I originally had my heads ported 3 years ago. They flow 271 CFM @ .550". My head ported firmly believed that intake CC volocity is more beneficial than overall volume. Because of this, my heads only have 175 CC chambers. The valves are undercut Manley 2.00" / 1.56".

With these heads, cc305, and a stock 95,000 mile bottom end, I went 12.0 @ 112 MPH with a 3448# full weight '93 Z.

The new combination is a 6" rod, 847 cammed, 12:1 355. My goals is low 11's on the motor (11.2-11.3). I'm trying to figure if these heads will work for the new combination.

Any thoughts about volocity vs. volume?

Frank

 

Airspeed and Velocity is crucial, but a 175cc head is going to be restrictive with a 350ci LT1 and your MPH shows it. Even though you ET well, you're lacking in the horsepower department. I also think those 271cfm @ .550 numbers were inflated.

Mike

 

Yeah, just alittle......

Pinch rod area is very important for correct peak rpm hp vs. cubic inches. With a good area (for your cubes) and proper porting, your cc's come out where they come out!!!!!!!!!!!!! For an LT1 head your looking at 195cc's as a minimum port, and more likely 205-210cc's Also a stock port LT1 cc's, is around 170-175ish by the way.

 

very true...the cc sounds kinda small...Did he actualy recontour the ports or just did a flap wheel and cross buff clean up?Its been about a year since ive hogged out a set of sbc heads but i do know they need more than a fancy pretty polish job...

Velocity is great when you can keep it up but you need to increase port volume to make power in the higher RPMs..Larger ports dont hurt the low end as much as people tend to think...Besides you can always loose a little down low since the car will hook better.

My last set of LT1 heads specced out to 198cc and flowed 285cfm at sea level on the intake 188 on the exhaust.That was with stock 1.94 intake valves and 1.625 exhaust valves.

 

Try telling that Advanced Induction's customers their top LT1 oem casting head is 190cc and they make power, Lloyd goes much bigger in port volume and his customers make power too. Chalk it up to more than one way to skin a cat ?

 

All depends on where the velocity is in the port...... if you can have the velocity thru the pinch and over the short side not get crazy then you can shrink the port. Lloyd also looks at some other things inside the port and makes the heads bigger so they work well in those areas.

Bret

 

OK AI head owners...your ports are tooo small.....

Actually, 190-195 sounds about right for their heads, I was off by 5cc's on my low estimation .....I'v skined that cat 100 times, can't believe it still needs more.....

Your right though, even those ports are one heck of alot bigger than 175cc's. I just can't see how with some standard porting measurements needed to make good airflow. How you could get that small of port, for some upper rpm breathing....(take that with a grain of salt, though as I'm certainly not a head porting guru) Maybe Lloyd or AI could pop on here and give the guy some help....

 

If I'm not mistaken, too much velocity is bad too. Too much velocity makes the flow "behave" badly.

 

velocity is important, for carburetors.

otherwise, the important thing is how much air is retained
when the intake valve closes.

if you want velocity, block off 99 percent
of the intake port on the cylinder head

 

When is the world going to realize that carbs and fuel injection motors don't need different things 99% of the time?

 

Gee, I was under the impression that carbs needed a pressure drop.

 

ok, the carb itself needs venturi-pressure-drop to function,
carb setups also need velocity to keep fuel
droplets from falling out of suspension.

other than that, the real issue is how much fuel-air
mixture is retained in the cylinder

 

I dunno...

I've seen some guys run pretty big carbs and have them do just fine.

Tuning a carb is more than just changing jets.