melle
South West
It'll come out in the wash.
Posts: 1,983
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All else being equal, does increasing intake valve diameter lead to higher compression? I'm talking about cranking compression, i.e., what you measure with a compression tester, not compression ratio. I'm assuming compression is defined by combustion chamber volume and intake charge volume? I'm working on the assumption that piston rings and valves can be left out of the equation for the sake of what I'm trying to understand, and cam timing can be ignored as well I suppose.
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www.saabv4.com'70 Saab 96 V4 "The Devil's Own V4" '77 Saab 95 V4 van conversion project '88 Saab 900i 8V
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No, airflow is minimal while cranking. At high RPM the valve would offer a restriction, so a larger one helps.
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melle
South West
It'll come out in the wash.
Posts: 1,983
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That makes sense truckdoctor. I'm trying to understand why a pair of rebuilt heads with slightly larger valves on a Saab V4 engine raised compression from 150-160 to 175-190 psi while nothing else was changed. NB: 150-160 psi is what I would normally expect from reasonable condition V4s with std heads. |
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Last Edit: Jun 6, 2022 18:50:11 GMT by melle
www.saabv4.com'70 Saab 96 V4 "The Devil's Own V4" '77 Saab 95 V4 van conversion project '88 Saab 900i 8V
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stealthstylz
Club Retro Rides Member
Posts: 14,833
Club RR Member Number: 174
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Did you measure the chamber volume? Might have had a load skimmed off in the past and bumped the static CR up, or the valves are sealing better than before being fresh.
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melle
South West
It'll come out in the wash.
Posts: 1,983
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Heads only had a clean-up skim I think. Yes, the valves will be sealing better for sure, but the compression is still a fair bit higher than what I would expect. Will compression test another rebuilt engine to compare readings when I'm back in my workshop next month.
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Last Edit: Jun 6, 2022 19:46:52 GMT by melle
www.saabv4.com'70 Saab 96 V4 "The Devil's Own V4" '77 Saab 95 V4 van conversion project '88 Saab 900i 8V
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is it the same type of head gasket? if its thinner that will up the compression a little
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Static comp,no,
Dynamic comp,yes
;-)
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Try another compression tester. The cheaper one's do vary the results.
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Still learning...still spending...still breaking things!
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melle
South West
It'll come out in the wash.
Posts: 1,983
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Thanks for all your feedback!
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www.saabv4.com'70 Saab 96 V4 "The Devil's Own V4" '77 Saab 95 V4 van conversion project '88 Saab 900i 8V
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“I'm trying to understand why a pair of rebuilt heads with slightly larger valves on a Saab V4 engine raised compression from 150-160 to 175-190 psi while nothing else was changed.”
Because the bigger valves are sitting higher in the seat, reducing chamber volume.
Nick
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1967 Triumph Vitesse convertible (old friend)
1996 Audi A6 2.5 TDI Avant (still durability testing)
1972 GT6 Mk3 (Restored after loong rest & getting the hang of being a car again)
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melle
South West
It'll come out in the wash.
Posts: 1,983
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I would think that in the grand scheme of things (>40cm 3 combustion chamber volume) a minimal head skim, potentially thinner head gaskets (of which I have no reason to believe they have been fitted), and the bigger valves sitting a bit higher in the seats as vitesseefi suggests, should play a marginal role. From the photos I've seen (not my engine and I'm not near to it), the combustion chambers have been slightly enlarged too to accommodate the bigger valves and avoid valve shrouding, which I would assume offsets the latter effect anyway. |
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www.saabv4.com'70 Saab 96 V4 "The Devil's Own V4" '77 Saab 95 V4 van conversion project '88 Saab 900i 8V
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My own experience with this, on a Triumph 6 cylinder head and with actual chamber volume measurements, was the effect of valve height was more than I’d bargained for. In my case lowering valves giving lower compression…..
Actually measuring the volumes is the only way to be sure.
Nick
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1967 Triumph Vitesse convertible (old friend)
1996 Audi A6 2.5 TDI Avant (still durability testing)
1972 GT6 Mk3 (Restored after loong rest & getting the hang of being a car again)
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