how much compresion is advised for an isky 278/278 234/234 .450/.450 106lsa?

It's a function of many things, including rod length, head material, etc. Take a look at Pat Kelley's discussion and try his excellent DCR calculator.

http://www.empirenet.com/pkelley2/DynamicCR.html

By way of example, my 383 SBC with aluminum heads and a hydraulic roller cam with 232/242 duration @ 0.050 lift has a static CR of 11:1, and a dynamic CR of about 8.3:1. With iron heads, you'd probably want to be more like 8.0:1 DCR.

By the way, the Isky catalog doesn't list a recommended CR for that cam - a Mega 278.

how much compresion is advised for an isky 278/278 234/234 .450/.450 106lsa?

I would run 10 to 10.5:1. I had a cam similar only ground on a 114o lobe sep. The idle was super smooth (on a 400 SBC) and vacuum was relatively high.

With your stock iron heads right? with 106ls, 9.5- 9.75 max. 114 would be a different story.

With your stock iron heads right? with 106ls, 9.5- 9.75 max. 114 would be a different story.

Yeah 114 and a 106 would be 2 different cams....
Mine is a 230/236dur, .450 lift (Im using 1.6 rockers tho=.485 lift) on a 108 and it runs great on 9.7:1 I can tell it would like more tho.

how about calling Isky?

The DCR calculator left dynamics out of the equation... It means nothing, once you reach your efficient rpm range (where your engine's cylinders will fill fully) then you better pray for a great combustion design and enough cooling effect from the incoming intake charge to not detonate.
LS2's have 11-1 compression (10.9 to be exact) with less than 208 degrees of duration at 050 on a 116 or 114 (can't remember)...I'd love to see what that "DCR" says...probably would scream "race fuel only"...interesting enough that today's combustion chamber design, cooling design and computer control will run that much CR on 93 all day long.

Forget DCR, cam according to the operating range you want to be in and then use as much compression you can without detonation on the fuel you want to run.

I run a 278 advertised duration cam with 10.2:1 CR. My cam is on a 108 lobe separation and is installled at 104. I have well over 200 psi cranking compression and it don't like timing. I rattle and ping over with over 32 degrees of timing. I have aluminum heads and a .039" quench also.

how about calling Isky?

The DCR calculator left dynamics out of the equation... It means nothing, once you reach your efficient rpm range (where your engine's cylinders will fill fully) then you better pray for a great combustion design and enough cooling effect from the incoming intake charge to not detonate.
LS2's have 11-1 compression (10.9 to be exact) with less than 208 degrees of duration at 050 on a 116 or 114 (can't remember)...I'd love to see what that "DCR" says...probably would scream "race fuel only"...interesting enough that today's combustion chamber design, cooling design and computer control will run that much CR on 93 all day long.

Forget DCR, cam according to the operating range you want to be in and then use as much compression you can without detonation on the fuel you want to run.

I can't leave this thread suggesting that DCR is meaningless without responding. The K-B Pistons site states the following about static compression and what they call compression pressure, which is virtually the same as DCR:

Compression Ratio as a term sounds very descriptive. Compression ratio by itself, however, is like torque without RPM or tire diameter without a gear ratio. Compression ratio is only useful when other factors accompany it. Compression pressure is what the engine actually sees. High compression pressure increases the tendency toward detonation, while low compression pressure reduces performance and economy. Compression pressure varies in an engine every time the throttle is moved. Valve size, engine RPM, cylinder head, manifold and cam design, carburetor size, altitude, fuel engine/air temperature and compression ratio all combine to determine compression pressure. Supercharging and turbocharging can drastically alter compression pressures.

The goal of most performance engine designs is to utilize the highest possible compression pressure without causing detonation or a detonation-related failure. A full understanding of the interrelationship between compression ratio, compression pressure, and detonation is essential if engine performance is to be optimized. Understanding compression pressure is especially important to the engine builder that builds to a rule book that specifies a fixed compression ratio. The rule book engine may be restricted to a 9:1 ratio, but is usually not restricted to a specific compression pressure. Optimized air flow and cam timing can make a 9:1 engine act like a 10:1 engine. Restrictor plate or limited size carburetor engines can often run compression ratios impractical for unlimited engines. A 15:1 engine breathing through a restrictor plate may see less compression pressure than an 11:l unrestricted engine. The restrictor plate reduces the air to the cylinder and limits the compression pressure and lowers the octane requirements of the engine at higher RPM.

At one time compression pressure above a true 8:1 was considered impractical. It still is in many cases with today's gas. The heat of compression, plus residual cylinder head and piston heat, initiated detonation when 8:1 was exceeded. Some of the 60's 11:1 factory compression ratio engines were 11:1 in ratio, but only 8:1 in compression pressure. The pressure was reduced by closing the intake valve late. The late closing, long duration intake caused the engine to back pump the air/fuel mix into the intake manifold at speeds below 4500 RPM. The long intake duration prevented excess compresson pressure up to 4500 RPM and improved high RPM operation. Above 4500 RPM detonation was not a serious problem because the air/fuel mix entering the cylinder was in a high state of activity and the higher RPM limited cylinder pressure due to the short time available for cylinder filling.

http://kb-silvolite.com/article.php?action=read&A_id=36

There's also a good discussion at ChevyTalk initiated by GrumpyVette, who's pretty knowledgeable himself.

http://www.chevytalk.org/fusionbb/showtopic.php?tid/92966

I'm not suggesting that DCR is the end all and be all of engine component selection; but to dismiss is as meaning "nothing" is, I think, misleading.

Finally, we all have to start somewhere and the DCR calculators are a pretty useful tool for the intial planning of an engine. I'd rather figure out that I could safely run 10.5:1 static compression ratio before I build my engine than after I've built it at 9.5:1.

I am at 11 to 11.25:1 compression. 220 psi cranking.

SBC 400
Iron heads
234o/244o @ .050
20o initial, 34o total with vacuum advance

91 octane, no detonation, no problems.

I am at 11 to 11.25:1 compression. 220 psi cranking.

SBC 400
Iron heads
234o/244o @ .050
20o initial, 34o total with vacuum advance

91 octane, no detonation, no problems.



It's strange how some of these engines work. Take your for example, 220lbs cranking, no detonation on pump.

My Luv engine, 11.2:1 (measured) Iron Eagles, 252/258 @.050", 215lbs cranking, 20 initial, 36 total, no detonation on pump

My Chevelle, 10.2 (measured) Aluminum heads, 238/246 @.050", 210lbs cranking compression, detonates, rattles, sounds like BB's in a can with more than 32 degrees timing. Can't run vacuum advance.

Both engines are zero decked and have .039" thick head gaskets. The only difference between the two is the 11:1 engine is running flat tops and the 10:1 engine is running -12cc dished.

Just goes to show how important a good quench area is worth I guess. Maybe if I had -12cc reverse or D shaped domes, it wouldn't detonate.

10:1 compression Robert

i have 9.7:1 compression would that work?

I am at 11 to 11.25:1 compression. 220 psi cranking.

SBC 400
Iron heads
234o/244o @ .050
20o initial, 34o total with vacuum advance

91 octane, no detonation, no problems.

It's strange how some of these engines work. Take your for example, 220lbs cranking, no detonation on pump.

My Luv engine, 11.2:1 (measured) Iron Eagles, 252/258 @.050", 215lbs cranking, 20 initial, 36 total, no detonation on pump

My Chevelle, 10.2 (measured) Aluminum heads, 238/246 @.050", 210lbs cranking compression, detonates, rattles, sounds like BB's in a can with more than 32 degrees timing. Can't run vacuum advance.

Both engines are zero decked and have .039" thick head gaskets. The only difference between the two is the 11:1 engine is running flat tops and the 10:1 engine is running -12cc dished.

Just goes to show how important a good quench area is worth I guess. Maybe if I had -12cc reverse or D shaped domes, it wouldn't detonate.

It all comes down to valve timing Gents. :D As long as you can get the valves to open at the right time you want them too, It wont matter what static compression is. Dynamic is what tells you if you can run pump gas or not. You can have an 11.25:1 SCR engine and because of valve timing, its only at 8.1:1 DCR. Thus allowing you to run 91 octane. I believe most of you know this....LOL :D

It all comes down to valve timing Gents. :D As long as you can get the valves to open at the right time you want them too, It wont matter what static compression is. Dynamic is what tells you if you can run pump gas or not. You can have an 11.25:1 SCR engine and because of valve timing, its only at 8.1:1 DCR. Thus allowing you to run 91 octane. I believe most of you know this....LOL :D


Some of it's valve timing. I also believe that quench area and piston design play big roles in it. My Chevelle engine at 10.2:1 has a dynamic CR of 8.325:1 My Luv engine at 11.2:1 has a dynamic CR of 8.540 Going by the DCR, the Luv should not be able to run on pump gas but it does great. The Chevelle has a lower DCR and aluminum heads but has detonation problems. The difference is the piston design.


rolling-robert

9.7:1 will work with that cam.

^agree^ piston and chamber design does have an effect on the detonation and flame across the surface of both. I believe the more efficient the burn the less likely you are to have detonation. I have been wrong before. However, this is a statement from Smoky Yunick out of a "How to build big-inch SBC's" book I have at home.

^agree^ piston and chamber design does have an effect on the detonation and flame across the surface of both. I believe the more efficient the burn the less likely you are to have detonation. I have been wrong before. However, this is a statement from Smoky Yunick out of a "How to build big-inch SBC's" book I have at home.

I also agree. As Mirage2991 pointed out, some of the LSx factory egines are running at 10.9:1 or higher. Their ability to do so has a lot to do with quench, good consistent AFR's in the incoming fuel charge, materials used for the heads, combustion chamber design, cooling design, and so on.

i have 9.7:1 compression would that work?

Perfect

okay great.

the 383 will come out and my 355 in, hope i have more luck with this one.

I have a competion cam solid roller in my 383 with 11.4 - 1 compresson. Duration at 50 is 268/274 and cam is advanced to 104. Block is zero decked brodix heads angle milled with srp flat tops.( very tight quench) Motor loves 93 pump gas set at 34 deg total. With the large cam cranking comp is 195.