idle vacuum

[Thomas G.]

For choosing a replacement cam for my 225 HP 56, I would like to know the idle vacuum at 600 rpm for an original cam (pn 3711354) engine. I'm considering a CS274 cam which should yield similar idle characteristics. Any response is appreciated.

Tom

[Edward J.]

Re: idle vacuum

Tom, do you have a shop manuel for your year? The spec.s should be in the rear of manuel. Generally most small blocks are about 15-18 inchs of vac., All factors are related to the engine timing and the sealing of of the piston rings and valves , and the camshaft profile.

[Duke W.]

Re: idle vacuum

For sixties/seventies vintage manuals, I've never seen an idle vacuum spec - at best just a recommended idle speed.

What Tom needs is someone with an known original cam '56 engine to state the idle vacuum @ idle speed in neutral with a manual transmission. Also, give us a subjective evaluation on smoothness. Is it smooth like later base engines, or does it have a little lope?

For example, the typical recommended idle speed in neutral for base 327s is 500 RPM and vacuum is typically about 18" Hg., so one would say 500@18", and the idle quality is very smooth and steady with no noticeable lope.

Whenever stating idle vacuum the idle speed must also be stated because vacuum will vary with the idle speed that the user sets.

Does the '56 owners or service manual state the recommended idle speed for the 225 HP engine?

Duke

[Thomas G.]

Re: idle vacuum

The owners manual and 57 shop manual give idle at 600 for dual carbs (225 hp) and either transmission, 475 for single carb, no mention of vacuum. 55. 56. shop manuals and 55-62 Corvette Servicing Guide have nothing on idle or vacuum.

Tom

[Duke W.]

Re: idle vacuum

Okay, so for a 225 HP 2x4 '56 engine that is known to have the OE mechanical lifter cam, we need the manifold vacuum at 600 RPM idling in neutral and a statement on idle quality.

Anybody have one they can measure and comment on?

Duke

[Gary C.]

Re: idle vacuum

Duke, all '56 cams were mechanical lifter cams - no hydraulic lifters in 56 Corvettes. Gary....

Sorry, Duke misread your post - please ignore, thanks, Gary....

[Duke W.]

Re: idle vacuum

Yes, I'm aware of that, and from the specs I have (that clearly have an error on the exhaust side since the specified duration doesn't match the number I compute from the specified timing points) it's probably a very mild cam - similar to later 3733431 and 3896929 base engine cams.

This is also supported by the specfied 475 RPM idle in neutral speed for the single four-barrel engine.

Tom can't find a NOS ...354 cam, there are no similar mechanical lifter cams offered by the aftermarket, and I'm not aware of any vendors who have lobe master for this cam to grind one.

So Tom's choice is to find a hydraulic lifter cam that offers similar idle and performance characteristics, and that's why we're asking for an idle vacuum reading, and he'll have to have the appropriate "notch" cut in the rear journal if he goes with the Seal Power CS274, which is an exact duplicate of the ...929.

Duke

[Gary C.]

Re: idle vacuum

Tom, FYI - if you're planning on trying to get a PV with your '56 - a hydraulic cam won't pass the PV. Might give Gary Pronesti (352) 638-5778 a call and see if he has one or knows where a '56 cam is. Gary....

[Thomas G.]

Re: idle vacuum

Thanks Duke and Gary.

I'm pretty sure 57 cam bearings were installed by my rebuilder 3 years ago along with the "too aggressive" Comp Cam replacement, which I now intend to replace. So, I shouldn't need the usual 56 notch. I'll make sure of that when I pull the cam.

I do hope to PV some day, but regardless, I want the whole car to be as close to original as I can reasonably get, which is why I love NCRS.

I hope to talk with Gary Pronesti later today. Thanks for that lead.

Tom

[Duke W.]

Re: idle vacuum

Tom, FYI - if you're planning on trying to get a PV with your '56 - a hydraulic cam won't pass the PV. Might give Gary Pronesti (352) 638-5778 a call and see if he has one or knows where a '56 cam is. Gary....

Why? As long as the idle characteristics are OE, why should it fail? Zero lashing a hydraulic cam will produce enough valve clatter to sound like a mechanical lifter cam, and a properly adjusted mechanical lifter cam should not have a lot of valve clatter.

Duke

[Duke W.]

Re: idle vacuum

Thanks Duke and Gary.

I'm pretty sure 57 cam bearings were installed by my rebuilder 3 years ago... So, I shouldn't need the usual 56 notch. I'll make sure of that when I pull the cam.

Tom

Is that really all that's required to run a non-notched cam in a '55-'56 block? I recall that this issue was discussed some time ago. The thread included some oil system diagrams, and my interpretation was that oil only flows to the lifter galleries when the notch is in a certain position, which means oil flow is intermittent, not continuous, and I thought that was really strange.

Duke

[John H.]

Re: idle vacuum

I'm pretty sure 57 cam bearings were installed by my rebuilder 3 years ago along with the "too aggressive" Comp Cam replacement, which I now intend to replace. So, I shouldn't need the usual 56 notch. I'll make sure of that when I pull the cam.

Tom -

It doesn't matter which rear cam bearing you have - either the 2-hole '55-'56 or a '57 where the single hole has been opened up to a full slot; you still need the notch in the rear cam journal.

[Thomas G.]

Re: idle vacuum

Tom -

It doesn't matter which rear cam bearing you have - either the 2-hole '55-'56 or a '57 where the single hole has been opened up to a full slot; you still need the notch in the rear cam journal.

I forgot to mention in my previous post that the 57 bearing was modified as you say. I did read the past threads on this, and I discussed it with my rebuilder.

My understanding is that the two openings in the 56 block and bearing holes, one at full oil pressure and the other feeding the lifter gallery, are "connected" when the cam notch passage is open to both, resulting in intermittent flow to the gallery while supplying continuous oil pressure to the cam journal. The 57 modified bearing still connects both block openings, but now continuously, with full flow oil delivery to the gallery and the journal through the bearing slot eliminating the need for the cam notch. With this bearing modification either a notched cam or unnotched cam can be used in the 56 block.

This bearing modification is described on page 80 in How to Rebuild your Small-Block Chevy book by David Vizard.

comments please,

Tom

[Thomas G.]

Re: idle vacuum

Tom, FYI - if you're planning on trying to get a PV with your '56 - a hydraulic cam won't pass the PV. Might give Gary Pronesti (352) 638-5778 a call and see if he has one or knows where a '56 cam is. Gary....

Gary Pronesti gave me 2 leads that I will try tomorrow, and I'll pass them on.

It seems now that I either choose a mechanical cam which is way too aggresive or a hydraulic one that performs close to OEM. Either way, I expect that it won't pass PV , and I'd rather have the latter. If anyone has a better option I'd sure like to know!

[John H.]

Re: idle vacuum

I forgot to mention in my previous post that the 57 bearing was modified as you say. I did read the past threads on this, and I discussed it with my rebuilder.

My understanding is that the two openings in the 56 block and bearing holes, one at full oil pressure and the other feeding the lifter gallery, are "connected" when the cam notch passage is open to both, resulting in intermittent flow to the gallery while supplying continuous oil pressure to the cam journal. The 57 modified bearing still connects both block openings, but now continuously, with full flow oil delivery to the gallery and the journal through the bearing slot eliminating the need for the cam notch. With this bearing modification either a notched cam or unnotched cam can be used in the 56 block.

This bearing modification is described on page 80 in How to Rebuild your Small-Block Chevy book by David Vizard.

comments please,

Tom

Tom -

The modification to the late bearing (creating the slot) simply allows the late bearing to be used in place of the 2-hole original bearing; in either case, without the notch in the cam journal, the only oil feed to the holes that feed the lifter galleries is from the thin oil film between the O.D. of the journal and the I.D. of the bearing, which is minimal. The only full-flow of oil occurs when the notch in the journal spans both holes in the original bearing (or the full slot in the modified later bearing), creating an intermittent full-flow passage from the pressure side to the lifter feed side.

Photo below shows the 265 rear cam bearing bore, with the pressure feed hole at 12 o'clock and the two lifter feed holes in the adjacent groove in the block; the only time those holes communicate with full oil flow is when the notch on the cam journal connects them.