My 365 That Isn't

[Joe C.]

Re: My 365 That Isn't

George,
Maybe these conflicting opinions your getting on detonation are due to different engine configurations. Some of these engines, especially the solid lifter SHPs have high static compression ratios but quite low dynamic compression ratios due to cam configurations such as overlap. Joe and Duke can give you more on these aspects and have already within some of their posts.
John

John,

Higher overlap causes lopey idle and lowered idle vacuum because of the period that both intake and exhaust valves are open simultaneously. Dynamic stroke, and thus DCR is determined by the intake valve (effective) closing event, which as defined by SAE J604d, is the point when it is closing and .004" off its seat. Engines with "big" cams have shorter dynamic stroke, due to later closing intake valves than engines with "small" cams, which is why engines with mild cams use lower SCR than those with higher duration cams. The proper way to design is to design for DCR in the 8.25 - 8.75 range, no matter what kind of cam you're using.

The cam in my 327, as installed 3* advanced rather than the 4* recommended has the intake closing (J604d) at 72* ABDC, which yields an average 8.53:1 DCR. There are some small variations on chamber sizing, and the cylinder with the tightest chamber develops a DCR of 8.65:1. This is very stout and I consider 8.75:1 to be the upper limit for running on pump gas.

[Duke W.]

Re: My 365 That Isn't

George,I have a 62 340 HP with 11.25 compression (effective compression is lower according to the "experts") and it runs fine on Shell 93 octane gas with ethanol???.As far as the cam goes I just had a 327 (337" with 0.060 pistons, flat top 10:1) using a L79 hydraulic cam which I choose due to a 114 degree LSA which should produce a good vacuum. I need the vacuum for my Hilborn injection which I modified to a EFI unit or I might just use one of my early Rochester FI units.As soon as I start the engine I will report on it's performance.

How do you know your '62 is 11.25:1? Did you disassemble the engine and make all the requisite measurements to determine the exact as-built CR?

During the 1962 model run, Chevrolet started adding and extra .018" shim gasket under each head of 340/360 HP engine. This continued through 1963 and reduced the CR about half a point, so depending on how high the decks are, the actual CR of these engines is in the range of about 10.2 to 10.7:1.

It's easy to measure head gasket thickness with a feeler gage. What do you measure?

I keep telling you guys that so-called 11:1 engines are not really that high and can be up to 0.5 lower than the advertised CR, which is about as realistic as the advertised gross horsepower rating, which average about 10 percent lower than advertised.

Also, virtually no known original "high compression" small blocks are known to detonate. It's a little more likely on "high compression" big blocks, but still not common.

Duke

[Joe C.]

Re: My 365 That Isn't

The Duntov cam was a very mild cam compared to the 30-30 and the LT1. It was designed to work in the earlier 283 engines, and was operating on the ragged edge of detonation as installed in the 62-63 327. Its intake valve closes at 63* ABDC............................very early as compared to the LT1 at 82* and the 30-30 at 85*. Chevrolet had no business running this cam in a 327 at their exaggerated 11.25:1, which was more accurately 10.75:1. This is why they added a second head gasket, to get the SCR down to a more comfortable 11:1 (more accurately, 10.5:1). The LT1 was the best designed, as far as CR is concerned, as 11:1 SCR (more accurately 10.5:1) put its DCR (somewhat higher in a 3.48 stroke 350 than in a 327 of comparable SCR) in a more desirable range. The L76/L84 for 64-65 used the 30-30 cam, and it should have been designed with more static compression............virtually impossible to get it to detonate as designed and built.

[Duke W.]

Re: My 365 That Isn't

George,
Maybe these conflicting opinions your getting on detonation are due to different engine configurations. Some of these engines, especially the solid lifter SHPs have high static compression ratios but quite low dynamic compression ratios due to cam configurations such as overlap. Joe and Duke can give you more on these aspects and have already within some of their posts.
John

Dynamic compression ratio has absolutely NOTHING to do with overlap. It's a function of two things - static compression ratio and the point that the inlet valve closes.

The only problem is that there is no industry standard for "point the inlet valve closes", so DCR calculations vary depending on the algorithm used, and sometimes it's not specifically stated, so I don't rely on DCR calculations other than a rough guideline.

My CR recommendations are from testing of engines with various camshafts and known, calculated CRs based on detailed and accurate measurements made during the rebuilding processs.

If you are restoning an OE configuration to the same OE configuration, take the measurements necessary to compute CR on teardown. If it didn't detonate before, there is not need to "lower the compression ratio".

If you're going to use different pistons, the deck clearance measurement taken during teardown will allow an accurate estimate of the new CR for any given head gasket, and final head gasket thickness is determined once the block is reassembled and the final deck clearances with the new pistons are measured.

Duke

[John M.]

Re: My 365 That Isn't

Joe,
Thanks for the education. What are the DCRs for the two versions of your engine, the 11.3 and the 11.5? I just want to add to my notes on this ongoing discussion. It seems to me that what you and Duke are saying is that these engines, in the stock mode and even with CRs a little higher than stock should not detonate on 93 octane. If they do, something is wrong or something has been changed. Makes those GM engineers of yesteryear look pretty smart and that George will be Ok whatever way he chooses to proceed; just replace the cam and lifters or go the whole route back to stock L84.
John

[Joe C.]

Re: My 365 That Isn't

Joe,
Thanks for the education. What are the DCRs for the two versions of your engine, the 11.3 and the 11.5? I just want to add to my notes on this ongoing discussion. It seems to me that what you and Duke are saying is that these engines, in the stock mode and even with CRs a little higher than stock should not detonate on 93 octane. If they do, something is wrong or something has been changed. Makes those GM engineers of yesteryear look pretty smart and that George will be Ok whatever way he chooses to proceed; just replace the cam and lifters or go the whole route back to stock L84.
John

John,
The answers are found in post nos. 61 and 63.

[John M.]

Re: My 365 That Isn't

Hey Joe,
I just noticed that you zero decked your block (is that the right term?) How did you handle the intake match up to the heads? I recently had an issue with a 327 that had heads that were shaved and the intake bolts weren't happy when I changed the head gasket from a thick composite back to the original shim type. So much of an issue that I went back to the thick gasket. I wasn't going to elongate the holes in the manifold. I could have milled the mating surfaces but was more interested in getting the job done than learning what should have been done. Just curious. Those were some pretty impressive stats for a 327, by the way.
John

[Joe C.]

Re: My 365 That Isn't

Hey Joe,
I just noticed that you zero decked your block (is that the right term?) How did you handle the intake match up to the heads? I recently had an issue with a 327 that had heads that were shaved and the intake bolts weren't happy when I changed the head gasket from a thick composite back to the original shim type. So much of an issue that I went back to the thick gasket. I wasn't going to elongate the holes in the manifold. I could have milled the mating surfaces but was more interested in getting the job done than learning what should have been done. Just curious. Those were some pretty impressive stats for a 327, by the way.
John

John,

"Zero deck" is used as a general term, and my piston heads are actually .004" - .006" above the decks, which were surfaced within .003" run-out in both directions.

The intake is a ported LT1, and its ports, although enlarged are still (intentionally) slightly smaller than the ports in the heads, which are matched to a FelPro 1206 gasket. Because the decks were cut, this causes the intake manifold to sit slightly high within the included angle between the heads. This is beneficial to the engine, as it provides a step in the bottom of the intake ports which serve to inhibit reversion pulses, a phenomenon which becomes more pronounced as cam overlap is increased. The way the head/intake ports line up, is that the rooves are a perfect match, with the floors of the intake ports being about 1/16" higher than the floors of the head ports. Since I used the mismatch to my advantage, I elongated the holes in the intake manifold slightly, in the outboard direction in order to line up the bolt holes. The intake manifold is ported and matched to a FelPro 1205 gasket, which is what I used for sealing.

[George C.]

Re: My 365 That Isn't

Dan,
Thank you for your response, I am really looking forward to getting this motor back toggether so I can drive it with the FI unit. Glad to see you drive yours, I want to do the same thing.

George

[George C.]

Re: My 365 That Isn't

Joe,
Thank you again for the details. I have the old cam out now, no decisions yet, but leaning towards the 30-30 and replacing the flat tops with the dome pistons like it came from the factory.
George

Was able to reuse my old pistons on latest build, so still using FM L2166NF-30. Dome volume is 5.3cc, combustion chambers are 63cc, pistons are .005" out of the hole, and head gasket compressed thickness in .038".

[George C.]

Re: My 365 That Isn't

Hi Guys,
I have removed the camshaft and the exhaust lobe on # 8 is hte only one showing destruction. Now I need to identify what the camshaft is, I have searched the board and hte archives, and the Corvette Forum and didn't find this number anywhere.
The number on the shaft is C1M302-R. The logo looks like an arrow in a circle.

I am almost afraid to ask, but does anyone have an idea what this cam is?

Thank you,
George

[Clem Z.]

Re: My 365 That Isn't

Hi Guys,
I have removed the camshaft and the exhaust lobe on # 8 is hte only one showing destruction. Now I need to identify what the camshaft is, I have searched the board and hte archives, and the Corvette Forum and didn't find this number anywhere.
The number on the shaft is C1M302-R. The logo looks like an arrow in a circle.

I am almost afraid to ask, but does anyone have an idea what this cam is?

Thank you,
George

sounds like a general kinetics number

[Clem Z.]

Re: My 365 That Isn't

sounds like a general kinetics number

i looked for my old catalog but looks like i tossed it. i used their cams when we ran the mushroom tappets in NASCAR

[Michael H.]

Re: My 365 That Isn't

Joe,
Thank you again for the details. I have the old cam out now, no decisions yet, but leaning towards the 30-30 and replacing the flat tops with the dome pistons like it came from the factory.
George

That sounds like the best idea so far.

[Clem Z.]

Re: My 365 That Isn't

That sounds like the best idea so far.

you will need to rebalance the rotating assy if you change piston unless they are the same weight