Honda engine specs and gear ratios

These are the numbers behind the presets in the compression calculator and the gearing calculator, written out so they can be read without opening a form.

They were recovered from zealautowerks.com before it goes offline. Dimensions of mass-produced parts are facts — a B18A crank is 89 mm however anyone feels about it — so this is a rescue of data rather than a copy of anyone’s work.

Read the dish sign carefully

The source wrote piston volume with positive meaning a dome. This site uses positive for a dish, because a dish is volume you are adding to the chamber. Every figure in the tables below has been negated accordingly.

That this is the right way round is checkable rather than a matter of opinion. A USDM B16A pairs a 42.7 cc chamber with the PR3 piston, listed in the source as 6.01:

Read as	Computed ratio	Published
Dome (as here)	10.25 : 1	10.4 : 1
Dish	8.7 : 1	—

No B16A has ever been 8.7:1. Picking those five parts in the calculator gives 1,595 cc and 10.25:1, which checks the recovered data, the sign convention and the formula in one go.

Complete engines

The calculator can be loaded with a whole engine — a stock GS-R, a Type R, a CR-V B20 — rather than six separate parts. Each of the 27 builds names the parts it is made from, so a correction to a part reaches every engine that uses it, and each carries the compression ratio Honda quote.

That published figure is a check, not an input: nothing computes from it. It is there because the recovered data has no deck clearance in it at all, and deck clearance moves compression more than anything else you cannot see. A stock GS-R built from these parts computes to 10.39:1 against a quoted 10.0:1 — and the gap is the interesting part, because it says the factory deck clearance must have been about 0.81 mm. The calculator will work that out for you, clearly labelled as derived rather than measured.

Two builds do not reconcile even allowing for that, and are marked in the data with the reason:

• ZC (JDM DOHC) computes to about 10.6:1 against a quoted 9.4:1, and reading the piston as a dish instead gives 8.3:1. Neither lands, so either the chamber or the piston figure is wrong.
• K24A1 computes to about 11.3:1 against a quoted 9.6:1. Every K24 piston in the data has a flat crown and every K head the same chamber, which cannot all be true.

Head gaskets are a choice, not a spec

The gasket tables are the one part of this that is not inherited. On the B and H/F series the source lists constructions rather than engines — OEM three-layer at 0.66 mm, two-layer at 0.49, one-layer at 0.25 — because fitting a thinner gasket to raise compression is a deliberate modification. On a B18C1 that swap takes the computed ratio from 10.39:1 to 10.86:1.

The D-series list is by engine instead, and the spread is wide: a D15B7 or D16A6 uses 1.22 mm where a D17A2 uses 0.66. Three builds — the D15Z1, the D16Y7 and the ZC — are not named in that list, so they carry no gasket and leave whatever is in the field alone rather than have one guessed for them.

What to trust

The source carried its own warning — “not all values are verified” — and that warning travels with the data.

Bore, stroke, deck height and rod length are dimensions of hard parts and are the most reliable — every one of the 27 builds reproduces its published displacement to within a few cc, which is a strong sign the geometry is sound.

Combustion chamber volume is the least reliable. Every K-series head in the recovered data carries the same 50.5 cc, across seven different engines; that is a placeholder, not a measurement. Elsewhere it varies with casting, with production tolerance, and with any skim the head has had in thirty years of service. If the ratio matters — and on a boosted engine it decides whether the thing survives — measure the chamber with fluid and a plate rather than trusting a table.

Gear ratios are printed in the service manual for every box listed. A ratio out by a hundredth moves a calculated top speed by less than a mile an hour, so an error here will not announce itself; check the casting code stamped on the case before ordering anything.