I'm new to Miatas and turbos. I come from the world of V8s and superchargers. With that said, I'm looking to turbocharge a '99 I recently purchased. I have a low mileage '01 VVT engine and forged internals that I plan to install before dropping the engine in. My experience has been, however much HP I have, I get bored and want more. So I want the engine to able to take whatever I may throw at it. I have read and reread many posts on turbo setups and have pretty much decided on the EFR 6258 setup from TSE. Finally to my questions. What effect does 5,000 feet have a a turbo engine. Let's say the 6258 makes 280hp at sea level at a given boost pressure. What has to be done at 5,000 feet to make the same 280hp? From what I've read, I need a turbo that's not out of headroom at sea level at the target HP (280-300) so it can be spun faster to compensate at higher elevations. It also appears the higher elevation increases spool time. Is this accurate? Am I on the right path? If not, please feel free to guide me in the right direction. Supporting mods will be, MS, ID 1000 injectors, 3" exhaust. Also, the car will primarily be a DD with occasional trips to the local road course in Albuquerque. Please, before bashing me and telling me to "Search", I have searched and found very little concrete information on turbo charging at high elevations. Primarily a bunch of differing opinions.

 

copy my car

 

Thank you. That's pretty much the path I was considering. I was debating between the 6258 and 6758. I do plan to go with E85.

 

Up at 5500 feet we lose about 3psi of atmospheric pressure. And we have less oxygen (17%) than sea level. So you have to compensate with more boost. I don't know the exact figure. But you can guess that 4ish psi will do the trick. But the problem is, at no/low boost, there isn't much you can do to improve the situation, so you spool that much slower.

If you want v8 response, buy a v8.

but I'd do the 6758 if you are going for big numbers.

 

Abandon the term "boost", it is meaningless.

Target absolute manifold pressure instead. Be aware that your spool RPM will move to the right by approximately (1-amb/sealevel). So for an ambient of 85kpa, your spool will happen ~15% higher in the RPM domain. The compressor will require more energy to make up for the increased PR, so hp will decrease. IOW, a 200kpa car at 5000ft will have lower whp than a 200kpa car at sealevel.

 

Thank you both for the information. As far as the V8 goes, that was my initial thought but I decided I wanted a new challenge. I'm looking forward to tinkering with a four cylinder and a turbo. I was thinking of running a high compression engine to help with the low rpm power, since I plan to run E85. Is this a good or bad idea? I have 8.6 CR forged pistons but I can always sell them off for a set of high compression pistons.

 

Stay with the 8.6:1 pistons if there is any chance of ever running gas and not e85. I don't think the spool difference will be enough to justify taking apart and rebuilding the motor. I would go with the 6758 in your case. It wont be v8 low-end, but will be pretty dann close.

 

I haven't assembled the engine yet, it's sitting in the garage waiting. Since I haven't assembled it, and I don't foresee running anything other than E85, do you think it would be worth it to run the higher compression pistons? As much as I enjoy wrenching, I want this to be done once and done right.

 

If you find that the turbo is too big, you could always throw a small wet shot of nitrous on to spool the turbo.

 

For a DD I would absolutely not go high compression. There are plenty of areas in the country where you won't find e85.

 

 

Honestly that is a great point i had never really thought of.

 

That is a good point and I have thought about that before. The car will probably never be driven far from home, where I currently have three stations nearby that sell E85. With that said, it would really suck if they went away and I had an engine with high compression pistons.

 

I'm running 9.5:1 compression (OEM 99-00 pistons) in my build. I'm at 7,200 ft elevation though, and am also going to be running e85 for high boost/track, so keep that in mind.

That said, the response is fantastic, even on my old 2860rs turbo, which is said to be lackluster.

EFR will certainly be glorious. BTW, I'm just a few hours north of you!

 

I think it comes down to that you are probably not going to be heavily racing the car and trying to win some class or series, so pushing the normal probably is not worth it. We have a common formula for built bottom ends that work. 8.6:1 supertech pistons and weisco rings. Forged rods of some sort. ARP main studs and head bolts. ACL race bearings. Boundary Engineering oil pump, and possibly some sort of front dampner. That combo is good for more then 500whp on a street driven car. E85 or 93 octane it can probably do it. Both the EFR 6258 and 6758 spool amazingly. Even at altitude you should be more then happy.

If you were racing and trying to be number one in a series then that would change the story and running the higher comp pistons would probably be worth it. In all other situations I think siding on the side of caution is probably the smarter bet.

Have you driven any turbo miata's at altitude near you already? I am just asking to see if you have any sort of baseline of what another turbo miata feels like in other configurations.

 

No I haven't driven any other Miatas so I don't have anything to compare to. You're 100% correct, I don't see myself in any serious competition, just a fun DD with occasional trips to the local road course. The recipe you described for an engine build is exactly what I have sitting on a shelf in the garage. I rounded up the engine parts but thought I had better ask for opinions before purchasing the turbo and assembling the engine. Everyone that's responded has given solid advice. I just wanted to make sure I'm heading down the right path.

Fireindc did bring up something I'm curious about. Stock, high compression pistons ('01 engine with 19,000 miles) and some version of a Garrett turbo. I gather the pistons will be fine to 300-350whp IF the tune is good (from what I've read in other posts). The Garrett turbo intrigues me because they can be found for a good price used along with the manifold. Is there a dramatic difference in performance between a Garrett and an EFR? Which Garrett would be a close rival to the 6758?

 

If you are worried about spool on an EFR then going to a Garrett will only make you cry. There are no Garretts available new that are cost competitive with an EFR that will hit 350whp.

Stick to what you had planned. You are on the right track.

 

Thank you. That's exactly the advice I was looking for. That makes me feel much better about my plan. Like I said, this is all new to me. I've been down this path with V8's I've built in the past. I no longer have the patience, time or money to "Guess" until I get it right.

Thank you very much to everyone that responded and thanks for not beating me up on my first build question. I'm sure I'll have more once I start putting it together.

 

You ask good questions and take the advice. That puts you above the majority of new posters on here. We are very glad to help and give good advice, you just have to not be a dumbass about it. And you are not.

If you have not already go make an intro post in the newb section. Post some pics up of the car and your plans and such.

There are a good number of mt.net people in AZ and you can probably find someone local if you look a bit to get a ride or drive.

 

I haven't posted pics but will certainly do so. I hope to meet some like minded locals in the ABQ area when I get the chance.