Thinnest performace head gasket

[ward00]

Steve C, in a weird kind of way I am back to my original question about the head gasket. Your measurements indicated that 1.2 mm compressed would be the best I could do for the head gasket. I have found one that purports to be 1.013 compressed, which would bump the compression up to about 10.03

Here it is:
https://www.cometic.com/i-24765786-lanc ... -each.html

Do you know anything about this gasket that would lead you not to use it?

[fiatfactory]

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[fiatfactory]

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[ward00]

SteveC, curious about other cam options. I have 40/80 80/40 cams and delta regrind 42/82 82/42 cams. Which in your opinion would provide more performance? This board also periodically suggests just changing the intake and leaving the exhaust cam stock? Is that something you would recommend. Keep in mind that I hope to be able to drive/idle this car on the street as well.

Also, just an FYI, I am having the head rebuilt right now to have larger valves. Rather than decking the head, and assuming I will have valve clearance, would your recommend taking something of the head surface to increase the CR.

[fiatfactory]

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[ward00]

Steve, will get the specs for the cams when i get home from the road this weekend. Thanks for the very undetstandabke info so far.

[fiatfactory]

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[ward00]

Yes, I'm tending to doing just that. I bought the block sight unseen from someone who represented the pistons as 10.4 cr and who based on his supposed racing experience should have known by just looking at the low dome height that the cr would not be that high. He was running a volumex supercharger with the pistons so for his purpose I suppose 10.4 would probably have been too high. It is what it is and decking the block and checking for what else might be hinky is in the cards.

[fiatfactory]

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[ward00]

unfortunately simply knowing the stated open/close timing values of a cam lobe isn't really enough information to make any sort of evaluation...

but if this timing value corresponds to actual running timing events (i.e. duration at running / cold lash clearance) then based on the dynamic CR calcs the performance would be lack lustre if used in an engine of around 9.8:1 static CR, as this would give a simple dynamic CR for the 40/80 of just 6.88:1 as I calculated before, and as the 42/82 closes a couple of degrees later it's dynamic CR will be slightly lower again.

cam choice shouldn't be based on anecdotal "evidence" that someone fitted this cam to their engine and got XXX Hp, there are far too many variables to be able to simplify the internal combustion engine to that level... differences in head flow, static compression ratio and actual VE of the engine changes everything.

Cam choice should be based on what sort of revs you plan to be turning, and what sort of air flow you think the head is capable of.

If you study the motion of the piston in a four stroke internal combustion engine, the speed of the piston as it travels up and down the bore is not linear... it comes to a full stop and reverses at TDC and BDC, yes, but the speed of the piston as it travels in one direction up or down is far from constant... the reason is kinematics...which according to wickepedia is "the features or properties of motion in an object."

Anyway I am not going to go thru the math here... too much information for nearly everyone... but I can give you a rundown on what happens in a 2 litre engine, and then try and explain how this relates to cam choice.

In a 2 litre the point of maximum piston speed is at 75 degrees after TDC, and if you can get your 2000 to spin at 7000rpm, the piston at this point is travelling 34.549 metres per second piston speed (instantaneous), the AVERAGE speed of the piston s travel at 7000rpm is 21.057 metres per sec

that's pretty high for a cast piston at nearly 35 m/s, in your case you have forged venolia's, so piston integrity at 7000 will not be an issue.

so lets say "air demand" is driven by piston velocity and piston area...

from around 70 ATDC to about 80ATDC the piston is imparting a heap of "demand" due it's high speed... in a 2 litre at 7000 rpm the pistons speed is above 34m/sec from 66ATDC thru to 83ATDC... all that demand is being drawn thru the partially open valve.

Changes in valve timing affects several things and to choose the right cam, IMO we want to look at what sort of air demand we have at this point around 75 degrees ATDC. One of the things that happens with a longer duration is usually more lift at this point, same with a cam with more total lift... at 75 ATDC more lift is more flow at the point of peak draw.

But opening the inlet valve earlier and earlier (before TDC) is only really effective at much higher RPM (think VTEC) and low speed power is poor with this sort of valve timing...remember that at lower engine revs, flow past the inlet valve before the piston passes TDC is only going to be initiated by a pressure inside the cylinder (at the moment the inlet valves lifts off it's seat) which is less than atmospheric, the only way this can happen is if the exhaust gasses have been scavenged effectively... so if exhaust flow / scavenge is poor for whatever reason (port/valve/seat throat/header/system) then this will affect how well the engine behaves / performs as the inlet valve is opened earlier. At higher RPM the air flow speed improves, as the inertia imparted to the air column allows it to keep moving forwards even when the valve is closed (effectively "piling up" behind the valve head for a millisecond) so at high RPM the flow actually becomes "continuous" due to the inertial effects, so then the initial low lift flow and VE improves and power benefits, some people might describe this as the engine "coming on to the cam" .... and can feel, in extreme cases, like the power band of a two stroke motor cycle!

So we want to calculate AIR demand at this 75 ATDC point and size the port/throat and most other parts of the inlet based on some number we get, an engine is after all... a glorified air pump!

Then based on the air flow at around 75 atdc ... based on the cam lift at around 75atdc... we work out the valve curtain area , the throat area, the port average cross section.. . we have an idea of the engines air flow cycling value (but it's never 100% of course) and know that the relationship between valve curtain area, port throat area , average cross sectional area of the port all have mathematical significance to one another... but there is no one number that is "magic" , it all depends again on the bore/stroke ratio the bore area to valve area ratio...and that difficult to measure VE.

But bottom line is, if flow at this lift, and this moment when the piston is moving fastest is really good... then the column of air gathers a lot of speed, even when the piston slows down and approaches BDC the column of air in the port still has loads of inertia and despite the piston reversing and heading back up the bore after BDC the column of air will flow virtually uninterrupted until the valve curtain area again becomes too small for the high air speed, the both port air speed and port flow drops rapidly after this point where the reducing valve curtain area becomes a restriction... so lift at BDC is also an important value for a cam, and I like to plot the lift values for each degree of lobe operation, and like to work out the valve curtain area / degree / lift curves and see where things intersect...

pittatore 77 grind / Alquati A16 has about 9mm of lift at 75 degrees, full lift 110 atdc of 10.35, inlet valve is closing quite rapidly at BDC but still about 5.75mm off its seat....

Can you tell me these sorts of numbers for any of the cams you have / are considering?

SteveC

Steve, the intake cam I plan to use is from Bayless and the first page specs are
duration 240 @ .050 lift
Lift 10.8
lobe center 106deg

at .015 lift, intake open 43.3 BTDC and Closes at 82.3 ABDC with an intake duration of 305.6
at .020 lift intake opens 33.6 BTDC and closes at 68.1 ABDC with an intake duration of 281.7

is this enough for you to tell me if it is a cam you recommend.

If not, I have the full spec sheet that I can email or upload to the Mirafiori site if you have a presence there.

Thanks

[fiatfactory]

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[ward00]

Steve, have sent the cam specs to your email