SCR, DCR, quench and cam duration- will someone PLEASE help me avoid detonation!!!! - Hot Rod Forum : Hotrodders Bulletin Board
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Old 09-06-2010, 01:39 PM
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SCR, DCR, quench and cam duration- will someone PLEASE help me avoid detonation!!!!

Let me start out by saying that this is a continuation of a conversation I was having with Bogie, Techinspector1 and others in a thread that I shameless hijacked. This whole exercise started when I asked Bogie about swirl port vs. Vortec heads for a low-end torque application. But through the course of the conversation it was pointed out that I have some other issues to address with my piston setup- I’m currently running “rebuild pistons” in an stock-deck block, which puts the crown of the piston .045 in the hole. I was planning on reusing the pistons, but looks like that might not be such a good idea because of excessive quench height.

Here are the specifics: I am rebuilding a 327 for use in a 1970 Toyota FJ40 (Jeep style 4x4 if you’re not familiar with them) with 4.11 gearing rolling 33” tires. Truck weighs about 3,800 empty, but add 500 for a couple of fat middle aged guys and their stuff. This engine will never see 4,500 RPMs and I'm building it for low end/mid range torque

If I install new flat top pistons with 2 valve reliefs (and correct compression height) and use a .015 shim gasket I can get my quench down to .038 but SCR is at 9.54. Using the old CR x 10 = octane formula I’m way out of the range for 87 octane.

I haven't started shopping for a cam yet, and since this is all new to me and I don’t have a clue whether a short duration torque cam will help or hurt DCR, I picked this one for modeling purposes: http://www.compcams.com/Company/CC/c...x?csid=94&sb=2 and plugged the intake close point (53 deg. ABCD) into the DCR calculator at www.wallaceracing.com and got a DCR of 8.25.

Also, I’ve swung back to using the swirl port “193” heads (this is a truck engine, and I don’t care about the lack of upper end performance), which means I will only have to use 30-32 timing advance.

Bottom line, the build I’ve spec’ed out has 9.54 SCR/8.25 DCR, .038 quench and total timing advance of 32 deg. Will this combination be good to go with 87 octane? Is there something else I need to be factoring in?

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Old 09-06-2010, 02:16 PM
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For 87 octane you need to drop to 8.5-9:1 compression in that truck.

Why a 327? If I were you I'd just throw an RV cam in a good used running 350 and call it good- it'll be better AND cheaper.
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Old 09-06-2010, 02:38 PM
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Just for a comparison try your numbers in this calculator:

http://www.kb-silvolite.com/calc.php?action=comp

don't for get to add the 15 to your intake closing when doing the DCR
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Old 09-06-2010, 03:22 PM
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Quote:
Originally Posted by Custom10
Just for a comparison try your numbers in this calculator:

http://www.kb-silvolite.com/calc.php?action=comp

don't for get to add the 15 to your intake closing when doing the DCR
I saw that calculator, but for the life of me I couldn't figure out what "Intake Closing Point @ 0.050 lift" is. I guess it's the point in the compression stroke when the intake is at .050 lift, but I don't see that value listed in the cam specs.
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Old 09-06-2010, 03:57 PM
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You don't have to use flat top pistons. Here is a D-cup with a 13cc dish from Keith Black that will make 9.0:1 SCR with a 64cc chamber....surely there must be other manufacturers with a similar product if you don't like KB....
http://www.kb-silvolite.com/kb_car/p...etails&P_id=43
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Old 09-06-2010, 04:24 PM
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Quote:
Originally Posted by Uncle Buck
I saw that calculator, but for the life of me I couldn't figure out what "Intake Closing Point @ 0.050 lift" is. I guess it's the point in the compression stroke when the intake is at .050 lift, but I don't see that value listed in the cam specs.
Normally, there are two different sets of cam timing figures, duration at seat to seat and duration at 0.050" tappet lift. Not to throw a monkey wrench into the works, but there are also 0.020" tappet lift duration figures for mechanical flat tappet cams.

Normally though, you will see duration at 0.006" tappet lift (think of this as seat to seat) and duration at 0.050" tappet lift (the industry standard) on the timing card that comes with the cam from the manufacturer. Sometimes though, if you don't have the timing card, you can only find timing figures on the cam at 0.006" lift or you may have no timing figures at all but you need figures at 0.050" lift for the calculator to work.

Let's say you have an intake centerline (not lobe separation angle, INTAKE CENTERLINE) of 106 degrees ATDC and an intake duration of 216 degrees @0.050 tappet lift. This tells you that the max lift point (or centerline) on the intake lobe will be at 106 degrees after top dead center.

On a piece of paper, draw a circle. Doesn't matter what diameter, bigger is easier to see for me, maybe 4-5 inches. Make a mark at 12 O'Clock and label it TDC. Make a mark at 6 O'Clock and label it BDC. Make a mark at 10 O'Clock and label it Intake Open, or simply IO. Make a mark at 8 O'Clock and label it Intake Close or simply IC. Now, make a mark at 4 O'Clock and write 106 beside the mark. That is the intake centerline point.

Now, if the duration is 216 degrees, then you know that the valve will be opening during half of that period and closing the other half of that period, so it will be opening for 108 degrees of crank rotation and closing for 108 degrees of crank rotation.

Therefore, if you start at 106 degrees after top dead center at the intake centerline point and come around the circle counter-clockwise by 108 degrees (half the duration), you will come past TDC by 2 degrees. In other words, from 106 back to TDC will be 106, then another 2 degrees after that will be your intake opening point. Beside your 10 O'Clock mark, write "2". This will indicate that the valve opens at 2 degrees before top dead center at 0.050" tappet lift. Now, from the intake open point of 2 BTDC, go clockwise around your circle to BDC at 6 O'Clock and add 180. This will be the right hemisphere of your circle from TDC to BDC during which time the intake valve is open and will be a total, so far, of 182 degrees, from the intake open point to BDC (bottom dead center).

Now, since the intake closing point is given in degrees after bottom dead center (ABDC), we will continue around the circle clockwise past BDC to find our intake close point. If the duration is 216 degrees and we have already used up 182 degrees of that getting to BDC, then all we have to do is subtract 182 from 216 and we find that we can write in 34 at the 8 O'Clock (Intake Close) point on our circle. To prove our calculations, we add 2 + 180 + 34 and find the duration of 216.

Just to go a step further, I'll explain about centerlines and lobe separation angle (LSA), also sometimes referred to as lobe center (LC). The intake centerline is expressed by a number after top dead center. The exhaust centerline is expressed by a number before top dead center. Adding the intake centerline and exhaust centerline together and dividing by 2 will give you the lobe separation angle. In other words, say, 106 + 114 / 2 = 110. OEM camshafts are ground to please little old ladies who want a smooth-running motor on the way to and from bingo, so they are ground on a wide lobe separation angle (114, 116, 118) for a dead smooth idle and high intake manifold vacuum to operate brakes and other items that require vacuum. Aftermarket cams are ground a little more agressively at 112, 110, 108, 106 and even tighter for a dedicated race motor. A tighter lobe separation angle will make more power at the low end, will build power quickly and peter out on the top. The tighter it is, the gnarlier the idle quality and the lower the intake manifold vacuum. A wider lobe separation angle, on the other hand will exhibit more power on the top end at the expense of low rpm power. The idle quality improves and manifold vacuum increases.
110 degrees seems to be a good middle ground for a naturally-aspirated street-strip motor.

I have explained this in a very elementary manner and hope I have not insulted your intelligence in the way I did it, but I wanted to make it very clear so that most anybody can understand it and graph any cam for their own satisfaction. The whole deal with the paper and circle just makes it bullet-proof when you can look at it and see what's happening. You can, of course, do it without the paper. You just need a better memory than I have. Hope this works out for you.
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Old 09-06-2010, 05:35 PM
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Quote:
Originally Posted by techinspector1
I have explained this in a very elementary manner and hope I have not insulted your intelligence in the way I did it, but I wanted to make it very clear so that most anybody can understand it and graph any cam for their own satisfaction.
No offence taken and thanks for explaining this. Based upon your info I was able to calculate valve close at .050 lift on my "model" cam and plug the numbers into the United Engine DCR calculator. Came up with 8.50 (as compared to 8.25 using the Wallace calculator.

Quote:
Originally Posted by techinspector1
You don't have to use flat top pistons. Here is a D-cup with a 13cc dish from Keith Black that will make 9.0:1 SCR with a 64cc chamber....surely there must be other manufacturers with a similar product if you don't like KB....
http://www.kb-silvolite.com/kb_car/p...etails&P_id=43
I assume you're confirming that the spec's I came up with are too hot for 87 octane? .

No problem with KB pistons, its just a function of cost. I was originally planning to reuse the pistons. I can purchase KB flat tops for about $200 a set, but the D-cups run a little over $400 a set. Ironically I can't find anyone else offering a comparable piston in 327. While I think a 327 has a certain "cool" factor (remember, this is a SBC powered Toyota!), rebuilding it as a 350 may be in the cards.

Dang..... I knew I was pushing it, but I was hoping the .038 quench and fast burn heads would be enough to make the 7cc flat top pistons work.

EDIT: Now that I can figure it, what is the max safe DCR for a truck engine with tight quench and fast burn heads and running 87 octane?

Last edited by Uncle Buck; 09-06-2010 at 05:47 PM.
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Old 09-06-2010, 06:10 PM
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Personally, I'd build the motor at 9.00:1 SCR and cam it at 8.00:1 on the KB calculator with a 0.035" to 0.040" squish. I would also build it as a 383, not a 327 or 350. Lots of grunt. For this kind of motor, I like to see 400 ft/lbs of torque at 2000 rpm.
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Old 09-06-2010, 06:44 PM
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I'm still looking for a way to do the 327. What if we used a longer rod and 350 piston? I guess it would come out about the same on money, but at least you'd have a better choice of available pistons.
http://cgi.ebay.com/ebaymotors/Eagle...Q5fAccessories
With a 1.560" 350 piston, the stack would measure 9.035", popping the piston up out of the bore by 0.010". If you then used this 0.051" head gasket, the squish would be 0.041" on a nominal 9.025" block.
http://www.summitracing.com/parts/NAL-12557236/
I realize there is no savings here, but I like figuring this stuff out, so here is a way to operate a 327 on 350 pistons.
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Old 09-06-2010, 09:05 PM
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Do you have access to a milling machine?? You could cut a small D-dish in the flat top piston.
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Old 09-06-2010, 09:45 PM
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Quote:
Originally Posted by ericnova72
Do you have access to a milling machine?? You could cut a small D-dish in the flat top piston.
Yep, good engineering practice dictates that the crown thickness of the piston will be about 7% of the bore size, so it should be about 0.280" thick in the middle of the crown.
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Old 09-06-2010, 09:51 PM
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You can also grind a few cc's out of the combustion chamber with a die grinder and a carbide burr. Don't limit yourself to just pushing the easy button with an open wallet. Spending a little time "massaging" things can make less expensive parts fit the bill.
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Old 09-06-2010, 09:57 PM
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Quote:
Originally Posted by ericnova72
You can also grind a few cc's out of the combustion chamber with a die grinder and a carbide burr. Don't limit yourself to just pushing the easy button with an open wallet. Spending a little time "massaging" things can make less expensive parts fit the bill.
Yep, bolt the head on the block without a gasket and scribe the bore onto the head from underneath with a sharp scribe or awl. Use Dykem Blue if you really want to see the mark.
http://www.amazon.com/Blue-oz-Can-Br.../dp/B002SLGAEO
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Old 09-06-2010, 10:16 PM
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IMHO This is the type of engine and application that can benefit from the Singh grooves. Build it as is with the "rebuider" pistons and swirl port heads and add the grooves. Enjoy.. The 327 Toyota could use a bit more rear gear with the tall tires and off road. if using a GM auto this is where a mild "2400" stall converter can help too.

Consider adding the whole TBI EFI system "top end" with the swirl port heads say with long tube headers, 454 BB throttle body.

Lot of advantages with EFI for offroading.

A TPI system top end transplant from a Camaro/Firebird is another option.

Google search Singh grooves and/or as Automotivebreath on this forum.

Last edited by F-BIRD'88; 09-06-2010 at 10:23 PM.
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Old 09-06-2010, 10:25 PM
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When you say you are "swinging" towards a particular head choice does that mean you don't have heads yet? I picked RHS Pro Torquers and they sell them in 64, 67 and 76 cc versions.
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