i need some enlightment please.
What is the benefit of increasing engine compression, and how is it done ? and when should it be done ? (whats engine mods are done to warrant the higher compression)
If someone can roughly expain it to me, because i know the concept of it, but dont actually know what i am talking about !
I'll ramble on.Compression is what is needed to get power/ combustion. On an engine with a fair overlap cam where air resonates in and back out of port at low speeds. Makes V.E low so compression is built up to get some bottom end rpm power.
The more compression the better combustion can push piston down the bore.In most cases its done on higher reving engines where inlet manifolds are shorter [Gen lll] and as said slightly lumpy cam. As in XR6... See the compression off the 290 kw?
Compression Ratio is deemed by the swept volume of air/fuel of the cylinder bore by the piston from Bottom Dead Center (BDC) to Top Dead Centre (TDC) called the Stroke Plus the area of the Combustion Chamber.
Basically if the compression ratio is 10:1 the amount of air/fuel mix at BDC is compressed to 10% of its original size at TDC.
Raise the ratio, the air/fuel molocules have less area to move around in. They get hotter by bouncing into each other more. When they are ignited they rapidly burn faster than they used to creating quicker and greater combustion expandig pressure. I say greater because no four stroke engine ever gets complete combustion.This forces the piston down quicker, hence getting more power. This is a more accurate description.
To achieve higher compression ratio? Some ways:
*Lengthen Stroke of the Engine
*lower the block deck height
*change pistons-many ways
*Change combustion Chamber, shape, size.
*Machine the head
thanks alot for that guys.
the reason i ask, is i am contimplating getting a 347 stroker kit for my XR8 and was told maybe a higher compression (compared to standard) would help too, but it sounds like it will already increase by doing this anyway.
oh yeah that was my other question.
but it seems like its pointless.
because as i said i am looking to stroke, and want to get some better heads, and thought that raising compression after i do this could give more HP, but by the sounds of it, stroke + heads will increase compression, am i right ?????
what compression did the au2 200kw 302 run at?
As you said, you don't have a lot of knowledge into what increasing compression ratio does, so don't leap into increasing it!
What has been said so far is correct, but what also is missing is some of the down falls.
Increasing compression can cause pinging or detonation if it is too high or other conditions are not optimum, then the need for high octane fuels to prevent this, otherwise engine failure is likely to happen.
High compression engines can be harder to start.
High compression engines can have less engine life.
High compression can sometimes increase engine temperature.
But as always, there is always pro and cons to anything. :s5
something that wasnt said yet is that stroking will effectively lower you optimum rpm. therfore if your peak hp was at around 5500 rpm then the stroked motor might peak at 4800 rpm which refers back to what was said about high compression being somewhat better for high rpm where you have less time to exchange fuel and air .
something that wasnt said yet is that stroking will effectively lower you optimum rpm. therfore if your peak hp was at around 5500 rpm then the stroked motor might peak at 4800 rpm which refers back to what was said about high compression being somewhat better for high rpm where you have less time to exchange fuel and air .
You are probably right here, but the bad thing is stroking a 302/5.0L would most likely decrease engine life, most stroker kits have a very poor stroke to rod ratio, increasing side loads and piston speed higher than optimum.
Exactly and if you turbo it if anything a "289" crank [longer rods, better piston speed,better pistons, rod angle etc etc]would be far better for the most boost you could run is around 12 Lp with good heads should yield around 380[509hp] rwkw well and truely enough to split a 5Lt Windsor into 2 four cylinders. So anything over 550 rwhp YOUR DREAMING !!!! Or going to have a very short engine life !!!! Believe me I have some links here some where??
I may be getting a bit more into detail but skim if you want.
An internal combustion engine (ICE) makes power by creating extreme pressures in the cylinder and forcing the moveable item (piston) down thereby driving the crankshaft. In this process there are many pressure points that affect the way an engine makes power (or more correctly torque or 'force').
The key factor in determining force is the pressure in the cylinder over the time of the combustion stroke. The engine gurus use a term called Brake Mean Effective Pressure (BMEP) which gives a value for this pressure over the combustion period. It doesn't however indicate peak pressures although a feel for the relationship can exist.
So, to gain maximum force for every cycle an engine designer needs to get the best possible BMEP (or if doing initial calculations iBMEP meaning 'ideal' BMEP) without exceeding set limits for the type of fuel, and the basic architecture of the engine (materials, combustion chamber etc etc).
Now it starts to get tricky because to gain the best airflows certain factors such as cam timing, inlet and exhaust, and compression ratio all affect BMEP at different points. You may have safe BMEP at say 2000rpm but come 3500rpm at good volumetric efficiency and an engine on cam the cylinders could be at their ideal point and the BMEP could be over the top. This can be evident in pinging or on a knock sensed engine the timing can be seen to be wound back by the ECU (in some cases to a retarded timing point!).
Having said that, an engine that has too low a CR will have a low base point for the cylinder pressures and even in combustion BMEPs may be excessively low causing a loss in efficiency and of course force, torque and ultimately power.
Some basic considerations:
1. Alloy materials provide a much higher heat transfer rate than iron inside a cylinder. This may seem a minor consideration but consider delta-Ts are in the 1000s and the energy loss can be severe. However, this also reduces hot spots and so detonation risks are reduced.
2. You must include the volume around the piston down to the first compression ring land when working out CR. Don't foget!
3. Sometimes higher CRs are achieved by wedge or other small combustion chambers. Make sure that if you want a performance engine that valve shrouding doesn't affect the issue.
Some basic rules:
A. Alloy heads - CR can go up
B. Longer stroke - CR can (will) go up (although piston speed at ignition is lower so mostly it should be wound back slightly)
C. Bigger duration cam - CR can go up
D. Open combustion chamber - CR can go up
Not exactly, close chamber heads, especially in Clevelands can have a higher compression ratio than open chambers because of the quelch area!(do I have to go into full detail about this or do you want too!) http://chevyhiperformance.com/techarticles/94138/ Don't flame me for using it, just it is a good article.
Related to this is the reason why you will get higher performance and can use higher compression ratio if if there is less piston to combustion chamber clearance, it creates a quelch area and more controlled combustion. You might notice that modern engines have no or very little piston to deck clearance compared to old engines, some performance engine builders actually end up with the pistons above the deck, carefull selection of head gaskets have to be choosen when this is done for obvious reasons.
i think what he was refering to was that the open chamber has less valve shrouding than a close chamber.
which can sometimes fill the cylinder quicker before the valves close and then give you the higher comp reading.
but thats not always the case uless you have a very efficient intake
Actually, I was referring to the fact that an open style chamber (such as a hemi or more modern designs) has less distortions in the path for things like flame fronts etc. Yes CR would go down but the engine as a whole can run a higher CR with less chance of detonation.
So, going on to what you guys are talking about, if you put a closed chamber head on an otherwise similar engine you will indeed get a higher CR.
Actually, I was referring to the fact that an open style chamber (such as a hemi or more modern designs) has less distortions in the path for things like flame fronts etc. Yes CR would go down but the engine as a whole can run a higher CR with less chance of detonation.
So, going on to what you guys are talking about, if you put a closed chamber head on an otherwise similar engine you will indeed get a higher CR.
What I was saying is the same engine can run a higher compression ratio with quench heads than open chamber heads i.e. Open chamber 4V 10.5:1 and close chamber 4V 11.0:1 using the same valve train etc.
The quench area allows you to use a higher compression ratio than an open chamber design.
In saying this, this doesn’t mean the performance from the quench head is better than the open chamber head, just because the compression ratio can be higher without pinging or detonation; it is a much bigger science than this!
Hemi head designs are harder to achieve as the spark plug normally needs to be fitted in the centre, lots of so called hemi heads are not hemi heads, as the chamber is not a half hemisphere.
hey i got a good one for you guys not to blow your mind but boggle it just a bit.
i have a 351 w and i put a set of windsor jr cast iron heads on it. i also had the block decked just a touch to make sure it was good and flat. i have the original style flat top pistons but they are also aftermarket with a little bigger valve relief and the heads have a 58 cc combustion chamber.
now i put it all together and set the ignition timing at 10 degrees and it knocked like hell on hills so i started running premium gas in it and that is all i use now to keep it from blowing apart.
here is the funny part i checked the compression which was supposed to stay at around 10:1 and cranking the starter with 7 plugs still in and not holding the throttle open i am getting a cranking compression of 193 psi to 195 psi.
oh i forgot to mention i ported the heads too, so how does this figure into the compression lesson ?
is this kinda high or just about right for the compression numbers or did i just to a really good job at porting the heads ?
In my experience that's big numbers for cranking compression especially with throttle closed etc. Did you do the proper CR calcs before putting it together?
hell no they told me at the machine shop it would be 10:1 with those heads and the trw pistons i used
they were the ones that bought the parts and i thought they would know since they build race engines everyday.
i knew they valves would never hit so all i did was put it together and now i cant even set the timing where it is supposed to be or it will blow up.
hell no they told me at the machine shop it would be 10:1 with those heads and the trw pistons i used
they were the ones that bought the parts and i thought they would know since they build race engines everyday.
i knew they valves would never hit so all i did was put it together and now i cant even set the timing where it is supposed to be or it will blow up.
Never take word of mouth mate, only way to check is to measure it.
(Salesman talk, “It will be right")
Checking cylinder pressures doesn’t prove much either, if it has a bit of blow by and high CR it would read probably normal, big overlap cam can also give you lower cylinder pressures even thou it has high CR.
Checking cylinder pressure is good for checking differences between cylinders, too low pressures and problems (i.e. wet test to check rings or valves etc)
Aussie Pete said:
Given this is a Windsor forum I agree your approach makes sense in this context. However, if we were talking Modulars.......
Got to love those modular's! One day I will own one! :wavey:
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