Question: RPM Air Gap Porting

[skinny z]

By most accounts, these intakes are pretty good out of the box. Which is how I've always run them.
As I'm slowly getting back into this next engine rebuild (or recreation entirely as the engine is out of the car but not disassembled for inspection) I'm exploring possible improvements in the overall package should I continue in the current direction.

This isn't so much a matter of whether the intake is best suited. This will be peak HP well below 6500 RPM (maybe 6000 plus overspeed) in a heavy car (3650 race weight for the 1/4) so the general consensus is that it's better suited than a single. This is also a Vortec platform (but not Vortec heads) so intake choices are limited.

So, the question is, is there much that can be done?

As Big Joe commented years back, skip the porting and move on to carb spacers. I can see experimenting with a spacer or two might prove beneficial. But as for outright porting as I've seen in single plane intakes, there doesn't seem to be much room.

Any thoughts?

[mag2555]

X2!
It’s the spacers that help to 1) get more air into the shallow plenum side, and (2) to set up the intake to wave tune better in the rpm range where the motor will spend most of its rpm time in during a run.
This all being said if your car is very steeply geared and will run in the upper 1500 rpm only of your Cams power band then a single plane type might serve you better , but with the info you have presented here, it does not sound that way to me.

[skinny z]

Definitely not set up to be a drag racer but the 1/4 mile is my favourite test facility. For a given chassis, changes in engine development become apparent.
I'm hoping to finish what I started with the 355. Best corrected time is 12.4x @ 109.
Other than a refresh, there'll be a new spec cam (to replace two previous versions, one tested as above, the other with no track time but it's toast now after the engine carnage).
I'd like a corrected 11.99 but it's not likely given the limitations of the car in general. But a couple of tenths would be nice.
Searching for gains, such as what a carb spacer might provide will be part of the program.

[F-BIRD'88]

" Corrected time". ...,?

The higher up the carb is, the better.
Especially if one of the spacers is a nitrous plate.

[skinny z]

" Corrected time". ...,?

Yep.
As I'm in a cross country competition with my former Ontario racing friends, I've taken to correcting my time slips to the DA of the day. That way the mine shaft air I often raced in at Toronto Motorsports Park or Grand Bend Motorplex (where I believe we met twenty years ago) can be compared on an equal Sportsman like footing with the 3000-45000 DA I race at here.

As an example, prior to leaving Ontario:
2014/08/23
12.641 @ 106.82, 8.043 @ 85.73, 1.85 60’
Toronto Motorsports Park info:
630’, 66.2F/19C, 30.09” Hg/101.9 kPa, 94% rh
DA: 1321 (<< and that's poor compared to -200 feet!)
Corrected.
12.461 @ 108.379 MPH corrected.

Next pass in Alberta went:
2015/06/05
12.93 @ 105.46, 8.23 @ 83.15, 1.85 60’
I thought I'd hurt the engine on the cross country trip.
But,
Castrol Raceway info:
2210’, 59F/15C, 29.91” Hg/101.29 kPa, 51% rh
DA: 2855
Corrected.
12.489 @ 109.264 MPH
Pretty much identical. So that explained a lot.

[skinny z]

The higher up the carb is, the better.
Especially if one of the spacers is a nitrous plate.

Don't tempt me....

[F-BIRD'88]

I think that you'd get the most ET gains by increasing air flow by using it more. Rev the engine up a lot higher thru the traps to benefit from "afterpower" increases of the airflow potential of porting.. Steep gears.
A 355 cid in a 3650# car needs gearz.

[skinny z]

I think that you'd get the most ET gains by increasing air flow by using it more. Rev the engine up a lot higher thru the traps to benefit from "afterpower" increases of the airflow potential of porting.. Steep gears.
A 355 cid in a 3650# car needs gearz.

Something I realize. But it isn't going to happen. The gears, trans and converter are fixed (new converter though and not yet tested).
Best 60' with the old converter (about 500-1000 less flash stall) was 1.7 flat. Street suspension. ET Streets.
60's fell to the 1.9x range as the tires went away, so if I can get that back, that's half the battle. New drag radials on the way.

Now, with this thread, I'm looking for the marginal gains here and there that'll make a difference.
Port the Air Gap? Not so much.
Try a 1" open spacer. Seems like the ticket.
Especially if the new cam spec brings the useable RPM range up. Was shifting at 5500-6000. Next step will raise that another 500 RPM or so.

[F-BIRD'88]

Improving the 1.80's 60 ft is good in any air.
Raw mechanical leverage (gearz) helps.
Opening up the manifold allows the engine afterpower to be realized, to pull it thru the traps because of the gears. Make most use of what you got.
Take your ET multiplied by your MPH.
A 1350 ish result is just average.
Get it down near 1300 ish. (ET X MPH)
By leaving harder pulling harder, reving higher thru traps
reducing 60 ft times will reduce ET.
Make the engine want MORE AIRFLOW.
Getting more flowbench flow has to be applied to the track by forcing the engine to demand →→more airflow on its 1/4 mile trip.
Make the car Work a lot HARDER.

[F-BIRD'88]

Got to find a way to restore that lost air DENSITY there.
Some airplanes use a supercharger to do that as they fly higher up. I heard it works.. Might just be a rumour.

[skinny z]

Make the engine want MORE AIRFLOW.
Raw mechanical leverage (gearz) helps.
Improving the 1.80's 60 ft is good in any air.

A few things were supposed to happen to bring this non drag racing package closer to the sharp end.
As of yet untested were:
Ported heads. Now 250~ at .550" which is up about 10%.
More cam. From 274/280 to 288/294. Both 110 LSA. The 288 on a 104 ICL.
More converter (as stated).
So that falls in line with airflow improvements and to some degree, addresses the gearing.
There'd have to be new tires too.
That package never made it to the track although it felt very strong in street testing.
The plan is now a refresh of the shortblock, new cam and go.

Anyway, to the point, looking at it further, the Air Gap is probably still the better choice. Spacer or not.

[skinny z]

Got to find a way to restore that lost air DENSITY there.
Some airplanes use a supercharger to do that as they fly higher up. I heard it works.. Might just be a rumour.

I've always liked the phrase, "Bring your own atmosphere."

[amc fan]

I cut the manifold base off of an AMC performer rpm intake to mount a 383 Mopar tunnel ram. I was surprised at the difference in area of the runners, the outside runners were considerably larger by 1/4 to 3/8 of an inch and they cast a ramp in the port to match the head floor. The widths were ok. Brzezinski ports dual plane intake manifolds and they claim Horsepower gains. You Tube search: Head bytes intake manifold porting.

[F-BIRD'88]

tunnel ram

Can ya say that around here?

[steve cowan]

Kevin,
I have mentioned this before,
In 2019 my 383 combo
10.6 comp
Dart cast iron 165cc (178cc)
Comp SFT 235-242 @ 0.050" 500" lift 106LSA-106INCL
As cast airgap 950hp carb
Tried alot of spacers the best I can not really say
Engine made around 430hp
Car weighs 3650 with me streeter 255/60/15 MT ET STREET
Testing for approx 9 months
11.75 - 113.23 mph
1.58 - 60ft
7.43 - 91.10 mph
Gear shifts @6000 rpm and 6000 rpm finish line
Engine falls flat at 6000
As far as porting a airgap good luck would be extremely difficult.
Same combo changed to Vic jnr was not port match and just removed casting flash no porting.
11.71 @ 114.48
1.60 - 60ft
7.44 - 91.73
Still 6000 rpm shift 6200 finish line
Had tested with shift points from 5800 - 7000 and 6000 seemed to be where it liked.
I understand you are set on gear and converter
I went from a 9" 4200 converter to a custom 8" 5600 converter and it is worth 3 tenths in the 60ft on all my different combo's .
450hp won't be hard for a 355 cube
But with a set rear gear and converter you will get to a point of flogging a dead horse so don't get caught up to much as it will wear you down.