This seems to be a complicated subject. I understand deck clearance and different compression height pistons, different gasket thicknesses, different size combustion chambers and so on. But doesn't a bored out engine (for example, a .060 over 390) have a little higher compression ratio than a standard bore 390 or even a .030 over 390. What about gasket bore size? Quench and detonation?
I just seem to read some conflicting opinions on some of this?

 

Deck height is the distance between the centleine of the crank shaft and the yop of the block head surface (for instance my current build) has a deck height of 10.300" a zero deck to the New pistons!! the pistons come right to the top of the deck (not above or below)

Your piston to deck height is what your piston is in relation to deck height of the block.. Most stock 390 are like .010", .020", .030" in the hole...

your Quench area is the distance the top of the piston is to the bottom of the head which is controlled by head gasket thickness and piston to Deck height..

Like my Latest build a 521" stroker... 10.300" deck height...6.800" rod, Zero piston to Deck height...-22cc Forged Dish piston, .038" Head gasket= .038" Quench area,and 11.15 to1 cr..

For Hi-performance Engines running a Quench area of .035" to .050" is preffered to keep detonation down to a minimum...and you also get a better spark distribution in the head chamber....

Hope this Helps some?? and I didnt confuse me or you?? LOL!!

Good Luck!!

Russ

 

PS...I got to rambling on in my last post..Cylinder Bore size has nothing to do with CR.. it just makes more cubic inches... Piston height to deck-Head Chamber size-and Head gasket thickness,and Piston flat top-dome cc-dish cc= CR..

Russ

 

What brand of head gasket are you using? I think the Mr. Gasket one's are .038 compressed thickness. I have the Fel-Pro Permatorque Blue on mine and they are .041 compressed thickness I believe.

 

I'll have to think about that a secound....LOL!! I have the pack outside in the shed...I believe there Fel-Pro marine gaskets if I remeber correctly...
They may not make them for a 390 application?? But even if you have a zero piston to deck height the .041"s will still put you in the .035 to .050 range!!

Russ

 

I had my block zero decked and the pistons actually come out of the block just a fuzz. Though my engine won't fire for the first time for another couple weeks, the machine shop said it's no big deal because the head gasket is thicker than how far the pistons come out.

I guess I'll find out when she blows up!

One thing about this though, the pistons are supposed to be 9:1 when zero decked, since they come up a little farther I assume I'll have a tad more than that. Assuming the CC on the heads are with-in reason. I'm not sure what they CC at though.

Jordan

 

I am using the felpro head gasket. On my set up. i am actually having the pistons cut to sit .005 above the deck to get a little more quench and compression. I am boring my 390 .040. so it will be 418 with the 428 crank. the felpro will handle the higher compression the specs on the gasket are 4.400 bore/.041 thickness.

 

Russ! Shame on you. Drag out your calculator and compute a flat top piston in whatever chamber and bore/stroke combination you want. Then change nothing but the bore and recalculate. More bore = more compression, all else being equal.

I just computed a stock bore, flat top 390 with edelbrocks. Then bored it .060 and computed it again. I got a tenth of a point of compression increase. Bigger bore = more air, compress that into the same size chamber, and VIOLA! You get more compression.

-Scouder

 

OMG!!!! Brian..Shoot your right!!! A whole tenth of a point!!! How the sam hell could I be so stupid???

I'am so glad you brought that to my attention!! LMAO!! But Scoot what you forgot to do was Translate that tenth into HP and TQ...

LOL!!

Russ

 

OK, OK, so it's only a tenth. It IS, however, sensitive to stroke. If I do the same calc with a 4.25 stroke the difference goes up to 3 tenths! So take that!

And, by the way, on a typical 300HP engine, 3 tenths would be a gain of about 3.6HP.

-Scouder

 

OK, Scoot, you got me by the short hairs!! Enough hp to run the CD player!! LMAO!!!

Well Know that I have the T-Ram and Carbs and headsAlmost done..I have to back off the Engine..and start on striping the Interior out to start on Chassis mods... Tubular K-Member and A-arms-Tubular rear upper and lower control arms,Track Bar,and Roll cage.Sub-frame connectors, etc... Are we having fun yet?? LOL!!

I dont think this Project will ever end.... I think last weekend I just made an easy $1000 ?? One of my Younger Uncle's saw an Old 460 block sitting in my shed...and asked me in front of his brother (another Uncle) if that was the block I was going to build for the Project...I said sure!! (I didnt say yes)
So he said he wanted to race his Vette against the Project!!

He has a B&M 6-71 on top a 350LT1 in a 74 vette..so he's probably running Maybe 550HP.....I told him sure..Just make sure to bring a grand with you to the track.... He jumped on that like a fly to a fresh pile of Crap!!

I just grined and said Fine!! He Hasnt seen the parts in the basement...anyway its been a trip playing with him and his vette trying to knock some of the wind out of his sail's!! LOL!!

Russ

 

Ok guys, I didn't measure my deck clearance when I assembled my engine the first time but with the upcoming cam swap I will. I know this isn't accurate but the pistons came almost dead even with the top of the deck. I would say that they maybe were .015 down in the hole at the most. .060 overbore with Fel-Pro gaskets (.041 compressed) and heads that were right in the 69cc range. I know I am right in the neighborhood of 10.1:1 compression.

Here is what seems like a science to me though. I know that an engine's ability to resist spark knock and detonation are left to many variables. But say I was .015 down in the hole and instead of using the Fel-Pro gaskets went to steel-shims... This would jump compression up 1/2 a point and you would think that it would ping easier, but I have heard guys talking that you want more quench and I know that would be one way to get more..... What do ya guys think?

 

Annihilator:
The steel embossed head gaskets will crush to .021". For a long run street motor they will start to weep and leak coolant and oil. I installed them on one motor twice, leaked both times after 10,000 miles and another 428 that leaked after 4,000 miles. Great for a tight quench provided your deck clearance is about zero.

Carl....=o&o>....

 

If it were me .... I think I would stay with 10 to 1 cr...for a street drivin Motor......The only reason I pushed the limiy with my Project...Is its going to be Pro Street/Strip and at 11.15 to 1 thats really pushing it even with aluminum Heads on pump gas...I think even as it is I'll have to run 93 with some 110 torco added to it?? I will find out I guess??

Alot of the Hardcore racers..will even run .005" + out of the hole to get even more Quench (meaning tighter) as more... But also remember the more CR you run the tighter the Quench you want .....

Russ

 

RapidRuss:
Years ago on my 69 AMX with a 401 the quench was .022". With iron heads back in the 70's I ran a CR of 10.70:1 on pump gas without detonation problems. Wish we had that good leaded gas today, progress my a$$.

Carl....=o&o>....