Any experience? - "G head" with "fast burn" pistons?

[johnl]

Got my 85GLHT running after years of sleep. Wow, ran great, better than I remembered. No wonder!! - knock sensor wire broken - cracked piston ring lands ensued shortly.

So, rebuild. Bores fine. Ordered standard bore Mahles. Finally received them. I am concerned though, because the volume of the dishes are greater than those that came out of the car.

Are these big dish pistons supposed to be coupled with a "fast burn" head and not my G head? If so, how much of a performance penalty is there? Is there an advantage?

I am converting to T2 and that will bring higher boost levels - 15s I suppose. I gather that lower compression ratios are good for higher boost levels but with 525 I do not intend to make huge power, yet.

Comments?

[turbovanman]

Oye!

An engine that has the 782 swirl head and then install the G head will lower the compression ratio. Unsure of the drop but its between .5 to point.

Pros-run more spark advance, can run lower octane gas, run higher boost on lower octane gas, better flowing head than a 782

Cons-less performance off the boost, lower bottom end torque.

[TurboShadowES]

im running a g-head with fast burn mahle's... havent cranked the boost above 14 psi but right now im REALLY likeing it!

[johnl]

Quote:

Originally Posted by turbovanman Oye! An engine that has the 782 swirl head and then install the G head will lower the compression ratio. Unsure of the drop but its between .5 to point. Pros-run more spark advance, can run lower octane gas, run higher boost on lower octane gas, better flowing head than a 782 Cons-less performance off the boost, lower bottom end torque.

So, I've got the greatest volume head and the greatest volume piston dishes for the lousiest streetability (crummy off boost low speed econo putting) but potential for high power via high boost ?

[johnl]

Quote:

Originally Posted by TurboShadowES im running a g-head with fast burn mahle's... havent cranked the boost above 14 psi but right now im REALLY likeing it!

I used to really enjoy driving my T1 GLHT around below the boost, short shifting, using minimal gas - - then fastdownshift and WHAM! Best of both worlds - econobox and sportscar - all in one. Kinda don't want to lose that by going to biggest possible combustion chamber combination - fast burn pistons and g head.

But, if it is truly a good high performance combination, and those that know on the list can confirm it, then I'll go for it. Just asking for validation of my thinking from those that know.

[Frank]

My pistons are Ross, but same deal. I have a G-head going on. What Simon says is correct... I think the little bit of less low end torque would be a good thing on the street.


Frank

[TheGinMill]

I have a G head, with venolias. Menegon prepped head, I have really decent power off boost. Like all others said, drop a gear and it really goes.

I wouldnt worry about it at all, you aren't losing all that much down low, keep the tires hooked up too.

[moparzrule]

I'll give a report in the next few days, I just got my G head back today andplan to have the car back on the road saturday. However my results might be a lot different since my head is ported and I'm running a much larger turbo than stock.

[turbovanman]

Sweet, I have a G-head sitting, hmmmmmmmmmm!

[moparzrule]

Got it today....pics
Ported and rebuilt by turbodynamx.net, super 60 springs installed.

[johnl]

Would everyone agree that the lower than stock compression ratio that the combination of a G head with "fast burn" pistons yields, also will give you a little bit more room for error with regard to the boost/compression/fuel/ignition complex at, or near, peak power?

Anyway, thanks guys, I'm feeling better about it.

[moparzrule]

Yes, lower compression means you can safely run more boost on a given octane and/or more spark advance.

[turbovanman]

Yes and no. If your at threshold with the 782 head and you bolt on the G-heasd then, then yes, you will have a margin of error but threshold on any head is threshold.

[kedavis]

Quote:

Originally Posted by johnl Would everyone agree that the lower than stock compression ratio that the combination of a G head with "fast burn" pistons yields, also will give you a little bit more room for error with regard to the boost/compression/fuel/ignition complex at, or near, peak power? Anyway, thanks guys, I'm feeling better about it.

this is what I found when going to a G-head I was able to run alot more advance with pump gas and even more with race gas your alot less likely to hurt something especialy under high boost.Didnt seem to lose any low end at all maybe even gained.

[Frank]

I was doing some reading in some books and online... remember Larry's (4sfed4) post about higher compression piston and back to back dyno test?

Anyway, here goes.

Changing compression ratio will drop your entire power curve, requiring more boost to meet the origional power. Of course you can now run more boost to compensate. How much boost you need to compensate depends on the engine and how it handles more boost (air flow, volumetric efficency, and the turbo's operating envelope). Depending on all of that, you can meet the same power, or go well beyond that who knows.

I dont see it effecting any specific region of power at all. The advancement of timing is nessecary and natural to bring the power to work efficiency back up - effecting regions of power. This is because it allows the gasses to burn sooner before applying significant force to the piston. Of course you will have a little more wasted energy, but allows for a more complete burn sooner. This actually lowers the EGT. Pretty wild, eh?

Of course, with only a 0.5 drop in compression, its almost a mute point in some aspect on our cars. This is more of what I have found over the past few days of reading while bored at work. I mainly just felt like sharing it.


Frank "BS'ing outload" Katzenberger