dan91

I've read Emilio700's excellent write up on engine building considerations.

https://www.miataturbo.net/engine-pe...g-ideas-83443/

I am building an engine now and one particular detail is causing me headaches: +2mm oversize exhaust valves. They are also mentioned in this thread from less than a year ago:

https://www.miataturbo.net/engine-pe...options-87635/

My build right now is a 2003 VVT engine, 9.0:1 Supertechs and Carillo Super-A rods along with new bearings and a general rebuild of the bottom end. I'll be running a completely stock head for now, and the next step is adding an EFR6258. After that I intend to get the CNC Whammy head that used to be available through 949 Racing with +1 intakes and +2 exhaust valves and Tomei cams. Hydra ECU. Final goals after the turbo and head are 8200 rpm, 280-320 whp with fast spool up, street car with occasional track days at a lower power tune. Burning 93 octane gasoline.

The issue is I need to cut larger reliefs in the pistons now for the future larger valves, or else pull the engine again in the nearish future, pull the pistons out, and then cut the larger pockets. If I go that route I might even need entirely new pistons since they will be coated.

My questions:
-949 Racing doesn't even sell +2mm exhaust valves. Where does one buy them? Wasn't it 949 that advocated using them?
-Are +2mm exhaust valves necessary at these goals? If so, how large are the benefits? (Faster spool? More horsepower/psi?) Those who have engines with +2s, please chime in
-If they are worth the expense, time, and hassle, can someone who has had Supertech pistons cut with larger reliefs tell me how they did it? And the name of the shop that cut them? I am hoping to ship the pistons to someone to cut the reliefs. My local machine shop does not seem interested in it at all. I'm in the Chicagoland area.

Schroedinger

From Emilio's thread:

"Valve size - Stock valve sizes work for any build. The greater the flow with say cams or F/I, the more will be gained by going oversize. Our CNC head has a +2mm oversize exhaust valve option. We have found that it is needed for max power output types of builds. Overkill for your 7400rpm 150whp pump gas HPDE build but just the ticket for your 8100rpm 180whp N/A or 300+whp F/I build."

He's probably the best guy to ask, but it doesn't seem to me like you're getting near enough to those limits to warrant the trouble of clearancing the pistons, etc.

dan91

I called 949 Racing but didn't get through, so I emailed the same basic questions. They answered quickly but said we don't and never have sold +2mm larger valves, consult with your engine builder. I would prefer to cut the reliefs now, even if I don't end up with larger valves. I guess my main question is how to go about it. If it's too difficult I'll give up and stay with +1mm.

Schroedinger

I'd follow 949's recommendations and talk to your builder. Any competent machine shop could cut the extra relief in the piston tops, but in doing so you're also adding a stress riser to a component that will see a lot of stress at high power. I would think they will need to do the valve work in the head first and see how much interference needs to be dealt with, so they can keep the cutting on the piston tops to a minimum.

Emilio's thread seems to imply that it's all a pretty long run for a pretty short slide.

ryansmoneypit

iTrader: (6)

I was right at the power you want, with a stock vvt head and had very fast spool with a 2560. Dont get over concerned with this if you goal is 300 ish.

psyber_0ptix

iTrader: (4)

8200 RPM for a street car on pump gas with easily obtainable power goals. Are you more concerned with transient response or just spool from a dig. Getting a CNC/ported head to actually benefit from the extra flow while modifying pistons to accommodate those fat valves? Fast spool is cool, drive-ability is cool too. I'm using peasant +1mm O/S valves. Soviet made gobs more power than me on a stock head.

chicksdigmiatas

iTrader: (11)

I have to agree here. Will the valves help power? Sure. But I think it is overkill. That head, that turbo, and those revs will have you at low 300's all day. This is advice coming from a guy that has stock valves, so take it as you will.

BogusSVO

iTrader: (1)

Keep in mind that most aftermarket forged pistons are already cut for larger valves than stock, so there may be no need to even fly cut the pistons.

Only way to truly know is to clay the piston, bolt the head down with the +2 valves and check clearance.

What would be more of a concern, and cost, is having to remove the stock valve seats, cut new counter bores and install larger exhaust seats to hold the oversized valves.

as long as the flow of the exhaust is about 85% of the intake, there will be no down side on power production.

dan91

Thanks for the replies. I won't be installing the ported head and larger valves for a while yet, so I'm not committed to anything yet. My goal is only to build in valve clearance on the piston now so that if/when I do go that route I won't have to pull the engine out of the car and take it all apart again.

I found a machine shop that can cut the reliefs for me, so I'm going to go ahead and do that.

I'm still interested in anyone's experience with using +2mm exhaust valves. The peak horsepower figure isn't my concern so much as spool and transient response. I know 300 whp is easily achievable, and I'm not worried about 4000rpm+. I'm looking to maximize power as much as I can in the first half of the tach as well.

psyber_0ptix

iTrader: (4)

Sweet! Twin scroll efr6758?

Madjak

I have no experience from a turbo point of view, but from a head building perspective you have lots of options.

If you are super keen you can use Supertech SR20 DE 30.65mm exhaust valves (+2.65mm oversize) or OEM SR20 DE Exhausts (+2.15mm oversize) or even the RB25 Exhaust 30.65mm inconel valves (but these are very long). You need to be running SuBs and source the matching Nissan keepers and shim the valve springs slightly as the keepers sit a bit lower in the retainer but other than that they are basically drop it. NOTE: SR20 DE not DET valves... the DET valves are sodium filled and so are a thicker valve stem, not that that is a major issue either. Ferrea also have some nice options too but they are a bit more pricey...

Or alternatively ask Supertech to make +2mm oversized miata valves... They have a custom order form.

chicksdigmiatas

iTrader: (11)

Like maximize how much? He made 200 tq at 3400 rpm. I think that is pretty damn good.

https://www.miataturbo.net/dynos-tim...-6258-a-81209/

mr.skywalker

I am not entirely sure when it comes to FI but here is what I do know from head work/valve sizing on bikes.

Generally if you are going for peak power you run a larger Intake diameter/exhaust diameter ratio, ie the exhaust size approaches the intake size so the ratio approaches 1. For example most engines are between 0.75-0.85 if i recall correctly. This is because at higher rpm you need to move more air so the larger diameter valve(and matching porting of course) allows more flow, but it actually can hurt you low end as the are is more "stagnant" and does not "pull" the air out as efficiently do to the dropped velocity of the air mass leaving. Now granted we would also run larger lift/duration cams that amplified this drop in low end torque

I can see how having a turbo provide resistance to flow you may want to completely minimize any restriction in the exhaust but I am not sure what point the directed airflow with higher velocity would affect turbo spool less that just max airflow. Just something to think about.

This is an interesting subject, has any one had any tests showing that bigger valves provide better spool/torque AND top end