Need help finding the right piston to achieve 10:1 compression. I have a 350CI which currently has 8.5:1 compression with 76CC heads. I plan to change to 64cc heads and a cam with .420 lift on both intake and exhaust. Would like to use a .040" gasket and keep the deck height standard at 9.025". Trying to keep a low buck upgrade to achieve some good low rpm performance so I'd like to find an inexpensive piston (possibly a standard flat top GM piston). Any suggestions?

Also any thoughts on improved performance changing out the Q-jet with a Holley (don't want to go FI)?

Thanks
Rich

 

Assuming standard bore
.038 head gasket
64 cc heads
deck height .025
then a +3cc flat top will give you 10:1 static.

if you’re changing the cam I would seriously consider a cam with more lift than .42”

Carb opinions here get spicy. Some die by Qjet.
I prefer Holley.

 

If you flat deck the block you will need ~+12 piston with a .039 gasket. That will give you better quench.

 

Thanks. Is there a way to find out I have enough valve clearance before buying the cam and piston combination? My current piston has a recess and 4 valve reliefs. I'm concerned that a flat top piston will hit the valve especially if I get higher lift cam.

The cam spec came from a Howards cam that David Vizard generally recommended. I have no real-world experience with cams so I just reading a lot and hoping I pick the right one. Love to get 300+ HP since the stock rating is 175 HP.

 

You can use true flat tops with one trough for a valve relief that might not have as many cc's.....but those usually cost more. A basic true flat top will get you there...BUT..pay attention to the piston height. Many low $$$ replacement pistons are made shorter to allow for rebuild shops to deck the block and not worry about things. With an undecked block it's easy to end up with the pistons .060" or more down in the hole which kills compression. So get the right pistons.

You can also use a .015" head gasket to keep quench around .040" ish if the piston is down in the hole .020-.035". The factory used thin shim gaskets and most all replacement composition gaskets are nearly double that..so more compression lost.

Cam lift has very little to do with piston to valve clearance. The actual valve events related to duration, intake centerline, lobe separation angle etc is what determines P/V clearance. You can go way up on lift with no issues in a "streetable" cam.

JIM

 

Go to summits website and look up the compression calculator. Take your info and plug it in and then find a flat top piston you can afford and plug in its info...easy as that.
If you arent running nitrous or boost the hypereutectic pistons are good.

 

Thanks guys for the education. I'll check Summit's website.

 

Zero deck two or four eyebrow bellybutton flattop piston will get you 10.44 to 1 assuming .038 gasket..... -5 cc eyebrows....
I just built one and left the piston in the hole with .015 shim head gasket for the vintage 186 64cc heads......
But as Jim states above....pay close attention to the C/D (compression distance) there are 1.540 C/D "rebuilder" pistons and 1.560 "performance" pistons......Speed Pro 4 eyebrow flattop is 1.56 C/D pt# H631CP 30

Jebby

 

Wiseco makes a 4032 forged two relief piston available that doesn't break the bank called Pro Tru Street Forged that would work well with a good ring 1/16 1/16 and 3/16 ring package than can have tighter piston fit with the 4032 than a 2032 Forged piston , Competition Products has them

 

300 horsepower? Flat tops a good cam 64 cc heads then pull 2 spark plugs out of the engine that should get you down to 300.

 

Here are 2 choices:
K-B Hyperuretic

ICON Brand forged

These CR calculations assume a standard deck block and .039 gasket per catalog notes. My math says the pistons will be .020 below deck. 9.025-3.48/2-5.7-1.565 = .020 (deck height, stroke, rod, compression height)

It is easy enough to download a catalog and browse.

It is not hard to get compression up with 64cc heads. You could even do it with the 76cc heads, just a diff piston, maybe a small dome.
But I would always double check both Compression and Dynamic Compression with a good on-line calculator, Wallace Racing has some very good ones.

Have the block carefully checked for flatness, by a machine shop, before determining it does not need to be decked. Buy pistons after that, not before.

All of this and your cam timing comes into play to stop your engine from pinging.
Keep your DCR below 8.4 with this calculator:
http://www.wallaceracing.com/dynamic-cr.php

You can build 350+HP into this very easy with 10:1CR
Try a 220 dur cam @ .050

The example above withe 9.025 deck and .020 in the hole with a .039 gasket gives you .059 quench. Which is OK but not great. With this quench keep the DCR below 8.0. With less quench you can run up the DCR some. Lots of places to juggle these numbers around.

DCR counts much more than static CR.

If your cam is too small with 10:1 you will detonate.

Al of these should be selected at the same time: deck height, piston, head size, cam duration.

 

I got some homework to do.

A little twist, I was going to buy 461 heads to get the 64cc chambers, but my machinist (he does only heads) says he can take my stock 399 heads (76cc) and resurface them to get me to 10:1. I'm hesitant to do that since the intakes are smaller. He said that if I were racing you would notice the difference but around town driving it wouldn't be worth it. I never used him but he's been super helpful and seems very knowledgeable (he's an old-timer). I feel like giving him work just because of his generosity. He also said doing this wont affect the sealing of the intake manifold. That makes me a little nervous.

I haven't seen this approach so what are your guys thoughts on this? I'm leaning to go with he's advice. Again, I'm not looking to get everything drop of HP but don't want to make an avoidable mistake.

 

First off....if either of the sets of heads in question are not absolutely pristine.....it is not cost effective to mess with either of them. Your "old timer" should know better too that the late model 76cc heads are the lightweight style from the mid 70's and not the hot tip for performance......and that the 461 is a better casting, but even so....either set is a total waste to put money into unless they are at least gasket matched and bowl blended......if the "399" (I think you mean 993 heads) are 1.72 intake valve then throw them away.
It is still possible to build a 400hp set of heads out of stock castings if you are good with a grinder with all new valves, springs, locks, retainers and seals for about $400 over above the purchase cost of the head....but like I say, those heads need to be pretty nice.....the most they could need is a quick cut on the seats.......another $200 will get you a 3 or 5 angle cut that really bumps low lift flow......I use the Elgin stainless "Argentina" valves and their "Z/28" spring lock retainer kit.......I spend four hours grinding on each head......but I only do this for my own engines or "shop" engines that I sell later on.......everyone else gets talked into an aftermarket aluminum cylinder head.........I don't want to R&R stock heads on a running car and do all that work to them....I come across a lot of pristine GM engine parts in my travels and ****** them up when I can.......that way I know I can build a reliable engine and keep costs down......this is how I did it in the 80's and 90's when aftermarket heads were still Blue Ticket ****......but these days the parts are drying up and it just makes sense to pop a new pair of heads on.......you will pay me $1000 to make a pair of pristine GM heads make 400hp.......

You "old timer" won't mill those 76cc heads to make 10 to 1 compression either.......at 1cc per .005 inch (rule of thumb) you will need 12cc or .060 mill....which is murder to a lightweight head and you will be cutting the intake to make it work.....

BTW....up to 400hp.....a Q-Jet is pretty awesome.....after that, there are better choices.....the Q-Jet can be modified for more power better there you are spending a ton of money and time on old stuff.......

A finally.....you will notice the power difference dramatically between a 1.72 intake valve head and a 1.94 intake valve head......the difference between 1.94 and 2.02 at this level is hardly worth talking about......

Jebby

 

Have to agree. Almost any Aluminium aftermarket head will flow better than a stock mid 70's -early 80's head. And be far more reliable.
yes you could mill your awful heads down to gain compression. But these awful heads are prone to cracking and of course there is the poor port design and the small valves. Not to mention that iron heads are more prone to spark knock than aluminium heads.
just my uneducated guess of course. But I am also a Gray haired old mechanic, as is Jebby, (I think he may be younger than I) BUT.
I would think you would be way ahead buying Cheap alloy heads over rebuilding your stock crappy heads.

 

The basic port design is not too terrible......but they just need a lot of grinding......you can make damn good street power with a stock head, but why? Another thing is that the new heads are SO much more efficient and resistant to detonation/knock.....you can build 10.5-10.8 to 1 engines on 93 octane no problem......compression on its own is worth power, but this is not why you put as much as you can in.....you do it to support better (larger) cam choices without sacrificing drivability......I like building the Street Rod engines that way.....too much cam and lots of compression....sound great and rev up quick......

Jebby

 

Jebby:

I doubled checked the casting and its 339X off of a 72 C-10 so these might not be light weight heads. Don't know if that changes anything in your response. Since I'm on the subject, is back grinding the valves worth it (will I feel the difference)?

This looks like a decision to spend more money than I wanted but its the prudent thing to do. I wanted to keep cost around $600 bucks all in.

 

im using 5cc speed pro hyper flat tops with .015 steel shim head gaskets (undecked block) and chinese 64cc heads (same casting as AFR uses for enforcers) to accomplish this.(cost me $700 to buy and build heads with decent hardware) The .039 quench from the gaskets and aluminum heads allow the use of 90 octane and even 89 without pinging for me but I chose a camshaft which allows a lower dynamic compression ratio in the recommended sweet spot to help.

these are the heads and the vendor I used to buy my heads 2 years ago. For some reason only the 190 version of this head has doubled in cost as 2 heads were about $350 shipped 2 years ago. I'm guessing they are the hot seller. ironically the larger 195-205 version are about the same price assembled. I used gatorman valves and comp springs/hardware. I watched youtube videos to learn to port and assemble and they outperform my old dart heads enough that I had to rejet the carb to fatter jets. https://www.ebay.com/itm/29369799873...3ABFBM6Pb3wdti

 

heres an old dyno software estimate for my engine.(with old dart iron heads I replaced the following year).. I ran different combos for months before settling on the comp 270 roller cam.. had I known I was replacing my rear end gears I likely would have gone to the 276 cam.. I ran a smaller lunati voodoo 262 cam also and the 270 was a noticeable improvement. im using 1.6 sharp roller rockers

these numbers are just for reference and comparision value I do not believe they are accurate as real world is typically lower.

 

1.72 intake valve on those.....garbage can.

Backcut on the valves is always preferred for low lift flow.......but if swapping the valves out for stainless, only do it if you bowl blend it or else it is a waste of time.....

Jebby

 

Resurrecting a cast iron head is not economical with todays available heads and the cost of machine wprk. Plus if he decks those heads he may jave to alter your intake to get the geometry to match. As much money as you are goingbto throw at cast iron you could get a good pair of heads, a used pair of great heads or good money towards new heads. I just swapped big block heads from old crap "performance" gm factory heads to good brodix race rite heads, lost 2 full points of compression and made 150hp just in flow alone with the same performance cam shaft. You cannot skimp on heads and if you get great heads now, have 10:1 compression, headers and a decent intake you can increase your horsepower down the line with just cam swaps. I did it for a couple of years on my 327 until i found a power level i could live with...which is about 425 hp as a minimum for me.

Just use high end bearings in your build. I have used good and name brand but not racing bearings and have had 2 engines fail on bearings