Opinions wanted on my compression ratio
- Thread starter duster360
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nm9stheham
Well-Known Member
It is indeed a number LOL. Intended use for the car? Cam? Heads AL or iron? I.e., all the usual suspects....
(And sorry I don't remember every detail on your car..... just can't track 'em all.)
(And sorry I don't remember every detail on your car..... just can't track 'em all.)
Good point. Lol. Just a street cruiser with an occasional pass at the strip for fun. Pump gas only.
67 Dart
340 ci
Comp cams XE275HL I-275* E-287* I&E 525"
Edelbrock RPM heads
Edelbrock Air Gap intake
Hughes roller rockers
650 DP carb
4 speed
3.55 gears
DesertRat
Leading edge boomer
Change deck clearance to .06??? There is definitely something amiss with the data entry but I cant read the small print. Old eyes-------and they aren't the first thing to go------
DR
DR
racerdan80
Well-Known Member
Should run on 91 octane and be streetable, almost the same as sbc in my s10 and it runs just fine on 91
Corrected and with a semi better picture.
It will run fine on pump gas. Just look at the DCR. It's plenty low for pump gas. It will be fine.
nm9stheham
Well-Known Member
I assume these are flat top pistons and the 5 cc is the valve relief eyebrows. We are running very close to that combo and expect to be OK but will count on 93 octane. Our cam is a bit smaller and so DCR is up in the low 8 range, and SCR at 10. Yours looks to be pretty safe per conventional wisdom.... and ought to be pretty dang strong for a cruiser!
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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I'd put it together with the .029 gaskets. You will still have .035 quench. And I would close the intake 2* sooner.
Yup that's what I would do.
Yup that's what I would do.
I already have the .039" gaskets ordered and I see your point on the quench. Being serious here, what would be the gain of advancing the timing 2*? More cylinder pressure? I am not an expert in this area and like to learn.
nm9stheham
Well-Known Member
Before getting too excited about quench, we need to know for sure which model of Edelbrock head you have. You list 65 cc's in the table above, and that number is for the ones with the combustion chamber opened up like an open chamber Mopar head. The depth of that open area is .060", so even with the .029" gasket, you still won't have a quench gap.
So what model number of head do you have?
So what model number of head do you have?
Why even worry with quench? Your DCR is under 8:1. It will probably run on 87 octane.
Head # 60779. In the book it says they are supposed to be 63cc. But I cc'd them myself and they come out to 65cc. I have read on this site many times that these heads are never at 63cc's as advertised. So I did it myself and they are definitely 65cc's. I did all eight chambers and they are all the same.
Wouldn't that be nice. I probably won't chance it, but it would nice.
I don't think you'll have any trouble at all running on 87. Aluminum heads and a DCR of under 8:1. I think you got it licked.
nm9stheham
Well-Known Member
OK, the 60779 is the closed chamber one so there is quench. Understood on the cc's ; ours cc'd out around 64 cc's average. It is easy to add a cc or 2 to such measurements; if the goop sealing the plexiglass to the bottom of the head is just .006" thick for open chamber heads (or about .008" thick on the closed chambers), you have just added a cc to the measurement.
I would only use the thinner gasket and tighten up the quench IF I had very thoroughly measured piston to deck clearance on each piston, at the upper and lower edge of each piston. There are a lot of variables in the piston-to-deck height, and the potential is very real that some pistons will be closer to the head than is calculated. (And this should be done anyway for any engine IMO with a tighter piston to head clearance.)
We found a total variation in piston-to-deck height of .014" when we first assembled our 340, and did some added work to get that down to .006" variation. You are going to find SOME variation..... you just need to know how much it is. And you need to know it at both the upper and lower edges of the closed chamber Edelbrocks because they have both the main quench area at the upper edge of the combustion chamber, and a 2nd minor area at the lower edge of the chamber, right by the plug.
When you check piston to deck, use a dial indicator on a mag base on the block surface, positioned to be very stable, and then check the piston height and rock it back and forth and take the average height on the dial indicator. Do it close to the upper or lower piston edge. Then swing the indicator to the deck and take a reading. The difference is the real piston-to-deck height for that piston at the top or bottom.
If you don't do this, then I would certainly stick with the .039" gasket. At .044" nominal quench (piston-to-head gap), you will should have a decent quench action (based on conventional thinking). It is there already with your parts selection as it stands.
I would only use the thinner gasket and tighten up the quench IF I had very thoroughly measured piston to deck clearance on each piston, at the upper and lower edge of each piston. There are a lot of variables in the piston-to-deck height, and the potential is very real that some pistons will be closer to the head than is calculated. (And this should be done anyway for any engine IMO with a tighter piston to head clearance.)
We found a total variation in piston-to-deck height of .014" when we first assembled our 340, and did some added work to get that down to .006" variation. You are going to find SOME variation..... you just need to know how much it is. And you need to know it at both the upper and lower edges of the closed chamber Edelbrocks because they have both the main quench area at the upper edge of the combustion chamber, and a 2nd minor area at the lower edge of the chamber, right by the plug.
When you check piston to deck, use a dial indicator on a mag base on the block surface, positioned to be very stable, and then check the piston height and rock it back and forth and take the average height on the dial indicator. Do it close to the upper or lower piston edge. Then swing the indicator to the deck and take a reading. The difference is the real piston-to-deck height for that piston at the top or bottom.
If you don't do this, then I would certainly stick with the .039" gasket. At .044" nominal quench (piston-to-head gap), you will should have a decent quench action (based on conventional thinking). It is there already with your parts selection as it stands.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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I'ma big fan of tight quench. My pistons were up,all over the place. I found the KB-107s bang on as to compression distance. But I found my rods were not all exactly the same length, and the reground crank throws were out a bit, and my decks were out a bunch. In the end, after a days worth of shuffling parts, I got the deck-heights evened up to .005 from the shortest to the tallest, but I hate expressing such a crappy number that way. I'm much happier saying they are out of the hole; .0025 +/-.0025 See doesn't that read much tighter. It's like buying something for 1.99 instead of 2.00. I mean who are we kidding.
So with the .039 gasket, I'm running a quench of .034 to .039, oops I mean .0365,+/-.0025! See how that works? My combo has never put the piston into the head. I ran it with the .029 gasket as well with no contact. I'm not doing the math on that cuz it sounds scary.
Tight quench is said to promote all kinds of benefits, to which some I can attest. With a HE2430 cam and a list 1850 carb(600vac,sec), and on the AG, I was able to get my S into 32mpgUS with no ill effects.And with a 3310(750) it went 12.9 with 3.55s(same cam). I like quench.
Of course I have no basis of comparison to say that .040 or .050 would be any worse. This is just how my combo came together, and I went with it.And I'm glad I did.
Then there's the extra compression with the .010 . This is worth a bit over 2 cc. You do the math.....
But more important is the earlier closing intake. This will trap a bit more mixture, and pump up the torque a few ftlbs right at the starting line, where the 340 needs it.And it will be noticeable any time you are running on the short side of the torque curve, namely idle to 3000 or so. Yeah you will be stealing it from the topend, but where will your engine be spending most of it's time.
I can guarantee you, in the long term, If you are a streeter like me,you will be happier with the extra at the bottom, than the extra at the top.
That's my thinkin an Ima stickin 2 it.
Well almost. I'm running a Scr/Dcr of 10.67/8.53 with a 360, an on 87E10. Your shorter stroke 344.5, could probably push a Dcr .2/3 higher than mine,on 89, and maybe .4/.5 on 91. That's the beauty of setting it up on the .029 gaskets. If you just can't tune for 91, then slide in the .039ers. I thought I was pushing the limits with my combo when I ran crs of 10.82/8.85, but not so. The only time I ran anything but the 87 was for the few track runs; 4 with the 223* cam, and 4 with the 230*.
Having BS'ed my way this far, I should add that this is how my combo fell together. I certainly wouldn't recommend that you spend a ton of green chasing perfection.On the street I doubt you would notice the performance difference in .3 or maybe even .5 in Dcr difference. I mean it might be the difference in spinning the tires 75% of the way through second gear or 50%.I mean either way, you will be speeding with 3.55s!
blah blah blah blah.
The thing is gonna run fine on 87.
The thing is gonna run fine on 87.
When you check piston to deck, use a dial indicator on a mag base on the block surface, positioned to be very stable, and then check the piston height and rock it back and forth and take the average height on the dial indicator. Do it close to the upper or lower piston edge. Then swing the indicator to the deck and take a reading. The difference is the real piston-to-deck height for that piston at the top or bottom.
If you don't do this, then I would certainly stick with the .039" gasket. At .044" nominal quench (piston-to-head gap), you will should have a decent quench action (based on conventional thinking). It is there already with your parts selection as it stands.[/QUOTE]
I checked the pistons exactly the way you mentioned in the above paragraphs. I also had the block square decked. It was out a lot from the factory. .006" was cut from one side to get it straight. Machinist cut .021" from the other side to get it down to the same specs as the other.
If you don't do this, then I would certainly stick with the .039" gasket. At .044" nominal quench (piston-to-head gap), you will should have a decent quench action (based on conventional thinking). It is there already with your parts selection as it stands.[/QUOTE]
I checked the pistons exactly the way you mentioned in the above paragraphs. I also had the block square decked. It was out a lot from the factory. .006" was cut from one side to get it straight. Machinist cut .021" from the other side to get it down to the same specs as the other.
nm9stheham
Well-Known Member
Cool! Sounds like you are set very well. Just curious....were the piston heights very consistent?
Gotta stick with the .039" gaskets since they just came in today, plus I am just an average tuner. But looking to improve that. Thanks for the detailed info.
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