Combustion chamber mods

[abently]

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.

That is correct. Most pro Head guru's will try and keep low lift flows to factory cfm. If you look at an F1 camshafts, you'll notice the duration is about the same between the exhaust and intake cams, except max-lift on the intake cam is pro-longed as much as physically possible.

It is also at low-lift where reversion (overlap) occurs.

[The Real Roadrunner]

dont mean to be rude but seriously whats the point in comparing F1 stuff to an engine that would be lucky to see over 9000rpm.
there 2 different horses for 2 different courses. 1 is an all out no compromise race engine the other is a modified factory engine that is compromised in alot of ways.

whats the rod/stroke and bore/stroke of an F1 motor? nothing alike to a toyota engine thats for sure.

yes, some theory can be transfered but not everything works the same.

for instance take a 4age 16v and 20v and build them with the same cam profiles, comp, timing and fueling. then fit them with some headers of 4-2-1 design and run them on the dyno, they will be different. then change the collectors over from 1/4 and 2/3 to 1/3 and 4/2 and the 20v will like it but the 16v wont.

they're effectively the same engine but respond very differently to the same mods, go figure?

cheers
linden

[clubagreenie]

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.

But when the piston reaches BDC, won't the vacuum created/pressure of FI fill the cylinder to max capacity. The piston rises lessening volume and potentially emptying the cylinder back into the inlet tract? (sort of reversion)

[MWP]

But when the piston reaches BDC, won't the vacuum created/pressure of FI fill the cylinder to max capacity. The piston rises lessening volume and potentially emptying the cylinder back into the inlet tract? (sort of reversion)

No... because the in flowing air still has enough force to continue to fill the cyl, even when the piston is on the way back up.

[clubagreenie]

Even in N/A?

[MWP]

Yes.

Of course, there is a point after BDC at which the piston will start pushing the air back out the inlet.
Finding this point is important in cam tuning, and is where adj cam pulleys and a dyno come in.

[jeffro ra28]

Why else would cam manufacturers want to close the intake valve a certain amount of degress after BDC. Sometimes it is quite consdierable.

[jeffro ra28]

That is correct. Most pro Head guru's will try and keep low lift flows to factory cfm. If you look at an F1 camshafts, you'll notice the duration is about the same between the exhaust and intake cams, except max-lift on the intake cam is pro-longed as much as physically possible.

It is also at low-lift where reversion (overlap) occurs.

F1 no longer use camshafts.
They use solenoids.

It is why low lift flow and velocity is important so that it prevents reversion.

[rms]

F1 no longer use camshafts.
They use solenoids.

It is why low lift flow and velocity is important so that it prevents reversion.

Please get your facts right - F1 still use camshafts !!!!!

What is low lift ??
With the inlet valve having 5mm of lift at TDC overlap and not closing until the piston is half way up on the compression stroke ( 90 deg ABDC) flow at 2mm lift is almost meaningless.

Reversion is more to do with inlet and exhaust lengths working against you.

[Ben Wilson]

No... because the in flowing air still has enough force to continue to fill the cyl, even when the piston is on the way back up.

Momentum is the key word here..

[abently]

for instance take a 4age 16v and 20v and build them with the same cam profiles, comp, timing and fueling. then fit them with some headers of 4-2-1 design and run them on the dyno, they will be different. then change the collectors over from 1/4 and 2/3 to 1/3 and 4/2 and the 20v will like it but the 16v wont.

they're effectively the same engine but respond very differently to the same mods, go figure?

cheers
linden

That's true. All I was trying to point out, is the current trend has been to minimise the amount of lift during the overlap period.

Do we all agree, that there is little to do to the combustion chamber on most modern Engines ?

The current trend has also been to flow the Cylinder Head backwards and try to limit this flow as much as possible without affecting the forward flow. If playing around with the combustion chamber helps this, then your heading in the right direction.

[jeffro ra28]

My apologies on F1 camshafts, i do recall tho somwhere along the line that BMW replacing cams with solenoids and pneumatics. I know Valentino Rossi's yamaha now uses a pneumatic a valve train.


Just a tad more information on the importance of low lift flow.
Quoted from Colin Loyd's write up in "dragster" magazine.

"The importance of high flow and velocity in the low lift portion of the exhaust cycle (to about 70 percent of the total exhaust lift) can't be emphasized strongly enough. This phase is called "blow down" and is where the piston is descending toward BDC and the cylinder volume is expanding. At the same time we are trying to expel the cylinder of waste combustion. High cylinder pressure is the only reason this takes place. The more we can get out before the piston turns around and has to pump it out, the better off we are. Hence the need for very good low lift exhaust flow."

[jeffro ra28]

Would i suprise anyone if i said 90% of the fuel in an efi engine is inducted when the valve first opens?

[rms]

Would i suprise anyone if i said 90% of the fuel in an efi engine is inducted when the valve first opens?

What are we talking about ?
If we are talking about an engine to meet emission requirements at part throttle .... no you would not surprise me.

If we are talking high performance engines at WOT ... yes, that would surprise me.

Injection timing I am currently useing at WOT and 8000 rpm is injection start - 19 deg BTDC, injection end - 60 deg ABDC.

Deliberately using large injectors to keep the injection period contained within the induction period. On lower throttle and/or less rpm injection period is centred on max piston velocity, around 102 deg ATDC.

[Ben Wilson]

Yeah, but your motor doesn't have to idle, so you can get away with using garden hoses in place of injectors...