MikeFleming
04-01-2013, 05:21 PM
I remember about 175 PSIG on a pretty fresh, stock 1984 engine when hot at cranking speed with all spark plugs removed, the throttle held wide open and at sea level. Here in Tucson (approx. 2800 feet) I expect that to go down by 10-15 PSIG to about 160-165. My engine reads about 155 PSIG now - the result of larger valves, later intake valve closing point and the higher altitude. I have included a table showing intake valve closing point (measured in crankshaft degrees after bottom dead center, ABDC) and nominal static compression ratio. These values were measured on air-cooled, multi-cylinder motorcycle engines with a cranking speed of 350 RPM.
CR ->
7.5
8.0
8.5
9.0
9.5
10.0
10.5
11.0
11.5
12.0
12.5
Intake ABDC
45
175
191
209
225
243
260
279
298
316
335
359
50
169
184
200
216
232
250
268
285
303
322
341
55
160
175
191
206
223
240
256
275
290
307
324
60
155
168
182
197
213
228
244
260
276
294
312
65
148
160
173
188
203
218
234
248
265
279
294
70
138
150
163
176
190
204
218
234
247
260
275
75
129
140
151
165
178
190
204
216
231
244
258
80
118
129
141
153
165
176
190
201
213
226
238
85
112
121
131
141
153
163
175
187
197
209
220
90
100
110
121
129
140
150
160
171
182
193
204
All values in PSIG
CR ->
7.5
8.0
8.5
9.0
9.5
10.0
10.5
11.0
11.5
12.0
12.5
Intake ABDC
45
175
191
209
225
243
260
279
298
316
335
359
50
169
184
200
216
232
250
268
285
303
322
341
55
160
175
191
206
223
240
256
275
290
307
324
60
155
168
182
197
213
228
244
260
276
294
312
65
148
160
173
188
203
218
234
248
265
279
294
70
138
150
163
176
190
204
218
234
247
260
275
75
129
140
151
165
178
190
204
216
231
244
258
80
118
129
141
153
165
176
190
201
213
226
238
85
112
121
131
141
153
163
175
187
197
209
220
90
100
110
121
129
140
150
160
171
182
193
204
All values in PSIG