Cylinder head port sizes.
 

I've had a couple things on my mind lately, in regards to cylinder heads. We all know the importance of airspeed/velocity, and the benefits of a smaller properly shaped port, on a N/A engine. The intake manifold design, port runner design, all come into play, to help pack the cylinder with air, when you are relying on 14.7PSI of atmospheric pressure, to fill the cylinders.

BUT, my question involves superchargers. Lets say a whipple, or roots blower in a marine application. I'll share my theories, or thoughts, and I'd love to hear whether I am right or wrong.

On a street driven vehicle, that often cruises down the road, under 2000RPM, in vacuum, might benefit from the smaller port size. To keep throttle response and efficiency up. Now, lets say a marine engine. That rarely operates in the 1200-2800RPM range. At 3000, 3500 Rpm, with roots or a whipple, how detrimental is port size? How about at 6000RPM?

I guess what I am getting at, is at what point, will you see a LOSS of power, by having too large of an intake runner on a blown application? Lets say one was building a basic 540CI blown marine deal. And he had two choices. Lets say a 355CNC Dart, or a 335CNC dart. Would the 355cc setup give up anything to the 335CC, or vice versa, if all other things were equal. Or, say a 320CC Dart, vs a 360CC dart. I'd love to hear/see some dyno observations on this topic.

I have an old B&M book here from 20 years ago. They talk about this, and one of the statements was, "in all of our testing, we have yet to see a drop in power, from using too large of an intake port" on their blown setups. I also remember an engine builder on here a few years ago, stating the intake port size, can in a way act as a restrictor plate, on a forced induction setup.

I understand the importance of head flow, exhaust flow, cam design, etc, I'm just trying to wrap my head around port size. I have heard many say "that head is way too large for a xxx engine", or ''That head is way to small for a xxx engine". Granted I dont believe slapping some 410cc BMF's on a 454 are the way to go. But I am curious if a 310cc will outperform a 345cc on a blown 540 or something like that. The idea that the larger head might be unresponsive and lazy down low, goes with theory. However, i've seen some marine blower engines with some large 360cc ports, that were far from being lazy in any way shape or form, when boost is added into the mix.
:helmet:


Maybe Haxby, or Eddie Young have some input on this?

The more power it makes na the more power it will make blown its that simple if your trying to get everything out of the motor. But if your not trying to get every last bit of power out of it and it is forced induction there is no sense in spending all the money to get the best heads money can buy.

Send a PM to Dustin. He can probably support each head build type with dyno data.

I am no expert by a long shot but I would say If I understand your thoughts correctly, that as long as you keep the intake plenum and runners under the boost level you want the size of the runner should not hurt the performance throughout the rpm band, and the intake signals wont effect the air volume as much in the plenum. it most likely will effect low RPM, low boost power/torque. I would like to see some test resuts if anyone has the time and means to do so!

I guess maybe it boils down to, at what point, does the intake port size become a restriction, causing a power loss. I mean, if lets say a 325cc port is not restricting the airflow, than going to a 345, should theoretically, not gain any power, and possibly cause a loss of low end/out of boost, flow and efficiency.

Lets say you have a N/A 540. And were to use a 305cc AFR cylinder head. That head, might work really awesome, and hands down be the head to use, from idle to 6000RPM on that build. But, you throw a blower into the build combo. Would that 305 still be the head to choose, or would the 325 version, or 345/357 be the head to choose.

Apparently what I am getting at, is, does the boosted engine, allow for a larger runner size, over a N/A, without negative results.?

My buddy built a 450" Chrysler twin turbo 60 pounds boost 350cc intake runner 3500 horsepower.

This is basics, so if someone wants more technical at this point don't get all rowdy.LOL.

SC/TC don't add velocity, even though you would think cause' more air in the same space should move faster right ?

Not really. Here's why.

They are compressors.

They take more air volume and compress it down to a smaller space then if it wasn't compressed.

Under boost yes but does that same theory apply at say 3200 rpm 0 boost?

Problem now is, that all SC and TC are not created equal.

Roots / Screw are positive displacement.
Screw compresses air in it's 'screws'
Roots compresses air in the intake manifold

===================================

Extreme example:

A 7.5:1 big headed 8-71 bown BBC can work pretty good.

How about a 7.5:1 big headed BBC with turbocharger or ATI/Vortec ? Dog for a while until the mojo starts to flow. Not so much fun.

ATI/Vortec/etc are centrifugal with gear ratios. IE: step up gears.

Turbochargers are exhaust (rpm/load/etc) driven.

Thus Roots/Screw are very popular in the I/O world because from idle to where ever, they respond 'right now.' Always full boost when the throttles are wide open now matter what rpm.

Centrifugals increase with rpm, again they have gearboxes.

TC needs rpm and load to make them to do more.

Not getting what I said in your quotes of me.

A compressor adds more air, but packs this larger amt smaller than if not compressed (ie: N/A). So......a blown engine may not require a larger sized cylinder head than it's N?A twin.

PS: Some Roots/etc add a lot of heat which will expand the air. So........bigger runner may be needed.
Remember, thus why intercoolers needed, cooler air is good for obvious reasons but one not so obvious is for the air/oxygen to stay 'smaller' and less likely to be restricted.

Again, don't think we should get too technical on this unless people want...but that may become never ending.

But it increases your mass flow rate.

The tighter intake runner is going to need more pressure to move the same mass into the cylinder.

In my mind, you'd want the intake runner sized for the best efficiency, which will be a mix of boost vs. port velocity.

These comments/ramblings are worth what you paid for then. I'm not an engine builder, just an engineer with a little flow experience.

I'm just asking so I understand
More air tighter in the same space would be under pressure wouldn't it? I get most of what you are saying. I just can't see how an 8-71 spinning efficiently at 3500 rpm and not making manifold pressure isn't increasing the air speed.

Ever try Dart for a good combo? They have to have some numbers for what's best at what rpm. I used to get a ton of info from comp cams until some jackass sued them.

Let me throw a curve ball here. Are you going to longer rods? On my blower apps I've always run higher wrist pins to accommodate a longer rod. Slower piston speed works well with blowers and makes better torque.

I've heard the same about stroke. Like a 4.250 bore, 4.25 stroke 496ci, would make more power, than a 4.00 stroke 4.47 bore, boosted. Whether or not thats true, I dont know?

Mass Air flow is up, but physical volume (size) isn't.

If you have a big innefficient roots without intercooler, than yah, physical volume will be. Heated air is expanded air.

We are not dealing with just how much oxygen (other gases too obviously) is going into the motor, but the size of the oxygen going in the motor.

200+F air is going to take up more volume than 30F air. That's for sure.


The volume correction factor of air at higher or lower temperatures than Normal Air can be found in the table below:

Air Temperature Volume Correction Factor
(oC) (oF) Correction Factor
-46 -50 0.77
-40 -40 0.79
-34 -30 0.81
-28 -20 0.83
-23 -10 0.85
-18 0 0.87
-9 10 0.89
-5 20 0.91
-1 30 0.93
4 40 0.94
10 50 0.96
18 60 0.98
22 70 1.00
27 80 1.02
32 90 1.04
38 100 1.06
43 110 1.08
49 120 1.10

We have used a compression ratio of less than 7.5:1. Our heads flowed just under 400cfm. They started out as dart 360's. We made boost at 1500 rpm's and made over 1000 ft lbs of torque at 2300 rpm's. Turbos are 75mm.

Two turbos ?

LOL. See. This is why I didn't want to get too into it because there are a quazzillion avenues to take with this subject.

How light to you talk about it ? LOL.

Try to make a general point and bang, someone has different info. Engine + turbo sizing / load / use / boat design / camshaft / etc,etc,etc

This is why a difficult subject to talk about without having an exact goal + vehicle to discuss.

BTW: I've followed your boats performance info (if you are the one I remember with checkmate and turbo motor) and it's impressive !

It is twin turbo in a 30'. It's been fun for sure.:)

What SB said.

its like a river flowing with NA.. You have water going down a river , it has to go up a slope then back down. To much going in would make a swirling mess as the right amount would flow faster.So the same size ports to exhaust matching is gonna work best. You can't cram too much air a a pocket that wont take it because it wil interfear with flow.. So yea NA maybe not good to go to big.

But Blown set up. You can never be too big because it is based on Pressure Not air flow.

Yeap More stroke makes more power than bore size

There are too many variables to make a blanket statement that a smaller head is as good as/better than a larger head. That 305 CC head may work great on a 540 with only 5 psi of boost. When you crank the boost up to 10 psi, the 325 head will make more power. I'm just picking numbers here for the sake of the example. You have to take everything into consideration when choosing the correct heads. This includes CI, intended rpm, boosted or NA, amount of boost, intercooler or not, etc.

I know this doesn't really answer your questions MT, but there are too many things to consider to make a blanket statement about which head will work better, the larger or the smaller one.
Eddie

No

and

No. - Separate topic that can be discussed in another thread..


We can get as technical with this as you want, but for now, like SB, I'll keep it simple. Joe, a blown motor is much more tolerant of an oversized port because of the SC's ability to force air into the engine. Just like it is more tolerant of a bad port, and even bad exhaust. However, just because it works doesn't mean it's optimized. The correct csa for your cubic inch and rpm will not change whether you run NA, or boosted. The port however does have to be good, ie localized air speeds. Some of the smaller high velocity ports have localized airspeeds (especially over the ssr) that are borderline, or a bit too high. On an engine running decent boost, I've found these ports get into trouble faster then when the same engine was NA. In a case like this, a larger slightly lazier port could make more peak power, and be easier for someone who doesn't have the ability to fix the smaller port.

On a 540 peaking at 6000rpm a good 315 head works great. If you put on a killer 357 head you will not make more power, and will likely lose average power, especially mid range.
I don't mind running a slightly larger then optimal NA head when you get over 30#'s of boost because I feel the additional fuel takes up a bit of the port volume, same with running Meth, or E85. I don't have the science to back that up, but it makes sense in my head. lol.

Forget about all that port size stuff Joe, on a BBC , combustion chamber improvements is where its at. :D

NA is under pressure too because Atmospheric pressure is positive pressure.

We just reference Atmospheric as 0, less than atmospheric as vacuum, and more than Atmospheric as PSI.

So, with boost, we just have more psi than atmospheric.

Soooo...................

(above is true - maybe this will change that thinking ?)

Good info guys. All valid points. Thanks for participating!

I always assumed, that a compressor helps move air thru the intake port. We've seen some impressive numbers from those small oval port modern heads, on some engines. I always assumed, that the small oval port design, was meant to improve velocity, to aid in cylinder filling when relying on atmospheric pressure. I've also always assumed, that running a small oval port, would not be the way to go, on a forced induction big block.

Ive seen alot of talk about the small oval like the AFR 265 being a great choice on the 454/496 combinations lately, in N/A form. So, is it safe to assume, based on what you guys are saying, that the same package, would also outperform a larger port on that engine, with a blower? I just cant wrap my head around the idea, that the ideal port size N/A, would translate to the ideal port size for blown. I'd have a hard time believing that a 496 with 265 ovals, would outperform, a 496, with say some 305 rectangles, with say 6-8lbs of boost.

I've got an old vintage book here, written by Jim Davis, back when he was at B&M, and they were in development with their supercharger systems.. Long before all the cool cylinder heads were available like now. They did alot of back to back testing on some stuff. They took a typical 8:1 454 chevy engine, with GM oval port heads (2.06/1.72 valves), installed a 420 Mega Blower, with around 10lbs of boost, and a blower cam. The engine made around 626HP at 6000RPM. They then removed the GM oval port heads, and installed stock GM Rectangle port heads (2.19/1.88 exhaust valves), with no other changes. The engine made 718HP at 6000RPM.

I know this is dinosaur stuff, but I thought it had some interesting information.

Yes, that is dinosaur stuff.

Just like
I $400 rebuilt a 7.8:1 flat top 454 with stock valve 781's. Ran 13.60's in that car. Mild cam...very mild. Installed larger valves in those same heads, no other changes. 12.90's and a bunch of mph.

Changed cam, still relatively mild. Ran 12.40's.

So.............what does that tell you ?

Oh man.. it's just like the day you fibd out your merc 575 BASE engines are 9.5 :1 and not the stocker 7.5:1 engine you can hold wide open all day

we must remember a few things,the above mentioned is not making boost at 3500 because the intake cycle is creating more vaccum than the supercharger can make boost because the throttle blades are barely open and restricting air flow.if you now open the blades all the way the restriction is removed and we make boost.no one has mentioned exhaust ports yet,dont forget we need to get the spent exhaust out of the cylinder before we can refill it with the next intake cycle.

i have to disagree,the pressure [or boost] is created by the restriction of flow.take a supercharged bbc with a set of good flowing heads that makes 10 lbs of boost,now put a stock chevy oval port head on it with no other changes and it will make more boost because the head creates more restriction.the supercharged engine makes power because more air/fuel enters the cylinder.

Great thread. Glad to see informative posts, without bashing and arguing.

I agree Very Refreshing!

Yes , I was starting to think these types of threads were gone forever . Great conversation , great information ! I'm all ears .

no bashing yet because the bashers have not found this thread yet.

Well don't incite them! I've lots to learn still!

My experience on a blown motor is put the best flowing heads your budget can afford. The more the heads flow, the less boost required to achieve your HP goals. I have upgraded heads on motors where everything remained the same except head port size/flow and experienced that boost went down at higher RPM and HP went up proportionally. That said, I agree that under most circumstances you cannot overhead a blown motor in a boat application unless the motor has an seriously undersized blower (blower is too small for the displacement). All componants do need to compliment and support each other.

dan,while combustion chamber is important as gm learned in the 60s,the best combustion chamber in the world will not cover issues caused from crappy port design.im not busting your balls im just stating facts.the pic of your heads looks good and im sure you are giong to see improvement ,your head guy did a really nice job.

1.5"Hg (vacuum on your guage) = 13.96 PSI actual
because
Normal Atmospheric pressure (O vac and 0 PSI on guage) = actual 14.7psi.

2 pounds of boost on your guage = 16.7psi
5 pounds = 19.7psi
10 pounds = 24.7PSI

You would only have 14.7psi if your cylinders were a perfect vacuum. You'll only have some differential pressure, not 14.7.

Yes! What you're seeing here is how different physical properties make the readings deceiving. Common logic is more boost = more air, and that's a bit misleading. More boost = more resistance to flow.

In your case your flow path became more efficient, thus lowering resistance to flow.

Beware, engineer ramblings below-

How come we (uhh, I mean those that have forced induction) monitor boost when we could be monitoring mass air flow? Obviously the technology exists... This would really only be good for testing, as once you have the runners/valves/cam dialed in, theyre not changing. Ambient pressure, especially for those on the sea, is pretty close to constant. Temp is almost moot.

You'd use a MAF setup to do real time flow testing of blower/head/cam combos, derive some constants/coefficients for the setup, and have a pretty damn good baseline.

I dunno, maybe people do this already,