[SD455DJ]

This conversation has got me curious…

For the 350 (354) and 400 engines with #11 heads to have the same advertised compression ratio of 8.80 to 1, the 350 heads would need to measure 77.2 cc’s and the 400 heads would need to measure 88.3 cc’s (both with ‘zero’ deck, 8 cc piston valve reliefs, and 0.040” quench using the Butler on-line compression calculator).

With Rocky’s information about the #11 heads having a design cc of 87.17, the 350 (354) would have 8.04 to 1 compression and the 400 would have 8.88 to 1 compression cc’s (both with ‘zero’ deck, 8 cc piston valve reliefs, and 0.040” quench using the Butler on-line compression calculator). Using a thicker head gasket (0.052” as per Rocky’s info), the 400’s compression drops to 8.69 to 1.

According to the Pontiac Parts Catalog – Models Thru 1975 (circa May 1979) - the “1970 400 2-B.C. w/8.6 ratio” uses #9790911 head gasket that measures 0.048”. This would increase the compression slightly to 8.75 to 1.

However, since factory engines were built with pistons at least 0.015”, or greater, below deck and head chambers 5-7 cc’s larger than advertised, the compression ratios are a good half point below advertised as well.

I have both the factory ’70 WU code 350 and XX code 400 2-bbl engines, so I’ll get them Whistlered to see what their actual compression ratios are.

Dennis

[Shiny]

Does a whistler measure static CR? I'm ignorant but curious how valve timing doesn't affect this?

[SD455DJ]

Originally Posted by Shiny

Does a whistler measure static CR? I'm ignorant but curious how valve timing doesn't affect this?

The Katech Whistler is an electronic device that accurately measures an engine’s compression ratio without disassembling the engine.

The Whistler Compression Ratio Tester is designed to determine the static compression ratio of virtually any engine quickly and accurately. It is widely used by race sanctioning bodies like NASCAR and SCCA to enforce compression ratio rules, as well as by engine builders to verify actual compression ratios without relying on volume calculations.

The device operates using acoustic principles. Air is injected into the cylinder through a spark plug hole, and the Whistler measures the resulting tone as the piston moves to top dead center (TDC). This tone correlates with the combustion chamber volume, which, combined with the number of cylinders, engine displacement, and temperature, allows the Whistler to calculate the compression ratio to the nearest hundredth of a point. Accuracy is reported at ±0.05 actual static compression.

The pictures below are of the RAIII in the '70 Formula; original engine before rebuild being tested on the Whistler.

Dennis

[Shiny]

Thanks Dennis

Wow, that's a fancy tool!

That 10 number must be nice to see!

Sounds like it is basically measuring air volume using some kind of sonic resonance response... then comparing max to min..

I'll look at their site to see if it spells out why valve opening doesn't affect it.

I predict 7.9 to 8.0 for a 70 350... just because I believe many #11 head measurements shared on this site that align with the engineering doc quoted above.

And the comment made about a gasket thickness difference between 350 and 400 2bbl engines in 1970 is curious... why would they do that?

Very bizarre to me that Pontiac would intentionally engineer different gasket thicknesses to somehow make the same head work on different engines, then advertise a known inaccurate number like 8.8... ??? Makes me think the transition to emission compliance caused a lot of decisions that were driven more by schedules than by design.

Mike

[SD455DJ]

I should have mentioned that you rotate the engine so both lifters to the cylinder you're testing are on the base circle (compression stroke). It is a resonance difference between the two extremes. You can hear a faint 'whistle' differences when testing.

Good questions that I certainly don't know the answers to. They sure played the numbers game for some reason that doesn't make sense on the surface.

Then 350 had its own unique head gasket for the smaller bore diameter. I don't have any information on the thicknesses unfortunately.

Dennis

[srmmmm]

Originally Posted by SD455DJ

Then 350 had its own unique head gasket for the smaller bore diameter. I don't have any information on the thicknesses unfortunately.

Dennis

The 350 block has chamfered cylinders for the large valve heads. That may be contributing to the actual/calculated compression ratios, as well as necessitating the special head gasket.

[SD455DJ]

I'm sure the double chamfer in the '70 350 blocks didn't help the compression any, except lower it a bit. 7.9 to 8.0 is probably right on.

Dennis