Dyno runs old and new

[Dr.Jeff]

No Dr. Jeff. It relates to all engines. If you want a copy PM me with your e-mail address and I'll send you a copy and you can check it out. GregS it will work just fine for you engine. I sent you a copy.

Charlie

Sorry Charlie (not Charlie tuna in this case), I wasn't asking about your spreadsheet. I was asking about the specific comment made by @GregS, "One promising method is with a graph plotting torque at the wheels to road speed, and to shift gear where the torque lines cross, trouble is with my engine the lines don't cross." Meaning, in his particular case the ideal torque curve does not match the ideal gearing for his vehicle. It was a bit of a rhetorical question, inferring that if the gearing was ideal for the particular engine's torque curve, then the two lines would cross at each ideal shift point.

 

[Old Fiat Driver]

Charlie the tuna here. Dr. Jeff, I don't really understand how the torque curves don't cross, not sure what that means, The spread sheet shows the torque in all gears, RPM and vehicle speed at the pavement. There is always an optimum point to shift and the spread sheet shows that. More gears, or a change in ratios, may, or may not, be ideal but that can be shown also. I sent the spread sheet to Greg, we'll see what he finds. I am a bit confused with the statement also but I suspect Greg will straighten me out as to the meaning.

Charlie

 

[GregS]

I have crunched the numbers, as I suspected it looks like I will have to run to redline in all gears. I have been using 7500 rpm as a fairly safe redline, that's 600 rpm past maximum power, although I could and have run above that in the heat of the moment. You can see from the numbers that in most gears it never gets higher wheel torque in the next gear up than in the lower gear, that's what I meant about graph lines not crossing if it was plotted (see example graph of what ideally it should look like). Apparently it's not uncommon for this to happen, it depends on the torque curve, gearing etc.
My car is close but not identical to when this dyno run was done, but it shouldn't be too far off. Also, the actual torque numbers in the spread sheet are about 13% higher than true because I just used the numbers off the dyno sheet which are in Nm at the wheels, so the high of 125 should actually be about 109 lbsft at the flywheel, but that shouldn't alter the shift points. As the readout cuts off at 7250 rpm I just estimated the torque for the last 500 rpm, but it should be in the ball park.

Just looking at the numbers again. 1st to 2nd ok at 7500. Third to 4th change and 4th to 5th probably ok at 7500 also.

My Torque at the wheel, very top line.

What it should look like when torque in next higher gear passes previous gear

By the way, my gearbox has the lower 4 gears from a Fiat Regata 100S (1600 twin cam), but retains the std X1/9 5th gear. this gives a big jump between 1 & 2 and between 4 & 5, but close ratios between 2, 3 and 4 which are the most commonly used gears on most race tracks I go to.

 

[Old Fiat Driver]

OK I see the problem now. It looks like you could make some improvements with gear ratio changes but that' a bit of a job. Don't forget that this chart doesn't take into account frictional losses which accounts for slightly higher numbers but they should still be relatively equal. That first gear is a low one. What kind of racing to you do, looks like fun?

Charlie

 

[GregS]

OK I see the problem now. It looks like you could make some improvements with gear ratio changes but that' a bit of a job. Don't forget that this chart doesn't take into account frictional losses which accounts for slightly higher numbers but they should still be relatively equal. That first gear is a low one. What kind of racing to you do, looks like fun?

Charlie

Yes, that first gear is a bit too low. I do occasional hillclimbs, which never use 5th gear, but mostly circuit sprints on various race tracks, where I get to use 5th on the longest straight. The gap from 4th to 5th is not ideal, but I wanted the tall 5th for 1 or 2 tracks with a long down hill fast straight.
Just out of interest I put standard X1/9 5 sp gear ratios into the spreadsheet and it didn't make much difference, it still doesn't get to the torque figures in the next higher gear except for 5th. And in all the lower gears the torque figures at shift points are lower than in the close ratio box. What it really needs is a calculator that gives acceleration figures for 0 - 200kph.

 

[Dr.Jeff]

Just out of interest I put standard X1/9 5 sp gear ratios into the spreadsheet and it didn't make much difference

What about playing with the final drive ratio? It would be interesting to see what that does to things in the spreadsheet.

 

[Old Fiat Driver]

Why is the engine being limited to 7750 RPM. I suspect it's a stroked motor but the breathing ability of the engine seems to be higher than the RPM which it's being run to. If the engine could turn in the 9000 RPM range things might work out. Is it because the crank, rods, or pistons would be suspect at that RPM?? What kind of head and cam are you using?

 

[GregS]

I see no point in revving past 7750 as the torque is mostly already matched in the next higher gear by then, except 1st to 2nd.
Still 1500cc, all major internal engine parts are still Fiat originals, just balanced, and a slightly lighter flywheel. Except the cam shaft, that is a Piper BP300, which is designed for a good spread of torque and power for rally/race, it's not a full race cam with all the power high in the rev range. The head is shaved to get about 10.7:1 CR, 39.5mm Fiat Tipo intake valves.
I suspect that the reason the spreadsheet doesn't show a bigger overlap of torque figures is because of the relatively constant torque between 5000 and 7000rpm, and the close ratio gears between 2,3 & 4.
I did try the spreadsheet with a closer ratio 5th, as expected that would give better acceleration after changing gear, but it would lose out in top speed as by then the engine is well past peak torque compared to my current 5th gear.