Roman's 4GR V6 Carina discussion thread

[mjrstar]

i think i remember there is some magic area ratio between in/out to manitain average velocity, that for instance if you just hog out the intake flange side and leave the valve area the average velocity is garbage, then if you hog out the whole lot then the average velocity is also garbage. 

 

how do you work out what optimal velocity might be, well i dunno. but yeah you want it to zoom in there real fast. so i think you want the port volume to be just enough to not choke power so the air gets maximum zoom through the valve seat..

 

i think my old mini suffered from massive port, big 296 cam and big valves to the point where there wasn't a lot happening below 6000 rpm. 

[Honda Ass Dragger]

I saw this and thought of your engine, did Toyota do things they way the did for emissions, using DI to using the minimal amount of air possible to make the required power

[BiTurbo228]

If you're looking at understanding porting and stuff I can recommend the Don Terrill's Speed Talk forums. Lots of proper good information on there, usually in a level of detail that's a step change from the typical blog posts covering the basics.

The stuff about the 'magic ratios' is a good example of the generalisation stuff I was talking about (though that was more about rod ratios, but the situation is the same).

Most of those were discovered through lots of empirical back-to-back testing, mainly by folks like David Vizard with smallblock Chevys and Ford Pintos (similar to what @kpr is doing with his 4AGEs). The principle is universal (there's a port size and shape that is optimum for flow and velocity for any given engine in any given state of tune), but what those ratios are varies around. Especially where engines operate on different principles (e.g. swirl vs tumble).

Though I don't want to discourage people from using them. If you don't have the setup for boatloads of iterative testing then they're reasonable starting points for making a bit more power and not screwing up your ports by hogging them out too much. They've proven to be broadly applicable to quite a few engines, so it's hard to go wrong with them. But they're not some 'golden rule', as evidenced by all the various different factory approaches that are still being iterated on (see nutty internal naca duct thing).

That naca duct is very weird, because the whole idea behind the port shape is to bias flow across the top of the valve for tumble, which makes more power due to improved combustion efficiency than it loses in flow. So Toyota is doing something odd there.

I wonder if DI doesn't need as much tumble because shooting the fuel in at stupid high pressure does a decent job of mixing the fuel anyway. Wonder what that might mean for having swapped back to port injection, if that's true of course. Testing that would probably mean epoxy filling which is a bit of a faff. Or reaming it out far enough that the naca duct is removed/reduced without changing the port orientation too much. How deep is it roughly?

Also interesting on the knock resistance of DI. That's probably worked in your favour as although you've lost that by going to port injection, your big cams will drop the dynamic compression closer to what you need anyway. Less need to skim the heads to match the big cam.

Neat stuff!

Edit: if you do try one with a reduced naca and it makes less power that might open up some interesting porting idea for other 4v engines. @kpr do you happen to have a junk head lying around that you want to do a back-to-back test with a naca duct in the SSR? 

[BlownCorona]

are you sure they didn't accidentally print those parts from titanium? the melted drill bits and broken taps was my exact experience why i had some bass guitar project parts made from titanium because they were tiny parts and the minimum price was the same between SS and Ti. 

it was my first lesson on how Ti work hardens if you look at it funny, i had a tough time drilling it with my cnc mill controlling the feed, this worked really well untill it didnt, instant work hardening and rapidly turned the small Ti parts into incandescent light bulbs. Tapping m4 & m2 was never going to happen.

I gave up an reordered in SS, even then i drilled out the M2 threaded portions and pressed in brass to accept the tiny brass adjustment screws. Aside from that i found 3d printed SS super nice to work with and i'm most of the way through designing a twin turbo exhaust manifold for the long forgotten blown(up) corona which will be getting printed as pricing is just awesome these days. 

lighting the titanium swarf on fire was a party trick that my daughter really enjoyed though.

[kpr]

Is a lot of talk about the intake side and not over sizing it these days.    so dont loose midrange.       But the more common mistake for peasants like us, is usually on the exhaust side.  people jump straight to the exhaust side and jam some massive primary headers on,  which usually does nothing for the engine other drop a bunch of midrange.   some of the tests on v8's, like on engine masters etc,  are misleading if you dont read into it. As the sizes they are testing are actually too small vs bigger for the cylinder capacity.   Not like a 1.75"  primary on a 1600cc making 130hp.  (yes thats too big) 
Have seen people go down the rabbit hole downsizing the intake (pre port)  to pick up midrange.  it works, but lose a bunch of top end doing so.       sort out the exhaust side on the same setup and can generally retain the top end while gaining a bunch in the middle. 
oem, who are chasing any kind of economy,  will often do both small intake and header primary.    since they weren't planning on some guy in nz to rev their 4gr to 10,000rpm.   they wouldn't have   been too worried about the top end loss 

moral of the story  = big intake  small exhaust primary.  not small intake  big exhaust primary

Also where the port ends and where the intake and exhaust start, isn't really a thing.   it just gets more important closer to the valve

 

 

[Roman]

On 21/08/2026 at 21:44, BlownCorona said:

are you sure they didn't accidentally print those parts from titanium? 

Definitely not, as I think the density of titanium is similar to aluminium. They were super heavy and the weight compared to the mm3 indicated on my model matched roughly what stainless would be.

The replacements that turned up were incredibly lighter. 

[Roman]

On 21/08/2026 at 21:15, BiTurbo228 said:

I wonder if DI doesn't need as much tumble because shooting the fuel in at stupid high pressure does a decent job of mixing the fuel anyway.

Improvement of the Intake Port

The fan-slit spray requires relatively little assistance from organised airflow to form a homogeneous mixture because of its flat shape and comparatively strong penetration.

However, Toyota found that increasing the intake-port tumble ratio was beneficial for achieving still faster combustion and higher thermal efficiency.

Testing showed that the benefit became progressively greater as tumble increased, with the largest benefit obtained at approximately a tumble ratio of 1.0.

The Usual Tumble-versus-Flow Problem

When an intake port is designed using conventional methods to increase tumble, the flow coefficient falls

With the conventional approach, increasing tumble requires weakening the discharge velocity from the intake-port side of the intake valve, relative to the airflow leaving the side closer to the exhaust valve.

The resulting directional imbalance produces more tumble, but because one side of the valve is flowing poorly, total airflow through the port is reduced.

During development of the 3GR/4GR-FSE, Toyota investigated numerous combinations of:

• port diameter
• port angle
• port shape
• other geometric parameters

using both CFD analysis and experimental testing.

Toyota ultimately found a port geometry that allowed strong discharge velocity on both sides of the intake valve while still generating the required tumble motion.

The result was simultaneous achievement of:

high tumble ratio + high flow coefficient.

 

[Roman]

14 hours ago, kpr said:

oem, who are chasing any kind of economy,  will often do both small intake and header primary.    since they weren't planning on some guy in nz to rev their 4gr to 10,000rpm.   they wouldn't have   been too worried about the top end loss 

moral of the story  = big intake  small exhaust primary.  not small intake  big exhaust primary

 

Hah yeah so in the documents they say "We aim for a tumble ratio of 1.0" whatever that means... and shows a graph at 4000rpm.
Maybe the air does a full loop back to intake valve by the time it reaches BDC or something.

But it probably means that it generates that much tumble at only 4000rpm or whatever. Maybe it gets more as rpm goes up? or maybe less. No idea.
Making port a bit bigger and might reach the magic number at a higher rpm perhaps.

It's also worth noting that part of the reason for the 4GR having the stupid big shape at the start of the port.
Is because it shares the same TVIS type manifold as the 3 litre 3GR engine. Which has bigger ports all the way down, and bigger valves.
I'll buy a 3GR at some point and see how far off it is from plonking the heads onto this. Will be interesting to compare the casting.
But I mean, the 3GR only makes 40? horsepower more from factory than a 4GR and it's got bigger ports on both intake and exhaust.
So maybe that's a clue about what would be good for a higher rpm motor thats smaller.

Either way, I'm definitely more in favour of real life testing over any level of speculation.
Because as we've seen many times. It doesnt really matter why something works, so long as it does. haha.

 

[BiTurbo228]

Not come across tumble ratio before. Seems to be the spin speed of the air compared to the speed of the crank. So a ratio of 1 at 4000rpm would have the air tumbling at 4000rpm too.

Still wondering if chopping a naca duct into another engine's ports might be an interesting experiment...

[Roman]

It probably only works in conjuction with the super tall port though, otherwise SSR messes it up.

And if its largely an economy type thing, unlikely to be worth the effort. 

[BiTurbo228]

On 22/08/2026 at 22:22, Roman said:

It probably only works in conjuction with the super tall port though, otherwise SSR messes it up.

And if its largely an economy type thing, unlikely to be worth the effort. 

...don't we know someone who's just spent a good deal of effort making sure his ports are super tall...

[fuzzy-hair-man]

Have you seen these guys channel / videos? The videos I've watch were dealing with issues such as the ones you were asking, minimum port cross section area, port taper, average port speed, maximum port speed and targeting specific rpm. Seems quite generous with the information.

https://youtube.com/@bainracing?si=cn_SA_LrSUm5Psdb