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High Compression spitfire pistons

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clshore Carter Shore
Beverly Hills, FL, USA   USA
Great stuff, glad it's suiting you.

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SpitMan Avatar
SpitMan Doug Walls
Brandywine, MD, USA   USA
1970 Triumph Spitfire MkIII "Lil' Red Fox"
1998 Chevrolet Corvette "Silver Fox"
2007 Chevrolet Silverado "Workhorse"
2013 Chevrolet Malibu "Pearl Baby"
Really interested in your results at the Dyno. Want to do mine some day when the car is all together and ready for it.

69Mk3Spit Avatar
69Mk3Spit Dino Finelli
Oakville, ON, Canada   CAN
We'll here it is!!!!!!!

I pretty happy the car runs great, however I need to start looking at leaner needles I could get up to 15 hp by getting mixture perfect. It will be hard though FI is the only way to get perfect mixtures.

The numbers are at the wheel and need to be multiplied by 1.2 to get flywheel.

I am running AAQ, does anyone have a recommendation?

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clshore Carter Shore
Beverly Hills, FL, USA   USA
The needles don't set the mixture strength, the jet does that.
What the needle taper does is match mixture strength to airflow volume across the range from idle to max RPM.
So with a perfect needle taper, if you set mixture for 14.7 at idle, it would at that value through the whole range.
Leaning or enriching the jet would raise or lower the AFR across the range.
In practice, no carb is perfect, and often the mixture is set richer at max RPM by a point or two to help cool things.

The AFR numbers are all over the place, from a high of 15.56 at 2000 to a low of 9.0 between 3600 and 4300.
I'd first try leaning the jets to establish a better AFR at high RPM before you go bounding off into the wide open unknown of needle choices.
But I'm guessing that you cannot just put your car on the Dyno whenever you want to measure the effect of tuning changes.
The 'butt dyno' is notoriously inaccurate and biased towards whatever you want to believe is true.
So you could use a stopwatch, or download one of the Dyno apps for your Smartphone.
An onboard Wideband O2 sensor is pretty inexpensive, compared to whatever you have spent so far.

Looking at the chart, I think the torque curve is suspiciously flat.
But the graph does pass the Dyno Chart sanity check:
Dynamometers do not measure Horsepower, they measure Torque and RPM.
Horsepower is computed from the mathematical product of Torque and RPM.
Stated another way, the measured Torque number at 5250 RPM, yields the same computed number for Horsepower.

Using Imperial units for Horsepower (lb-ft/sec) and Torque (ft-lb), the curves should cross over at 5250 RPM, as on your chart.

Not trying to rain on your parade, just offering observations to help you achieve the best results.

69Mk3Spit Avatar
69Mk3Spit Dino Finelli
Oakville, ON, Canada   CAN
RAIN ON MY PARADE?

Not a chance, I was hoping you would weigh in.

I have great improvements in performance and power, keeping up with a lot of todays cars and feel comfortable on the highways. I just want to ensure she is tuned to her best potential.

The car is still strapped up, I'm going in tomorrow morning to adjust the AFR at idle, set it to 14.7 then take it through the test again.

Are you thinking if I adjust the idle jets to 14,7 it will help the curve, I've talked to other people and they are recommending a needle change.

Hoping Joe C tunes in.

Tonyfixit Avatar
Tonyfixit Tony M
Duncan, BC, Canada   CAN
What other needles do you have at hand?

Your AF mix is all over the place off idle. I just wonder where the stock ABT needles would put you?
and it is strange that change at 4k rpm?

I diagree with Carter, I think the problem is your needles not the jets. But what needles??

69Mk3Spit Avatar
69Mk3Spit Dino Finelli
Oakville, ON, Canada   CAN
Hi Tony

I believe they are AAQ's

I have AAL,ADN and AAR on hand for the hs4s

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Tonyfixit Avatar
Tonyfixit Tony M
Duncan, BC, Canada   CAN
Are you familiar with the minty lamb compare o rama?

http://www.mintylamb.co.uk/suneedle/

claytoncnc Avatar
claytoncnc Gold Member Marcus Clayton
Melbourne, Ivanhoe, Australia   AUS
What I think you are seeing, is a resonance period just around 3000, and just under 4500 (there would probably be another at around 1500, the end of which could be the lean setting seen at the start of the graph.) This may be helped by changing the size and location of the balance tube, but as this is not practical picking a needle with correct calibration is the next best option.

It is because of these resonant effects inherent in most engines, that there are so many different needle profiles available.

If there were no resonance, then the air flow, fuel flow and therefore needle taper would have a fixed relationship, and all engines would have the same needle.
These basic 'linear' needles are the 'number (3,4,5 etc) needles', and are usually confined to race cars that have little need for part throttle or low speed running.

I would suggest a needle with a much more idealised profile, possibly starting with a GS or similar, and a heavier spring in the dashpot, to try to settle it a little.
Tune to 14.7 at idle and run on no load up to 6000rpm to establish the base tune.
Adjust the jet to achieve about 14:1 at its weakest point in the rev range, then try a power run, ignoring the possibly terrible idle.

Then superimpose the plot onto the needle station curve on one of the SU programs available, and fill in the gaps with your best fit of available needles.
Match the bumps and valleys of the needles with the changes in A/F ratio..
If it tends to go rich with RPM, choose a needle with a flatter curve, if it leans out, a steeper curve, but with the same basic profile of bumps and troughs as plotted.

You can pretty much ignore anything much above station 10 as HS4s on your motor will probably only open that far.

I agree that you have a lot of power to be unlocked, I would shoot for 13:1 at the richest, (top end) and 14.5 at the leanest, with the idle being where it works best, ideally closer to 15:1, but with a big cam, it could be a lot richer.

If you have not already done so, new jets are very desirable, they do wear and new ones can fix a poor idle with no noticeable effect on the power A/F ratio

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clshore Carter Shore
Beverly Hills, FL, USA   USA
Let me be clear, I am not suggesting that a different needle profile may not be required.
Only that based on the dyno results, there is no clear indicator of what direction to take for
determining what profile is needed.
Raising the jets leans the mixture, lowering the jets enriches it, over the whole range.
That is the quickest and easiest method of varying the mixture, and obtaining dyno run results
that will inform your choice of needle profiles.
Note that you choose the needle based on the profile, not the other way around.

I'm assuming that this is an inertia dyno, ie runs are performed by WOT acceleration from low RPM to max RPM?
They calculate torque by measuring the time required to spin up a heavy drum from one speed to another.
Inertia dyno are less useful for tuning, because they are incapable of steady state measurements.
An absorbsion dyno allows you to run WOT at various fixed RPM, and measure the torque.

Another issue with variable venturi carbs like SU and Stromberg CD is the piston/needle position vs RPM,
In transient situations like acceleration, the spring strength, and degree of dashpot damping affect the piston/needle position.
Once the piston is fully raised, that's it, whatever needle diameter is present in the jet orifice is what you get, and fuel
metered depends on the airflow velocity, which is a non-linear relationship.
This is why Clayton suggested a stronger spring in conjunction with a different needle, to raise the RPM where
the piston/needle reach their limit.

There is another issue using an inertia dyno with SU and Stromberg carbs, the transient response time.
Their dashpot dampers provide the means for transient mixture enrichment, by delaying the piston/needle rise.
That causes a temporary increase of airflow velocity across the needle/jet area, enriching the mixture.
Compare to conventional carbs with accelerator pumps actuated by opening the throttle, independently of airflow.
At the start of a dyno run, the throttles are snapped fully open and held there until max RPM is reached.
With conventional carbs, the accelerator pump dumps all it's fuel in one shot and that's it, no more enrichment.
With SU/Stromberg, enrichment can occur throughout the whole run, affecting the power developed and skewing measured AFR readings!

A question, what gear are the dyno runs being run in?
Because the higher the gear, the longer it will take for the run, and the less the effect of SU type mixture enrichment will have.

It will be useful to observe, and perhaps to record the piston positions during a dyno run.
If the pistons rise to the top early in the run, then consider spring changes.
That will likely require adjustments to jet mixture, and needles with a different taper.

69Mk3Spit Avatar
69Mk3Spit Dino Finelli
Oakville, ON, Canada   CAN
Ok Thanks

I put the needles through the calculator.

I was currently running the AAM not an AAQangry smiley.

I went with the AAL as it ran close to the GS measurements.

The AAQ will make it leaner, just ran out of time to test them out, might try them for the "butt dyno"

Naturally for these carbs a normal AFR curve is pretty hard to achieve, much easier with a FI system.

Happy where the 80+lbs torque and 82+hp came in at, and I do realize there is more to be had. drinking smiley

The test was done on 3rd gear of a T9 trans.

Anyhow I went a bit out of the box with this engine build, so happy with the work done by Active Engines, Wishbone Classics and of course the forum guys!

It's a good feeling to have numbers backing up performance!

Car ran great!


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69Mk3Spit Avatar
69Mk3Spit Dino Finelli
Oakville, ON, Canada   CAN
better pic of the needle comparison


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claytoncnc Avatar
claytoncnc Gold Member Marcus Clayton
Melbourne, Ivanhoe, Australia   AUS
Dino, I have to say congratulations on the result.
The engine has a very flat torque curve, and I bet that it feels very strong in your mk3.

There is a little more to be had as you know with just a needle change, but what you have already must be very pleasing.
I think that you and your team have done as good a job as can be done, with the spec you have chosen.

If you should ever feel ambitious, and rich, a pair of DCOE webers will pick up another 10 ftlb, and that mixture variation can be minimised.
Despite that I would (and I have on my own car) stick with the SUs

Just a point of information, and definitely not a criticism, because you have it pretty much right:

The resonance that is causing the mixture variation is inherent, and the rpm at which it occurs is a function of the intake tract length.
It can be tuned out, but it is not at all easy, requiring plenty of fabrication and hardware.
What is happening is that the air column is getting pushed out of the carb and then coming back in again, so the air is picking up fuel from the jet as it goes in, as it comes out, and as it goes back in again, making the mixture rich.

This is the reason I advise people to leave the inlet ports alone apart from a clean up.
Making the ports bigger and getting them to pass bulk air on a flow bench, also makes it easier for that air to flow backwards, and can greatly magnify that reversion effect, to the point where the car becomes extremely nasty to drive, and of course is made worse with more ambitious cam timing.
The inlet tract actually can have close to zero flow at some rpm due to reversion (so close to zero power)
A flow bench is a useful tool, but is steady state, and takes no account of the dynamics inherent in an inlet, (or exhaust) that your dyno sheet reveals so clearly.

SpitMan Avatar
SpitMan Doug Walls
Brandywine, MD, USA   USA
1970 Triumph Spitfire MkIII "Lil' Red Fox"
1998 Chevrolet Corvette "Silver Fox"
2007 Chevrolet Silverado "Workhorse"
2013 Chevrolet Malibu "Pearl Baby"
I have been following this Post for some time now. Although there is not always 100% agreement by all parties, there is a lot of commonality in this by all contributors. It has been a very good Thread.

As I read all of this I get a little anxious about my engine since I have done much of what Dino has done to his in some variation. I did not go to raised pistons but I did the block and head milling to get about 10:1 cpr and I did the polishing and matching of the intake and exhaust ports but stopped at doing too much more in the valve area. Running a Big Valve head and an Elgin Ground cam with dual Weber DCOEs and the Megajolt Ignition I think that the engine should do well. Hopefully as well as Dino's. Mine should be on the road within two months and we will see what happens. Currently, I have tested it around my farm, and what a little powerhouse! Like I say anxious!!

Very good work Dino. And very good Posts by the Contributors.


Doug/SpitMan

claytoncnc Avatar
claytoncnc Gold Member Marcus Clayton
Melbourne, Ivanhoe, Australia   AUS
In reply to # 1480192 by SpitMan I have been following this Post for some time now. Although there is not always 100% agreement by all parties, there is a lot of commonality in this by all contributors. It has been a very good Thread.

As I read all of this I get a little anxious about my engine since I have done much of what Dino has done to his in some variation. I did not go to raised pistons but I did the block and head milling to get about 10:1 cpr and I did the polishing and matching of the intake and exhaust ports but stopped at doing too much more in the valve area. Running a Big Valve head and an Elgin Ground cam with dual Weber DCOEs and the Megajolt Ignition I think that the engine should do well. Hopefully as well as Dino's. Mine should be on the road within two months and we will see what happens. Currently, I have tested it around my farm, and what a little powerhouse! Like I say anxious!!

Very good work Dino. And very good Posts by the Contributors.


Doug/SpitMan
Do not be too concerned.

The good news is that if the reversion becomes a bit of a problem for you, with a pair of webers, it can be tuned out to some extent by altering the tappet settings on the inlet valve (it alters the valve timing, therefore the resonance frequency), as you have 4 individual ports, each with their own set of jets.
I really do not think it will be a problem though, and you really need an engine dyno to do this effectively.

Back in the 70s, my business partner was preparing the factory race cars for Ford Australia (he also designed and built the exhaust and inlet manifolds fitted to the production versions). When tuning on the dyno, the engines picked up 25 ftlb of torque by adjusting the tappets. The variation was from .012, to .050, there were 11 different tappet settings for the 16 valve V8.
Reversion issues are far more complicated in a V8 with 4 barrel and a plenum manifold, and uneven induction events along runners of unequal length.


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