Inliners International Forums General Discussion Engines Dual Pattern Cam Grind for 1962 Chevy 261 I6

I’m upgrading to a dual pattern grind for my 1962 Chevy 261 rebuild. For background, this engine has a 848 head, Buick 455 springs, Bel Air Powerglide inlet valves, TRW 040-over forged pistons, an Offenhauser dual intake manifold and a C1 Corvette dual exhaust manifold. The backend has a 3.55:1 ring & pinion from Patrick’s. There’s also supposed to be a torque-tube converter for a Saginaw 4-speed from Patrick’s (paid over a year ago but still don’t have the product).

Goes into a 1950 ½ ton 3100 PU. The truck is a daily driver, no drag or track. About 80/20 highway/city, around 65 mph on the road. There’s some towing, but not heavy or long-haul.

I’ve drilled deeply into Inliners and other forums looking for cam recommendations, without much joy. Most posts are about single pattern grinds, with precious little about dual patterns. What there is (for Langdon’s proprietary “Bulldog”) doesn’t give enough grind specs, and understandably neither Langdon or Delta Cams will discuss.

My engine builder has recommended two grinds. The first has 244-3 lobe advertised duration (200@0.050) intake. The second has 252-7 lobe advertised duration (210@0.050) intake. Both I believe have 264 advertised durations (220@.050) exhaust, 0.435 intake and 0.440 exhaust lifts and 110 lobe separation. The second grind I think is very close to the Bulldog which has 254/264 advertised durations.

Ideally I’d like a smoothish idle, not too rumpy, with a power band suited to my application. Anyone care to weigh in on which you think is the better cam?

Thanks,
Mark

Hi Mark . . .

I am a fan of dual-pattern grinds myself. I have fitted one to my hopped up 216 - specs can be found here:
Inliners Post circa Jan '15
I have not put the engine back between the frame rails yet. I am at least two months from the break in run.

The reasons for a DP grind have been discussed in a variety of posts like:
Inliners Post circa June '07

In a nutshell - going for longer duration on the exhaust lobe gives the engine better volumetric efficiency through exhaust scavenging (because the EV remains open longer reducing the likelihood of orphaned exhaust gases remaining in the cylinder at the Intake Valve open event.) Because of the asymmetrical grind the amount of overlap is not significantly increased by the longer duration on the exhaust lobe - for a given LSA. And the overlap will mostly be at low Intake lifts - thereby limiting low RPM reversion.

For a street driven engine I would stick with stock LSA - not to mention it is easier for the re-grinder to work the existing lobe centers on the core.

regards,
stock49

Well I don't know the Stovebolt as well as I know the Nova Six, but here is what the DeuceCoupe Gonkulator says about your build, I am assuming stock head and 6.6cr but here is the cam effect. In each case I "installed" the cam 4 advanced ie 106 intake and 114 exhaust centers:

210-220-110
Torq 215 at 2000



Finally, I tried smaller still - the Gonkulator really does NOT like a split pattern, though this assumes open exhaust:

210-220-110 base case
Torq 215 at 2000
Torq 247 at 3000
Powr 172 at 4500

200-220-110
Torq 222 at 2000 +7
Torq 249 at 2900 +2
Powr 167 at 4400 -5

200-200-110
Torq 228 at 2999 +13
Torq 254 at 2800 +7
Powr 173 at 4300 +1

The split duration thing is kinda tricky - in general, single pattern is often best UNLESS - the exhaust ports are crummy and the pipes/mufflers are too small. But both of those are common conditions so sometimes it works.

If it was me, for your use, I'd go for the 200-200-110 if you can get that ground that way. Caveat, the Gonkulator is not as well wrung out on this engine as on the Nova Six so Just One More Opinion, hope it helps.

The 3rd generation 6 has a very different head design when compared to the stovebolt. In the 3rd gen the valves sit side by side in the roof of the combustion chamber. Whereas in the stovebolt only the exhaust valve is located in the combustion chamber - a chamber which cannot be fully swept by the piston at TDC. This is why the stock camshaft is a dual pattern grind - with a longer duration on the exhaust lobe. Advertized duration on the post war stock 216 cam is 220/231. The intake lobe duration is purposely shortened to eliminate overlap entirely - which is what would have happened had GM delivered a 231/231 single pattern grind on the same lobe centers.

If I remember correcty from the McGurk article, all his cams were dual pattern.

http://www.selectric.org/55chevy/soup.html

The number 5 cam seemed to be the winner in my book for a street engine. It had 245 degrees and .421 lift on the intake, and 284 and .410 on the exhaust.

The 235/261 heads have very poor flow. Not to mention the valve train geometry is terrible. You can't put alot of lift on the intake, because it's basically on the bottom of the head. The exhaust has tons of room for lift, but because it's canted, the area around it shrouds it and actually reduces flow. Unless you grind some material away from the exhaust valve pocket, adding lift is useless, adding some duration will always help though.

It's always worth a call to your cam grinder for recommendations too. I know Comp Cams still can do a 235/261 cam. You may have to give them a call to see what lobes are availible.

Other cam grinders, I'm not familiar with.

One thing you could also do is look into Manley's Street Flo valves for your exhaust valve. They claim a 14% flow increase at .400 lift and 93% flow increase at .150 lift. Seems like the perfect addition for a shrouded low lift exhaust. They're economically priced, and since the stovebolt exhaust valve is the same as a small block, you won't have to have them custom made.


http://www.manleyperformance.com/dl/2014/valves.pdf


http://www.jegs.com/i/Manley/660/10721-8/10002/-1

Good points - I just realized how much I am GUESSING at Stovebolt head flows - I tend to guess the exhaust flows about 2/3 of the intake, but maybe its more like 1/2 the intake?

Anybody have some solid head flow data on Stovebolt heads?

I am guessing from this thread that CNC-Dude may have done some:


http://www.teambuick.com/forums/showthread.php?15714-Buick-263-Head-Porting/page6

That would make a difference in cam choice of course

Here are the specs for the Bulldog cam ground by Delta Cams ...

Bulldog 254/264 dual pattern cam specs


Works well in a dual intake/exhaust bored 261 with milled 848 head, no overdrive trans and 3.55 rear, giving max torque around 3500 rpm.
Gets 20+ mpg and all the power and torque I need for a daily driver.