hey guys-
does forced induction or compression effect how your heads should be ported?

would you want to favor intake or exhuast?
bigger TB?
velocity greater or less?
flow?
intake runner CC?
any answers?

thanks
trey

 

I'll take a stab...

Leave intake alone, and make a big exhaust. Then have a tight cam with a high LSA so that you do not bleed off too much boost. Lastly, I'd guess you'd want the cam to have steep profiles to get the valves open asap.

I don't know about intake runner, if it's bigger than stock, it may resonate at a higher rpm, which would effectivly more the torque curve upwards (I think).

I would guess that you would want a bigger TB because even though your engine size is not getting bigger with a blower, it is consuming air at the rate of a larger engine, since you need xxx amount of gas & xxx amount of air for 500Hp, regardless if it's a 2.0L twin-turbo 4-banger, or a NA 502 BB chevy

my theory is:

- small intake = increased velocity = increased torque. Because the blower is pusing the air in, a bigger valve will only slow down the air, which would hurt torque.

- Big exhaust would be good because you want to get all the exhaust out very quickly (without necessitating for a "overlap" where you loose boost through the exhaust valve)

 

Ok, this is going to be a long post...but based on my experience so bare with me.

In regards to your first question it is kinda one of those yes and no answers. Yes, for porting heads for a boost motor and no not really for compression. I'll start with the compression...ideally you shouldn't "port" for certain compression ratios or use different head gasket sizes. You want to have the bulk of your compression come from the piston. Thats why a lot of "race" engines (even FI) ones have very very chambers because most of the chamber is the dish of the piston and you can control detonation/flame travel better. But since we have street engines, what you want to do is port the combustion chamber to unshroud the valves as much as possible but to keep the swirl in them to keep the flame speed up. You also want to keep your quench in the .035-.050 range and that will be determined by gasket thickness,and how far in/out of the deck the piston is.
Now porting a cylinder head for a boost motor is 2 fold. You are going to have people tell you that you dont need a great set of cylinder heads for a boost motor and those that say you need an awesome high flowing set of heads. In a way they are both right and you should choose one of the two ways below that fits your budget/motor plan. The one way to go is with a set of "decent" home ported heads and a lot of blower/turbo and a lot of boost to go with it to reach your horsepower goal. The problem with this route is the strenuous loads on the motor it will face. If you have to use a lot of boost to make your power you are going to find a lot of little problems that will have to be top notch meaning: ignition/fuel/possible o-ringing/octane/timing etc etc. It is a way to go but you have to realize all those things
The other way to go is with a great set of heads...the only thing a really good set of heads does on a boost motor is allow it to make the most available power at a lower boost/psi reading which is just the amount of pressure that air is backing up on the pressure side anyhow. Its really simple if you think about it, if the head flows more....it can use more of the airflow your turboo/blower is putting out without causing huge pressure in the intake (boost). This helps with saving head gaskets, both discharge temps out of the blower/turbo and the inlet air temps seen by the motor. I hope i explained that well.
In regards to porting a head for a boost motor you are going to have to port them to how you want the engine to perform. If you want to keep a lot of snappiess off boost power you are going to have to keep the intake port volume smaller and the compression higher, also more cubes help. If you port the intake port to a much higher cc you will feel a loss of low end torque but as i explained above it will make up for it on the top end when a lot of airflow starts going into the motor.
Lastly, in regards to throttle body size...there are debates about it EVERYWHERE. Personally i dont think you need a huge one, because the pressure that your blower/turbo is putting out is going to force its way into/through it anyhow and the money spent on the TB would be better spent on the heads/intake to flow the air. BUT, it is true the TB is on the pressure side...it is just my opinion it isnt as big of a bottleneck as some may make it out to be.

Hope this helps

And anyone feel free to challenge and call me an idiot it makes for a fun thread

 

Yeah your right this is going to be agood thread. I'm going to take a shot at this when I get home on my own account.

SStroker Ace

Bret

 

hehehe....i'm all about learning

I just hope you have more boost knowledge than your son has.... I wonder if Mr. SStrokerAce hiding

 

Spin-off questions:

How differently does a big dish/small chamber combo behave opposed to a small dish/big chamber set-up? Cup cars run, what, 30cc chambers?

For example, a 9.5:1 383 with a -31cc dish and 53cc chambers opposed to a 24cc dish and 60cc chambers. Blown of course

 

D'oh.. no time anymore to try to make posts worthy of a-tech.. but...


In short, air is air, and a good NA port is going to be a good FI port in just about every case. For FI we MAY go a little higher on E/I, but still... How big we go will also be affected by what the individual setup is.. mild or wild. Short answer

 

Vizard says:
To a smaller degree than with N02, we need to sacrifice a little intake valve size to accomodate larger exhaust valves.

N/A motors do very well with I/E ratio of 75%. But a Supercharged Application should be 80-85% ratio.

I'd say a good intake cleanup and small amount of massaging while "hogging out" the exhaust with an oversize valve would do the trick.

He also says: Very Important: is a good deshrouding cut to get the exhaust gasses moving for a quick enough "blowdown" before BDC.

 

AAAAANNNNDDDDD.. Vizard is wrong again. . Honestly.. his book has some good information in it, but alot of it is old theories from 30 years ago that people have long since realized is incorrect.

leave the intake alone? Hell, a blown motor will get a better gain from a good intake port than a NA one. As far as the exhaust, 70-75% is STILL enough. More wont hurt as much as it would NA, but I still wonder about overscavenging and losing some of that precious precious cylinder pressure.

That's not info that was read out of any book you can buy that i know of.. that's from the grinding jesus himself Greg Good. We've had Vizard ported heads.. and every single time theyve come through, the ports have been pretty but didnt flow, and he had his archaic valvejob on them. He knows alot and writes books, but he doesnt build anything that even comes close to competing with better race engine builders .


Stephen... I'd run as close to a flattop as I could.. cup cars.. eh.. 3X cc sb2.2...

 

Are you refering to a 30 degree angle on them?

 

hmmmmmmm some of Vizzards stuff has merit.


Yes in a centrifigal blower engine we find that more intake flow is very good for power, but the intake valve size should be made smaller since giving room for the exhaust valve is important. The Hogged out exhaust and large exhaust valve are very important. I would say a I/E of closer to 100% effective (not static flow) is even better. The reason for this setup is because, we want to have good ports that help the superchager put air in the chamber. We want exhaust ports that move alot of air out of the chamber, and we want a LARGE exhaust valve that gives extra valve area so we can have good low lift flow at the high exhaust pressures when the exhaust valve starts to open. What does that give us? a better cam choice with less exahust duration. You can take a whole lot of duration out of the exhaust with this setup, giving your precious cylinder pressure enough time to escape and you will have less overlap so you don't over scavenge. That's the point of it. The funny thing is when you put a bad cam with a I/E like this it will not work as well. If you are worried about overscavenging then you are running too much exhaust duration and it's in the wrong spot on the cam, and it's giving you too much overlap. Period.

Not trying to be a demostrative *****. Yes Vizzard has some things that are out there and some are not right, but the theroies behind his stuff make sense. I agree and disagree with Vizzard, but you need to read his stuff alot deeper to find the really good points.

On to combustion chambers. This is probably one of the better mysteries of engine building. Yes the Sb2 stuff is 30 some cc's, that because they like a piston with a dish in it to have a nice compact combustion chamber. The two examples one with a larger chamber and smaller dish and a smaller chamber and a "reverse dome" is a good point. The smaller chamber and revearse dome make better power because they keep the charge in the smallest spot possible as far as distance to burn the flame needs to travel to burn the air/fuel. If you could have a small ball that created the compression ratio you where looking for rather than a large flat horizontal disk, you would make more power. It's about the distance you have to travel, not the volume of the chamber. We can't increase the rate at which the flame front moves (6-7 mph) but we can put as much of it as possible close together as possible to make it accessable to the flame front as fast as possible. The crazy thing is that good Winston Cup Chevy engines might be at 28 deg of timing to get max power, while the Fords are bettering that by 2 degs. That's some secret I don't know yet.

The intake runner size is also a interesting point. I would say you want it to be the right size. Remember that the air is being crammed in. So the velocity is going to be high, getting it too high will reduce the VE because the MACH number is going to be too high. Make it large enough to work with the valve and small enough to help the velocity. It's almost a double edged sword for a blower engine. High velocity always helps, at a point it does hurt you though.

89ProchargedROC, your posts are really good. The TB size and flow means little in comparison to a NA engine. The ports and the cam all have to work together both NA and Blown. The point about the poor flow, more boost vs good flow and lower boost is good. Think about a World Rally Car. They have a restrictor limiting HP, so they just make more TQ. IF you have good heads, then you can run a smaller turbo (blower) to get throttle response up the lag down and the boost to come in earlier. They do alot fo things to increase the overall power, but working on that to increase the area under the curve is one of the reasons I think a Peugot is so good this year. BTW Old Stroker taught me well, he's a great asset and and even better ME, parts strength is what he worries about, I worry about stressing them more!

Bret

 

96, mayhaps

Brett,

I agree, alot of what he says holds true. I'm really only talking about most of his cylinder head stuff... I've read it, and taken alot from it. I just quit with his stuff when I had to relearn things b/c he was either wrong or way outdated. I'm not too smart, but I'm lucky that I've got buddies in high places who make Dave look like the local 17yr old browsing the ricer asile at pep boys. *shrug* As far as 100% E/I, i disagree with that.. I think it's too far, but that doesnt matter too much, bout all we can do is post what we think from what we've seen . I agree with what you said about chambers, it's something that almost everyone overlooks, but there's a whole lotta power to be had from knowing how to manipulate it .

I need to build the North Carolina version of Chuck a computer so he can get online and partake in some of these discussions .

-Phil

 

Just an addeed point.

As we know, boost is in a sense a measure of restriction.
The problem is that boost could be reaching 8 psi but not realizing that amount where it counts most, in the cylinder. 8psi in the manifold and 6psi in the cylinder isn't what you want. In this case you lose power fro not only the lack of filling but also due to operational power requirements to compress this air

So in regard to this discussion and FI, if flow is hampered by the runners or ports, then flow work is a necessity. We don't care about the ram effect or anything else as long as we can fill the cylinder and *bowl* with the mixture while at the same time having a minimal pressure differential between the intake manifold and cylinder. . You dont' want to take away any more material than to satisfy this runner/ port flow requirement.

 

I said effective E/I not static. Effective is what the engine is actually doing computed by valve area and flow.

I think we will disagree and agree on things, that's what makes us americans!

Bret

 

I love this section.

Mike