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Posted By: Ott3r valve timing vs collision! - Wed Mar 25 2020 06:53 PM
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 picture DSC_0002 (2).JPG
Attached picture valve timing.png
Posted By: 2ManyTrucks Re: valve timing vs collision! - Wed Mar 25 2020 07:00 PM
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
Posted By: Ott3r Re: valve timing vs collision! - Wed Mar 25 2020 07:02 PM
lemme go measure real quick...
Posted By: Ott3r Re: valve timing vs collision! - Wed Mar 25 2020 07:09 PM
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.
Posted By: Hotrod Lincoln Re: valve timing vs collision! - Wed Mar 25 2020 07:23 PM
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
Posted By: panic Re: valve timing vs collision! - Wed Mar 25 2020 07:24 PM
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
Posted By: panic Re: valve timing vs collision! - Wed Mar 25 2020 07:30 PM
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.
Posted By: Ott3r Re: valve timing vs collision! - Wed Mar 25 2020 07:31 PM
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 picture cam.png
Posted By: Ott3r Re: valve timing vs collision! - Wed Mar 25 2020 07:49 PM
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.
Posted By: 2ManyTrucks Re: valve timing vs collision! - Wed Mar 25 2020 08:05 PM
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
Posted By: Hotrod Lincoln Re: valve timing vs collision! - Wed Mar 25 2020 10:45 PM
Yes, the E-293S Elgin cam is the same one we used in John's engine. I purchased it from Internal Engine Parts Warehouse in Nashville and took it to Friendship Forest when we assembled the engine. To get an accurate lift curve on that cam, monitor the rate of lift AFTER the clearance ramp passes your dial indicator. You will see a rapid change in the rate of lift just as the clearance is taken up and the valve actually starts lifting off the seat. I don't recall the specified clearances Elgin recommends, but they're slightly different than the GM factory specs in the service manual. Elgin lists the cam as having a "290 degree" duration, but they're including the clearance ramps in that figure. The actual "valve open" duration is somewhere in the 260 degree range, give or take a little. Drop me a PM, and we can have a phone conversation about that cam if you like.
Jerry
Posted By: Ott3r Re: valve timing vs collision! - Wed Mar 25 2020 10:48 PM
Re: 2ManyTrucks
-s let me guess... Steve?

Thanks for estimating the numbers. I'd be interested if you have some kind of calculator online, or a spreadsheet so that I can play with that too!
Since i have valve reliefs, I'm wondering if I might benefit from a custom grind?

I'd hope to optimize for 1000-3000 rpm range
Copy the profile from a Jeep 4.0 or AMC 258?
http://www.angelfire.com/my/fan/Jeep4.0Camshafts.htm
Posted By: Hotrod Lincoln Re: valve timing vs collision! - Wed Mar 25 2020 11:06 PM
Ott, unless you're willing to sacrifice a LOT of low end torque and end up with an engine that has a very rough idle and won't run well unless it's wide open, stay away from long duration, high lift cams. The stovebolt cylinder head just won't allow enough airflow to make a lumpy cam worthwhile anyway.

Cam specs written for ".050" cam lift" are worthless for figuring valve to piston clearance. The rocker arm ratio on a stovebolt engine will have the valve open much further than that. You need a degree wheel, a dial indicator, and at least one piston and valve setup assembled and adjusted to monitor the piston and valve position in real time to get an accurate measurement. I use very weak "valve springs" so I can move the valves with thumb pressure while I'm checking cam timing and drawing a lift curve, just enough spring pressure to keep all the parts in contact with each other. As the parts approach "crash and burn" territory, I physically check the clearance with a dial indicator on the valve spring retainer by pushing the head of the valve into contact with the piston. I'd stay far away from a piston with a valve relief in it unless you've dons some radical head milling and/or block decking. On an OEM 235/261 block that hasn't been decked, the piston is about .050" or slightly more below the deck. Unless you've eliminated that fudge factor with your custom pistons, that needs to be taken into account when figuring valve relief cuts, also.
Jerry
Posted By: 2ManyTrucks Re: valve timing vs collision! - Wed Mar 25 2020 11:57 PM
right the valve will have started moving then,
my back of the envelope estimation didn't consider the 0.050 cam lobe lift specification which means the ramp starts even before that point a tiny bit on the lobe, and the rocker arm is already amplifying that travel by about 1.5x, i think a 235 is 1.477:1 ratio on the rocker?
so then the valve will likely be moving before TDC even on a cold engine
Jerry is right to measure it, because even after you have a model of all the curves of the lobes the tolerance stack up will be on the order of the distances we are discussing anyway.
engine to engine will be different too, that's why you always need to time them, not just assemble the parts to spec.
-stan
Posted By: Ott3r Re: valve timing vs collision! - Thu Mar 26 2020 12:40 AM
I really appreciate the input guys, & will definitely try out the lift curve measurement & valve clearance measurement.
Best regards
Logan
Posted By: Hotrod Lincoln Re: valve timing vs collision! - Thu Mar 26 2020 01:04 AM
A few more sneaky hotrodder tricks would include opening up an old timing gear until it's a snug slip fit on the cam instead of a press fit, and using offset Woodruff keys to tinker with advancing/retarding the position of the cam. Advance a cam for more low end torque, and retard it for more top end horsepower. An adjustment of 3-4 degrees can make a big difference in where the usable part of the power curve is located.
Jerry
Posted By: Dragsix Re: valve timing vs collision! - Thu Mar 26 2020 02:00 AM
A stock 261 piston sits over .100 in the hole. So there is more clearance then you think. On a 235 with a 848 head and flat tops, I have run cams as much as .480 lift 280 degrees duration without clearance problems. That was a fairly aggressive street cam. So that same cam would not create issues with a 261. But when you are hot rodding one of these motors, you really do need to check the clearance with the intake valve. Isky cams use to sell checking springs so you could degree your cam and at the same time graph the clearance at so much cam timing. The other old school way to do it is to clay up the pistons, install the head and gasket, rockers, adjust the valves and rotate the motor twice. Then pull it all apart and section the clay to find the thinnest part and measure.

Keep in mind, the 848 head has a combustion chamber of about 79.1 cc while the earlier 5913 head is about 86 cc. So the 848 head results in slightly more compression. That head entered service in 56 and the 235 motors were rated at 8.0 compression ratio. In 58 or 59, the 235 was rated at 8.5 compression ratio. I have never been able to sort out the difference as the motor specifications did not change. One of these days I will measure the combustion chambers of a 57 head and a later head to see if there is any difference.

In terms of flow, i have measured the intake and exhaust posts with calipers and was never able to see any difference in the port casting. So the flow is equally poor for either head. On the intake side, the port flows right to a dead end and bounces off, made worse but the short side radius design. On the exhaust side, where do I start, lol. Not great. But that is to be expected. These motors were work horses. So that was the design.
Posted By: Hotrod Lincoln Re: valve timing vs collision! - Thu Mar 26 2020 03:44 AM
Has anyone ever drilled and tapped the drive end of a stovebolt camshaft so the timing gear can be pushed on with a harmonic balancer installer instead of all the mickey mouse heat and beat methods that are sometimes used? It would seem to be a pretty good option, especially if the timing gears are being changed with the cam installed in the engine. I think I'll do that on the Frankenstein 216 I'm building so I can tinker with the cam timing between dyno runs. I'll add a thick flat washer and a tapered-head Allen bolt to retain the timing gear. That's one situation where a set of reversed bolts at the bottom of the timing cover might come in handy.
Jerry
Posted By: coilover Re: valve timing vs collision! - Thu Mar 26 2020 06:38 AM
Just a couple of $.02 advice from one that has spent much time with a degree wheel, dial indicator, and clay on top of pistons. First don't use clay but the much less messy Play-Doh. Second, valve-piston clash almost never occurs when the piston is on the way up or at TDC but rather the valve catches up to the piston on it's way DOWN. This can be from not enough deck height, pistons with no fly cuts, wrong cam with wrong lift amount or rate of lift (lobe profile), plus several other possible problems. For checking an assembled engine one can make up a lever to work the rockers since Stovebolts have very weak springs or even better is to pressurize the cylinder, pop the keepers off the intake and exhaust? (recessed) and then reassemble with very weak springs; this lets one work the rocker by hand. A 2' diameter degree wheel and a bolt in the front of the crank lets one move in very small increments. With a magnetic base dial indicator one can check the piston to valve clearance every 2* from 30* before TDC to 30 after in a short time. As was stated, any cam that needs piston to valve clearance checked on a Stovebolt engine is a waste of money since the head and manifold flow about the same as a flathead Briggs and Stratton.
Posted By: panic Re: valve timing vs collision! - Thu Mar 26 2020 02:02 PM
any cam that needs piston to valve clearance checked on a Stovebolt engine is a waste of money
McGurk didn't think so.
Posted By: Ott3r Re: valve timing vs collision! - Thu Mar 26 2020 02:19 PM
I just bought some play-doh yesterday. Good to hear there is another use for it!
Posted By: Hotrod Lincoln Re: valve timing vs collision! - Thu Mar 26 2020 02:20 PM
McGurk was like P.T. Barnum. He was selling hot air to people ignorant enough to believe his line of Bovine Scatology. I'm suspicious of anyone who promises to solve all my problems by selling me something. He just might have a tiny bit of an ulterior motive up his sleeve!
Jerry
Posted By: Ott3r Re: valve timing vs collision! - Thu Mar 26 2020 05:33 PM
Originally Posted by Hotrod Lincoln
Bovine Scatology.

I had to look that one up! 🤣
Posted By: Dragsix Re: valve timing vs collision! - Thu Mar 26 2020 10:00 PM
This is easy. If you don’t want to check for piston to valve clearance, use a stock car or truck cam. Then you don’t have to check. If you like to tinker and hot rod your motors like I do, then for some cams, you will want to check the clearance. These old style inline six motors respond to all of the same hot rodding things that other motors respond to. Better breathing, more cam, more compression, more carb, changes to the ignition curve. All kinds of things. But there is a limit to what you will be able to drag out of them torque and/or horse power wise. Do these kinds of modifications change some of the running parameters of the motor, sometimes. That should not stop you from tinkering if that’s what you like to do.

Hot rodding one of these motors a waste of time? Hardly. It’s actually a great deal of fun truth be told. Think you can build a stock 235 or 261 that will out run one of my modified motors, now that’s a waste of time, lol.
Posted By: panic Re: valve timing vs collision! - Thu Mar 26 2020 10:19 PM
So... the Mcgurk tests were all false, and the many cams tested in 1955 did nothing?
Posted By: Hotrod Lincoln Re: valve timing vs collision! - Thu Mar 26 2020 10:51 PM
Not false, just ineffective. They still couldn't outrun the flathead V8's. "All hat and no cattle" as the Texans say. They could rattle windows half a mile away with the exhaust note, though.
Jerry
Posted By: panic Re: valve timing vs collision! - Fri Mar 27 2020 11:34 PM
The admonition against hot cams in a stovebolt is, IMHO, based in part on the well known maxim that the better the head design the smaller the cam can be (and with better manners). The reverse case, as in the entire 216-235-261 series (bad intake flow requires rough cam for more power) also applies.
Posted By: brokenhead Re: valve timing vs collision! - Sat Mar 28 2020 03:02 PM
turbochargers all the way...
Posted By: Hotrod Lincoln Re: valve timing vs collision! - Sat Mar 28 2020 03:36 PM
"Turbos- - - - -We don't need no steenkin' turbos!" Stovebolters can develop enough high pressure hot air without any mechanical assistance!
LOL!
Jerry
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