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how to increase compression ratio on a turbo


Ruiz Racing

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You're at a higher altitude. What is your reasoning for not wanting to run more boost?

 

Why not just go for low compression and run higher boost? You must be at like 5,000 feet.

 

 

i was running 12psi. it made my car go very fast, but it took forever for the turbo to spool up. and i want to make the same power with less boost.

 

i kind of want to see what it will do. the only reson i know anything about my car is blowing it up and fixing it. its expenssive but it sure is fun.

 

i have a n-42 head in my garage without a cam.

 

what would happen if i put the turbo cam in the n-42, f-54 block and 10 psi. would it HOLD TOGETHER? it sounds fun. it will give me something to do this week.

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You can put that cam in the head, just check your wipe pattern. Make sure you have the correct sized lash pads for that cam. If you use the N-42 head than your compression is going to less then 10.5:1 as it has a 47cc chamber instead of the 38cc chamber that the Maxima n47 head has. Good luck!

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i was running 12psi. it made my car go very fast, but it took forever for the turbo to spool up.

 

Properly tuned? Boost leaks? Compressor wheel condition? Excessive plumbing/intercooler sizing? Etc? Etc?

 

 

Oh and isnt the mn47 head too tall to fit under the hood? Youll probably need to mod the hood...

 

No and no.

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Boost over compression. If your turbo was taking forever to spool, was your tune correct? Turbine selection? Were you trying to boost outside of the boost threshold?

 

I hit 22 psi of boost by 3200 rpm with a T03/04E 50 trim turbo.

 

It all really depends on your goals, whether you will run race gas, E85, methanol injection, or just pump gas. The power differential between a low compression motor (say 7.4) and a high compression motor (say 10) is not that significant off-boost. I have absolutely no problem with off-boost response with my setup.

 

Increasing compression is not the way to make more power with less boost. You need to increase the airflow to the engine though headwork, intake work, etc.

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WHOA....Lots of misinformation here.

 

First, the Turbo Cam CAN NOT go in anything but a TURBO head. The cam geometry is radically different, and you will wipe out a cam very very fast. The EXCEPTION is if your "turbo cam" comes from a P90 head that has solid lifters in it to start with. This was discussed in a recent thread, and has to do with the cam lift ramps (how fast the cam lifts the valve) needing to be much steeper to keep the hydraulic lifters happy.

 

Second, The MN47 head is exactly the same thickness and dimensions as every other L6 head, excluding the LD28 diesel head. It will not hit the hood.

 

Third, the L28ET pistons have a 10.9cc dish. The MN47 head is a 39.5cc head. On a STOCK F54 turbo block, i.e., one with the dished turbo pistons, using the MN47 in place of the stock P90 will result in a 8.8:1 compression ratio. The stock configuration is 7.4:1. On a FLAT TOP motor, the compression ratio with the MN47 head will be 10.75:1, all of this is calculated with a stock Nissan head gasket. With a Fel-pro gasket, the compression jumps to a hair over 11:1. (with the dished pistons, and the fel-pro gasket, the CR is 9:1)

 

As far as using the N42 head, the F54 dished piston turbo block, and a cam that is cut for solid lifters, your compression ratio will be 8.4:1. This is boostable, and with good fuel management, i.e., NOT the stock computer, it's cake, if others on this board are to be believed. You Might be able to do it on a stock computer, I don't know.

 

If your F54 block has flat top pistons, the N42 head will put you at 10:1 compression, back in the danger zone...

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It seems like you guys are making this too complicated. Ruiz Racing, if your turbine took too long to spool up, then get a smaller turbo. The reason you made so much hp was because of a big compressor wheel. OR, read Maximum Boost by Corky Bell or give him a call. Business phone is 830-980-8185. I'm sure he has a better answer.

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What dyno graphs did you see between the two?

 

The difference in thermodynamic efficiency (hence the power gain you could expect under ideal conditions) between 7.4cr and 10cr is about 10%. The L-series makes in the neighborhood of 100-120hp naturally aspirated when off boost (if this case probably up to ~3500rpm).

 

So at most, you could expect to be gaining 10 or so hp off boost, and giving up a lot more than that when on boost.

 

Also, raising the CR will not make the turbo spool earlier. If anything the increased efficiency from the CR will result in less energy being available to spool the turbo. You'll get through the off boost region slightly quicker due to the extra power output, but I'd bet it would be a wash, since the turbo would also spool a bit later, making the off-boost region a bit bigger.

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I think torque is what we would be interested in with the CR bump. I am not saying that a high compression turbo engine is built for a race car, just like I wouldn't say that a low compression engine running 30psi boost would be fun to drive to work everyday. My point is that if a person wants a fun commuter car I believe high comrpession is one way to go, or a really small turbine so you build boost at low rpms. Either one should be fun to commute in, neither one will have a big horsepower potential. I am not guessing about it, I am building one up to see. Hopefully the original poster will do the same. Then we can be two more guys saying that high compression and turbos is good or not good for our driving styles.

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I think torque is what we would be interested in with the CR bump. I am not saying that a high compression turbo engine is built for a race car, just like I wouldn't say that a low compression engine running 30psi boost would be fun to drive to work everyday. My point is that if a person wants a fun commuter car I believe high comrpession is one way to go, or a really small turbine so you build boost at low rpms. Either one should be fun to commute in, neither one will have a big horsepower potential. I am not guessing about it, I am building one up to see. Hopefully the original poster will do the same. Then we can be two more guys saying that high compression and turbos is good or not good for our driving styles.

 

The same ~10% number would apply to the off-boost torque.

 

All I'm trying to get across is that it seems to me that many people's expectations for increased CR are way overblown. Even with a moderately sized turbo like you mention above, what we are really talking about is giving up a pretty much imperceptible amount of hp/torque in the off boost region and gaining easily 50-70hp when on boost, and that boost would happen at the exact same or slightly lower rpm. I know which I'd find more entertaining.

 

BTW, my low compression engine running E85 and 26psi IS fun to drive to work, with the possible exception of having to cope with Michigan's crappy roads :mrgreen: (not a powertrain problem, except for when the big chunk of concrete flipped up out of the road and crushed part of my exhaust :fmad:). Off-boost drivability is just fine.

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