astrosurfer

Hi fellas, not posted in a while. Always coming reading. Love it here.
So here's my deal.
I bought a 350 block now to do the upgrade from the old tired 305.
I'm hot tanking and boring to .30.
My question is that if I use my current heads which are claimed to be 58cc, will I need dished pistons to run pump gas? If so, is there some sort of chart to tell me how much dish I need to maintain a higher ratio while using regular gasoline.
I'm not trying to build any tire melting machine but merely a decent reliable motor I can get on when I feel the need.
I looked at the compression ratio calculator, but was confused with it.
There's no deep pocket on the build. I already have a new crank and cam with 200 miles on them. Heads were machined also.
Locally can get a set of hypereutectic and rings for $120. $250 for block work.
As always...any help or advice appreciated.

vetteoz

iTrader: (13)

What cam?

astrosurfer

Sorry for the late response here....been busy on house projects....
It's a computer friendly cam I got from summit....it's pretty much the stock replacement for my 305. It may have a little more duration. Nothing that I need to make power with. I just want to make a reliable motor using pump gas. I'll go premium if I have to but whatever I got to do.
The shop that's going to do my block are good guys and he assures me all my stuff will fit except the pistons. I did not ask him about dished or flat top though.
So I thought it a good idea to ask here. People have been down this road before here and know what I need to know.
I don't want to over compress or nothing, so I am going to run with what I have and get new pistons and rings.
I know I will probably need a different computer from what I have read here on TGO.
Thanks

startat2

iTrader: (2)

Use summit's calculator, pretty easy to go by.

http://www.summitracing.com/expertad...on-Calculator/

Also if you dont know or aren't sure, talk to your machine shop. Those guys usually turn out quite a few motors and can usually tell you what you have for sure (mainly your heads since you said you think they are 58) and need.

astrosurfer

Appreciate the link. Thanks.
I tried another similar calculator...but I had a couple of values that I do not know.
How do I find my deck clearance?
How do I know my gasket thickness?
14022801 is my head casting #. 14093638 is my block. Also a new stock crank in the 305 going in.

vetteoz

iTrader: (13)

Std SBC deck is 0.025
But if the shop has to machine the top of the block to get it square then that #
will reduce (resulting in higher CR )

The maker of the gasket you choose lists that spec

( for playing around on the CR calc use 0.040)

InfernalVortex

iTrader: (4)

This is a very important part of the build, something I wish I had paid more attention to when putting mine together and a big reason why people are always unimpressed with their results. The other big reason is they use garbage 70s "from a corvette" or "202" heads or something like that.

Anyway, factory deck height is 9.025 inches.

If you have a 350's 3.48 stroke crank, 5.7 inch rods, and typical rebuilder 1.540" compression height pistons, you'll end up with pistons, theoretically (the tolerance stacks will come into play here, it can vary from bore to bore), .045" in the hole. That's (3.48/2 + 5.7 + 1.54 = 8.98, 9.025-8.980= .045 . The ideal distance between the cylinder head and the top of the piston pad is .040". That means you need a -.005 thickness gasket to get your quench right,a nd that's one of the most important parts of the build.

The key is to get pistons that are 1.560 (factory) compression height, and if you get the block decked, measure how far in the hole your pistons are. If they're even, then get a .040 gasket (the norm) and be done with it. If they're .010 in the hole, get a .030ish gasket. The previous poster is right, if they deck the block your compression ratio will go up IF YOU USE THE SAME HEAD GASKET THICKNESS AS THE FACTORY. If you use a thicker gasket to compensate for them shortening the deck, you'll be back where you started, and that's generally the idea behind cheap rebuilder pistons. You deck the block, and get shorter pistons to get back where you started, but the factory uses really thin steel shim head gaskets. You dont necessarily need to do that. It's better to get quality 1.56 height pistons and use a thicker gasket probably.

Make sense?

The key is to get the piston .040 below the cylinder head when everything is installed, and you tweak the head gasket and if necessary the piston compression height to get that quench distance right. Get the quench distance right, and then you tweak the compression ratio with piston dish. Most flat tops are 5ccs, dish pistons are between 8-20+ cc's.

Your best bet is to approximate factory specs if you're running a factory cam. If the factory pistons were dished, get dished pistons. A compression ratio in the low 9s or high 8's will be close to factory and should be good for you. I think LG4's are 8.5:1.

You sure you want to run with those 801 heads? They seem kind of old and outdated from what I can figure out. Even 305 TPI 081 heads or 416's are a good choice for 305s and should be cheap and available. But taht's neither here nor there. For all I know maybe the 801's are just a variant of 416s.

vetteoz

iTrader: (13)

Read this
http://members.uia.net/pkelley2/DynamicCR.html

You will see why cam specs affect what CR you can run.
The bigger the cam ( longer duration ) the more CR you can run without detonation on the same octane
You baby cam is going to limit your CR

If you can find your cam specs there is a calculator at the bottom of page
in above link that you can download and install
It allows you to input all your engine specs to see if DCR is within acceptable range
Once you have all the numbers in the calc ,
just change the piston dish size until you get a CR ( and resulting DCR ) that fits in the recommended range in the link

"It appears that most gas engines make the best power with a DCR between 7.5 and 8.5 on 91"

astrosurfer

@infernalvortex
thanks for your informative reply sir.
that all makes sense somewhat.
I'm thinking the heads are going to choke it with the smaller valve size of 1.72. Not sure how much that will affect things for my requirements.
I'm not thinking the price for block work is inclusive of decking.
So I guess I need to get the right pistons and then get the gasket. Now I still need to know whether I need dished or flat.
According to the calculator with all those figures I get a 10.89:1. That means premium right...anything over 10?
vetteoz...I'll look at that stuff tomorrow.

InfernalVortex

iTrader: (4)

Show me your list of specs. 10.89:1 is WAAAAY too high for a stock cammed car. You need to be around 8.7-9.5

Enter Cylinder Bore Size - 4.030
4.000 is stock, whatever they bore it over to, you increase it to that - larger bore = more compression. You'll likely end up at 4.030

Enter Piston Stroke Length - 3.480
This is your factory 350 stroke

Enter Head Gasket Bore Diameter - 4.1
http://forum.grumpysperformance.com/...er+coat#p11603 .020-.060 bigger than your bore size is preferable, but there's a big range here. One of the more common felpro 350 gaskets has a 4.166 bore.

Enter Compressed Head Gasket Thickness - .015
Distance in the hole subtracted from .040 or .045

Enter Combustion Chamber Volume In CCs - 58cc

Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-') - ( -5 )
Yes, negative 5 for a normal flat top with 5cc valve reliefs

Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') - .025
Factory deck, factory compression height pistons.

This yields a static compression ratio of 11.2:1 - WAY too high.

If you increase the piston dish to 10cc, you will get 10.5. If you increase it to 20cc, you get 9.4:1. That's the high end of what you want, but acceptable. I'd want the specs of your cam to compute the dynamic compression, though. You dont want to go bigger on the cam at all? A slightly bigger cam will give you a little more power and make it easier to run that compression.

So if you have to go .030 over, and you maintain a .040 quench, get a gasket that's around .060 larger than your bore, you can get a 20+cc dish piston and be in the right compression ratio range. This is where you just have to play iwth things.

You can go up to around .045 on the quench and still be good. So a .020 head gasket with 5cc pistons will get you a .045 quench, and a 11.0:1 compression ratio. Increase that head gasket bore to 4.166 on a .020 thick gasket, and you get 11.0:1 still. So as you can see there, head gaskets dont make a huge difference within the constraints you need to stay in. If you go to a .040 thick gasket with pistons .025 in the hole your quench will be terrible, but you will be down to 10.4. That's just not a good place to be though, you're better off having higher compression and better quench as far as running pump gas. The best way to lower your compression is to get dished pistons.

If you end up with some terrible rebuilder .045 in the hole pistons PLUS a head gasket on top, that motor will be prone to detonation AND be slow.

I would try to find some better heads. The 58cc chambers are just too small I think. the small valves will further kill performance.

For the record, your dynamic compression ratio will determine what gas you can run. If you run a stock cam and high compression you will need to run premium. If you run a big cam and stock compression, you can run crappy gas all day long but you'll never make power. Bigger cams bleed off compression with their overlap - this is one reason why the need such high compression ratios on purpose built race engines. The compression that is bled off is relative to your static compression, though, so it's stilli mportant to get your static into a good range, but the dynamic compression is more like the "real world" compression ratio after everything is put together. Your dynamic falls into the right range without worrying about it if you can get your static ratio right.

astrosurfer

That is what I got when I used the summit calculator from the link above...
here is what I put...
bore...4.030
stroke...3.48
head volume...58cc
deck clearance...0.045
gasket thickness...0.005
cylinders....8

that yields me the 10.89:1 CR.

EDIT** I may have done the gasket thickness wrong above...see I'm new to it...should have been -0.005 in the thickness ( to get the .040 quench). That actually now gives an 11.18:1 CR

I'll use that other calculator later today and play with that. I'm very new to this stuff. Didn't know it was all this involved to choose pistons and stuff. BUT I'm glad it is, and I'm glad I asked here before I get to doing this project.
It is good to be armed with sound advice from several sources.

Other thing is that I may be using the wrong cam. I am sure that when I ordered the cam I had made sure it was computer friendly. However looking at my order now the part # from summit is not. Which has me a little confused. I believe I have the 1102 according to my invoice.
So....apparently I have the wrong cam in my 305 right now. Ive only got 200 miles or less on it. I still have the same noise after many new parts and an infernal misfire....so I can only assume the block has a problem.
I'm already thinking I need to get better heads anyway, but If that's something I have to do then I just have to do it.

InfernalVortex

iTrader: (4)

You deck clearance is how far in the hole your pistons are. If you get good 1.560 height pistons AND they dont deck your block, that value will be about .025. In order to keep the pistons .040 from the cylinder head, you need a head gasket in the .015 to .020 range. They dont make negative thickness head gaskets. I dont think they make much of anything thinner than .015, but I havent raeeally looked either.

Is yout 350 block a roller block?

1102 is an old school, OLD very mild cam. You can go up to a comp xe256 and still have an extremely drivable car. It will sound and drive stock. I think it's 212/218 at .050 and get it in a 114 LSA and it should be fine with the computer.

That old school cam will work with a computer just fine probably, its very small, will probably make great vacuum. You just have room for improvement if you want to pursue it.

astrosurfer

Sorry for late reply on this sir.
Just been busy around here.
It's a roller block as far as I know. It has the mountings for the spider on top. I was told from the seller that either flat, like what I have, or roller will work.
It's going to be one of those "collect parts as I go type build".
I'm wondering If I should just get flat tops and upgrade the heads. Working the ratio with the head chamber.
It's all gotten a good bit more confusing now than when I just wanted to know about flat top or dish pistons.
Will be going with a roller set up at some point.