Porting sbc heads?

[88bluestar]

Ok, I must preface this question by saying this is a budget class so I’m trying to use what I have but make as much power as I can. Rules are:
Engine must be vacuum-pumped and measure no less than 14 inches of vacuum at 900 rpm. Must have a 3/16-inch port in the intake manifold to hook up vacuum gauge. 4412 Holley gauge legal allowed.

6,200 rev limiting device and must weigh a minimum 3,000 lbs., including driver after race. Must run factory cylinder heads (no aftermarket race heads allowed). Must run flat top or dish pistons. Must have a cranking compression no higher than 150. Must run factory-style stamped steel rocker arms.

What I have is an 350(010) block, 4340 used 3.750 stroke crank(20 main/30 rod main, and 991 open chamber heads.
3/8 mile dirt track. Using pump gas but eventually will be running E-85 fuel.

What porting would I need to do and other mods to make the 991 work as efficiently as possible within these rules?

[mag2555]

The first thing I would do is toss those open chamber heads for earlier ones and cut a dish in the pistons to make up the compression difference and of course rebalance.

If your stuck with those 991 heads then what are rules they have on the porting work?
There must be a CC limit to the port volume no?

Just note that the CCs of the chamber does have a effect on the vacuum level, smaller chambers more vacuum.

[Tom68]

Umm, dare I say Vortecs.

Still factory heads would include 292s, Bowties etc.

Aluminium heads ? You could stroke an LT4.

I swear some of these limiting rules make racing more expensive.

[mag2555]

With .270” longer stroke I don’t know that you will have a issue meeting a vacuum rule.

[perfconn]

I would spring for a set of large port vortec bowties.You can get 17" of vacuum with right camshaft.

[88bluestar]

Stock heads only. Vortecs not allowed. The 991 is what I have to work with. Porting allowed, chamber can be smaller so decking it to a certain degree is ok. I had planned to run a -5 flat top with them. 6.0 I beam stroker rods.

[88bluestar]

2.02/1.60 valves or stay with 1.94/1.50?

[Tom68]

2.02/1.60 valves or stay with 1.94/1.50?

Others will know better but I'd think the 2.02 would allow for better contouring in the bowl ??

[SoCalDave]

Go 1.94 intake 1.60 exhaust and a good valve job, concentrate on the bowl to get them to flow; cylinder wall side is important.

[mag2555]

To start with what is the limit to porting the heads, I mean there is no CC limit?

Also it’s very important to know what the numbers are on the worst flowing runner in the intake manifold you will be running.

To have the widest possible power band you do not want the manifold to be the choke point.

Having a condition like that will cut into any level of higher rpm Ram tuning you might have taking place.

These 991 heads stock flow near 200 cfm@.500”@28”.
A bowl port job can get them to 225 cfm.

200 cfm can make 400 Hp with a 4 bbl carb, non the less I would step up to a 2.02” intake and have a valve job applied to get the fattest flow numbers from .200” on up to .350” lift.

If you tap into high 130s cfm to low 140s @28” @ .200” your valve job is serving you well.

A 2.02” valve will need a back cut to help get these numbers as of course a 1.94” valve.

[88bluestar]

2701 edelbrock dual plane per rule. 1” spacer for 4412 carb. 1 5/8 header-3” collector

[88bluestar]

Was going to use the 1.94/1.50 valves I had left over from vortec heads I had that were no good. Sadly both had exhaust seat cracks.

[steve cowan]

Ok, I must preface this question by saying this is a budget class so I’m trying to use what I have but make as much power as I can. Rules are:
Engine must be vacuum-pumped and measure no less than 14 inches of vacuum at 900 rpm. Must have a 3/16-inch port in the intake manifold to hook up vacuum gauge. 4412 Holley gauge legal allowed.

6,200 rev limiting device and must weigh a minimum 3,000 lbs., including driver after race. Must run factory cylinder heads (no aftermarket race heads allowed). Must run flat top or dish pistons. Must have a cranking compression no higher than 150. Must run factory-style stamped steel rocker arms.

What I have is an 350(010) block, 4340 used 3.750 stroke crank(20 main/30 rod main, and 991 open chamber heads.
3/8 mile dirt track. Using pump gas but eventually will be running E -85 fuel.

What porting would I need to do and other mods to make the 991 work as efficiently as possible within these rules?

How much power do you think is required to be competitive?

What cam do you have,the cranking compression rule will be the one to watch.
I don't know those heads but I suspect smogger type.
Depending on your cylinder head budget as well I spose.
Valve job,angle milling, crack testing etc.
I would run the smaller valve initially, for a 6000rpm 383 you need approx 290 cfm.
Won't be easy with those castings and dual plane intake.

[steve cowan]

And to clarify-
383 cubes needs 290cfm @ 6000rpm before anyone loses there mind.

[88bluestar]

Jones cam. 242 @.050. Roughly.500 lift,110 LSA +4
43@.050 INT closing

3200 lbs race car to clarify