I am trying to calculate the compression ratio for a 399 stroker I'm building. I am using a 3.85 stroke crank with 5.956 length rods and 302 flat top pistons. The box lists a size point of 1.89 for these pistons, which I am thinking is the same thing as compression height. I need to be able to convert the listed 302 compression height to actual compression height in the stroker application, assuming normal deck height. Also, does size point=compression height?

 

what heads are you going to use?

 

Well I guess that would depend on what kind of compression ratio I was gonna get, which is one of the reasons for this question. Unless someone who knows can answer this question, I can't calculate the compression ratio with any degree of accuracy, and ultimately, completely accurately, untill the crank and at least one rod is installed in the block to verify deck height, but if I could do that, it would be pointless to ask this question. I know this much, if the 302 piston would develop 9:1 in a 302 it would be over 10:1 (maybe 11:1) in the stroker.

 

If that number is the comp. height of those pistons, they wont work. your piston will be .268 out of the hole. stock comp. height of a 302 piston is 1.6 with flat top pistons your comp. ratio will be in the neighborhood of 10.5 (give or take a little), even having 69cc chambers. That also depends on how much you mill of the deck.

 

Will you be using stock 351W rods? I believe the length you wrote down (5.956") is for them. Check the specifications of the stroker crankshaft, some use stock 351W style rod journals while others use special 2.1" or other size.

I have no idea what a "size point" is.

 

there are about a thousand calculators online....

if you use a flat top 302 piston you will be way over anything streetable....

look at some of the kits online... most have a 22cc dish 302 piston....to get a 9to1 with 60cc heads....

 

I went back and checked the box to make sure I wrote down the right #, and I did. Is it possible size point is the distance at which the pistons should be measured for boring the block? I have a general idea what CR I'll be running, I also am familiar with the formula and all the information necessary to calculate CR. What I need to know is if the listed compression height for a 302, (lets say it's 1.605) is the same number I'll punch in for compression height in the stroker, or will it change? That's why I listed the stroke and rod length. The way I see it, I'm adding .175 to the stroke but only dropping .114 CH, (if CH is the ONLY diff in 302 and 351w pistons.) These are Speed-Pro H273CP's, so they SHOULD be 1.605 302 pistons. Scott9320- I'm hoping the block hasn't been decked, if not, I'll be running closer to 10:1. Beanscoot- No, but they will be stock length. HemiEater- My set-up is based on a kit. Do you think 10:1 or so is too much for a hot streeter?

 

No it's a 302 piston...1.605.... if it was not it wouldn't be a 302 piston...

do the math

1/2 the stroke 3.85/2 = 1.925
plus the rod 5.956
plus the comp heigth (302 piston)1.605= 9.485

351W block 9.50

 

doug, the comp. height of a piston never changes regardless of the stroke of the crankshaft.

 

HemiEater- I had to read the 302 piston thing twice, but I think I understand what your saying- if they are 302 pistons they HAVE to be 1.605 CH and vise versa. So a 1.719 is a 351w and that is only difference. Assuming stock deck height of 9.503, I get .18 below deck, right? I think I get it now, the simple answer to my original question would be listed 302 CH = actual CH in the stroker, there is no difference in ratio involved. So if I replaced a stock 302 rod with one .114 shorter and used a 351w piston the top of the piston would be exactly the same distance below the deck. I should be able to calculate a fairly accurate anticipated CR now. I'll have to verify deck height and actual head CC's before I can get totally accurate. I'm sorry I chunked the old piston/rod assemblies now, because I could have used them to measure what my piston height would be before boring my block, knowing what I know now. Thanks a lot guys, I'm gonna crunch some numbers and I'll get back to you. P.S. I guess size point does not= CH, it might be a measurement location , because you're not supposed to mike the skirts on these coated pistons, but that's just my guess.

 

A couple more points to consider:

Early 351W blocks (before 1972 or so?) had a 9.480" deck height.

Replacement cast oversize pistons were commonly "destroked", ie the compression height was reduced .020" or similar to account for the increased cylinder volume to retain stock compression ratio. I think (and hope!) this practice has been phased out, though.

 

to build a 393 stroker you have to use the 351W rod.... and a 302 piston... they don't make a shorter rod that will work.

and it will put the piston .018 down the hole at tdc.. not .18

 

I have gone here twice and I don't see the last thread I posted listed, so if I'm repeating myself, I appologize in advance.Beanscoot- I am unsure of the exact year as well, but it's somewhere around that, mine is a '79. I sure hope these pistons aren't .020 taller, I like to keep my pistons and cylinder heads seperated. All the H273's I've ever seen were listed at 1.605 CH and thanks to HemiEater and Scott9320 I am sure that is what these are, because while the box didn't list CH, it did identify these pistons as 302. HemiEater- I was talking about a 302 motor with a 351w piston in a hypothetical situation, I'm sure they don't make a rod for that either. It was late , I meant .018. Kemicalburns, I can kinda answer your question now, if you even care- Looks like I'm probably going with some stock iron ported and polished heads that I estimate to be 70cc combustion chambers, most likely with 1.94/1.60 SS swirl polished valves installed. Many Thanks to ALL who responded to my inquiries, you have been a big help. The final anticipated CR # on this 399 stroker: 10.072197:1. I wasn't trying to be a smart *** when I asked HemiEater if he thought 10:1 was too much for a hot streeter, while I suspect that is not the # he anticipated, I would still like his opinion and any others out there on what CR to target, as now is the time to make those adjustments.

 

9 to 1 should do it.......

 

The H273CP is listed for 8.6:1 with a 64CC on a 302, My 399 also uses those same pistons and from what I came up with, it oughtta be 10.8:1 with the 60CC Aluminum heads im gonna run. GT-40's are also 60CC, where exactly are you finding iron heads with 70CC chambers?