BBC iron oval intake

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LaVelle
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BBC iron oval intake

Post by LaVelle » Wed Oct 04, 2017 11:59 am

What is the performance potential of the 60's era spreadbore, dual plane, high rise, oval port intakes, -vs- a Performer 2-0?
I have a 3931067.
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Researching into doing a peanut port 454 for a 65 GMC 3/4 ton to tow the car trailer.
Budget is limited, so I'm trying to use as many parts as I already have.

Plan is;

8.5-9.0 to 1 comp.
Ported #236 peanut port heads. ( possibly 2.19/1.88's )
Cam around 220 @ .050 / .470-.500 lift.
The 3931067 intake.
Q-jet or Holley #6210.
1 3/4 headers.

Any thoughts or suggestions?
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Re: BBC iron oval intake

Post by F-BIRD'88 » Wed Oct 04, 2017 1:05 pm

These GM BBC intakes work fine.

One thing you can do is Siamese the primary and secondary throttle holes in the carb flange.
Do not remove the center divider. Just join the front and back barrels.
You want to keep the divided 180deg plenum.
This will allow you to use a Qjet carb spacer to get some carb height and a bit more power.
use the common 4 hole Q jet adapter spacer and siamese the primary and secondary barrels by hand
also. This adds some plenum volume but keeps the divided 180deg separation of a dual plane manifold.

See the heat track channel carved in the carb flange. This requires a GM stainless steel shim plate
to seal up the exhaust track or you will have a exhaust leak at the carb.

You do not want to or need to eliminate the carb heat track but you can adjust the amount of exhaust heat flow +/- by restricting the exhaust hole in the centers of the intake manifold gasket.

I suggest blocking off one side and restricting the hole side of the other gasket using a thin piece of sheet metal as required. This will give you some carb body heat but not excessive.

The modified Siamesed Qjet 4 hole carb adapter will do the job for this in place of the OEM GM SS carb shim shield plate.

Towing is all about low and midrange torque and fuel efficiency so don't get all wrapped around "horsepower"

A Qjet is fine for this. It may well need different jetting and metering rods to re calibrate with the headers
but is probabily your best bet. I like the old school 4MV style pre 1975 BBC Qjets that came on that exact intake.
68-69 ish 396 325 and 350hp 427 360 and 390HP. it will work great once dialed in for the headers.
you can even shorten up on the cam duration a hair shorter 214-218 ish duration @.050" and lots of lift .500"+ lift
Examples are Comp Cams # 11-231-3 and # 11-235-3
Again for towing it is all about TORQUE.

Modify the carb flange opening like this.
Do the same to a common 4 hole Qjet carb adapter spacer so it matches.
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Last edited by F-BIRD'88 on Wed Oct 04, 2017 1:15 pm, edited 1 time in total.

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Re: BBC iron oval intake

Post by ProPower engines » Wed Oct 04, 2017 1:10 pm

I would use the 2-0 intake. They are much better then the iron intake.
I just finished a 454 oval port build with 781 heads and the stock aluminum snow flake intake which looks the same as the Edelbrock performer was about 70 hp better then the tall iron intake with a Q-jet carb. Took a bit to tune but was worth the time and effort.
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Re: BBC iron oval intake

Post by tenxal » Wed Oct 04, 2017 1:12 pm

That intake was used on the '69 396-325 & 350 horse engines and the low horsepower '69 427's. They do a nice job and provide lots of torque. Downside is they are heavy and have a divorced choke (if that's an issue for you). I'd opt for the QJet and 1 5/8 headers (shortie stainless). Did a few of these years ago for pick ups and Suburbans with the Comp 262 High Energy cams...worked well.

If pushed for a different intake, the G.M. Performance 12363420 would be my choice.

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Re: BBC iron oval intake

Post by F-BIRD'88 » Wed Oct 04, 2017 2:06 pm

I also like the G.M. Performance 12363420 intake.
I'd prefer it had a fully divided plenum for your purpose.
I like the Weiand street warrior too. It has a fully divided plenum
and a universal carb flange. ('d try a spacer on this one too.)

The carb flange holes modification of the OEM cast intake with a added spacer
is a effort to move in this direction for a bit more power.
The adapter spacer can be a hybrid style too.
You can make this by hand starting with a common 4 hole QJet carb spacer/adapter.
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Re: BBC iron oval intake

Post by pamotorman » Wed Oct 04, 2017 3:54 pm

GM later did away with the heat groove because if there is a carb fuel leak it could cause a fire. just drill and tap the 2 holes at the end of the heat groove for a socket head pipe plug to plug them off

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Re: BBC iron oval intake

Post by LaVelle » Wed Oct 04, 2017 5:50 pm

F-BIRD'88 wrote:
Wed Oct 04, 2017 1:05 pm
These GM BBC intakes work fine.

One thing you can do is Siamese the primary and secondary throttle holes in the carb flange.
Do not remove the center divider. Just join the front and back barrels.
You want to keep the divided 180deg plenum.
This will allow you to use a Qjet carb spacer to get some carb height and a bit more power.
use the common 4 hole Q jet adapter spacer and siamese the primary and secondary barrels by hand
also. This adds some plenum volume but keeps the divided 180deg separation of a dual plane manifold.

See the heat track channel carved in the carb flange. This requires a GM stainless steel shim plate
to seal up the exhaust track or you will have a exhaust leak at the carb.

You do not want to or need to eliminate the carb heat track but you can adjust the amount of exhaust heat flow +/- by restricting the exhaust hole in the centers of the intake manifold gasket.

I suggest blocking off one side and restricting the hole side of the other gasket using a thin piece of sheet metal as required. This will give you some carb body heat but not excessive.

The modified Siamesed Qjet 4 hole carb adapter will do the job for this in place of the OEM GM SS carb shim shield plate.

Towing is all about low and midrange torque and fuel efficiency so don't get all wrapped around "horsepower"

A Qjet is fine for this. It may well need different jetting and metering rods to re calibrate with the headers
but is probabily your best bet. I like the old school 4MV style pre 1975 BBC Qjets that came on that exact intake.
68-69 ish 396 325 and 350hp 427 360 and 390HP. it will work great once dialed in for the headers.
you can even shorten up on the cam duration a hair shorter 214-218 ish duration @.050" and lots of lift .500"+ lift
Examples are Comp Cams # 11-231-3 and # 11-235-3
Again for towing it is all about TORQUE.

Modify the carb flange opening like this.
Do the same to a common 4 hole Qjet carb adapter spacer so it matches.
I was wondering about removing the material between the primary's and secondary's opening it up like an aftermarket.
David Vizard talks about that in one of his books.

The heat channel I was going to leave unplugged and make a 1/8 steel sandwich plate to cover that area and eliminate adaption problems.
(I could even go with an adaptor and square flange carb at that point but would like to stay with the spreadbore.)
Using the restrictors that come in the intake set.
I'll check into the spreadbore spacers.

Believe me HP isn't game plan,I'd be using the 781's and RPM intake I have, with a lot a lot more cam and compression if I was.

Cam choice has yet to be made.
I stated those figures as a ball park example of the area I was considering.

Thank you for the input.

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Re: BBC iron oval intake

Post by LaVelle » Wed Oct 04, 2017 5:55 pm

ProPower engines wrote:
Wed Oct 04, 2017 1:10 pm
I would use the 2-0 intake. They are much better then the iron intake.
I just finished a 454 oval port build with 781 heads and the stock aluminum snow flake intake which looks the same as the Edelbrock performer was about 70 hp better then the tall iron intake with a Q-jet carb. Took a bit to tune but was worth the time and effort.
I just finished one like that, 781', a Performer, 230 @.050/ .550 lift cam.
Does 11.38 - 12.43 in my 3700 lbs. Pontiac

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Re: BBC iron oval intake

Post by LaVelle » Wed Oct 04, 2017 6:02 pm

tenxal wrote:
Wed Oct 04, 2017 1:12 pm
That intake was used on the '69 396-325 & 350 horse engines and the low horsepower '69 427's. They do a nice job and provide lots of torque. Downside is they are heavy and have a divorced choke (if that's an issue for you). I'd opt for the QJet and 1 5/8 headers (shortie stainless). Did a few of these years ago for pick ups and Suburbans with the Comp 262 High Energy cams...worked well.

If pushed for a different intake, the G.M. Performance 12363420 would be my choice.
Weight is of little concern.
It's a tow rig, not the hot rod.
The 1 5/8 headers might work better but I have the 1 3/4's.
Good to hear I'm trying to copy a proven combination.


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Re: BBC iron oval intake

Post by cpmotors » Thu Oct 05, 2017 2:24 pm

The first 396 I did for my girlfriend/wifes car was a '68 325HP 396 with Qjet and the iron Qjet high rise. Swapping to a Performer made zero difference on her car than ran 13.50's.
I did a mild 496 for a 1966 390HP/427 Corvette a few years ago with the Holley version of that intake. I only bored it out to fit an 850 on it, otherwise it was stock on some mildly ported 781 heads and 235° HR. It made excellent torque down low.
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Re: BBC iron oval intake

Post by F-BIRD'88 » Thu Oct 05, 2017 4:05 pm

If you already have the Performer rpm BBC oval intake manifold on hand why not use it.

But make up a custom carb spacer adapter that restores the plenum divider wall height
so the plenum under the carb is fully divided. Can use a Q jet carb or a square bore carb.
This carb spacer/adapter can be 2 Siamese holes or a hybrid type with a Siamese hole on the short side and a 2 hole on the deep side of the plenum.
Avoid a open spacer for low rpm driving/towing where low rpm and part throttle driving efficiency matters.

Its going in a truck so hood clearance is generous. Most all the chevy intake manifolds that were limited by passenger car hood clearance can and will benefit from adding a carb spacer ( divided or hybrid type).

This added carb height above the runners and helps to make the left and right side plenum(s) volume more equal. It also adds total induction tract ( runner and plenum of each side of the split 180deg plenum(s)) tuned length which should help low/mid rpm torque. The longer tuned length "tunes" *resonates* right where you need it to for a big low/mid range toque bump.

The (best) camshaft has to match up to this to get max gain benefit. the best valve timing will be shorter than you think...

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Re: BBC iron oval intake

Post by LaVelle » Fri Oct 06, 2017 7:24 am

F-BIRD'88 wrote:
Thu Oct 05, 2017 4:05 pm
If you already have the Performer rpm BBC oval intake manifold on hand why not use it.

But make up a custom carb spacer adapter that restores the plenum divider wall height
so the plenum under the carb is fully divided. Can use a Q jet carb or a square bore carb.
This carb spacer/adapter can be 2 Siamese holes or a hybrid type with a Siamese hole on the short side and a 2 hole on the deep side of the plenum.
Avoid a open spacer for low rpm driving/towing where low rpm and part throttle driving efficiency matters.

Its going in a truck so hood clearance is generous. Most all the chevy intake manifolds that were limited by passenger car hood clearance can and will benefit from adding a carb spacer ( divided or hybrid type).

This added carb height above the runners and helps to make the left and right side plenum(s) volume more equal. It also adds total induction tract ( runner and plenum of each side of the split 180deg plenum(s)) tuned length which should help low/mid rpm torque. The longer tuned length "tunes" *resonates* right where you need it to for a big low/mid range toque bump.

The (best) camshaft has to match up to this to get max gain benefit. the best valve timing will be shorter than you think...
The 781's and the Air Gap RPM are for the race motor.

Building a low budget tow engine out of an old motorhome 454/T400 for the 65' GMC, and trying to keep costs down by using parts on hand over purchasing others without a major power loss.
During the rebuild was going to bump the compression a little. ( since I'll be buying pistons )
Will be porting and doing the heads myself. ( haven't decided on leaving the heads (#236 peanuts) 2.06/1.72 or going 2.19/188 )
The 3931067 has a runner shape similar to the RPM and thought it might be well suited for this application.
With minor modification it should fit the bill.

I'm doing away with the 350 sbc and granny 4 speed.

I found a newer low rise smog intake that is factory with the primary's and secondary's Siamesed.
Looks like maybe the factory learned something.

For the cost of a 4 hole spreadbore spacer and then modifying, it would be a lot cheaper for me to get a remnit piece of aluminum and make one.
How thick of a spacer?
I can get material in 1/8" increments up to 1" thick.

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Re: BBC iron oval intake

Post by mag2555 » Fri Oct 06, 2017 8:26 am

In regards to the peanut port heads,other then bigger valves gaining you some compression and having fresh valve seat material to work with if your just going to apply a bowl porting job stick with the stock valve sizes or maybe use the 1.88" Exh valve to gain better low lift flow at blow down.

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Re: BBC iron oval intake

Post by F-BIRD'88 » Fri Oct 06, 2017 12:45 pm

For what is the best carb spacer height I cannot tell you exactly. You are going to have to try a few and see.

I suggest 1" minimum up to 2 1/2" or so. A stack up of 3/4" spacers is easier to carve out by hand with a die grinder. The typical cast alumunim Qjet carb adapter spacer that you see is 3/4" thick.

If you have access to a mill or at least a big drill press it makes carving the holes a lot faster.

You can make up low cost carb spacers out of wood (good plywood) to test with without braking the bank
or spending more money that nessessary. You are going to need to do some drive testing with the spacers
to find the best combo of spacer(s) height and type(s). Be prepared to rejet the QJet carb too.
Even a simple low cost narrow band O2 sensor on the exhaust can really help you dial in the carb.

Qjets are great carbs when properly jetted for the specific engine and bad carbs when not properly jetted.
That is why GM had so many different setups for the Qjet.
I wish I had kept my old 1968 396 325HP 4MV Qjet carb. it was a good one on many of my BBC and SBC motors, way back.

Here is the thing...You have the RPM intake on hand to test against as a bench mark to compare.
I think with that, you can make the factory hi rise cast iron intake with this mod and spacers
do the job very well.
if I was going to do large valves at all I would not go up to a 2.19". I would go 2.12" 2.14" at max.

Calculate the road speed and the resulting engine running rpm at that speed and keep that in mind
* the rpm will be lower than you think) so you don't get all crazy on camshaft duration.

On the heads avoid the thought of hogging out the bowls too much removing that funnel venturi under the
valves. on a mild cam the valve stays close to the seat (relative) and that funnel venturi shape helps the air go around the valve ( at low and mid lift) and go into the chamber. This venturi shape just under the valve matters. When you port the valve bowls think velocity and air flow quality around the valve and pressure recovery at low and mid lifts...( because that is where the valves spend most of the time with a mild cam VS a larger duration/overlap high lift racing cam for a higher rpm race motor.

On a (Mild low rpm) motor where the valves stay relatively close to the seat most of the time there is more to it than just getting more raw flow. And the motor spends most of the towing trip time running at part throttle, not WOT. I believe the BBC vortec L-29 heads are good for this.

One of the smart guys here that does BBC's will tell you which valves ( part numbers) {2.12" to 2.14") to get .
* GOFAST or Joe Sherman or.... VortecPro or

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