Cam Specs for 69 & 70

[Duke W.]

Re: Cam Specs for 69 & 70

Yes, and I should also add that in my San Diego lecture I presented a technical and economic argument for raising the compression ratio of post-1970 engines to their 1970 antecedent levels - not the "advertised" value, but the value they were built with, which is typically about 0.5 less than advertised.

This change will improve torque and power across the range by 7-10 percent. It will require premium rather than regular fuel, but premimum is now only 5 percent more than regular, so you get improved across the range performance and lower fuel operating cost.

It's a no-brainer!

Duke

[Pamela H.]

DO NOT CAM
Corvettestation wagon

[Duke W.]

Re: Cam Specs for 69 & 70

Any competent machine shop should be able to measure deck height on a bare block. I think the nominal spec for the big block is 9.800". Then you subtract crank throw radius, rod length, and piston compression height to get the new nominal deck clearance. Now you can use the CR calculator with various head gaskets to get to the target CR.

It's all in the documents I sent you.

The '66 to '72 Corvette big block hydraulic lifter cam is no "station wagon cam". It's a lot closer to the L-79 small block cam than the 327/300 cam, which is very mild.

I'm not sure exactly what the changes were to the big block cam beginning in '73, but I expect it was emission related, which is why I recommend the earlier cam.

Forget the Lunati and Edelbrock cams. They are similar in duration to the above cam, but probably have more overlap, which will screw up the idle quality and loose low end torque. They probably also have more aggressive dynamics, which will place excess load on the valve train. For a road engine, most OE cams are nearly impossible to beat in terms of torque bandwidth and valve train durability. They were very well engineered.

If you want "more power" without screwing up the idle and low end torque, massage the heads and install the cam four degrees retarded from the standard indexing.

I'm not sure of the head cc. Joe or Clem might know, but you should have them measured in any precision engine restoration.

Duke

[Pamela H.]

Re: Cam Specs for 69 & 70

Duke, can you tell me the source for the cam you recommended.(3883986)..checked with Napaonline and in the store as well and they can't find it either. I also check GMPP with no luck.
The other question is that the machinist is reporting a significant failure rate with KB pistons and recommends against their use. We wanted to go with KB 203 that you recommended. I personally have never heard of a failure with a KB poston under 700hp. Our application prob won't be 450hp. What's your opinion...besides get a different machine shop?
Thanks!

[Duke W.]

Re: Cam Specs for 69 & 70

From the napaonline.com com home page type in CS1093M in the search box. That's the Sealed Power replacement for the '67-'72 Corvette big block hydraulic lifter camshaft. The high level specs match, and it is built to the original GM print for the later 3904359 camshaft that is the same as the ...986, other than not having the groove in the rear journal, which was required for '65 and '66 big block camshafts.

The Web is full or horror stories about KB pistons including pictures of broken ring lands. KB hypereutectic pistons have a very high mounted top ring. Because the ring is closer to the crown than typical OE pistons it runs hotter and needs greater than OE end gap.The reasons why you see pictures of broken upper top ring lands is that the moron who put the engine together DIDN'T READ AND FOLLOW THE INSTRUCTIONS TO ENSURE THE TOP RING END GAP WAS ADEQUATE.

Top rings in standard ring sets may require filing the to achieve adequate gap.The instructions should be included with pistons and they are also downloadable from the Web site. Do this and have your "engine builder" sign them with the understanding that he has read and understands the instructions and will follow them to the letter when he assembles the engine.

I DID NOT recommend a specific KB piston. You have the information from me on how to use piston volume and head gasket thickenss along with your measured deck clearance and head chamber volume to achieve your target compression ratio.

Duke

[Pamela H.]

Re: Cam Specs for 69 & 70

Got it! Thanks for your quick reply!

[Pamela H.]

Re: Cam Specs for 69 & 70

Do you recommend getting the pistons coated? Heads, Skirts or both?

[Duke W.]

Re: Cam Specs for 69 & 70

No, I do not.

[Gene M.]

Re: Cam Specs for 69 & 70

Pam,
That EDELBROCK 218/228 @ .050 500/500 has 114 Degree lobe centers and idles very smooth. Over all duration is 292/302 so ramps are gentle. I have it in my 57 Chevy, oval port aluminum heads and 435 hp TRW pistons. Runs strong and has good idle with strong off idle torque. My vacuum at idle is 16 and steady.

[Pamela H.]

Re: Cam Specs for 69 & 70

What your your opinion of KB257 cast piston (.250 dome) with total seal top ring?

Would this negate the chance of piston failure due to ring isssues?

Can you advise on the nomial OE production tolerance specs for deck clearance?

[Pamela H.]

Re: Cam Specs for 69 & 70

For my further education could you explain your thoughts on this?

[Duke W.]

Re: Cam Specs for 69 & 70

What your your opinion of KB257 cast piston (.250 dome) with total seal top ring?

Would this negate the chance of piston failure due to ring isssues?

Can you advise on the nomial OE production tolerance specs for deck clearance?

I have no opinion because I have not researched that piston and don't have the specific dimensions for your engine to run all the data through the CR calculator. That's your job.

Each top ring must be checked for adequate end gap and filed, if required, to achieve the minimum specified by KB.

I believe the nominal deck clearance for big blocks is .020", but I'm not sure. Maybe someone else can confirm. As with small blocks, most big blocks are probably greater due to higher than nominal height decks, and up to .015" higher than nominal decks is not uncommon.

I've noted that KB small block pistons have a slightly greater specified compression height than OE pistons.

Final head gasket thickness to acheive the target CR can only be determined once actual deck clearance is measured on the assembled block and actual head cc measurements are determined.

Duke

[Gene M.]

Re: Cam Specs for 69 & 70

Pam,
The hypereutectic pistons need to be fitted tighter to the bore than the original forged pistons.

[Duke W.]

Re: Cam Specs for 69 & 70

The 427/390 use cast, not forged pistons. KB piston fit specs are on the Web site.

Duke