Head flow and header choice!

[73calroadrunner]

Alright, I just got my heads back from the machine shop. They are '72 cast iron J heads fully ported with a five angle on them. My question is what should be my header choice. The ports are nearly twice as big as stock (yes I know port size has nothing to do with header choice) and the flow numbers are as follows: 270 on the intake side and 175 on the exhuast runners. The cam I am using is an isky hydrualic with 485 lift across with 240* duration @ .05 . The valves are the stock 1.88 intake 1.60 exhaust. Compression is 10.2 to 1. So my question is should I go with the hooker comp or super comp headers? Even though I now have more head flow Will the super comps with there 1 3/4" primaries be too much for this setup?

[dwc43]

If they are fully ported and as big as you say they are you need a 2.02 valve or it wont do you much good at all. The 1 3/4 will be fine for an all out race car. You did not mention if it was street or race only. If street use a a stepped 1 5/8 to a 1 3/4 header for better low and mid range. Also make sure you use a merge X pipe from www.drgas.com for added tq and hp gains. You really need to get that 2.02 valve installed though.

[73calroadrunner]

That was the numbers I was given by the shop who did them. On there flow bench (L&S automotive). And yes this is a street weekend racer setup. I have yet to install the heads so I may take your advice and have the valves upsized. This can be done right?(this is my first motor build and I want to start with a bang) And I hope that would be correct because I laid down quite abit to have it done, in retro spect I should have just bought eddy heads off the bat.

[dwc43]

Yes, the 2.02 valve can be installed in that head by your shop. Quite easy to do too. If you had went with the edels you would have had to up your compression another full point just to get the same performance you will get with these heads at the cr you have now.

[sanborn]

dwc43 is about right on header tube size. He has much more experience with those heads than I.

But, head flow has everything to do with header
tube size. The tube is sized according to intake/exhaust port flow relationship and based on actual flow of the exhaust under W.O.T. conditions. And also, you work with head flow at actual cam lift at the valve when determining this.

[sanborn]

dwc43 is about right on header tube size. He has much more experience with those heads than I.

But, head flow has everything to do with header
tube size. The tube is sized according to intake/exhaust port flow relationship and based on actual flow of the exhaust under W.O.T. conditions. And also, you work with head flow at actual cam lift at the valve when determining this.

[fastmopars .inc]

but the edelbrocks would have outflowed that measly 175 cfm on the exhaust side at about .400 lift, and that's with a stock casting.
for an extra 300 dollars from hughes engines, you could get another 35 cfm out of those edelbrock heads.
that 270 cfm on the intake doesn't mean anything unless you've got about 200 on the exhaust.
i suggest you look into a custom cam with lots of exhaust duration, maybe something like a 230/245 or something along those lines. depends on what you want to do with the car.

[73calroadrunner]

Well I figured I was going to use a split cam later have it custom ground and change the rocker configuration with higher ratio on the exhaust side. Thank god my dads best friends have connection at Isky and can get me a discount on a custom cam.

[dwc43]

The dual pattern cam is a good idea. I have also used different ratios on exhaust and intake rockers before too. I have used a dual pattern cam from crane that we designed on a computer. We used them since they had a cam simular to ours and used there blank to grind it from. IT has a real flat torque curve and works well with modded X heads,

[sanborn]

I've always found the ideal intake/exhaust flow relationship to be about 70-72%. Yours is 175/270=65%. That means your exhaust needs a little help- or said another way, your exhaust can't get everything out in order for the intake to do everything it can.

Before I go any further, all calculations should be at NET valve lift- because at your cam lift the intake/exhaust relationship may be different.

The easiest way to help the exhaust is with duration. And at a 65% relationship, an extra 10 degrees on the exhaust would not be too much. Rocker ratios can help but you have a lot to make up and I don't think rockers alone can get enough.

And, on a street/weekend engine combination the intake/exhaust relationship is even more important especially at part throttle.