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Greg, I do fully understand. That cam lobe shape was designed for maximum performance, as Strosek has pointed out. It is a shape that tends to out-perform others, but it's a bear on valve train parts. We used it, and it worked for a year and we made 900 BHP and later 960 BHP with those cams.
It was not accident. We went for ultimate performance, and encountered this problem. I just wanted to share this experience so possibly, those going for the .500 lift numbers will avoid the same issue I encountered.
Reading above, they have contemplated this issue and are working around it by installing larger diameter buckets and changing their cam lobe profiles. Perfect.
With all this flow potential and stock bottom end not being able to use it, here's one idea: how about porting the heads for high tumble design? Motorcycles that have huge valves and short stroke often use this design. The idea is to intentionally bias the high-lift flow towards the outside to get an efficient tumble motion in the cylinder. 39mm valves and new seats might allow one to do that?
The reason I've been thinking about the "tumble" motion recently is that it's one of the main determinants in a four-valve head of how the charge burns. The four valve head with more tumble motion burns both faster and more biased to the exhaust side:
So , what MINOR porting is recommended for a stock 4 valve head ?
What camshaft profiles? What bore, stroke, peak-power rpm, and redline rpm? What intake manifold? It seems to me that what one should do to the heads depends entirely on what the rest of the engine is and what it is intended to do.
The reason I've been thinking about the "tumble" motion recently is that it's one of the main determinants in a four-valve head of how the charge burns. The four valve head with more tumble motion burns both faster and more biased to the exhaust side:
you might be able to make use of this if you aren't familiar with it already...
from Subaru WRX/STI engines, called the TGV (tumble generator valve).
in their application it is an emissions aid for cold engine/low load running, tumbling better mixes the fuel into the air.
the "butterfly" is D shaped to match the upper D shaped passage - when activated the butterfly shuts and all incoming air has to go through the narrow slot at the bottom of the photo.
these TGVs mount directly to the cylinder head, and the intake manifold goes on top of these (think like a CIS 911 fuel injector mount + upper manifold).
you might be able to make use of this if you aren't familiar with it already...
from Subaru WRX/STI engines, called the TGV (tumble generator valve).
in their application it is an emissions aid for cold engine/low load running, tumbling better mixes the fuel into the air.
the "butterfly" is D shaped to match the upper D shaped passage - when activated the butterfly shuts and all incoming air has to go through the narrow slot at the bottom of the photo.
these TGVs mount directly to the cylinder head, and the intake manifold goes on top of these (think like a CIS 911 fuel injector mount + upper manifold).
The tumble generator valves are useless at high loads and high rpms. Too much pressure drop, so it's always off at high loads and high rpms. If you are porting stuff for performance use, that's the first thing you'll get rid of.
The 4V heads are usually making tumble by default. Also, my understanding is that dished piston top will allow tumble to spin up until point of firing the mixture
whereas flat piston top tends to kill the tumble before mixture ignition point.
Thinking of my early -87 S4 combustion chamber, I would like to have additional squish bands as you have in one of your heads.
One of the advantages adding additional squish bands is that compression ratio can be set closer to 11:1 with stock pistons.
After all, my -87 will have 11:1 compression, optimally dished piston top, some tumble and more squish velocity. All this together with reduced intake port CSA, 110 LSA modified cams, good headers
and 270 - 280 CFM flowing S4 intake should help to pass the 500nm torque along with closer to 450ehp, I hope.
For a normally aspirated five liter motor, here's probably what I'd do now, although this is day to day... and what do I know:
I'd try to port the valve seats to flow equally around all sides of the valve at very low lifts. I'd bias the high-lift (7mm+) flow towards the long side of the intake port to generate efficient tumble. This biasing without hurting overall flow coefficient would probably require a slight increase in the throat diameter, moving the seat contact area to edge of the valve, and using a narrow intake seat. I'd sink both the exhaust and intake valves close to the wear limit to improve tumble generation and to prevent compression of the hydraulic lifters. I'd shave off some from the heads for higher compression and use the thin head gasket to minimize the squish clearance to safe minimum. I'd take out all the sharp edges from the pistons and combustion chambers.
Take a look at the large-eddy simulations of modern heads and I think you'll end up convinced that horizontal squish zones in the intake and exhaust sides work well. They are fully sufficient to break up the tumble into right-sized vortexes. If it's a pure dump port with intake valve to intake port angle very narrow and thus very low tumble index, then maybe the addition squish area would be useful? By my reading anyway.
You don't have an exhaust tailpipe emissions test, right? (It's been too long, I have no idea what's going on in there...) If you have the option to do so, I'd close in the LSA on the cams for more overlap because it's going to make more torque with a little higher emissions at a little rougher idle.
Originally Posted by simos
The 4V heads are usually making tumble by default. Also, my understanding is that dished piston top will allow tumble to spin up until point of firing the mixture
whereas flat piston top tends to kill the tumble before mixture ignition point.
Thinking of my early -87 S4 combustion chamber, I would like to have additional squish bands as you have in one of your heads.
One of the advantages adding additional squish bands is that compression ratio can be set closer to 11:1 with stock pistons.
After all, my -87 will have 11:1 compression, optimally dished piston top, some tumble and more squish velocity. All this together with reduced intake port CSA, 110 LSA modified cams, good headers and 270 - 280 CFM flowing S4 intake should help to pass the 500nm torque along with closer to 450ehp, I hope.
The 4V heads are usually making tumble by default. Also, my understanding is that dished piston top will allow tumble to spin up until point of firing the mixture
whereas flat piston top tends to kill the tumble before mixture ignition point.
Thinking of my early -87 S4 combustion chamber, I would like to have additional squish bands as you have in one of your heads.
One of the advantages adding additional squish bands is that compression ratio can be set closer to 11:1 with stock pistons.
After all, my -87 will have 11:1 compression, optimally dished piston top, some tumble and more squish velocity. All this together with reduced intake port CSA, 110 LSA modified cams, good headers
and 270 - 280 CFM flowing S4 intake should help to pass the 500nm torque along with closer to 450ehp, I hope.
It is wise to choose a larger LSA (less overlap) in combination with the stock intake manifold. With ITBs the engine will drive like a lamb (quote Mike Simard) even with much overlap.
Åke
For a normally aspirated five liter motor, here's probably what I'd do now, although this is day to day... and what do I know:
I'd try to port the valve seats to flow equally around all sides of the valve at very low lifts. I'd bias the high-lift (7mm+) flow towards the long side of the intake port to generate efficient tumble. This biasing without hurting overall flow coefficient would probably require a slight increase in the throat diameter, moving the seat contact area to edge of the valve, and using a narrow intake seat. I'd sink both the exhaust and intake valves close to the wear limit to improve tumble generation and to prevent compression of the hydraulic lifters. I'd shave off some from the heads for higher compression and use the thin head gasket to minimize the squish clearance to safe minimum. I'd take out all the sharp edges from the pistons and combustion chambers.
Take a look at the large-eddy simulations of modern heads and I think you'll end up convinced that horizontal squish zones in the intake and exhaust sides work well. They are fully sufficient to break up the tumble into right-sized vortexes. By my reading anyway.
You don't have an exhaust tailpipe emissions test, right? (It's been too long, I have no idea what's going on in there...) If you have the option to do so, I'd close in the LSA on the cams for more overlap because it's going to make more torque with a little higher emissions at a little rougher idle.
Tuomo, the Gerolamy heads you have for the other engine do have much squish area but it would be interesting to know how much the valves shrouding is affected. It is always a compromise between high flow and a compact combustion chamber. It would be interesting to flow one of these intake ports.
Åke
Ake -- your definition of little overlap is not exactly the same as most other people's definition of little overlap! ;-) We need numbers!
I'm thinking turbo heads right now, so those Gerolamy heads are waiting on hold. They are more appropriately sized in terms of combustion chamber volume, for example, for a normally aspirated application. But on a later date, once the blue turbo engine has been run thru the gauntlet and the next version of the turbo motor is being built with the information gained, that's maybe the time to think N/A motors...
Spencer -- I'm not a believer in the staggered closing of the intake valves. I believe (not to be confused with knowing) that simultaneous close and the appropriate amount of tumble from biasing the intake port and squish will give the same burn characteristics with better cylinder filling. Just my opinion, I have not proof of it, and some people think I'm nuts to think that.
After all, my -87 will have 11:1 compression, optimally dished piston top, some tumble and more squish velocity. All this together with reduced intake port CSA, 110 LSA modified cams, good headers
and 270 - 280 CFM flowing S4 intake should help to pass the 500nm torque along with closer to 450ehp, I hope.
Simos what piston are you using to obtain this thanks ?
03-11-2017, 11:45 AM
