Anything wrong w/ flat tops and turbo?
Turbo Mini Forums

Background: This is for a budget-build track-only 998 turbo build-- and it's a 24-hour long event...

I have an A+ 998 with flat top pistons and I have a couple 12G295 heads available to me, so basically I can make it whatever static compression ratio I should need (I have a grinder and am not afraid to open up a chamber MORE if needed).

What's the general consensus on flat top pistons and turbocharging? Our biggest enemy thus far (With stock dished pistons and about 9:1 compression using 12G940 heads) has been sorting the timing to prevent detonation as track temps rise during the event.

We have a chance to make a more purpose-built 998 turbo engine(s) using a gt15 turbo as well as a metro T3. The flat top engine is already together, the dished piston motor will take me a bit to get it to the same point. I'm feeling lazy, but want to know if I should work to shake off the sloth a bit faster.

At the risk of sounding like I know what I'm talking about, is there an issue with "flame front" when using flat tops thereby increasing the risk of detonation when compared to an identical motor that uses dished pistons instead of flat tops?

It's compression ratio that important, I run flat tops as do many others without issue.

The hill climb boys will set you straight regards the actual figures required for race car conditions as this differs to road use where we want the turbo spoiling earlier.

You may want a lower compression ratio with more boost but that may require the gear ratios to be altered to suit the turbo spool.

I noticed on track last weekend that I needed a lower gear than that of a na car to make sure I was spoiling the turbo out if the corners

I don't know about the flame front, but I've used flattops in some of my 998 builds no problem.

Even the ones that have dishes I have had dished, so they are really 'stepped flat tops' rather than having a genuine dish.

If the CR is right I don't dee a problem.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

I ran flat tops in a STD engine at 15 psi with enlarged chambers during a hillclimb without a hint of det. You should be fine with the 295 if it's not mega skimmed. Make sure you sort your fuelling and timing do you have an intercooler?

Thanks for the responses!

We do not have an intercooler (yet). Wasn't in the budget and small displacement turbocharged cars aren't in large supply here in the US breakers. But it's on the to-do list for this go-round. I'm told this will help a lot with detonation and the amount of timing we can run.

the comments about gearing are quite relevant to our experience. We've had lag that doesn't give us anything below 3500, but once we start getting close to 3500, it starts to really take off (I've had a thread on the 998 section "holy balls"). Putting our close-ratio gearbox back in would help, but until we're done with the experimentation I'd rather not risk dropping bits down into it (again)

We're wanting to move everything down the rev range. I don't want to take it much above 6K simply because we're using stock valves and have had head/valve/spring cap failures (even before we went turbo). I'm a little worried that the gt15 will go too far in the opposite direction, but it's what we've got and it's all an experiment anyway.

Some of the comparisons to hillclimbing and track sessions are helpful but at the same time a little off--- we are going 8-9 hour days of racing under multiple drivers-- sometimes a straight 24 hours with the only stops being for fuel and driver changes. We bring in a hot car after a driver's been out for about 2 solid hours, then take 10 minutes or so to refuel (have to cut off the car) and change drivers and do a quick check and then it's back out on the track again. Restarting is always difficult. When we go to megajolt, we're hoping we can dial in timing advance for startup so we don't have to manually advance it every time like we do with our current setup. The top cars (turbocharged miata, bike-engined cars, and some V8 cars) are blowing by us at 120-130 on the straights and we've no expectations to stay with them. But we're hoping that getting more punch out of the corners will take us to a mid-pack car.

Back to the original topic-- thanks for reinforcing my laziness! I'll go ahead and get the flat top engine ready for a possible turbo transplant.

As long as you can get the CR then you should be fine.

Squish will not be as good as with a properly dished piston, but depends on your chamber shape.

I ran flat top pistons in my engine (1293) and made 200bhp at 20psi.