Inliners International Forums General Discussion Hi Performance Help to Compression and Piston issue

Need some advise on these issues

I'm rebuilding and spicing up a Chevy 292. I got all parts and advise from Clifford. I have learned after, that they advise totally different than the gurus.

What I was told to do:
mill head to 63cc chambers
O-deck the block
1.94/1.50 valves.
.030 bored

The rest of the setup is:
2 x 32/36 Webers
264 Clifford cam
Alloy pistons with a 16cc chamber

All the machine work is done.
I just measured the chambers in the head to be 66cc and the pistons to have a chamber on 16 cc.

Is this in your eyes a too high compression?

Another problem is that the #1 piston top is .011 over block (it does not flush the deck) the #6 piston does flush the deck perfectly.
#2-5 all is a little over the deck hight.

Should I mill the pistons so they all flush the deck to get an even compression or does it not matter?

I'm NOT going to race the engine, but use it in my 1950 Chevy PU. I'm going to use it 3-5000 miles a year (summer)

Hope you have time and want to comment my little issues..

Thanks.

Yes, that is way too high a compression unless your planning to run high octane racing gas or additives. It is around 11:1, and i'm guessing from your comments you are wanting something more in the 8-1/2-9:1 compression range. The stock 292 pistons have around a 27cc dish in them and with a 72cc head this will put you right in the compression range you want. As for the pistons sticking above the deck surface, I would trim them to make them all consistent at zero deck height.

I would also run the 1.6 on the exhaust. I would also check the pistons ( from skirt to top ) and make sure they are the same and If so I'd be taking a close look at the Crank. Because I wouldn't think they would be that Much difference on the pistons Above the deck.

Some incoherent thoughts follow:

Maybe the deck was milled at an angle by accident.
Maybe the block was line bored at some point, and ended up crooked. Not sure how someone could screw it up like this.
Also thought maybe you had mismatched pistons.
Check the bearings too and wrist pins for play.

Your compression ratio is around 10.3:1 with zero deck, not 11.1 (you must add the head gasket crushed thickness to your deck height, which is usually around .040").
That's still too high for what you are building the engine for, and that MILD CAM. If you built a hotter engine with a much bigger cam, your compression would work fine on pump gas even.


With the bigger 27cc piston dish your compression ratio would be a more reasonable 9.3:1 and that would probably be ok with high octane and a little bigger street cam. I set mine to about 9.5:1 with a bigger cam than yours, but I haven't finished the build yet. I am prepared to commit to using high octane pump gas only.
I don't build engines to run the low octane stuff. The cost difference in fuel is recovered by higher efficiency, more power extracted form the same amount of fuel because you can compress it more.

Here's a calculator for trying out different numbers: http://johnmaherracing.com/calculators/engine-calculator/

to lower the compression ratio further, you now have a wonderful opportunity to optimize the combustion chamber by unshrouding the valves by removing material from the "tub". I would not hog it out wider, just deeper and open up the area around the valves so the mixture has somewhere to go once it passes the valve.
You could probably also dig the piston dish deeper some, without risking anything, considering your mild build and intended use. I would try to see if you can mirror the combustion chamber shape on the piston dish if possible, so the dish isn't wider than the chamber tub unnecessarily.

One of the advantages of what you now have is the zero deck, it alone allows for higher compression ratio, everything else being equal.

I was using the measurements he provided and even taking into consideration his positive deck clearance. Also, many calculators don't allow for the actual head gasket volume and only use the thickness to calculate the compression. This is often misleading because most head gasket bores are not round, such as in a BBC, or a Flathead Ford and many others. So taken all those factors into account, my calculations came to 10.78:1.

Oh yea I just went with zero deck (plus head gasket) which was/is his goal. Forgot to mention that. Before shaving the pistons to set the deck heights even to zero across all of them, I would spend a moment trying to figure out what is causing the difference. Can you think of any other ill effects if the deck was milled at a slight angle? It's late, my head doesn't work right anymore... if it ever does :P

Actually you were spot on for some of the causes for the discrepancy, but his readings aren't uncommon. You also have the factory machining tolerance for the rod length to factor in. It is +/- .005", so that in itself could account for the variance. Also the stroke of the crank has a +/- tolerance of a few thousandths, not to mention if it had ever been reground before and was off a little by whoever ground it. So, a lot of little factors adding up to still be in spec by GM's guidelines, but not so much when you try to tweak the engine a little better for performance purposes.

ahh so maybe that explains why mine are all level with the deck top, since I had the crank reground and the block milled.... Maybe the crank grinder made sure the journals were ground to the same plane /stroke, so to speak? I'm no machinist I don't know how they do it. When I took the engine apart, I found two journals undersize and the rest untouched... After I had my junk machined I didn't have this height difference. I didn't think to measure it before teardown. My pistons are a couple years old, the flat top ones with 4 useless (to me) valve pockets. Forgot what they were or where I got them from. The car has not been on the road since 2011..... ugh.

THANK you so much for your inputs..

The only thing comes through my mind is, why is it I'm adviced to do what I have done. I mean there was no doubt in the way I got told to do my block and head.
What I'm asking is, they must have tried it since they tell me (and on thier homepage) to do it that way.

Another thing which make me sad is that I have posted a lot of money in this project - got it all shipped to Denmark (Europe) paid taxes etc. and now it seems like I'm ending up with an engine ready to junk.

On one hand I feel like an idiot only asking one guy (company) and then ad a little unluck with the machine work (maybe)

Option one:
Sand/mill the top of the pistons so they flush the deck - put the engine together - done deal.

What kind of engine do I then have?
Unrealiable?
Not working at all?
Cracking block/head?
Anything else??

Option two is:
Buy new Pistons with a bigger dish, work the head chamber bigger again

What kind of engine do I have?
Even more expensive!
More realiable?
Do I get value for the extra money?

On the other hand I want to believe that the setup I have is the right and the work done the right way - with people done it before.

I am in doubt??

With pistons machined to where they all have Zero deck, you still have too high compression for your CAM and FUEL. Your cam is very mild, and it means it does not "lose" compression through open valves. BIG cams keep the valves open longer, and some compression pressure is lost, so to compensate for the loss, the compression RATIO needs to be higher, just to maintain the same actual cylinder pressure.

So your engine is not junk. It's just a combination of parts that can't work together. You basically have two choices (Extremely simplified): Lower the compression ratio to match the cam, or change the cam to a much bigger one to match the compression ratio.

Those two pieces of your puzzle need to match.

Because you already have the cam ($$$) and modifying OTHER parts of the engine to support the much bigger other cam is expensive, and you are not building a race engine, it means you really only have one option: reduce the compression ratio.

Now you need to figure out HOW, and it may need a combination of several methods:

1. piston dish needs to be bigger. With the MILD engine you are planning to build, you could make the existing piston dishes larger. (new pistons = $$$)

2. remove material from the combustion chamber to make the volume larger. This is beneficial too if done right, improving flow around the "tight side" of the valves, also known as "unshrouding".

3. Use a thicker head gasket. Your "deck height" with a normal gasket is about .040" (1mm) which is fine for a performance engine, a little on the tight side. You could use a gasket that is maybe .050" (1.3mm) thick. This you would need to have made for you, using a stock gasket as a template. You could have it made of copper, it would also be reusable. Much cheaper than new pistons or cam or changing your whole engine design.

4. maybe not a real option, but changing the cam timing also affects cylinder pressure. You would need an offset cam or crank gear key.

5. Change the cam to something similar but with bigger LSA (lobe Separation Angle) to reduce cylinder pressure. You can ask a cam manufacturer to make a mild cam with a bigger than normal LSA for you. Unfortunately this means buying a new cam.

I just ran out of time, I have to run to work. Others please correct me where needed and continue with your ideas

Agree with above posters, Check stroke on the crank, block deck height, and piston compression height. Could also be rod length but that seems unlikely. My bet is block deck was milled crooked for some reason.

If you are prepared to modify pistons to adjust compression height, consider increasing the dish as well. Not a big deal if you have access to simple mchining equipment. No reason you couldn't do it by hand with a die grinder in desperation...

If you end up shaving the piston tops down, the dish would get smaller too wouldn't it, throwing your compression ratio off

Yeah it would reduce the dish by some amount. I would cut all the pistons that need it and then figure out how much more dish is needed to reach his goal. Trying to open up the head that much is a different matter. It might be more cost effective to search for an open chamber head than to try and modify the one he has. Maybe Larry could do a head for him and ship it over.

someone might be interested in that high compression head if it's otherwise in good shape

If it was mine, I'd fix the crooked deck (assuming that is the problem), then modify all pistons the same. Shoot for 0 deck and open the dish a little bit, leaving the piston top flat around the perimeter for squish and quench.

Now, to get from 11:1 down to 9:1 may require more material off the piston tops than you are comfortable with...

Trimming .010 or .020 inch off the piston top, then make the dish deeper by .050 or so I would be totally comfortable with. Not sure how close that would get you. Just want to point out the possibility.

I am only getting around 10 to 1 compression with this calculator. http://www.rbracing-rsr.com/compstaticcalc.html
I did average the deck height to be in the middle at .005 out of hole on all cylinders.

I say work on the pistons to lower the compression.

It is too bad that Clifford told you to cut so much from the head. At times he gives bad advice. The only one who pays for it is the customer.

When life give you lemons, make lemonade.

In that calculator Tom, you have to enter the deck number as a "-, or minus" value since the piston sticks up above the deck surface. Once you re-calculate, it comes out to 10.67 using that calculator. Also where many calculators have an error is in the head gasket portion of their formulas. And if it only asks generically for the gasket thickness, it will not be accurate. More accurate formulas need to also ask the head gasket volume and gasket bore size.

I agree with the method with head gasket. But in this case it will be close enough.
Ran it again 10.09

3.905 bore
4.12 stroke
.040 gasket
-.005 Deck Height (the average)
16cc Piston
66 CC chamber

just BEGGING for a bigger cam

You might try pulling all the pistons and remove them from the rods. Then using a deck gauge, slip the piston pin out just a speck and measure the piston from the top of the piston to the top of the pin. That ought to tell you whether it's the pistons. If they all come up to speck, then you will move to the rods and measure the rods to make sure they were not machined wrong, i.e., not all the same length.Then I think you are down to checking the crank throws to make sure the crank throws are equal and the crank is properly spaced apart degree wise, 120 degrees.

Thanks again guys!

I was told from Cliffords that the engine would produce 325 HP when done. I know I said that I'm not gonna race it, but the HP/torq is always welcome for fun.

I can read a lot of different opinions on my issue here.

Am I right when all can agree that the pistons need to flush the deck.

When I have measured the pistons, rods, crank and block. And maybe found the problem, I believe from the forum posts that I have more options..

If the block is milled in an angle:

One option:
Mill more off of the block and then mill the pistons to flush the deck and then dig in the pistons to get a lower compression?
What about the stroke then?

Another option:
Let the block be crooked, mill of the pistons seperately so they flush the deck - dig the piston dish to aim for a lower compression.
This option gives me 6 different pistons in weight and dish size. What does that mean?

Other options are welcome -
I know that it is me that have to take the last decision, but it helps with your inputs.

Let us say that I end up with a compression around 10:1 - will I need a bigger cam?

the engine would produce 325 HP

Don't count on it, and not with that cam.

Hi Guys.. THINGS has changed!

I hope that you will come back with new inputs..

My block had a hugh crack in one of the hyd. lifter cylinder wall.. The machine guy didn't spot it and is willing to pay back for the block work.

So now I need a new block machined.

So from earlier discussed what will you recommend hence to the 66cc chamber and the 16cc dished pistons.