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Rod Ratio, Combustion Chamber, CR and stuff

Rod Ratio, Combustion Chamber, CR and stuff

Rod Ratio, Combustion Chamber, CR and stuff

(OP)
Hello,

I've been lurking on this forum for a few weeks, and I love it. I wish I could go back to school :) ...  Anyway .. I love to read up and shape myself into a more knowledgeable and better technician. Of course, everything new I learn is directly translated to a use or application in my own projects. My newest project is a killer horizontally opposed VW/Porsche combination. I found that the block width of their 4 cylinders is the same as their 6 cylinders, and I came up with the idea to build the 4 cylinder using new factory Porsche parts of the 6 cylinder, and use the same geometry to stay within the limits of these parts and reduce the chance of breaking something later on. Reliabillity, power, and high revs are keywords in this project. Whenever I see a calculation or equation pop up here I apply it to my project and compare the data I find with similar engines. I love this stuff :) ..

Question: Would one prefer a different rod-ratio in a 6 cylinder engine VS a 4 cylinder? I mean ... is the result different in either a 4 or a 6?

The Porsche six has a 76.4mm stroke and a 127mm rod lenght, it makes for a rod-ratio of 1.66. Rather short. I must note that the Porsche bottom end is very very strong and seemingly has no problem with this rod ratio, it is not evident in either crankshaft problems or piston/cylinder wear, these engines occasionally see 7000rpm.

My intitial goal was to exactly copy Porsches geometry into my 4 cylinder. But ..  you guys got me thinking (uh-oh!) ..  :) .. I would love a very reliable strong engine to be installed in my 0-miles 2002 -classic- VW bug (not the "New Beetle"). I would love a well balanced, light running, high revving engine.

Instead of 76.4strokex127rod, I could go with
71x130 which gives a 1.83 rod ratio. OR 74x128.5 which gives 1.73.

A smaller stroke would give me less displacement, yes, but it would also give me less stress on the crank, and a stronger crank to begin width, higher revs, less flex..  No? Yes?

I am just thinking, maybe going to the max displacement one can get within the confines of a given engine is just not always the way to go...

Bore = 100mm
valves are 48x39
induction is 2x48ida Weber or equivalent throttle bodies.

Compression Ratio, there is a lot of controversy regarding CR in the VW world. The old testament of the VW bible is: Lower CR for todays fuels, combine with big hemi-like combustion chambers and run much advanced timing, into the 35deg btdc region max..

The new testament: BS! Run high CR, and retard timing as necessary, high squench combustion chambers, the increased combustion speed wants initial ignition to be later, so as to have optimum combustion pressure at 15 deg ATDC. Max. ignition advance in the 25 btdc region.

How do I know if the ideal combustion pressure is happening at 15 deg ATDC?? Dyno set timing?

Detonation will kill one of these engines in no time. Does 9:1 in an aircooled engine with 93 fuel sound bad? (Taken into account that the combustion squench area has been pretty much destroyed because of displacement increase and thus requires bigger combustion chambers..  but then is not 'hemi' shape, just the factory squench areas came out about 2mm higher..  is this still a squench area to speak of?)

Is there anyone who wants to hire me to mess with stuff like this all day and pay me for it??  hee hee .. (I'd LOVE too!)

I'd love to babble some about this, I can get you most data you may need. This is not a theory only project, it will be built :)

Thanks for reading this far,

Brian.

RE: Rod Ratio, Combustion Chamber, CR and stuff

Detonation will kill one of these engines in no time. Does 9:1 in an aircooled engine with 93 fuel sound bad?
-------------------------------------------------------

A "real" measured 9:1 CR with 93 Premium gasoline
is compatible with cam duration @ .050" of at least
216 to 221 degrees

its possible to run real 10:1 on 93 premium
with cam duration measured @ .050" of at least
225 to 235 degrees depending upon centers

check with cam people !

Larry Meaux (meauxracing@mindspring.com)
Meaux Racing Heads
MaxRace Software
ET_Analyst for DragRacers
http://www.mindspring.com/~meauxracing/

RE: Rod Ratio, Combustion Chamber, CR and stuff

(OP)
Larry, Thanks for your reply. It gives (once again) food for thought ..  Thanks!

Brian

RE: Rod Ratio, Combustion Chamber, CR and stuff

You can work for me, Brian but, you must accept the same pay scale as I get!  


Rod

RE: Rod Ratio, Combustion Chamber, CR and stuff

so as to have optimum combustion pressure at 15 deg ATDC. Max. ignition advance in the 25 btdc region.
-----------------------------------------------------

Brian,
personally, i think if you can't run at least
a minimum of 28 degrees advance
(and i personally prefer at least 32 deg BTDC ignition
with a particular octane fuel) ....the fuel/air mix is burning too quickly ....it will force you to make sure you are ontop of your "tuneup" at all times

if you have an certain gas/octane that you can only achieve max torque/power with only 25 BTDC ...then i would increase octane to run at least 32 BTDC to make max torque/power curve

its just my experience..maybe some one else can add more info ?

Larry Meaux (meauxracing@mindspring.com)
Meaux Racing Heads
MaxRace Software
ET_Analyst for DragRacers
http://www.mindspring.com/~meauxracing/

RE: Rod Ratio, Combustion Chamber, CR and stuff

(OP)
Thanks again Larry. This really helps putting things into perspective.

Rod, I'll think about it :)

Brian

RE: Rod Ratio, Combustion Chamber, CR and stuff

Brian ,
i have had a few engines , like the 9:1 Comp Ratio rule
engine make more HP/Torque with Exxon 93 premium
and 36 deg BTDC ...than it did with VP C-14 race gas and
timing increments tried on dyno tests all the way up to 46 to 48 !

in other words with the Dart/Buick Chevy V8 heads
and roller cam, true 9:1 CR , it made more HP/Torque with
93 Exxon + 36 deg BTDC .....trying VP C-14 race gas ..we kept increasing timing 2 degrees at a time till we got to 46 deg BTDC were it made its best HP/Torque curve

putting too high octane gas for a certain Comp Ratio can hurt you ...no matter if you try to compensate with additional timing advance .  

the 36 deg was better in this situation , but its quite far away from the 25 deg Total timing you are asking about ?
25 is very quick and rough combustion rate
very edgegy to tune at all times , not recommended !



Larry Meaux (meauxracing@mindspring.com)
Meaux Racing Heads
MaxRace Software
ET_Analyst for DragRacers
http://www.mindspring.com/~meauxracing/

RE: Rod Ratio, Combustion Chamber, CR and stuff

(OP)
This is such interesting matter ..  see.. I'm still of the "more or bigger is better" ..  or at least ..  I used to .. Thanks!

Brian

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