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Old 10-21-2010, 04:22 PM   #1
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So I'm building a motor with a Miata head and gtx bottom end so 7.8:1 CR. Everybody on my local forum is saying the car is going to drive like *** low speed no boost, and that I will want to tear my engine apart ad put in higher CR pistons? What do you guys think?
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Old 10-21-2010, 04:51 PM   #2
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It will definitely not have as much response down low. You are trading that for safety/cushion space in boost.

If you know what you're doing tuning wise you can definitely gain back a bunch of the lowend with good spark tuning.

I don't think it will be nearly as bad as they say it is. Just me though


*edit: it would also help us help you if you listed the whole setup including turbo specs. a huge laggy turbo and a small quick spooling turbo will only amplify the effects of teh low compression, for worse or for better respectively.
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Old 10-21-2010, 05:02 PM   #3
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2560 ball bearing, ms2 diypnp w/ebc, 99-00 265cc injectors, lc1,vtps, gtx rods/pistons, new bearings all over and rings. goal of max 200-220whp
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Old 10-21-2010, 05:05 PM   #4
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I don't see the need for such low compression with such modest power goals, but it doesn't hurt. Should you want to step it up later you'll have the headroom.

Why not just refresh the stock long block, throw in some rods and call it a day at 9.1:1?
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Old 10-21-2010, 05:06 PM   #5
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2560 ball bearing, ms2 diypnp w/ebc, 99-00 265cc injectors, lc1,vtps, gtx rods/pistons, new bearings all over and rings. goal of max 200-220whp
Those pistons I sold you are going to be woefully wasted at that power level.

The only reason to run compression that low is if you plan on running 20+PSI on pump gas with no meth. I myself hit 240 at the wheels with stock 9.4:1 pistons, and I know of several other 1.6 guys on this forum that have made that much or more.
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Old 10-21-2010, 05:09 PM   #6
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99-00 265cc injectors
Everything else sounds good as far as I know.. but these injectors seem pretty small for 200 HP. Better to step up and run less duty cycle.
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Old 10-21-2010, 05:14 PM   #7
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My plan is to start small and after some more time and money going higher but I want to do it while I'm in there. Later going for rx8 secondarys and a fuel pump maybe some better spark then 250-280.
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Old 10-21-2010, 05:22 PM   #8
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Why secondaries and not just bigger injectors?
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Old 10-21-2010, 05:24 PM   #9
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Why secondaries and not just bigger injectors?
Rotary engines tend to use staged injection. The secondary injectors are generally larger.
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Old 10-21-2010, 05:31 PM   #10
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450cc's yellow body
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Old 10-21-2010, 05:33 PM   #11
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FWIW I am putting miata pistons in my GTX. Think about off boost drivability, spool and overall power. 9.4:1 is seriously going to be a better choice for your application and miata pistons are cheap.
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Old 10-21-2010, 05:47 PM   #12
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With that small a turbo you won't be off boost as much as guys with a big one. I wouldn't worry about it much. I've run the difference on Desktop Dyno in the past and it is only a scant few foot lbs of torque difference on our little engines. As it has been stated many times in the past, power is only a downshift away. I'd rather have the headroom for the future.

You might even be able to get away with running 87 octane at low boost levels!
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Old 10-21-2010, 05:48 PM   #13
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Rotary engines tend to use staged injection. The secondary injectors are generally larger.


Doesn't make sense, unless he means that he will replace the current injectors with a set of secondary injectors from an rx8.


I'm sure I'm just daft.
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Old 10-21-2010, 05:53 PM   #14
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Old 10-21-2010, 08:08 PM   #15
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I really want the big power but seeing as "I'm in school its not feasible right now'' Fiance's words not mine, I think I will go for the low compression and when I can afford the extra add-ons to make 300+whp I will buy them. 7.8:1 will be slow going from the get go but it will scream at 20+ psi 93+octane. Thanks for the advice and opinions everyone. I'm always open to opinions.
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Old 10-23-2010, 01:08 PM   #16
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Personally, I see compression as "free horsepower" on naturally aspirated engines but with forced induction it's really just a matter of managing all the parameters.

I understand the school that says a piston swap is easy and can be cheap when the engine is apart and pays dividends forever but everyone has different needs.

I see a guy who is in school, has no real "extra" money, is carving out a place for his "hobby" with a future wife (which you have to do, or there won't be a hobby), probably needs a working car no matter what and might want to take a few drives in the summer away from the easy availability of quality high octane gasoline.

Luckily the Miata is a free revving engine out of the box, so he won't have to spend money to actually use the higher power band.

Seems like a sensible tradeoff. In particular, an engine grenade experience is definitely going to force him to dig in with the future wife, who may be the most sweet, understanding woman on Earth, but has other ideas about what a family's priorities should be. Not her fault, just the way it usually is. Better the whole thing is painless, with no surprises, so I say conservative is fine. For now.
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Old 10-23-2010, 09:46 PM   #17
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Yeah, I'm building it for big things but, for the time being + some, it will do.
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