Change of plans... Does it call for a change of heads?

[ChevelleNut]

Alright, I've had a change of plans. I have all of the stuff to build a 12.5:1 489 BBC for my bracket car, but I've since decided that I'm going to install my 396 in the car and sell it. I'm still going to build the 489, but drop the compression to 10:5.1 and put it where the 427 sits in my street car.

Here's the question....I have some Brodix RR BB2 plus 312 rectangle port heads for it. The engine will now be mostly a street engine with a little track time. Should I see if I could swap these 312 rectangles for some 270 oval Race Rites? I have a solid flat tappet cam that I'm not going to run now. I'm going to go with a smaller hyd roller and won't be turning it past 6,500. Engine will be in an all steel, full interior 68 Chevelle. Th350 w/ Gear Vendors OD and a 3.70 rear gear. 275 rear tire.

*Edit - I also have intake manifold options for either set of heads. A Team G single plane and a Team G dual plane for the rectangle port heads, A Team G single plane and Performer RPM for the oval port heads.

[lefty o]

489" motor , dont choke it down with smaller heads.

[mag2555]

As you probably know hooking up that cid motor on the street even with those size rear tires will not really take place when using either set of the heads your considering.

Also seeing as your going thru the trouble of running a over drive I would try to make the 270 cc heads the ones you get on that 489 motor ( there only 15% smaller then the 312 heads ) which will keep the average torque band wider down a big lower, and I can’t see the 270 heads choking off more then 10 hp way up top with the Cam I bet you will be running, as the intake manifold you will run will have a bigger impact on the top end hp numbers,

I also hope that TB 350 is built to take the torque of that 489 motor, I know the Hone over drive cam as a buddy of mine has one behind the 4 speed in his 508 motor making 725 hp.

[rustbucket79]

The way I see it you’re trading in the neighborhood of 40/50 lb ft for the same amount of horsepower, take your pick.

[ChevelleNut]

The way I see it you’re trading in the neighborhood of 40/50 lb ft for the same amount of horsepower, take your pick.

That's kind of what I'm thinking. The flow numbers are the same on both sets of heads. I was thinking that with the 270's the port velocity would be better and not as lazy.

[ChevelleNut]

As you probably know hooking up that cid motor on the street even with those size rear tires will not really take place when using either set of the heads your considering.

Also seeing as your going thru the trouble of running a over drive I would try to make the 270 cc heads the ones you get on that 489 motor ( there only 15% smaller then the 312 heads ) which will keep the average torque band wider down a big lower, and I can’t see the 270 heads choking off more then 10 hp way up top with the Cam I bet you will be running, as the intake manifold you will run will have a bigger impact on the top end hp numbers,

I also hope that TB 350 is built to take the torque of that 489 motor, I know the Hone over drive cam as a buddy of mine has one behind the 4 speed in his 508 motor making 725 hp.

Yeah, the 427 that's in it already makes short order of the tires now. Lol.

I don't see it choking any horsepower off really. The flow numbers are identical between both the 270's and the 312's. My thinking was that the port wouldn't be as lazy on the 270's making for a happier engine. I could be overthinking it though.

By the time the 489 is ready to go in, I'll have a Th350 that will hang with it. I would TH400 it, but I have a new in the box Gear vendors for a TH350.

[mag2555]

Your thinking is spot on!