Upping the compression?
- Thread starter Vito93
- Start date
There was a huge thread about compression ratios over on Modfords.
It depends on what type of supercharging you will be doing. An Eaton, Whipple big or small, centrifugal, or turbo.
People were getting good results from 9.0 all the way up to 12.0
It also depends on what you will be using for fuel. Here in California we only have 91 octane and no E-85 so our engines will be at the low end of the ratios.
People that are running E-85 full time have gotten away with 11.0 and a Whipple 3.4
It depends on what type of supercharging you will be doing. An Eaton, Whipple big or small, centrifugal, or turbo.
People were getting good results from 9.0 all the way up to 12.0
It also depends on what you will be using for fuel. Here in California we only have 91 octane and no E-85 so our engines will be at the low end of the ratios.
People that are running E-85 full time have gotten away with 11.0 and a Whipple 3.4
Like its been said above, it all depends on the type of power adder and fuel being used. If its gonna be a PD or twin screw blower then compression really wont matter since youll have the low end power no matter what. If its a turbo or centrifigal youll want to bump the compression to help with the low end off boost throttle response. Just keep in mind fueling will be a determining factor for safety.
I agree with above. Upping the compression closes the margin of error a little bit. But if your running huge boost does it really matter? Usually higher compression requires less boost to get good hp. But that is not the only factor. Heads, cams, intake, and fuel all play a important role too.
Pretty much this. If you have good fuel available, I would up the compression no matter the power adder. We run E85 with a 4.0L Whipple and 10.5:1 compression on our 2013 GT500, and basically the same combo on our 2010 GT500 but 8.5:1 compression and the 2013 makes over 150 more RWHP with 5-6 psi less boost.
I agree
Raise it to 10.5:1 and run E85 or race gas :banana:
Not something I would ever do but that's just me.
Raise it to 10.5:1 and run E85 or race gas :banana:
Not something I would ever do but that's just me.
Could you show us the dyno charts?
"good fuel available" equates to hard to find and expensive race gas or E85. This is the one detail that seems to get downplayed or even outright ignored when this topic comes up and is very misleading. If race fuel is not available/affordable upping the compression to this level is a bad idea. The tables on that engine would be reversed if it had to rely on pump gas.
I'll see if we have anything available.
Even if 91 octane is all that's available, I would still raise the compression at least a half point from stock (.5:1 increase).
A friend raised his compression on his ported engine to 9:1. He got it re-tuned and he gained less than 10hp.
Aren't stock motors like 8.8:1? So he raised it .2?
Anabolic
Banned
i agree with everything the first poster said BUT would never go to 12. That would be well.. stupid.
9.5 is a good ratio. i replaced mine with 8.5
9.5 is a good ratio. i replaced mine with 8.5
I would aim to keep the dynamic compression similar to stock for a street car.
So decide what cams you want to run and where the centerline will be and adjust the compression from there.
So decide what cams you want to run and where the centerline will be and adjust the compression from there.
I think its a bad idea using dynamic compression when selecting piston dish volume.
This is an FI engine which makes its compression ratio "dynamic" way more than the camshaft could ever hope. I would use static compression ratio when selecting pistons which is a good "fixed" baseline number to work with rather than the valves opening and closing that can't account for the actual volume of charge being packed into the cylinders at any given time.
well even if race gas isnt anywhere around you couldnt you just do a water meth kit on 91 pump gas with lets say 9.5:1 and call it a day? from what i read water meth kits raise the octane in your fuel a long with better intake temps ect....
The use of meth injection as a crutch to allow and engine to survive on low octane fuel is a recipe for disaster should the system fail or the reservoir runs dry. It is good method of insurance should an injector go lean or a bad batch of gas. Best to build and tune an engine to run without it than add the meth injection after.
:rockon:
Isn't the 2013 a 5.8 and the 2010 a 5.4? Not entirely a fair comparison although I agree results would be similar. High compression and less boost means less work turning over that blower.
To each their own right? Anything can fail on your car. I don't know about calling it a crutch. For those of us with only 91 readily available it's a great alternative. Put standard fail safes in place like you would for any power adder so if the meth fails your car shuts down (in boost) then I don't see the problem. My car can withstand water/meth failure or a fuel pump failure to name a couple with no harm.
Just like when you put on nitrous you put in some standard fail safes. My nitrous only sprays if WOT, clutch is not depressed, within a certain RPM (window switch), A/F is not leaning beyond a certain point.
I think it's the guys who just bolt on a water/meth system with no monitors or fail safes in place and then have a mishap and blow their motor that give it a bad name.
Can your car run safely at max boost without meth injection?
Depends on timing. There's a certain line I cross where the answer becomes no. I believe it's around the 20 degree mark but I'm still working that out as discussed in the other thread a few days ago.
Depending on how you tune your car and set it up one of two things could go bad when water/meth fails, or both. You could go to lean or you could start knocking (detonation) or even both. You'd want a fail safe for both possibilities. Something to stop you from going lean and something to stop you from detonation.
I don't think I did a good job asking the question :smmon:
Better put: Would you build and tune your car to the point where the engine could not survive without meth injection (a simple "yes' or "no' will do)?
Having read your post all these years and how methodical you are I don't see you doing this and I think for good reason. I posted this in response to Vito93 about adding injection to allow the CR to be raised and "call it a day". You know as well as I do such a simplistic assessment can lead people down the wrong path often with disastrous results. There is a lot more to consider here which is what we are discussion and I'm sure the people who are reading this are looking at it from a different perspective.
Upping the compression on a blown application seems to me more pervasive in the NE Cobra site where most of the guys don't want to change out their pistons and are advocating high compression. Some of them were loath to admit after boxing them in a corner that without special high octane fuel this was not a good direction to take and was actually counterproductive to their goals. I have never seen a 10:1 Cobra engine running pump gas make much more than 425 hp where an 8.5:1 engine will make close to 600 as a matter of routine.
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