SonOfThomp
Well Known Member
What is the point of them? Lack of power and low efficiency (which has a negative effect on fuel economy).
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Low Compression Engines
- Thread starter SonOfThomp
- Start date
oil4kids
Well Known Member
except for one thing, a lot of the power in a motor is lost in during the compression stroke
we may not realize it but it does take power away from the crank to compress the air in a motor 10 times
fyi
Standard Air Pressure at sea level= 14.7 psi x 10:1 compression ratio = around 150 psi per cylinder
we may not realize it but it does take power away from the crank to compress the air in a motor 10 times
fyi
Standard Air Pressure at sea level= 14.7 psi x 10:1 compression ratio = around 150 psi per cylinder
bubbletop1961
Well Known Member
Nothing wrong with low comp. my car runs 12.30's with a 9:1 sbc. use pump gas all the time. It was possibly the most fun change I ever did. No more running friday after work to buy enough race gas to last a weekend. Not to mention the extra cost. Not as fast, but just as much fun.
SonOfThomp
Well Known Member
Why did they make them? Advantages, disadvantages?
If they are, in fact, relatively inefficient, what's the point?
Do they last longer?
If they are, in fact, relatively inefficient, what's the point?
Do they last longer?
Lower compression was used in the truck engines because you can lug them harder. Back in the day you could stand on the throttoe of a 409 truck engine at 2000 RPM all day and not hurt it. An 11 to 1 engine would cook itself.
Dave
Dave
The trucks were 7.75 compression .
static compression and dynamic compression ratios
two different things going on in that fire breathing monster under the hood!
Good link is
http://en.wikipedia.org/wiki/Compression_ratio
I stay away from 11 12 to 1 stuff anymore and rather go after a better burn to build power. Plus the gas sucks today!
two different things going on in that fire breathing monster under the hood!
Good link is
http://en.wikipedia.org/wiki/Compression_ratio
I stay away from 11 12 to 1 stuff anymore and rather go after a better burn to build power. Plus the gas sucks today!
SonOfThomp
Well Known Member
But . . . but . . WHY a low compression ratio? Like the 8.25:1 in smog motors.
How is inefficient combustion supposed to reduce emissions?
WHY GO LOW?
How is inefficient combustion supposed to reduce emissions?
WHY GO LOW?
I'm certainly no expert on this but I think the problem is that high compression causes high combustion pressures and that causes high NOX emissions. http://en.wikipedia.org/wiki/Nitrogen_oxide#NOx
They've obviously found a way to reduce the NOX emissions in newer engines because many of the new engines are running in the 10 or 11 to one range.
I think that low compression was a quick fix to meet the emission standards.
I also agree with Dave that low compression was used in truck engines because you can lug a low compression engine all day long without hurting it.
bigblockwilly
Well Known Member
they sloved the NOX problem with the EGR valve. But now they are elminating the EGR and using VVT to control NOX. They hang the Exhaust valve open longer so when the pistion starts on its intake stroke is sucks some of the exhaust back in the cylinder to re burn it.
Compression ratio was lowered as an easy fix for oxides of nitrogen (NOx) exhaust emissions. NOx is formed when the combustion temps exceed about 2500 degrees f. Less compression means lower cylinder pressures and temps leading to reduced NOx production. Another easy fix was to retard the cam timing also reducing cylinder pressure. Exhaust Gas Recirculation (EGR) is also used to reduce NOx production by lowering combustion chamber temps. Exhaust gas is inert and has no impact on the air fuel ratio, but it does add mass to the intake charge. This spreads the energy of combustion over a larger mass which reduces the temps produced during combustion. Modern vehicles manage NOx by the use of EGR, Variable Cam Timing, and combustion chamber design. On a twin cam motor it is easy to alter cam timing to create an EGR effect. The systems I'm familiar with will advance the intake cam by 40 degrees or more when EGR is needed. This increases overlap and bleeds exhaust gas back into the intake, which is then drawn back into the engine on the next intake stroke. If you look at modern head designs the combustion chamber shapes have changed and they generally have gotten smaller. The longer the combustion event takes to complete, the greater the chance for combustion to become unstable, leading to spikes in cylinder chamber pressures and temps. Fast burn chamber designs allow increases in compression without dramatic increases in NOx or detonation.
Is this why W's like a tight quench? Whew! I didn't intend to write a book.
John
Is this why W's like a tight quench? Whew! I didn't intend to write a book.
John
Great explaination John....... so a tight squish and a lower engine temp will help reduce NOX??
.......... if I read it right
Dave
.......... if I read it right
Dave
Yep, anything you do to stabilze the combustion process which reduces pressure and temp spikes, and a cooler running engine will reduce NOx. Richer mixtures will also generally reduce NOx. Basically, if there is not enough oxygen to go around to complete the combustion process, there no extra oxygen to combine with the nitrogen to produce NOx. As mixtures lean out past 14.7 NOx will continue to increase until misfire is reached, then NOx will drop. If it's misfiring it's not burning, and if it's not burning is not creating heat. I didn't mention catalytic convertors because the original question was on engine designs.
John
John
Where did that John guy come from??? Pretty much nailed it.
Here's something to think about. Water injection can do the same thing EGR does.
The oxygen thing has to do with gas engine. They try to run at stoichiometric. 14.7 air fuel ratio.
Now diesels..... WE always have extra air. Everyones know diesels are using EGR now, right?
VVT - don't remember any Americans using it ....yet.
Here's something to think about. Water injection can do the same thing EGR does.
The oxygen thing has to do with gas engine. They try to run at stoichiometric. 14.7 air fuel ratio.
Now diesels..... WE always have extra air. Everyones know diesels are using EGR now, right?
VVT - don't remember any Americans using it ....yet.
I've been a member for a couple of years, asked a few questions, but mainly come here to get ideas on what to do with a pile of 348/409 parts I've collected over the years. The current post with the dyno results for the 348 stroked with the 409crank is the build I've been looking for. A nice mild cruiser, but yet plenty of power to blow the tires off my 50 Chevy PU street rod. Thanks for all the info everyone on this site provides.
John
John
oil4kids
Well Known Member
If you went to the 348/409 Convention Lamar Walden did a hour long talk about 409s and told one of the best street motors is the stroked 348 motor.
Maybe Phil Reed can transfer that part of his dvd to youtube and let all of us hear Lamars talk. Ive listened to it 3 times and the only exception i have with it is his take on synthetic oil which was the standard BS from Crane Cams.
Maybe Phil Reed can transfer that part of his dvd to youtube and let all of us hear Lamars talk. Ive listened to it 3 times and the only exception i have with it is his take on synthetic oil which was the standard BS from Crane Cams.