Highest compression
Intermediate
Joined: Nov 2005
Posts: 28
Likes: 0
From: South Carolina
You are right about it having to be below 11:1. I'm building a 10.5:1 engine and I was told it should run on pump gas, but i would imagine with a good build and tuning then 11:1 could be possible. Worst come to worst just add some octane booster.
Race Director
Joined: Jan 2002
Posts: 12,264
Likes: 88
From: O'Fallon Missouri
What the highest compression to run depends on the engine, LT1/L98/LT5, heads al/cast, camshaft, ect ,ect. There is no one formula for every engine. I've seen some LTx/LT5's running 12+:1 compression on pump gas.
Safety Car
Joined: Apr 2005
Posts: 3,624
Likes: 431
From: Dale City VA
STL94 is correct.
There is alot more to it than just picking a SCR (Static Compression ratio) number and thinking your safe.
Do a search on DCR (Dynamic Compression Ratio). Learn everything you can about it - but don't think it's the entire answer either.
They are both guidelines to building engines for a specific purpose and are effected by enough variables to fill up the internet.
10.5 SCR is a good guideline for pump gas considering:
1) The timing events of MOST off the shelf camshafts.
2) Combustion chamber desgin and heat disipation of MOST Alum Cyl heads.
3) The burn rate of SOME 93 octane unleaded fuel with those variables above in play.
Lets not forget engine temp, air,fuel charge temp, vaporization of the air/fuel charge, the amount of load being placed on the engine, the amount of swirl gernerated by the intake port, the amount of spark advance...
Like I said none of them are the ONE answer, but all of them together are.
Will
There is alot more to it than just picking a SCR (Static Compression ratio) number and thinking your safe.
Do a search on DCR (Dynamic Compression Ratio). Learn everything you can about it - but don't think it's the entire answer either.
They are both guidelines to building engines for a specific purpose and are effected by enough variables to fill up the internet.
10.5 SCR is a good guideline for pump gas considering:
1) The timing events of MOST off the shelf camshafts.
2) Combustion chamber desgin and heat disipation of MOST Alum Cyl heads.
3) The burn rate of SOME 93 octane unleaded fuel with those variables above in play.
Lets not forget engine temp, air,fuel charge temp, vaporization of the air/fuel charge, the amount of load being placed on the engine, the amount of swirl gernerated by the intake port, the amount of spark advance...
Like I said none of them are the ONE answer, but all of them together are.
Will
Thread Starter
Team Owner
Joined: Oct 2004
Posts: 35,065
Likes: 0
From: Bergen County, NJ Democrats, doing for the country what they did for Michigan
STL94 is correct.
There is alot more to it than just picking a SCR (Static Compression ratio) number and thinking your safe.
Do a search on DCR (Dynamic Compression Ratio). Learn everything you can about it - but don't think it's the entire answer either.
They are both guidelines to building engines for a specific purpose and are effected by enough variables to fill up the internet.
10.5 SCR is a good guideline for pump gas considering:
1) The timing events of MOST off the shelf camshafts.
2) Combustion chamber desgin and heat disipation of MOST Alum Cyl heads.
3) The burn rate of SOME 93 octane unleaded fuel with those variables above in play.
Lets not forget engine temp, air,fuel charge temp, vaporization of the air/fuel charge, the amount of load being placed on the engine, the amount of swirl gernerated by the intake port, the amount of spark advance...
Like I said none of them are the ONE answer, but all of them together are.
Will
There is alot more to it than just picking a SCR (Static Compression ratio) number and thinking your safe.
Do a search on DCR (Dynamic Compression Ratio). Learn everything you can about it - but don't think it's the entire answer either.
They are both guidelines to building engines for a specific purpose and are effected by enough variables to fill up the internet.
10.5 SCR is a good guideline for pump gas considering:
1) The timing events of MOST off the shelf camshafts.
2) Combustion chamber desgin and heat disipation of MOST Alum Cyl heads.
3) The burn rate of SOME 93 octane unleaded fuel with those variables above in play.
Lets not forget engine temp, air,fuel charge temp, vaporization of the air/fuel charge, the amount of load being placed on the engine, the amount of swirl gernerated by the intake port, the amount of spark advance...
Like I said none of them are the ONE answer, but all of them together are.
Will
I am looking at new head gaskets. Stock are .051 new ones are .015
Stock compression is listed as 10.25
I am thinking that going with the .015 will bring me up too high. That .039 may be a better choice.
That's why I ask.
Safety Car
Joined: Apr 2005
Posts: 3,624
Likes: 431
From: Dale City VA
GM's advertised SCR has been proven on quite a few different engines at least 1/2 a point less than advertised.
If you own a burrett (or know some one who does) I suggest you cc your heads, you piston valve relifs and dish, then caluclate your true SCR first. I would not go changing things around until I knew first hand what I had to start with.
Once you know your actual true SCR, you can adjust it it with the head gaskets of your choice.
I have been following the head gasket thread. Why are you limiting yourself to Fel Pro? Victor, ROL, GM (McCord), Cometic and Corteco also make some very good head gaskets that could give you more possibilities in your search for the perfect combination.
Will
If you own a burrett (or know some one who does) I suggest you cc your heads, you piston valve relifs and dish, then caluclate your true SCR first. I would not go changing things around until I knew first hand what I had to start with.
Once you know your actual true SCR, you can adjust it it with the head gaskets of your choice.
I have been following the head gasket thread. Why are you limiting yourself to Fel Pro? Victor, ROL, GM (McCord), Cometic and Corteco also make some very good head gaskets that could give you more possibilities in your search for the perfect combination.
Will
Thread Starter
Team Owner
Joined: Oct 2004
Posts: 35,065
Likes: 0
From: Bergen County, NJ Democrats, doing for the country what they did for Michigan
GM's advertised SCR has been proven on quite a few different engines at least 1/2 a point less than advertised.
If you own a burrett (or know some one who does) I suggest you cc your heads, you piston valve relifs and dish, then caluclate your true SCR first. I would not go changing things around until I knew first hand what I had to start with.
Once you know your actual true SCR, you can adjust it it with the head gaskets of your choice.
I have been following the head gasket thread. Why are you limiting yourself to Fel Pro? Victor, ROL, GM (McCord), Cometic and Corteco also make some very good head gaskets that could give you more possibilities in your search for the perfect combination.
Will
If you own a burrett (or know some one who does) I suggest you cc your heads, you piston valve relifs and dish, then caluclate your true SCR first. I would not go changing things around until I knew first hand what I had to start with.
Once you know your actual true SCR, you can adjust it it with the head gaskets of your choice.
I have been following the head gasket thread. Why are you limiting yourself to Fel Pro? Victor, ROL, GM (McCord), Cometic and Corteco also make some very good head gaskets that could give you more possibilities in your search for the perfect combination.
Will
I haven't purchased anything yet and I'm open to suggestions.
The heads are already off, so getting a preliminary SCR is out of the question at this piont.
Safety Car
Joined: Apr 2005
Posts: 3,624
Likes: 431
From: Dale City VA
Actually now that the heads are off, this is the perfect time to take a burret and CC everything. You will know, down to the decimal, precisely what your compression ratio was and will be.
Will
Will
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Team Owner
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From: Athens AL
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It is true that the measurement of the chamber cc on '113s varies, so the SCR may have been different than 10.25 initially.
Thread Starter
Team Owner
Joined: Oct 2004
Posts: 35,065
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From: Bergen County, NJ Democrats, doing for the country what they did for Michigan
So I have to assume since they are brand new heads, which are being milled to a specific spec, they will meet that spec.
Weather the compression is 10.25, 10.5, 10.75, 11, 9.75 is not terribly important to me.
What is important is knowing that in a theoretical vacuum assuming control conditions of a 58cc chamber, will the .015 gasket put me at borderline pump gas?
Safety Car
Joined: Apr 2005
Posts: 3,624
Likes: 431
From: Dale City VA
Jesup, to determin that I need some info.
1. What pistons do you have?
Dish w/2 or 4 eyebrows (if so whats the cc of the dish) or Flat top w/2 or 4 eyebrows.
How far in the hole are the pistons (deck clearance)?
2. Camshaft?
I need the seat timing for the intake lobe (advertised Duration).
I need the Intake Centerline as installed in your engine (or the intake valve closing angle).
3. Headgaskets?
I already have the ones your considering.
4. Combustion Chamber?
Your Brodix's are going to be cut to 58cc right?
Will
1. What pistons do you have?
Dish w/2 or 4 eyebrows (if so whats the cc of the dish) or Flat top w/2 or 4 eyebrows.
How far in the hole are the pistons (deck clearance)?
2. Camshaft?
I need the seat timing for the intake lobe (advertised Duration).
I need the Intake Centerline as installed in your engine (or the intake valve closing angle).
3. Headgaskets?
I already have the ones your considering.
4. Combustion Chamber?
Your Brodix's are going to be cut to 58cc right?
Will
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
You don't need most of that. Certainly not all the factors in #1. You know #3 and #4. The heads are being milled to match the ccs of the stockers. So the only change here is the head gaskets. If a .051" gasket makes a 10.25:1 compression ratio, then a 1094 gasket makes a 11.25:1 compression ratio, and a 1010 gasket makes a 11.00:1 ratio.
To figure the DCR all you need from #2 is the intake valve closing point.
RACE ON!!!
To figure the DCR all you need from #2 is the intake valve closing point.
RACE ON!!!
Thread Starter
Team Owner
Joined: Oct 2004
Posts: 35,065
Likes: 0
From: Bergen County, NJ Democrats, doing for the country what they did for Michigan
How far in the hole are the pistons (deck clearance)?
2. Camshaft?
I need the seat timing for the intake lobe (advertised Duration).
I need the Intake Centerline as installed in your engine (or the intake valve closing angle).
I need the seat timing for the intake lobe (advertised Duration).
I need the Intake Centerline as installed in your engine (or the intake valve closing angle).
3. Headgaskets?
I already have the ones your considering.
I already have the ones your considering.
4. Combustion Chamber?
Your Brodix's are going to be cut to 58cc right?
Thread Starter
Team Owner
Joined: Oct 2004
Posts: 35,065
Likes: 0
From: Bergen County, NJ Democrats, doing for the country what they did for Michigan
You don't need most of that. Certainly not all the factors in #1. You know #3 and #4. The heads are being milled to match the ccs of the stockers. So the only change here is the head gaskets. If a .051" gasket makes a 10.25:1 compression ratio, then a 1094 gasket makes a 11.25:1 compression ratio, and a 1010 gasket makes a 11.00:1 ratio.
To figure the DCR all you need from #2 is the intake valve closing point.
RACE ON!!!
To figure the DCR all you need from #2 is the intake valve closing point.
RACE ON!!!
However, the 1010 seems like a "safe bet".
CFI, what would YOU do? I'm thinking that at 11:1 I'm still on pump gas with no real problems.
Safety Car
Joined: Apr 2005
Posts: 3,624
Likes: 431
From: Dale City VA
You don't need most of that. Certainly not all the factors in #1. You know #3 and #4. The heads are being milled to match the ccs of the stockers. So the only change here is the head gaskets. If a .051" gasket makes a 10.25:1 compression ratio, then a 1094 gasket makes a 11.25:1 compression ratio, and a 1010 gasket makes a 11.00:1 ratio.
To figure the DCR all you need from #2 is the intake valve closing point.
RACE ON!!!
To figure the DCR all you need from #2 is the intake valve closing point.
RACE ON!!!
As for DCR you are smart enough to know that if the cam is installed advanced or retarded from the intended Intake Centerline you change the intake valve closing angle. Thats why I asked for either the seat timing AND installed intake centerline (so I can calculate the closing angle) OR just the valve closing angle.
Why all the static? I'm just trying to verify what he has and what he will have. He's probally safe (as the factory is slighly optimistic on their SCR advertisement) but I don't like to guess.
Will
Yes the deck clearance in
Race Director
Joined: Sep 2000
Posts: 17,298
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From: The Top of Utah
Originally Posted by rklessdriver
Why all the static? I'm just trying to verify what he has and what he will have. He's probally safe (as the factory is slighly optimistic on their SCR advertisement) but I don't like to guess.
Will
Will
Originally Posted by rklessdriver
YOUR SCR calculations are based on the assumption that his SCR actually was 10.25 to begin with. It very well may have been, maybe it was close or maybe it was off in left field.
RACE ON!!!