Best compression for 4.0?

[2jxj]

Hi guys,got a 96 sport,that ive been havin some fun with in the local gymkanas.Its pretty mild,with just a good set of headers,2.5 inch system,63 mm tb,and spacer,but goes well enough to be competitve in the standard class.Im really wanting to extract some more power,but dont have the cash to stroke it. Ive got a late model intake and injectors to go on,but want to give it some compression at the same time.Will it be ok just to deck the head 40 thou? I believe this will raise it to around 9.6 to 1. Im ok with running premium gas at 98 octane.Does this sound ok,or am i missinformed about my final compression ratio,could i go even further with the head or would that be on the limit for pump gas.Its still got to be reliable,and not ping and not run too hot. Any input would be great...
Thanks

[Cheromaniac]

I suggest you deck 20 thou off the block instead and use a Mopar/Victor 0.043" head gasket. That'll reduce the deck clearance to near zero, reduce the quench height to 0.045", and raise the static CR from 8.8 to ~9.35:1.

[SilverXJ]

x2. .040" is a lot to take off the head and probably won't be much use on a stroker once you go that route.

[2jxj]

Hmm,I wasn't really planning on pulling the motor,I have a spare motor that I will be stroking on the side. What are the cons if any,of decking the head this much,I have a victor reinz,hg I will be using with it.I really want to give it a kick along,and thought a bump in compression,would be a good help.. Will it give me the 9.6 ratio that I'.m after? Thoughts?

[CobraMarty]

... and raise the static CR from 8.8 to ~9.35:1.

To only raise the compression 0.5 points, what's the point? Will only add about 2%HP. A lot of work for little gain.

[Cheromaniac]

To only raise the compression 0.5 points, what's the point? Will only add about 2%HP. A lot of work for little gain.

Yes but it's also an opportunity to port the head while it's off and that would improve things quite a bit.

[Retlaw01XJ]

Changing your gearing could give it more low end punch. What's in it now?
Cam swap could help too. An intake valve that closes earlier than stock will create more cylinder pressure, effectively raising the Dynamic CR (DCR).

[2jxj]

Its currently stock,3.55 gears,however I usaully race in low range,which is still good for 100 km an hour with 235"s.So im happy with gearing,just want it tp pull harder.. Has anyone out there just decked the head for some compression ? Surely some one has...

[racing89]

.110 " off my head, 12 years running. Stud kit highly recommended.

[2jxj]

Well,that's sounds like a lot! Do you know what the compression ends up at? Do you still run on pump or race gas only? Any other mods needed to make this work reliably?

[racing89]

Pump gas with 10.75 :1 and positive deck clearance (although it responds well to 100 oct.) 2.5l pushrods (approx. .100 shorter). I understand what your are trying to accomplish, as planeing the heck out of a $50 head and buying a gasket are inexpensive compared to other proposed performance upgrades and as Dino stated ,its a good time to do some grinding n polishing n port matching cuz thats where alot of potential power is on a stock 4.0l NOT saying this is best for your application , but on my highly modified 306ci race engine I havent had any problems.

[2jxj]

Well,as usuall everybody has a different idea,am thinkint at this stage ill just deck it about 60 thou and,and whack it back on,hopefully this will give a good whack of compression without been too radical...

[racing89]

It will work

[BADASYJ]

I zeroed the deck, took 30 from the head and it gave me roughly 10.75-11 to 1 comp. I run at least 93 oct and have not had n issue yet. Knock on wood. But I am looking for more. Get a valve job while you got the head off.
Hey 89 what are the cc's on your combustion chamber with that mutch milled from the head?
How much lift does your cam have?

[2jxj]

Hmm,that sort of compression would surely need 98 Oct,to avoid pinging,are you your sure your compression is that high?