Are our internals horsepower limited or boost limited?
 

To better phrase that, if some are safe running 12psi out of large-ish turbos like T3S60s and stuff, am I safe running 15-17psi for short bursts out of a T25 as long as I can keep the intake temps down (using WI)? Are the rings in the Miata motors limited by actual pressure or power?

You are really limited by the quality of your tune up to about 270-300hp. That is what it seems like. Remember that there are people that blow up their motors at less boost and less hp.

boost indirectly limits how much you can safely run through the motor. Two things cause great stress for the motor given it is tuned right: high hp AND high rpm. The rods are the first to go.

You will be pushing the envelope on stock rods at 275+whp but any increase in power will results in added stress to the motor in any case.

Of course proper cooling, etc have to be present to support the power level you are running.

Actually I believe the two would actually be torque and high rpm. Horsepower is a function of torque and rpm whereas torque is a calculation of how much power each stroke of a piston is putting down, which in turn estimates the ammount of force pushing down on the piston (which pushes on the wristpin, which pushes on the rod, which pushes on the crank). High rpm causes a slightly different sort of stress.

of course hp is derived from torque but I mentioned hp since it is directly related to torque and is always the higher curve past 5252 RPM.

and high RPM simply caused the moving parts to be accelerated faster and stress is thus applied and removed at a higher rate causing .... well more stress on the metal.

you have a good point. I can see how a constant torque would create far greater stress at 7k rpm as opposed to 4k rpm due to the rate at which force is applied and removed. So maybe hp is a better general indicator of the capabilities of a motor given a good tune?

I'd venture to guess that 90% of snapped rods in Miatas are from knock. Not many people run race gas in their Miatas, especially non-built ones, but I bet you wouldn't see the same failures at 300whp on race gas as people wouldn't be knocking.

Frank

True nock and shock laoding like the ones who where running 270+ whp and put in a wet shot of NOS.

well the torque and hp and boost and knock are all going to have an effect on.... cylinder pressures. while power can influence them, it wont have nearly the effect of detonation, preignition, or knock. compressing an explosion too early is not good for rods.

but there are also reports of missed-shift rod failures. which makes me wonder if the high vacuum forces are slightly stretching them and weakening them.

high vacuum at high RPM is very detrimental to rod bolts due to stretching - this is why it is recommended to change the rod bolts when doing an engine build.

However, like it was said, a lot of the failures are due to a poor tune or something going wrong. You don't need race gas to run 20psi on the Miata motors, just enough ignition retard and fuel.

ignition retard whats that we wanna advance ours :D

unless you're ignition is behind LPP, then output falls off with diminishing returns.


Is this where lowering compression will add power...because now more boost can be added without detonation or pre-ignition?

boost effectively raises compression ... it has a similar effect to running a higher static compression ratio. So you have to make a compromise at some point between boost, compression ratio, and ignition timing. I run 7 degrees advance timing at my maximum torque point and boost to be on the safe side. If you really want to run high octane, run some natural gas as fuel :) Or you can do some work on the combustion chamber to remove hot spots.

But realistically, a 9:1 compression ratio will let you run lots of boost ... easily over 20psi on 94 octane pump gas and well into the 300whp territory. Also throw enough fuel in there to keep combustion fairly cool. The stock rods will go before you've maxed power due to detonation.

this thread is making me question my supertech 8.6:1 piston purchase.

8.6 compression is not that low - it should be ok on and off boost. A slightly higher compression helps spoolup time and torque off boost while the slightly lower compression will give you a bit more of a safety margin and may allow you to use cheaper gas depending on your tune and amount of boost you run.

I went for the forged lowered comp pistons because I track the car, and want the extra safety. I thought it was good insurance, and I want to avoid rebuilds.

That's exactly why I run race gas, to not have to retard as much..

Fuck it, if 93 is 3.65, and race gas is 4.85, I know what I'm doing, drinking one less bottle of gray goose per month.

Unnecessary cost. The pistion is unlikely to wear out and require replacing, rings on the other hand ... But really, do you think you'll own this car for 10 more years?

ignition retard sucks ass, i know why the WI love it sooo much now.

what is your guys' minimum ignition advance under boost?

yeah, why would I sell it?

:inout:

19°

4k, 8psi.

4500rpm, 93 pump gas
17° - 8psi
14° - 12psi
12° - 15psi

I was hitting 21-22° at 12.5 psi at 4500RPM, probably have room for more advance

15° minimum, ~15 psi @ tq peak

brainy i'm around 17-18 at 12psi. no we didn't push it until detonation, but adding more timing didn't make more power.

hmm, looks like I can safely add quite a bit more timing, especially since I am running on 94 octane and 8.2 compression.

I used flyingmiata's ignition map as a base.

Awesome shit, guys. That's what I wanted to hear. My goal is to have two maps, a normal 12psi 91 piss-water map and a 15, mabye even 17+psi map once I get a water injection setup. I probably won't push past 275rwhp or so until the motor is built, but I think I can hit that number even on a small T25 and big boost as long as I can keep the intake temps and detonation down (perfect use for WI).

yeah, that was my problem...I also had a 2° retard (i suck) so I was closer to 19-20°

right now my targets are 20° @ 180 kPa; 18° @ 210; and 16° @ 230

I was still making more power as I advanced it, but the returns were starting to become dimishing, my fuel was also way off, so it will probably be altered some. KnocksenseMS is 85% wired up, so I'll be able to make sure all is kosher and ready for my real dyno tuning session.

Hmm, so why is FM using so much retard on their big turbo kits?

cuts down on "FM blew up my engine!!!11!!one!!!" complaints.

seriously tho they make it for bad gas and lots of margin of safety.

i'm still using FM's stock map..

That's OK, you can run 89 octane then. :rofl:

i advanced it 2 degrees, i think i can go a bit more.. i have no knock as far as the knock sensor sees anyway..

Ben and Brain, do you guys mind posting up a screenshot of your current ignition maps (

should be in my "why i think timing is important thread" but here:

https://www.miataturbo.net/braineack.../diytiming.jpg


Ben's should be in his "I'm squirting" thread, should look similar.


I have yet to do much testing of this table as I'm not currently driving it untill I get my LC-1 back from innovative. Then I'm getting on a loaded dyno to do some comprehensive tuning. But as is at 12.5psi, I had no signs of knock and the power output seemed butt-dyno friendly.

Thanks! how come you are running 14 degrees timing at idle?

My map is close enough to brainy's. Why not run 14 deg?

I thought stock was around 10 and was wondering if there was a reason for the 14.

stock base timing is 10, but without TEN and GND shorted, it varies. measure it sometime.

Can you send me a 50 gallon drum of race gas at 4.85 a gallon? Hell, I'll pay 5 bucks. $6.50 here for Sunoco 100 octane. Otherwise it's 7 bucks for VP 100.

What octane are you getting for 4.85? Unleaded?

I might just have to go with 750 injectors and run E85.

Frank