Why build a high compression motor?

[DRRummel]

I was going to order some 10.5:1 to 10.9:1 pistons for my next motor.
But I was not building the motor to twist much above 5300- 5500 RPM. Stock ignition system and inexpensive rotation assembly.

Then someone said that it was stupid to build a high CR motor and not design it to be run at 6500 RPM. Maybe that is true for a drag car. But wont the high CR pistons give me more power though out the RPM range?

[ModularMike]

Most likely, but why go through all of that to build an engine your not going to rev over 5500rpm? Are you doing a lot of towing with your car?

[DRRummel]

No towing. Mostly around town driving. I run it between 2000 and 5000 RPM. Usually between 3000 and 4500 RPM with 4.10 gears.

I was hoping to keep the low end grunt for stop and go driving. I was concerned an intake designed for volume would help get power at 5000-6200 RPM but lose off the line power, and that is not where this car runs, except for three time a year at the track.

The Gt-40 will restrict the top end, but keep the low end velocity up.

Or so I have heard. I am learning as I go.

[TooFast98Cobra]

Low RPM is no good for street racing up in Glen Burnie. lol-my fav place to street race.

[Balaska347]

I would not considers those numbers to be high compression. I would say once you start getting around 11.5 and up you can consider it high compression. I would go 10-10.5 maybe a little higher if your running an aluminum head and it will be tuned. I dont care what rpm range you wanna run in compression will give you power especially torque.

[SonofaBish]

I would not considers those numbers to be high compression. I would say once you start getting around 11.5 and up you can consider it high compression. I would go 10-10.5 maybe a little higher if your running an aluminum head and it will be tuned. I dont care what rpm range you wanna run in compression will give you power especially torque.
i agree... if you're building the motor anyhow, there's no reason not to up the compression unless you are planning to go FI in the future.... Just don't want to get too high of a CR and then end up needing some sort of fuel additive (or different fuel altogether) to suppress detonation... but at 10.5 - 10.9 you're nowhere near that point yet

[DRRummel]

Thanks. I figured I would gain power thoughout the curve. I just did not know how much. A local builder was implying that most of my power gains would be in the very upper RPM range. Right where the GT40 would start to choke.

[Killercanary]

Compression should help pick up torque and therefore power everywhere. Do some calculating to see what static compression ratio would be needed to arrive at the desired dynamic compression ratio based on your cam profile. There are a lot of online calculators to help with this. If you want to build it for power then this is the proper way to choose your static compression ration. It has been too long for me to remember what a desirable dynamic compression ratio is for an NA combo but I am sure you can find it out. I suggest reading up on dynamic compression ratios to familiarize yourself with what it is, why it is important, and what effects it before you decide on what static compression you build into the engine.

[yeahloh95]

I agree with Paul his 9-1 motor acts like a 10-1 just because of his cam

[somethingclever]

I agree with Paul his 9-1 motor acts like a 10-1 just because of his cam


Paul's camshaft makes compression?


Can I have one too? :hyper:

[Mark Aubele]

Thanks. I figured I would gain power thoughout the curve. I just did not know how much. A local builder was implying that most of my power gains would be in the very upper RPM range. Right where the GT40 would start to choke.

If a builder was telling you that, I suggest finding another.

[yeahloh95]

Yes call fti and he will make a custom one for you too