4g61t.org

Guys I wanted to run this by you guys to see what your thoughts are...

What if:

A 4G61T motor was bored .060 over, and had forged 9.0:1 pistons and rods....

Would we see any possible MAJOR horsepower gains?


Geoff

Sounds good to me but the power gains from a 4g63t would be better, for the price of the forged pistons and rods you be making alot more bang

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Adam
93'FD Touring non-seq
05'WRX

Yo, Geoff, Everything I've heard is make power from boost, not internals. In fact, I believe that the better method is to lower compressiopn. Check out: www.sdefi.com/techmods.htm under "turbo for street" (about seven para. down). Also note that several vendors are pushing thicker head gaskets to aid in a lower comp. ratio. So... make it strong (forged pistons, even overbore), clamp it with with ARP and blow
^%$& out of it.
A bit more, the advantage is operational, more than power, per se. The pistons are incredibly light and are each balanced in grams. I was told that our rods are forged anyway, so the only advantage would be in weight. Mine were shot peened and edges were ground. They were each balanced at 681 grams (before, they ranged from 682-685 grams). Also, you want to be sure that the cylinders are each true and vertical Bill Spruill, oldest Colt

That theory is old skool! The new theory is: use high compression AND high boost. Marco from Magnus Motorsports will sell you a built 4G63 engine designed specifically for turbo that uses 10:1 flat-top pistons.

Just about every man and his dog with a built motor I know of are using 9:1 pistons. Plus, if you look at the history of the EVO motors you will see that the compression trend is upwards. Currently I think the latest EVOs are running 9:1s as well.

There are alot of benefits to running a flat-top piston with the head design of the DOHC 4G6x motors. The standard dished turbo pistons use virtually none of the quench pads built into the head whereas a flat-top piston uses all of it. Quench and squish play a large part in detonation suppression by introducing a high degree of 'artificial octane' which essentially means you can run the same boost levels as a similar decompressed motor but with potentially even less detonation which results in far less lag and alot more torque.

Im running custom flat-tops on my 4G61T which gives a CR of ~8.7:1. Im using the 4G63 inlet manifold as well and the response is still very good. The pistons have ceramic coated crowns to aid in detonation suppression as well as keeping the piston cool (since I dont run any failure prone oil squirters).

Yup, low compression is just so you don't risk funning-low on fuel and detonating. I know that a couple of guys have turbo '64s with stock internals (9.5:1 CR) running low boost (6-8 psi) with the stock fuel system and they're find. 9:1 would be fine for a turbo app as long as you're paying attention and have plenty of fuel. I saw a turbo motor somewhere running like 12:1 CR, I forget the details though. But I think it was a full race motor.

I agree with bill. Just make it handle more boost. I know alot of guys running 9.0:1 comp, but I'm still a firm believer in the ideal 8.5:1.

It is true that the Evo's went up for a bit, but they went back down. I believe that the Evo IV was as high as 9.5:1 and they had loads of problems, so they took it back down to the 8.5:1. The current Evo 8 is 8.8:1.

I say if it makes you happy, build it. Afterall, no one here has done it so why not?

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Jason Veraldi
Scammer
http://www.4g61t.org/forum/viewtopic.php?f=3&t=34532

Dont get me wrong. High CR and turbo will not work on all motors. It just so happens that the DOHC 4G6x heads have design features that will allow a relatively high CR without detonating given the correct tuning. Correct tuning doesn't mean throw alot of fuel in as that will cause detonation as well. Also, attention to detail is extremely important when building a high CR turbo motor. It takes only one sharp edge in the combustion chamber to create what is termed a "heat riser" or "hot spot" which will lead to an engine that will detonate very easilly under high boost.

If people weren't sometimes morons and fuel quality could be guaranteed you would probably find cars like the EVO running even higher compression than what they do now.

Also keep in mind that at least here in California, our so called "Premium" fuel is actually 91 octane camel piss. I imagine that in Oz you smegs get the choice 98 octane like most of Europe.

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Jason Veraldi
Scammer
http://www.4g61t.org/forum/viewtopic.php?f=3&t=34532

I get ultra 94 at least

bah, we get absolute crap fuel here in Oz. Most performance cars that come out here and especially turbos only arrive in a detuned state. The best fuel we can get here is 98 (BP Ultimate, Shell Optimax) except that is measured using the RON standard and not the more accurate MON standard used in the US. Our 98RON fuel is about equivalent to your 93MON fuel.

I refuse to buy anything except BP Ultimate for my cars (superior quality control than Shell) of which ATMO I pay AUS$1.11/litre(US$2.57/gallon). Plus, given that Perth is the most isolated capital city in the world (closest capital Adelaide is 2800km or ~1800miles) away makes for expensive travelling (not that Im making day trips to Adelaide or anything ).

Excuse the OT.

the only thing higher compression does for a turbo engine, is give a less laggy feel, a high compression engine tends to rev faster, until boost buidls tehn takes off

a low compression engine will feel like a dog, until boost hits, tehn it takes off


a high boost engine on the street would ideally use 8-8.5 pistons, for longevity, but a rac engine may use as low as 6-7, more space to cram more air and fuel, making a bigger bang, and ultimately more power, but on the street until boost hits, its gonna suck for off boost acceleration

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If speed kills, then i shoulda been dead awhile ago. There is no such thing as "Too Much Power". There is no excuse for a lost race. Do you view the rev limiter as a fun limiter?or as a shift point? And we all know, more boost=more fun.

Ultimately it isn't purely static piston CR alone that determines the ultimate dynamic compression CR which is far more important. Cam profile and phasing also play a huge part. Essentially it is possible to dial up or down dynamic CR (if it is too high) by altering the cam timing.

However in general, a high static CR motor will always produce more torque than a similar low CR motor given the same air flow and produce it earlier in the rev range.

It is not peak torque or power that defines a truly powerful and flexible motor rather its the 'area-under-curve' from the dyno plot.

Ultimately it isn't purely static piston CR alone that determines the ultimate dynamic compression CR which is far more important. Cam profile and phasing also play a huge part. Essentially it is possible to dial up or down dynamic CR (if it is too high) by altering the cam timing.

thank you xlr8, i completely forgot about that one as well, good catch,

that is entirely true, thats why some big american v-8 guys can run 13:1 + compression on pump gas, they just have to run race fuel to race the thing to keep detonation to a minimum, LOTS of overlap at low engine speeds.

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If speed kills, then i shoulda been dead awhile ago. There is no such thing as "Too Much Power". There is no excuse for a lost race. Do you view the rev limiter as a fun limiter?or as a shift point? And we all know, more boost=more fun.

Oh Jason....thats what makes california so great

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