Cranking compression results are in

[SilverChicken]

I posted a while back after doing some dynamic compression calculations involving my LT1 that on paper seemed too high. As I said I would but would be delayed in doing so, I finally was able to do my cranking compression check but, as always seems to be the case, there were some oddities while doing the test. To start off with what has been done since the engine last ran, Lloyd Elliot ported and decked the factory heads and did some chamber work to them but I never cc’d them to know their new volume. Different cam installed from the previous GMPP Hotcam, the Howards 180885-10 (225/225@.050, .532/.532 w/1.52:1 rockers, 110LSA). Finally I’m running Morel 7717 lifters than have been preloaded with 1 full turn for about .042” on 3/8” studs), spec says .060” standard preload. SCR calculated out to around 11.5:1, with above cam DCR was anywhere from 9.1:1 (Wallace) to 10:1 depending on the calculator. Now for my findings today. First off, I had to do the test cold but I did do it with wetted cylinder walls, and the lifters are not pumped up, but maybe now they are because of how many cranks it took just to build pressure. She didn’t want to build up cylinder pressure and the throttle is open. Each turn of the engine only raised the needle 5-10psi. The ring seal has never been an issue prior. Last, the final pressure once it stopped building was 230PSI!!!!! That seems to be an insane number, like I’m running a factory cam on that compression! Experience and knowledge would be appreciated with the info I have here as well as advice. Thanks y’all.

[F-BIRD'88]

Ir reguardless of the crankiing compression test if the compression ratio is 11.5:1 it is not going to operate on street pump gas without retarding the timing at least some.... THis engine needs 98-100 R+M/2 octane fuel. 92 octane is not going to work without retarding the timing.
On a street car anythjing above 10.5:1 can and will get real dicy on good pump gas 92 octane.
The tune has to be just right and the timing may/will need to be compromized a bit.
Building for 11.5:1 on pump gas is a mistake for a street driven car. The operating conditions on the street in real world traffic are JUST TOO VARIABLE.
You can make it "run" as is but the timing will need to be compromized to keep it alive on the street on pump gas.
If you want to let the lobster loose at the track use 100 (or more) octane gasoline. ( max power ignition timing for best engine power)

[F-BIRD'88]

Your cranking compression test may have been fubared by the lifters BUT the truth does not change. 11.5:1 is excessive for street pump gas.
If you run it balls out with full power WOT timing it will be a very SHORT car ride. Bring cab fair.
You can and will make a TON of POWER ( reliabily) with a real 10:1 to 10.5:1 engine compression ratio.
Put the compression testetr away and re- think this.

[rebelrouser]

I posted a while back after doing some dynamic compression calculations involving my LT1 that on paper seemed too high. As I said I would but would be delayed in doing so, I finally was able to do my cranking compression check but, as always seems to be the case, there were some oddities while doing the test. To start off with what has been done since the engine last ran, Lloyd Elliot ported and decked the factory heads and did some chamber work to them but I never cc’d them to know their new volume. Different cam installed from the previous GMPP Hotcam, the Howards 180885-10 (225/225@.050, .532/.532 w/1.52:1 rockers, 110LSA). Finally I’m running Morel 7717 lifters than have been preloaded with 1 full turn for about .042” on 3/8” studs), spec says .060” standard preload. SCR calculated out to around 11.5:1, with above cam DCR was anywhere from 9.1:1 (Wallace) to 10:1 depending on the calculator. Now for my findings today. First off, I had to do the test cold but I did do it with wetted cylinder walls, and the lifters are not pumped up, but maybe now they are because of how many cranks it took just to build pressure. She didn’t want to build up cylinder pressure and the throttle is open. Each turn of the engine only raised the needle 5-10psi. The ring seal has never been an issue prior. Last, the final pressure once it stopped building was 230PSI!!!!! That seems to be an insane number, like I’m running a factory cam on that compression! Experience and knowledge would be appreciated with the info I have here as well as advice. Thanks y’all.

230 psi seems a lot if you only have 11 to 1 static compression. I have built a lot of 10 to 1 street engines with cams similar to what you are using, and I seldom get more than 150 psi cranking. The other thing I might mention is do you have a different compression gauge to double check the readings? You should max your reading in 3 or 4 pulses. Compression gauges have a check valve, in the hose usually, that holds the pressure so it can keep building a reading, these valves look like a valve stem core except that the spring is very weak, pieces of carbon and gunk can cause these valves to fail. I used to teach at a Technical College, and I would have 20 students running compression test at one time in the shop and every year at least one gauge would mess up, 99% of the time it was the check valves.

[SilverChicken]

F-Bird, this is a gen2 LT1. They start at 10.5:1 with god awful quench from the factory and a tiny cam and have no problem running on crap 87 octane fuel. Factory cranking compression is 185-200psi on these engines. With aluminum heads and reverse flow cooling these things break the rules of conventional carbureted, iron headed gen 1 small blocks as engineered. People regularly run these over 11:1 even on the factory cam. So 11.5:1 is high for sure, maybe the edge of streetable for this platform, but then there are modern engines that do the same without any super fancy technologies like variable cam timing or direct injection.

[Dave B]

That comp and short jabby cam, I would expect 225-230.
Ic is 39ish. I have ran 11.5 on street with 220 lbs . Is it ideal? Of course not but your results are dead on where I would expect them to be. You better have all sharp edges out of comb area and I would run colder plugs than normal. Watch timing around peak torque area.

[SilverChicken]

“230 psi seems a lot if you only have 11 to 1 static compression. I have built a lot of 10 to 1 street engines with cams similar to what you are using, and I seldom get more than 150 psi cranking. The other thing I might mention is do you have a different compression gauge to double check the readings? You should max your reading in 3 or 4 pulses. Compression gauges have a check valve, in the hose usually, that holds the pressure so it can keep building a reading, these valves look like a valve stem core except that the spring is very weak, pieces of carbon and gunk can cause these valves to fail. I used to teach at a Technical College, and I would have 20 students running compression test at one time in the shop and every year at least one gauge would mess up, 99% of the time it was the check valves.”

It was the valve. When fully open it seals off as if it was closed. But the cranking compression stayed the same. Unfortunately I don’t have another 300psi gauge to check it against. I was also surprised by that number when the gauge finally stopped moving. I expected 210-220psi, hoped for less.

[F-BIRD'88]

F-Bird, this is a gen2 LT1. They start at 10.5:1 with god awful quench from the factory and a tiny cam and have no problem running on crap 87 octane fuel. Factory cranking compression is 185-200psi on these engines. With aluminum heads and reverse flow cooling these things break the rules of conventional carbureted, iron headed gen 1 small blocks as engineered. People regularly run these over 11:1 even on the factory cam. So 11.5:1 is high for sure, maybe the edge of streetable for this platform, but then there are modern engines that do the same without any super fancy technologies like variable cam timing or direct injection.

When you do run these stock LT1 engines with just 87 octane gas the computer dials back the spark timing @ WOT to avoid detonation. You will no make full power if on 87 octane gas.
Again do not get all carried away on compression on 92 octane gas. People do this an claim that. Lots of bragging.
Most will never tell you that the engine ate itself in a short time.
Or ring seal quality went away in a short time. A fresh engine suddenly needs a re ring job in a short life because it was detonating.
Some people who attempt to then hone and re ring it find that the "freshened up engine) has evern MORE Blowby now as the Piston ring lands are now DISTORTED from high speed detonation.


Yes these engines have a great reverse cooling system. and computer control. Keepte real compression ratio under 11:1
(With good quench) and run it on approriate fuel octane. Forget the 87 octane BS.
You can make a TON of reliable power @ 10.5:1 cr. Make the engine process MORE AIR AND FUEL.

This engine as it is builtr if the CR is 11.5:1 won't last very long on pump gas. Use 100 octane unleaded if you want:
A: Maximum performance. B: a decent service life.
There is no free lunch.

ALL the factory SHow czr classes that rin these high compression ratio factory cars includiing the LS engine cxars run them on 98 to 100 octane unldeaded.. These cars cost a fortune and cost a furtune to rebuild. No body races them on pump gas.

[F-BIRD'88]

Variable valve timing and direct injection is not what saves the modern latest LS engines. Its the ability to sense detonation and retart timing on the fly. YUp when that occurs these engines ALL Make REDUCED in car engine power.
Again NOBODY road races these engines on pump gas. They run on 98-100 octane "Showroom stock class" fuel.
If yu want to race this 11.5:1 engine and have it last Use this fuel or blend your own using 92 pump gas blended with 110 octane unleaded racing gas as required.

I could see decking the block some to improve NET piston-head quench clearance (.040" ish Net) but to then shave down the heads was a big mistake if the chamber volume is now less than stock. You could revisit the cylinder heads starting wih accurate Chamber CC'ing. Make be able to then correct it with chamber volume "deshrouding" and pick up some flow too.
Get the real CR under 11:1. Tell your friends and competitors YA its 11.5:1 cr.!!
Let them all burn thru numberous engines trying to catch you.

[Stan Weiss]

Bore = 4"
Stroke = 3.48'
Rod = 5.7"
CR = 11.5:1
Cranking Compression = 230 psi
Temp = 60F
BP = 29.92 Inches Hg

DCR = 8.8539:1
IVC = 68.878

Stan

[cab0154]

Building for 11.5:1 on pump gas is a mistake for a street driven car. The operating conditions on the street in real world traffic are JUST TOO VARIABLE.
You can make it "run" as is but the timing will need to be compromized to keep it alive on the street on pump gas.
If you want to let the lobster loose at the track use 100 (or more) octane gasoline. ( max power ignition timing for best engine power)

interesting. i guess my 12:1 4.145 bore small block doesnt know that it isnt supposed to have pump gas in it. when i run it around on the street part/mid throttle, light load it is way safe on 93. some will, some wont. depends on your build, cam, chambers, cooling, timing curve, ect.

[Dave B]

F-Bird, this is a gen2 LT1. They start at 10.5:1 with god awful quench from the factory and a tiny cam and have no problem running on crap 87 octane fuel. Factory cranking compression is 185-200psi on these engines. With aluminum heads and reverse flow cooling these things break the rules of conventional carbureted, iron headed gen 1 small blocks as engineered. People regularly run these over 11:1 even on the factory cam. So 11.5:1 is high for sure, maybe the edge of streetable for this platform, but then there are modern engines that do the same without any super fancy technologies like variable cam timing or direct injection.

When you do run these stock LT1 engines with just 87 octane gas the computer dials back the spark timing @ WOT to avoid detonation. You will no make full power if on 87 octane gas.
Again do not get all carried away on compression on 92 octane gas. People do this an claim that. Lots of bragging.
Most will never tell you that the engine ate itself in a short time.
Or ring seal quality went away in a short time. A fresh engine suddenly needs a re ring job in a short life because it was detonating.
Some people who attempt to then hone and re ring it find that the "freshened up engine) has evern MORE Blowby now as the Piston ring lands are now DISTORTED from high speed detonation.


Yes these engines have a great reverse cooling system. and computer control. Keepte real compression ratio under 11:1
(With good quench) and run it on approriate fuel octane. Forget the 87 octane BS.
You can make a TON of reliable power @ 10.5:1 cr. Make the engine process MORE AIR AND FUEL.

This engine as it is builtr if the CR is 11.5:1 won't last very long on pump gas. Use 100 octane unleaded if you want:
A: Maximum performance. B: a decent service life.
There is no free lunch.

ALL the factory SHow czr classes that rin these high compression ratio factory cars includiing the LS engine cxars run them on 98 to 100 octane unldeaded.. These cars cost a fortune and cost a furtune to rebuild. No body races them on pump gas.

Have you ever tried this or do you just chatter? All kinds of vehicles run 11.5 to 1 comp on pump swill. We have 6 grain trucks from the 90s, BBC with that comp and 215 lbs cranking compression. When loaded we run the shit out of them. They are not short lived and have yrs and 1000s of miles on them. AGAIN do you build anything or parrot bs you read?

[Dave B]

I have a corvette, 355 that's 11.5 to 1 comp 208-215 lbs cyl pressure between 8 cyls. 6-71 blower and it has run on 89 pump gas since the 90s. 9 lbs of boost and a vertex mag locked at 33 degrees timing. Old Chevy cast 041 heads. So how does this work. Dcr is the most over rated ,out of place formula out there. It's as over rated as quench. Again f bird, do you have anything that has a performance engine ? Do you build any engines? Or do you parrot bs you hear or read?

[F-BIRD'88]

Building for 11.5:1 on pump gas is a mistake for a street driven car. The operating conditions on the street in real world traffic are JUST TOO VARIABLE.
You can make it "run" as is but the timing will need to be compromized to keep it alive on the street on pump gas.
If you want to let the lobster loose at the track use 100 (or more) octane gasoline. ( max power ignition timing for best engine power)

interesting. i guess my 12:1 4.145 bore small block doesnt know that it isnt supposed to have pump gas in it. when i run it around on the street part/mid throttle, light load it is way safe on 93. some will, some wont. depends on your build, cam, chambers, cooling, timing curve, ect.

As long as you never open the throttle.

[F-BIRD'88]

I have a corvette, 355 that's 11.5 to 1 comp 208-215 lbs cyl pressure between 8 cyls. 6-71 blower and it has run on 89 pump gas since the 90s. 9 lbs of boost and a vertex mag locked at 33 degrees timing. Old Chevy cast 041 heads. So how does this work. Dcr is the most over rated ,out of place formula out there. It's as over rated as quench. Again f bird, do you have anything that has a performance engine ? Do you build any engines? Or do you parrot bs you hear or read?

I don't use a "DCR FORMULA snd you are full of it. Been doing this since I was 13.