Rodrigues

How important are high flowing heads when it comes to an engine producing the maximum of horsepower possible. Would one just be wasting money putting large amts of time into the heads to achieve flow numbers over the 320cfm mark with aftermarket castings with lets say a t88? How much horsepower gain is possible with a ported stock casting over a ported aftermarket casting at similar cylinder pressure.

carnutz

Im interested too as I need to figure out what to do for heads, I need a 76cc chamber so I'm pretty much stuck with AFR's. Im trying to decide how much head I need on a 383 and a T76GTS.....195's? 210's?? 220's???

Cerwin Vega Fan

If you have an LT1 look into some Trick flow heads. They are made for blower applications and will lower your compression ratio.

Fast Caddie

How important? VERY IMPORTANT. Some people will say "it's a waste, just turn the boost up". You will want the largest ports and biggest valves you can fit in the thing without compromising drivability while not boosting. Boost is just a measure of restriction in flow. The less restriction, the more air you get into the cylinders and the quicker you get it out. And lesser the restriction, the less heat is made. You want the incoming charge to be as cool as possible since cool air is denser than warm air (more oxygen per unit volume). You also let more air out of the cylinder faster to get into the exhaust system in route to the turbine of the turbo. If you let more air out faster, you'll allow more air to move across the turbine where it will spin the compressor at a faster rate. It's a cycle that builds on itself. More air in means more power, more exhaust gasses out faster means more air in faster. The more you improve this overall flow rate, the more power you make. You also increase your potential for more power with better flowing heads.

An arbitrary example: Say you have a built, 9:1 CR 355. Let's say you can make 500rwhp at 15psi with the stock heads and cam. You port your heads and now at 15 psi you make 600rwhp. You made a larger volume for the air to move through. And since you're under boost, that air moves easier with less restriction. You don't have to worry much about port cross-section and velocity like in a NA setup. The air under pressure will naturally move to areas of lesser pressure (the combustion chamber). In a NA engine, you have to make sure your velocity and such are acceptable so that air will move easier under the influence of vacuum. Ports and valves too large can hurt low end performance and might not always make an improvement in other parts of the powerband. With boost you enjoy the benefits of not having to be a slave to all these limits. Just make the path is easier for the air to move, get it as cool as possible (use an intercooler or alcohol injection, or even nitrous, and make the ports and valves larger to lessen restriction and make more valve curtain area). VCA is the cylindrical area made by your valve when it is open. It's important since all the air you force into your engine has to go through these small areas to get into the cylinder, where it has to be to make power. Making them larger via larger valves and bigger lift cams will make a larger area for the air to flow with less restriction, so more power can be made.

Then if you were to go with another cam ground for boost with a little more duration, a lot more lift, and very little overlap. You now lessen the restriction further and give the incoming charge a longer period of time to enter the cylinder.... and make 650-700rwhp at 15psi. Not only do you let more air into the cylinder, you let the air in easier, which will be cooler. And from here you can turn the boost up (provided your bottom end is built to take it) and at 20psi you might be making 750-800rwhp.

Remember the cycle, and if you improve those areas you can go to a larger turbo (which will be more efficient at higher boost levels) and make more power. If you're not afraid to rev it high, you can make more power at higher rpms with more boost since you're moving and burning more air and fuel at a faster rate... and horsepower is a measure of rate.... the rate at which the torque you make is being used to perform work... and that work is moving your car faster.

This is why the japs developed the supra and skyline engines.... they have 4 valve heads to move a lot of air and turn many rpms to use the torque they make at a faster rate. A 900rwhp supra might make 500rwtq (as an example), but since they turn 7000+ rpms they use that torque at a faster rate and make a lot of rwhp. A 900rwhp LT1 might make 800-900rwtq, but since they turn fewer rpms on average, the split in their hp and tq numbers aren't so large. There are many, MANY things to consider.

Better head flow + more boost + higher rpms = much more power, no matter what displacement you have. Kenny Duttweiller is making an estimated 1500rwhp with a little 4.5L Buick V6. FMS has a 1200rwhp supra at a mere 3L of displacement. And Murrillo's turbo mustang (4.6-5.0L? mod motor) is running 6's at over 200mph. Just goes to show that with a properly designed boost-utilized system can make as much power as you'd never dreamed of with any engine. Just make sure the block you're using can handle that much power

Which leads to the issue of block strength. If you go with.......

Aw the hell with it.... i've had a little too much to drink and can't shut up. Sorry for rambleing. Hope this helps

carnutz

So your implying that you cant get a head that is too big?
I had a 383 with 235 proactions on it and it was a pooch,(no boost tho, so I am asking here not flaming) I think you can go too big on heads even with a force fed application. Cruise mode will more than likely be little or no boost right? so It still has to have some velocity correct? I can see if your building a race only car, you really dont care how it drives to the staging lanes as long as it hauls when you mash it. But For something that sees street duty you gota drive the thing too.
Your analogy seems correct for a race only set up........

Fast Caddie

Not entirely. Those 235s might have been too big for that 383, although we're not sure what cam, compression, timing, etc was done do it. Vacuum fed engines are a lot more picky as to the size port, valve, and cam characteristics they like than a force fed engine. The pressure (boost) that is generated in the intake will force the air to push against all walls of the intake, and will move to areas of less pressure, like when your valve opens and the cylinder is at lower pressure than the intake. It will naturally flow into the cylinder because it is being forced in there. When talking NA engines, you have to "persuade" the air in by means of better flow and optimal port velocity because you can only "stretch" air out so much. What i mean is after a certain point the air just will not flow any more, which is where the term "port stall" comes from. To get more flow you need to make the path easier to flow, but keep the port size and cross-sectional area minimal to retain port speed... and more port speed (to a point before port stall) means more momentum for the incoming charge to force it's way into the cylinder even though the pressure differential between the port and cylinder has equalized.... which is where "ram tuning" comes from. I believe it was an article written by David Vizzard, where he stated that in a NA engine, you can get as much as 5psi of pressure inside the intake ports when the valve opens if ram tuning effects have been optimized. This is why port speed and cross-section are so important in a NA engine.

In a forced induction setup you don't have to worry about that..... that turbo is constantly acting like "ram tuning" during the whole period of boost. You really just want the pressurized air to have the easiest path possible, which is where bigger valves and ports come into play. I followed a 4.1(?) litre stage 2 buick v6 during it's build up a couple of years ago. After the engine was finished, they were just waiting for the turbos to come in. To get a base line for the engine they put it on a dyno..... it was a 7000rpm built engine, and NA it made somewhere around 300rwhp. Sucks huh? Well once the turbos came in they put it back on the dyno and it pulled over 1000rwhp at 30psi. The intake ports on the stage 2 heads are enormous.... you might confuse them with BBC ports they look so big on those little V6 heads.

For a normal street engine you generally want a larger port to a point where it doesn't compromise driving while not under boost. With the right cam, a ported AFR 210 or 220cc head can work very well with a 350 or 383 for daily driving (in a FI setup only). And once you get under boost, you reap the benefits of those larger heads by getting more power per pound of boost.

[edit]- one person that comes to mind is INTMD8, you should research his setup and talk to him about it. His car has a 355 LT1 topped with ported AFR heads (finished port at ~215cc). That's a pretty big port. And a baby cam with int and exh. durations not even in the 220s. From all the videos i've seen and hearing others talk about it, that car purrs like a kitten at idle. But when he romps on it, he'll have about 850rwhp in just the blink of an eye. Since it's a low compression big headed engine, he might be lucky to hit 350rwhp without the turbos (just speculating). But the proof shows for itself.

carnutz

So it is still posable to get too big of head BUT a FI isnt as "picky" as to port size. Correct? So how do you go about sizeing the heads for your combo?

Fast Caddie

It's really more of a trial and error process... there's no set path to the right setup for a turbo project. Look at what others have done, make some assumptions about what you want from your particular setup, and start going for it. See what happens and make some changes, then try again. Some turbo setups have hit 500rwhp on completely stock-headed LT1s (as in UN-ported heads). The LT1 port can go out to ~200cc when fully ported and might hit 270-280cfm on the flowbench. Not sure what the hp limit is on a set of maxed out LT1 heads, but i'm sure you'd be happy with them up to about 5-600rwhp. After that, IMO, it'd be better to go for trick flows or AFRs since they'll make it easier to acheive those higher goals and the heads themselves are just thicker and stronger. Don't want to build an expensive engine just to have a cheap factory head warp on you and ruin it all.