Well, I was talking to some people about this. It's no secret that aluminum heads allow for higher dynamic compression ratios without detonation. As we all know, higher compression ratios usually have a direct increase in power output of the engine that normally equates to 1 full compression point is worth about a 4% increase in power. Now, the issue that I am having is that I wanted around 10.2:1-10.3:1 compression on my 383 stroker but people are telling me that 9.7:1 is the upper limit of a GEN 1 small block with aluminum heads. I said how is that possible because my LS1 has 10.2:1. Supposedly the reverse cooling of the newer style small blocks LT1/LS1 allow the coolant to travel to the cylinder head first resulting in lower cylinder head temps. This is what allow higer compression ratios without detonation. With the old school small blocks, the heads recieve coolant last, and therefor are subjected to getting less compression because of it. So what do you guys think? What is the higest compression ratios that you guys are suing with aluminum heads, and on 93 ocatane? And are you running full advance without detonation? Thanks for any information!

 

Im currently running 91oct on my newest creation. Its an iron head GenI, static compression is 10.8:1.

Most important is the dynamic compression, mine is fairly low. I cant remember exactly wat it was now but if i recall it is 8.8-9:1 which is definately iron head pump gas friendly.

So calculate your dynamic compression then you will know for sure if your good or not!

 

well, i know it's only my personal experience, but my 305 has the 113 aluminum heads and is pushing 10.4:1 static compression with 18* initial timing and 36* total all in by 3000rpm (no vacuum advance) without any ping on 93 octane.

how do you calculate dynamic compression?

 

There are lots of guys running pump gas Gen 1 small blocks with over 11:1 compression. To do this, you have to minimize quench area (.035-.040 range) and use a cam that puts your dynamic compression ratio (DCR) no higher than 8.2:1 - 8.3:1. Here is a link to a great DCR calculator.

Dynamic CR

 

I run 10.0:1 on SBC1 iron heads, 91 octane fuel. .054" quench distance IIRC. 36* timing at WOT, no pinging at 4000' altitude. I have 8.5:1 dynamic CR.

 

I was running 10.3:1 with a 350 and aluminum heads. Whoever told you 9.7 is the limit didnt know what they were talking about. The cam plays a huge role in what ratio you can and should run. I dont think 10.2 is going to cause you any problems unless you are running some sort of tiny cam in that 383.

LS1's arent reverse flow so its not the way the coolant flows but the overall design. The heads promote swirl and a better burn is a result and allows more compression.

 

8.5:1 is ragged edge for a aluminum head motor, but your altitude may play a role. Are you sure you're at 8.5:1? Using the specs in your sig, I calculated 8.27:1 DCR.

Intake Closing Angle is 60
Dynamic Stroke Length is 2.812
To get 10:1 static compression, I assumed .039 compessed gasket thickness, 4.160 gasket bore, 4 cc chambers, and flat tops with 5cc valve reliefs

 

heck you could run pretty much any static compression so long as your using a cam that will bleed off enough pressure...

I agree that the guy that told you 9.7:1 on aluminums needs a knock over the mellon...

 

i've got a similar build to 84z, but he has 10.8:1 and 9:1 DCR, almost same cam!
so i'm a little confused...

neil, you're about bang on on that. here's how I did the math:
using this site:
United Engine and Machine Co.

 

I tried the calc again using this simplified DCR calculator and got 8.27:1 again.


Then I plugged in your 4000' altitude and got 7.47:1.
----------
Here's the link....

Boost Compression Ratio Calculator</title>
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<p><font size="6">RSR Advanced Dynamic Compression Ratio Calculator</font></p>
<div align="center">
<hr>
<p><font size="5">ABDC Inlet Closing Figures, Evo Big Twin</font></p>
</div>

<div align="left">
<p><font size="4"><b>S&amp;S Cams:</b></font><font size="5"> </font><font size="3">502 (40 deg), 520 (40 deg), 546 (55 deg), 561 (40 deg), 562 (55 deg), 563 (64 deg), 600 (55 deg), 631 (61 deg).</font></p>
<p><font size="4"><b>Andrews Cams:</b></font><font size="5"> </font><font size="3">EV13 (31deg), EV27 (36 deg), EV46 (41 deg), EV59 (48 deg), EV51 (44 deg), EV72 (54 deg), EV38 (37 deg).</font></p>
<p><font size="4"><b>Crane Cams: </b></font><font size="3">H290-2</font><font size="3"> (43 deg), H286-2B (43 deg), H296-2 (49 deg), H304-2 (50 deg), H314-2 (54 deg), H306-2 (54 deg), H310-2 (63deg), 2000 (22 deg), 300-2B (34 deg), 310-2 (37 deg), 316-2B (43 deg), 326-2 (49 deg).</font></p>
<p><font size="4"><b>Comp Cams: </b></font><font size="3">EVL-2000 (38 deg), EVL-2015 (24 deg), EVL-3010 (39 deg), EVL-3020 (42 deg), EVL-3030 (44 deg), EVL-3040 (45 deg), EVL-3050 (50 deg), EVL-3060 (56 deg), EVL-3070 (61 deg).</font></p>
<p><font size="4"><b>Sifton Cams: </b></font><font size="3">RE-145 (42 deg), RE-140 (42 deg), RE-143 (35 deg), RE-144 (46 deg), RE-141 (41 deg), RE-146 (45 deg), RE-142 (52 deg), RE-147 (64 deg).</font></p>
<p><font size="4"><b>Red Shift:</b></font> <font size="3">559V2 (46 deg), 575V2 (54 deg), 625V2 (59 deg), 626V2 (54 deg), 653V2 (52 deg), 654V2 (59 deg), 655V2 (65 deg), 715V2 (71 deg), 785V2 (66 deg)</font></p>
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Sorry, the link got messed up and I couldn't edit it. Here is the link....

Boost Compression Ratio Calculator</title>
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<p><font size="6">RSR Advanced Dynamic Compression Ratio Calculator</font></p>
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<p><font size="5">ABDC Inlet Closing Figures, Evo Big Twin</font></p>
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<p><font size="4"><b>S&amp;S Cams:</b></font><font size="5"> </font><font size="3">502 (40 deg), 520 (40 deg), 546 (55 deg), 561 (40 deg), 562 (55 deg), 563 (64 deg), 600 (55 deg), 631 (61 deg).</font></p>
<p><font size="4"><b>Andrews Cams:</b></font><font size="5"> </font><font size="3">EV13 (31deg), EV27 (36 deg), EV46 (41 deg), EV59 (48 deg), EV51 (44 deg), EV72 (54 deg), EV38 (37 deg).</font></p>
<p><font size="4"><b>Crane Cams: </b></font><font size="3">H290-2</font><font size="3"> (43 deg), H286-2B (43 deg), H296-2 (49 deg), H304-2 (50 deg), H314-2 (54 deg), H306-2 (54 deg), H310-2 (63deg), 2000 (22 deg), 300-2B (34 deg), 310-2 (37 deg), 316-2B (43 deg), 326-2 (49 deg).</font></p>
<p><font size="4"><b>Comp Cams: </b></font><font size="3">EVL-2000 (38 deg), EVL-2015 (24 deg), EVL-3010 (39 deg), EVL-3020 (42 deg), EVL-3030 (44 deg), EVL-3040 (45 deg), EVL-3050 (50 deg), EVL-3060 (56 deg), EVL-3070 (61 deg).</font></p>
<p><font size="4"><b>Sifton Cams: </b></font><font size="3">RE-145 (42 deg), RE-140 (42 deg), RE-143 (35 deg), RE-144 (46 deg), RE-141 (41 deg), RE-146 (45 deg), RE-142 (52 deg), RE-147 (64 deg).</font></p>
<p><font size="4"><b>Red Shift:</b></font> <font size="3">559V2 (46 deg), 575V2 (54 deg), 625V2 (59 deg), 626V2 (54 deg), 653V2 (52 deg), 654V2 (59 deg), 655V2 (65 deg), 715V2 (71 deg), 785V2 (66 deg)</font></p>
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----------
Having trouble with the link. Click on the blue text above.

 

Sonix,

I see the difference. You used the .050 duration numbers and got 39.5 IVC. Then you added 15 degrees to get 54.5. I used the Lunati advertised diration numbers of 268/276 and got IVC of 60 degrees.

 

cool, based on that calculator, i'm running 8.27:1 DCR and 10.2:1 SCR

 

Sonix, we are running the same cam arent we? XE268...

I just start out with a bit more compression than you, i still drop 2 points on the dynamic like your engine does. I am also pretty much at sea level compared to your 4000'



Whats up with the links lately? Some of mine have been doing that...

 

i'm using the lunati equivalent of the compxe 268. @.050 it's 227/233 compared to 224/230. Just a smidge more aggressive. I also have the 1.6 rockers, giving me .521/.535" lift

Those links are pooched... But they take into account altitude? most excellent, I didn't think it'd drop THAT much....

which is more accurate, the @.050 IVC +15* fudge factor, or the "true" IVC of 60*?

oh, my bad 84z, I thought you said your DCR was 8.0, not 9.0, ok, then we both have a sound grasp of math

so the link is:
http://www.rbracing-rsr.com/comprAdvHD.htm

oh, and that link asked for IVC @.050, in which case my cam is 39.5, so i'm right back up to 8.45:1

 

[B]heck you could run pretty much any static compression so long as your using a cam that will bleed off enough pressure...[/ B]


It don't quite work that way.

 

Pretty much...

If your cam can bleed off enough pressure that you end up with a reasonable (sub 9:1) compression i dont see the problem there.

Just like the old "muscle car" engines, some of them were way up there in compression (somethin like 12:1+) but the cams bled off enough cylinder pressure that they were pump friendly chunks of iron!

 

You're looking at it from a very, very limited point of view. Not considering all the factors. I'm from the "Muscle car era" and can tell ya that the high compression cars all had to run on high octane gas. (been there done that)
Most of the muscle cars only had about 10.5:1 advertized cr. Actual measured cr of these production line motors was more often a little less when actually measured. (blue printed)

11:1 required 102 to 104 octane. 12.5:1 needed about 110 even with the aluminum heads. remember this was leaded gas.

A motor that has a big cam (the wrong cam) will bleed surely off cylinder pressure at low rpm and still bleed off pressure at high rpm. (over scavageing) But by the very fact of bleeding off usable pressure, the motor will not make the power it could if it had "the right cam" in it and more realistic compression.
Builds a ****ty engine. (does not perform in the real world and eats a lot of gas)

Except for a very specialized motor that say will run for a few seconds under controled conditions on a dyno ( engine masters dyno contest) all the pro engine builders build their "true pump gas motors" with very modest static compression ratios. rarely exceeding 10:1. They make the power (more power than you will) by consentrating on induction and exhaust tuning matched with dyno proven cam timing. In a word, they make the motor more volumetricly efficient. The real world operating conditions the engine sees has nothing to do with the conditions on a engine dyno.

An engine being fed 60*F air on a dyno will be much more detonation tolerant than the same engine in your car taking in 110*F air. (usually hotter).
the real world cr has to be lower for the motor to live.

A motor with improved induction/ exhaust tuning with the right cam will have higher actual in cylinder pressure and make more reliable power than a motor that overcamed with a mismatched combination and excessive cr.

Your simple dynamic cr calculation based on simple intake valve closing point does not take into consideration volumetric efficiency (ram tuning effect) or egr effect or exhaust/ induction scaveging or lack of it. (thru the mufflers)

The advertized duration or the .050" duration +15 spec does not indicate the actual intake on seat crank angle in a "running engine" especially with a hyd lifters. If the valve is bouncing on the seat actual styart of compression will be delayed.
Big cams do not tolerate closed exhaust well ( manifolds, headers without proper collector length, or mufflers. much of the scavedging effect if lost with many mufflers. (reduced actual power)

You can make a ton of real world usable street power @ 10:1 compression reguardless of therory. you'll go a lot Farther, faster that way.

 

what I don't understand is those are called dynamic compression calculators but their result isn't dynamic but still static

 

I wasnt saying that a 12:1 pump gas engine would make more power than one at 9:1. Im sure theirs lots of N/A 8:1 static CR motors that put out alot more power than mine!

Just saying that it is possible to run a higher compression deto free on pump gas with a cam that will bleed off some of that pressure


Now if you built a 18:1 motor with a cam that bled off enough pressure to get you down to 9:1 i dont think it would run all that well

 

groan....

no no, if you look at it closely, the number is moving, it's just changing so fast you can't see it.... Say, 60 times per second or so....

 

why are you groaning?

how is that number anymore dynamic then the typical static compression calculations?
all it does is add a few more things to give you a static number.

 

i'm groaning because you are nitpicking, and hence you are a **** disturber.

So it's certainly not perfect, and to be perfectly correct, I guess it would be another static compression ratio, just taking into account more variables. But it does the job. A dynamic compression calculator, would be like a compression gauge...

 

For some reason I couldn't get the picture to load from that website so I just saved it as a test file with all the information. I did two different tests. The first was with a normal .041 thickness gasket and the sencond is with a .015 gasket. Which one do you guys think that I should use???

 

Thats going to depend on how far below the deck your pistons are when they are all the way up. If the pistons are level with the deck and you run that .015 gasket... you're going to have problems.

The gasket thickness should be chosen to provide proper quench, not a different compression ratio even though that is affected by the gasket thickness.

 

As far as I know, the machine shop parallell decked the block by .010 so that leaves me with a deck height of 9.015 inches. There are actually a couple of different gaskets that I can use on this motor. I just opened up the Jegs catalog and found the smallest and largest compressed thickness gaskets that they had to run the tests with. Honestly, seeing as how my last engine grenaded due to piston to valve clearance(or lack there of) I would be more prone to using the .041 gasket because it gives me a little bit more clearance at the valves. I'd like to try to stay as close to 10.0:1 as possible, and if I can get that compression with the .041 gasket, then I'll be using it.

On a side note, how do you calculate quench and/or figure it in with the rest of everything else?

 

Crap!!!! This whole time I have been calculating with the 68cc chambers when I will actually be using the 74cc units. That changes everything. Let's see.....

OK, plugging in the other numbers but changing the combustion chamber cc I get...

9.8:1 if I use a gasket with a compressed thickness of .032 and bore size of 4.060

AND

9.589:1(call it 9.6:1) if I use a gasket with a compressed thickness of .041 and bore size of 4.166

If I use the copper head gasket that has the .032 compressed thickness and a 4.060 bore, the quench distance is .047. If I use the gasket with .041 compressed thickness with a 4.166 bore the quench distance is .056. Now, I did some research, and supposedly a quench distance of .045-.060 is ideal so either gasket would fall on the maximum/minimum side of that spectrum. Is that still OK??? If it came down to it, which quench distance would be more ideal? Now I'm worried that the copper headgasket might not be the better choice, but I'm not sure if I am gonna get the compression that I want out of it.

 

it's not that hard to get a dynamic compression if you know what your VE% is through the rpm range


and come on everyone can get something that reads VE%


but at least you know what I'm talking about

 

if you run a flat top with a modern shaped chamber, minimize quench, and just pay attention to the details, you can safely run mid ten to one ratios on pumpgas... if you're sloppy about it, and mismatch stuff, you'll can have it detonate at ten even... like most other engine things, the devil is in teh details.. now, if you pay close attention and knwo what you're doing, you can run up into the elevens, but that takes away the safety factor some..

btw, whoever it was that said LS1s are reverse cooled, they're wrong. it has standard coolent flow.. only diff between it and the SBCs is that the thermostat is on the intake of the pump, and that theres an airbleed at the top of each head.

 

haha, yea, let me find my VE%-o-meter... Probably right next to my trusty tire-smoke-o-meter, and thrill-o-meter......

paul, i'm losing my memory here, please fill in the details:

383 motor
AL heads, you have these on hand yet or not?
camshaft is...?

If you use AL heads, and a big cam, 9.5:1 CR isn't high enough really. I mean, it'll work, but it won't be ideal.
oh yea, with a decked block like you have, and flat tops, on a 383, usually the CR ratio is quite high.. You're using 3.75" stroke right?

.035-.040" quench is BEST. Up to .055" is still where you have use of a quench area, with flat top pistons. Over that and it starts to fall apart. If you can use a .030" gasket, with your .010" deck heigh, you'll be peachy.

hey mr. dude, which chev motor was it that DID have reverse cooling then? LT1?

 

I don't know about the other two, but my car has a VE% meter, except it's called a MAF sensor.

 

Sonix wrote:

Yeah, sure. I have the 383 stroker so 4.030 bore 3.75 stroke with 6.00 inch rods. Flat top pistons with 5cc valve reliefs. Heads are AFR 195 74cc chambers camshaft is a COMP cams unit .520/.540 lift @.050 and 236*/242* duration @ .050 with a 110 LSA

See, I have emulated this new engine against almost every single 500HP 383 stroker engine out there. They are all using the 383 bottom end with AFR 195 heads, COMP cams part number 12-433-8 which is the cam I have now, and a 9.5:1 compression ratio. The reason why I was initially shooting for 10.0:1 was because I know that when they dyno test these engines they do it in perfect air with no accessories, open headers, etc. So I wanted to use a slight increase in compression to make a little bit more power to offset the fact that I have an alternator/water pump/power steering pump that are driven off of the engine so I can get close to the same power level.

 

so run a gasket with the thickness of .015"-.028"

 

yup, LT1, and it was more headache then it was worth.. GM ditched it completely. if you look at other engine makers, they went thru a similar thing... most ditched reverse cooling schemes after a couple years.

 

Another problem!! Seems like that's all I have these days Anyways, I called the machine shop back and asked them to send me the sepcs sheet for the build up. Come to find out that the piston is only in the hole by .005. Not the .015 that I originally thought. So now I'm back up to using a thicker gasket. Using a Fel Pro pre flattened gasket with a .039 compressed thickness and a bore soze of 4.166 my quence ends up at .044 with a CR of about 9.8:1. Looks like that's the best it's gonna get

 

with an .028" gasket, your quench is .032 (which is plenty acceptable, i was told anywhere from .025-.050 was good) and your compression ratio will be right at 10:1

 

I think that's bang on perfect for what you want. Thats a cheap, run of the mill head gasket too, good sealing and common.

 

Yes, but now the question stands that will that gasket be able to reasonably handle 500HP/TQ on a daily driver???

 

You can get a MLS (multi layered steel) gasket from Cometic, Fel Pro, Mr Gasket and at least with the Cometic you can get a pretty wide variety of compressed thicknesses. I dont think that the .028 GM gasket is an entirely bad idea if in fact ALL of the pistons are at .005 down.

Considered milling the heads down some to lower the CC a bit? Sucks youve got 74cc heads... those would be good for a larger dished piston and boost or a dome piston, neither of which you have.

 

Why not? Head gaskets don't really have a HP rating on them. If I was you, i'd use ARP head bolts at least, and some washers, and just make sure you torque down the heads correct. I'm running 10:1 CR, stock head bolts, MRG washers, and teflon sealant on the threads, torqued in sequence correctly.

 

I've just built almost the exact same motor. Only difference is the heads were milled to get up to 10.4:1 compression. Been running it with 40* total timing and no noticeable detonation problems. I do have a huge cam though... running the 294S which is 248/248 @ .050". According to WallaceRacing that brings my DCR way down to 9.15:1 (if I calculated my intake closed ABDC correctly)

Planning on that combo making 500HP/TQ N/A? I'm hoping for that with 10.4 CR - not sure if it'll happen at only 9.8 (will post my dyno results once I run the thing)

 

I'm hoping so. When I initially did my research on the 383 build, I came across several know places that were advertising that engine at that power level. What sold it for me was I went to AFR's site, and they have the dyno charts and info to prove it. I even called them to ask about it, and they told me that it was a tried and true package. A few people on this site have also put together a 383 using those same parts and are running pretty much on par with what AFR says it will. The plus is that it was designed to make this power on 93 octane at 9.5:1. I think that it will be very happy on the street.

 

I agree.

 

My engine is 11.5 with afr 227 heads and a .650 lift cam. It makes 650hp. I run 93 octane @ 36 degrees timing. With no knocking.

 

great... think your TOOTIng your own horn there?

my engine makes poptarts try beating that!