Interesting discussion, but I still want to get back to what started it. I'd like the theory people here to expound on what the original question boiled down to: with the available 23 degree LT1 heads what is the optimal port size for a daily driver limited to 7,000rpm? I still say that there is such as thing as ports that are "too big". Heads with ports that are too big will be "lazy". Throttle response will be poor, the engine will not run well at low speed/part throttle, torque will be decreased. If you accept the idea that it is possible to have a port that is too big, how big is "too big" for a 383 or 396 LT1? If you don't believe that ports can be too big, please explain.

Rich

 

Rich-
In my very unexperienced opinion, couldnt you cam the motor so that the intake valve opens later, creating enough of a vacuum with the piston moving down to make a pretty good intake velocity charge, regaurdless of the port CC? Guess that would be the intake centerline?

 

Making power to 7k, a 350ci engine should be able to make ~600hp. It could make more, but 600 is a decent target for a street cam setup IMO. If it's a well built street engine with near-optimal intake and exhaust system, it's going to need a little better than 300cfm from the cylinder head. Maybe 315-320 as a target.

The 383/396, having 9-13% more air demand will obviously not make it to that rpm with the same head. If the port velocity is as high as it can possibly be (great VE) without choking on the 350, it will definitely choke at a lower rpm on the 383/396.

Now we're kind of generating an idea on the size here. How many 190cc port heads do we know of that will flow 315-320cfm? What will we need to make the 383/396 turn the same rpm?

David Vizard gives us a few calculations for sizing the minimum cross section area of a port so we don't choke the port. He says, somewhere around 690 fps and others say ~720, but they're all in the same ballpark.

My intuition and having seen good 383/396 combos that make power to 7k+... I say the port is going to need to be 220cc or better. That whole cc thing is misleading any way you look at it because everyone has slightly different port lengths, bowl depths, etc., but I don't believe I've ever seen a 383 pull to 7k with a port smaller than 210ccs for what that's worth.

If you look at the 396 like you would a big block 396, which big block head would you run on it to turn 7k?

You'd have a tough time finding heads smaller than 250cc.

My preference in choosing a head for a build like this is to take a smaller head that has lots of work-room and have an experienced porter get the port to size and SHAPE for best power. So, I'd choose a porter and talk to him about the choices needed to get there.... then get the head he suggests and let him have at it.

-Mindgame

 

Mindgame thank you that i understand.

So this looks like a 210 or 220 and have a pro fig out the combo

Can an Lt1/4 pull all the way to 7000rpm with that intake or is it not the intake but the head 23*

thx

 

The intake port is simply an extension of the cylinder head port. May need a little welding on top to make it work if the port is high but a good porter should be able to get it to compliment the cylinder head.

My LT4 intake is still a "cast" intake with nothing but straight forward porting, a little welding and spacers. It does fine to 7k and that doesn't have anything to do with the extra length a spacer provides.

That's just me. There are a lot more guys doing 7k on the stock casting so take it for what it's worth.

Yes, discuss port size and the head you intend to purchase with the porter first. You want his opinion on what he'd like to work with for best results. May even let him do the purchase if he can.

-Mindgame

 

MG hit the critical point. It's air flow that a motor of a given displacement and rpm will need, not port size per se. A port CAN be too big. You want it to be as small as possible for the air flow that you need. Shurter: if you are buying LT1 heads get the 210cc heads and have them ported.

Rich

 

It will be a combo heads/intake/cam Now all i have to due is build the 383 that will be another post

Rich it's Schurter my last name thx

 

Saying that Guys like WJ and Anderson ( would add Sonny and Kaase in there too, and might as well throw in Maskin and Grumpy since we are talking high end NA drag motors here) I find it hard to believe that they don't look at MACH numbers. Max port velocity has a huge amount to do with RPM range that a motor will work in. Then again it's relative to the cubes pulling on it too. If the min cross sectional area, cubes and RPM band are not matched then the motor is not going to work up to it's potential.




Bret,
Thats what I have said over and over in this post.
IMO if you take a 230-250CC port that will make power at 7000 and put it on a SBC it will never attain .5 Mach in port velocity.It don't have enough pull on the intake port.
If you built a small port to attain that .5 number,it wouldn't be large enough to make proper power at 7000RPM's
When you reach that .5 number the air is moving so fast through the engine it is hard to trap,and you have to cam it so radical it becomes non street drivable.Cross section in relation to port flow is the big thing.You can have flow without volume and you need both.As you know.

 

May not... may have too large a cross section area (csa). That's why you size the csa to provide .5-.6 Mach at your torque peak.

Sure it would if it's properly sized. May not be real gung ho for anything above 7200 or so but we didn't size it large enough for that in the first place.

"So fast that it's hard to trap"?

You mean it's going so fast that it's just going right by the rings? Where's it going??
If it's high enough in velocity it will just fill the cylinder. We're talking charge inertia here. If we invision air molecules as steel pellets with mass, when they reach high speeds (inertia) how are they just going to turn and go out the exhaust?
No, they're going to travel a straight path and fill the cylinder. If the speed is good .5-.6, the fill will be even better and you'll have higher VE and more avg torque and hp throughout the curve. Not new science.

That part is correct.

-Mindgame

 

You might have forgotten the exhaust is open when the cyl starts to fill.It has a good pull on the cyl and the air will go right out the exhaust with the intake "speed" helping it,hense the radical cam to slam the exhaust closed as to keep it in the cyl.High speed air wants to follow a streight line and not turn easily,like into the cyl.Speed is no substitute for the proper port size and flow.You have to have a certain amount of speed to get the flow,and 245CC's are not to big to make power at 2000 and will still make good power at 7000,figure it any way you want.
IMO you don't have enough "pull" on the port with A SBC to reach .5MACH with a 230-250CC runner.You might do it with a 195 or smaller runner and then you are not making the proper power at 7000.You can do it with a blower though. Steel Pellets???? come on.

 

1racerdude what head(s) would you pic? I am say 7000rpm because that is the limit of the stock pcm and this is a "street car" and 7000rpm should not be the hard on parts...

I will see when i start talk to the head and cam guys on what can be done and how much cyl head i need and cam....

any info would be great

thx

 

Haven't forgotten.

So all this time our discussion is based around maximum port speeds and now you want to throw in EVC and the fact that there is some overlap.

What does that have to do with max piston speeds and the corresponding max port speeds that occur some 20-60º after the exhaust valve closes?

Why does the cam need to be "radical" to close the valve at the right time so as to allow optimal cylinder fill in an engine operating in the 3500-7000rpm range?

The point on high flow airflow "turning to go into the cylinder" is totally wrong.

Vizard states that flow from around mid-lift is highly biased towards the center of the bore and high-lift/high-speed flow is predominant across the valve and away from the exhaust valve.

It'd have to make a 180º turn to make is out the exhaust port of a wedge style head. I think you're confused.

WRT to your 245cc port and speed vs flow.

What happens to your big port when engine speeds are slow? Where does the charge go when the piston rises on the compression stroke and the inertia is low in the port?

Soggy bottom.

Ok, let's see the math behind the opinions. I'd really like to know where you're coming from for once. Give me a cross section are of your choice and the rpm, displacement, etc and tell me what the speed is through that area.

-Mindgame

 

Like I said before, a 220cc port will make the power you're after if it has the airflow.

220 could very likely have a port min-cross section of ~2.3".

Max power rpm = (2.3 x 185,000)/(3.875 x 4.03^2)

Max power rpm = 6730

edit: That 185,000 constant is based on camshaft designs from 10 years ago. I wouldn't be surprised if that could go 187,000 or somewhere in that area.

If you push that port to 7000rpm you get...

(.00353 x 7000 x 3.875 x 4.03^2)/2.3 = 676fps through the port min-cross section area or ~.56 Mach

Imagine that.

-Mindgame

 

A port that starts at 210cc will be ~220-225 after porting.

Rich

 

I personally think that you can't get to much head.The bigger the head the smaller the cam for a given HP level.Mine are 245CC's and flow in the 330'sCFM 23*.You have to decide how much power you want to make and how much you want to drive the car on the street.If you have a good head it is a lot easier to make HP.Buy the heads first,build the engine around them,and buy the cam last.Remember you have to have an intake that will flow with the heads from the air filter to the piston.