General engine questions/issues that aren't specific to the other categories
By JohnRM
#331156 I found an interesting article about spark plug heat ranges and an equation used to determine the heat range for a certain application.

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Spark plugs with the proper heat range make a world of difference in the final outcome of the tune. If the spark plug heat range is too hot the plug cannot dissipate heat quickly enough and you'll have a source of pre-ignition in the engine. Earlier we published an article about how to read spark plugs. You can read that article by clicking here. Even if you know how to read plugs, this article will give you a rough idea on where to start.

The strongest correlation to spark plug heat range is BMEP or MEP which both can be found as a function of horsepower and displacement or torque and displacement. What we did is plotted RWHP/Displacement (CID) against the heat range we found to be appropriate to nine different engines. While I do not expect the results to be linear the correlation is fairly strong. Also we took all the examples as approximate RWHP as tested on a dynojet dyno. Do not think that what we did here can be taken as the Bible, but do use it as an approximate starting spot. Follow our other article How to Read Spark Plugs to fine tune the proper heat range plugs in your application.

To find the starting heat range plug for your application....

Determine approximate RWHP expected
Divide RWHP by displacement in CID. For example 430 RWHP / 376 CID = 1.14
Multiply result found in (2) by 3.07 - Continuing example - 1.14 * 3.07 = 3.5
Add 3.15 to result found in (3) - Continuing example - 3.5 + 3.15 = 6.65
Round result found in (4) up! So suggested starting heat range is 7
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So I tried this with a stock d15b7 and the assumed whp to be ( I need reference to this) 92. According to the equation,
91.1 CID ( cubic inch displacement) / 92 WHP (wheel horse power) = .9902
.9902 x 3.07 = 3.04 ( rounded up from 3.0399)
3.04 + 3.15 = 6.19 heat range. Round this number either up or down depending if its greater or less than .5
So using heat range 6 is the proper range for a stock d15b7.

Now here is the problem, the OEM style spark plug for this engine is
ZFR5F-11. The 5 in the NGK's part number is the heat range and the -11 is the gap ( 1.1mm converted to inches is .0433 inches ). Although their equation is just a test, I wanted everyone's input on this and to ask get some accurate numbers like the whp of a stock d15b7.
What does everyone think? Here is the original website.
http://www.pcmforless.com/index.php?opt ... &Itemid=56
By HeikDiesel
#331166 If that was the case I'm suppose to run (12) spark plugs lol
By JohnRM
#331169
HeikDiesel wrote:If that was the case I'm suppose to run (12) spark plugs lol


I also tried it with bigger numbers and it doesn't make sense(or does it?) maybe I'm doing something wrong or I'm missing a step. I went to my auto parts store and bought some ngk spark plugs with a heat range of 7. I'm going to test these and see how it goes for a month. My original plugs that i took out looks normal but I'll still test this.
User avatar
By JUICE
#331170 From personal experience..

Stock- 6
Boosted stock on pump gas @ 270whp- 7
Built/boosted/leaded race gas @ 547whp- 8



And interestingly enough, my buddy made 850whp on ethanol with 7's.