Bumping up compression ratio

[73cudakid]

Im planning on bumping up the c.r. on a 400 and was reading the mopar B-RB engines guide. Its says if i cut the heads 1/16" then the c.r. would be bumped up to almost 10:1 with a stock bottom end. I did my calculations and taking off that much from the heads would make the heads roughly 75cc (based on a 452 head guessing 90ccc) and with the pistons down in the hole .1175" (think thats what it is) then my compression ratio is only 8.5:1. Can anybody else confirm this? Thanks in advance.

[Chryco Psycho]

sounds right ot me why not use a set of the 516 or 915 heads & mill those down , the killer is the .110 down the hole on the piston , you reall need to change them to make decent compression

[73cudakid]

I would change the pistons, but i have a set of nos pistons, rings & rods with a good crank. Plus one of my buddies works for a machine shop (not an engine shop thought) so i think i can get night shift decking, wink wink, aka free. I think Im gonna shave the heads and intake and have him zero deck the block for me.

[Chryco Psycho]

I do not believe you can zero deck the block , by the time you remove over .100 the deck will not be thick enough , also I would mill the intake face of the head Not the intake itself so any intake can be used

[moper]

Excessive milling causes other expense and issues of it's own. The best way is pistons. Otehrwise, stick on closed chamber heads, run the steel shim, and dont worry about the static ratio. Get a cam that works like you want.

[oldkimmer]

           If u get ur crank offset groung .060 u will gain 1.2 points comp + gain a few more cubes  bcause u increase stroke x2 so u have  an increase of .120 stroke... u should rebalance the assambley....Lots of ppl think going .060 weakens the crank, but u r slowing down the bearing speed which takes stress out of the rotating assambley...kim...

[ntstlgl1970]

  Another thing, taking that much off the head or block could cause rocker geometry problems too. I think replacing the pistons or going to a stock iron closed chamber head would be the simplest and most cost effective solution.

[cudabob496]

Just bumping the CR is not going to be much of a power increase, compared to a better cam, better breathing, etc.

[nivvy]

Just bumping the CR is not going to be much of a power increase, compared to a better cam, better breathing, etc.

I disagree... every point in compression is appx. a 25hp gain!  and a nice cam helps also 

[cudabob496]

I went to a site called Performancetrends.com. They have a program for computing engine performance you can down load for free. Anyway, they said a 1 point increase in compression ration gives a 4% increase in power. So for a 400 horse engine, thats 16 hp, and for a 600 horse engine thats 24 hp.

A person can get the same increase in power just by going to cold air induction (ram air box). I put one on my Cuda, and it is supposed to give 1% increase in power for every 10 degree drop in inlet air to the carb. So, I ram cold air from the front of the car, and say its 50 degrees colder outside than under the hood, that gives a 5 % power increase, and on a 600 horse car, that's 30 hp. I also get a mini supercharger affect from the design I have, as the air pressure to the carb is increased.

I agree that increasing CR can help, but it will effect so many other things, and if your engine isn't matched to the CR, like in the cam department, you may do more harm than good. Even though you bump CR 1 point, if you don't have adequate cam duration, your peak cylinder pressure may go way up, and cause detonation, which is a major power robber. Milling the head can also mess up valve train geomitry, etc. so, increasing CR is good, provided everything else in the engine is designed accordingly.

[bb71challenger]

The problem is what Chryco stated, pistons too far down in the hole. I had a set of 906 heads milled .050 and the intake sides milled .062 to keep it all square for an intake and even with some light porting(MP porting templates), new 2.14/1.84 stainless valves, .020 steel shim head gaskets and a nice cam the engine did not really wake up much for my taste. The next time I break that engine open it will be to at least put some pistons in there and most likely to stroke it out to a 451 or 474. Pistons in the hole defeat quench and I found out the hard way that unless you get the pistons right you are kind of pissing in the wind for any sort of a serious power build. P.S. I kept my pistons because it was a new engine with forged but stock compression pistons. When I first factored the compression it was a weak suck 7.6:1 

I now have a set of 516 heads now and I wish I had found them before I dumped all that money milling a set of open heads. You end up with the same problem of an open head but more compression. Now I am going to have to do it all over again to get the advantage of the closed chamber heads because the 516's have such tiny exhaust valves