Thread: last ? on compression

Tweet

Originally Posted by DennyW

ps: Just don't run it with a 100lb boost.

POSTED BY DENNY....
"You didn't just ask that ??"

No Denny, I didn't. Don't try to make me look any crazier than I am please

O.K., here's the way I see it. One point of compression equals 3 to 4 percent more power. A fellow could build a real nice 9.0:1 motor, use a short cam so that the torque would be down low and the motor would run great on regular pump gas. With a 268 cam, it would also work with the stock torque converter. Let's say this motor makes 350 horsepower.

Or, you could build the motor 10.0:1. The increased compression is begging for more cam (I'm bettin' the motor would rattle like a can of rocks at 10.1 with the short cam on pump gas) which moves the effective maximum torque up in the rpm range, away from where it is needed on the street and also requiring a looser torque converter. ...(more heat in the trans fluid, requiring a cooler). With the longer cam, pump gas would work and the motor would make 360 to 364 horsepower, all other things being equal.

To me, it just doesn't make sense to build a 10.0:1 motor for the street.
O.K., I'm through.

I would see if they offer a little bigger dish on those pistons,to drop the compression some.You could probably make the motor run at 10.1 :1 with the 268 cam,but the tune had better be perfect ,if not KABOOOOOM!!!!If you plan to ever drive that outa state you better bring your own gas ,because that motor would never run on cali pump gas. The 268 cam is perfect for your motor ,other than the compression and the cheapest and best way to lower the compression would be a bigger dish on the piston.
The balls in your court now,try to "maybe" be able to run it on pump premium or know that it will run on regular pump gas anywhere you go.By the way ,massive goverment emissions laws have already been passed and go into effect in 2009,look for 87 octane to be pump premium,we are at 91 max octane here in cali already ,before the laws go into effect!!!

ok now im really confussed if that makes the compression so high why dont i keep my flat top pistons and buy the set 441 heads next door for only 100 bucks 76 cc 9:5:1 compression or close and still make the 360-400 hp where i need it in the low rpm range

Has anybody explained to you about dynamic compression ratio ? It is totaly different from static compression ratio. Dynamic compression ratio concerns when the intake valve closes in tha cam timing event and cylinder compression begins.

Have a read of this link http://members.uia.n

Originally Posted by maineevent

ok now im really confussed if that makes the compression so high why dont i keep my flat top pistons and buy the set 441 heads next door for only 100 bucks 76 cc 9:5:1 compression or close and still make the 360-400 hp where i need it in the low rpm range

The 441 heads will only support about 300 hp max in stock form and will take more money than you save not buying the vortecs and intake to make the 441 heads flow what the vortecs do stock.the vortecs have 30 year newer combustion chamber design and are brand new.The motor will run at 10.1:1 ,but you better carry a test kit for gas ,1 bad tank from the quickie mart and K A B O O O M!!!!!NO MORE ENGINE.The tune will be very very sensitive to fuel ratio,carb gets out of adjustment BOOOOOOM,timing gets out a little and BOOOOM,a little water in the gas and boooooom ,head gaskett blowing water and oil everywhere do ya feel lucky?,......well do ya ? The engine can seem like nothings wrong at all till it blows,but careful inspection of the plugs with a magnifying glass will show melted piston sticking to the insulator on the plug from detonation you cant hear or feel,then one day,..BOOOOOM,then the guesses start ,was it this ,was it that,no it was this ,no i had to be that,then you buy more parts and build another engine,or since you do not have alot of exsperiance you can lower the compression some and not worry about havin to be mr.goodwrench to keep it from blowin ,or always scared that next tank of questionable gas from the next town over wont blow your engine up,like Techinspector said your lookin at 10 to 15 hp different in the compression change ,I will give up 10 hp all day long to have a reliable engine,after all hp comes in with rpm,torque is lower in the rpm band and is what a 400 is designed for low rpm torque same thing the vortecs are designed to do .You will be lucky if a 400 would rev past 5000 rpm with the 441 heads because they max out at 200 cfm,the vortecs are 240 cfm,just about the perfect amount to make 360 hp and probably well over 400 ftlbs of torque below 3500 rpm.Why worry about if todays the day the temps going to be just hot enuff to detonate the motor apart for 10 extra hp your hardly ever going to be in the rpm band of to use.If this were a dragstrip only motor that ran on pump gas sure you would not care about having to do hours a week in tunning and checks to stave off the detonation,or puting in the wrong temp thermastat and when the motor starts smokin and detonating a hole in the piston cause you were 20 degrees to hot.This mostly probably sounds like a forieng language to ya and hard to comprehend all the little things that make difference and how they affect performance and maintenence,whick is understandable.My suggestion to you would be to spend 20 bucks on real good book.How to build max performance shevy smallblocks on a budget,by David Vizzard.Excellent book that goes into how and why ,not just do this or do that ,and you will understand more of the advice thats being offered to you and may understand enuff from readind that book to not even need our help and be able to offer others help from your new found knoledge!1A good second book is Smallblock chevy engine build ups,by chevy high performance magazine.Here is some good cam reading for ya to help.http://www.popularhotrodding.com/tec...ics/index.html

Sorry Shawnlee, I was typing while you had already posted.

1. 76 cc heads with your flat-tops will make 10.15:1 c.r.
2. 441 heads will not support 360-400 horsepower or anything close to it.

Let me outline a recipe for you that will work:
400 block bored 0.020" or 0.030" (don't bore the block any more than is necessary to clean up the bores. These pistons are available in either size.)and block cut for zero deck (crown of piston exactly even with the block deck at top dead center).
Keith Black hypereutectic pistons, part no. KB126, 30 cc cup, 9.1:1 c.r. Use with 5.7" 350 rods. Pin end of rod may require clearancing to eliminate interference with underside of piston crown. Big end of rods will need to be clearanced to eliminate interference with the cam, (slight grinding near the bolt head). Slight grinding may be required at the pan rail where the oil pan bolts up to provide clearance for the big end of the rod.
Scoggin-Dickey part no. SD8060 Vortec head kit. Includes 2 heads fully assembled, head gaskets, rocker arms, Edelbrock Performer Vortec intake manifold, intake gaskets and 12-point intake bolts. Make a deal with them to trade out the 350 head gaskets for 400 units, 0.039" compressed thickness. Drill the heads for 400 steam holes before you install them. Use your 268 cam and enjoy. Just make sure it has a lift of 0.450" or less. The stock Vortec heads will only accomodate that much lift. If you want a cam with more lift than that, you'll have to change springs in the heads.

thank you NOW I uunderstand I did'nt want the motor's compression that high just didn't know how to bring it down I havent bought the cam the only thing I have is the block etc an these pistons the flat top ones I'll sell them no way do I want my motor blowen up that easy so if the shorter 268 a work better than the 270 xtreme cam with the blue print you gave me I'll get that an then get the vortecs an vortec intakes thanks my head has stopped spinning now

cool

Originally Posted by shawnlee28

The 441 heads will only support about 300 hp max in stock form and will take more money than you save not buying the vortecs and intake to make the 441 heads flow what the vortecs do stock.the vortecs have 30 year newer combustion chamber design and are brand new.The motor will run at 10.1:1 ,but you better carry a test kit for gas ,1 bad tank from the quickie mart and K A B O O O M!!!!!NO MORE ENGINE.The tune will be very very sensitive to fuel ratio,carb gets out of adjustment BOOOOOOM,timing gets out a little and BOOOOM,a little water in the gas and boooooom ,head gaskett blowing water and oil everywhere do ya feel lucky?,......well do ya ? The engine can seem like nothings wrong at all till it blows,but careful inspection of the plugs with a magnifying glass will show melted piston sticking to the insulator on the plug from detonation you cant hear or feel,then one day,..BOOOOOM,then the guesses start ,was it this ,was it that,no it was this ,no i had to be that,then you buy more parts and build another engine,or since you do not have alot of exsperiance you can lower the compression some and not worry about havin to be mr.goodwrench to keep it from blowin ,or always scared that next tank of questionable gas from the next town over wont blow your engine up,like Techinspector said your lookin at 10 to 15 hp different in the compression change ,I will give up 10 hp all day long to have a reliable engine,after all hp comes in with rpm,torque is lower in the rpm band and is what a 400 is designed for low rpm torque same thing the vortecs are designed to do .You will be lucky if a 400 would rev past 5000 rpm with the 441 heads because they max out at 200 cfm,the vortecs are 240 cfm,just about the perfect amount to make 360 hp and probably well over 400 ftlbs of torque below 3500 rpm.Why worry about if todays the day the temps going to be just hot enuff to detonate the motor apart for 10 extra hp your hardly ever going to be in the rpm band of to use.If this were a dragstrip only motor that ran on pump gas sure you would not care about having to do hours a week in tunning and checks to stave off the detonation,or puting in the wrong temp thermastat and when the motor starts smokin and detonating a hole in the piston cause you were 20 degrees to hot.This mostly probably sounds like a forieng language to ya and hard to comprehend all the little things that make difference and how they affect performance and maintenence,whick is understandable.My suggestion to you would be to spend 20 bucks on real good book.How to build max performance shevy smallblocks on a budget,by David Vizzard.Excellent book that goes into how and why ,not just do this or do that ,and you will understand more of the advice thats being offered to you and may understand enuff from readind that book to not even need our help and be able to offer others help from your new found knoledge!1A good second book is Smallblock chevy engine build ups,by chevy high performance magazine.Here is some good cam reading for ya to help.http://www.popularhotrodding.com/tec...ics/index.html

Good post.
One thing I might add is rpm's is more of a function of valve train rather than head flow.
His engine may rev past 5,000 but it might not make any hp up there.