[pizzi-man]

I have -16cc srp pistons in my 383 and was wondering what I would gain by using flat tops with -5cc srp pistons. The assembly is balanced for the -16cc pistons with a 403 gram weight and the flat tops have a 416/419 gram weight Is it worth the change or should I leave it as is. I am using AFR 195 street heads

 

[richietables]

Hell, Don! We car guys can never just leave well enough alone----:noway:

How much performance will you gain, ballpark?

 

[jeff swisher]

not enough weight there to make a difference.


My 350 345np pistons flattops are 30 grams lighter than the stockers.I never ballanced my 350's...the H345np's are 30 grams lighter than the regular cast 345np
Now you end up with a big overballance when the piston is lighter.
I have seen 7500rpm many many times and logged over 75,000 miles on the first round with them singing to 6500-7200 with the small cam.
Never ballanced never wore anything out.
I feel if you are running sustained above 6500 ballancing may be benificial,,,or shift points way above 7500rpm a ballancing may help keep stuff alive longer.

 

[jeff swisher]

If the tune can tollerate the compression boost then DO IT.
I love the most compression i can get by with.

 

[Philip]

If you wanted more power why give away the the 23 cubic inch to build a 383?

 

[pizzi-man]

Here is the build, AFR 195, 65cc chamber with a LT1 383 forged rotating assembly. Cam is 230 /236 @ .050 lift is .510/.520, ls is 110. Piston is .025 in the hole.
I was going to supercharge but with money being tight I was thinking of just boosting compression. With the pistons I have now the compression is about 9.2 and with the flat tops would be about 11. The flat tops are 16 grams heaver. I think that the way it’s built I should be at close to 400hp and was wondering if changing the pistons would be worth it or not. This is my daily driver and will see no track time. Motor is on the stand and ready to go so if I change something now would be the time.
Sometimes I wish I could leave well enough alone.

 

[Philip]

What is the compressed thickness of the head gasket?

 

[jeff swisher]

I like compression it creates better mileage and increases power through the entire rpm range...I think you should like the added boost.

My daily driver 3200lb truck with 325 gears has a tad over 10.3 with old iron heads and a 218-218 454-454 lift on a 110,,,she runs really good and i get 20mpg,,91 octane
I would like to see your combo in the 10.5 to 11.0 range ..9.2 seems well for a blower , like you stated..but a tad low if you just plan on running it as is.....

What is the rule ..about 3to 4 percent gain with 1 point of compression..I dont use that because there are way too many variables to consider,,could be much more, could be less if it you cant tune it.

I had one 355 with mid 11's on compression with a 224-224 470-470 on a 110,,,she had 245lbs of cranking pressure,,hardest pulling 355 i had out of the hole,,with pump gas...iron heads
I say do it
BUT if you are not racing it much and just tooling around and do not plan on taking full advantage of it then why bother

 

[pizzi-man]

What is the compressed thickness of the head gasket?

.039 with the felpro now and I have the embossed .019 shim on hand.



"BUT if you are not racing it much and just tooling around and do not plan on taking full advantage of it then why bother"

I guess mabe for the same reason I took the 88 out of my Indian Scout and replaced it with a 100 inch power plus and a 6 speed I don't know and that's why the post

 

[jeff swisher]

Hands down i would go for the compression boost.

I have a cherry 350 block and crank sitting here i have flattop hyper pathetics on some good rods..some ported iron heads with 62cc chambers..these will be milled to get more compression or find a higher comp piston.....I want 11to 1 or better,,in fact 11.8-12.1 would absolutely thrill me,,,'

I have the cam 252-252 at .050 and .536" in and ex lift on a 106LS ..
I could put it together and run it but i want AT LEAST another 1/2 point of compression..
If i get over 12.1 to 1 i have another cam in mind a 260-260 at .050 and .555" lift on a 106..all solid flat tappits......I think these would make a good weekend engine...
DO THE COMPRESSION UPGRADE:thumbsup:,,,or you will wish some day you would have just done it to begin with

 

[Philip]

As it is built now the compression is not the issue but the quench area is too large for optimum tuning. It will limit your timing and cause poor fuel mileage.

Here is an excerpt from an article in February 2009 of Popular Hot Rodding by Steve Dulcich.

Quench It If You Can
The quench effect on engine efficiency has been well documented and researched since early in the last century. What is the quench effect, you ask? Simply put, it is designing in a close clearance between a substantial portion of the piston area and the bottom of the cylinder head when the piston is at top dead center. A closed-chamber head has a large flat area, the quench surface, over a substantial portion of the bore. It has been found that if the piston rises to within .050 inch or closer to the flat of the head, good things happen in the combustion process. The effects here are multifaceted. First, is the squish effect, wherein as the piston closes the gap in the quench portion of the head as it approaches TDC the combustible mix in this portion of the chamber is rapidly displaced, creating combustion-promoting turbulence, speeding the burn. In the compression process, the gasses in the chamber reach a very high temperature. As the propagating flame front expands, the pressure can get high enough to auto-ignite the end gas at the far side of the chamber. Since with a tight quench clearance, most of these end gasses are squeezed out near TDC, the chances of auto-ignition (detonation) are greatly reduced. The temperature of autoignition is approximately 1,375° F. Clearly, the cylinder head temperature is significantly cooler than the end gas temperature at or near autoignition levels. Due to the temperature differential, the thin layer of detonation-prone gasses at the extremities of the chamber are actually cooled by the proximity to the head, further diminishing the tendency to detonate. It is from this cooling effect that the term “quench” is derived.
An engine with an effective quench will be more detonation resistant, and it is typical for surprisingly substantial improvements in torque to result from the more efficient combustion. Most builders consider .040 inch or so to be an effective target for piston-to-quench-area clearance, a spec easily obtained with a closed-chamber head, a piston at zero deck, and a standard FelPro .039-inch compressed thickness gasket.

Full article here

The .019' gasket would get the quench better and raise compression, but IIRC no one makes one for a 400 block. When I layed out the build for the 406 it was necessary to have the blocked decked to achieve and optimum quench area.
The extra machining was well worth the cost. I am running 10.4:1 compression, 50° total timing, tons of low end torque and no detonation issues.
The full specs are 400 block bored .030” over and decked for .010” piston height, stock balanced bottom end, 12.5 cc dished piston Lunati hydraulic flat tappet cam with 110° lobe separation, lift of .489” intake, .504” exhaust, duration 268° intake, 276° exhaust installed straight up. Intake is an Edelbrock performer with a modified Quadrajet carb. 1 5/8” Hooker headers and a 2.5” Magnaflow stainless steel x pipe exhaust system. Initial timing is 18°, mechanical is another 18° for a total of 36° all in by 2200 rpm. The vacuum advance bumps the total to 50° and is hooked to a full manifold source.
The engine has amazing power from idle to 6000 rpm’s, can get close to 20 mpg, has great vacuum and no problems running on the freeway. Last fall I drove it to CA with the AC on at 70 – 75 mph and averaged 18.8 mpg for the 1300 mile trip. That included some spirited driving and a few burnouts

If you do nothing else get the quench area from .064” down closer to .050” to take advantage of all the power your engine is capable of. If changing the pistons stay with the dished configuration and use one that has a raised height to bring it closer to a zero deck height. I do not believe that a steel head gasket is recommended by AFR to use with their heads.

 

[pizzi-man]

The head gasket is a felpro 1094 .015 shim and was told that this would be a good gasket to run. That would give me a .039 quench. You know how stuff snowballs, starts with a head gasket and the next thing is you question the pistons then you start to question the cam and while I'm in here and on and on

 

[Philip]

The felpro 1094 only has a bore of 4.100". A 400 block is 4.125" plus any over bore that would have been done.

 

[pizzi-man]

Philip,
This is a 350 bored .030 over with 3.75 stroke and 6 inch rods= 383

 

[Philip]

oops sorry, I had the bore and stroke backwards. The gasket change should be a good move for both quench and compression. Run the numbers for compression through your favorite calculator and make sure it gets you where you want to be.

 

[pizzi-man]

Ran the numbers through several caculators and come up with Static Compression @9.806 with a dynamic compression of 9.026 using the 1094 head shim. Maybe I'll leave it at that.

 

[Philip]

The SCR is good for pump gas, the DCR is real good for power. The engine should run well with the gasket change.