Dave,
I always enjoy reading the engineer take on these discussions. I don't
argue with anything you have said. What I put forth on degrees of
rotation in the combustion process comes from text books that I have
taught from, those of course, being related to stock engines. I haven't
a clue what the real numbers are.
I can speak from experience, however, and as you know, I often do. On
gasoline, I have run ignition timing from 36 degrees to over 50 degrees
on the Salt in my Chevy small block. Different conditions of engine
build and atmospheric conditions have required different settings. Over
time I have learned that major advances in timing usually signal an
internal engine breathing or pressure problems. However, my tune-up has
always been such that meets the circumstances. I advance timing until it
doesn't help, back it up a couple of degrees and then work on something
else. I haven't had the luxury of coming to the Salt with dynoed engine
on many occasions. This year was an exception, but our dyno tune-up was
not what was needed at the Salt. Redding is 500 ft and we went to 4300
ft. which requires some adjustments. Even on the dyno, the recommended
38 degrees of timing didn't work. We picked up 30 hp by going to 42
degrees. I am a firm believer in "combinations", what works for me may
not work for anyone else, but I don't argue with success. Mostly, we
have been successful. Sure we have broken lots of parts! If I wasn't
breaking things, then I would assume that we were leaving too much on
the table.
I hope that everyone else is enjoying this banter as much as I. It is a
great help for everyone to chime in on these topics. We all learn
something that we can take home.
Tom, Redding CA - #216 D/GCC
Dave Dahlgren wrote:
>wow have been tied up the last couple of days but here is my take on this
>thread
>
>Jon
>Maybe the expert tune up does work fine for what they have tested, about 1/4
>mile..It probably makes the most power for that distance and runs the best for
>that distance but if you change the distance or time on I think you need to
>change the tune-up some too. Kind of like running nitro 100% might be fine for
>1/4 mile or less but for 5 miles might be pretty hard to make it to the other
>end. If your wrong tune up involves running slightly less timing and slightly
>more fuel then that wrong tune up is probably the safer one in my mind. I can't
>remember if it was last year or the year before we talked a while about your
>tune
>up, I had suggested starting back at a lower setting and working your way back
>up
>in power levels plus at the time you were running a lot of ignition timing with
>the nitrous if you compared the original output of the engine compared to the
>size of the hit. Are you now running a lot less advance than you were? If you
>don't want to reply to this on list I understand but am a little curious if
>that
>is the case. The whole nitrous tune up is relative to the state of tune of the
>engine before the nitrous, if you have 14/1 compression and on the edge of
>detonation with the ignition advance and everything is optimized for running at
>100 deg F and 4500 feet. then the changes you will have to make to run a big
>load
>of nitrous are a lot more drastic than a 9/1 engine setup to run at 70 degrees
>and sea level. Also if I add 100 hp to a 100 hp engine then if you think
>taking 4
>degrees of ignition advance is going to work you are sadly mistaken. I would
>take
>out at least 1/2 of the total timing to start. If you add 100 hp to a large
>engine that is very inefficient
>then taking out 4 degrees makes sense.
>The only other question I might ask with your 'unusual tune up' is what is the
>bottle pressure during the run?? Are you chasing the bottle pressure down the
>track.
>
>
>Tom you are only giving us about 10 degrees at best to extract the power out of
>the gasoline(or alky or nitro..) does that make sense to you? With your numbers
>peak pressure is at 20 degrees ATDC and it is over at 23.. Might have to spin
>that one real high and need a lot of fuel to even make it run..
>
>Keith where do I start LOL...
>at the beginning I guess..
>Ve is interesting but there is mechanical, combustion, thermal to consider too.
>They all have to be good. You can pump lots of air and make no power I have
>done
>it and it is easy to fall into that trap.
>
>With any engine you have to start the combustion so that peak pressure is about
>17 degrees ATDC. Before this point you are actually starting to fight to a
>certain degree the expansion of the gases. That is just the best compromise
>that
>makes the most torque(which is what we are trying to do make torque hp is
>merely
>a mathematical manipulation to express it in a different way for other
>comparisons). To take that to a different viewpoint you are oxidizing a fuel
>to
>make heat that will cause expansion by heating up the inert gases and making
>them
>expand. If you have no inert gases then you have less expansion. That is why
>nitrous is a case of diminishing returns in the first place. You have a lot of
>heat and nothing to expand, so things melt, the heat has to go somewhere. If
>you
>insulate and coat everything it might make it out the tailpipe else something
>will get too hot and fail.
>
>Turbos and blowers..
>Air pressure has nothing to do with anything at all in this...
>It is all about air density. A supercharged engine is no more than a naturally
>aspirated engine running at a different altitude period... It is all about air
>density, the Ve of the engine does not change one bit you are just pumping
>denser
>air. It has more weight per cubic foot. Same as when you go up in altitude you
>have less and make less torque. Well supercharging is going down in altitude
>you
>have more per cubic foot that is pumped. If you have more mass then it takes
>longer to burn so the power stroke is longer. Also you have more gases to
>expand
>so that takes longer as well and will extract more energy from the combustion.
>Further for your theory of air pressure to work inter coolers would be both
>un-necessary and actually cost power. They convert hot compressed air to cooler
>air at a lower pressure. In this process they increase the density. The cooler
>the air and the denser the air the better. As an aside to this as the charge
>air
>density goes up the speed of the combustion process also speeds up, that is the
>reason you run less timing with a supercharged engine as well as to a certain
>degree a nitrous engine( this is why the size of the engine run on nitrous has
>an
>effect on timing irregardless of the size of the hit). To carry that to an
>extreme put a 50 hp hit on a 1100 ci engine and what are you going to do with
>the
>tune up? Probably nothing, ok now put it on your lawn mower...
>
>90 degrees ATDC...
>Nothing special about this number other than it is about 18 degrees past peak,
>the best mechanical advantage is about 70 to 76 degrees ATDC with 72 being most
>typical. It is a relationship of stroke and rod length. Next time you have the
>pan off and are degreeing a cam or have a degreed damper turn the engine till
>the
>rod on #1 cylinder is 90 degrees to the crank pin. Ought to be around 70 to 76
>degrees ATDC.
>
>Nitrous is easy to explain
>All bang no push. Reason being it has a lot of oxidizer and fuel. But it has
>less
>gas to expand, so it gets done sooner. Having a more dense fuel/oxidizer mix it
>requires less advance because it burns faster, plus it is potentially cooler
>further increasing density..
>
>It is all about air density, pressure recovery and thermal efficiency....
>
>Ok class dismissed.................
>
>Oh as for Dave Freiburger and Hot Rod Magazine
>
>Copyright 2002 Engine Management Systems . All rights reserved.
>
>Dave Dahlgren
>(The 'Flame suit' is on now) LOL
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