Skip to main content
15 votes

Why is it that when supercharging, it's preferred to compress the input, rather than pull a vacuum on the exhaust?

The power of an engine is not determined by the difference in pressure between the combustion chamber and the exhaust. Power is determined by how much energy one can put into the combustion chamber ...
fred_dot_u's user avatar
  • 4,560
13 votes

Why is it that when supercharging, it's preferred to compress the input, rather than pull a vacuum on the exhaust?

OP's question states: It should follow that turbocharging should be equally effective pumping air out the exhaust, than in the inlet. No, it is not as effective. You can't reduce the pressure to ...
Weather Vane's user avatar
  • 2,314
9 votes
Accepted

Why is it that when supercharging, it's preferred to compress the input, rather than pull a vacuum on the exhaust?

IMO this is not a stupid idea, however it doesn't actually make sense for multiple reasons: A naturally-aspirated Otto or Diesel engine by itself doesn't expand the gas even to atmospheric pressure. ...
leftaroundabout's user avatar
7 votes

What actually affects engine compression ratios?

Put simply, an engine's compression ratio is the ratio between the volume of a cylinder with the piston at the down position (position 1 and 4 in the pic below) and the volume of the same cylinder ...
tlhIngan's user avatar
  • 12.2k
6 votes

Shouldn't a loss of compression equal a loss of power?

Those numbers do not seem horrible, unless the 140psi is low. You might want to repeat the test with the engine warm. I generally feel that a cold test only reveals inter-cylinder differnces (in ...
SteveRacer's user avatar
  • 12.4k
6 votes
Accepted

Shouldn't a loss of compression equal a loss of power?

Yes, it does lead to a loss of a power, but your compression is not worth worrying about. Every engine will loose some compression after a few years. New rings wear out faster at the beginning and ...
Arturs Bolsunovskis's user avatar
6 votes

How can a model with a smaller engine produce more power?

I am guessing that the smaller engine has a turbocharger and the larger engine doesn’t. This would fit with the smaller engine having a lower compression ratio. The turbo compresses the air entering ...
HandyHowie's user avatar
  • 25.4k
6 votes

Why is it that when supercharging, it's preferred to compress the input, rather than pull a vacuum on the exhaust?

There is no direct path from the intake to the exhaust, at least one set of valves will be closed at all times. The exhaust valves open and the piston pushes the exhaust out of the cylinder, then the ...
GdD's user avatar
  • 17k
5 votes
Accepted

If an engine starts does that mean compression is fine?

You cannot assume just because it starts it has good compression. It can have minimal compression and still start and run just fine. That minimal compression is going to be different for every engine, ...
Pᴀᴜʟsᴛᴇʀ2's user avatar
4 votes
Accepted

What actually affects engine compression ratios?

I always assumed that the less "square" (or more "undersquare") a motorcycle engine was, the higher the compression ratios it can have. In order to maintain the same in-cylinder displacement a ...
Zaid's user avatar
  • 39.2k
3 votes

How can a model with a smaller engine produce more power?

how is it possible? Turbo/ Supercharging, higher tuning, there are a lot of possibilities to make more power out of an engine. what are the advantages & disadvantages of less cc more power ...
Daniel's user avatar
  • 1,072
3 votes

What actually affects engine compression ratios?

Caveat lector: I am neither an engineer nor engine builder. I just read a lot. You have received correct descriptions of static CR. But you framed the question with concerns about preignition and how ...
Shannon DeWolfe's user avatar
3 votes
Accepted

How to leak down test a cold engine?

So searching the Internet, I've found some conflicting opinions, with an occasional person saying either that cold leak down tests are completely invalid, or that cold numbers will be two to three ...
Robert S. Barnes's user avatar
3 votes
Accepted

Why can't they add more gas particles to the pressure higher to force a piston down!

What you are proposing has been done a few times: there are cars that run on compressed air. The problem with that is that compressed air has a low energy density, comparable to lead-acid ...
Hobbes's user avatar
  • 4,412
3 votes
Accepted

No compression on my two stroke

TL;DR: Either there's a problem with the head gasket, or with the piston rings. Remember, the piston rings fly over the openings in the cylinder wall, and there's no radial force onto the section ...
sweber's user avatar
  • 7,057
3 votes
Accepted

How does compression tests work inside of an engine?

The pressure gauge has a one way valve in it so that it does not follow the pressure in the cylinder at all times - it is designed to record the maximum pressure over several cycles of the engine. ...
Solar Mike's user avatar
  • 34.6k
3 votes

Why is it that when supercharging, it's preferred to compress the input, rather than pull a vacuum on the exhaust?

Since the question doesn't specify any type of engine... Funnily enough, there are engines which pull a vacuum on the exhaust. They are steam engines, so the vacuum is created by simply adding cold ...
user_1818839's user avatar
3 votes

Why is it that when supercharging, it's preferred to compress the input, rather than pull a vacuum on the exhaust?

Pumping air into an engine requires work, but each joule spent pumping in pre-combustion air will increase the amount of work that is produced downstream by more than a joule. The effective power ...
supercat's user avatar
  • 291
3 votes
Accepted

Leakdown Test | EJ205 on Engine Stand and replaced all Valves

You state you "lapped the valves" when replacing them? If you have replaced the valves, you need to do more than just lap the valves. You actually have to do a valve job. This entails ...
Pᴀᴜʟsᴛᴇʀ2's user avatar
2 votes

Does a warped engine block affect compression test?

You should have checked the flatness of the block deck with a feeler gauge and a straight edge before installing your new equipment. For instance one of my engines has a maximum value of .002" of "...
Matthew Bossard's user avatar
2 votes

Why can't they add more gas particles to the pressure higher to force a piston down!

The amount of energy required to force that amount of gas into the cylinder is immense. Causing a large enough pressure difference to force the piston down and drive the engine, and also the vehicle, ...
raydowe's user avatar
  • 3,012
2 votes

What actually affects engine compression ratios?

There's actually another factor to consider in addition to the bore and stroke - the size of the space above the piston when the piston is at TDC is also a factor. (that dome shaped area you see ...
Trimble Epic's user avatar
2 votes

Honda dirt bike with clone engine lots of problems

Your bike is full of deposits -- carbon and varnish -- that are byproducts of petrochemical fuel and years of combustion. Deposits clog fuel lines and carburetor jets, cause piston rings to stick, ...
Carguy's user avatar
  • 1,967
2 votes
Accepted

Simple Question: During combustion stsgep do the piston go up all the way?

TL DR: You are absolutely correct: the piston does not completely close the gap. There are two points in the rotation of the crankshaft where the piston stops momentarily as it changes direction: ...
Pᴀᴜʟsᴛᴇʀ2's user avatar
2 votes

Low compression on all cylinders - Flooded Engine - Maybe Washed Cylinders?

If you attempted to run the engine with the valve train improperly aligned, you may have caused valve damage to your head, as this is an interference motor. This would definitely affect your ability ...
Pᴀᴜʟsᴛᴇʀ2's user avatar
2 votes

Low compression on all cylinders - Flooded Engine - Maybe Washed Cylinders?

Does my hypothesis sound reasonable? What doesn't sit well with me is that the engine has already been cranked a lot (when the cylinders were flooded). Regardless of whether the engine started, ...
Zaid's user avatar
  • 39.2k
2 votes
Accepted

Low compression on all cylinders - Flooded Engine - Maybe Washed Cylinders?

A teaspoon of oil is sufficient, but this trick does not work if the pistons are dished... the small amount of oil won’t be a problem, it will fall down / burn away.
Solar Mike's user avatar
  • 34.6k
2 votes
Accepted

Can I use cylinder compression to conclude the valves are adjusted

No. A compression test is used to measure the sealing in the cylinder, both through the valves and the rings. You cannot confer anything through a compression test as to the adjustment of the valves, ...
Pᴀᴜʟsᴛᴇʀ2's user avatar
2 votes

How does compression tests work inside of an engine?

The maximum pressure obtained inside the cylinder is achieved at top dead centre (when the piston is at the top of the stroke). There is a very small volume of gas in the cylinder at this point. ...
HandyHowie's user avatar
  • 25.4k
2 votes

Low compression in one cylinder with a misfire code, but no smoke in the exhaust

Pull valve cover for #3, remove both rocker arms for #3, pull the spark plug and apply compressed air (100lbs) into the cylinder #3, listen for a air leak sound in the exhaust tail pipe, Intake ...
Moab's user avatar
  • 7,990

Only top scored, non community-wiki answers of a minimum length are eligible