Forum: HoseHeads Sprint Car General Forum (go)
Moderators: dirtonly  /  dmantx  /  hosehead

     What effect does compression have on the performance and cost of a sprint car engine?

Compression makes a huge difference in the performance of any engine. When you make the space for an explosion smaller, you make the explosion more violent. Think about a firecracker, in an open hand there is no damage, close your hand and you lose a finger. The more compression you make the more horsepower you can create. 

On the flip side of that... the engine is stressed more an more as compression rises. This can cause rod, wrist pin, piston, ring and crank failures. As compression goes up, failure times for engine parts goes down.

The cost is something you will pay for up front. But it is the rebuild cost that gets you. An engine with a moderate amount of compression could be strong for 20-25 races. This engine may not have the most hp, but it will live longer. A high compression engine may need a rebuild after 9-10 race but has a huge amount of horsepower. 

We have a about average compression engine and get 20-25 races. But it also race primarily on dry slick tracks so it does not see 9000 rpm, ever. 

Very insighful answer, thanks for sharing.

Two roads diverged in a wood, and I—	
I took the one less traveled by,	
And that has made all the difference.

About how much of a horsepower difference would we be talking about between the engine needing rebuilds at 20-25 nights verses 9-10?

This is an excellent and insightful answer.  Just to add for the OP keep in the mind the Compression Ratio for a 410 Sprint Car engine is about 16:1.  That's ridiculous.  Just to throw a non comparison, comparison out there a ZR1 Vette has about a 9.1:1 compression ratio if I remember correctly.  Yes I know purpose built vs factory built consumer brand car, but just to give you a "real world" idea with something you'd see everyday (a Vette).   

Good comparison.  What makes that even more ridiculous?  Going from 9.1:1 to 10:1 is a HUGE jump let alone up to 16:1

The observatons are correct and pretty well described.  However the other way to damage a motor is to have too much gear and over rev the engine.  Proper Gear selection for both motor and track conditions are crucial in how long your motor will live.
Most engines have prime operating range and while you want to be able to get off the corner good, You also don't want to be max RPM a quarter way down the straight and producing long max rpm times.

Champphotos nailed it!!

Agreed.  When I explain sprint cars to my gearhead buddies (who don't follow the sport, that is until I give them free tickets and they and their kids become hooked) and say the compression ratio is 16:1 their jaws drop. Literally.  Their minds are blown.  

Another non comparison, comparison.  A Nascar engine compression ratio is about 12:1.  I had to look that one up as I don't follow Nascar outside of checking to see how Larson, Kahne and Stenhouse did.  Again, two different forms of racing, two different engines, bla, bla, bla but just for comparison's sake I throw it out there.  

As we've noted 16:1 is absolutely ridiculous.  

I can't answer about sprint car engines, but I can tell you about 302 Fords. I had a1970 Econoline, 9 to 1 compression, 205 HP, could beat anything but a Porsche or Vette over independence Pass, highest paved road in USA ( my timing was way advanced) It eventually blew up, got a 1974 Econoline, also 302 but "post oil embargo": 7.5 to 1 compression, 139 HP, 66 HP loss (32%) It was a dog going over the pass....

Rebuilds? I Lost track of how many times I turned over on the odometer on the 1970, either 352,000 or 452,000 with no rebuild. It blew up doing 50 in 2nd gear going up last big hill before Red Rocks (Stevie Ray Vaughan show, didn't miss a note, friends right behind me on highway!) But it never got ANYWHERE near 9,000 RPM!

We are around 14:1 and I honestly don't know the HP, best guess is around 800 but again no clue. Both 410's and both 360's have a little less compression and the reality is we don't need more. Ohio is dry and slick fairly quick in the evening. A huge high compression engine is not a big advantage in Ohio. Unless you racing Eldora or Atomic if they are prepped decent.

DA

Its actually amazing what new cars are starting to squeeze out for compression. For example, a 2017 Chevy Malibu with a 1.8L 4 cylinder is squeezing 12.5:1 compression. 20 years ago that was a full blown race engine. Now its just an every day car. 

Has that been made possible by all the computerized stuff?

Yes the computer systems have made huge advances along with the engineers having a much better understanding of how to find that magic combination of air, fuel and compression. The computers can monitor about everything you can imagine, such as spark knock which helps these run right on the edge of efficiency and power. These new engines are running 11.0:1 compression on turbo engines now as well. Years ago if you were over 9:1 with boost it was a death sentence.

Very interesting.   With 12.5:1 is higher octane required or does the computer somehow control the timing advance to deal with the potential for pre-ignition?   

My '17 6.2 silverado has 11.5-1 on 91 octane. Aluminum heads/large overlap cams and- direct fuel injection all allow more compression ratio all else being equal

Gravel snapped his crank bouncing through ruts wide open. When tires leave the surface engine spins up then when tires come back down it stresses crank........snap. Experience wins

     How does that compression compare to say, a common 360 sprint car engine?

There arent any specific recommendations on fuel such as 91 or above. And yes the computer watches the spark knock and other inputs to adjust timing accordingly. With each cylinder having its own coil its easy for the ecm to control when the coil fires the cylinder. Pretty cool technology compared to dinosaur ignition systems we have in race cars today. Ive actually worked quite a bit to try and introduce these modern LS style engines that utilize these systems into dirt track racing.