Quote:
Originally Posted by SD455DJ
He also "whistled" the engine to see what it had for compression and were hoping to see low 10's (10.75 advertised), but came in at 9.4 - 9.5 to 1 on both sides, so we're surmising that it was rebuilt for lower compression to be more street friendly and was possibly punched .030" over. The only way the Whistler came be wrong is if the valves are not completely shut, but that isn't the case with this motor.
Dennis
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Never heard of that machine before, I looked it up, I guess it measures the maximum and minimum psi to get the ratio?
That might be the right compression btw, I just rebuilt a 69 428-360, she was a never rebuilt virgin and we carefully measured and calculated the cc's and the stock ratio was about 9.7 to 1 versus 10.5 advertised (it had the original head gaskets and their crushed thickness made sense). I thought it might relate to it being a small valve engine but when we did the same measurements on a set of virgin 62 heads from a 69 428HO they were almost identical in cc's. I then measured a set of 48 heads from a WT 400 4spd (also virgins) and they were 5cc less because of a different shape to part of the chamber.
The fact that Pontiac claimed that both manual and automatic GTO's had the same 10.75 in 69 even though they had different cc heads and inversely they had mechanically identical engines rated at 10.75 and 10.5 in different market applications puts their ratio credibility into serious question!
Maybe that's partly why places like Royal could wake up these engines by blueprinting them, just raising the compression from 9.7 to 10.7 alone probably developed 25 horses.