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Turbo race car, high or low compression ratio?


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For a 300-350whp turbo race car what would be a good strategy for compression?

The race car will live at 3000-6000 RPM and hopefully be on boost all the time? Does it make sense to run 7:1 compression to really really avoid getting detonation? 


Most posts on this forum indicates that >8:1 compression ratio is very doable with modern engine mangement and good intercooler.  It significantly Improves off-boost conditions, but unless drivers scews up there shouldn't be any off-boost conditions.



From what I understand a 300whp NA race car also sucks under 4000 RPM.


Would be on high octane pump gas and Megasquirt fuel and spark.



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Stock L28ET makes those numbers easily.  So do many other engine options including any turbo L28.  Generally speaking, compression ratio is not a major consideration when considering that minor amount of power from a turbo engine.

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Modern combustion shapes that are not detonation prone  and cooling of head really affect this. I do know that some of the 510 crowd are running really big compression with AV gas or racing Fuel ( VP C16 ) . E85 would work as well.


Example 1:  Turbo 2.1 Liter 510 with 10.3 CR on 16 lbs boost making 300 RWHP.  Track day, Autocross and Hillclimb car. Can be driven on street. Water/Meth injection . Owner brews his own street fuel in garage for cruises!!! Not the safest or most practical option. No E85 here. VP C16 for track.  Carburated!! But very well sorted. 


Example 2: Local 240 Z  on low compression . L28 all the tricks including big intercooler and C16  Race fuel, but can run on pump gas with reduced Boost and Water/Meth.  Makes 450 RWHP. Can't remember exact boost, but I think it was around 15 lbs. MS2 ECU. 

Edited by Chickenman
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I will be using a 

stockish N47 head or maybe a P79 if I am lucky. Either flattop or dished pistons.

So no modern stuff here. I cant run water injection either and no race gas.

Its in Texas and it does get warm during summer.


Seems like going low on compression might be the best strategy, the extra power from higher compression is not worth the devastation of detonation. Street car maybe but not enduro race car.

Even if I get a stable tune things can break and mess things up, so having something robust is not bad.


Also 300whp is plenty for a 280zx amateur race car weighing 2700 lbs with driver and fuel.  (My current racing league doesnt allow cars below a PWR of 10:1, so probably need to detune it even)..

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I'm planning on doing something similar as your setup, and so far I have a forged piston L28et (roughly 8:1 cr) on megasquirt, spearco intercooler, and plan on running e85 since we have easy access to that stuff all around austin. I haven't run mine yet to test it out, but I honestly think you'd be fine with just a stock bottom end as long you as run a quality intercooler and e85. E85 obviously isn't necessary, but it's definitely a godsend to help keep detonation at bay. Oh and I'd highly recommend finding a p90 head. I've ran a p79 with on a  turbo setup before and it worked fine, but some people talk about the exhaust liners coming apart. I couldn't care less if it was an n/a build, but I wouldn't think you'd want that shrapnel going through the turbo. 



For longevity of the turbo, I'm also running a water-cooled t3, larger aluminum radiator, and an external oil cooler.

Edited by zeeboost
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Haven't driven my car much in the past few years, more of a garage queen at the moment, but when I hydrolocked my turbo block I swapped an NA into it.  I've been running it on 16 lbs of boost out of a Holset HY35 w/ a sequential MS3 setup.  Haven't dynod it but based on peak fuel consumption I think it's making about 360whp.  No major problems in a few thousand miles since the swap.

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when you think of compression, it helps to think of how much boost you think you`ll need to achieve your hp goal.  to get 300-350, you need something like 10-15psi on a t3-t4 turbo.   so you need to think of how much compression you get can away with for that psi...  8.1 is PLENTY low enough for those numbers if you manage your gas well.  I think mildly dished  pistons gets you that.  I`m pretty certain flat pistons get you something like 9.x CR..


you might also want to think of which turbo you`re getting.  if you want too spool time and low rpm boost threashold, you need a smaller torbo.  that would be great, but you`d have to take into consideration that smaller turbo create a bit more heat (bigger intercooler OR lower CR ratio) and run out of breath earlier, although if you`re building an engine that won`t see anything much above 6000 rpms, you can probably get away with a lot.  



I`d call a turbo shop (one of the big company) and tell them what you want so you can get an idea of what turbo you need and what kind of psi you`ll need at 6000 rpm to get 350 hp (maybe ask for 4000, so you have some margin of error, and room to grow). 


if you run on e85, or on a mix of gas/r85, some ECUs will adjust the AFR accordingly (maybe even boost level).  running e85 will get you less detonation, so you can run higher boost/get more hp with it.  this way, you can get e85 for races, and still get gaz anywhere if you`re simply cruising on the street.   but I'm not certain MS has that yet..  

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Run your 8:1 compression.

Spend your money on a proper porting and cam, as well as a properly matched turbocharger to make that power at as low a boost pressure as possible and you will be very happy with the results (though you may be revving to 6,500~6,700 due to how it pulls. This is a case where a cam that supposedly peaks at 7,500 or closer to 8,000 will pay big dividends because of it's high lift breathing down low. It is entirely possible to make 350hp at 8psi in an L28 without boring or stroking. Or an intercooler...but it's good insurance...

With an RPM cap of ony 6,000-7.000 there is no reason to run bigger than a 0.63 (stock) turbine side A/R. Preserve your boost onset as low as possible, the torque you will be rewarded with will be prodigious. There will be no 'lag' so to speak but expect 4-5 mpg on-track. Horsepower costs fuel, period. This goes for a Geo Metro. Yes, you can get 5 mpg in a Geo Metro if you are using all it's horsepower, the ratios of air to fuel don't change, the Z will just be going much faster than the Geo getting the same mileage...LOL

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