1st Dyno run complete - analysis help
04-11-2019, 02:51 PM
#31
Rennlist Member
I may have an idea of what's going on...
I remember a while back when the compression numbers were posted for this engine, and to my eye, they were very very low.
In fact now that I'm typing this, I recall that they were 40-50 psi low across the board - a HUGE discrepancy.
My recommendation at the time was to advance the cam timing (...close the intake sooner) so that the resultant numbers would then mimic a GT-cammed engine, around 180-185 psi.
I am pretty sure this is the root problem, and better yet, easily fixed.
As it is now, the current cam-timing's dynamic compression ratio is too low and is sacrificing power.
I remember a while back when the compression numbers were posted for this engine, and to my eye, they were very very low.
In fact now that I'm typing this, I recall that they were 40-50 psi low across the board - a HUGE discrepancy.
My recommendation at the time was to advance the cam timing (...close the intake sooner) so that the resultant numbers would then mimic a GT-cammed engine, around 180-185 psi.
I am pretty sure this is the root problem, and better yet, easily fixed.
As it is now, the current cam-timing's dynamic compression ratio is too low and is sacrificing power.
The following users liked this post:
Marti (05-03-2023)
04-11-2019, 03:44 PM
#32
Former Vendor
I may have an idea of what's going on...
I remember a while back when the compression numbers were posted for this engine, and to my eye, they were very very low.
In fact now that I'm typing this, I recall that they were 40-50 psi low across the board - a HUGE discrepancy.
My recommendation at the time was to advance the cam timing (...close the intake sooner) so that the resultant numbers would then mimic a GT-cammed engine, around 180-185 psi.
I am pretty sure this is the root problem, and better yet, easily fixed.
As it is now, the current cam-timing's dynamic compression ratio is too low and is sacrificing power.
I remember a while back when the compression numbers were posted for this engine, and to my eye, they were very very low.
In fact now that I'm typing this, I recall that they were 40-50 psi low across the board - a HUGE discrepancy.
My recommendation at the time was to advance the cam timing (...close the intake sooner) so that the resultant numbers would then mimic a GT-cammed engine, around 180-185 psi.
I am pretty sure this is the root problem, and better yet, easily fixed.
As it is now, the current cam-timing's dynamic compression ratio is too low and is sacrificing power.
The following users liked this post:
Marti (05-03-2023)
04-11-2019, 03:48 PM
#33
Rennlist Member
I may have an idea of what's going on...
I remember a while back when the compression numbers were posted for this engine, and to my eye, they were very very low.
In fact now that I'm typing this, I recall that they were 40-50 psi low across the board - a HUGE discrepancy.
My recommendation at the time was to advance the cam timing (...close the intake sooner) so that the resultant numbers would then mimic a GT-cammed engine, around 180-185 psi.
I am pretty sure this is the root problem, and better yet, easily fixed.
As it is now, the current cam-timing's dynamic compression ratio is too low and is sacrificing power.
I remember a while back when the compression numbers were posted for this engine, and to my eye, they were very very low.
In fact now that I'm typing this, I recall that they were 40-50 psi low across the board - a HUGE discrepancy.
My recommendation at the time was to advance the cam timing (...close the intake sooner) so that the resultant numbers would then mimic a GT-cammed engine, around 180-185 psi.
I am pretty sure this is the root problem, and better yet, easily fixed.
As it is now, the current cam-timing's dynamic compression ratio is too low and is sacrificing power.
S3 cams seemingly appear to test at around 225 psig and they have the same profile as the GT cams but they have a wider LSA - that does not make my test pressure range of 185 to 188 psig falsely low but it is noticeably different.
04-11-2019, 06:16 PM
#36
Race Director
When my 88S4 auto made 279 to 286whp (with intake spacers) it did have the S4 cams set slightly retarded by Marc Thomas and had at least 195psi per cylinder
04-11-2019, 07:03 PM
#37
Rennlist Member
Also, according to the workshop manuals, the S3 intake closes 8 degrees later than the GT intake, so assuming the cam-timing-specs are the same between an S3 and GT, the S3 cams will bleed off more compression than the GT cams. In other words, the S3 cams will have a lower dynamic compression ratio versus the GT cams, yet yours is considerably higher.
Again, I've never tested an S3 before so I don't know first-hand. With that said, your S3 at 225 psi versus a GT at 180-185, the numbers just don't seem to add up.
04-12-2019, 03:38 AM
#38
Rennlist Member
Fred, I've never tested an S3 engine, but 225 psi seems really high; are you sure this particular engine is timed correctly? The numbers imply the cams are really advanced.
Also, according to the workshop manuals, the S3 intake closes 8 degrees later than the GT intake, so assuming the cam-timing-specs are the same between an S3 and GT, the S3 cams will bleed off more compression than the GT cams. In other words, the S3 cams will have a lower dynamic compression ratio versus the GT cams, yet yours is considerably higher.
Again, I've never tested an S3 before so I don't know first-hand. With that said, your S3 at 225 psi versus a GT at 180-185, the numbers just don't seem to add up.
Also, according to the workshop manuals, the S3 intake closes 8 degrees later than the GT intake, so assuming the cam-timing-specs are the same between an S3 and GT, the S3 cams will bleed off more compression than the GT cams. In other words, the S3 cams will have a lower dynamic compression ratio versus the GT cams, yet yours is considerably higher.
Again, I've never tested an S3 before so I don't know first-hand. With that said, your S3 at 225 psi versus a GT at 180-185, the numbers just don't seem to add up.
It was not my motor - I think it was Dave C who mentioned this in Marti's thread. I rationalised that it is probably down to the LSA being wider thus less overlap and maybe this also advances the inlet cam but I have not studied the actual timing per se- it simply caught my attention.
My S4 motor has the GTS inlet cam fitted with the S4 exhaust- something Louie recommended for me and my motor [warm] generates compression pressures in the range 185 to 188 psig using my rig. I have seen folks claiming numbers like 210 psig that I thought were suspiciously high at the time.
Marti's numbers certainly caught my eye as to how low they seemed but they were very consistent so never gave it a thought and put it down to the specifics of Colin's design profile. Never thought to give him a prod to check the numbers out but...?
04-12-2019, 08:23 AM
#39
Rennlist Member
Fred, LSA is meaningless and has zero effect during the compression stroke, therefore does not affect the engine's dynamic compression ratio.
On the compression-stroke, all we care about is when the intake closes, and that is shown in the WS manuals.
On the compression-stroke, all we care about is when the intake closes, and that is shown in the WS manuals.
04-12-2019, 09:39 AM
#40
Rennlist Member
TDC and the inlet cam opens earlier than TDC and needless to say the more overlap there is the less compression you get at cranking rpms. Different story at higher rpms where the momentum of the gas streams induces more airflow.
Thus if you have the same motor and one has the GT cam fitted and the other has the S3 cam fitted- same profile but with greater LSA and do the same compression test procedure will they give the same numbers and if not why?
The only other variable is static compression ratio- I am not at all sure what pistons the S3 runs - I have a feeling they may be 10.4 to 1 items. If the S4 or GT generates 185 psig could an increase in static compression ratio generate 225 psig?- simple answer is "no" as Boyle's law tells us that it should increase to 193 psig. The GTS has 10.4 to 1 pistons and generates this kind of pressure.
Thus, if the S3 genuinely can make 225 psig how does it do it? Cam timing surely changes things- advance the timing and the compression test numbers will increase but how much does the timing have to change to record significant differences- 10 degrees?
Last edited by FredR; 04-12-2019 at 03:33 PM.
04-12-2019, 12:45 PM
#41
Pro
Thread Starter
I thought previously that I had cocked up the compression test as I did not hold the throttle open during the test and I could not see the gauge to see whether it was still increasing in pressure on the gauge. So I just went out this afternoon and took a couple of random cylinders and recorded 179 and 171 psig - confirming that I had cocked up the first test.
So now I need to do the rest of the engine and post back. Both the plugs I pulled looked a good colour which was comforting to see.
Sorry for the mis-direction on the cocked up CR test. I might be struggling to post back for a week due to the holidays.
So now I need to do the rest of the engine and post back. Both the plugs I pulled looked a good colour which was comforting to see.
Sorry for the mis-direction on the cocked up CR test. I might be struggling to post back for a week due to the holidays.
04-12-2019, 12:50 PM
#42
Pro
Thread Starter
So long as the Compression numbers are balanced left to right that is most important.
Out of curiosity what is the number that you are seeing for compression?
Has the ignition system been refreshed?
I wouldn't worry about it pulling right up to redline these are large cams and the only restriction left is in the rest of the system.
With the Hall (cam) sensor. Did you replace the plug, or buy a new one?
If you replaced the plug you can reverse 2 of the wires which throw the computer into a fault mode without really showing it......
I agree with recommending doing a couple street pulls from 3k rpm to redline to get a better idea as to how the car is performing.
Out of curiosity what is the number that you are seeing for compression?
Has the ignition system been refreshed?
I wouldn't worry about it pulling right up to redline these are large cams and the only restriction left is in the rest of the system.
With the Hall (cam) sensor. Did you replace the plug, or buy a new one?
If you replaced the plug you can reverse 2 of the wires which throw the computer into a fault mode without really showing it......
I agree with recommending doing a couple street pulls from 3k rpm to redline to get a better idea as to how the car is performing.
04-12-2019, 01:30 PM
#43
Rennlist Member
I thought previously that I had cocked up the compression test as I did not hold the throttle open during the test and I could not see the gauge to see whether it was still increasing in pressure on the gauge. So I just went out this afternoon and took a couple of random cylinders and recorded 179 and 171 psig - confirming that I had cocked up the first test.
So now I need to do the rest of the engine and post back. Both the plugs I pulled looked a good colour which was comforting to see.
Sorry for the mis-direction on the cocked up CR test. I might be struggling to post back for a week due to the holidays.
So now I need to do the rest of the engine and post back. Both the plugs I pulled looked a good colour which was comforting to see.
Sorry for the mis-direction on the cocked up CR test. I might be struggling to post back for a week due to the holidays.
The only people who do not make mistakes are the ones that do nothing except bullsh*t all the time- Plenty of those in Westminster!
I always do my compression tests with my motor warm, the throttle fully open, fuel pump/ignition disengaged and about 5 cranks of the motor. It always helps if there is someone monitoring the gauge to ensure it has maxxed out. I once tested the same cylinder with different numbers of cranks and found that by 5 it was maxxed out.
04-15-2019, 10:15 AM
#44
Rennlist Member
We were talking about this over the weekend and somone noticed the Correction Factor is zero - are you sure it is orrect and supposed to be zero?
04-15-2019, 01:20 PM
#45
Former Vendor
Flexplate preload and crankshaft endplay been checked?