Pelican Parts Forums - can somebody explain the effect of compression ?

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can somebody explain the effect of compression ?

i've heard lot's about high compression , and the requirement for twin plugging if you go high.

but i'm not really sure what the effects are from doing it
and how to get it..

could somebody explain

1 how can you raise compression on an engine

2 what performance gain does it give

for example if what would the difference make if you would raise compression on a 190HP 2.4S engine

3 any do's and don'ts involved?

i'm not actually planning anything ( yet) but i'de like to understand what it does or doesn't do

my 2cts:

You can raise the compression ratio on our engines in a number of ways:
* use 2.2 pistons in 2.4 engines (or use custom pistons)
* resurface the cilinder heads
* bore the little end on the rods off centre

A rule of tumb is a increase of 2 % to 3 % for every point increase of CR

If you get it up to high, detonation will occur which is not exactly nice on pistons and bearings. Twin plugging and good gas helps to prevent detonation.

My 2,2 (2195cc) puts out 180hp, all things being the same, a 2.4 (2.341) with the same compresion ratio (9.8:1) would put out 192hp. The 2 hp yours is "lacking" can be attributed to the lower compression ratio (8.5:1)

So the performance gain is not so dramatic.

Others will probably be more helpfull

so if i get this right,

2.4 @ 8.5:1 now gives me 190
2.4 @ 9.8:1 should give me 1.3 times 2-3% increase ? 6 extra ponies ? ( i calculated with 2.5% as average for the increase)

indeed, not that impressive , but interesting to know how it actually affects power output

so what's the difference between a 2.2 or a 2.4 piston then?
is it the shape or something?

EDIT , not that impressive considering the costs and labour involved for the twin plugging and all

Stijn;
The affects of increasing the CR are easier to see on the torque graph then the HP graph. Basically increasing torque by a certain percentage will move the torque curve up by a similar percentage. So an increase of the CR from 8.0 to 9.0 (as 12.5% increase in torque) will move the torque curve up across the board by something less then 12.5%. On the other hand an increase in the CR from 10.5 to 11.5:1 will only result in something less then a 9.5% increase, so there are diminishing returns. There are a few other twists...

* Cams have an impact. Cams with long duration actually have reduced static compression at low revs because the mixture only compresses (at lower rev's) once the intake valve is closed. Increasing the CR helps offset this somewhat. Once the rev's pick up inertia can also help to compress the mixture which means at higher RPM's the actual cylinder pressures can increase.

* Air flow has an impact too (which is where CR really affects HP). HP is a function of how much air an engine can combust in a certain amount of time. All engines reach a point of diminishing returns. In many street engines the induction system or valve size ultimately limits the air flow. Once the air flow is constricted torque starts to drop. Once the torque drops faster then rev's increase, HP goes down. Increasing the CR can "prop-up" the torque curve some at high rev's and delay the drop in the curve some. If you can delay or reduce the downward slope of the torque curve, you'll increase HP. But keep in mind that this only occurs in the upper 3rd of the rev range where most people don't spend a lot of time. The NASCAR restricter plate motors take advantage of this by running CR's up in the 16:1 or higher range.

The big caveat is that once the CR gets above 10:1 on street gas, you really need a good engine management system to prevent combustion problems like pinging and detonation. Twin plugging in engines with hemi heads (like the 911) helps. Engine's with compact combustion chambers (like SBV8's or 4 valve/cylinder heads) are a little bit more forgiving because of the reduced flame travel and increased turbulance. With race fuel and no intake restrictions, the general limit seems to be +/- 12:1.

Disclaimer: I'm no professional engine builder and of course the devil is in the detail. A good and clever race engine builder might be able to build you a happy street motor with CR of 11:1. I'm just sharing what I've found from analysing a whole lot of different engines.

left to right
2.7RS (similar to 2.4S), 2.4T and 2.2S

The 2.2 piston has the bigest dome.
http://forums.pelicanparts.com/uploa...1089145751.jpg

You would not get 9.8 with the 2,2 pistons as there is also the difference in crankshaft (stroke) for the 2,4.
With this CR there is no need to twinplug.

A lot of the 2.2 piston upgrade is discussed in this thread: http://forums.pelicanparts.com/911-engine-rebuilding-forum/164556-using-2-2s-pistons-2-4s-engine.html

Good luck with your engine drop and lift tomorrow. Realy hope it workes out this time!

wow , John , heavy reading !!

i read it like so ( correct me if my interpretations is wrong )

CR isn't really a HP thing , it's more effective as a torque thing

Long duration cams ( i think that's what i have in my S ) don't have much Compression in low revs beause they don't compress till the the intake is closed

so for my cams , increased compression rate works nicely torque wise

airflow wise : it could give some more HP in close to redline, on streat engines, but since mine is an S, i would benifit less than on an engine that is more tuned for the street, on a car with a dyno chart with more low end grunt > not S which only delivers from 4000 and upwards

is my readback somewhat right?

thanks Peter, this thread is giving me several new insights
especially on stuff i haven't messed with , and can't see without cracking my case ( which i wasn't necessary , and hopefully won't be any time soon )

BTW , if my black one is running , i'll come around and show it to you , it ain't far, and you know a lot more from it than i do , so you can gimme some honest and objective feedback on it :D

It's actually not so simple. You can see it like this:

When ignited, mixture will not explode but burn. That "wall of fire" which we call flame-front will move at finite speed. If your piston is domed, combustion chamber big and plug offset-placed it will take long time for flame to reach other side and ignite every little hamlet in combustion chamber.

Pressure-peak will rise lazily and you have to use lot's of advance. Problem with advance is that you ignite before TDC, so flame-front is moving toward perifery and piston is moving upwards at the same time. So ignition started on the one end but on the other you have mixture being squeezed by the piston. If squeezed too much, this mixture will explode (detonate) and you have knock.

By lightening the mixture from both sides, flame-front will need to cover just 1/4th of distance instead of 3/4th. You have pressure peak that rises quicker and can use less advance and more of pressure will be used to press piston downwards instead of lightening up while piston is still moving upwards and hoping that it won't detonate.

Domed pistons will make flame-front "go around the bend" and take even more time before it lightens up everything.

Best practical combustion-chamber design is bowl-shaped piston with cloverleaf combustion-chamber, centrally placed spark-plug and four vales: http://project911.luminasweden.com/images/aad.jpg

this reminds me to back in the days wheni was beginning in my career of networking and puters
i was all gung ho and interested, till somebody started explaining packet traces and OSI layers... wooooahh , to much info ...

now i need to figure out what the use and effect is off advance
man , i opened up a can of worms here:D

The static compression should be the final parameter decided when building an engine.
Some things to consider ,
how is the car driven? Will the engine see full throttle below the torque peak?
How much does the car weigh? What is the expected load on the engine?
What grade fuel will it run? Are you willing to mix in a little racing fuel for track events?
Combustion chamber and piston dome shape?
MFI, EFI, Carbs? Exhaust system?
What camshaft? Deciding the camshaft has the closest relationship with setting static compression. The cam should
be chosen for RPM band, and airflow expected.

I usually get called when the customer it finished buying everything and needs camshafts. With the cost of pistons,
the cams sometimes have to be a compromise.

Weber 40 IDTP 13 C

Can somebody help me find a repair kit for Weber 40 IDTP 13 C carbs? I have been having no luck. Car runs only with full throttle and low speed needles are not having any effect on mixture. I had no luck with the Pelican parts counter and Red Line told me "not available"!

Hi Gene:

I'd wager you that your idle passages are clogged and you should:

1) Thoroughly clean out the fuel tank and make sure its totally rust free

2) Install a very high quality fuel filter

3) Remove the idle and main jets so you can blow out all those passages with clean, dry air.