope this answers all the critique?
Regards David
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David Allison
United Kingdom
665 Posts |
Posted - 02/07/2008 : 14:07:58
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| Peter - yes I did know about the smaller blower from Dereck but the Volumex was in Tims garage and much cheaper! |
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etlanpa
United Kingdom
560 Posts |
Posted - 02/07/2008 : 15:46:01
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I believe Derek can supply 2, 3 or 4 lobe rotors.
However, it's worth bearing in mind that more lobes require more power from the engine to drive them... |
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MDP
United Kingdom
35 Posts |
Posted - 02/07/2008 : 16:23:03
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You csn use petrol with blowers of around 30 psi, it's not just ordinary petrol though.
Mark |
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David Allison
United Kingdom
665 Posts |
Posted - 02/07/2008 : 16:29:44
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Remember - the more lobes the more power you get too!
Alfa Romeo had a 2 stage blower system on the 159 GP car and some bright spark pointed out that although it produced 400 or so BHP (was probably more than that but this is just for instance) the blowers required nearly 60 bhp to drive them.
What the twit forgot is - that without the blowers the engine does not produce aything like the horsepower it does with them!
If a blower takes 15 BHP to drive it - but gives you an extra 50-60 BHP then it is worth pulling the thing around with you - anything that requires more power to drive it than it produces sounds a lot like a horse - and are we not trying to power a horseless carriage?
MG managed around 140 bhp from 750 cc pre-war - this was with a Zoller vane type compressor running at high speed.
This is nothing short of amazing - however 230-270 bhp from 800 cc without a blower is even more clever and shows what is ultimately possible in engine tuning.
As I have said before - supercharging was a means to an end - a convenient way to gain horsepower with little effort - as an engineering solution though it was a blind alley!
The future of engine technology was more revs - modern race engines just keep going up and up with F1 now capped at 19,0000 rpm.
But with blowers you dont get nothing fer nothing!
To get horsepower - you need horsepower to drive it.
Even if like the idiot concerning the Alfa engine you use a second engine to drive the blowers and guess what you get more horsepower!
Well DuRRRRRRRR!
The thing with a blown MMM car is you need to carefully assess what it is you want to achieve and stick to that goal.
Then it is all very simple - they even write books telling you how to do it.
I am ranting sorry guys!
It is a humour filled rant though!
Regards David |
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kimber
United Kingdom
1529 Posts |
Posted - 02/07/2008 : 17:07:28
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quote:
Originally posted by David Allison
MG managed around 140 bhp from 750 cc pre-war - this was with a Zoller vane type compressor running at high speed.This is nothing short of amazing
I believe that Messrs Dark/Piercy have exceeded that output from a 750 by some margin.
Have you thought of using carburettors?  |
Edited by - kimber on 03/07/2008 07:27:54 |
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Richard Hardy
United Kingdom
2165 Posts |
Posted - 03/07/2008 : 11:39:21
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David, I think you will find that the manifolds you have fitted are not mine as they are not quite ready yet. The name "Volumex 7 Fin" will avoid any confusion with others as there are several major improvements to asist in easier fitting, beneficial mods and general asthetics for a period look. The will upload more photos shortly. The cast aluminium cover may be one of mine but I am aware that someone else has already copied it!! Made me chuckle.
I am enjoying reading the technical info exchanges on this and other topics though and really highlights the benefits of the Forum site especially for others considering uprating their cars.
Keep going with the technical talk David!!
Regards
Rich H |
Edited by - Richard Hardy on 03/07/2008 11:42:17 |
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David Allison
United Kingdom
665 Posts |
Posted - 03/07/2008 : 12:34:44
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I have been corrected by Oliver who did all the work - and is a complete hero by the way!
The manifold was bought from Ron Grant and the brackets from Peter Greens son in law.
Oliver fabricated the carb mounting himself.
Apologies for giving the wrong people applause - Allison's are always fair and never take plaudits for the work of others!
Regards David |
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timmetcalfe
United Kingdom
2 Posts |
Posted - 03/07/2008 : 16:13:47
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This is a very interesting discussion and one I have a keen interest in as sponsor to, and occassional second driver of, my wife Jane's N Type referred to above.
I see a lot of talk about blower pressures, but my experience, at least with my three blown Lagonda Rapiers (two single seaters with Chinn blowers and a Volumex blown sports racing car), is that compression ratio is equally important and it is getting the right compression ratio/blower pressure combination that is key. I have always been a believer in keeping Roots type blowers to about 16 psi maximum (as per the perceived wisdom on efficiency), but then increasing the compression ratio as much as possible to maximise power. I have never had the time or funds to really work out whether this is the right approach!
Using this maximum pressure formula with the Rapier's we have about 140bhp (at the wheels with a pre-select box) from 1100cc on petrol with 14psi boost (from a Volumex)and 7.5:1 compression, or 160 bhp on methanol at 16psi (from a Chinn blower)and 8:1 compression. As a comparison my 1500cc normally aspirated Rapier runs on 4 Star or super unleaded with 11.5:1 compression and has 100bhp at the rear wheels.
To answer another question I have used both 3 and 4 lobe Chinn blowers and with the same blower pressure on the same engine (obviously at different times!) and could see no measurable power difference between them.
I believe Jane's N-Type has about 6:1 compession (from the time when it ran on methanol at 25/30psi boosrt pressure)and in my view could cope with much higher, given there is only about 15psi boost maximum and we run 100 octane (FIA approved "pump fuel") or when allowed 105 Octane racing petrol. Any views on what the compression ratio could sensibly be increased to with an N Type engine and this sort of blower pressure? Also what compression ratios are other people using on their MMM cars? I have no idea what power the N is currently putting out, but the car is currently 10 seconds a lap slower round Donnington with me driving compared to the Rapier with the same engine capacity and blower (Volumex), boosting at the same pressure (they also weigh much the same and handle pretty similar).
Being a competitive sort I always want more power and also think we can shame quite a few bigger cars in the longer distance pre-war sports car races given the N handles so sweetly on 4.50/500 18 inch Blockleys. Any views on how to achieve this!
Tim
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Gordon
United Kingdom
693 Posts |
Posted - 03/07/2008 : 19:03:49
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In my simple view talking about the pressure that a given blower installation gives is not correct but what matters is mass flow. PSI is much easier to measure and "understand"(!) but needs to be linked to adiabatic efficiency amongst other factors.
With regard to the maximum cr for a given blower/ engine combination this depends on the charge temperature in the cylinder at the end of the compression stroke. I believe that a rough guide is about 1075 C max if one is using standard advance of ~37 degrees if knock is to be avoided. Of course you can retard the ignition but this throws away power. It is for these reasons that the turbo people use intercoolers, alcohol users overfuelling to cool the charge and petrol users run their engines rich! If your thermdynamics is good you can calculate from first priciples! None of this is intended to detract from the hard experience and wisdom of the contributors to this thread.
Sorry for being pedantic.
Gordon
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MDP
United Kingdom
35 Posts |
Posted - 04/07/2008 : 14:04:34
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I believe that a rough guide is about 1075 C max if one is using standard advance of ~37 degrees if knock is to be avoided. Of course you can retard the ignition but this throws away power
Gordon - I don't agree with advance/retard statement above...
Mark
ps - I still have another racing MMM car that we expect to easily exceed the R types performance with! |
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George Eagle
United Kingdom
3246 Posts |
Posted - 04/07/2008 : 14:44:36
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Mark
What car is that? Must be some motor if it outperforms your magnificent outings with the R type!!
Regards George
PS I bought my Volumex new about 18 years ago from a local Lancia dealer for something like ú250. As one of our respondants states - that must be inflation!! |
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Peter Green
United Kingdom
1682 Posts |
Posted - 04/07/2008 : 16:34:21
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Mark,
I think I know which Triple-M car you are referring to but when are we going to see it out on the track? Were you not hoping to have it at VSCC Silverstone in April?
Peter. |
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