How to determine best throat size?

[travis]

I work on small single cylinder 2 valve Honda bike engines.

One highly developed inlet port on a Japanese aftermarket head has an 83% or so throat to valve ratio.

This is an engine that is making over 150 hp per litre with good driveability.

It’s starting to seem like maybe this intake/throat percentage really doesn’t mean much by itself, and the MCSA or throat area is really the more important number here. Does that sound about right?

[RevTheory]

What if you were to decide what top cut and width worked best with your chamber wall, worked your way backwards through the seat angles and widths and rolled the bottom cut into the bowl around .090 and the throat diameter is what it is?

Dumb idea?

[David Redszus]

I work on small single cylinder 2 valve Honda bike engines.

One highly developed inlet port on a Japanese aftermarket head has an 83% or so throat to valve ratio.

This is an engine that is making over 150 hp per litre with good driveability.

It’s starting to seem like maybe this intake/throat percentage really doesn’t mean much by itself, and the MCSA or throat area is really the more important number here. Does that sound about right?

Actually, throat to head ratio means little and neither does MCSA by themselves.

Valve diameter can only be used if the lift is more than 25% of the valve diameter; which occurs only
once per revolution. The better area measure would be valve curtain; valve head perimeter x valve lift.

But, even then, the ratios are static, not dynamic. The will work for a specific size engine at a specific
rpm. The ratios would be sub-optimal at all other conditions.

Even a stopped clock is right twice a day.

[steve cowan]

What if you were to decide what top cut and width worked best with your chamber wall, worked your way backwards through the seat angles and widths and rolled the bottom cut into the bowl around .090 and the throat diameter is what it is?

Dumb idea?

No not dumb idea but we have to ask what happens if throat gets blown out?
In theory we want the best cfm/in2 as possible at the throat,Good velocity gradient around intake valve ( it is not consistent on a 23 deg head)
To help with filling the cylinder after BDC when piston going wrong direction.
Dave R makes good points about curtain area etc but this is all puzzle pieces in a big box,application,compromise etc.
I have been working on my alloy dart 180 heads went to a 40tc and 50 seat on the exhaust no problems
The intake a different story- the 40tc does not sit well with me because of the chamber is really scalloped out so maybe 2 x top cuts required??
Next is apex of SSR is really low off the deck and getting a nice shape to the bottom cuts/ throat leaving me scratching my head but that's just an experience thing I guess

[steve cowan]

Steve, I’ll ponder on that more once I’ve had some sleep. Using this calculator http://www.wallaceracing.com/ca-calc.php it says for a 4.03”x3.5” bore and stroke, for 5000 rpms needs a MCSA of 1.63”. Using an average port length of 5.25”, it comes up with a 140cc port…which sounds pretty realistic. But…the heads I’m working on right now have a 1.78 intake valve, and a throat of 1.63” diameter, which is about 2.08^2”. I’ll circle back to this later.

But in the meantime, another question. These heads I’m working on have a 1.78” intake valve, and a 1.63” throat. This comes out to about 91.5%. Now if I move up to a 1.84” valve, and leave the throat at 1.63”, this comes out to 88.5%…theoretically a good place to be. What difference would this make in the way the engine runs? Would it make any difference, since the choke size is still the same?

Depending on application and what you wanting to achieve going to bigger valve to get throat smaller should be fine.
Depends how bad chamber/bore shrouding is I guess.
If it is a low end deal you probably won't notice any difference really.
Or run it with the bigger throat ??
I'm sure there are plenty of engines out there that run fine with a bigger than 90% max theory throat

[PRH]

If I do my normal “bowl blend” to a later factory BBM Head, while retaining the stock valve sizes, the throat diameters end up around 86%.
That work will bring the flow @.500 lift up about 25cfm for the intake, and 35cfm on the exhaust.
I realize there are many heads out there that don’t respond as enthusiastically to minor mods like this........ but for the 1-ish hp/ci BBM crowd I don’t think bringing the throat out to 90% would do anything for that type of build.

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[frnkeore]

How does valve pocket angle, to throat angle & area play into this?

[PRH]

In one of the how to hot rod your Pontiac books, the fairly detailed porting chapter has you bring out the intake bowls to 1.70”.
That’s not a tiny amount of porting to those heads.

At 1.70” bowl diameter, with the stock 2.11” intake valve....... you’re at only about 81%.

I’ve done Pontiac heads using the basic guidelines in that book, and the motor those heads went on ran quite well.

[SpeierRacingHeads]

With all this said, I just sized a 2.02 valve Super Stock head to 93% with a 45º seat and 40º top.

[Momus]

I work on small single cylinder 2 valve Honda bike engines.

One highly developed inlet port on a Japanese aftermarket head has an 83% or so throat to valve ratio.

This is an engine that is making over 150 hp per litre with good driveability.

It’s starting to seem like maybe this intake/throat percentage really doesn’t mean much by itself, and the MCSA or throat area is really the more important number here. Does that sound about right?

I don't know.
The engine I referenced has a single 28.5 mm intake valve, a 23.8 mm throat and a tapering port runner that starts at 29 mm. The port is fairly flat and the turn reasonably abrupt with a pronounced snake neck. The throat/bowl is quite deep/long about 1.25 valve throat diameter andfollows the necked 4.00 mm valve angle. The exposed guide has a longish streamlined fin before. It runs best with a slide carburettor venturi of 32 mm.

[frnkeore]

In one of the how to hot rod your Pontiac books, the fairly detailed porting chapter has you bring out the intake bowls to 1.70”.
That’s not a tiny amount of porting to those heads.

At 1.70” bowl diameter, with the stock 2.11” intake valve....... you’re at only about 81%.

I’ve done Pontiac heads using the basic guidelines in that book, and the motor those heads went on ran quite well.

For this thread, are we talking area or diameter?

The area for a 2.11 valve by 1.70 throat = 3.405 by 2.178 = 64%, allowing for valve stem dia.

I would, again like to ask about the effect of the transition angle from the pocket to the throat diameter, assuming that the throat starts in the 60 to 70° range.

[PRH]

Obviously I’m talking diameters if 1.70 is roughly 81% of 2.11(that, and I used the word “diameter”).

Ideally, IMO the angles involved should be such that they promote the flow within the targeted lift range of the cam that will be used(and the intended use of the engine).
I expect they could end up being different depending on what head you’re working on.
For the milder bowl blend type jobs on factory American v8 ohv type heads, sometimes there isn’t a lot of meat in the bowl, or bowl depth to work with.


The best throat diameter percentage and approach angle for a Big Chief head running a .900 lift cam that goes on a 632 and turns 7500rpm+ isn’t likely going to be optimum for a factory Pontiac head for a 400 street build running a cam with .450 lift where the emphasis is on low speed driveability.

Which to me means....... as with almost all of it........ the “best” throat diameter percentage is........”it depends”.