[quote="MrBo"]FWIW, I bought some Ultra 94 on Monday and measured it today.
I measured 6% ethanol. I did it twice, both times 6%.
I have no idea if that would have a noticeable effect on lowering the octane though.[/quote]
[quote="wil8115"]Ethanol raises the octane not lower it. It burns cleaner then the Eth free gas.[/quote]
Ethanol raises the knock tolerance however doesn't it also reduce the BTU rating of the fuel? That is to say, in engines that don't need the detonation resistance of a given blend, more horsepower can be made with pure gasoline than with a gasoline/ethanol blend.
Compression Ratio Limits and Iron Heads
Moderator: Team
Re: Compression Ratio Limits and Iron Heads
[quote="F-BIRD'88"]Is this the same engine that you previously posted about having a off idle stumble
carb tuning and drivability problems with?
if the off idle transition is wrong and the cylinder go momentary lean
(does not matter if it is below the converter stall speed or not) as the engine is accelerating.
This can cause brief knocking. This knocking , once started does not want to stop.
as it rapidily heats the combustion chamber.
(especial y if the vacuum advance is too aggressive, often all made worse by using full manifold vacuum . At a 10:1 compression ratio, you were ok and below the knock threshold.
Now at 10.37:1 you are now a hair above the knock threshold.
The trouble with a little it too much compression ratio is a little nit too much is a lot too much.
(cause it knocks). I bet your distibutor mechanical and vacuum advance curves rates need more work. you have not got it right yet.
and the carb idle off idle circuit probabily is not correct yet either.
@10:1 you ahe a window of tolerance and did not hear any knocking, but now you have a higher cr so the tolerance of incorrect carb and distibutor tune up is gone.
Start with adjusting the vacuum advance rate and max amount.
Limit the max to 12deg max at high vacuum, not 20+.
Then try a different mechanical advance curve with more initial at idle, but less centrifical but the same total max 36deg.
more initial base timing (20-22-24 at idle) and a slower advance curve is usually better.
Then use ported vacuum as a vac source.
Fix the PCV valve ( it effects the carb idle and off idle circuit afr)
The wrong PCV valve creates off idle lean spots, drivability issues and knocking.
and all kinds of carb tuning headaches.[/quote]
Yes it's the same engine I had posted about earlier however my tuning has solved some of the driveability issues.
The off idle stumble is all but gone and that's due largely to a switch to full manifold vacuum.
The part throttle stumble that I was experiencing with the accompanying lean spike seen on the AFR gauge was cured with a 10.5 power valve. Now mid throttle acceleration, with manifold vacuum dipping to 10" or less is smooth and knock free. Insanely rich at high 11:1 but knock free. I can tune that richness as I progress.
Those issues appear to have been resolved.
As for timing, yesterday I reduced my timing by 4 degrees across the board.
Timing before:
Initial: 12
Vacuum advance: 23 (full manifold vacuum adjusted to be all in 1" below idle vacuum which is 11" at 800 rpm).
Total idle advance: 35.
Mechanical: 20 (all in by 2900 rpm).
Max WOT timing: 32.
Maximum available cruise timing: 55 (at 2900 rpm and part throttle 11"+ vacuum)
Timing now: reduced 4 degrees across the board.
Initial: 8
Vacuum advance: 23 (unchanged)
Total idle advance: 31
Mechanical: 20 (unchanged)
Max WOT timing: 28
Maximum available cruise timing: 51
I agree that the vacuum advance is a little aggressive at 23 degrees however, connected or disconnected, it hasn't made an impact with the detonation. I do intend to reduce it to more sensible 15 degrees. I may even switch back to ported vacuum however the engine really likes the 25+ degrees of idle advance. Smoother, higher vacuum and cooler running.
The old engine, with 10:1 SCR, with similar heads and cam, liked 16 degrees initial, 18 mechanical and ported vacuum advance that was in the 20 degree range. No knock (or at least very little and the engine gave a lot of service miles before I finally wore it out) and great mpg.
I strongly believe that I may be able to address the initial hit knocking with some accelerator pump tuning. It would probably also address the tip-in stumble off idle and mid-throttle that the full vacuum advance has had an impact on.
carb tuning and drivability problems with?
if the off idle transition is wrong and the cylinder go momentary lean
(does not matter if it is below the converter stall speed or not) as the engine is accelerating.
This can cause brief knocking. This knocking , once started does not want to stop.
as it rapidily heats the combustion chamber.
(especial y if the vacuum advance is too aggressive, often all made worse by using full manifold vacuum . At a 10:1 compression ratio, you were ok and below the knock threshold.
Now at 10.37:1 you are now a hair above the knock threshold.
The trouble with a little it too much compression ratio is a little nit too much is a lot too much.
(cause it knocks). I bet your distibutor mechanical and vacuum advance curves rates need more work. you have not got it right yet.
and the carb idle off idle circuit probabily is not correct yet either.
@10:1 you ahe a window of tolerance and did not hear any knocking, but now you have a higher cr so the tolerance of incorrect carb and distibutor tune up is gone.
Start with adjusting the vacuum advance rate and max amount.
Limit the max to 12deg max at high vacuum, not 20+.
Then try a different mechanical advance curve with more initial at idle, but less centrifical but the same total max 36deg.
more initial base timing (20-22-24 at idle) and a slower advance curve is usually better.
Then use ported vacuum as a vac source.
Fix the PCV valve ( it effects the carb idle and off idle circuit afr)
The wrong PCV valve creates off idle lean spots, drivability issues and knocking.
and all kinds of carb tuning headaches.[/quote]
Yes it's the same engine I had posted about earlier however my tuning has solved some of the driveability issues.
The off idle stumble is all but gone and that's due largely to a switch to full manifold vacuum.
The part throttle stumble that I was experiencing with the accompanying lean spike seen on the AFR gauge was cured with a 10.5 power valve. Now mid throttle acceleration, with manifold vacuum dipping to 10" or less is smooth and knock free. Insanely rich at high 11:1 but knock free. I can tune that richness as I progress.
Those issues appear to have been resolved.
As for timing, yesterday I reduced my timing by 4 degrees across the board.
Timing before:
Initial: 12
Vacuum advance: 23 (full manifold vacuum adjusted to be all in 1" below idle vacuum which is 11" at 800 rpm).
Total idle advance: 35.
Mechanical: 20 (all in by 2900 rpm).
Max WOT timing: 32.
Maximum available cruise timing: 55 (at 2900 rpm and part throttle 11"+ vacuum)
Timing now: reduced 4 degrees across the board.
Initial: 8
Vacuum advance: 23 (unchanged)
Total idle advance: 31
Mechanical: 20 (unchanged)
Max WOT timing: 28
Maximum available cruise timing: 51
I agree that the vacuum advance is a little aggressive at 23 degrees however, connected or disconnected, it hasn't made an impact with the detonation. I do intend to reduce it to more sensible 15 degrees. I may even switch back to ported vacuum however the engine really likes the 25+ degrees of idle advance. Smoother, higher vacuum and cooler running.
The old engine, with 10:1 SCR, with similar heads and cam, liked 16 degrees initial, 18 mechanical and ported vacuum advance that was in the 20 degree range. No knock (or at least very little and the engine gave a lot of service miles before I finally wore it out) and great mpg.
I strongly believe that I may be able to address the initial hit knocking with some accelerator pump tuning. It would probably also address the tip-in stumble off idle and mid-throttle that the full vacuum advance has had an impact on.
Re: Compression Ratio Limits and Iron Heads
55deg is way too much at cruise. yes the vacuum advance is WAY too agressive.
All engines like a lot of timing at idle. But you are getting carried away with the vacuum advance.
recurve it with more base initial timing. What cam is in this motor.?
All engines like a lot of timing at idle. But you are getting carried away with the vacuum advance.
recurve it with more base initial timing. What cam is in this motor.?
Re: Compression Ratio Limits and Iron Heads
I meant that you can’t necessarily assume the 94 fuel has 10% ethanol in it. The amount of ethanol seems to vary.wil8115 wrote:Ethanol raises the octane not lower it. It burns cleaner then the Eth free gas.MrBo wrote:FWIW, I bought some Ultra 94 on Monday and measured it today.
I measured 6% ethanol. I did it twice, both times 6%.
I have no idea if that would have a noticeable effect on lowering the octane though.
.
"I promise you Sheriff, I won't throw one more rock... Didn't say nothin' 'bout no brick!" --Ernest T Bass
Re: Compression Ratio Limits and Iron Heads
I agree on the 55 degrees. It's been reduced to 51 and I feel that it could probably be dialed back a little more. I going to go for 15 vacuum degrees however I'll keep the WOT throttle total low (via a reduced initial setting of 8 degrees) until the knock is gone.
Cam specs are as follows:
Comp custom hydraulic roller.
Intake lobe XFI profile 3015
Exhaust lobe XFI profile 3036
Adv dur. 274/282
.050 dur. 224/230
Lift in/ex. .574/.569
LSA 110
ICL 106
Degreed and installed as per spec.
Cam specs are as follows:
Comp custom hydraulic roller.
Intake lobe XFI profile 3015
Exhaust lobe XFI profile 3036
Adv dur. 274/282
.050 dur. 224/230
Lift in/ex. .574/.569
LSA 110
ICL 106
Degreed and installed as per spec.
Last edited by skinny z on Thu Sep 12, 2013 11:00 am, edited 1 time in total.
Re: Compression Ratio Limits and Iron Heads
[quote="MrBo"][quote="wil8115"][quote="MrBo"]FWIW, I bought some Ultra 94 on Monday and measured it today.
I measured 6% ethanol. I did it twice, both times 6%.
I have no idea if that would have a noticeable effect on lowering the octane though.[/quote]
Ethanol raises the octane not lower it. It burns cleaner then the Eth free gas.[/quote]
I meant that you can’t necessarily assume the 94 fuel has 10% ethanol in it. The amount of ethanol seems to vary.
.
[img]http://i956.photobucket.com/albums/ae45 ... c1a3e0.jpg[/img][/quote]
Interesting.
I measured 6% ethanol. I did it twice, both times 6%.
I have no idea if that would have a noticeable effect on lowering the octane though.[/quote]
Ethanol raises the octane not lower it. It burns cleaner then the Eth free gas.[/quote]
I meant that you can’t necessarily assume the 94 fuel has 10% ethanol in it. The amount of ethanol seems to vary.
.
[img]http://i956.photobucket.com/albums/ae45 ... c1a3e0.jpg[/img][/quote]
Interesting.
Re: Compression Ratio Limits and Iron Heads
For reference: post the carb model number and how you have it set up. jets,PV, IAB's HSAB's IFR etc.
Did you try going to a different Petro-Can station to get your Ultra 94?
How did you raise the compression ratio from 10:1 to 10.37:1 just by changing from one vortec head to another vortec head?
is it possible the real compression ratio is really a lot more than 10.37:1?
Sharp thin machined edges on the combustion chamber that were not dressed with a file after machining.
Cam installed wrong. Vacuum leak, oil in manifold. intake gasket leaks.
So what is the result of the 4degree timing reduction? Is it still knocking?
Did you try going to a different Petro-Can station to get your Ultra 94?
How did you raise the compression ratio from 10:1 to 10.37:1 just by changing from one vortec head to another vortec head?
is it possible the real compression ratio is really a lot more than 10.37:1?
Sharp thin machined edges on the combustion chamber that were not dressed with a file after machining.
Cam installed wrong. Vacuum leak, oil in manifold. intake gasket leaks.
So what is the result of the 4degree timing reduction? Is it still knocking?
Re: Compression Ratio Limits and Iron Heads
I suggest you change the timing curve to something real close to this.
20 to 26deg initial base
34 to 36 total centrifugal maxing out at 3200-3500 rpm.
That means a shorter 10-12deg mech advance curve. limit the travel of the mechanical advance system.
Be sure the advance curve is smooth and progressive without jumps in timing.
And slow down the progression rate so it peaks out a little latter (3200-3500rpm)
If it does not have smooth advance progression, fix that. Some of the aftermarket distributor's advance systems really suck for quality.
a max of 46deg total combined mechanical + vacuum advance advance at max vacuum cruise ( light load)
That means a 10 to max of 12deg maximum of vacuum advance MAX. The rate in and out based on throttle load
has to be found by drive testing. Limit the travel of the vacuum advance system to 10-12deg max
Then verify and correct the distributor cap/rotor phasing alignment.
The vacuum advance has a big effect on this. If it is out of wack the spark will crossfire and the engine will knock.
Use ported vaccuum advance.
Be sure the carbs primary and seconday throttles T slot exposure is correct at idle.
You need the right amount of initial base timing and correct carb throttle opening T slot exposure at idle.
This is a critical factor.
I bet the PCV valve needs some attention.
20 to 26deg initial base
34 to 36 total centrifugal maxing out at 3200-3500 rpm.
That means a shorter 10-12deg mech advance curve. limit the travel of the mechanical advance system.
Be sure the advance curve is smooth and progressive without jumps in timing.
And slow down the progression rate so it peaks out a little latter (3200-3500rpm)
If it does not have smooth advance progression, fix that. Some of the aftermarket distributor's advance systems really suck for quality.
a max of 46deg total combined mechanical + vacuum advance advance at max vacuum cruise ( light load)
That means a 10 to max of 12deg maximum of vacuum advance MAX. The rate in and out based on throttle load
has to be found by drive testing. Limit the travel of the vacuum advance system to 10-12deg max
Then verify and correct the distributor cap/rotor phasing alignment.
The vacuum advance has a big effect on this. If it is out of wack the spark will crossfire and the engine will knock.
Use ported vaccuum advance.
Be sure the carbs primary and seconday throttles T slot exposure is correct at idle.
You need the right amount of initial base timing and correct carb throttle opening T slot exposure at idle.
This is a critical factor.
I bet the PCV valve needs some attention.
Last edited by F-BIRD'88 on Thu Sep 12, 2013 11:16 am, edited 2 times in total.
Re: Compression Ratio Limits and Iron Heads
Since PetroCan took over all the Sunoco gas stations I wonder if the (PetroCan) Ultra 94 gasoline is STILL the same as
the good old Sunoco Ultra 94 gas , or if they have let the quality and octane slip.
On many cars it would support engine compression ratios at or very near 11:1 cr. And sometimes more.
I had one racy 355 SBC that was 12.65:1cr and it would run with limited max timing of 30deg-31 deg BTDC
as long as you keep the timing dialed back a bit it was fine on 94.
Needed 110 octane unleaded to run at full max power timing of 36deg BTDC. It really liked the 110.
But some cars can be pesky.
Try blending in some of this Mark II 110 octane unleaded. Say 10% 15% added in to spike the fuel octane.
Does it eliminate the knock?
http://www.proracingfuels.ca
it is very effective when blended in with our pump gas, when increased octane is needed.
Did you CC these new heads yourself? are they much smaller chamber than spec.?
Your engine is acting as if it has in excess of 11:1 compression ratio.
the good old Sunoco Ultra 94 gas , or if they have let the quality and octane slip.
On many cars it would support engine compression ratios at or very near 11:1 cr. And sometimes more.
I had one racy 355 SBC that was 12.65:1cr and it would run with limited max timing of 30deg-31 deg BTDC
as long as you keep the timing dialed back a bit it was fine on 94.
Needed 110 octane unleaded to run at full max power timing of 36deg BTDC. It really liked the 110.
But some cars can be pesky.
Try blending in some of this Mark II 110 octane unleaded. Say 10% 15% added in to spike the fuel octane.
Does it eliminate the knock?
http://www.proracingfuels.ca
it is very effective when blended in with our pump gas, when increased octane is needed.
Did you CC these new heads yourself? are they much smaller chamber than spec.?
Your engine is acting as if it has in excess of 11:1 compression ratio.
Re: Compression Ratio Limits and Iron Heads
skinny z wrote:I agree on the 55 degrees. It's been reduced to 51 and I feel that it could probably be dialed back a little more. I going to go for 15 degrees however I'll keep the WOT throttle total low (via a reduced initial setting of 8 degrees) until the knock is gone.
Cam specs are as follows:
Comp custom hydraulic roller.
Intake lobe XFI profile 3015
Exhaust lobe XFI profile 3036
Adv dur. 274/282
.050 dur. 224/230
Lift in/ex. .574/.569
LSA 110
ICL 106
Degreed and installed as per spec.
Just looking at this I would have not built this engine with a 10.37:1cr using this camshaft in it.
It is a bit too agressive for every day street driving. Thats the trouble with pushing the envelope on engine cr.
A bit too much cr is a LOT too much, once you cross the knock threshold of the fuel in the tank.
I have been using the same Ultra 94 octane gas as you, for over 25 years.
Knowing that you cannot always find a Sunoco/PetroCan Ultra 94 octane gas station, I built mine vortec head motor at a modest
9.80:1 cr. If the recent Ultra 94 octane is now slipping, this may have been a wiser choice than I thought.
The next best pump gas I found here, was the Pioneer Magnum 93. It was pretty good stuff. The 91 octane gas here plainly sucks for real engine knock resistance.
Try some Pioneer Magnum 93 gas.
Re: Compression Ratio Limits and Iron Heads
if this engine is now still knocking @ WOT with only 28deg max WOT timing
( leave the vac adv disconnected) you got something seriously wrong.
( leave the vac adv disconnected) you got something seriously wrong.
Re: Compression Ratio Limits and Iron Heads
F-BIRD'88 wrote:For reference: post the carb model number and how you have it set up. jets,PV, IAB's HSAB's IFR etc.
Did you try going to a different Petro-Can station to get your Ultra 94?
I've tried many. All across southen Ontario.
How did you raise the compression ratio from 10:1 to 10.37:1 just by changing from one vortec head to another vortec head?
New short block with a cut deck, larger bore and a smaller valve relief in the piston. Piston below deck went from .030" (avg) to .014" (all). Bore from 4.020 to 4.030 . Valve relief from 6 to 5. Heads were not cc'd and were advertised as 64 cc. Some valve unshrouding done as well.
is it possible the real compression ratio is really a lot more than 10.37:1?
Cranking compression is only 190 +/- 3% so I don't think my SCR is anything over 10:37:1. Possibly even less.
Sharp thin machined edges on the combustion chamber that were not dressed with a file after machining.
Well dressed and all precautions taken.
Cam installed wrong. Checked and re-checked...several times. Vacuum leak, oil in manifold. intake gasket leaks. There are no vacuum leaks apparent. No oil other than were it should be.
So what is the result of the 4degree timing reduction? Is it still knocking?
Yes. No change other than the driveability has changed ever so slightly with a small tip-in stumble off idle.
Re: Compression Ratio Limits and Iron Heads
[quote="F-BIRD'88"]if this engine is now still knocking @ WOT with only 28deg max WOT timing
( leave the vac adv disconnected) you got something seriously wrong.[/quote]
WOT is fine. No knocks and there never has been. Even with the advanced timing. At least not that I can hear. AFRs are very rich in the low 12s.
The only knock is on the initial hit either from a standing start (with as much throttle as can be applied without seriously spinning the tires) or from a rolling start at 60 kph. That's were the problems are.
There used to be an issue with an intensely lean conditon and part throttle acceleration at a medium vehicle speed. Example would have been when gently passing another vehicle from 80 kph and trying to accelerate gradually. That's been resolved with a different PV.
So now it's just the initial hit.
( leave the vac adv disconnected) you got something seriously wrong.[/quote]
WOT is fine. No knocks and there never has been. Even with the advanced timing. At least not that I can hear. AFRs are very rich in the low 12s.
The only knock is on the initial hit either from a standing start (with as much throttle as can be applied without seriously spinning the tires) or from a rolling start at 60 kph. That's were the problems are.
There used to be an issue with an intensely lean conditon and part throttle acceleration at a medium vehicle speed. Example would have been when gently passing another vehicle from 80 kph and trying to accelerate gradually. That's been resolved with a different PV.
So now it's just the initial hit.
Re: Compression Ratio Limits and Iron Heads
[quote="F-BIRD'88"]
Just looking at this I would have not built this engine with a 10.37:1cr using this camshaft in it.
It is a bit too agressive for every day street driving. Thats the trouble with pushing the envelope on engine cr.
A bit too much cr is a LOT too much, once you cross the knock threshold of the fuel in the tank.
I have been using the same Ultra 94 octane gas as you, for over 25 years.
Knowing that you cannot always find a Sunoco/PetroCan Ultra 94 octane gas station, I built mine vortec head motor at a modest
9.80:1 cr. If the recent Ultra 94 octane is now slipping, this may have been a wiser choice than I thought.
The next best pump gas I found here, was the Pioneer Magnum 93. It was pretty good stuff. The 91 octane gas here plainly [b]sucks[/b] for real engine knock resistance.
Try some Pioneer Magnum 93 gas.[/quote]
That's just it. My old Vortec had a 10:1 SCR and the timing specs as this cam although with less lift (hence the custom grind using the XFI lobes). That thing ran just fine on 94 and the Pioneer 93 and whatever they serve at the UMI in Listowel. (that's on my way to Grand Bend).
This one just seems over the top and I may have gone too far.
I will keep at the tune for a while yet but I'm seriously thinking about taking some of the bite out of it with either reduced compression or retarding the can by 4 degrees. That may make the difference for least impact on my resources (time and money).
Just looking at this I would have not built this engine with a 10.37:1cr using this camshaft in it.
It is a bit too agressive for every day street driving. Thats the trouble with pushing the envelope on engine cr.
A bit too much cr is a LOT too much, once you cross the knock threshold of the fuel in the tank.
I have been using the same Ultra 94 octane gas as you, for over 25 years.
Knowing that you cannot always find a Sunoco/PetroCan Ultra 94 octane gas station, I built mine vortec head motor at a modest
9.80:1 cr. If the recent Ultra 94 octane is now slipping, this may have been a wiser choice than I thought.
The next best pump gas I found here, was the Pioneer Magnum 93. It was pretty good stuff. The 91 octane gas here plainly [b]sucks[/b] for real engine knock resistance.
Try some Pioneer Magnum 93 gas.[/quote]
That's just it. My old Vortec had a 10:1 SCR and the timing specs as this cam although with less lift (hence the custom grind using the XFI lobes). That thing ran just fine on 94 and the Pioneer 93 and whatever they serve at the UMI in Listowel. (that's on my way to Grand Bend).
This one just seems over the top and I may have gone too far.
I will keep at the tune for a while yet but I'm seriously thinking about taking some of the bite out of it with either reduced compression or retarding the can by 4 degrees. That may make the difference for least impact on my resources (time and money).
Re: Compression Ratio Limits and Iron Heads
If your "64cc heads" are actually 62cc the cr is 10.60:1 If your "64cc heads" are actually 60cc the cr is 10.86:1
IMHO that is really pushing it with that cam. Especially if there is some build/carb tune/dist timing set up error.
You don't need to "deshroud" the combustion chamber of a SBC Vortec head.
Clairify: it is knocking @ WOT with only 28deg BTDC timing @ WOT. yes/no
it is knocking @ 31deg but not at 28deg..@ WOT.. yes/no
It also knocks on part throttle to full throttle roll in from cruise yes/no
But this stops after reducing the timing to....________..
All this tested with vacuum advance disconnected (for now)
is the balancer/timing tab TDC location ( still) accurate?
IMHO that is really pushing it with that cam. Especially if there is some build/carb tune/dist timing set up error.
You don't need to "deshroud" the combustion chamber of a SBC Vortec head.
Clairify: it is knocking @ WOT with only 28deg BTDC timing @ WOT. yes/no
it is knocking @ 31deg but not at 28deg..@ WOT.. yes/no
It also knocks on part throttle to full throttle roll in from cruise yes/no
But this stops after reducing the timing to....________..
All this tested with vacuum advance disconnected (for now)
is the balancer/timing tab TDC location ( still) accurate?