304 rebuild - Contoured piston

[jauringer]

Hello,

I've recently picked up a spare engine for my Scout II and plan to rebuild it from the ground up for both experience and to have a good engine with a known history to go in the Scout one day.

This will be my first rebuild. I've become pretty familiar with the sv engine and done quite a bit of work on one but not a full rebuild. My goal is to have a nice, solid performer in the end. Nothing crazy, safely bump compression and run a mild cam.

I've done quite a bit of reading and the flat top piston set up is covered very well. I've got a pretty good grasp on calculating cr and the process for zero decking.

My question is, how does the process change for calculating deck height when working with contoured pistons. There are variables here that seem to add to the equation.

1. Dome volume: I haven't been able to find any information regarding this number in a factory piston, or how to calculate it myself.
2. Deck clearance: same as above, I haven't been able to find any info on this.

Any help would be appreciated.

Thanks,
jason

 

[Robert Kenney]

Good questions. The first thing you need it the compression height datum used for the published data. In your case and for a non decompressed piston the ch is 2.2 for a contoured slug. For an flat top "e" piston it is 1.891.
So try and get close so we can determine the datum start with what I know to be true on a stock deck 304 e. The piston is .025-.030 in the hole. 2.2-1.891 is .309. Subtract the in hole dim and you get from .284 - .279. Find the point that measures
.279 above the deck on the contoured piston and you should be able to id the datum for your 304...

The dome volume is measured the same as if it were a pocket. You cc it. But for a dome the value will be a + number in a combustion chamber or fixed volume calculation.

Assuming you want to measure the dome volume with the engine assembled.

First seal up the gaps between the piston and engine cylinder with grease. You don't want the cc'ing fluid to leak by.

Find a cylinder (clear preferably) that will fit around the piston and not so big as to overlap any deck hole.
Scribe a line on it (like 1 inch up the inside) and calculate the cylinder volume if filled to the scribe Mark.

Now place the cylinder over the piston and seal it to the deck with grease.

Using a graduated fluid measuring device fill the cylinder on the deck to the scribed line and see what volume it takes. Subtract that from the calculated cylinder volume and that is your dome volume.........

 

[jauringer]

Robert, I'm happy to finlay have the pleasure of speaking with you. I appreciate all the information you provide to all fellow binder drivers around the country.

Thanks for tackling my questions as you are the reason I posted here. I've actually been playing with water, measuring cups and my piston this afternoon so I had just about everything ready when I saw you're post. I felt much more comfortable with this method you posted for obtaining a volume as my results thus far have varied greatly.

I did just as you stated above and was able to obtain a (edit)* 25cc *difference in filling a cylinder with a flat bottom vs filling it over my piston @ tdc. I'm not sure that sounds correct to me so I'll need to do this a few times to verify my results

I also cleaned and cc's my combustion chambers last night. Most came in @ 83cc's with a couple in the 84-85 range. These particular heads do no have the recessed (when looking at it from the combustion side) spark plug hole so they're not as big as I was anticipating.

I was also able to find a point along the piston @ .279.in the pic below you can see the black Mark showing my .279 Mark. Flat raised portion of the piston is slightly shorter but just as you start following it up the ridge there, it hits the Mark. ~.350 being the highest point in which the cylinder is raised out of it's bore.

 

[Robert Kenney]

Glad it helped. There are more ways then one to accomplish what I described but this way give's true volume intrusion @ tdc then others while you have the engine together.

For decking using a pop up piston, clay is used to acheive colision potential areas. Ideal quench might be possible in some areas with teh contoured piston if you use heads for the contoured piston (non "e") engines. Be sure to map using clay, the valve to piston clearance. The chamber intrusion you will see can greatly limit your valve lift maximums @ tdc and just after. Rembmer that the intake valve will catch up (close in on) with the piston just after tdc on the intake stroke. Be sure to map between tdc and 15* after before decking.

 

[jauringer]

That makes sense, however, it's sounding like decking the block with these pistons might not be something I want to shoot for being so new at this. The last thing I need is to spend $$ just to hear my piston colliding with the heads and or valves.

I also caught something you mention that I didn't consider. The contours in the piston don't give an equal quench distance so not all of the piston will be in an ideal position?

I might need to reconsider just cleaning the surfaces up and go with a mild cam or look at converting to a "e" style head and flat top pistons.

Unless of course there's a way to incorporate flat tops with these heads? From what I've read, it looks like there's roughly a 13cc difference in the combustion chambers on a 304 and it's not possible.

Thanks a bunch.

Jason

 

[Robert Kenney]

The contoured design leaves more to investigate. The typical way to check it to use modeling clay and place little balls in a grid around the piston crown of #1 and with the head and gasket in place and a working valve train and super light valve springs, turn the engine through two complete turns. Then measure the clay height. This will give the info on what amount of decking you can do.

The short of it is that the e or flat top piston/head combo is a better setup from a efficiency and torque aspect but the contoured head and open combustion chamber flows better but is considered a non quench design.

If you are willing and able the e setup is a better choice.
Zero deck (piston flush with deck @tdc) and composite head gaskets will run well and can net better fuel economy and drivability.

 

[jauringer]

good questions. The first thing you need it the compression height datum used for the published data. In your case and for a non decompressed piston the ch is 2.2 for a contoured slug. For an flat top "e" piston it is 1.891.
So try and get close so we can determine the datum start with what I know to be true on a stock deck 304 e. The piston is .025-.030 in the hole. 2.2-1.891 is .309. Subtract the in hole dim and you get from .284 - .279. Find the point that measures
.279 above the deck on the contoured piston and you should be able to id the datum for your 304...

Quote]

One more question for you Robert. When calculating cr, should this .279 deck height be negative since it's out of the hole?


Thanks,
jason

 

[Robert Kenney]

After you measure the volume intrusion (say for reference sake 20cc's) subtract the 20 cc's from the combustion chamber volume. So yep it is a negative volume. If you cc the combustion chamber and get 75cc's it will net 55cc's after factoring the piston intrusion volume if it were 20cc's.

 

[jauringer]

Alright, I cleared my head and put a little more data together. (I've decided not to give up on these contoured pistons just yet) the whole story is here Btw.

I kept getting varying numbers for my dome volume. I'm dealing with a relatively large volume of water so 20cc's added to the cylinder doesn't make a huge difference to my reference line. I figure since I had deck height now I could back into that number using all stock data.

Head volume: 84cc'c
piston dome volume: ? Variable
gasket thickness: .020
gasket bore: 4.0
bore: 3.875
deck height -.279
stroke: 3.218

oe cr: 8.19:1

backing into the numbers gives me a dome volume of 52.37 cc's.

So now I've got all the variables to start playing with the numbers.

I've got to consider the composite head gasket @ .040 as well as the deck height to obtain a ~9.0 cr.

With those two variable in mind, I'm getting a change in deck clearance from .279 to .350. Basically a .071" of material removal.

Max height of the piston (the very top of the ridge) was .350 so it would now be .421.

Also, my bore gauge came in so I had a chance to spec out the cylinders. There is no more than .00250 max taper and .00125 max out of round. My initial issue was wear as it was .02425 - .02600 which threw me for a loop! I dug through my pistons to find one that wasn't cratered, cleaned the top and noticed it has 20 over IH pistons! So, these cylinders are bored.

Taking that into considerations, I'm still looking at cylinder wear from .00425-.00600 so it still looks as though I'll need to open them up more to obtain a recommended piston clearance of .0035 which would then change my numbers above a little more.

So using everything above plus an estimated .030 over bore
(3.905") my new deck height is .340 @ 9.1:1 cr, shaving .061 off the deck. If I stayed with the original .071" off the deck cr would be @ 9.3:1.

I hope I explained this well enough to make sense? Does this all sound correct?

Thanks,
jason

 

[Robert Kenney]

Jason,
you know a guess is just that a guess. You need to successfully measure the intrusion. It is really cut and dry simple. Piston must be at tdc. Cc the dome like was instructed. Use that and. .040 hg @ 9cc of fixed volume. Must factor bore/over bore in the swept volume value. I feel you should bore it as .005 clearance is to much. After a hone you are bigger than that. .030 over bore min 3.905 is close.

 

[jauringer]

Thanks Robert,

ok so backing into the calc is no good. No problem, I'm going to refine that method for obtaining dome volume untill I can start getting a consistant number.

It's good to get some verification on the cylinder wear as well. I'll get it bored out and honed.

Thank you,
jason

 

[jauringer]

Alright mr. Kenny,

I took a trip to the hardware store and picked up a pvc coupling with an od of 4.5. I had been working with way too much water so as I said above my numbers where all over the place. This time I cut it down to an inch and used some plexiglas to cover the top to create a chamber. I also used a smaller syringe, which went slow but it was much more accurate.

So, I finally have some real numbers. They're surprising to me but I know they're accurate. I ran the test 3 times with numbers @ 6.5, 7,& 8 cc's difference. The interesting part to me was it actually took more water to fill the the piston than it did the flat surface. I didn't realize the dished portion was that deep.

Piston @ tdc w/ dial indicator.

You were correct about backing into the number as this is no where near what that calculation gave me.

Since I had the water back out, I re-tested the heads too just to make certain I had done it correctly. They were all the same. 83-85 cc's.

I guess my next question is now what? I have all the numbers now to calculate cr but coming up to something like 14.9:1 cr. I must be doing something wrong.

Here's what I using:

head volume: 85cc
piston volume: 7cc
gasket thickness:.040"
gasket bore: 4"
bore: 3.905"
deck clearance. -.297"
stroke: 3.21875"

here's silvolite's cr calculator I'm using. It didn't ask for the hg fixed volume you mentioned.

I appreciate your time,
jason