The Stovebolt.com Forums Home | FAQ | Gallery | Tech Tips | Events | Features | Search
Time to gathering together!
Fall Happenings
That time of year with some last chances to get together in bunches. Time to check out:
THE SIDE LOT
From Coast to Coast and some International gatherings listed, too.

Next up: the ODSS gathers this week (Sept 22-24) at a yearly ATHS show in Winchester VA.

Since we put up this promo last week, a bunch of new postings of Swap Meets, Cruisin's, and more
in the SIDE LOT.
See what's happening with a Stovebolt Collective near you
.
Searching the Site

Get info about how to search the entire Stovebolt site here. To do a search for just the forums, get those details in the IT Shortbus fourm.
Old Truck Calendars
Months of truck photos!
Nothing like an old truck calendar

Stovebolt Calendars

Check for details!


Who's Online Now
2 members (Unsupervised_aga, 1 invisible), 83 guests, and 3 robots.
Key: Admin, Global Mod, Mod
Forum Statistics
Forums60
Topics129,094
Posts1,046,667
Members46,191
Most Online1,229
Jan 21st, 2020
Step-by-step instructions for pictures in the forums
Previous Thread
Next Thread
Print Thread
Page 1 of 3 1 2 3
#1351620 Wed Mar 25 2020 06:53 PM
Joined: Aug 2019
Posts: 244
O
Ott3r Offline OP
'Bolter
On my 848 head the intake valves are nearly flush with the deck... which makes sense why the stock cam for 848 heads waits till 1° after top center before opening intake valve.

'59 valve timing [chevy.oldcarmanualproject.com]

However, the 261 cam is listed as opening the intake 11° before top center... so it sounds like a recipe for disaster when used with an 848 head.
I'll assume that the 261 head has a more recessed intake valve?


So now I'm confused:
There are several stories of performance builds using fabled 261 block, with 848 head, or the corvettes using the 261 cam in a 235.
Are these just myths, or is it common to cut intake valve reliefs in the pistons?

Attached Images
DSC_0002 (2).JPG (127.22 KB, 176 downloads)
valve timing.png (79.11 KB, 175 downloads)
Last edited by Ott3r; Wed Mar 25 2020 06:59 PM.

'59 Chevy Suburban, NAPCO
Ott3r #1351621 Wed Mar 25 2020 07:00 PM
Joined: Aug 2018
Posts: 696
2
'Bolter
i'm guessing here, but i could do the math if i had all the dimensions...
the valve starts to open early at 11 before TDC, but it's not like an on/off switch, likely the valve still has plenty of clearance by the time the piston passes TDC as the camshaft has only rotated 5.5 degrees into it's opening for the 11 degrees of travel the crank had to do.
-s

Ott3r #1351623 Wed Mar 25 2020 07:02 PM
Joined: Aug 2019
Posts: 244
O
Ott3r Offline OP
'Bolter
lemme go measure real quick...


'59 Chevy Suburban, NAPCO
Ott3r #1351624 Wed Mar 25 2020 07:09 PM
Joined: Aug 2019
Posts: 244
O
Ott3r Offline OP
'Bolter
With a straight edge across the head, I get some variation of valve clearance. Some have 0.01" clearance and a couple actually protrude by a few thou.

So the only saving grace is my 0.05" head gasket. It doesn't seem like that's worth 11 degrees of rotation?

This head has been re-surfaced, but I don't think it's wildly different than original.

Last edited by Ott3r; Wed Mar 25 2020 07:10 PM.

'59 Chevy Suburban, NAPCO
Ott3r #1351626 Wed Mar 25 2020 07:23 PM
Joined: Feb 2004
Posts: 24,968
H
Kettle Custodian (pot stirrer)
The 261 engine we assembled for John's big pickup had an 848 head on it. I degreed the cam (a brand new Elgin 261/Corvette) and had no piston/valve interference at all. If you're concerned about things, do a temporary assembly without a head gasket, and as the piston approaches the top of the block at the end of the exhaust stroke, start pushing the intake valve down against the top of the piston and measuring how far it has to travel before contacting the piston. If you can make a full cycle without getting any piston/valve contact, the thickness of the head gasket will be your safety margin. That's how we did it on small block race engines to see how deep we needed to cut the eyebrows in the pistons. On one memorable roller cam engine with 1.6:1 ratio rocker arms, the valve reliefs and what was left of the piston dome computed out to be the equivalent of a flat-top piston!

That engine IDLED at 2500 RPM- - - -slow it down to 2000 and it didn't lope or run rough, it just quit running. We couldn't make a dyno pull at any speed below 3,000, but from 3K to 7K RPM it pulled like a freight train!
Jerry


"Freedom's just another word for nothing left to lose"
Kris Kristofferson

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self.
Ernest Hemingway

WAG MORE- - - - - -BARK LESS!
Ott3r #1351627 Wed Mar 25 2020 07:24 PM
Joined: Dec 2008
Posts: 1,731
P
'Bolter
All of those cams are very mild with very slow motion in inches of lift per degree of rotation.
The 235 & 261 connecting rod to stroke rod ratio is fairly long at 1.73:1, which makes piston motion near TDC relatively slow (and faster near BDC) in inches of lift per degree of rotation.
At 11.5 degrees from TDC the piston has moved about .051".

For any the following changes reliefs in the dome may be needed:
1. milled block
2. milled head
3. larger valve OD
4. higher rocker arm ratio (than 1.477:1)
5. cam advanced
6. more intake duration
7. more lobe height

Ott3r #1351629 Wed Mar 25 2020 07:30 PM
Joined: Dec 2008
Posts: 1,731
P
'Bolter
IMHO one reason why the valve is so close (and its relief pocket in the head such low volume) is to increase local vacuum (directly adjacent to the valve curtain) as the piston passes TDC. With the low static CR these engines all have, and the valve farther away, that suction would be lower.

Ott3r #1351630 Wed Mar 25 2020 07:31 PM
Joined: Aug 2019
Posts: 244
O
Ott3r Offline OP
'Bolter
Whoa, the roller cam engine sounds agressive!

I wonder if Jerry used the same Elgin cam that I have (E-293-s)... I'm having one heck of a time figuring out what it's specs actually are.

It's supposed to be a 261 cam, yet the website says it opens intake at 12° after top center?

Attached Images
cam.png (71.3 KB, 141 downloads)
Last edited by Ott3r; Wed Mar 25 2020 07:34 PM.

'59 Chevy Suburban, NAPCO
panic #1351631 Wed Mar 25 2020 07:49 PM
Joined: Aug 2019
Posts: 244
O
Ott3r Offline OP
'Bolter
RE: panic

Sounds like the valve clearance can get pretty small in some cases... especially for hot-rods
I'd assume that OEM designs typically guarantee at least .05" wiggle room?

I have some custom pistons with 0.2" valve reliefs in them... seems like quite a waste if my cam is only opening 12° after top center.

Any recommendations for cams that open the intake before top center?
I'm looking for something with good low end.

Last edited by Ott3r; Wed Mar 25 2020 07:50 PM.

'59 Chevy Suburban, NAPCO
Ott3r #1351636 Wed Mar 25 2020 08:05 PM
Joined: Aug 2018
Posts: 696
2
'Bolter
seems impossible to find the 261 cam lobe flank and opening ramp shape online

but for a rough estimate (maybe i screwed this up...):
assume it is an raised inverted sine shape and using 0.42 maximum valve lift for a modified 261 cam
the first 5.5 camshaft degrees of rotation would only lift the valve by about 0.002-0.010 inches of travel,
in a cold engine with max valve to rocker clearance, the valve might not even begin moving until the piston reaches near or at TDC.

could put the engine together with a carefully placed blob of putty on the piston, and rotate the engine slowly
pull the head and measure the clearance
-s

Page 1 of 3 1 2 3

Moderated by  Phak1, Woogeroo 

Link Copied to Clipboard
Home | FAQ | Gallery | Tech Tips | Events | Features | Search | Hoo-Ya Shop
Powered by UBB.threads™ PHP Forum Software 7.7.5