Dip in dynograph.....Input?
10-13-2010, 01:43 AM
#1
Staging Lane
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Dip in dynograph.....Input? Im hoping someone can maybe give me some insight here. I got my setup tuned/dynoed. My buddy that does all the tuning in town tuned it and he is very good/experienced so I know the tune is out of the question. The car made good power, runs good, but for the size of the cam it seems to just not pull high and has a pretty aggressive dip at around 6500rpm. Heres some info.
L92 stock bottom end
AFR 225s unmilled with their upgraded dual springs
242/248, lift is in the low 600s on a 112.
Kooks 1 7/8
FAST 102
42lb injectors
NW 102
Dual 3" exhaust
Speed density tuned
489/434
Heres a pic of the graph
L92 stock bottom end
AFR 225s unmilled with their upgraded dual springs
242/248, lift is in the low 600s on a 112.
Kooks 1 7/8
FAST 102
42lb injectors
NW 102
Dual 3" exhaust
Speed density tuned
489/434
Heres a pic of the graph
10-13-2010, 01:01 PM
#5
That dip is very common with smaller engines (under 400 CID) and 1.875 tube headers (which normally have shorter primary lengths).
You may be able to tune some of it out playing with timing and A/F ratio's (note your A/F curve in the same area of the dip) but honestly its almost impossible to remove.....you can only minimize it.
Cam selection will also slightly effect it as well....
Keep your eye out for other 346 - 383 CID engines with 1.875 headers....you will see a very similar trend.
Hope this helps
-Tony
You may be able to tune some of it out playing with timing and A/F ratio's (note your A/F curve in the same area of the dip) but honestly its almost impossible to remove.....you can only minimize it.
Cam selection will also slightly effect it as well....
Keep your eye out for other 346 - 383 CID engines with 1.875 headers....you will see a very similar trend.
Hope this helps
-Tony
10-13-2010, 01:24 PM
#6
Staging Lane
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Thanks tony. Ill try to get the full cam specs. The cam was actually just something off the shelf really. Ive had smaller cube motors with smaller cams that seemed to pull further than this thats why it concerns me.
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10-13-2010, 07:41 PM
#8
Staging Lane
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That dip is very common with smaller engines (under 400 CID) and 1.875 tube headers (which normally have shorter primary lengths).
You may be able to tune some of it out playing with timing and A/F ratio's (note your A/F curve in the same area of the dip) but honestly its almost impossible to remove.....you can only minimize it.
Cam selection will also slightly effect it as well....
Keep your eye out for other 346 - 383 CID engines with 1.875 headers....you will see a very similar trend.
Hope this helps
-Tony
You may be able to tune some of it out playing with timing and A/F ratio's (note your A/F curve in the same area of the dip) but honestly its almost impossible to remove.....you can only minimize it.
Cam selection will also slightly effect it as well....
Keep your eye out for other 346 - 383 CID engines with 1.875 headers....you will see a very similar trend.
Hope this helps
-Tony
10-13-2010, 08:45 PM
#9
Take the time to look at other smaller engine cars with 1.875 headers on them.
You will see right away what Im discussing here.
The bigger motors are flat in that area (usually) and then ramp up to torque peak. They aren't as sensitive to that tube size and length.
A 1.75 tube has a dip in the 3200 range and is exploding at 4k (like an inverted sine wave when you see both headers on the same engine), but....the 1.875 will usually hang the torque and power curve out further netting more power which is why you go with the bigger tube in the first place.
Just like cams, big and small headers will always have strengths and trade-offs.
Hi Performance engine building is ALL about managing the trade-offs when you really break it down. Designing a package or combination to benefit from as many of the "upsides" as you can while minimizing the trade-offs and doing so in such a way the engine in question is optimized for you particular application and driving style....making these decisions much more personal.
-Tony
10-14-2010, 12:11 AM
#10
whats the compresstion of the engine with the unmilled AFR?
AFR 225 comes 72cc right Tony?
What transmission and rearend do you have??
Good numbers IMO Congrats.
Looks like smoothing 5 when I saw the graph I thought it would be 0 or the car is a lot of KR.
AFR 225 comes 72cc right Tony?
What transmission and rearend do you have??
Good numbers IMO Congrats.
Looks like smoothing 5 when I saw the graph I thought it would be 0 or the car is a lot of KR.
10-14-2010, 12:32 AM
#11
Staging Lane
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The nature of that header tube length and diameter (dealing with wave tuning, scavenging, etc.) always creates that dip you see around 4K.
Take the time to look at other smaller engine cars with 1.875 headers on them.
You will see right away what Im discussing here.
The bigger motors are flat in that area (usually) and then ramp up to torque peak. They aren't as sensitive to that tube size and length.
A 1.75 tube has a dip in the 3200 range and is exploding at 4k (like an inverted sine wave when you see both headers on the same engine), but....the 1.875 will usually hang the torque and power curve out further netting more power which is why you go with the bigger tube in the first place.
Just like cams, big and small headers will always have strengths and trade-offs.
Hi Performance engine building is ALL about managing the trade-offs when you really break it down. Designing a package or combination to benefit from as many of the "upsides" as you can while minimizing the trade-offs and doing so in such a way the engine in question is optimized for you particular application and driving style....making these decisions much more personal.
-Tony
Take the time to look at other smaller engine cars with 1.875 headers on them.
You will see right away what Im discussing here.
The bigger motors are flat in that area (usually) and then ramp up to torque peak. They aren't as sensitive to that tube size and length.
A 1.75 tube has a dip in the 3200 range and is exploding at 4k (like an inverted sine wave when you see both headers on the same engine), but....the 1.875 will usually hang the torque and power curve out further netting more power which is why you go with the bigger tube in the first place.
Just like cams, big and small headers will always have strengths and trade-offs.
Hi Performance engine building is ALL about managing the trade-offs when you really break it down. Designing a package or combination to benefit from as many of the "upsides" as you can while minimizing the trade-offs and doing so in such a way the engine in question is optimized for you particular application and driving style....making these decisions much more personal.
-Tony
running through an m6 and 10 bolt currently.
10-14-2010, 12:55 AM
#12
I see what ur saying. So would a 2" header have a dip say around 4800rpm or so or will it be too big for that motor and have a deinite tq decrease across the board? From the looks by the graph does it look like it could benefit from a better spring? One other thing is Im using a 7.4 pushrod which barely preloads the lifter. Theres no excessive valvetrain noise either. I thought having less preload would allow it to pull a little harder up top? I just dont fully understand why with that size cam, heads, etc its dipping that hard that early.
running through an m6 and 10 bolt currently.
running through an m6 and 10 bolt currently.
You realize you have very low compression right?
A lighter valve may have helped....I recommend it to everyone that wants to buzz them pretty good.
Are the heads new....do you know what shape the springs are in? You could shim them higher for more pressure....also what lifters are you using....cheap lifters could be bleeding down.
Lots of situations could cause what your looking at.
What rockers as well?
I assume the FAST isn't ported?
-Tony
10-14-2010, 04:00 AM
#13
Staging Lane
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2" header is out of the question....how thick are your head gaskets?
You realize you have very low compression right?
A lighter valve may have helped....I recommend it to everyone that wants to buzz them pretty good.
Are the heads new....do you know what shape the springs are in? You could shim them higher for more pressure....also what lifters are you using....cheap lifters could be bleeding down.
Lots of situations could cause what your looking at.
What rockers as well?
I assume the FAST isn't ported?
-Tony
You realize you have very low compression right?
A lighter valve may have helped....I recommend it to everyone that wants to buzz them pretty good.
Are the heads new....do you know what shape the springs are in? You could shim them higher for more pressure....also what lifters are you using....cheap lifters could be bleeding down.
Lots of situations could cause what your looking at.
What rockers as well?
I assume the FAST isn't ported?
-Tony
