Porting heads

[cjjtulsa]

People always over cam and over carb; too much google university. Current one I'm doing has a 110 LSA; was going to go 108, but figured I'd stick with 110. Mild cam, but went probably a little big on carb (650) for what I'm doing - Ford 331. Light car, though, so it should do OK. Just hoping that carb doesn't drag it down. Probably should have even went as low as a 500cfm, but we'll see how the AVS 650 works out. Smaller primaries (almost a spread bore), and supposed to be very tunable in the secondaries. We'll see.

 

[kwaynem]

I have a 010 350 block stuck back for a build I plan doing this summer haven’t decided on heads yet I would like to find a good set of old camel hump heads I have a holly 750 that I would like to use but will decide when I figure out what pistons and cam I’m going to use

 

[O4L]

Not to derail this thread but I have a question you guys may help me answer.

Many years ago I raced roundy round on dirt tracks in the Street Stock division.

I put together a small block Chevy 400 that was bored .030 over with flat top 10:1 pistons and a high lift Lunati hydraulic cam. (I don't remember the specs).

Per the rules we couldn't use any after market carbs, intake, exhaust or heads.

With .194 heads I was getting a little over 7000 rpm from this combination.

A guy I knew loaned me a set of .202 heads that had been polished to try out and without any other changes, I lost nearly 1000 rpm from the new combo.

After one night of racing I changed back to the .194 heads and never looked back.

So why the big drop in rpm?

It's probably a simple answer but one that I never really looked into to figure out what caused it.

 

[doctorjj]

Not to derail this thread but I have a question you guys may help me answer.

Many years ago I raced roundy round on dirt tracks in the Street Stock division.

I put together a small block Chevy 400 that was bored .030 over with flat top 10:1 pistons and a high lift Lunati hydraulic cam. (I don't remember the specs).

Per the rules we couldn't use any after market carbs, intake, exhaust or heads.

With .194 heads I was getting a little over 7000 rpm from this combination.

A guy I knew loaned me a set of .202 heads that had been polished to try out and without any other changes, I lost nearly 1000 rpm from the new combo.

After one night of racing I changed back to the .194 heads and never looked back.

So why the big drop in rpm?

It's probably a simple answer but one that I never really looked into to figure out what caused it.

First off, a bad porting and polishing job can be worse than stock. It seems as though those heads he loaned you didn’t flow as well as your own heads. Bigger isn’t always better as well.

 

[O4L]

First off, a bad porting and polishing job can be worse than stock. It seems as though those heads he loaned you didn’t flow as well as your own heads. Bigger isn’t always better as well.

Yeah, I've seen some messed up heads but he had used these heads on his car and they seemed to do very well.

He might have done a bad job on them though because I never got into modifying heads and couldn't tell if what was done had been an improvement or not just by looking at them.

 

[Mad Professor]

Not to derail this thread but I have a question you guys may help me answer.

Many years ago I raced roundy round on dirt tracks in the Street Stock division.

I put together a small block Chevy 400 that was bored .030 over with flat top 10:1 pistons and a high lift Lunati hydraulic cam. (I don't remember the specs).

Per the rules we couldn't use any after market carbs, intake, exhaust or heads.

With .194 heads I was getting a little over 7000 rpm from this combination.

A guy I knew loaned me a set of .202 heads that had been polished to try out and without any other changes, I lost nearly 1000 rpm from the new combo.

After one night of racing I changed back to the .194 heads and never looked back.

So why the big drop in rpm?

It's probably a simple answer but one that I never really looked into to figure out what caused it.

Ohhh. You are one of those cheaters!

The increase in valve size itself should not cause a dramatic decrease in flow velocity anywhere on a 406” Chevy.
I suspect the head was dramatically different in port volume or more likely combustion chamber size. The change in compression ratio can be extreme in a flattop 406. 2 full points between a 58cc and 70cc head. God forbid it was a 76cc or larger head.
The 4.155 bore of a 406 also allows a little unshrouding of the intake, more so than a 4” 350. Same with a 358” 4.155 bore engine.

I had a really nice set of 292 Angle plug turbo castings with 2.05” intakes on my 400. The chambers measured 59.5 after unshrouding and milling. They were Crane Fireballs” built from GM castings and ported. I want to say they were made in the late 70s maybe early 80s before the Bowties were released. A much better head than the 461s we used to search for in the stock head classes. I didn’t have to worry about rules on the street!! LoL

 

[O4L]

Ohhh. You are one of those cheaters!

The increase in valve size itself should not cause a dramatic decrease in flow velocity anywhere on a 406” Chevy.
I suspect the head was dramatically different in port volume or more likely combustion chamber size. The change in compression ratio can be extreme in a flattop 406. 2 full points between a 58cc and 70cc head. God forbid it was a 76cc or larger head.
The 4.155 bore of a 406 also allows a little unshrouding of the intake, more so than a 4” 350. Same with a 358” 4.155 bore engine.

I had a really nice set of 292 Angle plug turbo castings with 2.05” intakes on my 400. The chambers measured 59.5 after unshrouding and milling. They were Crane Fireballs” built from GM castings and ported. I want to say they were made in the late 70s maybe early 80s before the Bowties were released. A much better head than the 461s we used to search for in the stock head classes. I didn’t have to worry about rules on the street!! LoL

That makes sense. I never considered the change in the compression ratio.

Are you saying the heads with a little spit shine put on them was cheating...

I don't remember the rules well enough to say one way or the other but my cam might have been a little hot. IIRC it was the highest lift hydraulic lifter cam I could find at the time.

In my defense, I was driving a heavy ass tank because most of the guys thought we were having a demolition derby instead of racing.

 

[Mad Professor]

That makes sense. I never considered the change in the compression ratio.

Are you saying the heads with a little spit shine put on them was cheating...

I don't remember the rules well enough to say one way or the other but my cam might have been a little hot. IIRC it was the highest lift hydraulic lifter cam I could find at the time.

In my defense, I was driving a heavy ass tank because most of the guys thought we were having a demolition derby instead of racing.

Not sure what rules you were under. But most rules would not allow any polishing as it was the easiest way to cover up port work. Some would allow you to go so far above the valve seat to allow blending.

The heads I had were off a Champ Sprint. We ran them a couple of years before we were allowed to move to Aluminum heads. Then back to Steel, then Aluminum again (but a spec head)

 

[kwaynem]

How often did they pull a head at the track?

 

[Mad Professor]

How often did they pull a head at the track?

Not that often, but 20 years ago or so bore scopes started showing up at the track and being used for inspections. I think some may have concurrently served day duty at local hospitals. Way more crude than what we have available now. It was pretty much limited to bore scope, puffing a cylinder for displacement, and fuel checks. That is unless a protest was filed.