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Old 08-22-2008, 11:32 AM   #1
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Lightbulb noobish Question(high end power)

My 92' Miata 1.6: Greddy 15g-td40h, 8 psi, no brain tune/just FMU 12:1 disc, no IC, 8 degrees from base timing, aluminum 8lbs. Flywheel, A/C removed
has decent low-end and good midrange power. But top end, it's laggy in my opinion and is revving high around 4000 rpm at 60mph. It's not been properly tuned either. Is it normal for miata's to be revving that high? I never had/drove a stock miata so it seems so.
Where do I start? I can't seem to find anything helpful. What's everyone's take on this. I am retarded I know.
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Old 08-22-2008, 11:40 AM   #2
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Read stuff on this forum for days. Then you'll know.
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Old 08-22-2008, 11:46 AM   #3
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WTF is revving high???

The redline is a 7k. With your current setup you really can't tune it (get a MS). Your setup could be decent if you had any amount of actual control over the engine.
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Old 08-22-2008, 12:00 PM   #4
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If your rev's are too high at 60mph it means you are running too rich which can be fixed by pulling timing.

Edit: Check turbo fluid too... could be low.

Last edited by stevenh; 08-22-2008 at 12:18 PM.
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Old 08-22-2008, 12:09 PM   #5
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No thats not how gears work.

If you engine is spinning at 4k (assuming no clutch slippage) there is only one speed the rear wheels can rotate at. The ratio between the engine and rear wheels is fixed (in any one gear) and doesn't change.

If you have a stock clutch its very possible its slipping which would cause you to be at a higher rpm than you should be at a given speed. This should only be an issue under hard acceleration.

As I stated previously, get a MS. Then you can know what your engine is doing and change it as needed.
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Old 08-22-2008, 12:21 PM   #6
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My bad steven, you were right.

Take stevenH's advice. He knows his ****.
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Old 08-22-2008, 12:22 PM   #7
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Have you tried shifting to fifth gear? 4,000 rpm at 60mph in a 92 with stock wheels/tires and rear is 4th gear.
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Old 08-22-2008, 12:39 PM   #8
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If I remember right, at around 65-70 I'm around 3200 rpm in my 91.

When my stock clutch was slipping, it was WAY more likely to slip in 4th and 5th than lower gears. Simply trying to accelerate while in 5th would cause the clutch to slip if I wasn't very gingerly.

RPM aside, it makes sense because the clutch would "see" more torque in a higher gear than a lower one...same reason it is easier to start a car rolling in first than in third.
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Old 08-22-2008, 01:14 PM   #9
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Quote:
Originally Posted by miatamoxie View Post
RPM aside, it makes sense because the clutch would "see" more torque in a higher gear than a lower one...same reason it is easier to start a car rolling in first than in third.
The clutch is 1:1 with the engine. There's no gearbox between them that would cause any torque multiplication.
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Old 08-22-2008, 01:58 PM   #10
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no but to accelerate in 5th your using more throttle than to accelerate the same amount in 4th. Therefore you are using more 'area under the curve' and applying more torque. But for a specific throttle postition and RPM in any gear, your right, it would be the same
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Old 08-22-2008, 02:18 PM   #11
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Quote:
Originally Posted by patsmx5 View Post
The clutch is 1:1 with the engine. There's no gearbox between them that would cause any torque multiplication.
The clutch is 1:1 with the engine but the clutch is not 1:1 with the wheels. The torque the clutch sees is the force of the engine against the wheels. It's just like the clutch experiences no torque (except for moving the internals of the transmission) when the transmission is out of gear, even though the clutch is spinning at the same rate as the engine, but it does experience differing amounts of torque depending on what gear the transmission is in accelerate or decelerate the car.

This same issue has an effect in transmissions for DC electric motors, which have nearly flat torque curves (actually somewhat backwards from ICEs).
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Old 08-22-2008, 02:41 PM   #12
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thanx for all the feedback.
I have a 6 puck Spec Stage 3 carbon/metallic clutch. It's a good clutch, and not used that much 5k miles maybe? But It may be that my flywheel is too light(8lbs.), plus I got all aluminum wheels that weigh just 10lbs each.
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Old 08-22-2008, 02:49 PM   #13
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You have got to be shitting me. Thanks for the laugh.
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Old 08-22-2008, 02:52 PM   #14
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If the engine's changing rpm in high gear without an increase in speed then the clutch is slipping. Timing and fuel mixture have nothing to do engine rpm at a given speed in a given gear.
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Old 08-22-2008, 03:05 PM   #15
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Quote:
Originally Posted by Mobius View Post
Timing and fuel mixture have nothing to do engine rpm at a given speed in a given gear.

Are you kidding? The more timing the car has the more times the motor times the motor spins for each rpm. Duh. How do you think it got the name "timing"? You should also check that your lug nuts are tight? Maybe they are loose and the wheels aren't turning as much as the hubs.
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Old 08-22-2008, 03:14 PM   #16
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Quote:
Originally Posted by stevenh View Post
Are you kidding? The more timing the car has the more times the motor times the motor spins for each rpm. Duh.

There are 360 degrees in one rotation of the crankshaft. When you change the timing from 10 degrees BTDC to 14 degrees BTDC you actually move the point where the spark is fired from 350 degrees of rotation to 346 degrees of rotation of the crank. Imagine the end of the crank being a clock dial with 12 o'clock being 0 degrees. At 11.58 and 20 seconds the ignition process starts at 10 degrees BTDC. Change the timing to 14 degrees BTDC and the ignition process starts at 11.57 and 20 seconds.

When the crank reaches the timing point an electric current is fired off to the spark plug. The spark ignites the air and fuel mixture. This ignition starts at the spark plug and flame fronts travel up to the top of the chamber, out to the sides of the chamber and down to the top of the piston. When the flame front reaches the top of the piston it pushes it down, the crank rotates and there is movement in the drive train.

Whereas the engine revs can vary between 900 and 7,000 rpm; there are a constants for the speed at which the current passes to the spark plug, the ignition process and the burn rate of the fuel.

There is a point in the piston's downward movement where the flame front impacts on piston. For this example we assume that this point is when the crank has turned to be 15 degrees after TDC. On a 1.6 litre engine the location of the piston is now 1.865mm below its TDC position on its downward stroke. To check this use the formula {HT = (r + c) - (r cos (a)) - SQRT(c^2 - (r sin (a))^2)} where HT = position location ATDC, r = stroke/2, c = con rod length, a = radians of crank angle. For a 1.6 litre the stroke is 83.6mm and the con rod is 132.9 (+/-0.05) mm

At 900 rpm the crank rotates one degree in 0.18519 milliseconds.
The crank turns 360 degrees in one rpm. Therefore 360 degrees X 900 rpm = 324,000 degrees per minute = 5,400 degrees per second = 1/5400 of a second per degree = 0.00018519 seconds or 0.18519 milliseconds per degree.

By advancing the timing by 4 degrees you start the ignition process 0.741 (0.18519 X 4) milliseconds sooner at 900-rpm (0.111 milliseconds at 6,000 rpm).

At 900 rpm it takes 4.629 milliseconds for the crank to travel the 25 degrees from 10 degrees before TDC to 15 degrees after TDC. At 6,000 rpm this movement takes just 0.694 milliseconds. Remembering that the flame front has a constant burn rate then to have the flame front and piston converge when the piston is just after TDC requires the combustion process to start earlier in the cycle at higher rpm. Otherwise the piston would be lower down in its location before the flame front reached it.

Therefore as the rpm increases the ECU changes the timing so that, in a stock 1.6 litre, the advance increases through the rev range to a maximum of 36 BTDC at 5,500.

If you advance the timing at idle to 14 degrees BTDC then you increase the timing by 4 degrees across the rev range.

In our stock model we had the flame front meeting the piston when the crank is at 15 degrees ATDC (when the piston is 1.865mm on its downward stroke). Therefore by advancing the ignition process by 4 degrees the piston receives the flame front 4 degrees sooner. Ie at 11 degrees ATDC. The piston is located 1.00753 mm from TDC at 11 degrees ATDC.

When the combustion commences there is an expansion of the volume (remember the formula for volume of gasses at various temperatures from our first year physics classes). The rate of expansion is determined by the burn rate of the fuel. In our model this is constant.

Combustion actually commences while the piston is on its upward journey. The time it takes for the flame kernel at spark plug to expand to the top and sides of the chamber permits the piston to pass top dead centre before it receives the flame front. The expansion of the gasses during the period from ignition point (BTDC) to flame front reception (ATDC) increases the volume in the chamber and effectively creates higher compression.

Now we can see why it is that when the timing is advanced the higher position of the piston at 11 degrees ATDC (compared to 15 degrees ATDC) increases the effective compression and gives more power on the downward stroke. That equals more torque.
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Old 08-22-2008, 03:19 PM   #17
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Quote:
Originally Posted by bboybe16 View Post
thanx for all the feedback.
I have a 6 puck Spec Stage 3 carbon/metallic clutch. It's a good clutch, and not used that much 5k miles maybe? But It may be that my flywheel is too light(8lbs.), plus I got all aluminum wheels that weigh just 10lbs each.
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Old 08-22-2008, 03:25 PM   #18
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Lol, I was TOTALLY ******* around with my responses. Good explanation though

Quote:
Originally Posted by Braineack View Post
There are 360 degrees in one rotation of the crankshaft. When you change the timing from 10 degrees BTDC to 14 degrees BTDC you actually move the point where the spark is fired from 350 degrees of rotation to 346 degrees of rotation of the crank. Imagine the end of the crank being a clock dial with 12 o'clock being 0 degrees. At 11.58 and 20 seconds the ignition process starts at 10 degrees BTDC. Change the timing to 14 degrees BTDC and the ignition process starts at 11.57 and 20 seconds.

When the crank reaches the timing point an electric current is fired off to the spark plug. The spark ignites the air and fuel mixture. This ignition starts at the spark plug and flame fronts travel up to the top of the chamber, out to the sides of the chamber and down to the top of the piston. When the flame front reaches the top of the piston it pushes it down, the crank rotates and there is movement in the drive train.

Whereas the engine revs can vary between 900 and 7,000 rpm; there are a constants for the speed at which the current passes to the spark plug, the ignition process and the burn rate of the fuel.

There is a point in the piston's downward movement where the flame front impacts on piston. For this example we assume that this point is when the crank has turned to be 15 degrees after TDC. On a 1.6 litre engine the location of the piston is now 1.865mm below its TDC position on its downward stroke. To check this use the formula {HT = (r + c) - (r cos (a)) - SQRT(c^2 - (r sin (a))^2)} where HT = position location ATDC, r = stroke/2, c = con rod length, a = radians of crank angle. For a 1.6 litre the stroke is 83.6mm and the con rod is 132.9 (+/-0.05) mm

At 900 rpm the crank rotates one degree in 0.18519 milliseconds.
The crank turns 360 degrees in one rpm. Therefore 360 degrees X 900 rpm = 324,000 degrees per minute = 5,400 degrees per second = 1/5400 of a second per degree = 0.00018519 seconds or 0.18519 milliseconds per degree.

By advancing the timing by 4 degrees you start the ignition process 0.741 (0.18519 X 4) milliseconds sooner at 900-rpm (0.111 milliseconds at 6,000 rpm).

At 900 rpm it takes 4.629 milliseconds for the crank to travel the 25 degrees from 10 degrees before TDC to 15 degrees after TDC. At 6,000 rpm this movement takes just 0.694 milliseconds. Remembering that the flame front has a constant burn rate then to have the flame front and piston converge when the piston is just after TDC requires the combustion process to start earlier in the cycle at higher rpm. Otherwise the piston would be lower down in its location before the flame front reached it.

Therefore as the rpm increases the ECU changes the timing so that, in a stock 1.6 litre, the advance increases through the rev range to a maximum of 36 BTDC at 5,500.

If you advance the timing at idle to 14 degrees BTDC then you increase the timing by 4 degrees across the rev range.

In our stock model we had the flame front meeting the piston when the crank is at 15 degrees ATDC (when the piston is 1.865mm on its downward stroke). Therefore by advancing the ignition process by 4 degrees the piston receives the flame front 4 degrees sooner. Ie at 11 degrees ATDC. The piston is located 1.00753 mm from TDC at 11 degrees ATDC.

When the combustion commences there is an expansion of the volume (remember the formula for volume of gasses at various temperatures from our first year physics classes). The rate of expansion is determined by the burn rate of the fuel. In our model this is constant.

Combustion actually commences while the piston is on its upward journey. The time it takes for the flame kernel at spark plug to expand to the top and sides of the chamber permits the piston to pass top dead centre before it receives the flame front. The expansion of the gasses during the period from ignition point (BTDC) to flame front reception (ATDC) increases the volume in the chamber and effectively creates higher compression.

Now we can see why it is that when the timing is advanced the higher position of the piston at 11 degrees ATDC (compared to 15 degrees ATDC) increases the effective compression and gives more power on the downward stroke. That equals more torque.
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Old 08-22-2008, 07:59 PM   #19
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Now that's what I call help. It makes sense. I am not btdc. I actually moved a couple degrees too far. Thanks for the 'splanation, now to open the covers again and doing it right this time. Thank you all!
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Old 08-22-2008, 08:15 PM   #20
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Quote:
Originally Posted by miatamoxie View Post
The torque the clutch sees is the force of the engine against the wheels.
The motor produces a torque. The clutch couples the engine to the transmission.

Quote:
Originally Posted by miatamoxie View Post
It's just like the clutch experiences no torque (except for moving the internals of the transmission) when the transmission is out of gear, even though the clutch is spinning at the same rate as the engine, but it does experience differing amounts of torque depending on what gear the transmission is in accelerate or decelerate the car.
Any time you accelerate in the positive direction (we assume forward to be the positive direction) with the transmission in gear, torque is transmitted from the flywheel/pressure plate to the clutch disk. The amount of torque transfered is whatever the motor is developing irrelevant to what what gear you're accelerating in. Torque multiplication occurs in the transmission, which is AFTER the clutch. Maximizing torque at the wheels is the objective. That's why we have a transmission.
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