Dynamic compression

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twayne24365

twayne24365

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I was talking to a local engine builder today about compression and whats pump gas safe with iron heads. He told me the static compression doesn't really matter as long as the dynamic compression is below about 9.5:1..

I gave him my engine for example, its about 11.2:1 with a 590 cam, I read that the intake closing of that cam is 80* so he plugged it in his computer and it came out to 8:1 dynamic compression and he said it's pump gas safe....

Any comments on that one?
 
8.1 is cheap pump gas safe.

I have a 10.17 dynamic small block I am going to throw in my farm truck, 50/50 mix race gas required.
But it should pull good.
Plus I only put 1,000 miles on it a year.
 
twayne24365 said:
I was talking to a local engine builder today about compression and whats pump gas safe with iron heads. He told me the static compression doesn't really matter as long as the dynamic compression is below about 9.5:1..

I gave him my engine for example, its about 11.2:1 with a 590 cam, I read that the intake closing of that cam is 80* so he plugged it in his computer and it came out to 8:1 dynamic compression and he said it's pump gas safe....

Any comments on that one?
Click to expand...

It is only true at low engine speeds. At higher engine speeds the engine will see the full 11.2 compression and will detonate.
 
9.5 is too high a DCR with iron heads and pump gas. 8.5 is usually considered the limit.
 
RustyRatRod said:
9.5 is too high a DCR with iron heads and pump gas. 8.5 is usually considered the limit.
Click to expand...

What would you consider safe with aluminum heads, and my 10.17 dynamic?
 
Take this chart with a grain of salt. Got it from a big block Ford site.
 

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AndyF said:
It is only true at low engine speeds. At higher engine speeds the engine will see the full 11.2 compression and will detonate.
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That makes sense, what person wouldn't use the upper rpm range, I'm sure my 4200 converter will like the upper rpms.

Do you think I would be safe with a 50/50 of cam2 and 93?
 
I run a blueprinted 11.5 with iron heads and 93 octane no problemo, with quench and a 4 gear standard and 3100 lbs. and 4.30 gear
 
But at higher RPMs, while it is true the engine will see it's real compression ratio, it's also true that the engine is under WAY less load. Just something else to consider.
 
Little off topic but....

Hey rusty, did you ever get your 383 finished? I think I remember you said you were building one with those kb400 pistons?
 
twayne24365 said:
Little off topic but....

Hey rusty, did you ever get your 383 finished? I think I remember you said you were building one with those kb400 pistons?
Click to expand...

No. That's the engine baracus is building. He bought it.
 
I always understood it as 10.5:1 static on iron heads and pump gas and aluminum can go a whole point higher on pump gas. By pump gas I mean 93 octane.
 
From what I've read, guideline is 8:1 dynamic for iron, 8.5:1 for aluminum to run pump gas safely (whether that's 91 or 93 octane, I do not know). Mine is right at 8:1, cranking pressure is only 150 psi, I run 91 in it and don't have issues unless I get a bad tank of fuel (which has happened). It will click off 10's all day long in a full street trim Dart to boot. I do retard the timing back to 28 degrees to run the 91 octane, but it will still run 10's with only 28 degrees timing. It might accept more timing with 91, but I know 28 degrees works without detonation problems and it still makes enough power.
 
alpha13 said:
I always understood it as 10.5:1 static on iron heads and pump gas and aluminum can go a whole point higher on pump gas. By pump gas I mean 93 octane.
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Always look at the dynamic. The static won't matter as much as dynamic will. I have seen 12.5:1 with aluminum heads that ran pump gas but they pulled back a lot of timing to accomplish this. I do not know what the dynamic was. Mine is 11:1 iron and is fine on 91 because the dynamic is 8:1.
 
Using the wallace calculator i figured my iron head 383 build to be 8.5 dynamic and it's measured at 9.7:1 static, estimates 172 cylinder pressure. What do you guys think about that? I can still use a thicker head gasket to lower it if needed.
 
baracus
That would have to be a real small cam.The 8.5/9.7 split is too tight, unless it has a very early ICA
I did the math;With a 4.27 bore,it comes to 50* ICA; that's 8 * smaller than the factory cam(58*ICA). Are you gonna use it to power a tow-truck?
 
AJ, its very likely i did that wrong. .030 over stock stroke 383. 9.7:1 compression. Hughes STL 3842B53-9. 238, 242.
 
baracus said:
AJ, its very likely i did that wrong. .030 over stock stroke 383. 9.7:1 compression. Hughes STL 3842B53-9. 238, 242.
Click to expand...
To do the calcs you will need the advertised intake duration and the LDA.And Hughes does not publish advertised on their website, so you gotta call. I do see they publish it as a 109LDA.
You probably entered the 050 numbers and that does not compute. Adding an arbitrary 40 to 50 degrees for ramps is only guessing.
However 238* is pretty big, and I would guess wants close to 11/1 Scr, perhaps a bit more.And that, with iron heads might put you over the edge, as to detonation,with pumpgas.
Do you know that 9.7 is a fact.If it is, and is not easily adjusted, then a much smaller cam might be worth a look, or one with really fast ramps.

So get the facts; exact Scr, and ICA of that cam in advertised duration, as close to zero lift as Hughes will offer. .006 is typical of other manufacturers. Hughes likes to give it out at .008, which means you can't compare theirs directly to anyone elses, and makes their clearance ramps seem shorter than they really are.
 
Thanks AJ, I was confused on that. I will call them and get the correct info. That is the cam they recommended to me over the phone.
 
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