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SBC intakes Holley 300-25 vs 300-2

General engine tech -- Drag Racing to Circle Track

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dave brode
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SBC intakes Holley 300-25 vs 300-2

Post by dave brode »

All,

How do the two compare? Is the 300-25 for the most part the same as the -2, except for a taller plenum, or are the port entrance larger too?

Or is the 300-2 a lesser manifold altogether? If so, how mild would the combo need to be on say a 355 to make the 300-2 be a better choice?

TIA

Dave
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Re: SBC intakes Holley 300-25 vs 300-2

Post by falcongeorge »

dave brode wrote: Fri Sep 20, 2013 6:21 pm All,

How do the two compare? Is the 300-25 for the most part the same as the -2, except for a taller plenum, or are the port entrance larger too?

Or is the 300-2 a lesser manifold altogether? If so, how mild would the combo need to be on say a 355 to make the 300-2 be a better choice?

TIA

Dave
Back when they were originally designed, the 300-2 was intended for stick drag race applications, the 300-25 for automatic applications.
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Re: SBC intakes Holley 300-25 vs 300-2

Post by dave brode »

thank you
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Re: SBC intakes Holley 300-25 vs 300-2

Post by F-BIRD'88 »

Both are full race very high rpm type. Not for a mild motor.

If you did want to make both these more friendly to a "Mild motor" install a plenum divider.
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Re: SBC intakes Holley 300-25 vs 300-2

Post by Walter R. Malik »

dave brode wrote: Fri Sep 20, 2013 6:21 pm All,

How do the two compare? Is the 300-25 for the most part the same as the -2, except for a taller plenum, or are the port entrance larger too?

Or is the 300-2 a lesser manifold altogether? If so, how mild would the combo need to be on say a 355 to make the 300-2 be a better choice?

TIA

Dave
There were two different -2 intake intake manifolds released, the later one simply superseded the first design and had the same part number.
The later one had much shorter runner dividers and runner entries were rounded at the plenum roof.
The -25 is like an EARLY -2 but, raised with longer runners; (the plenum of the -25 manifold IS an early -2 but, raised and turned around from front to rear).
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Re: SBC intakes Holley 300-25 vs 300-2

Post by dave brode »

Mr. Malik,

Do you know if this one is 1st or 2nd version of the -2? Did both have the boss for the oil fill tube? My 300-25 does not.
Dave
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Re: SBC intakes Holley 300-25 vs 300-2

Post by dave brode »

F-BIRD'88 wrote: Wed Jul 29, 2020 7:23 pm Both are full race very high rpm type. Not for a mild motor.

If you did want to make both these more friendly to a "Mild motor" install a plenum divider.
F-Bird,

I would like to see a test of how a typical single plane with such a divider [fully divided] would effect the curve. I realize that a fully divided need more carb, but it would be interesting.

Dave
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Re: SBC intakes Holley 300-25 vs 300-2

Post by dave brode »

fwiw,

The 300-25 did really well on this hot street 406 test [19 intakes] Better than the Dorton 300-110.

https://www.hotrod.com/articles/19-sing ... akes-test/
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Re: SBC intakes Holley 300-25 vs 300-2

Post by F-BIRD'88 »

dave brode wrote: Thu Jul 30, 2020 8:02 pm
F-BIRD'88 wrote: Wed Jul 29, 2020 7:23 pm Both are full race very high rpm type. Not for a mild motor.

If you did want to make both these more friendly to a "Mild motor" install a plenum divider.
F-Bird,

I would like to see a test of how a typical single plane with such a divider [fully divided] would effect the curve. I realize that a fully divided need more carb, but it would be interesting.

Dave
The tests I have seen it helps the midrange torque.
Not quite like a dual plane does but helps.
It is the street driving improvement that you really notice.
A dyno test does not show this stuff
But a in car real acceleration test shows a lot. Wether the divider should be full height or... depends on each situation
It is a street tuning tool..
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Re: SBC intakes Holley 300-25 vs 300-2

Post by Walter R. Malik »

dave brode wrote: Thu Jul 30, 2020 8:00 pm Mr. Malik,

Do you know if this one is 1st or 2nd version of the -2? Did both have the boss for the oil fill tube? My 300-25 does not.
Dave
That manifold in the photo is the original design before the changes. I don't remember about the other parts of the manifold other than the plenum and runners.
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Re: SBC intakes Holley 300-25 vs 300-2

Post by dave brode »

thanks
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Re: SBC intakes Holley 300-25 vs 300-2

Post by 1980RS »

I know one thing now for sure, my 300-36 dual plane was real close to the 300-25 that I tested back to back. The 300-25 was 1 mph better and 2 tenths quicker but to be fair had I been able to get the car to hook up with the 300-36 I think it may have been a wash between the 2 intakes. If i test the 300-36 again I may cut down the divider.
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Re: SBC intakes Holley 300-25 vs 300-2

Post by dave brode »

1980RS wrote: Wed Aug 05, 2020 12:43 am I know one thing now for sure, my 300-36 dual plane was real close to the 300-25 that I tested back to back. The 300-25 was 1 mph better and 2 tenths quicker but to be fair had I been able to get the car to hook up with the 300-36 I think it may have been a wash between the 2 intakes. If i test the 300-36 again I may cut down the divider.
Please describe the car and combo
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Re: SBC intakes Holley 300-25 vs 300-2

Post by 1980RS »

dave brode wrote: Wed Aug 05, 2020 10:08 am
1980RS wrote: Wed Aug 05, 2020 12:43 am I know one thing now for sure, my 300-36 dual plane was real close to the 300-25 that I tested back to back. The 300-25 was 1 mph better and 2 tenths quicker but to be fair had I been able to get the car to hook up with the 300-36 I think it may have been a wash between the 2 intakes. If i test the 300-36 again I may cut down the divider.
Please describe the car and combo
Car is an 80 Camaro that is a .040 350
Flat top pistons, PM Rods with ARP bolts
Cast GM crank, 4 bolt main
Sportsman II mild ported heads roller rockers and a stud girdle
Engle .600 lift cam 250° duration at .050 both sides on a 108° lca
Holley 300-36 first test and Holley 300-25 for the 2nd test
850 Mighty Demon A/D booster carb
MSD locked in distributor and blaster 2 coil
Turbo 400 with a Jeg's XHD 10" converter that flashes to 4100 stall
4.10 gear on 30" slicks
Best time with this combo was 11.29@118 and change.
Wait until I post what it ran with the 406 back in it today. Very surprising results.
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Re: SBC intakes Holley 300-25 vs 300-2

Post by GOSFAST »

Not sure tonight the difference in the two intakes but I can give you this much on the 300-25, back in the mid '80's we began welding in some "fins" and did a little porting and was able to pick up about 30 HP. We saw it on the dyno and some close customers who used it picked up 3-tenth's at the strip!

See the photo below, on this one the "fin" work is finished and ready for the porting!

Thanks, Gary in N.Y.

P.S. Back in those days the 350" was much more prevalent than the larger SB inch units, we never got to really test it much on the 400" platform but figure it would perform as well with the "fins" installed. We still do a couple during the year, but not as many as in the past!
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