| Title: | Musclecars |
| Notice: | Noter Registration - Note 5 |
| Moderator: | KDX200::COOPER |
| Created: | Mon Mar 11 1991 |
| Last Modified: | Mon Jun 02 1997 |
| Last Successful Update: | Fri Jun 06 1997 |
| Number of topics: | 182 |
| Total number of notes: | 5467 |
< cross posted in mechanix_3 and carbuffs >
Hi,
Does anyone know the compression readings for a 1979 firebird with a 301 cin
engine installed. In the Haynes manual it only gives a reading for the 350
400,454. The spec for these guys is 120-160 psi @ 155-175 rpm. If you dont have
it is there any way I can work it out from the following info.
BORE STROKE COMP RAT.
301 4.00 3.00 8.2
350 3.88 3.75 8.0
400 4.12 3.75 8.0
455 4.15 4.21 8.0
My guess is that the 301 should be slightly higher as the compression ratio is
greater than these. I have measured my compression with the following results.
1 125 psi
2 120 psi
3 120 psi
4 110 psi
5 125 psi
6 125 psi
7 120 psi
8 100 psi
The engine is old 120K miles but runs real pretty well, I am considering a
rebuild but just want to know how bad it really is before I go and spend money.
The plugs all looked nice and brown appart form no 8 (no surprises) which had
some signs of fouling but it wasn't too bad).
As the car runs well I would like to see an increase in my fuel consumption
(currently around 10-12mpg) or performance ( the car feels really slow 0-60) is
around 20 seconds.
Any suggestions would be appretiated.
John
| T.R | Title | User | Personal Name | Date | Lines |
|---|---|---|---|---|---|
| 110.1 | BARUBA::REARWIN | abolish the Registry | Mon Jun 14 1993 11:03 | 3 | |
Not enough information is available to determine what you want. Needed are the piston dome/dish volume and the compressed thickness of the head gasket. Matt | |||||
| 110.2 | Formula for pressure | ZUR01::SCHMIDTHE | Wed Jun 23 1993 08:26 | 25 | |
Dear John,
the formula to get the compression ratio is cylinder volume plus head
volume divided with head volume.
That means it doesnt matter how the piston head looks like or how thick
the gasket is. The volumes are mesured with oil or calculated including
piston head , gasket and plug.
The highest pressure arise at the highest torque rpm.
The pressure at starter rpm is 10 to 20% less with hot engine and
throttle valve fully open.
example:
Assume the comp.ratio is 8.5 that means the whole volume cyl and head
is 8.5 times compressed to head.
14.7lps/si(pressure absolute) * 8.5 -10% = 112.5 lbs/si
In this example you would get a read out 112.5 lbs/si. However. In
fact you onli need the compress.ratio to calculate th proper pressure
read .
Henry/Switzerland
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| 110.3 | Why so high? | MSDOA::SCHMIDT | Thu Jun 24 1993 09:24 | 17 | |
Henry / All,
I've been struggling with this same concept. I have a '73 Chevy
Caprice Convertible with a 400 small block - aka the Great Red Shark of
Hunter Thompson fame. It's listed at somewhere around 8.0:1 or 8:5.1
compression from the manual. In doing a compression check ( throttle
open, engine mildly warm, plugs out, cranking at starter speed, going
through 3-4 complete compression cycles using a gauge that screws in
and measures the highest compression seen ) - I get a reading of
between 150-160 psi in each cylinder. This in on an engine with 140,000
miles wear and tear. I'm assuming some level of carbon build-up (
honest I try to keep it blown out!! ). The engine runs fine on 87 pump
gas with the timing slightly advanced from factory settings.
Any ideas why my readings are 150-160 rather than 110-130?
Chuck
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