I am building an 350 with vortec heads for my 70 my compression ratio will be 10.25 to 1 with .040 of quench. Does anyone have a similar setup? I was thinking about using an xe268? Any feed back on cam selection would be appreciated. I currently have 3.08s and 2400 stall

Probably not enough cam for the compression but let somone else chime in.

I wouldn't run anything smaller than a 276º cam. And probably not smaller than a 278. Then you get into gearing issues. If you can drop the CR to 9.9, the 268 will work.

I tend to agree with the above.

Joe, what octane are you planning on running? I don't think the 268* would be enough cam to bleed off that compression. Might be detonation issues unless you have a crisp tune. Also might not tolerate running at higher temps. I do like the quench, tho.

I think Pat hit it pretty good. Bigger cam to bleed off compression (unless real good gas), but only to a point. Then your gearing would be a hindrance unless you run an a OD auto. What tranny do you have now?

What are your goals and intended usage for the car?

Am I in the ballpark guys?

unless you have CC'd everything, your CR will be different than advertised. My 64cc dart heads were 69cc..my 5cc VR pistons were 7cc There was 2cc's from the compression ring to the top of the piston...the dish/margin of the valves will change the cc's too...my supposed 10.2-1 CR would of turned out to be 9.3-1.

We had a 10.3:1 355" Remosa motor with an XE268H cam and iron heads that ran very well on 91 octane California gas.

How can you bleed off compression if there is no compression to bleed off? I would think a more correct term would be to delay the compression.

Jim, that is really a lot different than advertised specs. Another reason to measure everything.

onovakind67, was this a measured CR? If not, you might have the same situation as Jim.

Bleeding off compression refers to reducing the theoretical static CR with a late closing of the intake valve. A 10.5:1 SCR engine will have about 8.4:1 CR after the intake closes (64º intake closing ABDC), bleeding off the CR from 10.5 to 8.4. The compression isn't just delayed, it is reduced.

I once ran iron non vortec heads with a 10:1 compression, and the smaller 262 cam in my 355 and 92 octane gas and never picked up any knock on the sensor.

The compression isn't just delayed, it is reduced.


So why isn't it referred to as 'reduced' compression? Bleeding off imlpies letting something out, but if it's not there to let out....

What happens later in the rpm band when the VE increases, i.e. if you were to spin the motor at peak VE, would the compression increase?

Actually the late closing does let it out, through the intake valve. The compression ratio never changes (it's fixed when the engine is built), cylinder pressure does change, though. Having 10:1 at idle is still 10:1 at 7000 rpm. The amount of mixture introduced into the cylinder can and does change. But it still is compressed at the same ratio. Closing the intake late positions the piston farther up the cylinder when actual compression starts. This causes the effective CR to be lower than the static CR. Increases in VE will increase cylinder pressure and increase power but the CR is still the same. Don't forget, a ratio is the comparision of two volumes. In the case of static CR, the sweep volume of the cylinder to the chamber size (accounting for crevice and other spaces). For DCR it is the comparision of the cylinder volume using the position of the piston at intake closing to the chamber size. None of these volumes change while the engine is running. The bleeding off refers to the lower effective CR the engine sees after the intake valve closes.

Thx for all the feedback. Right now i have a th350 and use the car as a weekend toy and run 92 octane fuel.I am planing bigger gears down the road as well as on day an OD trans. I have measured the deck height and was using stated numbers for other volumes i guess its time cc everything to get a more accurate CR. I also have a cowl induction set up do you think that will make a significant difference in octane requirements?

Closing the intake late positions the piston farther up the cylinder when actual compression starts.

I'm still confused. If you don't have any actual compression, how can you bleed it off?

onovakind67, youve been on steves nova site havent you..... i got into an arguement with a guy about this over there, he liked to ramble on and on with his fancy terms and it seemed to me like he was trying to impress someone. The fact of the matter is, "bleeding off compression" is a term that is very commonplace among enthusiasts. BUT for the guy who dosent understand swept volume, volumetric effecency, and all those fancy terms. Its a simple way of saying all of what pat just said. Call it a slang term, maybe not proper, but slang. Anyone ever... "bang some chick" or did you, as my boy Mike Tyson likes to say.... "Fornicate"

Originally posted by onovakind67:
The compression isn't just delayed, it is reduced.


i.e. if you were to spin the motor at peak VE, would the compression increase? Doesn't matter, motors don't make compression, engines do. ;)
Just messin' with you to illistrate the point that ESPECIALLY in the auto world, we don't always call things as we should. We tend to be lazy and say things which get to the point but aren't technically corect.
As long as you get the idea, what does it matter what it's called?

Jeff

personally i think 10.25 to 1 would work perfectly with the XE268, and especially with those gears, i definately wouldn't be wanting a bigger cam than that until i got some steeper gears and a little more convertor.

Ok it might work, but why would somone want to push the DCR limit with a total street car. For that last 5 hp and 5 lb/ft of torque you will never notice or miss???

Would this be the post from Steve's?

http://stevesnovasite.com/forums/showthread.php?t=10097

Those Rousch Racing guys tend to be sticklers for detail. Our Remosa engine was a true 10.3:1 engine. This engine was made in the Remosa plant in Mexico City, and since the altitude there averages about 7000', high compression engines are the norm. If I run the engine through EA3.0 I come up with a DCR of 7.61:1.

Static compression ratio is what you build into the engine by way of dome volume, combustion chamber volume and compressed head gasket volume. Dynamic compression ratio is the maximum pressure that develops inside the cylinder during the compression stroke. Dynamic compression ratio is influenced by the overall engine breathing efficiency (engines with good flowing ports along with an equally matched induction system) and also by the camshaft profile (how much air the camshaft allows to enter the engine and when it enters the engine). Ring seal is also a factor (as rings seat, they leak less, and increase dynamic compression ratio; as rings wear, they leak more, which decreases dynamic compression ratio). Air density is the final factor in the dynamic compression ratio equation. The "better" the air, the superior the dynamic compression ratio. All of these elements must be taken into account when determining the engine compression ratio. The ultimate goal, according to Reher-Morrison is to create as much dynamic compression ratio as possible, without creating detonation.

http://www.hotrod.com/tipstricks/34219/index5.html

It's important to make a distinction between static compression ratio and dynamic compression ratio. The static compression is what you measure on an engine stand with a burette and graduated cylinder. It's a simple mathematical comparison between the volume above the piston at the bottom and top of its travel. Dynamic compression is much more difficult to measure because it depends on how efficiently the cylinders are filled when the engine is running. Factors such as camshaft timing (specifically the intake valve closing point) and the tuning of the induction system have a major impact on dynamic compression.

Dynamic compression ratio is what really counts in a racing engine because it determines the actual cylinder pressure. Think of two engines that are identical except for their camshafts. The engine with the shorter camshaft duration will typically have higher dynamic compression at low rpm because the intake valve closes earlier on the compression stroke. The engine with the longer duration camshaft will have less dynamic compression because its intake valve closes later after the piston has traveled farther up the cylinder. On the other hand, if the long-duration cam does a better job of filling the cylinder at high rpm than the short-duration cam, more air and fuel will be trapped in the cylinder and the resulting dynamic compression ratio will be higher.

http://www.rehermorrison.com/techTalk/08.htm

Onovakind67, are your initials MG? I knew that I knew you from the nova message boards several years ago.
Getting back to the original question ;) I think the xe268 would just work with a really sharp tune with a true 10.25-1 compression. I run a measured 9.4-1 355 in a 4500 pound truck with 3.08's and mild convertor, and I run it on 89 octane. Cranking compression is 180psi. I run 36* total timing (34* runs better on the top end, but the low end is stronger with the higher initial). I also have horrible quench with cast flat tops .040 in the hole, plus head gaskets. One thing though, my engine runs very cool. Too cool really...typically in the 140-150* range, which helps.

This is the problem. Terminology!!!. As said in many places the term dynamic compression ratio is not a great one. Effective Compression Ratio would be a better one.

In any case, a ratio does not change because of the amount of mixture introduced into the cylinder. How can the volume of a cylinder compared to the volume of a chamber change while the engine is running? The amount of gas stuffed into the chamber can change but it is still compressed the same amount no matter how much is there. If the volume in the cylinder is 100 cubic feet and that is compressed 10 times to 10 CF then the cr is 10:1. If there is 10,000 CF in the cylinder and that is compressed to 1000cf that is still 10:1.

The statment above that the dynamic compression ratio will be higher at high rpm is just wrong. The cylinder pressure will be a lot higher because there is more in the cylinder to be compressed but the gases are still compressed the same amount. This ratio cannot change. It is built into the engine just the same as the static CR is. 8:1 at idle is still 8:1 at 7000 rpm or 17,000 rpm. If he had said the Dynamic Compression was higher, he would be correct. But the ratio doesn't change.

BTW, I still think a 268 cam is too small for a 10.25 CR. You'd be courting detonation problems. But some have got away with it. A fellow on Chevy Talk claims to run 87 octane with an 8.5 DCR. I know I wouldn't do it.

This ratio cannot change. It is built into the engine just the same as the static CR is. 8:1 at idle is still 8:1 at 7000 rpm or 17,000 rpm. If he had said the Dynamic Compression was higher, he would be correct. But the ratio doesn't change.



Ever hear of variable valve timing? Rhodes lifters?

Originally posted by onovakind67:
This ratio cannot change. It is built into the engine just the same as the static CR is. 8:1 at idle is still 8:1 at 7000 rpm or 17,000 rpm. If he had said the Dynamic Compression was higher, he would be correct. But the ratio doesn't change.



Ever hear of variable valve timing? Rhodes lifters? Valve timing DOESN'T change the STATIC compression ratio. It's pure math.
http://members.cox.net/jpeterson40/comp.jpg
Valve timing would only affect the dynamic ratio. You wouldn't even need to have the cam in the engine to get the static ratio.

Exactly.

I knew someone would say somethng like that smile.gif . Yes, that will effect it. Volvo or Saab (I forget which) has a variable compression engine they are working on that changes the chamber size while running. But my statement stands with those caveats.

Originally posted by Pat Kelley:
I knew someone would say somethng like that smile.gif . Yes, that will effect it. Volvo or Saab (I forget which) has a variable compression engine they are working on that changes the chamber size while running. But my statement stands with those caveats. Yes, I saw that Saab(?) engine in a magazine a long time ago. IIRC, the head gasket was kind of like a bellows and one side of it would expand and contract to alter the chamber volume. Not surprisingly, they were having sealing issues.
Nice idea if they can get it to work.

Jeff

Onovakind67, yep thats the conversation.....