The Stovebolt Site Forums The Shop Area Engine Shop Picking out a cam for my 250

Hello all. I've been researching for a while on here and other sites, and I figure I'd finally tell about my build and get some advice from the pros.

Overview:
Vehicle - 1947 Chevy truck, M2 front suspension, dropped S-10 leafs w/8.5" 10 bolt w/ 3.42 posi in rear. Going to use an S-10 T5 (haven't gotten yet) and around a 28" tall tire (225/75 or 235/70 on 15x7)

Engine:
1972 250 inline 6, which is at the machine shop getting freshened up:
- .040 over flat tops (~9.3-9.5:1)
- new bearings/rings
- lump ports installed in the head with 1.94 valves
- new oil pump, timing gear...
-Offy 4bbl intake, Holley 600 carb, Flow Tech headers (1.5" primary with 2.5" collectors, dual exhaust

But I'm torn between two different cams for this combo...
-Crower 03242 Level 3
lift intake - 0.478
lift exhaust - 0.490
adv dur int - 262
adv dur exh - 272
dur @ .050 - 204
dur @ .050 - 212
lobe center - 112
rpm range - 1800-4500, peak of 5000

AND

Comp 268H -
lift intake - 0.499
lift exhaust - 0.499
adv dur int - 268
adv dur exh - 268
dur @ .050 - 218
dur @ .050 - 218
lobe center - 110
rpm range - 1200-5200, peak of about 5500

I'm leaning towards the Comp cam, especially since it comes in a kit with everything that's needed, for good price too. I'm shooting for about 250hp and about 275ft lbs of torque. Will that get me there?
Thanks,
Wayne

ps, here's a link to some pics from last fall when I was test fitting some body parts after the suspension was finished and engine was in it.

My '47 Chevy truck

I haven't used the Comp cam for the 250 6 cylinder, but their 268H V8 cam is one of my favorites. It's got plenty of power without being too lumpy to behave well in traffic, and it even works well with an automatic transmission if the converter stall speed is increased slightly. I've been building race engines with Comp Cams since the mid-1980's with very few problems.
Jerry

Thanks Jerry. That's good to know. I went ahead and got the Comp. That was one that I've been keeping an eye on anyway. Plus it coming in kit form saved me some money. I needed to get parts gathered up and get them up to the machine shop soon. He's got my block bored and is now waiting for parts. I just got a set of Sealed Power flat tops for a 307 from Rock Auto for $12.64 each. I almost got a set of 8 from Summit for $137 but figured I'd check all my sources...good thing I did. $88 shipped. Cam kit k61-244-4 ordered from Comp...everywhere else was backordered. Also got bearings, rings, gasket kit, and oil pump from Summit. Now I just need to find the right T-5, preferably with the 0.72 or 0.73 5th gear.

With a little bit of tinkering, you can install screw-in rocker arm studs and roller-tip 1.6:1 ratio small block V8 rocker arms on the 250 head. That will really wake up the engine.
Jerry

Do those pistons have the big chamfer at the top around the OD?

I don't think one could get to the horse per cube normally aspirated could you?? I've seen them do it with turbo or a supercharge.

John

Oh sure it can. With the lump ports and bigger valves, the head flows like a Vortec 096 head... ~250cfm intake.

Watch this...
Tom Lowe's 250 that makes over 285hp NA

It looks like they do in the picture, but they only have 4 valve reliefs in them. I looked at my pistons today and they also have the chamfer around the rim, but they have a huge dish in them too. These will have to have more compression, just by the difference in the dish vs. the reliefs.

Here's a link... Flat top 307/250 piston

How do they put in the lump ports?? Also do they put in dividers on the intake ports??

John

They cut the intake "bosses" out (where the head bolt goes through), smooth it out and insert the lump. It gets drilled, tapped, and bolted from the bottom, and short head bolts gets used, and then the top hole gets tapped and plugged.

I'll check with the machinist about the screw-in studs and see if he thinks I need them. He is an old Super Stock racer (he actually still races-1968 AMC AMX called "The Wild Child"). He and his brother (they're twins) are pretty slick engine builders, and Harry (the one doing my 250) is known for his cylinder head work.
I did buy new stock rocker arms when I was buying parts, but the 250 rocker arm has a ratio of 1.75. I'm not sure if they interchange with the sbc or bbc.

Those 1.6 rockers will definitely help. Even on the stock cam, you'll see improvements. What's the lsa of the cam?

The lsa is 110, and the int cl is 106.

Question...If the stock rocker ratio is 1.75, how would a 1.6 ratio rocker help? Wouldn't that make the cam smaller?

is your truck a 47 ?? looks like the cab has vent windows

Yeah, it started out as a '47, and my title says '47, but when I started to do work on it, the floors and lower doors were so rusted and actually gone, that I found a complete cab from a '51 2 ton truck that was in really good shape...for $300. It made life a lot easier. Besides, I wanted the wing glass doors anyway. The only thing missing is the driver side cowl vent. I could've taken it off of the '47 but didn't think it was worth the trouble.

Wow 1.75 ratio already? Maybe just find a roller set of the 1.75's. Just the temperature difference in a roller set up will help. There's lots of sets out there for under $200. But for the most part 1.6:1 ratio produces the most power. With the 1.75's you may experience some valve float at higher rpm. Just a thought.

Thanks for jogging my memory about the 1.75:1 rocker- - - - -it just might be that we used a big block roller tip rocker on those hotrod 250's. I remember thay we found a compatible roller unit to duplicate the stamped steel original equipment rocker. There might have been some pushrod guide plates involved, also. One of my students built a hotrod 250 in a Chevy Nova, and his buddy with a Firebird with a Pontiac 350 couldn't outrun him.
Jerry

I thought about the roller tip, but didn't think the cost offset the power gain. So I bought a set of the stock replacement 1.75 from Comp, along with the recommended pushrods.

Hollow65, the 1.75 is the stock, factory ratio for a 250, so it wouldn't be worth it to me to go "smaller" just for the roller function. The cam is rated 1200-5200 rpms, and I don't plan to take it past 5500, probably not past 5000 too much actually.

Just the temperature difference in a roller set up

Too small to measure.

Thanks panic.

I just got back from the machine shop to drop off all my parts and talk with the machinist. We decided on 1.84" intake valves and 1.50" exh valves. He's going to unshroud around the sides of the valves a bit for better flow. We put both a 1.94 valve in and a 1.60 valve in to see how much unshrouding would be needed, and he wouldn't be able to do much without getting into the head gasket area. Since he's been doing heads for like 40 years, we both thought that the slight increase in valve size and limited amount of unshrouding wouldn't be anymore beneficial than just doing unshrouding and using the slightly smaller valves. There would be more area around the valves for flow with the 1.84/1.50 combo. He can do some bowl blending and smooth out the radiuses and get a great flow in conjunction with the lump ports.

This combo will probably work better with the Offy intake too. Might help with some midrange power.

SBC rockers will not work correctly on the Chevy inline 6 cylinder head. The geometry is completely wrong for this head. The BBC rocker is the only correct substitution for it(Chevy-wise) but it has less ratio at 1.7 than the 6 cylinder has with a 1.75, but has the correct geometry. Roller rockers for the BBC are cheaper than the 6 cylinder also if your price shopping.

I have a buddy's head in hand and a bag of BBC? studs from Tom Lowe and am unsure how much to shorten the rocker studs. Leo's book says between .1" and. 4" depending on pushrods used. I'm assuming he wants rollers??? Any tried and true combinations here?

Curt, do you mean stud boss? It's usually the thickness of the hex on the stud plus the thickness of the guide plates if your using them, so a total of about .400" is common when using both screw-in studs and guide plates together.

I took some lump port info and both of my Leo Santucci Books Vol. 1 & 2 up to my machinist so he can look over all of the inline info. He agreed that the 1.60 exh valve was the way to go. So it's gonna have 1.84-1.60 valves with porting and unshrouding.
He's got my short block about finished, then he's going to work on the head. I guess I should order my Offy intake and headers soon.

Sorry for the late update. I finally got my engine back home. I had my main machine shop do the long block and porting and valve work on the head, and I had another shop, friend of my dad, do the lump ports. He had a mill where he could do a better job of the cutting of the bosses and the spot facing. Turned out great. I also painted it the early Chevy truck medium gray. Looks good. My goal is to have it dressed up like an early 216/235, with stickers and air cleaner, but be a hopped up 255 underneath.

I'm going to break the cam in with the original intake/exhaust manifolds and 1 bbl, then swap the Offy intake and Holley 600 and the headers on. That way I know it should run ok for the break-in. Pics to come.

Got the engine running. Here's a link to the videos and a few pics. Breaking it in with the Offy intake and new Holley 600, but stock exhaust manifold with single pipe and muffler. Headers wouldn't fit on the stand. It's snappy, and it should sound even better with the headers and dual exhaust.

Wayne's 250 Pic and video thread

You won't get far enough to see geometry with SBC rockers - the tip won't reach the valve, it's too short.
On paper, the BBC isn't that close a match either (but it is longer), the valve stem angles are very different, but apparently it's not significant.