We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we discuss the basics of compression and how it relates to your fuel octane.
T.C. • Dayton, TX
Q: I have a 1993 Chevy Silverado Z71 and according to the repair manual, my small block’s compression is 8.2:1. What factors determine the ratio? Also, I know a lot of drivers who want a high compression engine that’s pump friendly. What does the compression have to do with the fuel octane?
A: Here’s a quick rule of thumb: the smaller the combustion chamber, the higher the compression. The ratio is determined by comparing two measurements—the volume of the engine cylinder with the piston at BDC to the volume of the cylinder with the piston at TDC. So your choice of piston will affect the ratio. For instance, a flat top piston will create a different ratio than a dome top piston. A thinner head gasket can change the compression very slightly, and your stroke will affect it as well (longer stroke = high compression).
Pump-friendly high compression engines can be easier on the wallet. That’s why many gearheads choose that project path. High-octane fuel resists preignition, so you can run higher compression (more power) without the risk of early detonation. On 93 octane, most cast iron cylinder heads can support up to a 9:1 compression. However, aluminum cylinder heads on the same fuel can support up to a 10:1 ratio and produce more power because they dissipate heat more efficiently.
For more information, check out this handy compression calculator.
Compression Test: shows 80% of New Compression… Wont pass smog! ( RUNS GOOD.~ Mid range Gas ) 110,ooo Mi. 20/50 W … Will 50 only help compression? Motor Honey, S. T. P.?
Bert, thank you for your question. This is a topic I’d recommend running past the Summit Racing tech guys: 330-630-0240.
I dont agree with that at all. Cast iron heads can only support up to 9:1 compression with 93 octane? The FACTORY GM 454 Vortec and 8.1L engines were 9:1, with cast iron heads, and ran on 87 octane. Those engines were used in trucks and motorhomes, and extremely stressed. Many more engines came out of detroit with iron heads, and more than 9:1 compression.
I have yet to see any data proving you can run a full point more static compression, based on switching to aluminum heads.