Compression ratio for modified Mi16

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GRAHAM WALLIS

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I have found that the Mi16 engine in my 309 is just not satisfactory. It is a standard XU9J4 with 10.4 pistons and 45 mm throttle bodies, great top end but what you gain there you more than lose on low speed corners. It won't even spin the wheels at a start control until it gets up in the revs. Would it be better with a standard inlet manifold? I remapped the ignition timing last week and it looked to have a positive effect on the dyno but on the road was worse if anything.
I have a new engine ready to assemble, Piper Ultimate Road cams with 285 duration and 430 thou lift, what compression should I aim for? I have a set of 11.8 to 1 Wossner pistons which can be reduced in height to adjust the compression. Even with the increased compression there is a chance it will be worse than what I have now?
I will probably drop in an XU10J2 in for the time being if the inlet manifold will fit as I have some long distance events coming up soon. Driving the donor car, 405 Sri, makes the 309 feel slow in many situations!
 
GRAHAM WALLIS said:
standard XU9J4 with 10.4 pistons and 45 mm throttle bodies, great top end but ...won't even spin the wheels at a start control until it gets up in the revs.
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It sounds that the venturis in the throttle bodies are too big, perhaps you should start there before getting into deeper surgery

GRAHAM WALLIS said:
Piper Ultimate Road cams with 285 duration and 430 thou lift... 11.8 to 1 Wossner pistons which can be reduced in height to adjust the compression.
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These numbers are right on the edge as far as a match is concerned. The compression is a tad too high for the cam duration, but I doubt you will have a problem with good quality fuel. I would start there and if you do get pinging, you can use a thicker head gasket. More reversible and less risky than machining forged pistons.
 
Thanks Thanos, I was thinking of putting the standard inlet manifold back on but borrowing a set of smaller throttle bodies may be more productive.
 
GRAHAM WALLIS said:
Thanks Thanos, I was thinking of putting the standard inlet manifold back on but borrowing a set of smaller throttle bodies may be more productive.
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Graham

You should also be able to change the venturis in your current throttle bodies. Yours are probably 38mm, you can try 34mm and 36mm.
 
ThanosK said:
You should also be able to change the venturis in your current throttle bodies. Yours are probably 38mm, you can try 34mm and 36mm.
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Fuel injection throttle bodies don't have venturis. There is no need for them.

Graham, changing the throttle body size will do buggar all on that standard motor. You need to change the runner length or diameter to get any kind of response. You are better off refitting the standard inlet or a similar inlet such as a GTI6 or S16. You might loose 5kW up top but gain 10-20kW in the low-mid range and driveability.
 
Yes, just a straight tube.
I can't get any more length, the distance from trumpets to head is about 15mm shorter than the runner length in a standard manifold. How do you think torque will go with the different cam and increased compression?
The power curve is almost vertical at just above 5000.
 
GRAHAM WALLIS said:
Yes, just a straight tube.
I can't get any more length, the distance from trumpets to head is about 15mm shorter than the runner length in a standard manifold. How do you think torque will go with the different cam and increased compression?
The power curve is almost vertical at just above 5000.
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I was looking at throwing this partial gti6 manifold would it be any good to you Graham to make a custom manifold?
Runners approx 100mm long and needs a flange repaired
 

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Got the pulleys, will go on the new motor, which brings me back to the original question, compression ratio for that?
Maybe bring the manifold down on Saturday Lauree, we could compare with the stock XU9J4.
By the way John will be at my place at around 0900.
 
These are a lot shorter than the stock Mi16 manifold, would be the high rev tract I'm guessing.

djvu205 said:
I was looking at throwing this partial gti6 manifold would it be any good to you Graham to make a custom manifold?
Runners approx 100mm long and needs a flange repaired
Click to expand...
 
And?, fit a 4.7 or 4.9 and use 1 to 4 then fit a fifth gear for road use. We did this on rob Riggs 205.

Gearing will make the most difference.

Why not fit the pulleys to this motor. Move the power band to where you need it to work. Not that much work for the gains it makes.

Inlet manifold won’t make that match difference. Leave it alone for now.

Comp should be fine in the motor you have.

Think you need to fix what you have rather than chasing a new motor and forever chasing set up.
 
Your intake to the butterfly is longer than my race car set up and it only a makes power to 7000
 
I have a gearbox with this setup, albeit with 4.43 that you did for me. On fast roads you do need to use 5th though, so there is an annoying gap. Present box is 205 1.9 ratios with 4.43 FD.
This present motor is a stop gap after the original track day motor cracked a liner, since then have noticed that the ring lands are damaged, both due to too much ignition advance and hence pinging.
The new engine is ready to go after the compression is decided on but won't be fitted until the present one starts to fade so what you say about the pulleys is a good idea. At the moment other work is required before the rallies coming up but my be able to fit in some cam timing trials late May. We managed to get some gains, particularly in part throttle yesterday, with another tuning session.

Adrian Wuillemi;1564555]And?, fit a 4.7 or 4.9 and use 1 to 4 then fit a fifth gear for road use. We did this on rob Riggs 205.

Gearing will make the most difference.

Why not fit the pulleys to this motor. Move the power band to where you need it to work. Not that much work for the gains it makes.

Inlet manifold won’t make that match difference. Leave it alone for now.

Comp should be fine in the motor you have.

Think you need to fix what you have rather than chasing a new motor and forever chasing set up.[/QUOTE]
 
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GRAHAM WALLIS said:
This present motor is a stop gap after the original track day motor cracked a liner, since then have noticed that the ring lands are damaged, both due to too much ignition advance and hence pinging.
Click to expand...

Sounds like you've been trying to run 8V timing numbers on a 16V engine.
 
If you're looking for torque, the last thing you want is Piper's matching exhaust cam. You need to use an exhaust cam which has a duration 10 deg smaller at 0.050" lift than the inlet. Opening the exhaust valve too early just throws away torque. The exhaust ports don't flow any better after 0.370" lift, so having a monster exhaust cam is pointless.
 
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