Intake port velocity?

Status
Not open for further replies.
Joined
Jun 3, 2003
Messages
4,413
Location
BC, Canada
How can intake port velocity be improved without using a smaller port or larger engine?
1. A higher compression ratio?
2. A shorter connecting rod?
3. A larger intake valve?
4. A later intake valve opening point?
5. A faster intake valve opening rate?
6. A larger bore for the same displacement?
7. All of the above?

Let the BITOG SAT begin.
 
At what RPM?. from the throttle body [carb] to the end of the exhaust pipe all has to be working as a unit.
 
I know one thing for sure: large intake ports and long duration cams will kill velocity!

Never put a long duration cam in a 351C-4V. I learned that one the hard way.
 
Any RPM above idle will do. CT8, you are bringing sonic and resonance tuning to the table. At any rpm, as in game theory, one could be working against the other.
1st; Do what is good for yourself, the intake port must generate it's own velocity to be of any use to the other players or the engine.
 
The first two replies are heading you in the right direction. If the whole system doesn't work together, it doesn't work.

Some builders have (once upon a time) used a longer intake track to get a more laminar flow, I don't see that much these days.

The reversion that other posters are referring to is entirely real. The most obvious example is on old HD Big Twins, on a fresh motor with a perfect valve job, if you take the air cleaner off, at a certain rpm, usually a fast idle, you will see gasoline mist blowing backwards out of the carburetor. That is reversion.

I have tinkered with track shape and port wall smoothness/roughness, HD figured out in the late 1970s that a little smoothness on the long side of the intake track, and a little roughness on the short side of the intake track, with a little subsequent turbulence in the boundary layer, did actually help flow. The best heads I've used are not too smooth and not too rough in the intake track. I know, real scientific. Valve guide shape seems to matter also.

A given air filter shape, carburetor throat diameter, intake port configuration, valve head configuration, cam lift, duration, and timing, compression ratio, piston dome configuration, exhaust port shape and size, and exhaust pipe size and length, will combine at any given rpm to form the conditions in the intake port that you are concerned with.

It's tricky.

When the slug of exhaust gas hits the end of the pipe it goes from a certain shape, size, density, velocity, to what is for all practical purposes infinity (outside the pipe).

You then get some sort of reversion, depending upon the particular application. And the harmonics that other posters have mentioned.

After years of screwing around with the intake side, I finally realized that the result I was looking for was really in the exhaust. So I pick a cam that I like for the application, do a careful valve job that works for that cam, and then tune the exhaust.

I'm a big fan of anti-reversion cones immediately outside the exhaust port, others may disagree.

Now there is some length of pipe that spoils the harmonic we're concerned with. So for example, after I get everything else squared away, with 1-3/4" pipe (shorter than I would run) I slide a slightly larger pipe over it, just enough for a slip fit, and slide it back and forth like a trombone, at various rpm.

If you feel anything about motors, you will feel the sweet spot where the exhaust is the right length for that rpm. You can't miss it. The whole track, from the air cleaner to the end of the exhaust , just starts working together and you can feel it, hear it, it's palpable.

That's just one small piece of a big puzzle, but it is a way to get to the conditions in the intake track that you're asking about.

It's also the logic behind the slash-cut straight pipes from the 1960's.

Now this is all about old Harleys, but the logic pertains to any motor.
 
Shorter rods are somewhat bad as the increased thrust angle increases friction and wear. If wear matters,
 
CCI; I get all of that, but to achieve "ram tuning" ABDC, intake port velocity must be very high.
Which brings me to my next question;
With DI, valve overlap should become less of an issue, because the exhaust will not pull fuel through the engine.
1. Can the OEMs bring overlap back to valve timing with DI?
2. Is port velocity important in a DI engine?

Edit; CT8, will longer rods help port velocity?
 
Last edited:
I don't know anything about DI in gasoline engines. I've owned a few Caterpillar powered machines (six, actually) but I suspect the application you're looking for is very different from the long-stroke low-rpm diesels I was running.

I know there is a lot of tinkering happening with variable cam timing and intake port configuration, and I have my doubts. Complexity brings its own troubles.

The biggest thing I ever saw that made a difference in DI engines is boost. For sure, all of the mapping makes a difference, a skilled tuner can do a lot with a laptop, but in terms of bang for the buck I think the turbocharger is that first place to look.

If you check on Youtube you will see people doing things with dump trucks that defy all credibility except it's real. I have seen (in person) dump trucks that are sending over 1,000 hp to the rear wheel and are driven to work every day. A lot was done with injection timing, a certain amount with cam timing, but a real lot of it in in the turbocharger, too.
 
Oversquare - larger bore, shorter stroke.

Piston velocity is highest at mid stroke. Time valve events accordingly. Steep ramps would be nice, but they beat on the valve train hard.

Minimum overlap for modest RPM motors.

I prefer a blower if all else fails
laugh.gif
 
Agreed on all points, especially boost. There's a point of diminishing returns on stroke; the piston speed gets way too high and that change in directions (inertial load) at the top and bottom is tough on everything once you get up into the higher rpm range.

Back in the late seventies / early eighties I ran a cam that was real steep/long duration, darn thing looked sorta square. Beat the heck out of the motor, longevity was nil on tappet rollers and rocker arms, but it did produce the power at the higher rpms.This profile was square enough that you could see the tappet bounce in the wear pattern.

It's always a balancing act.
 
Oversquare; displaces more area above the piston per degree of rotation than under-square engines.
A short connecting rod will move that piston away from TDC more per degree of rotation than a long connecting rod.
Maximum lobe lift rate is limited with roller lobes; by side loading, FT cams; by lifter edge riding.
What is the currant rocker ratio in happy pro-stock and Nascar engines?
 
Last edited:
userfriendly said:
CCI; I get all of that, but to achieve "ram tuning" ABDC, intake port velocity must be very high.
Which brings me to my next question;
With DI, valve overlap should become less of an issue, because the exhaust will not pull fuel through the engine.
1. Can the OEMs bring overlap back to valve timing with DI?
2. Is port velocity important in a DI engine?

Edit; CT8, will longer rods help port velocity? [/quote ]intake tuning is important for cyl filling. But then so is exhaust, I had a big block chev in a boat I had that with a dual carb tunnel ram and headers with good mufflers would jump out of the water with a romp on the pedal out of the hole and hit close to 8,000 rpm pretty quick and the same engine with log style manifolds and over the transom pipes and a dual plane manifold and a 750 double pumper Holly seems like it made half the power. The difference was the intake and exhaust. That is not what you were asking though. Before I sold the boat I was looking at a 600 ci World Products 1000 hp on regular gas super charged engine with a 2 year guarantee. I always wondered how it would have worked out.
 
The BBC was criticized when rolled out for it's short un-skirted block.
Without going into history, the short CAR block limited stroke and connecting rod length.
The thinking back in the day was the opposite; tall blocks, long rods and lots of piston above the pin.

Fast forward 25 years to the Olds pro-stock short deck, big bore 500 CID engine with monstrous (for 500 cid) Pontiac heads.
The way to get those ports moving, other than RPM, was over-square short rod.

More opinionated and less informed;
On the exhaust side, exhaust valve open duration has more to do with sonic tuning than the need to exit exhaust gasses.

We have an engine that achieves 100% VE @ 4500 and 9000 rpm.
But, at 4500 rpm the same charge of exhaust has twice as much time to exit the cylinder.
Because the exhaust charge is the same at both 4500 & 9000 rpm, the exhaust velocity leaving the cylinder should theoretically be the same.
Reduce the VE, and the exhaust velocity should drop accordingly.
At low rpm, a drop in VE would reduce time required to efficiently exit exhaust gasses.
 
Last edited:
My over-square short rod example mechanically increases the intake stroke duration by moving the piston quickly away from TDC.
The piston in this engine will dwell longer ABDC, than an under-square long rod engine of the same displacement.

Peak port demand is higher with under-square long rod engines because the piston speed is also higher.
In this example, the piston will dwell at or around TDC, then quickly move down the bore.
In addition, the piston will turn around sooner ABDC, mechanically ending the intake stroke.

With the over-square engine, the total intake demand is higher than the under-square engine.
Peak intake demand is higher with the under-square engine, but the total demand is less.
 
Last edited:
Status
Not open for further replies.
Back
Top