Anyone have any opinions on compression ratio in a fully built 2.4 hybrid engine? Is 10:1 just asking for trouble? 9:1?

It depends on what you are looking for. Are you building a race car? How much street duty do you want it to see? What are you going to use to tune it? Standalone wideband etc. If I was building a race car I think a 2.4L 4g64 would be GREAT! The Honda guys seem to be making fantastic power with 10:1 CR. But if you were to do something like that you would probably want a turbo the size of a T66 anything else would be totally pointless. 10:1 is not a CR for big pump gas numbers. It is for running big boost on a huge ass turbo on c16. Even though everyone THINKS they want to have the “fastest” or make 600 hp on boost the reality of the situation is that most people wont be able to commit the dollars to hit their end goal.

I decided on 9:1 on my 64 because I wanted a strip/street car. 9:1 is higher than stock but still low enough to make good power on pump gas and it should spool my big ass 60-1 around 3800 rpms or quicker while making about 550hp. 64 low end grunt with a SM intake manifold and HKS 272s for top end breathing. Sounds like a powerband to die for to me

I love rtrdk's answer's...they're always correct (especially his first paragraph). But here's the best way of thinking of compression ratio (I think). More compression is going to give you higher HP and higher exhaust gas velocity (ability to spool those turbos)...but the higher in CR you go up the smaller the tuning window becomes (c16 manditory)...so it's easier to blow motors.

Thanks guys. I'm in the planning stages of putting together a 46g4 for the street. Don't plan on running race gas too much and driveability is more important than peak BHP. I guess 9:1 is the way to go.

If you just want a 400whp 2.4L street/strip car that has a great powerband you should consider a 4G64 or 4GCS from a junkyard and leave the internals alone. 9.5:1 CR and 2.4L will spool a big turbo and with the boost low it should stay together.

I went with ross pistons for a 4gcs block for the shop talon, they are supposed to be 8.5cr with a .4" headgasket.

I tend to err on side of saftey with the tuning window...you can always make more power with nitrous.

just watch out which rods you go with though, some of them dont clear the block and youll need to do a little bit of grinding to make sure every thing clears. even a 9:1 comp ratio will spool a pretty good size turbo pretty quick. talk to magnus, they spooled there gt35 turbo at 3100 rpms, that was 30lbs of boost at 3100 rpms. whitch is more than most people will ever need on the street. they had all the parts I needed, and were great to talk to as far as what else I would need

I've not actually picked a turbo yet, but I'm looking hard at a GT30 with a .63 A/R turbine and a 56 trim compressor wheel. Hoping to hit full boost below 4000 RPM at 18-20 PSI on street gas (which is 94 octane where I live). But that may be wishful thinking. Was going to get a used L2R, but the GT30 new isn't that much more. The final set will be something like:

- 4g64 block with ross pistons, eagle rods, arp hardware, 9:1 compression, etc.
- FMIC (Spearco), 3 inch turbo back
- stock 1G head (!) except for HKS regrind 272/272s
- eight 450 cc injectors (second set only kicks in when needed; better driveability than four monster injectors), fuel pump, etc.
- custom tubular manifold
- Tial HP 35 mm external
- GT30 (?)

Comments are welcome.