How much will 70cc heads raise my compression? Also how much lift?

-
C

chrisd426

Well-Known Member
Joined
Jun 17, 2015
Messages
65
Reaction score
2
Location
mississippi
Ok as the topic states, by putting 70cc closed chamber heads onto a basically stock piston 400 with the slugs .100" down the hole, flat tops with no valve reliefs, how much will I raise my compression? Saying the stock heads had around a 90cc chamber, and a .020 head gasket and around 8:1 compression, what would my compression be with a chamber that's 20cc less and say a .025 cometic gakset? Or .039 felpro gasket? I tried to do a compression calculator online but I don't know the dome height and all that of the stock pistons.

Now my second question how much lift can one get away with on a camshaft with no valve reliefs in the piston? Also around what duration and lsa is good for a hot street/strip 400? Heads flow around [email protected] lift. Holley 830cfm double pumper, 3300 stall, 3.91 rear gear, alum heads. Car has been lightened up a little.
What brand of cam would you prefer? I got a buddy that works at a parts store so I can get an off the shelf cam such as a comp, lunati, crane for about half price.
This is just a temporary engine until I get my 440 block redone. Or I may end up putting a 440 crank in this 400 after it goes, who knows. Haven't decided yet. Really just wanna get it back out there whooping some ford's and chevys soon. Lol I can think about all that other engine later lol.

Another question what kind of rpm can a low deck handle? Say cast crank, cast pistons, factory forged rods, arp main studs, oil pickup opened up to 1/2" soze, hugh volume pump, 6 or 7qt pan, (using the 7qt if it fits, if not I'm going to use my 6qt hemi pan) roller rockers, heads setup for external oiling. Some say they spin stock 383 and 400s to 6500, and even 7,000 and even higher, I honestly don't believe the part about 7,000 on a stock bottom end. In a way 6500 sounds fishy to me, I know when I had my cast crank 440 I was told not to go much above 6500 and I had kb pistons and a balance job on it.
 
I kinda like to know the answer to this too,i have indy sr heads that are 65cc,.039 head gasket,and flat tops with valve reliefs,and .050 in the hole,my car runs 5.80s
 
Impossible to answer accurately without knowing your present compression ratio or measuring, BUT it will be somewhere north of 9:1 if I had to "guess".
 
chrisd426 said:
Stock 400 bottom end so wasn't the compression around 8:1 on them?
Click to expand...

That's what Chrysler advertised it as. Their machining tolerances were really bad though. Deck heights were tall and uneven, combustion chambers were big. That means compression was down into the 7s. So, you don't know what you have unless you measure.
 
Measure what? As I said pistons are .100 down the hole, which from what I've read is about average for the 400. I'm just trying to get an idea how much would 20cc less of combustion chamber volume would raise my compression? A ratio? 2? 3? 20? Lol.. I guess anything would be better than the crappy 7.8:1 or so they actually had from factory. I'd be happy with 9.5 or more, which thats around what my guess would be but I'm no expert and have no idea what the dimensions are of the stock pistons as far as dome volume, etc to plug into a compression calculator, Google hasn't answered the question much either. I know there's guys out there that's put closed chamber heads onto these low compression boat anchors before.

Also when loozing for a camshaft selection, from what I've heard it's easy to over cam a low deck, like you don't want to go much over a .235 duration, but is that still the case with high flowing cylinder heads? Also what's the most lift you can get away with, not having valve reliefs in the pistons? I've read in some forums of guys using cams like the .528 purple shaft with 1.6 rockers which comes out to .560 on stock piston 440s and 400s before but Idk if I want to risk going that high... it would be nice to though, I would just like to have an idea before I purchase a cam, lifters, springs, pushrods to match, and find out the cam won't work, and then stuck with a cam I can't use or return being it has been installed.
Thanks for your patience and forgive my ignorance, my experience with the particular engine, and combo is limited.
 
If you don't know what to measure, maybe you should take up another hobby.
 
With the 70cc head , pistons down .100 @ TDC and with no valve reliefs with a standard .039 Felpro head gasket, I get 8.874:1 which is really about all you want with a street engine with iron heads with no quench.
 
With the RSR static compression calculator, .039 felpro gasket I got 8.9:1

with a .020" shim gasket I get 9.26:1 - if it all fits I'd go this route

If this is a budget build I wouldn't waste a ton of money on expensive head gaskets - the shim's are $25, and the Mopar's are $63 (<- $80 Felpro's)
 
This is temporary until funds allow me to stroke the bottom end. Would it be safe to run steel shim head gaskets with aluminum heads? And felpro head gaskets are 40 bucks thru 440 source. But I already have a set of them
 
RustyRatRod said:
With the 70cc head , pistons down .100 @ TDC and with no valve reliefs with a standard .039 Felpro head gasket, I get 8.874:1 which is really about all you want with a street engine with iron heads with no quench.
Click to expand...

Dang, that's lower than I was expecting, considering 20cc less on the combustion chambers... stock wit the 90cc chambers was a hair under 8:1. I'm also not far above sea level, maybe 100ft above, and when I go to new Orleans to run I'll be about 40ft below sea level lol. Also it's aluminum heads. What I meant by guys using 915 heads I was using as a comparison. Which now I've heard and read on forums just by going from the open chamber stock heads to a closed chamber 915 or 516 and a little milling would bring 9:1 or more, and I know my chambers are smaller than that, then again what some claim by saying something like that don't mean it's true... and on roadkill with their crop duster project they put the eddy rpms onto a low compression 440 and they have 84cc or 88cc chambers and they said the compression was like 8.4:1 I think.
 
zac_F71 said:
With the RSR static compression calculator, .039 felpro gasket I got 8.9:1

with a .020" shim gasket I get 9.26:1 - if it all fits I'd go this route

If this is a budget build I wouldn't waste a ton of money on expensive head gaskets - the shim's are $25, and the Mopar's are $63 (<- $80 Felpro's)
Click to expand...

I couldn't find a calculator online that didn't ask for piston dome height, etc. And I don't know all those numbers all I know is the bore stroke and how far. They are in the hole, which really measures .90 something but I rounded to .100 just to give it the benefit of the doubt. Do you have a link to this calculator?
 
a flat top with no valve reliefs is 0 dome volume/height. if U use the shim gaskets spray both sides with Permatex copper coat
 
I remember a while back seeing multi layer steel head gaskets that were like .025 compressed and were around the price of regular head gaskets I don't remember if it was mopar that mad them or who? Since then I'm having trouble finding them, I've never messed with cometic head gaskets but aren't they a multi layer steel?

Would it be safe running steel shim head gaskets on aluminum heads? They wouldn't warp my heads or anything? Say if it gets a little hot? The block looks like it had been resurfaced at one time, engine looks fairly fresh I think someone must have did a rebuild with stock Pistons or replacement Pistons.

I'm running stage 6 heads that I got a pretty good deal on, they had been done by David porter, and at first I was going to run some iron heads with the steel shim gaskets and shave them, then I managed to get a little more $$ for my budget and was going to get a set of stealths or sidewinder heads but I ran across these for what a set of stealths would cost and I figured in the long run being they already have better flow numbers than the stealths, and luckily I'm running a low deck block so header fittment won't be as much of an issue with a high deck block with raised ports. they would be a lot better head for when I stroke it, which I'll probably either send them back to porter and have more extensive port work done and maybe even bigger valves installed.
 
If that block looks ok with no ridge and still has a cross hatch with little to no taper I would look into some real Pistons. Maybe you can get away with a rehone.
 
Yeah bores are perfect, no ridge, and the mich checks everything good and the crank and all looks like they did a balance job on it. It's still stock bore size I'm thinking it was a low mileage rv motor and they did somewhat of a build to it they had a 7qt milodon oil pan, tapped the oil pickup to 1/2" hemi size, .509 purple shaft cam, Holley street dominator intake, 906 heads that had been shaved a good bit. The Pistons appear to be forged from what I could tell looking at them with the oil pan off...
 
And when it comes to time for a tear down and replacing Pistons I'm going to go ahead and make it a stroker is my idea, right now I'm going to get it together as is to get the car up and going, then I'm gonna build the bottom end in the future, stroker, girdle, cross bolt main caps, and nitrous. I'm doing a little at a time main goal now is to get the car running, get the chassis figured out and save up the money to do the bottom end. At first I was just going to run some ported iron heads then some budget opened up to allow a little more so it was between a bottom end rebuild, or heads, being I plan on a stroker later on, and the bottom end was fine i didn't see much sense in rebuilding a good bottom end for now just for basically a piston change while heads could help the compression some and really help flow a lot better than any factory heads would and can be used when I build the stroker. And I got a deal on the heads I couldn't pass up
 
Heck I could actually still see hone marks in the cylinders the guy I got it from had pulled it out of a charger that he bought as a project car and he wanted to put a 383 back in it so that's what he did and I picked the 400 up for 275 bucks and a 2 hour drive off craigslist so the motor hadn't been ran much at all I don't think
 
8.8:1 is a very good ratio for a street engine. Plenty of power can be made.
 
If it looks that good I would talk to a machinist and see if you could just slide in a set of KB Pistons for around $300. I ran close chamber 915 with flat tops 10.3 to 1 ran great on 93 with a nice .039 quench
 
I remember reading a while back where these guys took a motor home 440, put on a good set of ported 516 heads, 509 cam, intake and headers, and played around with different parts and made close to 500 hp without going above 8:1 compression, said the compression could've been more but they did a lot of valve deshrouding so I'm hoping with only 40 less cubes and a better head and maybe a better cam I'm hoping I'll get at least 450 and its also going to be a lot lighter than an iron head 440, stock vs stock I think the 400 is what like 40lbs less, then the alum heads will take off another 40 also the 400s ability to spin more rpm and shorter stroke, lighter rod, crank, and piston, I guess we'll see, and if she's still a little slow I do have 2 sets of solenoids, maybe put a 75 shot on it lol
 
EL5DEMON340 said:
If it looks that good I would talk to a machinist and see if you could just slide in a set of KB Pistons for around $300. I ran close chamber 915 with flat tops 10.3 to 1 ran great on 93 with a nice .039 quench
Click to expand...

Most kbs I've found were around $469 or more thru summit then another $300 in bearings and rings $550 for a balance job just for a little bit more compression. Would it really be worth it? I was actually thinking spending 80 bucks and get a little shaved off the heads and as small as the combustion chambers already are on the stage 6 heads even shaving .010 or .020 or .030 would reduce the cc a decent bit and still leave plenty of meat for future milling.
 
You must log in or register to reply here.
-

Back
Top