Don Terrill’s Speed-Talk

Rectangle port Intake on Oval port Heads, I have heard this combonatation tends to work or vice versa... is there any reason that it does work or solid proof that it does? Like an A vs. B tests at the track, same everything except manifold switch... now my thoughts are it could be that the runners have or could have been filled to keep velocity up, and perhaps prevent reversion if that is an issue with bbc. I am very new to bbc and racing engines in general so if there is anyone who can enlighten me on this phenomenon or is it just one guy tried it and it worked for him? I always thought that any disruption in flow causes turbulence or unwanted /change in airflow at certain places in the port is detrimental to power output.... maybe the people it worked for made a round port exit out of the rect. port manifold.??

Thoughts, experience is what I am looking for.

thanks

while either way it will physically run, neither is the best route to big power. oval intake on rect heads is much better than a rect intake on oval heads though.

okay thanks, would you mind elaborating on that a bit for me and who ever else is interested?

I've seen very good power, oval port head, rectangle port dual plane, around 740 on a 467, and it was tearing the cam during the process, along with other problems.

again, I guess what you are saying that it does make power with consequences, but I don't understand tearing the cam or what that entails other than I assume cam bearings. Potentially moving, the cam flexing too much making contact turning too high an RPM. I guess I am just going to have to try to read between the lines a bit better sometimes it is hard to sift through the bs with a coarse strainer...

Generally it is a better choice to have the smaller manifold opening into a larger port than the opposite. The air fuel mixture flows better when it goes into an void instead of hitting a wall that it has to go around.

Fordracer347 wrote:Generally it is a better choice to have the smaller manifold opening into a larger port than the opposite. The air fuel mixture flows better when it goes into an void instead of hitting a wall that it has to go around.

Was at a engine performance class this past weekend and Harold Bettes mentioned that is would be better to have the manifold slightly larger than the head. Phil Martin on the air flow cd's also mentioned the same thing that they made more power with a slightly larger intake. Air hates sudden expansion vs contraction. I have not tested this myself but I think Harold would know.

I agree. We aren't talking about slightly though, we are talking about 4 fairly large protrusions in the corners of the port though. The air that hits the void will slow down substancially but I still think its a better choice than having large protrusions. I am talking about out of the box use, now if you are willing to port on either its an intirely different situation.

Ford347 say you are willing to go in there and start moving material around what are your suggestions then? pm if you feel you need to.

thanks

just clarify they are oval heads possibly a rectangular port... also I may of missed it but is there any firm logic that why this power level increases?

If you do a search you'll also find that this topic has been discussed at length, by some key players, in the past.

Manifolds are cheap. There's no reason to mis-match.

In the 60's there were few aftermarket bbc intakes available. It is the nature of the racer to re invent the wheel. Back in those times it was found to be advantageous to install the bbc rectangle port intake on the oval port head. As mentioned, this subject has been discussed in depth here. Use the search feature and have fun reading. That combo works well.

I have done this several times in the past. I dont understand why people would not purchase a new intake but I will let you know as I am preparing to put one on the dyno for a customer. He has oval port heads with a stock early model Chevrolet rec port intake. He says he does this quite often. My feelings are thgat he is passing an engine off as a hi po engine. U can trace the intake or even see it but you cant the heads unless u remove the valve covers. He always get the HP numbers to show how good it runs. The last one I did for home was a stroked 454 to 496. We reworked the heads quite a bit and it got 627 ft lb torques at mid range power. Not as much HP but did make good Tq. Was a dead duck by 6500 (Memory thinks that was the rpm). dont know which quit first , int or heads. Had reworked Quick Fuel. Jetting was a little screwy..
reed

Way back in the day we used to run the big intake on the oval heads port matching the heads to the intake, working the bowls and going with bigger intake valves on the 427's. That combination made more power than the rectangle port stuff until you got over 6.000 rpm. On the street it was cheap speed, those heads were a dime a dozen. Sorry, just an old school flashback.....

One thing to consider, many aftermarket oval port BBC heads do not match with oval port intake
manifolds. The ports in these heads are actually rectangle port heads with a reduced corner radius.
That is the case with Profiler heads, the oval port intake does not match at all, but the
rectangle port intake is a close fit.

Edelbrock makes an intake with reduced corners;
Victor Jr. 454-R (with 3/4" radius filled-corner runners) #29020*