Big Block Chevy Oval Port vs Rectangle @ 20 PSI Boost

[Craig J]

Tall Deck BBC
4.280 Bore
3.76 stroke
Small solid roller (less than .700" lift)
Single Turbo: 88 x 124 mm compressor, 96 x 88 Turbine, 1.32 AR T6
10.3:1 Static CR
EFI with E100 fuel
20 PSI Gauge Manifold Pressure

Below 7000 rpm will a 310 cc intake rectangle port head make more power than a 265 Oval Port?

I assume the intake port will not make any difference for such a small engine with this much boost?

Thanks
Craig

[Orr89rocz]

What do the heads flow?

[Craig J]

What do the heads flow?

At 20 psi? I have no idea.

[tt 383]

S488? Are you limited to 1 turbo?
Doesnt forced induction (at least with a turbo) basically add to what the NA "version" would do? It doesnt really *change the the power curves much just multiplies them by whatever psi/bar you run at any given rpm...

[Orr89rocz]

What do the heads flow?

At 20 psi? I have no idea.

I meant whats the heads in question? What does the rect port flow and what does the oval port flow? Do you know valve sizes and port areas? This may help give indication of what it might do with either head.

Bit bigger head may make it abit softer under peak torque but a turbo car with that small turbine and bigger cubes wont have any issue developing torque, so losing some lower end response and power could make for a better car at the track or on the street, whatever you are doing. Bigger head and flow may make more power at peak rpm. 310cc does seem big for 433”. Either will work imo, as the turbo will likely be limited by the 96mm turbine wheel. The 88 as a single really needs the 100+ mm turbine options some manufacturers have developed for that turbo frame

[Craig J]

What do the heads flow?

At 20 psi? I have no idea.

I meant whats the heads in question? What does the rect port flow and what does the oval port flow? Do you know valve sizes and port areas? This may help give indication of what it might do with either head.

Bit bigger head may make it abit softer under peak torque but a turbo car with that small turbine and bigger cubes wont have any issue developing torque, so losing some lower end response and power could make for a better car at the track or on the street, whatever you are doing. Bigger head and flow may make more power at peak rpm. 310cc does seem big for 433”. Either will work imo, as the turbo will likely be limited by the 96mm turbine wheel. The 88 as a single really needs the 100+ mm turbine options some manufacturers have developed for that turbo frame

I didn't buy any heads yet, but it will probably be 290 Brodix or 265 AFR.

My assumption is that smaller port would improve engine response from 2500-4500 rpm, and once it starts building some boost the oval ports will not really have any disadvantage up to 7000 rpm. From a practical standpoint, if the oval port is good for 600 hp NA, and I increase intake density by over 200%, I sort of expect a component like a head gasket or a connecting rod will fail before running out of intake flow.

[Orr89rocz]

Oh in that case an aftermarket head, theres alot of guys running oval ports in the 265-290’range and making great power. That should be more than sufficient. Gotta compare areas and valve sizes since port shape will change volume but not necessarily mean bigger or smaller in area. Like i mentioned earlier i think the turbo may limit things before the heads are an issue

[ptuomov]

One gets a very different cylinder head answer if one has to limit the manifold pressure to a specific value. If yes, then best normally aspirated engine makes the most power turbocharged (to a close approximation). If no, then one can increase boost at various points of the torque-rpm curve and usually the best pruning heads make the most power, even more so than with normally aspirated engines. With free boost, it’s ok to sacrifice flow for better burn. My opinion.

[Alaskaracer]

A supercharged engine is no different from a na engine. Airflow is airflow. Heads that flow (quality, not always numbers) better will make more power, period. Something to think about it this: What are you measuring when you measure boost? You're measuring how much pressure is building up in the intake tract of the engine. High boost is typically related to high hp, but that's not the whole picture. Boost is relative. You can have a turbo, blower, whatever that displaces a certain amount of air at a given operating range, and have completely different boost numbers depending on intake restriction. Higher boost is an indicator of higher intake restriction, and will not always mean more hp. The advantage to running a supercharger of some sort is you can overcome a lot of the restriction in the intake tract that an na combo cannot. Case in point, my buddy's funny car. He was making about 35 lbs of boost on his combo before he had head work done. NOTHING changed with the combo but head work.....intake ports, valve job, exhaust ports.....he's only logging about 25-28 lbs of boost now but the car is WAY faster....more of the air that is being forced into the intake is making it into the cylinders as a result of the porting, allowing him to add more fuel. He had to go up about 5-10% on fuel delivery to compensate....

One thing boost will allow you to do is run a bigger head than you can na and still have good drivability. The forced induction overcomes the loss of air velocity through the port due to being oversized. If I'm supercharging an engine, regardless of method, I'll never sacrifice flow...flow is everything...burn quality has to do with chamber shape, proper a/f ratios, good ignition, piston dome.....but the bottom line is it's all in the combo.....

Last thing is don't look at port volume....it can be very misleading....just because a port is at a certain size doesn't mean it's capable of flowing more than a smaller port. Also don't look at flow numbers directly either, they can mislead as well..If it's an na combo, it's better to error to the smaller side to help keep port velocities up.....on a supercharged deal, you can get away with more head to a point......

[ptuomov]

“ One thing boost will allow you to do is run a bigger head than you can na and still have good drivability. The forced induction overcomes the loss of air velocity through the port due to being oversized. If I'm supercharging an engine, regardless of method, I'll never sacrifice flow...flow is everything...burn quality has to do with chamber shape, proper a/f ratios, good ignition, piston dome.....but the bottom line is it's all in the combo.....”

I don’t know how applicable this is for the case at hand, but pump gas four valve head engines seem to make more power when the raw flow numbers are sacrificed to make the chamber burn faster. That sort of engines are knock limited and the turbo version likes to be on the different point on the flow coefficient vs tumble index efficient frontier.

[Alaskaracer]

Four valve engines are very different animals than a BBC....Chamber shape, port size, volume, plug location, piston dome shape, etc...can't make a fair comparison from one to the other as each will behave differently due to engine design....Four valve engines tend to have more efficient chambers than BBC. They make better use of the a/f mixture. On BBC with more efficient chambers, the brake specific fuel consumption per hp will change, but airflow still applies...reduce airflow capacity, even with more efficient chambers, power goes down due to loss of oxygen for the combustion process. Raw flow numbers only tell part of the story...what's the airflow velocity on the four valve heads in comparison to a BBC? They tend to be better at filling the cylinder simply due to design....valve area is where they win over two valve heads, so raw flow can be misleading......On a two valve head, you can't fit a big enough intake valve in them typically....but there is a lot that can be done to aid in cylinder filling. Even with smaller valves, a four valve head generally has significantly more valve area which is a great benefit....

I hope I explained that...I know what I wanted to say in my head but not sure it came out right!!!

[n2omike]

I would think a larger head would be fine... but you better have a big enough turbo to feed it.
As we are all familiar... engines require a matched combination. Adding a turbo just adds another variable to the mix.
To know if a larger head would help, the entire combo needs to be discussed.

[ptuomov]

Makes sense that if you’re limited by the intake valve and not by knock, the solution is similar to a normally aspirated engine. If you are not valve limited and instead knock constrained, I think the turbo port should prioritize tumble/swirl over flow, compared to normally aspirated port.

[BLSTIC]

Makes sense that if you’re limited by the intake valve and not by knock, the solution is similar to a normally aspirated engine. If you are not valve limited and instead knock constrained, I think the turbo port should prioritize tumble/swirl over flow, compared to normally aspirated port.

Do you think this engine will be knock limited? 10.3:1 is quite high for 20psi, but e100...

It could also be limited by both. In which case I'd sacrifice flow for a smaller port that burned faster and probably lose top end for throttle response.

Is that turbine going to be too restrictive at that power level? if so this could all be academic with the charge contamination and lack of scavenging limiting top end power anyway

[Orr89rocz]

Intercooled it will be fine at 10.3:1 on e100

Without intercooler, i think it will be ok if run richer but hard to say. Iat will be rising with time so longer you pull it the hotter it gets the more at risk it will be. But some guys are near that boost pressure and beyond on E fuels and are ok.