idle vacuum

[Cecil L.]

Re: idle vacuum

56-57 tune up specs iincluding vacuum.

http://chevy.oldcarmanualproject.com/engine/56etu/images/56etu53.jpg

[Duke W.]

Re: idle vacuum

Thanks for posting the link. That Web site is a great resource for '50s vintage owners!

Here's the link to the first page of the section:

http://chevy.oldcarmanualproject.com/engine/56etu/56etu51.htm

The second page has the photos and the third page is what you posted.

Note that even though both the single 4-bbl. and two 4-bbl. engines have the same camshaft, the single carb version vacuum range is 16-20", but only 14-18" for the dual carb. engine.

Based on these manifold vacuum ranges, the ...929 cam with a suitable notch in the rear journal should provide very similar idle and overall operating characateristics.

I take it from reading the instructions that the 2x4 engine has no idle speed adjustment screw on the throttle linkage and that idle speed is contolled primarily by the idle air bypass screws on the carbs -items 5 and 6 in Fig. 73. It that correct?

Duke

[Cecil L.]

Re: idle vacuum

Thanks for posting the link. That Web site is a great resource for '50s vintage owners!

Here's the link to the first page of the section:

http://chevy.oldcarmanualproject.com/engine/56etu/56etu51.htm

The second page has the photos and the third page is what you posted.

Note that even though both the single 4-bbl. and two 4-bbl. engines have the same camshaft, the single carb version vacuum range is 16-20", but only 14-18" for the dual carb. engine.

Based on these manifold vacuum ranges, the ...929 cam with a suitable notch in the rear journal should provide very similar idle and overall operating characateristics.

I take it from reading the instructions that the 2x4 engine has no idle speed adjustment screw on the throttle linkage and that idle speed is contolled primarily by the idle air bypass screws on the carbs -items 5 and 6 in Fig. 73. It that correct?

Duke

Duke,
56 and early 57 had idle air screws.
Lots more info, just about everything you could want to know about all 56 models including a section on Corvette from the original factory specs.

http://chevy.oldcarmanualproject.com/chevyresto/56index.htm

Also for 55 at the same site.

[Stuart F.]

Re: idle vacuum

Every 56 Dual Quad Vette engine I worked on back in the day had the air idle screws, and I loved them. I suspect they were a carry over perhaps from Caddy El Dorado's which was one of the first WCFB dual quad applications GM had. Those Caddy setups; however, ran both primaries together. Although they had a square base mounting pattern which required an adapter to use them on a Corvette manifold, their square 1-1/8" venturis provided a lot more top end trap speed at the strip (for me). Running both primaries together required some getting used to off the line for the extra torque caused some traction problems, even with two size over doubleagles. My 57 B/S 2-door post (292ci+)would typically pick up 4 to 6 mph through the traps, which was great when the paid for top speed in class.

Stu Fox

[Thomas G.]

Re: idle vacuum

John,

Thanks for the photo. That really makes it clear without the bearing in place. Picture the modified 57 bearing in place. The bearing slot alone creates a new passage which connects the pressure feed hole with the groove. This passage in cross section (looking from the rear) is like a curved rectangle with top being the block bearing bore and the bottom being the un-notched cam journal. The sides of the rectangle are the sides of the bearing slot which are as high as the thickness of the bearing. This passage is always open, with or without the journal notch.

Another method of converting from intermittent to full flow in a 56 block would be to grind a new groove in the block bearing bore between the pressure feed hole and the lifter gallery groove. In this case, the 57 bearing would not need to modified. Of course, this should only be done on a block that is competely disassembled and can be adequately cleaned of debris from this grinding.

Why did Chevrolet design the 56 block with intermittent oil supply to the lifter galleries?

Why did they change to full flow in 57?

Does anybody care?

Tom

[John H.]

Re: idle vacuum

Another method of converting from intermittent to full flow in a 56 block would be to grind a new groove in the block bearing bore between the pressure feed hole and the lifter gallery groove. In this case, the 57 bearing would not need to modified. Of course, this should only be done on a block that is competely disassembled and can be adequately cleaned of debris from this grinding.

Why did Chevrolet design the 56 block with intermittent oil supply to the lifter galleries?

Why did they change to full flow in 57?

Grinding the new groove works perfectly, and eliminates the need for special bearings and cam journal notches.

Have no idea why they designed the '55-'56 lifter feed oil system that way, but they did an equally oddball lifter feed oil path on the first ('65-'66) big-blocks with the circumferential groove in the rear cam journal, which they fixed for '67. The big-block and small-block engine design groups were completely separate, and apparently didn't talk to each other.

[Joe C.]

Re: idle vacuum

The 55-56 cam and lifters were different. The cam has a notch in the last cam bearing journal to flow oil between the 2 adjacent oil holes in the block and cam bearing(if properly installed). This is an impulse oiling system that is regulated by that notch. Hydraulic cams in the full size cars have a long deep notch which deliver more oil to the hydraulic lifters. The solid lifter cams have a much smaller notch. The lifters have the oiling hole in a different place than the 57-up lifters, although they are all edge orifice type, and not piddle valve. They flow much more of their oil because they only get oil intermittently. If you were to build a 57-up engine and use 55-56 lifters you would flood the heads with oil and overpower the valve seals. That won't happen because those lifters are not available. The easy way around all this is to cut out the area between the 2 rear cam bearing holes to allow oil to flow all the time and then the later cam and lifters will work properly in the 55-56 block.

[Thomas G.]

Re: idle vacuum

The 55-56 cam and lifters were different. The cam has a notch in the last cam bearing journal to flow oil between the 2 adjacent oil holes in the block and cam bearing(if properly installed). This is an impulse oiling system that is regulated by that notch. Hydraulic cams in the full size cars have a long deep notch which deliver more oil to the hydraulic lifters. The solid lifter cams have a much smaller notch. The lifters have the oiling hole in a different place than the 57-up lifters, although they are all edge orifice type, and not piddle valve. They flow much more of their oil because they only get oil intermittently. If you were to build a 57-up engine and use 55-56 lifters you would flood the heads with oil and overpower the valve seals. That won't happen because those lifters are not available. The easy way around all this is to cut out the area between the 2 rear cam bearing holes to allow oil to flow all the time and then the later cam and lifters will work properly in the 55-56 block.

Thanks Joe. I knew somebody had the answers.

Tom

[Thomas G.]

Re: idle vacuum

Thanks to all that replied. You have been a great help.

I decided to follow Duke's recommendation. I purchased cam CS274 with hydraulic lifters from Napa. I found that I did NOT have the modified 57 bearing (have original style 2-hole) when I removed the notched cam. I plan to have the new cam notched as shown in the photo supplied by Joe for the 55-56 passenger car with hydraulic lifters. If anyone thinks this is a bad idea, please respond.

Tom

[Joe C.]

Re: idle vacuum

Tom,

According to what I found on the Chevy website from which I culled the camshaft photo, it was pointed out that the 1955-56 lifters flowed more oil than the 1957-up lifters, which necessitated the intermittent flow designed into the older cam bearing journal. The difference between the 55-56 and the 57-up lifters, both piddle valve type, was supposedly in the location/size of the oil orifice, which caused the earlier lifters to deliver more oil to the rocker arms.

Of course, the 55-56 style lifters are no longer available, so you'll be using the late style piddle valve lifter which flow less. Considering that the journal diameter is 1.868",the hydraulic slot appears to be (about) 1.25" long. I do not have a measurement for you.

Since your engine block is unmodified, you will not be modifying it, and you are using unmodified 55-56 cam bearings, then you would be well served to have a slot or shallow channel machined into your CS274 which is at least 1.25" long, but preferably longer.

Joe

[Thomas G.]

Re: idle vacuum

Tom,

According to what I found on the Chevy website from which I culled the camshaft photo, it was pointed out that the 1955-56 lifters flowed more oil than the 1957-up lifters, which necessitated the intermittent flow designed into the older cam bearing journal. The difference between the 55-56 and the 57-up lifters, both piddle valve type, was supposedly in the location/size of the oil orifice, which caused the earlier lifters to deliver more oil to the rocker arms.

Of course, the 55-56 style lifters are no longer available, so you'll be using the late style piddle valve lifter which flow less. Considering that the journal diameter is 1.868",the hydraulic slot appears to be (about) 1.25" long. I do not have a measurement for you.

Since your engine block is unmodified, you will not be modifying it, and you are using unmodified 55-56 cam bearings, then you would be well served to have a slot or shallow channel machined into your CS274 which is at least 1.25" long, but preferably longer.

Joe

Thanks Joe. What do you think about converting to full flow by cutting a groove/channel all the way around the journal. It looks like that would require removal of about half the surface area of the journal, perhaps too much? Also, is there danger of creating too much flow capacity to the lifter galleries, or do the small bearing holes provide sufficient limit? This engineering is beginning to feel uncomfortable for me. How critical is this metering of oil? I thought this would be much easier than pulling everything apart to replace the rear bearing, but now I'm not sure.

I started a new thread today "56 hydraulic cam" because I thought this discussion under "idle vacuum" would get lost to other readers. Now I have 2 threads on the same subject! ooops

Tom

[Joe C.]

Re: idle vacuum

Thanks Joe. What do you think about converting to full flow by cutting a groove/channel all the way around the journal. It looks like that would require removal of about half the surface area of the journal, perhaps too much? Also, is there danger of creating too much flow capacity to the lifter galleries, or do the small bearing holes provide sufficient limit? This engineering is beginning to feel uncomfortable for me. How critical is this metering of oil? I thought this would be much easier than pulling everything apart to replace the rear bearing, but now I'm not sure.

I started a new thread today "56 hydraulic cam" because I thought this discussion under "idle vacuum" would get lost to other readers. Now I have 2 threads on the same subject! ooops

Tom

As I stated in the email, I would recommend a full annular channel, which would provide full time flow, and is more in-line with the modern hydraulic lifters, only if the width of the required groove were not as wide as the required notch. The limiting factor is the force vector in the lateral direction, which would determine how much pressure per unit area would be applied to the modified journal under maximum load. This would in turn affect boundary lubrication on the sliding surface of the cam journal. Because the engine will not be disassembled, the block and/or rear bearing cannot be modified. This necessitates having to present a full width depression in the cam journal in order to enable oil to flow from the main gallery to each of the lifter galleries. This, of course requires engineering analysis and is beyond what I am willing or able to speculate about.

Short of an engineering analysis, I would think that a Chevy small block, with its inherent copious top end oiling, would be able to tolerate a small deficit in such oiling as a result of your exact emulation of an approximately) 1 1/4" long original design journal notch. Anything more than that might be gambling with the durability of the rear cam journal.

There might be some racers' tricks that can be used to enable you to cut a full width annular groove, but I am not qualified to offer any. Try going back to the website where I found the cam photo and ask the Chevy guys there. They will have more experience with such a situation as most here will have.

[Thomas G.]

Re: idle vacuum

There might be some racers' tricks that can be used to enable you to cut a full width annular groove, but I am not qualified to offer any. Try going back to the website where I found the cam photo and ask the Chevy guys there. They will have more experience with such a situation as most here will have.

Thanks Joe. Please give me that website address or name. I'll check it out.

Tom

[Joe C.]

Re: idle vacuum

Thanks Joe. Please give me that website address or name. I'll check it out.

Tom

Here you go:

http://www.chevytalk.org/fusionbb/sh...hp?tid/232629/

[Bill H.]

Re: idle vacuum

Several years ago I restored a 56 225 horse and needed a cam. I was fortunate enough to have known Frank Burrell and Dave Bartusch -- Dave got an NOS cam for me from Frank along with a set of NOS lifters as I recall. That engine ran smooth as silk -- no loping. It was a great motor. I sold the car years ago and do not recall the buyers name, but if VIN 3197 is out there perhaps he will respond and provide you what you are looking for.