[Murfys_Law]

all being the same - flow rates at a certain lift, comustion chamber size, etc, etc, what does the intake runner size do for you?

If a certain cylinder head with a 170cc runner head flows 210 @.500 and
a 200cc runner head flows 210@ .500 what head would you get? Who cares what the intake runner size is then right????I am confused

[pete b]

I'm not an expert but I'll give it a try- the smaller intake runners will help keep the speed of the incoming fuel mixture up, which is a plus on a smaller motor or one with less cam. If you put too big of a head on a smaller motor you lose fuel charge speed and the quality of the fuel charge can suffer. The gas can actually start to fall out of the fuel mix. The same thing applies to intake manifolds- if you use too big of a manifold you lose fuel speed and end up with wet fuel in your chambers. The size of the intake runners and the size of the manifold also affect the carburetor's signal, although I don't know how to properly explain that part of it. I'm sure there are guys on here with a lot more experience than me who can. I hope I made some sense to you!
Pete

[69SSRS]

Murfy,

You are exactly right. This whole industry standard of stating intake runner volume is stupid. What they should be telling the customer is the minimum cross sectional area of the runners. With this information, and the right equations, you can calculate mixture velocity. This is the determining factor for calculating the rpm at which torque peak occurs. Some manufacturers, if asked, could probably tell you this information, but IMO it should be the standard by which heads are measured. Take care.

------------------
69 SSRS Frame-off Resto
81 Z-28 377ci Drag Car

[JohnZ]

Previous posts are correct - runner volume by itself doesn't mean much; it's shape that counts, and that's more black art than science. Good example is the Ford 351 Cleveland, revered by many Ford-lovers as the "end-all" small block (which it isn't - the 351 Windsor is a much better engine). The 351 Cleveland had "2-barrel" heads with normal ports, and had "4-barrel heads" with HUGE intake ports. The "4-barrel heads" were designed strictly for Trans-Am racing and were excellent above 4000 rpm at full throttle; below that the car was a lousy-running "stone" due to low gas velocity in the ports and the resultant poor vacuum signal to the carb's metering system. For the street, the "2-barrel heads" were just right. Same is true for carburetors on 90% of mildly owner-modified street engines; most have WAY too much carburetor, and have lousy throttle response and driveability - but they love to pull the air cleaner at shows and look at that BIG Dominator with no choke horn and 4-corner idle they don't know how to set. Stock Gen I Z28's (like mine) are the same way to some extent - there's no way a 302 needs an 800CFM carburetor for the street (they run much better with a 600, and won't starve for air at full throttle until about 3000 rpm above red-line with one, which they'll never see anyway). But, we leave them alone because that's the way they came originally. I did the 600-instead-of-800 swap on a friend's 302 a couple of years ago (same 68/76 jetting) for several months, and it ran much better, had nice, crisp throttle response, and picked up about 3.5 mpg in the process; then put the 800 back on because that's the way it came. Who said this hobby required logic or reason?

------------------
JohnZ
'69 Z28 Fathom Green

[davidpozzi]

John,
I'll bet that many stock first gen Z/28's probably never have the secondaries get fully open.

A dyno operator/engine builder once told me not to compare an Air flow research head to a Brodix track 1 because the AFR head had straighter ports and would come out smaller in CC's but might have a larger cross section. The intake does not have as large a hump as a Track 1. But he said the Track 1 did better when a little clean up porting was done than could be gotten from an AFR head.

It seems like every time you learn something, it changes. I guess the rule of thumb is... There is no rule of thumb anymore!
David

------------------
Check my web page for First Gen Camaro suspension info:
David's Homepage
67 RS 327
69 Camaro Vintage Racer
65 Lola T-70 Can Am Vintage Racer

[zzre]

1. Limiting Port Velocity:

LPV = [0.00353 x stroke x (bore x bore)]
divided by:
(CA)
CA = Minimum port cross-sectional area in square inches

2. Formula for finding minimum port cross-sectional area for peak power at a target RPM:
CA = [0.00353 x RPM x Stroke x (Bore x Bore)]
Divided by:
690

3. Maximum RPM allowed by the pushrod constraint:
MAX RPM = CA x 184,136 (see note)
divided by:
stroke x [(bore)(bore)]
note:
Use 184,136 for endurance race roller cam
Use 195,558 for drag race Pro-Stock type roller cam.
Use 177,780 for a flat tappet cam.

[zzre]

The advertised cc of a cylinder head is a good indication of airflow potential. More important, the size of the intake runner should not sacrifice good port induced swirl. Porting heads to achieve better port induced swirl will allow you to run greater cylinder pressure with no ill effects. A smaller intake runner which exhibits an overall great port induced swirl is far greater efficient than just a cylinder head whose runner is “hogged out” for the sake of volume alone.

Gas/air fills up a combustion chamber by pressure equalization. The piston doesn’t suck it in. Fuel Injection, Super-charging, Turbo-charging, Nitrous, etc., all achieve the same goal: they improve the Volumetric Efficiency. Sizing the intake runners on normally aspirated engines is a combination of math and physics. Every component of the induction system is a dependent variable. A camshaft is designed to operate within a certain RPM band. Within that RPM band the engines requires a certain amount air/gas for optimum performance. How much gas an engine can effectively consume is dictated by how much air it can consume. By researching how much air an engine can consume within your specified RPM band you can set up your cylinder head and carburetor accordingly.

This is a pretty deep subject and more variables exist. Your local library would be a great place to start for more information on this. There are chapters on Volumetric Efficiency in most Thermodynamics books. Or cruise around the web, check out:
www.auto-ware.com/combust_bytes/eng_sci.htm


[This message has been edited by zzre (edited 09-21-2000).]

[pdq67]

ZZRE,

Now thats technical. It's going to make me think, Darn, it will hurt my head.

Doesn't this type stuff also come into play when looking at max torque production given a certain engine combination???

Keep it coming!!! pdq67

[JohnZ]

Built a 351 Windsor (for a Cobra replica I built for him) for a customer about five years ago; he wanted World Products Windsor heads (iron), as they were much better than any Ford or SVO heads at the time (factory heads had the smallest, crookedest (?) exhaust ports ever designed). He wanted every last bit of power he could get without adding the weight of an FE big-block and budget wasn't a problem for him. Dynoed the motor with the stock World Products heads which are pretty nice out of the box, made 370 hp. Pulled the heads, sent them to Robert Yates with (his) $2,000 check - got them back a month later, dynoed again with no other changes, made 430 hp. The Yates guys are wizards, and obviously don't just "go at it with a grinder". Building 750 hp 358 Windsors has obviously taught them a few things about ports and breathing.

------------------
JohnZ
'69 Z28 Fathom Green

[Murfys_Law]

Excuse my ignorance, but who is robert yates?

[pete b]

John Z you're right- Robert and Doug Yates are freakin geniuses. I worked for Wilson Manifolds for 6 years and they sent us some really wild heads to build manifolds for. Their restrictor plate stuff is unreal- amazing flow from tiny ports. And all within 2% from port to port.
BTW it is really cool to read your posts about the assembly of our beloved Camaros. Thank you for sharing the stories. Pete