Highest CR on 91 octane
Highest CR on 91 octane What is the higher CR you can run on 91 octane. I'm running a daily driver so I'm not going to push anything too far. If you're going to throw out numbers please back it up and have a number accurate at least to the tenths place. I know this question has been asked before but I can't find them.
Ive heard so many things on the subject its hard to get a head around it all. DCR is more important than SCR in the scheme of things, and Ive been given different numbers by different people about whats safe and whats not. For my build I settled on 10.9:1 for my 402, but was told could have went as much as .2 or .3 higher since the larger cam needed for the 402 will bleed off more compression resulting in a safer DCR.
Either way, from what Ive heard, each full compression point is worth about 2%-4% increase, so the 10 or so hp I left on the table in my case was worth the peace of mind. But of course the higher the compression the "peppier" the car is going to be as well, and its basically free power, but still Its a DD so didnt want to have to rely on a **** on tune in order to run my car safely on 91 in 110degree temps.
Just my .02
Either way, from what Ive heard, each full compression point is worth about 2%-4% increase, so the 10 or so hp I left on the table in my case was worth the peace of mind. But of course the higher the compression the "peppier" the car is going to be as well, and its basically free power, but still Its a DD so didnt want to have to rely on a **** on tune in order to run my car safely on 91 in 110degree temps.
Just my .02
TECH Resident
Joined: Dec 2004
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From: K-W, Ontario
DCR is more important than SCR in the scheme of things.
Static compression means nothing without* dynamic compression when figuring
fuel type.
Comrpession can't start until the vavles close (in this case, intake valve).
You'll need to know the IVC - intake valve closing for the camshaft you
select when you design your setup.
Last edited by Adrenaline_Z; Jul 19, 2010 at 07:43 AM. Reason: *changed with to without
But for building an engine up, I need to calculate SCR first since I'm working out the heads, block, piston, and rods combination I'm running now. I'll figure out DCR when I get a cam. I think I'm right on thinking this way but feel free to correct me.
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From: K-W, Ontario
If you started out with 13:1 static compression, you have limited yourself
to the intake valve timing events that you can safely dial in before DCR
becomes an issue.
You'll need to know your power goals, what range of RPM you would like
to make power safely and then forecast the IVC (intake valve closing) and
static compression to make it all work.
9/10 there are proven combinations that will be close enough that you
don't have to think much about it (for generic street/strip motors anyway).
If you're asking to know how to arrive at figuring out a specific static
compression ratio, then you should approach the build using all of the parameters.
My opinion is: I would estimate how much power I wanted to make and
what approximate peak. From there, I'd select a duration, lntake center
and then optimize the exhaust lobe using LCA and duration.
Once knowing the ICL and IVC, the DCR value is figured with the
complimenting SCR value to use with the fuel type.
Of course that's my method and it lends itself to carte blanche with camshaft
specs as opposed to limiting the options after the rotating assembly has been assembled.
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Here's my problem, I'm kinda stuck with a certain setup and the compression is high. Right now I think it will be 11.8 or something near there. I'm wondering if I can get a different gasket to help me out. The original owner had 10.9 SCR with 67cc heads and I have 59cc heads. Therefore, I think the compression is going to be sky high. All I need to know is can I get by with altering the gasket because its going to be expensive to change out pistons or rods. I know the optimal situation is to probably pick the cam specs and then build up the short block from that but I don't have the money to do that. so I just need that one magic number please. I can alter everything after that and I am at sea level if that helps.
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Verdad Gallardo TECH Resident
Joined: Dec 2004
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From: K-W, Ontario
Actually sea level doesn't help in your case...because it improves cylinder
filling and increases the chances of combustion problems with a high compression
ratio.
That's the science and theory, but if you can set up the motor with a larger
duration intake lobe and maybe retard the cam phasing you could end up with a well
working motor.
With 11.8 Static, the cam will require a later intake closing to keep the dynamic
compression within a safe window. This will ultimately shift the torque
peak up higher (depending on what the rest of the valve timing is like).
I would suggest a custom grind for you which will allow you to move the
valve timing to work with your high compression and still have a good street
mannered engine.
You can use a thicker head gasket, but don't go too crazy. Excessive 'quench
height' is counter productive to efficiency and preventing detonation.
My advice to you is: measure the compression values (heads are off right?).
Get the chambers cc'd. Rent/borrow a dial and figure out the piston to
deck clearance.
If you're not sure, ask a friend how to use the tools. Once you have all
the relevant numbers, you can insert a value for compressed gasket thickness.
Keeping the gasket thickness to approximately 0.045" in your equation is pretty good
if the rotating assembly uses stock components. 0.050-0.055" is acceptable,
but not ideal. Anything over that is less efficient and could lead to detonation.
Your combo is not bad overall; if you can cut half a point from the static
numbers, there's still hope for street setup that will kick-butt when you
hammer on it.
P.S. Consider engine cooling mods to keep temps a bit cooler.
filling and increases the chances of combustion problems with a high compression
ratio.
That's the science and theory, but if you can set up the motor with a larger
duration intake lobe and maybe retard the cam phasing you could end up with a well
working motor.
With 11.8 Static, the cam will require a later intake closing to keep the dynamic
compression within a safe window. This will ultimately shift the torque
peak up higher (depending on what the rest of the valve timing is like).
I would suggest a custom grind for you which will allow you to move the
valve timing to work with your high compression and still have a good street
mannered engine.
You can use a thicker head gasket, but don't go too crazy. Excessive 'quench
height' is counter productive to efficiency and preventing detonation.
My advice to you is: measure the compression values (heads are off right?).
Get the chambers cc'd. Rent/borrow a dial and figure out the piston to
deck clearance.
If you're not sure, ask a friend how to use the tools. Once you have all
the relevant numbers, you can insert a value for compressed gasket thickness.
Keeping the gasket thickness to approximately 0.045" in your equation is pretty good
if the rotating assembly uses stock components. 0.050-0.055" is acceptable,
but not ideal. Anything over that is less efficient and could lead to detonation.
Your combo is not bad overall; if you can cut half a point from the static
numbers, there's still hope for street setup that will kick-butt when you
hammer on it.
P.S. Consider engine cooling mods to keep temps a bit cooler.
12:1 SCR and 8.7:1 DCR here and running fine on 91 octane pump swill. Quench is .040", pistons are coated flat tops. In my experience, this is fairly close to the ragged edge for 91 fuel.
