| [ Total Views: 1884 | Total Replies: 24 | Thread Id: 514 ] |
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 bugmeister
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| posted on September 10th, 2002 at 05:15 PM |
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Compression ratio
I know those Americans like running 9:1 with their mega-overlap cams, but I was wondering what maximum compression ratios people are happily (without
pinging and overheating) running on their daily (I stress daily) drivers on unleaded fuel using a mildish cams (think Engle 100).
Cheers.
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 Bizarre
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| posted on September 10th, 2002 at 08:25 PM |
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I am running about 8.3:1 on my 1776 with a AP2 cam which is fairly stockish. Between a 100 and 110 Engle. I run Super Unleaded and have no troubles.
Drive it everywhere - easy 400k each week mainly city / suburd driving :thumb:thumb
Futue te ipsum!!!
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70AutoStik
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| posted on September 11th, 2002 at 11:06 PM |
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I know people are gonna jump up and down about this, but cam timing has nothing to do with what compression ratio you should use in a healthy engine.
There is a kind of exception, if you are building an all-out race engine - e.g. camming it to produce power from 5000-8500rpm - you would need a
higher compression ratio to achieve a fast enough burn to produce power in the working part of the stroke.
I know people are going to jump on this, saying longer valve openings reduce the effective compression, but if the cam is "working" i.e. the
flow through the intake and exhaust tracts is using the overlap and early opening/late closing of the exhaust and intake valves, this effect
disappears.
Start the flame wars now - check with an engineer later...
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Bizarre
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| posted on September 12th, 2002 at 09:07 AM |
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Uh - oh.
I can see a "dynamic" Vs "static" compression ratio war comming on 
Somebody contact Jake and Dan!
Futue te ipsum!!!
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bugmeister
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| posted on September 12th, 2002 at 09:08 AM |
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What do you reckon your recommendation would be? I ask because I have to re-ring the 1915 soon and if I can increase efficency by bumping up
compression then I'll do it at the same time.
Also I'm not and engineer, but the valve overlap reducing dynamic CR does seem to make sense to my feeble mind...and goes someway towards
explaining how Jake Raby and the like get away with 9-9.5:1 on the street.
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Bizarre
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| posted on September 12th, 2002 at 10:37 AM |
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What compression are you running now?
The cam - is it a E100 that you have there now?
If it is i probabally wouldnt go over 8.25:1.
Unless............how a bout a full strip and change cam while your at it. Then something more exotic in cams and a matching increase in CR 
Futue te ipsum!!!
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bugmeister
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| posted on September 12th, 2002 at 02:53 PM |
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About 7.5-7.5:1. The cam is CB and has (from memory) 234 degrees@0.050 and .417 lift at the valve. I doubt i'd really want to change the cam at
this point. All my simulations in Desktop Dyno didn't give me a major increase with a cam cange to justify the effort, although DTD may not be
very correct.
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jakriz
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| posted on September 12th, 2002 at 08:27 PM |
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comp ratio
I'm running 9.0:1 & I use Optimax & it runs beautifully & stays cool, even after many laps on the track or idleing along in traffic.
I'm going to bump it up to 10:1 next time its apart.
regards
Jak |
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OvalGlen
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| posted on September 12th, 2002 at 09:31 PM |
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Jak , from memory youre running something like an FK89 ? . What is the duration if you dont mind.
For a performance engine with short manifolds / big carbs / long duration cam , I would be using 9 : 1 compression. For the milder cams , I would be
lowering it to 8.5 , thats my 10 cents worth.Glenn.
Regards,Glenn>
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70AutoStik
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| posted on September 12th, 2002 at 10:50 PM |
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OK, I'll bite, I know this is a bit long, but some of the BS about CR really bugs me. Note: re-shaping the combustion chamber, e.g. takig off
any sharp edges or hemi-cutting, will allow higher CR without pinging, but will still tend to increase head temps.
To understand why the “dynamic compression” arguments pan out, you need to look at the pneumatic operation of an engine. I've written a highly
simplified explanation below. For those with an engineering or scientific background, look for texts on “Fluid Dynamics” and possibly pneumatics.
To chose a starting point, I'll use ignition: the plug fires, the flame starts spreading and combustion chamber pressure starts rising. This
pressure acts through the conrod, at an angle, to turn the crank. By the time the crank reaches about 45o BBDC, these pressures have decreased and
the rod is acting at a less useful angle on the crank, so the exhaust is opened. The pressurised gasses start flowing into the exhaust and as the
crank passes BDC the piston starts to help them out. As gas has weight, the motion causes an effect called inertia, which means they want to keep
moving, so the intake is opened shortly before TDC in order that the vacuum created behind these exiting gases can help to start drawing mixture into
the cylinder. Shortly after TDC (the point depending on the speed of the engine) this effect becomes minimal, so the exhaust valve is closed and the
mixture continues to be drawn in by the now falling piston. Again, inertia comes into play, this time in the intake tract; so the intake is kept open
for about another 45o ABDC (also, the piston is not making any meaningful travel up the cylinder for much of this period.) The intake closes, ideally
with a bit of positive pressure and the piston with much of it's travel left to be made, and the compression stroke begins. The mixture is
compressed and the plug fires a little before TDC to allow the burn to begin. Obviously the actual valve event timing is highly dependant on what
engine speed we optimally want these effects to work at, and I have ommitted the effects of CR on flame rate for simplicity.
In the static model (think of a very slow turning engine,) the optimum points would be: Exhaust opens BDC, Closes TDC, Intake opens TDC, Closes BDC.
Proponents of the so-called dynamic compression thought mistakenly take the dynamic points (& often ignore how little the piston actually travels
at the extremes of it's stroke) and apply it to the static model, figuring that if the full stroke isn't available with extended exhaust
closing time, the effective CR will be lower; therefore must be compensated for by providing a higher “static” CR. Obviously, if the engine is
correctly cammed for it's operating range, this will provide an increased “real” CR. So, choose your CR for what will produce the best
compromise of power and heat for the rev range your engine will be working at and choose a cam that will produce it (and a higher octane fuel if you
then need it.)
As for the original question, the cam mentioned is very close to stock in timing (stock 1600 is 224 intake, 234 exhaust [at 50thou] and .328” lift)
and would probably work best with close to stock CR, a lower CR will produce a little less power but improve economy (and run cooler.)
:o
Now, start your flame wars...
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aussiebug
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| posted on September 13th, 2002 at 01:26 PM |
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I agree with most of 70Autostick's big post (very clear description of valve timing), but have a small probolem with the last sentence
"lower CR will improve economy...".
A high compression ratio increases the efficiency of an engine, so if it's producing the same power (say at a steady speed) it will actually use
a little less fuel. But of course in a practical sense, if you have a little more power available, then you WILL use it, so naturally that will
increase your fuel consumption.
And just for a practical example of increased CR, I have 1600 P&Cs in an otherwise stock 1500, and it still has the 1500 heads (I've had this
car from new).
This increases the compression ratio from the stock 1500s 7.5:1 to almost exactly 8:1 and the engine now pings just a little on normal unleaded (91
octane), but runs fine on a 50/50 mix of unleaded and premium (octane rating now 93) and of course, runs fine on premium unleaded too (95 octane).
The general run of thumb for aircooled VWs is 7.5:1 needs 91 octane. 8:1 needs 93 octane. 8.5:1 needs 95 octane, and 9:1 needs 98 octane (Optimax,
or Mobile Synergy 8000 for example).
It will be interesting to see what comes out of the "ethanol in petrol" argument raging about Canberra at present.
Alcohol has a high natural octane number, so the petrol component can be a cheaper refined version with a slightly reduced octane number, and the
resultant mix will still meet the 91 or 95 commonly used in Aus for unleaded and premium unleaded.
Adding alcohol to petrol can make for a cleaner burning mix (although I believe that to get more than a trace of alcohol to mis with petrol you need a
mixing agent like Benzol added - and Benzol is a known carcenogenic. Anyone out there have more info on this point?
And adding alcohol would undoubtedly give the QLD sugar carne farmers a boost in sales since the alcohol is usually made from sugar/molasses; but it
comes with a cost for carburetted cars like our VWs.
Alcohol contains 39% used oxygen, which cannot be burnt again, so you need MORE alcohol fuel for the same power output.
Adding 10% ethanol to 90% petrol will make the VW engine run 3.9% leaner than it should, and lean means hot and hot means pinging and pinging means a
higher octane is required...... you get the picture?
So if they DO add 10% alcohol to petrol in future, don't be surprised if your VW runs like c**p.
The solution is to increase the usual idle jet size from a 55 to a 60 and the main jet increases 2 sizes - for example from 125 to 130, or 127.5 to
132.5.
Each increase in main jet size increases fuel flow by approiximately 2%. So your fuel consumption will increase but the engine will be happy again.
Alcohol has a very high latent heat of vapourisation, which means it absorbs a lot of heat as it evapourates - so the engine may in fact run a little
cooler when using an alcohol mix fuel, so long as the mixture is in stoichiometry (those larger jets mentioned above).
Of course the modern cars with computers and FI simply adjust themselves on the road - they increase the fuel flow automatically until the exhaust gas
mixture matches the mapping in the computer. But our older carburetted cars can't do that, so carby adjustments will be needed.
Rob
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555bug
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| posted on September 13th, 2002 at 03:35 PM |
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so how do you think a 10% toluline mix would effect a dub? I know that my drive car 11.5:1 is happy on 16 static 30 deg max, however with the toluline
I can afforde to run a couple of degrees extra. I've also used this to good effect in friends turbo donks to slow down the flame front on there
highly boosted engines. A lot cheeper than 105+ stuff too!! |
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70AutoStik
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| posted on September 13th, 2002 at 09:49 PM |
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The increase in fuel economy comes from peak torque occuring at a lower engine speed and from a cooler running engine being more efficient. It
won't be a drastic improvement in most engines, I admit, but it is there. 
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Baja Wes
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| posted on September 14th, 2002 at 10:18 AM |
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Time for an engineers opinion.
A lower compression will lead to a decrease in efficiency. lower combustion temperatures also lead to a decrease in efficiency. That is why so many
people are trying for ceramic engines or at least ceramic coated combustion chambers and piston heads, to increase the combustion temperature and
decrease heat loss. lower the temperature of the combustion gas lowers it's pressure (PV=nRT). A lower pressure means lower force on the piston
and therefore less power. A hemispherical combustion chamber is more efficient than a beetle one because the hemispherical shape has the lowest
surface area to volume ratio you can get. This means there is less surface area (per volume of gas) for heat loss to occur, leading to higher
combustion temps and also less heat being lost into the head. Therefore more power, higher efficiency, and lower head temps.
There is obviously the point you reach where the temperatures are too high and the engine is overheating, in which case the internal clearances go to
crap and then you find the efficiency will reduce again. But you should not run a compression high enough to encounter those probs.
From experience I also believe LRP makes the engine run slightly leaner. When I started using it, the brochure I got on the new LRP said it may run
funny on older cars and they may require a tune up. This immediately said to me that LRP will effect the mixture ratio. When I first tried it in my
Baja I noticed the webers popped back through the carbs more often when cold, and occasionally when hot. So I think LRP makes the engine run slightly
leaner. I run optimax or BP Ultimate now.
Autostik, you are right about the effects of dynamic compression being lessened by the momentum of the intake air. There are also pressure pulsation
resonance to think about. My new engine has a variable resonance intake system which is a very fancy set-up. Check this site for more info
http://www.geocities.com/mikey9t6/car_uvwxyz_vris.htm
Now back to "the effects of dynamic compression being lessened by the momentum of the intake air". But that only happens at higher RPM. At
lower RPM the momentum of the intake air is not there, so you actually get combustion gases being pushed up the intake when the intake valve opens
early. It soon gets sucked back in once the piston goes past TDC, but not much intake gas makes it in, so the dynamic compression is low again. This
is why a big cam runs so crap down low. The volumetric efficiency of a big cam engine is terrible down low, and stays crap until "it comes on
cam", which is the point at which the momentum of the intake air is enough to force itself into the cylinder even when the valve opens early.
So at low RPM the dynamic compression of a big cam motor is lower than a smaller cam motor. So a big cam motor will have less pinging problems AT LOW
RPM, and that is when most cars will ping. So yes a big cam will definitely have less pinging probs than a same compression ratio motor with a tractor
cam.
You also need to think about carbs. A stock carb will have much more pinging probs than twin webers. This is mainly due to the long stock intake
letter the intake charge heat-up a lot more before it even enters the cylinder. The hotter the intake charge the more chance there is the engine will
ping.
So that's all the theory. As for my experience, I ran an 1835 at about 8:1 with a mild cam and a stock carb and it pinged bad. Then I rebuilt it
to a 1776 at about 7.5:1 with a mild cam and a stock carb and it still pinged but a lot less. I jetted it a little richer and it reduced the pinging a
little more so it only happened crawling offroad. Then I bought kadron carbs and the pinging stopped.
Then I got a 1915, 8.5ish:1, bigger cam, twin webers. It basically never pings. The only time it does in crawling over obstacles offroad doing about
500rpm and using a lot of throttle. But you can't blame it under those conditions.
So what carbs do you have?
[Edited on 14-9-2002 by Baja Wes]
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bugmeister
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| posted on September 14th, 2002 at 10:18 PM |
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Great info guys. As for Baja Wes's Q; Dual 40mm IDFs at the moment, and eventually 45MM DCOE (don't ask) throttle bodies later on (with
Wolf3D ECU).
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Baja Wes
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| posted on September 15th, 2002 at 09:16 AM |
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I'd probably stay around 8.5:1 then (with the twin carbs).
But when you go to EFI you can bump it up higher. The EFI will keep the right mixture all the time. You could then even go to computer controlled
spark timing and add a knock sensor and handle an even higher compression.
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jakriz
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| posted on September 15th, 2002 at 10:03 AM |
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c-ratio
I'm running an Engle K-8 with 298 duration Glen
Jak |
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70AutoStik
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| posted on September 15th, 2002 at 07:27 PM |
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Baja Wes, do you still have your little piece on compression ratio vs power posted somewhere? This is probably getting a bit too technical for this
forum, so I'll post a more detailed explanation of what I was trying to show elsewhere and provide a link.
In short, though, I'm not trying to sell everyone on running stock or less CR - but to run an appropriate CR for their engine. Higher CRs are
appropriate for wilder cams, just not for the reasons stated by some. From observations and discussions with experienced people, example guidelines
are about : 7.3-75 with stock cam, 7.5-7.8 Engle100, 7.8-8.0 Engle 110, 8.5-9.5 Engle FK series. These higher CRs will cause detonation with stock
combustion chambers, plugs and standard fuel, as you found.
Some tips for those wishing to up their CR are:
"Hemi" cut combustion chambers.
Cooler plugs (especially ir you will be running the engine long at high revs.) Try switching from the stock W145T to W175T to start with, if the
engine is really "hot," you may need to go to 225s.
Smooth the edges of the combustion chamber. The edges between the chamber around the valves to the deck are sharp because this is a lot easier and
cheaper to make with machinery. If you smooth them off (removing as little material as possible) with a file or the judicious use of a die grinder,
then smooth them over with sandpaper (down to 120 grit is fine,) you will drastically reduce the chance of pinging.
Retard your timing (a couple of mods to a type 3 carb can actually produce better performance than a 009.)
Jet your carbs a little richer. If you are not actually running lean, this is probably the least desireable option; as it can cause other problems
such as carbonisation and fuel wash.
But remember, higher CR will still make your engine run hotter (as in, it won't live as long.) How far you want to go is your personal choice.
And, last but not least; pinging is not annoying, it's a warning that your engine is trying to "suicide."
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Baja Wes
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| posted on September 16th, 2002 at 01:53 PM |
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| Quote: | Baja Wes, do you
still have your little piece on compression ratio vs power posted somewhere?
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I'm not sure what you mean?
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KruizinKombi
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| posted on September 16th, 2002 at 07:31 PM |
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Cam timing
Cam timing DOES affect the compression ration you can use. A long duration cam can allow more mixture to escape the combustion chamber at slower revs
effectively lowering the compression ratio (and hence detonation) down low, where you need it the most.
[Edited on 16-9-2002 by KruizinKombi]
Kruizin Kol
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70AutoStik
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| posted on September 17th, 2002 at 10:34 PM |
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Sorry, Wes; your quote of that pressure equation led me to believe you posted a link on the previous forum stating some theoretical figures for power
production based on combustion pressures. I will still get around to posting a more detailed explanation of the pneumatic description of the
operation of a four-stroke engine in order to try to enlighten people of a more complete picture of what we need to attain in our engine design.
Always remembering - the VW forgives a multitude of sins! 
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Baja Wes
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| posted on September 18th, 2002 at 12:49 PM |
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as any engineer, chemistand possibly physisist will tell you, PV=nRT is the ideal gas law. You can use it to figure out how pressure, temperature and
volume relate to each other. I implied it shows that by reducing the temperature of a gas in a constant volume you also reduce the pressure. Less
pressure means less power. Do a search on "PV=nRT" or "ideal gas law" on the internet and I'm sure you'll find
something.
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Buggy Boyz
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| posted on September 18th, 2002 at 12:57 PM |
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Geeeeeeeeeek
Man you have way too much time on your hands if you are writting replies that long Mr Wes, maybe you should spend it helping me build my buggy in time
for fraser .. hint hint wink wink .... |
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aussiebug
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| posted on September 18th, 2002 at 01:41 PM |
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Just one added comment from me (oops - it ended up about 3 comments :-)
Both Wes and autostick70 talk about hemi-heads being more efficient.
I think anyone contemplating getting their heads hemi cut need to read Sir Harry Ricardo's books on engine design.
(Ricardo Engineering - now part of the UK BMW group...he designed the WW1 Brit tank engine, the variable compression test engine, did the first
measurements which lead to octane ratings and a host of other great stuff, designed a "reverse stirling engine" to produce liquid nitrogen
from air in a single step, and even got 168mpg out of a 900cc motorbike in 1905!)
He's the guy who invented the "squish" combustion chamber, which the VW heads (and most engine designs) use. He found that although
this shape is not the ideal theoretical shape (the hemi is better in that regard), the turbulence created by the squish shape in the heads increases
fuel-burn efficiency and also allows a higher compression ratio than a hemi-head in the same engine.
He came across this design in (I think) 1925 and the first big user of the new design was the London Bus Company, which paid 5000 pounds for the right
to convert their petrol powered buses, and they saved 100,000 pounds in fuel costs in the first year of operation (nothing else was changed). They
were rather pleased :-)
He had a Brit patent but did not have the funds to apply for a US patent, and so the US car companies started using his designs without permission (it
was good enough for them to steal it!). He took them to court in the UK, won hands-down and the US companies were forced to pay him a royality for
every car they sold in the UK, or alternatively fit their UK cars with less efficient non-squish heads.
VW use this design because it allows them to have a slightly higher compression ratio (read Wes's stuff about efficiency and CR above) than
otherwise, and since the hotter running VW engine has to use a lower compression ratio than an equivalent (cooler running) water cooled engine, every
assistance in keeping the comrpession ratio up a bit was useful.
And Wes - re the LRP lean burning problem. You are probably right.
The most common LRP ingredient used in Aus it MMT - Methylcyclopentadienyl Manganese Tricarbonyl. I don't know how much of it is needed, but any
"non-fuel" additive added to fuel DOES lean the mixture out, because there is less "fuel" in the fuel.
Just talk to the US drivers about MTBE in their fuels (11% MTBE in California fuels results in a 2% lean mixture - my source is the California EPA).
Rough running in older cars (needing engine mods) was a common problem when this stuff was introduced.
This is even true for adding alcohol to petrol - the proposed additional of 10% alcohol to Aussie fuels will result in 3.9% less "fuel" in
the fuel (alcohol is 39% used oxygen), so if that one is forced on us, we will have to increase the jet sizes in our carbies (up 2 sizes for Solex
carbs from 125 to 130 or 127.5 to 132.5 as examples) to get the mixture back in Stoich (each increase in the Solex main jet size increases fuel flow
by about 2%).
Modern computer equuiped FI cars can adjust the mixture on the run to cope with different fuel formulars, but our carburetted VWs can't do that,
so WE have to reset the carbs for any major changed in fuel formulation.
Rob
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70AutoStik
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| posted on September 18th, 2002 at 11:54 PM |
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Actually, I majored in Applied Physics at uni; hence I not only know the formulae, I learnt a lot about how to apply them to the real world...
Oh, and I didn't mean to say hemi cut heads are inherently more efficient - but they are an effective way to reduce compression, and the slower
burn rate they produce seems to suit the rev range of a VW engine. A computer designed CNC combustion chamber would be better, but pushes the cost up
to an unreasonable level... I did say that lower CRs can produce better economy, but this is not due to "increased efficiency," but simply
because torque at a lower rev point allows a higher gear to be maintained at lower speeds.
[Edited on 18-9-2002 by 70AutoStik]
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Baja Wes
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| posted on September 19th, 2002 at 08:22 AM |
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A hemispherical combustion chamber is more efficient due to the reason's I have already stated. That is why all of today's leading high
performance motors are of the basic hemi design. It works even better with 4 or 5 valves per cylinder.
The old squish design was to promote richened areas which did help prevent detonation. However that has nothing to do with efficiency. It was a method
of making an ancient engine handle respectable compression ratio's. Changing compression ratio's and deck heights on a beetle motor stuff
the squish shape anyway.
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jakriz
Wolfsburg Wizard
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| posted on September 22nd, 2002 at 01:33 PM |
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c/ratio
enough said really?
Jak |
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Baja Wes
Bishop of Volkswagenism
V6 Baja Pioneer
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| posted on September 23rd, 2002 at 08:23 AM |
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I only give long replies when the post contains inaccurate, misleading or unclear information. There are people reading these posts that are trying to
learn and it's important they don't learn incorrect or misleading stuff.
Just trying to help :thumb
So in your own funny way it sounds like you agree with what I said anyway Ben.
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