Ziggy

I'm researching alloy heads at the moment and had a question for you relly keen cluey fellas. Minimum cross section spec, is it better for this number to be higher or lower and what will it affect.

if you could explain this in a simple form it would be great. thanks

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Authuorised GM Destroyer

xdclevo

The smallest Cross section with high airflow is considered better.
Put you lips together and make a small hole and blow hard. This will make for high volocity or speed of the air, but only flow small amounts of air over a certain amount of time. Open you mouth wide as you can and blow. There will be a lot of airflow, but low in speed or volocity.
The object is to move a lot of air very fast, with a small port volume.
This is all assuming you'r rev range isn't from 5000rpm to 8500 rpm.

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10.70 @ 125.00 on the black fun top
408 Cube Cleveland.

brenx

Small cross sectional area is supposed to be better as xdclevo's said but there are limitations. It'll give better power/torque down low basically upto 5500rpm or so. Beyond that I found a larger cross sectional area will make more power and better torque up higher. For example I'll use CHI's hp flow figures. The 185cc head flows 590hp (1.9) and the 218cc head flows 680hp (2.1).

The basic jist of it is 351's supposedly like about 1.9 and strokers (378 and up) like 2.1.

I've got a set of 218cc 2.1 cross sectioned CHI 3V's on a 351 too My signature tells the story. I'll update that sig with some faster times. Yes, I am that confident I am one who tends to run against the grain and get results instead of following others and running all the same parts as other people. Being a sheep ain't my thing.

My personal opinion is 1.9 cross section is great provided you don't want to rev past 6500rpm max. Most decent cams like to go 7000+ with a good flowing head. Thats where the bigger cross sectional area comes in. More rpm and more flow = more power/torque.

Brenden

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XB Fairmont (street car) 11.07@123.02mph.

Ziggy

This is also wat i was thinking, but my prob is it's a v8, at the moment stock bottom end eventually 393 stroker and lets face it, proably wont spin any more than 6000rpm the chi heads r actually 2.1 for the 185's and 2.4 for the 215's on the other hand afd's are 1.9 and down on air flow so i'm a little bit more toward the 185chi but im scared of a torque drop. the afrs give the torque but its a lot of money if they don't flow enough air????

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Authuorised GM Destroyer

brenx

I stand corrected By the way the big CHI's are 218cc not 215cc. Either way on the 351 the 2.4 cross sectional area is prooving not to be any disadvantage. I gained 49rwkw just by bolting the 218cc CHI's on. I hope to do a 300rwkw+ dyno run in Jan without a massively loud exhaust etc. I've only got to gain 11rwkw, so not far to go. As I can't be bothered with a twin 3" system. My twin 2 1/2" with triple flow mufflers works well.

If your doing a 393 you'd be wasting $ on a set of 185's. I am looking starting a 408ci Clevo next year to go under the 218cc CHI's. You could still bolt the CHI's on and you'd get a power increase also. Just not as much if everything else was done too. The 218cc heads will work fine with a 393 as all the 218cc's need is cubes even if your not reving past 6000rpm.

I know a person who's building a mild 408 with 218cc CHI's. How's 590hp @ 5900-6000rpm sound? The 408 I build will be a tad more savage than that as I have beyond 600hp aspirations.

Either way the CHI heads have prooved a worthy investment world wide and @ the Engine Masters comp.

Brenden

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XB Fairmont (street car) 11.07@123.02mph.

XD408

Just a question regarding rwhp converting to flywheel HP, everyone says that on average u loose a 1/4 of your power thru an auto and stall set up and about 1/5 with a manual. When on a chasis dyno some dyno dynamics dynos give an estimation of flywheel HP, mines pumped out 459rwhp on U98 but it reckoned it was only 553fwhp??? wouldnt it be closer to 600+ being 3/4 being = to 459 rwhp and the extra 1/4 being a 1/3 of that figure ie.. 153 equaling 607fwhp?? which is correct

brenx

On a dyno dynamics they say 33% with autos roughly and 25% roughly with a manual.

Brenden

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XB Fairmont (street car) 11.07@123.02mph.

STROKEXD

33% seems like a lot of wastage in HP.. Seems too high

Because XD408 that means you have 3 x 459/2 = 688 FWHP which seems a bit much too me ie as a check 688HP - (.33 x 688) = 459 HP.

On the Moroso calculator I have 402 RWHP to do a 106 mph pass in a 3600lB car. This works out as 603 FWHP, sure I don't have that much - more like 530-550 FWHP.

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xdclevo

No dude, it is 685hp. (685 fwhp less 33% = 459 rwhp)
You are finding 33% of the smaller fugure od 459 which is way low.

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10.70 @ 125.00 on the black fun top
408 Cube Cleveland.

STROKEXD

Actually I work out the driveline losses for my car as 24% - C4 auto.

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