compression = horsepower query

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TimDart

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Giys, i've been reading many threads on here, but am wondering if thee is an easy answer to the following question.

Is there a ratio by which HP increases with increased compression if all other factors are equal e.g if a 360 is changed from 9.0 comp to 10.0 comp is there a direct effect on hp ?

I appreciate that there are many variable in making power, just wanted to know about this one:read:!
 
Yes...and no.
The increase will raise HP. Theres a given of approx. 3% per point, but this is subject to engine size, and everything on it like carb/cam/exhaust.

Stock will show so much, modified will show more, race preped will show even more.
 
Thanks guys, i can se where this thread will head!

I rebuilt my motor 2 years back with a modest power increase. Thing is, ever since i've been kicking myself for not going for KB107s and zero deck to get good compression. I really can't/won't spend a lot on heads as i had a fully rebuilt, slightly ported, valve jobbed smog heads put together for the motor.
So....if i am unwilling to splash $s on Eddy heads etc, I guess i may as well stick with what I have.
Or, to put it another way, probably not worth spending cash on block work, new pistons etc if I dont want to alter the heads?
 
Keep in mind that the NASCAR truck series was limited to a 9.0-1 ratio and made over 700 HP. There intake and head system would basicly be a full tilt operation on a street engine. There cams were roller and valves big for what they did.

If you have even a modest 8.0-1 ratio now, a chamber reduction on the curent heads or new heads will help in the power department.
If you invest in new heads, and your compresion is low (9.0-1 or lower) I would try and stick with an iron head with reduced head cc's.

Port and equip accordingly. A small chambered W-2 is an idea. Now, does anyone know off the top of there head if the W-2's still come in a 55 cc chamber?
That would be a very good option.

You could go from 8.93 to 10.70 with a 20cc reduction.
8.43 to 9.97
8.0-1 to 9.34
 
Thanks Rumble, one of my problems is that i dont know the exact comp ratio. I used Sealed Power pistons (H405cp), which as i understand it, provide more compm than the original pistons, but not a whole lot more. Beyond that, the heads were lightly skimmed and the cc is 68 as I recall. Head gasket (compressed) .027.
Here in the UK heads are limited in their availability, which in turn limits my options
 
Pistons 'in the hole'

IMG_0148.JPG
 
You can keep an eye on your valve clearance and maybe take a little more off the heads and deck combined with a .019 gasket?
 
it looks like you have a good foundation so suggest giving us a rundown on your engine build and maybe get some feedback on some changes that will give you the best "bang" for the buck so to speak. then its up to you.
 
Hey Tim. I did notice your in the UK. I'll keep that in mind. Your local Chysler/Dodge dealer will carry MoPar stuff. Or I should say at least order it for you.

Measure how far down the hole they are and with the known head cc amount and gasket dimentions, you/we can come up with the exact ratio.
They do not look to far down in there, but the reliefs on both sides have a cc amount that should be added in.

See if you can find something out at the Federal mougal site about it. Or measure it yourself if you can.
 
Guys,
the pics are from 2 years back when i built the motor, its fully assembled currently and runs well. I checked the Federal Mogul site last night and could not find the dimensions i need (the valve relief cut out x cc) on the pistons.

Having read about 30+ posts on 'what cam' etc, I suspect i could replace the current XE262 cam for the 268 version, which would put some more power in.
I'm also changing the stock convertor for a 2800 stall.

I dont really want to cut the heads too much or i'm into cutting the inlet manifold as well.

You know how this works, Winter comes around and we /I start to think about rebuilds more power etc...limited as ever by budget/practicality etc..

longone, didn't know there was a .019 gasket, who supplies those?
 
Hmm, did some more reading. should have fitted the H116C (sealed power) pistons as they provide between 9.5-9.8 compression with my set up.:eek:ops:
Our fuel is rated at min 95 and up to 97 is available so higher compressions are possible.
 
From what little I know, the stock high compression early 340's had 10.5 to 1 and were out of the deck about .026. The 8.5 360 I tore down was about 1/4 of an inch down with dished pistons. So i would guess you could be somewhere around 9.5 now, as you look to be about .040 below deck. This is just an uneducated guess, and my .02
walt:sleepy1:
 
longone, didn't know there was a .019 gasket, who supplies those?[/QUOTE]

The cometic gaskets in copper can be had as thin as .005, check with
FBO. This brings up another issue with the head gaskets available for small blocks. Unless you have a set made for your engine with it`s specific bore,then you`re probably getting the "one size fits all" gasket which is made to accomodate a 340 bored .060 over. There`s a lot to gain by finding/getting a head gasket made to fit the exact bore you have.
 
If it was mine, I'd cut the heads. With the explosion of aluminum heads on the market, iron heads aren't really worth much except to restorers, so there really isn't much demand for them. Just try to sell a set of iron heads today. Mopar performance sells the .019 head gaskets. Your machine shop should be able to cut the intake side of the head so you can still use a standard intake manifold. Go for it. BTW there was an article about 3 years ago in I believe Car Craft where they tested a big block with 8.5:1 to 11:1 compression by stacking copper head gaskets.
 
I remember that test Don. That was a good illistration (sp) on how things work.

Street demon is right. Go for a big fat flow curve on the heads.
 
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