N/A high-comp question

[livinglegend2100]

hey guys its been awhile since ive been on Nico, used to be over in the 240 forums for the longest time and ended up selling the car. long story short i have an rx7 that im in the process of putting a VG into. i have the motor and everything mocked into the car, and after i get everything straightened out and running, i would like to rebuild the motor. im not looking to make a power-house of a VG, just bring it to life a little bit (mild cams, tube headers, lightweight fly, just little bolt on junk). i did a little searching through the forums, but most of what came up was unrelated to what im looking for, or the builds never went anywhere.

-High Compression. any idea what sort of ceiling im looking at for compression on a ~stock-ish vg? Will new OE headbolts handle it or would i need to go ARP? What about OE rings? Is there much to be gained from shaving the heads? reason i ask is im pulling the heads off for some mild porting, so im entertaining the idea of shaving them and upping compression a bit since they will already be at the shop.. but i cant find anything on it and im keeping stock management for now so i dont want it to run like a bag of d!ck.. any information is welcome.

there was a bunch more i had to ask, but it escapes me at the moment so ill get back to it later. thanks in advance guys

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[themadscientist]

You can likely get away with something in the 10~ish:1 range if you give it good gas and watch your timing. Stock head bolts can contain boost, they can handle a high-comp NA motor, no worries.

New OEM rings or the rings on the pistons you buy will be fine, but make sure to gap them properly and if you can find a place with the gear and can foot the bill be sure to have the cylinders done with torque plates so when assembled the bores are truly round. No point in pumping up the compression just to let all the pressure escape past a weak ring to cylinder seal.

If the heads are flat they only need a quick cleanup pass, but you could increase the compression by shaving them. BE AWARE when you do this you just f*** up your timing. The belt is now stretched across a shorter span between the crank and first timing gear. You will need adjustable gears on all the cams to degree them back in. IMO, it's a lot of investment for very little gain.

Porting is always good, but be thinking about your intended use. If you will be using the power on the highway go to town and open up the intake ports, but you can go too far and create a dog motor that can't pull from a stop because the air column takes forever to get going. Talk to your head porter and make sure that first, he knows what the hell he is doing. Check out his work on somebody else's stuff before you let him near yours with a grinding stone. Second, that he understands your goals so he knows what type of work need to be done.

IMO, on a low-expectation build like this I would just have him clean up the ports, not really open them at all and smooth out the shortside radius, check the valve seating and correct if they are not seating cleanly, check the heads for warpage and deck them if necessary, bore the cylinders just enough to get them straight and round and spin balance the crank.

[livinglegend2100]

man. thank you. first bit of decent information ive gotten so far. And as you put it, its a "low expectation build". im not looking to make a ton of power over stock. i dont really want to d!ck around with cam gears and all that if i dont have to so ill probably just deck the heads/block when i do gaskets. The porting isnt gonna be crazy either. more or less just eliminating all the casting and making it pretty. To be honest i didnt even think about bringing the cam gears closer to the crank with shaving the heads down, so im glad you pointed that out.

What are the odds it would hold 11:1 pistons with OE hardware and gaskets? Would i be moving into e85 and piggyback territory then?

thanks again for the info

[NolimitZ32]

I cant tell you what kind of compression is will handle but I would imagine that you could go 11s or maybe even 12s with a very well balanced rotating assembly and head work, these bottom ends have seen 600+ whp before so I don't think you would get even close to braking anything as long as you had better pistons, and kept all the factory tolerances.

[InitialForce]

So a little more clarification:

If it's a stock N/A motor (not a TT with the turbos removed) you are already running 10.5:1 compression. I don't see why the heads couldn't handle 12:1 or even 13:1. I wouldn't take any material off the heads, get pistons and do it right if you like to go that route.
If your running a TT swap minus the turbos your current compression is 9:1 if i remember correctly. (which btw is a HORRIBLE idea... I will never understand why people swap a TT motor without turbos because your running like 175 HP at the motor without the turbos due to its lower compression ratio. If it's a TT swap without turbos definitely change pistons to at least the stock N/A 10.5:1 ratio)

My personal opinion, get a turbo motor. You are going to spend A LOT more money on high compression and all the mods you listed then getting a Turbo motor.

Stock N/A at 10.5:1 compression: 222 HP at the motor
Stock Turbo at 9:1 compression and stock boost levels: 303 HP at the motor.

In the end its your car and your project but in my opinion doing major motor work isn't worth in on an N/A considering the prices and you can get a lot more HP out of a turbo set up for cheaper then an N/A build.