Combustion chamber mods

[jeffro ra28]

yuh sorry about that. All 2v stuff.

Motorbikes mate. motorbikes.

[abently]

There is very little to gain in combustion chamber mods...... and this only applies to very low lifts, which I doubt will apply to your case.

With regards to squish, you need it in N/A Engines but not so in Forced Induction.

Just concentrate on the Intake ports as this will be where you gain the most positive results. The exhaust should be fine with just a stage 1 (clean-up casting) type port work.

[jeffro ra28]

There is very little to gain in combustion chamber mods...... and this only applies to very low lifts, which I doubt will apply to your case.

With regards to squish, you need it in N/A Engines but not so in Forced Induction.

Just concentrate on the Intake ports as this will be where you gain the most positive results. The exhaust should be fine with just a stage 1 (clean-up casting) type port work.

I beg to differ entirely with what you have said.
Very considerable gains are made in chamber work. Not only can you increase intake flow, but you can increase exhaust flow aswell. Whilst reducing detonation risk and allowing more ignition timing.

You also need squish especially in forced induction engines because under high boost, there is that much fuel going into the cylinders the need for squish is still very evident.

How can you justify concentrating on the intakes only?
Engines are specifically designed with a ratio of flow between the intake and exhaust, it is important to keep this ratio! So if you are increasing the intake flow by say 20% then it is aboslutely nessacary to increase the exhaust flow by 20% to keep this ratio.

N/A engines generally should have a ratio of 70-75%. Forced induction engines should have from 80-85%

If otherwise, please justify your reasoning.

[jeffro ra28]

low lift flow is also very important. As on one lobe opening it will go past this low lift area twice. Once on opening and once on closing, where as at full lift it only see's it once!

It also helps to keep the charge flowing past the valve when the engine is past BDC. The more air and fuel that can be jammed into the cylinders the better.

[Sam_Q]

I have to agree with jeffro on this one, if the vlaves are shrouded or they have things in the way of the air flowing out of the valves then it will hurt it. As for low lifts thats all relative to what you call low, I am so far looking at the stock 7.8mm then later to possibly 9mm inlet + 8mm exhaust

[Ben Wilson]

low lift flow is also very important. As on one lobe opening it will go past this low lift area twice. Once on opening and once on closing, where as at full lift it only see's it once!

I disagree, increasing flow at low lift has pretty much the same effect as putting in a cam with greater duration. All engines love lift but it is really hard to achieve without having excessive duration (even on a modern multivalve engine). If you manage to reduce flow at low lift while increasing flow at high lift, you can run more duration without any real detrimental effects and therefore get away with more lift without having to spend gigabucks on valvetrain hardware.

Of ourse if you just want a motor which is gutless down low and takes longer to come 'on cam', by all means port the head for extra flow at low lift....

[jeffro ra28]

I disagree, increasing flow at low lift has pretty much the same effect as putting in a cam with greater duration. All engines love lift but it is really hard to achieve without having excessive duration (even on a modern multivalve engine). If you manage to reduce flow at low lift while increasing flow at high lift, you can run more duration without any real detrimental effects and therefore get away with more lift without having to spend gigabucks on valvetrain hardware.

Of ourse if you just want a motor which is gutless down low and takes longer to come 'on cam', by all means port the head for extra flow at low lift....

Could you please explain why low lift flow is detrimental? And why it simulates a long duration cam? Im a bit keen to see

I see it like this. Increasing low lift flow would decrease the need for high duration and high lift camshafts. Hence the motor is more likely to pull down low.

I realise what im about to say is like comparing apples with oranges, but just some food for thought. You could spend $20 000 on the latest racing v8 2v heads. Which are designed to turn upwards of 9000rpm, but at low lift, they actually flow less then my 1uz heads in bog stock form.

May i also add, that most engines do not benefit from more lift unless the ports have been modified to compensate. Most of the time, the flow of a stock port basically plateus when its reaching full lift anyhow, meaning that it is wasted energy opening the valve any further if the port isnt going to flow any more with the extra lift.
Recently i did a 4 cylinder in an aircraft, the thing actually lost flow after using the last .050" of lift.

[jeffro ra28]

low lift flow is also very important. As on one lobe opening it will go past this low lift area twice. Once on opening and once on closing, where as at full lift it only see's it once!

It also helps to keep the charge flowing past the valve when the engine is past BDC. The more air and fuel that can be jammed into the cylinders the better.

I disagree, increasing flow at low lift has pretty much the same effect as putting in a cam with greater duration. All engines love lift but it is really hard to achieve without having excessive duration (even on a modern multivalve engine). If you manage to reduce flow at low lift while increasing flow at high lift, you can run more duration without any real detrimental effects and therefore get away with more lift without having to spend gigabucks on valvetrain hardware.

Of ourse if you just want a motor which is gutless down low and takes longer to come 'on cam', by all means port the head for extra flow at low lift....

Ok sorry, so are you trying to say that low lift flow is NOT important?

[Ben Wilson]

That's exactly what I'm saying. I've had this argument in the past from your point of view, and it took a while for me to get my head around it.

Valves are at low lift twice in a cycle where they are only at full lift once, so you'd think that low lift flow would be critical, right? All good so far, but where is the piston when the valves are at low lift? It's either at the top or bottom of the bore and not moving very fast..

That means there is bugger all potential flow compared to full lift. The flow bench measures flow at constant vacuum. Max lift occurs pretty close to max piston velocity, and that's where you find power.

[oldcorollas]

i would have thought the end of the intake stroke is also important.. you want the air, that has velocity in the port, to keep going in for as long as possible..... of course getting that velocity in the first place is also important...

[myne]

I still get a kick out of reading this :
http://theoldone.com/archive/pro-sto...ing-engine.htm

But you're probably more after the ones like this :
http://theoldone.com/archive/quench-area.htm

I've always wanted to try staggering the individual valves with different profiles.
Say inlet 1 and exhaust 2 (diagonally opposing) share one common 'wild' profile with appropriate seperation and inlet2/exhaust1 share a milder profile. should create swirl, extend the torque band a little and reduce the lope on idle from what'd otherwise be a lumpy engine if they were both on the wild profile.

[jeffro ra28]

I agree with you on that side of things ben.
I still find it hard to beleive that it is NOT benifitial. Which is what you are basically telling me.
As Stu said, you still want the air to keep flowing after the piston has reached BDC. Which is why most camshafts close so many degrees after BDC. Because the air keeps flowing, one would say "the more the better right"? This is why i argue that low lift flow is still important, because at this time where the air is still flowing at low lifts but the piston is on its way back up the bore. This is called charge motion.

If you have good charge motion, the cylinder will continue to fill even after the piston hits BDC and the air/fuel charge will evenly disperse in the chamber. This make horsepower.

Then we can factor into the eqation of valve overlap. It may seem like all our low lift flow is going straight out the exhaust?
On a recent REAL engine that i modelled on EAP, it told me that 15% of my fuel air mixture was going straight out the exhaust on overlap. This is fine, great for cooling down the entire system, and keeping your EGT's down, for turbo application probly even more beneficial. Obviously not built for efficeincy tho.

[jeffro ra28]

I still get a kick out of reading this :
http://theoldone.com/archive/pro-sto...ing-engine.htm

But you're probably more after the ones like this :
http://theoldone.com/archive/quench-area.htm

I've always wanted to try staggering the individual valves with different profiles.
Say inlet 1 and exhaust 2 (diagonally opposing) share one common 'wild' profile with appropriate seperation and inlet2/exhaust1 share a milder profile. should create swirl, extend the torque band a little and reduce the lope on idle from what'd otherwise be a lumpy engine if they were both on the wild profile.

I see what your saying,
2 valve chambers swirl, however 4 or 5 valve chambers tumble. But, if you could do what you are saying, it may mean both!? Even better. Just a mass of turbulence!? This however soaks up energy, fine for enginesdesigned for efficiency.
However, swirl or tumble is only benefitial in the first 4-5000rpm of engine speed.

[Ben Wilson]

I agree with you on that side of things ben.
I still find it hard to beleive that it is NOT benifitial. Which is what you are basically telling me.

It has benefit in the same way overlap has benefit. Too much overlap and you lose bottom end power. Too much low lift flow will have the same effect.

[Sam_Q]

I agree with that, too much lift and you go downhill again even if the flowbench says otherwise. Roughly 1/3 of the valve diameter is a guide according to what I am reading now.