Head flow for autocross torque

[SRD Art]

My current SBC 406 has roughly 325 ponies and about 400 ft lbs. It runs a stock bottom end with 8.5:1 dished pistons, an XE268 cam, and self ported factory 400 "low performance" smogger heads. This motor is approaching the 100,000 mile mark and although it runs great I am after more power and a beefier bottom end.

I already have a fresh flat top pistoned short block 406 rebuilt and ready to go and was curious how my research compares to you all. The goal is approach 1 hp per cube and 500+ ft lbs to move my 3600 lb wagon around the autocross at a decent speed. I have been looking at bigger cams and budget head options and and like the Vortecs but rebuilding a junkyard set wouldn't save that much over a set of say Summit heads which are $700/pr.

The summit heads are 165 cc intake ports. Obviously small ports create velocity and = torque but 165 sounds really small. Most of my experience in the past has been drag racing and starting with a 200cc runner and porting from there. Those motors also launched at 4500, had a torque peak at 5000 and I shifted at 6500. I want to keep the power band low, like 2500-5500 and hopefully torque will be flat and peak around 3500-4000.

In your experience can a small 165cc port head handle the power goals I'm after?

Anybody know who mfgs Summit's heads? Thanks for insight!

[DB Z28]

Your 165 cc heads will run out of wind about 4800 rpms you need about 210cc intake runners ,AFR heads or Brodix are good.

[SRD Art]

My current motor runs out of breath at 4500. I want to run a bit steeper gear so I'm thinking around 5000-5500 peak would be good. 210 cc sounds big for a lower rpm range torque motor, no? My drag motor seems to pull past 7000 with world products 200 cc heads but with it's cam and all makes little power below 3500.

I like the 195 AFRs but them and and Brodix are not in the budget I want to spend- those ATS spindles sound awfully good when compared to spending more $ for a few extra hp.

[Che70velle]

Heres a NOS set of Vortec heads, 906 stamping. They have never been on an engine. Excellent price, but shipping might be steep. Don't let the Ebay listing scare you. These guys are in North Carolina, and sell mostly new and used race stuff. I've bought from them, and they are legit.
Compression is an easy way to gain power...torque AND horsepower. Your currently pretty mild on the compression side. Raise it up to say 10 or 10.5:1. Keep the intake runners on the smallish side, talk to an engine builder that understands camshaft theory, not just sells camshafts. Talk is usually free, so no harm there. Tune your stuff. I can't say enough about tuning your carb and ignition to work in harmony with each other.

http://www.ebay.com/itm/NEW-Pair-SB-...item5422f49c74

[Sieg]

Originally Posted by Che70velle

Tune your stuff. I can't say enough about tuning your carb and ignition to work in harmony with each other.

Amen! I really wonder how much performance has been left on the table or perfectly good parts tossed and replaced because of improper/unknowledgable tunes.

That said obtaining the flow/power numbers are pretty much a mathematical exercise.

[SSLance]

The Vortec heads flow great but machine work needs to be done to them to accept a larger lift cam...and you can be limited on your intake manifold choices when using them as well.

My engine is in the same boat as Bens, 400 ft lbs to the rear tires at 2000 RPM and 300 hp at about 4000 RPM. It starts running out of breath after 5,000 RPM. It's fantastic for smaller tighter autocross courses but is limited on the larger national style courses by the lack of high RPM hp.

[Ben@SpeedTech]

This is really intriguing as I'm digging more. DB Z28 really got me wondering about larger port and whether a larger motor, 383+ can perhaps still handle the larger ports and make good low end torque.

I found this article searching around. Speaking about matching larger ports to larger engines...

"While greater port size does not guarantee increased airflow, properly matched port size and design promotes increased airflow without sacrificing velocity. This equals improved torque."

The cam they're using is only one step bigger than mine, however I'm currently running 1.6 rockers so my lift is roughly around .500+

http://www.superchevy.com/how-to/eng...formance-test/

I also noticed in the GM performance parts catalog the ZZ383 engine with a relatively low duration cam and their 210cc intake port heads makes 400 ftlbs at 2000 and up to 450 at 4500. Talk about a flat torque curve, even with "big port" heads. It peaks at 435 hp at 5500. Man that sounds like a great reliable autoX/ street driver engine...

http://www.chevrolet.com/performance...ck-zz-383.html

Lance- If you're talking RWHP then you're roughly about 480 ft lbs and near 400 hp at the crank, that's pretty impressive and sound like you're almost there. All the numbers I'm stating so far including my current engine are crank numbers. Your heads are Vortecs? Maybe they're choking you out as the rpms rise?

Thanks for the ebay link Che70. Looks like those heads have everything I'd need done to them except drilling the steam holes for the 400. The question remains then will the Vortec heads have too small ports for the bigger motor?

[WSSix]

someone forgot which screen name they were logged in on, lol. threw me for a loop at first, Ben.

You don't have to worry too much about going too large because you're honestly not going that big in the grand scheme of things. The engine works as a system and needs to be designed as such. Focusing too much on one aspect will cloud the overall picture. Too much cam with too small of a head will kill low end just as well as too big of a head for the cubic inches will. In your case, I would go with a 200 or 210cc runner head and match the cam accordingly. You'll also have to match the intake, carb, and exhaust.

For reference, look at what LS engines have for runner volume and where they make power.

Also, I'll be going with 200cc heads on my 383 LT1. I'd go bigger but I'm limited due to being an LT1. The motor should make about the same power you're looking for as well. My intake and cam are coming from the same company that does the heads. They've proven their setup works. My headers and exhaust are sized for street power which is why I only have 1-3/4 primaries and 2.5 inch exhaust.

Good luck, Ben. Remember, average power under the curve is everything.

[SSLance]

Originally Posted by Ben@SpeedTech

Lance- If you're talking RWHP then you're roughly about 480 ft lbs and near 400 hp at the crank, that's pretty impressive and sound like you're almost there. All the numbers I'm stating so far including my current engine are crank numbers. Your heads are Vortecs? Maybe they're choking you out as the rpms rise?

My crate engine is rated at 430 ft lb, 340 hp. It has a really small cam, pulls 21 inches of vacuum at idle.

Camshaft Lift (in.): .431 intake / .451 exhaust
Camshaft Duration (@.050 in.): 196° intake / 206° exhaust
Cylinder Heads (P/N 12558060): Vortec iron; 64-cc chambers
Valve Size (in.):1.940 intake / 1.500 exhaust
Compression Ratio: 9.1:1
Rocker Arms (P/N 10089648): Stamped steel Rocker Arm Ratio: 1.5:1

What I like about it on the autocross course is it pulls like a train out of corners and is very repeatable. I know what it is going to do every single time I stab the go pedal.

Ryan said after his second run last weekend "I think I hit the rev limiter on the straight after the slalom". I checked the gauge, 5690 RPM. "No, you didn't hit the 6,000 RPM rev limiter, it just ran out of breath..."

A bigger cam would really wake it up but I'd have to pull the heads and have them milled...and I'm just not ready to crack the seal on it yet. I would like to try the 1.6:1 rockers I have sitting on the shelf first just to see.