Engine Tech Tip: Measuring Blowby

Engine Tech Tip: Measuring Blowby

You’ve built an engine exactly the same as the last engine, but the power seems to be down 10 to 20 horsepower on the dyno. Could it be excessive blowby because of a ring sealing problem? One way to find out is to measure crankcase blowby.

By Larry Carley
Technical Editor

You’ve built an engine exactly the same as the last engine, but the power seems to be down 10 to 20 horsepower on the dyno. Could it be excessive blowby because of a ring sealing problem? One way to find out is to measure crankcase blowby.

A blowby flow meter can tell you precisely how much blowby is occurring inside the engine. Unlike a cranking compression test or a static leakdown test, a blowby test actually measures the volume of gases that are entering the crankcase past the piston rings. The flow meter allows you to measure blowby from any engine speed, all the way from idle to wide-open throttle.

A blowby test requires a blowby flow meter. The meter measures airflow, and is attached to either the crankcase vent on a valve cover breather, or the PCV valve fitting. On a V6 or V8 engine, the opening on the opposite valve cover must be temporarily blocked so all the airflow from the crankcase will flow past the meter.

When the engine is running, all blowby that leaks past the rings will flow through the crankcase, out the valve cover opening and through the blowby flow meter sensor. The meter outputs an analog voltage signal that ranges from zero to five volts. The display can then be converted into units that show you the volume of airflow per unit of time. Most engine builders typically display the reading in cubic feet per minute (cfm), though heavy-duty engine builders more often use cubic feet per hour (cfh).

One supplier of blowby flow meters said contrary to what many people think an engine typically has more blowby at idle than at higher rpms. As the speed goes up, the rings actually seal better and blowby drops.

How much blowby is normal? Dividing an engine’s maximum horsepower output by 50 will give you a ballpark number for how much blowby you would normally expect to see. For example, a street performance engine that makes around 500 horsepower will typically have about 10 cfm of blowby with conventional pistons rings and ring end gap tolerances. Higher performance engines that are built to tighter tolerances will usually have less blowby, as might those with gapless piston rings. An 800 to 900 horsepower NASCAR motor, for example, might only have 5 cfm of blowby.

Less blowby means more usable horsepower. Being able to baseline the actual blowby in an engine means you can then go back and try different ring configurations, ring types (conventional or gapless), different ring end gap settings and cylinder wall finishes to see which combination gives the best seal and the least amount of blowby.

Measuring blowby has been one of the best kept secrets with performance engine builders because it allows them to see how well the rings are or are not sealing. It also allows them to detect any ring flutter that may be occurring within a particular rpm range, and to then change the mass or end gaps of the rings to minimize the problem.

You May Also Like

Brake Pad Edge Codes – What Can They Tell You?

The edge code is a language written by engineers, federal entities and industry associations.

When a brake pad is manufactured, one of the last steps involved is to print a series of letters and numbers on the edge of the friction material. This code has been on brake shoes and pads for more than 60 years, but what does it mean?

The “Edge Code” can tell you information about a brake pad’s friction material. These letters and numbers can help you to select the correct brake pads or shoes for a vehicle. But, the edge code can do only so much. 

USPS to Buy Six Canoo EVs

The Postal Service will take delivery of six right-hand drive versions of Lifestyle Delivery Vehicle 190 in Q1 2024.

Tool Time Podcast: Clore Automotive

Jim O’Hara from Clore Automotive joins Nadine Battah and Eric Garbe.

Ride Control For Electric Vehicles

Replacement units are available from sources other than the dealer.

CITGO Enters EV Space with Pilot Program Launch

The CITGO location on 11 Mile Road in Michigan now offers both regular transportation fuel and EV charging stations.

Other Posts

MEMA Aftermarket Suppliers Becomes UAF Gold Lifetime Trustee

The partnership with UAF is a commitment to the future of the automotive aftermarket, MEMA Aftermarket Suppliers said.

Castrol Turns 125, Debuts New Market Strategy

As part of the new strategy, Castrol will explore opportunities in battery thermal management, digital and service solutions and data center immersion cooling.

Brake Problems

Reducing brake drag on late-model vehicles is not accomplished by a single component; it takes a system.

FCS Introduces 18 New Numbers in February

The new release includes 4 shock absorbers and 14 suspension struts.