2T-G/3T Hybrid Compression Problems

[Rodger]

Sillycar chick,

The 2T-G/3T combination is and always has been a good thing. I agree, it makes for a very drivable car. Always makes me wish I had kept my set of Wisco 89mm 3T pistons I got sent instead of the 89mm 2T pistons I asked for, all thoughs years ago for my race 2T .

What is the induction system?

There are several Solex things I can help you with to help reduce the potential for "pinging" if and when you plan to get a dyno done.

I ran twin 44mm Mikuni/Solexes on my 1640cc 2T with 10:1 compression, regular unleaded with no pinging and got 74kW at the rear at the last CCG Dynoday.

As Owen is saying, I concur as far as the issues about higher compression and reliability. The 2T series motors a well designed and built. Try doing these things to an old Holden or Ford six cylinder these realy needed big money to raise compression reliably.

There are other small issues that reduce the reliability of the 2T series motors but these only really start to appear when used regularly at the higher RPMs.

Regards

Rodger

[sillycar chick]

I have Twin Solex Sideys off a 88220 head (that's whats stamped on the intake) that my dad used to have on his TA22 years ago. I think they have already been tuned up, but will probably need another going-over on a different engine...?

[TheToyman75]

Sillycar Chick,

What Crank are you using ? Is it a 2T or 3T crank ???
Which pistons did you use ??


I hope those carbies arn't off your Dad's TA22 GT or I might just go cry in the corner ????

[YelloRolla]

Hmmm,
First things first.
Sillycar chick - you keep referring to cylinder pressure and equalling approximately xx:1 compression ratio, this is just plain crazy speak.
Cranking cylinder pressure is just that. Referring to what the book says the cranking cylinder pressure ought to be is just a waste of time - unless the book is referring to your modified engine. My point is this - cranking cylinder pressure is generated at whatever speed your starter can turn it over, your engine NEVER operates at cranking speed EXCEPT when you are trying to start it. Cranking pressure is not there to determine whether or not you are running a desirable comp ratio, but rather as a means of determining whether or not something is amiss with the engine (like a smashed ring land will show up with a lower cylinder pressure). Personally I wouldn't raise too much of a sweat about the 210psi.
You can try a simple change of cam timing and re-check the cranking pressure to see what the net result is, you may find are number that you are happier with.
Static compression ratio - as determined by cylinder volume + clearance volume/clearance volume is also NOT at the operating speed of the engine - it is static, motionless, doing nothing.
If you go big on the CR, then the cam timing (duration and overlap) can be changed to bring the DYNAMIC compression to where it needs to be (this is why big cams are always associated with raised CR). Dynamic CR will be a determining factor in whether or not your engine will ping.

I am not suggesting that 11:1 will be fine with your combo, as pointed out already it is quite an archaic combustion chamber design, BUT do not get too carried away with your loose compression ratio calculations.

I hope that this is of some help.

[gianttomato]

I was about to say exactly what Yellorolla said - nobody actually knows what the static CR of this motor is. I can't see where anyone has measure combustion chamber volume. All we can say is that dynamic compression correlates positively with compression ratio. Dynamic compression will be affected by so many other things. To say that cranking cylinder pressure x means that the engine has static compression ratio y is just plain nuts. No offence.

If you must know what the CR of the engine is, then use a burette to fill the combustion chamber of a cylinder whilst the piston is at TDC compression stroke (don't pull the head off - just use oil and suck it out when you are done). If you don't have a burette, just use some 20 mL syringes from the pharmacy - accurate enough.

Like Yellorolla said, change your cam timing or even your cam profile. Run some cooler plugs. If you didn't already, then you should have deburred all the sharp edges in the combustion chamber.

[sillycar chick]

Like I said earlier....

I cc'd the head chamber and top of piston using burette and some acrylic plate on top, and when I calculated out the comp ratio, I got somewhere around 8.9:1. So the 210 psi doesnt seem to make sense, if its 8.9:1 it should be around 170 psi? May be I stuffed up somewhere...

Does anyone actually know what compression ratio to expect from bolting a 2T-G head straight on a 3T block, crank, pistons (without shim)??? And I would like to hear from anyone who has actually DONE this. I have a lot of theoretical figures thrown around, thanks guys, but there is also conflict from other ppl who think the theory side is crap...

And can anyone explain why I had 210psi without the 1mm shim, then when I put the 1mm shim in I got 170 psi? If compression pressure is not proportional to compression ratio, why then has the readings changed? Thanks...

[YelloRolla]

Like I said earlier....



Does anyone actually know what compression ratio to expect from bolting a 2T-G head straight on a 3T block, crank, pistons (without shim)??? And I would like to hear from anyone who has actually DONE this. I have a lot of theoretical figures thrown around, thanks guys, but there is also conflict from other ppl who think the theory side is crap...

I have actually done this - twice. I have a 2TG head to my 3T GTE. This was done with 1mm oversize pistons with a 6cc dome. This brought the comp ratio to about 8.8:1 (or a bit over). This depends on the volume of the chamber used and the head gasket thickness (but you already know that). I didn't do a compression check, BUT, I do pump 25psi of boost into it (I imagine that the combustion pressures are quite high to what may be normally seen).

And can anyone explain why I had 210psi without the 1mm shim, then when I put the 1mm shim in I got 170 psi? If compression pressure is not proportional to compression ratio, why then has the readings changed? Thanks...

If all else remains equal (and that is VERY important), then the cranking pressure will go up, BUT who says the pressure shouldn't be 210psi with 8.9:1? Because this is VERY dependant upon cam timing.

Personally I wouldn't raise too much of a sweat about the 210psi.
You can try a simple change of cam timing and re-check the cranking pressure to see what the net result is, you may find are number that you are happier with.
Static compression ratio - as determined by cylinder volume + clearance volume/clearance volume is also NOT at the operating speed of the engine - it is static, motionless, doing nothing.
If you go big on the CR, then the cam timing (duration and overlap) can be changed to bring the DYNAMIC compression to where it needs to be (this is why big cams are always associated with raised CR). Dynamic CR will be a determining factor in whether or not your engine will ping.

[sillycar chick]

I have set the cam timing on this engine to standard as per the toyota engine manual. Cams are standard for 88261 head (as far as I know).

Also, I found this information on the net from Puma Race Engines, just as a bit of research... http://www.pumaracing.co.uk/comp.htm
Scroll down a bit to "So What Cranking Pressure Can We Expect To See Then?", which I found quite interesting, and does line up with what I have been thinking for some time...

Also, from what info I have on the 2T-G, it should have about 8.4:1 compression ratio and about 165 psi pressure, which definately lines up with the theory in the link above.

[Rodger]

sillycar chick,

Nice Link.

I have always figured there is a relationship between compression ratio and pressure and is as stated in the article. It is the first time that I have seen something written down in all my years of thinking about it . Also I have always been too lazy to research it.

Thanks.

Regards

Rodger.

[sillycar chick]

Thanks Rodger, good to see Im not the only one thinking hard about the theory behind how engines work. You might also find this link interesting....
http://www.andrews-products.com/moto...compressio.htm
I think it brings together the views of EVERYONE who has replied to this thread...

[YelloRolla]

They are informative articles - thanks for posting them.

Basically - and I like it when peple do their own thinking, if your cam timing was not suitable (like if you made a mistake assembling the engine) it COULD go as far as 210psi (the first part of the Puma article states that 300psi is possible from a true 10:1 CR), OR your calculations for CR were flawed. My first reply was that to prod your thinking back in the direction of where it could go wrong; like a slipped harmonic balancer placing the TDC mark in the wrong location, using the wrong timing cover (there are 2 that I have seen with radically different locations for where TDC is).
I recommend trying this as a quick check - wind no 1 to TDC compression (dial gauge down the bore), check that number 4 valves are open about the same amount. While the cam cover is off check all of the valve clearances.

Forgive me if you have written this elsewhere in this thread, but, what was the volume of the piston dome? what is the volume of the combustion chamber? what is the HG thickness? Is the bore and stroke still 85 x 78mm?

There is a javascript CR calculator on carfor.com

[YelloRolla]

http://users.erols.com/srweiss/calccr.htm

edit: here it is

[sillycar chick]

Bore 85mm, Stroke 78mm, Head Chamber 62cc, Piston dome 15.6 cc BUT I am convinced I stuffed this one up with the burette. 3T piston domes are conical, with diameters of 70mm at top and 76mm at bottom so by calculating volume of cone it should be 25cc (less the valve releif volumes - but these will only reduce final comp figure anyway). I used 25cc in my calcs as I know I cant have stuffed this one up.

So by my calcs I get 10.97:1 compression ratio with just a compressed head gasket at 1.3mm, and 9.8:1 with the head gasket and 1mm shim. When related with the compression pressure as I thought and as explained in the article links, it all seems to make sense.

[YelloRolla]

That all makes sense. Those approximate calculations for cylinder pressure vs CR set forth by Puma racing are clearly shown here to be accurate enough for the application and I consider myself more learned from the experience.

[Born-Enthusiast]

just a quick question how where do you measure the size of carbies from i also have a pair of twin side solex's but i have no idea what size they are?. how much difference does different sizes make