Cylinder Head Question

[358T]

Hello all,

First post here, been lurking for some time though. First off, I'd like to give some info about myself. I'm am not a proffsional engine builder but just a enthusiastic drag racing hobbyist and a weekend bracket warrior. Just like any other gear head, I'm always looking for ways to go faster.

I've posted the following question on another board and got some great info but this board seems to be a better place to ask since a lot of engine builders and head porter post here. I also like the no nonsence approach the people on this site have.

On a 427SBC ( 4.125" bore x 4' stroke ), what intake port minimal cross sectional area will produce the most power in the 5800 to 7300 rpm range? For instance, one head has 247cc intake runners and the other has 262 intake runners. The MCSA's of the heads are 2.51" and 2.75" respectively and the heads are the same design. The flow#s of the heads are within 6cfm throughout the entire lift range ( .700 max )with the larger runnered head lagging slightly behind until .500" lift and then pulling slightly ahead. The exhaust port flow the same on both heads.

The reason I ask is because I'm considering having my heads opened up some and the flow tendencies mentioned above are how these heads normally respond to the type of porting I'm considering. My combo is:

-427ci SBC ( 4.125 x 4 )
-Brodix -12P heads 247cc w/2.51" MCSA intake runners. 59cc combustion chambers. 2.15"int/1.6"exh valvles
-14.3:1 compression
-2" headers
-Ron's Flying Toilet Alky Injection on a single plane Brodix intake. 3.44GPM @ 8000 pump with .035 nozzles and a .062 pill
-1750lb RED w/driver
-powerglide w/5800rpm 8" stall converter
-33x16 tires
-3.89 gear
-Solid Roller 312/325, 279/286, .700/.700, 108ICA, 108LSA
-Head Flow#'s @28" are
Lift Intake Exhaust
.200 159 127
.300 226 170
.400 275 194
.500 310 221
.550 320 230
.600 330 238
.650 335 243
.700 342 251

This engine runs pretty good the way it is, but seems to start running out of steam, or at least flattening out, at about 6800 - 7000rpm's. It has run a best of 7.85 @ 169 while shifting at 7000 and crossing the stripe at 7000.

Will I be wasting my time?
Is this engine being port limited?
Are there any gains to be had by switching to a cam with a 110lsa while keeping everything elso the same?
Do the durations look correct for my operating range?

Thanks,
Scott

[speedtalk]

Hey Scott, Thanks for joining the forum. I'm glad we got you to be more than a lurker. Now for my opinion:

Will I be wasting my time?

Probably

Is this engine being port limited?

Yes, most engines are.

Are there any gains to be had by switching to a cam with a 110lsa

Zero. What I think you need is more lift. Make sure the heads don't back up at .800+ lift

Do the durations look correct for my operating range?

Maybe a fuzz big, but probably ok.

Before I read your other post I was going to ask if you had enough spring pressure.

[358T]

Thanks for your respnsonses.

The valve spring question was inspired by a recent event. My old springs had 220lbs on the seat and 580lbs over the nose. I broke a spring after countless passes so I changed all the springs. The new springs are 260lbs on the seat and 630lbs over the nose. The car seemed to slow about .05 after the spring swap. But overall the engine's powerband was the same with either spring. That is the reason I'm asking about minimal cross sectional areas and such in this thread.

Scott

[maxracesoftware]

On a 427SBC ( 4.125" bore x 4' stroke ), what intake port minimal cross sectional area will produce the most power in the 5800 to 7300 rpm range? For instance, one head has 247cc intake runners and the other has 262 intake runners. The MCSA's of the heads are 2.51" and 2.75" respectively and the heads are the same design
=======================================

you will have almost twice as much Methanol in the Port (-vs- Gasoline)
choking up the Port Cross-sectional area

the 2.750 Minimum Cross-Sectional Area (MCSA)
should work the best, if everything else is OK


>>>This engine runs pretty good the way it is, but seems to start running out of steam, or at least flattening out, at about 6800 - 7000rpm's. It has run a best of 7.85 @ 169 while shifting at 7000 and crossing the stripe at 7000. <<<

for a 2.150" OD Intake valve
a gasoline engine would need at least .37 Lift/Diameter Ratio
to make good HP and good port velocity...Methanol would like a little more Lift if the MCSA is not choking

2.150 x .37 = .796" lift

2.150 x .41 = .882" lift ...the engine should keep responding to this much Lift if you're OK with valve spring life ???

the Exhaust Valve Lift would be approx. .95+ % of Intake Lift
= .837 to .756

Durations at .050" look OK for 7300 RPM w/427 cid SBC
Centers need to be spread
you could discuss your Combo with Cam Motion as far as Cam Specs

[maxracesoftware]

i use .87 PerCent as rule of thumb

your .700" Lift cam times .87 % PerCent = .609"

so approx. .609" Intake Lift would be considered the highest effective Lift your engine would effectively use thru the Induction System

.700" Lift cam - .025" Lash = .675" Lift
.675" Lift minus .030" or so valvetrain deflection/rocker arc motion = .645

so the valve is at .645" for just a few degrees
the .609" would also account a little for Intake/Carb PerCentange Losses

at .609" or .600" rounded-off your Heads flow = 330.0 CFM

330 CFM is just enough for a 427 cid at 7300 RPM


if 330 CFM times .285 Flow Factor = 752.4 HP potential then

then a .805 " lift cam times .87% = .700" lift
and that gives you 342 cfm

342 cfm times .285 Flow Factor = 779.8 HP potential

a gain of 27.36 HP and reduced Spring life

[358T]

Thanks fo the replys maxrace.

Since I'm bracket racing mostly, spring life is a concern. Allthough I will keep the valve lift relationships in mind in furture endevors. With that being said, I would like to keep the lift at a conservative .700 for now. Since I prefer to keep the lift at .700, would you agree that I need to get the heads to flow as much as possible by .600 lift? Are you familiar with these heads? Could the flow be increased siginificatly is this area while opening the MCSA up? Overall, I do not want to have to turn the engine higher than 7300 to acheive best performance.

Thanks,
Scott

[Guest]

Are you familiar with these heads? Could the flow be increased siginificatly is this area while opening the MCSA up? Overall, I do not want to have to turn the engine higher than 7300 to acheive best performance.
Thanks, --Scott
==================================

yes, i currently have a pair in the Shop for a refreshen Valve Job

but they are for gasoline SBC around your 427 cid size
i'll Post some Dyno tests info later on today


>>>This engine runs pretty good the way it is, but seems to start running out of steam, or at least flattening out, at about 6800 - 7000rpm's. It has run a best of 7.85 @ 169 while shifting at 7000 and crossing the stripe at 7000. <<<

but thats why its running out of steam
for 7300 RPM "Limit" , your pretty close with what you have now
as far as Flow CFM demand and cam specs for 427 cid @ only 7300