9.6/1 vs 9.9/1 Compression Ration

[82 sportsc]

Guys
Deciding for my 2.5 weber carbs , DC30 cam, 70.4 crank
Have new under fender cooler
Whats your thoughts ?
Thanks
Frank

[Gonbad]

You won’t probably notice a difference in power

[Henry Schmidt]

Originally Posted by 82 sportsc

Guys
Deciding for my 2.5 weber carbs , DC30 cam, 70.4 crank
Have new under fender cooler
Whats your thoughts ?
Thanks
Frank

Lean towards the lower compression ratio for three reasons.
First, with Mod Solex cams, dynamic compress will be higher than with most larger spec cams so detonation will be a issue.
Second, you will not notice a performance difference between 10:1 and 9.5:1
Third, as soon as the green people realize that the best way to get old cars off the road is to restrict octane, be on the look out for liberal states restricting octane to 85?

[PeteKz]

Thanks for that "Third" reason. I hadn't considered that as a method of getting old cars off the road.

[Jeff Alton]

Measure the CR, do not rely on the "advertised" CR, it is rarely correct.....

[Henry Schmidt]

Originally Posted by Jeff Alton

Measure the CR, do not rely on the "advertised" CR, it is rarely correct.....

As "advertised" might as well say as "fantasized".
We're building a 906 engine and the "10.4:1" Mahle pistons aren't even 9.2:1.
The big valve 906 chambers are close to 74cc and CP tapped out trying to find 9.6:1. We even went to Wyseco for their 13:1+ super pistons and they weren't even 10:1.
It all reminds me of dyno numbers after the "correction" is applied.

[Jeff Alton]

^^^^^^^ this...