Intake pulse waves causing tuning issues with Fiat SOHC 1.9 Stroker

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elderair
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Intake pulse waves causing tuning issues with Fiat SOHC 1.9 Stroker

Post by elderair » Tue May 17, 2016 6:47 pm

So, I have been running my 1.9 Fiat SOHC stroker for about 2500 miles now. The engine is a monster from 4,000 rpm through 7500 or beyond. The issue is with the 2500-4,000 rpm range. Running a Pittatore #87 grind 42/82 camshaft timed for f/l at 110deg. ATDC crank. When I initially start the engine and look into the carbs and rev the engine it has a smooth venturi spray pattern. Once the exhaust system warms up say in 3 or so minutes I get sputtering through the 2,000-4,000 range unless I rev the engine slowly. You can physically see the intake wave push the fuel in the venturi of ALL barrels and make it a pulse. It has consistently done this from the beginning and I have driven around it and tried a few different jet settings and different venturi sizes. I haven't tried advancing the cam timing to close the intake valves sooner. I haven't tried putting a spacer under the carbs to change the intake runner length to tune the pulse out. I haven't tried removing the exhaust system at the collector. The header is an Allison short tube unit with step up tube design to mimic a long tube header and the exhaust system is an Allison design with a hush power super quiet muffler.

As a refresher from previous post the engine is as follows: 1.6 punto sporting block with a Stilo 78.4mm crank and 128.5 mm ZRP rods. 27.5mm c/h Iapel flat tops with large valve reliefs and the block is decked .023" for .6mm pop up. A navalineasport non integrated fire ring head gasket at 1.9mm thick which is about 1.75mm after head is torqued. This gives approx. .045 inch of squish. Tipo head with 39.5mm intake valves and 35mm exhaust valves ported and smoothed pockets with intake port measurement of 30.5mm at the head to intake mating surface. Intake is matched to the head and matched to the carb base gasket at the carb flange. Ignition is Computronix with 10 deg. crank timing with a total of 28 deg. timing.

I am sure there is a mathematical explanation for this phenomenon but I am slightly challenged in that area. Any thoughts and ideas would be greatly appreciated as I am trying to prove to the X community that this combination is a viable option for those not wishing to do an engine transplant from some other species of automobile. This engine has produced 136.24 whp, 130.19 lbf ft torque on the rolling road at 12:l A/F across the rpm range and it has potential for a bit more properly tuned. Intake is a Sprint Filter for dual 40 DCNF's port matched at both ends and carbs are Italian Weber 40 DCNF-12's with 34mm Venturi, 145 mains, 210 a/c's F24 emulsion tubes, 47 idles and 4.5 aux venturi. 45 accel pump nozzles. Float level is 50mm and float drop is 58.5mm.

The problem is worse with the 30mm venturi's perhaps due to a stronger signal in the venturi pulling more fuel into the pulse wave. Yesterday with the airbox off was the first time I tied the problem to engine temp and visually noted the pulse wave in all the barrels coincide with the stumbling and sputtering. Under load this equates to bucking and jerking if not driven through carefully.

Sure is a kick in the unacceptable to drive except around town.
Last edited by elderair on Thu May 19, 2016 8:44 am, edited 2 times in total.

Guy Croft
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Re: Intake pulse waves in Intake on SOHC 1.9 Stroker

Post by Guy Croft » Tue May 17, 2016 7:19 pm

give me the header configuration, primary & secondary lengths..

in fact give me every dimensional data you can from head to exit tailpipe please.

The phenomenon you are seeing is not entirely unfamiliar to me.

With respect the rest of your tech data is interesting but NOT the cause of the fuel-standoff problem.

G

elderair
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Re: Intake pulse waves in Intake on SOHC 1.9 Stroker

Post by elderair » Tue May 17, 2016 10:10 pm

Tomorrow I shall take a vernier caliper and seamstress tape and record all that I can see. Thanks in advance.
Kent
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elderair
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Re: Intake pulse waves in Intake on SOHC 1.9 Stroker

Post by elderair » Thu May 19, 2016 1:36 am

Today I took some measurements of the exhaust system and the header primary tubes. The Exhaust is 2" o.d. throughout with a 4" straight section just after the collector. Then bends past 90 for 16.5" section into a 6" radius 180 turn back. 4" straight into a 19 x 5 x 4" muffler 5" straight section to a 90 deg. bend and 3.5" straight out. Having done the measurements I started the car with the system removed at the header collector and let it warm up 3 min. then 5 min. and under rapid acceleration no pulses in the inlet. The only stumble is if it is at 1,000 rpm and you just stomp it there is a tiny burst of air up through the inlet of all barrels. If you go to 1,500 then do it there is no lag. . Total length of the system from the collector to the end is around 73".

I measured the #1 cyl header primary and the others seemed to be similar. Off the flange is 1 3/8" o.d. pipe for 8.5" into a 1.5" pipe for 14 3/4" into the collector which has a 2 3/8" i.d. The collector is 4" long.

There is a visible repair to the exhaust system which had to be made almost immediately after installation on the vehicle which could have been an indication of a problem with the flow. Or it may have been a bad design on the manufacturers part. They have since changed the mounting of that hangar.
Attachments
004 (640x360).jpg
Header where it can be seen.
004 (640x360).jpg (183.52 KiB) Viewed 2947 times
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2" exhaust system with hush power muffler
20160518_192045[1].jpg (4.6 MiB) Viewed 2947 times
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Guy Croft
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Re: Intake pulse waves causing tuning issues with Fiat SOHC 1.9 Stroker

Post by Guy Croft » Thu May 19, 2016 11:54 am

help me simplify this please?

The header is 4-1, with total head to collector pipe length - of 23.25"?

G

elderair
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Re: Intake pulse waves causing tuning issues with Fiat SOHC 1.9 Stroker

Post by elderair » Fri May 20, 2016 12:14 am

That is correct. Not sure how the various step up sizes of pipe play in to the equation but perhaps you could answer that as well.
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Re: Intake pulse waves causing tuning issues with Fiat SOHC 1.9 Stroker

Post by Guy Croft » Fri May 20, 2016 12:00 pm

I have no direct experience of primary pipes set out in the way you have described - where the pipe diameter is increased at some point betw head and collector.

I would expect some effects on pressure waves and flow of exhaust gas - but I simply could not predict what they might be. Without modelling the layout using very advanced simulation software - based on your exact engine - nobody could predict what would happen. Moreover unless the system was bench-dyno tested back-to-back on the same engine one could not assert that it was beneficial either.


Optimum bhp/torque and bmep results have been obtained on units built by me where simulation has been employed to model the exhaust header (manifold) and similarly many, many GC prepped cylinder heads have found their way into service via the same regime. In every case the conventional route of keeping the pipe diameter constant between head and collector (be it a 4-1 or 4-2-1) worked perfectly.

I think the fundamental problems are caused by the primary pipe length and the silencer. I emphasise 'think' because these things can be something of an interation to solve - ie: try this - try that..

I would never use a 4-1 with less than 27" chord length. It might be 'ok' at 23" (or so) but I would not expect optimal results in terms of throttle response or output. There's just too much risk of wave interference cylinder-cylinder. The shorter the primaries (especially on a 4-1) the worse the throttle response becomes. The effects get worse the more valve overlap is employed.

The silencer width of 5" on your oval design is too small in my professional opinion. I would want 8" width. I do know (and I published this in my 1995 book in the chapter on exhaust systems) that if the volume or cross-sectional area of the silencer is too small relative to the size of the perforated tube you will get adverse back-pressure. The more highly tuned the engine the worse the problem.

It is well-known that exhaust silencer back-pressure can be very damaging - to rings, valves and valve seats, and certainly fuel standoff at the carb on throttle opening is never a good sign or situation.

GC

I hope this helps to some extent at this time.

elderair
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Re: Intake pulse waves causing tuning issues with Fiat SOHC 1.9 Stroker

Post by elderair » Fri May 20, 2016 7:55 pm

That gives me some idea of what may be ahead for the exhaust. Perhaps a long tube 1 3/8" Vics Header and maybe a custom exhaust hooked to it. Which of your mufflers would be the choice to install into the system if I fab up an exhaust? Thanks!
Kent
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Guy Croft
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Re: Intake pulse waves causing tuning issues with Fiat SOHC 1.9 Stroker

Post by Guy Croft » Fri May 20, 2016 8:19 pm

email me,

be warned I am no fan at all of Viccs.

My US sole agent is Autoricambi in Texas, with whom I correspond almost every day. And for good reason. They trust my technical expertise, again, for good reason.

No 'off the shelf solution' will help you here, and whilst I can guide you, I cannot guarantee success.

That is of course a 'flying mile' from saying that solution X or Y from anyone else will be better than one proffered by me. It will not be.

If you don't have access to a superbly skilled ex header mfr who will FOLLOW a design regime set by me - we are both wasting our time..

...there are many so-called 'experts' in the UK but in reality (in my view) most of them are no more than ex-industry pipe-fitters with welding experience - who know nothing about actual exhaust system design other than that gained by experience at client expense.

GC

elderair
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Re: Intake pulse waves causing tuning issues with Fiat SOHC 1.9 Stroker

Post by elderair » Fri May 20, 2016 8:57 pm

I do agree about not being a fan of "Vics" Autoricambi has been my go to for all things 124 but they are not ones to offer help on X 1/9 issues. I would very much like to find a header maker that could build a specific header based on your recommendations. I will do some checking here in the states. Thanks again. I will shoot you an e-mail.
Kent
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Guy Croft
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Re: Intake pulse waves causing tuning issues with Fiat SOHC 1.9 Stroker

Post by Guy Croft » Fri May 20, 2016 10:09 pm

OK!

G

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Re: Intake pulse waves causing tuning issues with Fiat SOHC 1.9 Stroker

Post by Guy Croft » Sun May 22, 2016 11:46 am

Here is a software optimised header for a SOHC unit for which I prepped the head some years ago. It looks like a 4-1 but is actually a 4-2-1 with very short secondary pipes.

The tubing and pre-formed bends were bought from a dairy tubing supplier online, as were the collectors. The engine is smaller than yours but that won't affect performance per-se. It also has a more radical cam than yours but again the layout will still work - I believe better than yours.

I would start with a better silencer like the AX891 I use. Not stainless but - right!

G
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AX891 silencer (2).jpg
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AX891 silencer.jpg
AX891 silencer.jpg (110.51 KiB) Viewed 2895 times

elderair
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Re: Intake pulse waves causing tuning issues with Fiat SOHC 1.9 Stroker

Post by elderair » Thu Jun 23, 2016 2:06 am

I received my completed X1/9 exhaust assembly tonight and installed it to the Allison header on my X for a test run. The exhaust is now a 2 1/4" system with the Croft silencer and has a very nice mellow sound. After about ten minutes of warm up and repeating the 2,000 to 4,000 rpm revving I found the fuel pulsing and spitting out the carb barrels has come to an end. There may be a tad bit more playing with jets and some more dyno trips with a degree or two of camshaft advance to be played with but I am very pleased with the results of the new system. I will drive it in the next few days as time and weather permit and note any issues that still remain in the throttling. Once again thanks for some very good and sound advice. I am sure that a long tube header properly designed for this engine would yield a much better result but for now at least slow speed driving as needed is possible without bucking and jerking. Here are some pics of your sllencer integrated into a nice 2 1/4" system.
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Guy Croft
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Re: Intake pulse waves causing tuning issues with Fiat SOHC 1.9 Stroker

Post by Guy Croft » Thu Jun 23, 2016 11:43 am

Thanks for publishing that interesting report Kent!

G
Guy Croft, owner

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