JesasaurusRex

iTrader: (3)

Got my sb 400 back from the machine shop today. Told me deck height is 9.020 after he square decked it. Now i got the 3.8" stroke crank to auto zero deck it but with this deck height it means the piston is sticking up about .005 above the deck. So with 76cc heads and a 4cc dome piston, i had a .040" head gasket which would put my quench at .035" with a static compression of 11.56 roughly and a dynamic compression of 9.8 (6" rod and abdc 55). He told me id be better off getting a .051" head gasket which would put my quench at .046" with a static compression of 11.24 and a dynamic compression of 9.6

Cam is 264/272 @.050
649/649 lift with a LSA of 107

AFR 220 heads and a Super Victor Intake manifold

I wanted her to run on 91 octane with some timing retard but that was with it having a 9.025 deck instead of the 9.020 and an optimal quench of .040" . So my question is which head gasket should i throw on. Would i be better off with a .035" quench or a .046" quench also what do you guys think the possibility of her still running on 91 is? Thanks fellas.

ME Leigh

Don't listen to him, does he even know what quench is?

AsphaltAssalter

I would run the .035 quench, that cam you have should bleed off plenty of pressure to run it on 91, it may even run fine on 87. I'm running .032 quench with 10.75 compression with aluminum heads and a much smaller cam and I can run on 87 all day with max timing of 29 deg. I tried running more timing and higher octane but it didn't help power any.

RB83L69

I'd use the thicker one (1044)

JesasaurusRex

iTrader: (3)

Who? The machinest? I donno he just suggested the .051 head gasket. Which would you run and why?

JesasaurusRex

iTrader: (3)

Why?

JesasaurusRex

iTrader: (3)

you really running a .032 quench with no issues? hmm anybody else have any impute?

AJ_92RS

I concur.

JesasaurusRex

iTrader: (3)

Can anybody actually give me an actual reason for why they would run the thicker head gasket? or the thinner one for that matter?

RB83L69

I'd be looking for a DCR around the high 8s with aluminum heads, at the highest, for use with pump gas. IMHO 9.6 is still too high. But your odds of success are slightly improved.

Running too much compression and then backing off the timing does 2 things: 1) it makes the engine run REAL lazy; and 2) if it ever gets hot, it detonates anyway. It's a bad idea.

.032" clearance cold from the piston to the head deck is cutting it mighty close besides. It will close up some when it gets hot.

I don't think you have a very good combo of stuff for your intended purpose. Looks to me like this won't quite qualify as "streetable".... which at its most basic, apart from one's tolerance for big cam type things, means "survives use with pump gas".

Flattening the domes off of the pistons would be a VERY wise move.

AJ_92RS

Better safe than sorry.

.035" is pushing the limits of clearance for no good reason.

FWIW, .035"-.045" is the target range for optimum quench. Why .010" variation? Because anything below .045" does little more to help, if any help.

Why not tighter than .035"? Because pistons and heads don’t play well together.

.045" is confined enough where the gasoline won't burn and cause problems. That's the real reason to run such a tight clearance; to keep the fuel in that "dead space" from burning and upsetting the flame propagation within the combustion chamber.

The "squish" effect promoting swirl and better atomization is simply a side benefit.

ME Leigh

I'd use the .035 if it was me but i would also reshape the combustion chambers to reduce the compression.

JesasaurusRex

iTrader: (3)

Thanks for the info fellas, looks like ill be using the .051 head gasket then. So who wants a set of 4.2" bore .040" compressoin copper head gaskets

AsphaltAssalter

Yes it's running fine for years with no problems, I wanted it set up to have .035 to .040 quench but the machinist decked the block too much so I decided to run it anyway as I don't need to rev it over 6000 RPM. And I have read in some mags of some professional engine builders using a lot less quench, but I don't trust much of what I read in those rags.

ME Leigh

Alot of race cars and engine builders, build motors with the tightest quench possible. Usually around .025"

JesasaurusRex

iTrader: (3)

And the advantage of such a tight quench would be?

JesasaurusRex

iTrader: (3)

K guys im having issues with my dynamic compression calculators. One says use cam timing abdc and which puts my dynamic compression at 9.9 and the other says use cam timing abdc + 15 degrees which puts my dynamic compression at 8.8 . This is a big difference and could easilly mean pump gas or no pump gas. Anybody think they can calculate my dynamic compression for me so i know which formula is correct.

Static compression is 11.59
Rod Length is 6"
Intake Closing Point (degrees)
ABDC @ 0.050 is 55

Would really appreciate it thanks fellas.

Motor City Mike

It's been proven in dyno tests numerous times, the tighter the better, makes more hp (better combustion). However, I would limit myself to .040", just to be on the safe side.

rx7speed

dynamic compression is more then just when your intake valve closes

dynamic compression can be effected by many things.
what rpm range your running at
what heads your using
what your exhuast size is and intake size
valve sizes
all that effect dynamic compression


it would be like trying to figure out volumetric efficiency just by by the same

part of the reason they call it dynamic compression is cause it changes with rpms and many other variables

JesasaurusRex

iTrader: (3)

You are confusing dynamic compression with cylinder pressure. Dynamic Compression never changes just like static compression.

rx7speed

isn't that the meaning of dynamic though? change

what is dynamic compression? first lets go into static compression.
static compression as you prolly know is a ratio of expanded volume to compressed volume
so a 100ci motor with 10:1 compression means the compressed volume is 10ci
for simplicity sake think of a single piston motor here

now for dynamic compression
isn't it the ratio of the volume of air brought into the motor vs compressed volume?

ie a motor is 100ci it's compression ratio is 10:1. lets say it is running at 50% VE that would mean it is drawing in only 50ci of air correct?
so lets see you are taking 50 ci of air that gets compressed into the same 10ci space

50:10 ratio correct?
or 5:1 dynamic ratio
this changes through the rpm range
huge lift long duration cam will bleed of cylinder pressure at lower rpms so your dynamic compression will be lower but at higher rpms there isn't as much time to bleed off the pressure the dynamic compression ratio goes up




the words alone should be clue enough
dynamic
http://dictionary.reference.com/search?q=dynamic
"Characterized by continuous change, activity, or progress"
"marked by continuous usually productive activity or change"

static
http://dictionary.reference.com/search?q=static
"Having no motion; being at rest; quiescent.
Fixed; stationary."
"characterized by a lack of movement or change"

even as one of the examples under static
adj 1: not active or moving; "a static village community and a completely undynamic type of agriculture"


a motor with a specific cam will always be drawing in different amounts of air based upon rpms as a main factor but also other things like the heads, cam exhuast/intake manifold that can either restrict flow or aid in flow. your throttle also plays a huge part in dynamic compression

if your at idle with that same 100ci motor you might only be pulling in around 15ci of air (example I would have no idea how much it would really bring in ) so that 15:10 would be a 1.5 dynamic compression ratio cause again the throttle is being a major restriction of air flow

and yes all of these will effect cylinder pressure because cylinder pressure works with dynamic compression

just like taking a motor and increasing the static compression by shaving the heads or putting dished pistons in there bu tkeeping the rest the same is going to increase your cylinder pressure as well

if you hav eever seen a dyno chart where they have the ability to read volumetric efficiency (VE) take a look at it and notice how the number changes all through the rpm range? like that motor using 100ci with a 75% VE is only going to be drawing in 75ci of air or 7.5:1 dynamic compression but up another 1000rpms it might draw in 80ci of air. you get the idea

and sorry if it seems like I'm trying to make you out to be an idiot I'm not. just giving the info out for others who might see this as well and going into a little more detail for those who might not know.
figure just cover my bases here for one to back up what I was saying and two for those who might ask questions seeing the post.


but yeah the dynamic compression calculators might give ROUGH idea but there is more to it then just taking static compression and then figuring out when the cam closes to figure out true dynamic compression

JesasaurusRex

iTrader: (3)

Here ya go buddy

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

ME Leigh

The dead space volume created by the head gasket is the prime location for detonation to originate. Many of the engines built for the Engine Masters Challenge had such tight quench areas that the pistons were hitting the heads.

Streetiron85

I have a 327 that I deliberately decked to leave the pistons .004" out of the hole, leaving a quench clearance of .035". It had cast pistons and a fresh rebore with the recommended piston/bore clearance, or perhaps slightly looser.
This was an engine that I consdered expendable, so I just thought I'd try it out to see what would happen.
The project was moved to the back burner due to a cracked block which was unrelated to the quench clearance... I should have magged it before it was bored.
But it did have some run time before it was shelved, and there were no problems with piston dome to head contact in the time it was running.
I did the project thinking that if there was any sound of metal to metal contact that started developing as the engine wore in, I would correct it with the thicker gasket.
My suggestion is to try it with the thinner gasket if you're feeling adventurous. Or if you want to play it safe use the thicker gasket.
Yuo can calculate, and speculate, and debate dynamic CR til you're blue in the face, and then end up being proven wrong in an actual road test. It sounds like your DCR is in the ballpark, give it a try and see what happens.
If the quench clearance is too tight, what you can expect is that there won't be any complications for several thousand miles, after the piston to bore clearance loosens up a bit, and the pistons rock in the bores. It will be a tapping sound from the top end that becomes louder. It won't destroy your engine if you fix it early by placing a thicker gasket in there.

Good Luck - Have Fun

Motor City Mike

One of the big factors determining how much squish/quench clearance you can get away with is how well the piston fits in the bore. A sloppy fitting piston will be more apt to make contact with the cylinder head, due to the piston rocking. Forged pistons and/or bigger bore sizes are more intolerant to this also.

Streetiron85

DITTO

After some thought, as a PS to my previous post I should add, There's a lot of difference between a cast 327 piston like the one I was using in my engine, and the forged stroker pistons you're using.
I believe the true issue here isn't your CR, but whether there will be metal to metal contact between the piston and head.
It's easy to understand why your machinist suggested the thicker gasket for more clearance. Still, if you're in the mood to experiment, you could try the thinner gasket.

rx7speed

I found one myself
http://www.rehermorrison.com/techTalk/08b.htm


but why would one make dynamic compression nothing morethen how much the piston goes up in the bore before valve seats and nothing more?

if that was the case how much compression you run doesn't mean much at that point being cylinder pressure is part of what is going to cause detonation


and dynamic being change
also think about how much volume of air your bringing in vs how much it gets compressed
that would make sense for dynamic compression wouldn't it?
cause the amount of air you bring varies from rpm to rpm

JesasaurusRex

iTrader: (3)

True the amount of air you pull in varies with RPM but wouldnt each cylinder still pull in the same amount of air during its intake stroke, it might be doing it faster now since the engines spinning faster but it shouldnt change per cycle...should it?

JesasaurusRex

iTrader: (3)

Im also confused now on the dynamic compression/pump gas thing. Donno if you guys know the guy on this forum, IHI is his name on here. Has a red third gen fbird with drag lites all around runnin 10.9s and he says hes running on 91 octane. So i pm the guy asking him what his dynamic compression and quench is. Didnt know his dynamic compression but has a static compression of 11.5 . I asked him for his cam abdc which he told me was 49.5 . This puts him at a dynamic compression of 10.1ish to 1 . Duration on his cam is like 255/260 somethin and hes got some afr 210 heads and its a 388. His dynamic compression is higher than mine would be with the .039 head gasket (originally thought it was a .040 now i took a look and its a .039, which would move my quench down to .034 ) . So what gives? How come hes the pump gas king with a dynamic compression of 10.1 : 1?

AsphaltAssalter

I would think the cylinders would fill a little better at high RPM by getting a little bit of "RAM AIR" by the increased intake velocity because the air doesn't have time to slow down as much between cycles. Correct me if I'm wrong.

dimented24x7

iTrader: (2)

The ammount of air, or more appropriately, the volumetric efficiency, changes with the engines rpms. The VE, which reflects how much air the engine can move in relation to its own displacement, is entirely dependant on the engine setup.

rx7speed

thats part of the reason why that intake valve will stay open after BDC when the piston is moving back up again.
air has some inertia to it at at high enough rpms itwill still be able to enter the cylinder while tha tpiston moves up
look at some nascar motors they can pull in more then there rated displacment at high enough rpms
so like lets say they are running a 358 motor they might be breathing 380ci of air per cycle because of that ram air effect
but at lower rpms the cam bleeds off enough pressure and lacks

or think of some other things
put some 215cc heads on a 305 your going to kill intake velocity so no ram air effect and the car is going to act very soggy through most of the rpm range untill way up high

or what about the 350tpi motor the opposite is true it breaths real good and has good cylinder flow through the lower and mid rpm range but what's it do up top? starves to death trying to get air in there. the intake velocity is getting to a point where it is too high and chockes cause the restriction so now the cylinders don't fill as much as they did to the lower rpm range.

a lot of that is the induction system made for a 305 put on a 350.
not just the cam

JesasaurusRex

iTrader: (3)

Wont forged pistons expand less than cast? Or am i just sounding stupid? Also being the heads are aluminum and the pistons are forged aluminum shouldnt they expand at the same rate? Or not because one is forged and the other is not? Another thing is ok the piston expands, wont the head expand too? Also how bout piston to valve clearance? .649 lift is a lot. The valve reliefs are pretty decent on the SRP pistons but still.

AsphaltAssalter

Forged pistons expand more than cast pistons do, that's why they need more clearance when they are fitted cold, cylinder heads expand too but they don't expand down into the combustion chamber so it doesn't affect quench any.

JesasaurusRex

iTrader: (3)

I see...so who thinks i can get away with the .034" quench

F*ck it ill try it, what should i look/listen for after i assembled it? Before i run it im talkin about.

rx7speed

if you hear the sound of valves going out the exhuast that's not a good sign


but no listen for tinking noises I would assume.
you said you ran a clay test on it right? maybe start the motor up let it warm up and listen for noises and give it a few revs
take the head off check the valves and pistons for damage if good you might be ok but this isn't my area of knowledge I'm just assuming here

Streetiron85

With your shortblock assembled, put a dial indicator on the edge of the piston dome 90deg to the wristpin when the piston is at TDC.
Then rock the piston from one side to the other and see what you come up with.
If your pistons hit the heads, it won't be solid metal to metal contact, so it won't self destruct immediately. But it's not something you want to let go for any length of time, especially racing.

Edit:
I'm pretty sure rods expand more than pistons do, at least in relation to the deck height.
I'm not sure how much the quench clearance closes up with the operating temp of the engine, that sort of stuff is a little bit too involved for my level of experience.

It would be interesting to find out how much those pistons of yours rock in the bore though.

JesasaurusRex

iTrader: (3)

I ill take the rings off one and do it, ill let yawll know.

Streetiron85

I was just thinkin, if you rock the piston all the way from one side to the other, the dial indicator reading will be double what the actual difference would be if you were comparing against the TDC deck height.
See what I'm getting at?
I hope so because I'm not sure how to explain if you don't.

JesasaurusRex

iTrader: (3)

why would it be double?

katman

My 359ci, KB D dish hyper pistons, cast iron bowtie heads, .564 lift, 264 @ 50, 106 lsa, 1.5 roller rockers, pistons are out of the hole .008, .041 gasket. Previous owner ran 1/8 mile for several years and I've run it for 2 years. Car runs in the 12s, I shift 6200-6500 and go through the traps 7200-7300. Previous owner was a auto tech teacher and he said the bottom end was stock reconditioned parts with a balance.

Streetiron85

Imagine a teeter totter (sp?) and lets say you want to measure how far it rocks in relation to the neutral point (or the center).
If you measure the full stroke on one end. That would be twice the amount you're actually seeking.
You're only interested in finding out how high the edge of the piston comes up from the neutral point (or .005" above the deck)

Sounds like katman has the most useful info yet. But his is a 3.5" stroke (longer piston skirt) with hyper pistons (less bore clearance) and those would both be factors to keep in mind.

But then, ideally the running clearance of a forged piston skirt at normal operating temp should be the same as hypers... Isn't that the idea?

JesasaurusRex

iTrader: (3)

i ment why do you not want to take into account how far it rocks from side to side and not only half way, i understand the doubling concept.

JesasaurusRex

iTrader: (3)

No didnt run a clay test yet, i havent got my spirolocs for my piston pins yet so i havent been able to get that far yet. Listen for noise, this is gonna be fun on a solid cam with that much duration and that lsa

JesasaurusRex

iTrader: (3)

Oh one more thing, when measureing how far out of the hole they are (might as well double check what hte machinest told me) which part of the piston do i put the dial indicator on? The dome part?

Streetiron85

A depth mic is a better tool to use for measuring the piston height above or below the deck. Although I've used a depth ga on a digital caliper cause I don't have a depth mic.


You lost me on that question

JesasaurusRex

iTrader: (3)

So lets say its just sitting perfectly in the cylinder. You can rock it one direction or another (the double thing) why would you only care about how much it rocks to one side and not take into account it rocking the full length?

Streetiron85

Because of the shape of the combustion chamber, it's open on the outside half so it wouldn't matter if the piston rocked up on that side, cause there's nothing there to make contact with.

I'm still not sure we're on the same page here. But basically it doesn't make any difference if the piston rocks downward or if it rocks upward on the open side of the chamber. Only if the piston rocks upward on the quench side of the combustion chamber, cause that's the only place there could be metal to metal contact.

JesasaurusRex

iTrader: (3)

Got cha makes sence now

Streetiron85

Whew!! words ! ! !
I wish I had some sort of graphics program on this dinosaur computer, it would make explaining things so much easier.