I'm trying to do CR calcs with 64cc heads on a stock roller 351w and I need to know the stock piston to deck clearance and the amount of dome/dish/valve relief (+/- cc's) with stock flat top pistons...

Anybody know these specs??

 

Stock CR is 8.8:1 so you can work backwords in DD if you like. As far as I know the spec is zero deck height and the pistons have something like a 10cc dish...

 

So, the pistons end up flush with the deck on a 351w at the top of its stroke??

 

Or you could add half the stroke (1.750") to the compression height of the piston (1.774"), add the rod length (5.956") and subtract this from the deck height (9.503").

Which gives .023"

 

I get ~9.7:1 static CR with a 4.000 bore 3.5" stroke 4.1" head gasket bore .042" thk. 0 deck clearance and 10cc dished pistons using the calculators linked below

Compression Ratio (CR) Calculator
RSR Static Compression Ratio Calculator

 

Beanscoot, thank you. (DUH!!, I can't believe I didn't think of that).

Conanski, I don't have my chilton around but I thought the 351's were 9.2:1, and the 302s 8.8:1...? That what I get on those CR calculators with the same numbers except a .023 piston-deck clearance

 

Either of you guys know what the volume is for the flat top pistons (right?) in a stock late 351w? (IE 4cc for valve reliefs)

I'm getting my afr 185s this weekend, they have HUGE combustion chambers (79cc), I'm just wondering where I'd be on compression with these if I were able to use them with stock pistons (even though I'm pretty sure I cannot because of the 2.02 valves)

Trying to decide whether to run these N/A or just save 'em for when I put on the supercharger.

E7te's SUCK, the intake ports are too small to be used with a 351w gt40 lower, so I'm debating whether or not to change out or fly cut the stock pistons so I can use the 185s and gt40 setup. I would only try to do it if I could reasonably cheaply fix the piston-valve clearance issue, and keep the compression up around 8.8:1-9.5:1 with the 79cc chambers.

 

A stock motor is about 9:1 CR with 64cc heads so there's no way you will be anywhere close to that the 79c heads on a stock bottom end. I have some stock 351 pistons in the shop I'll try to get a quick measurement and get back to you.

 

Thanks!

I wasn't really sure what 15cc more per chamber would mean to my compression but I didn't think it be much below 8:1. I was thinking maybe I could bump my compression back up a bit with smaller headgasket bores (4.06") and .021" compressed thickness as opposed to the stock .039" thk. / 4.1 bore gaskets. Even then, it might not be enough.

 

It's easy to find out with a CR calculator, just input some numbers to get 9:1 with 64cc heads and 5.8l displacement and then change the chamber volume to 79cc.

 

You need domed pistons to get any kind of compression out of those heads, which is a pretty expensive stop gap if you're planning on forced induction.

Don't fool with thinner gaskets, you need at least .05" between the piston and head.

Save the heads.

 

The tool in DesktopDyno says 7.86:1 with those heads.. that's way low.

 

Ok so playing with the numbers to yield 9.1:1 CR, I get 11.7cc of dish for the stock piston volume if I assume 4" bore 3.5" stroke 4.1 gasket bore .039 gasket thickness and .023" deck clearance. Does that seem right?

(assuming all the same as above) Using the 79cc AFRs, it works out to 7.9:1 (matches what Conanski got) with a stock headgasket. A set of KB hyper domed pistons (KB-136) with only +1cc of dome gets me back up to 8.9:1, though making the switch could definitely be cost prohibitive.

What, other than rings, would I need to buy in order to change pistons? Would I need new rod bearings? Could i reuse my OE pins?

I'm just trying to consider my options.

 

All this makes me wonder what your reasons were for those buying heads. There's nothing wrong with putting a blower on a stock bottom end you just have to keep boost levels moderate with the cast pistons and stock rods. At this point unless you are going to rebuild the bottom end and add the blower you might as well use the stock heads and leave it NA.. even if you have to take the grinder to the ports and open them up a little to match the intake... E7's can use a little hoggin anyway.

 

Well a couple of things came into play in the decision.
*The high cost of getting iron gt40s fully redone.
*I don't often see pedestal mount afr 185s up for sale. Especially at less than $1100.
*Kenne Bell rates the superchager I have as efficient from 8-16psi so I figured the low compression ratio the big chambers will give me would be ideal for the higher boost levels if I can tune it correctly to run that much boost safely.

I'm most likely running the rebuilt roller 351w in NA form for a while and slowly getting familiar with tuning on that first. Once I get bored with that/decide I want more, I'll probably throw in some forged pistons&rods that'll allow me to use the afr 185s and slowly creep up the boost to maybe 12psi (if I can get the belt not to slip!). I'm going to try and keep it below 5500rpm for the most part and just try and see what can be done in terms of the torque curve from 2-4.5k.

Hopefully by the time I've gotten that far, I'll have more money to throw at it, and I can start thinkin' bout stroked aftermarket blocks & larger displacement twin screws and 8 rib belts and such. I like to dream big?