Why SUs will work well on a Stroker?

[steve260z]

Building my 3.1L and plan on using my SU Carbs. I've read a number of posts and various opinions but let me pose this question and someone more knowledgeable than me will hopefully enlighten me!

 

Q:

These SU carbs were originally designed for a 2.4L.(Which doesn't necessarily mean they won't work well on a larger motor) A 3.1L is obviously 30% or so larger. I do have SM needles which I plan on using/trying.

 

Can the SU deliver the additional volume of air for the increased displacement? Does the SU have that much "headroom" so to speak?

 

The SM will certainly deliver more fuel at a higher RPM but can the SU provide the air? There's certainly a volumetric max for any carburetor design.....

 

Just a question that's been running through my head.

 

thanks!!!

SS

[johnc]

The answer is... it depends. SUs work on a 2.4L engine that makes around 240hp and spins to 8,000 rpm.

 

 

If you're just going to bolt-on some junkyard SUs to a generic stroker then you might have problems. If you're willing to spend some time and effort on research and tuning you can make SUs work fine on a stroker. Especially a street engine that sees full throttle about 1% of its life.

[steve91tt]

I used to run my su's with sm needles on my stroker. The car ran great but had very little high rpm power due to the limited air flow potential of the su's. Great torque though.

[morbias]

Maybe pick up some 2" SUs if you are intent on running that type of carb

[BlueStag]

Maybe pick up some 2" SUs if you are intent on running that type of carb

 

It is also not unknown to run three SUs! Quite the blast.

[Z Greek]

The answer is... it depends. SUs work on a 2.4L engine that makes around 240hp and spins to 8,000 rpm.

 

 

If you're just going to bolt-on some junkyard SUs to a generic stroker then you might have problems. If you're willing to spend some time and effort on research and tuning you can make SUs work fine on a stroker. Especially a street engine that sees full throttle about 1% of its life.

 

That is a beautiful sounding engine.

[Tony D]

It's funny how after John C posted an actual example of SU's obviously flowing more than sufficient air people continued with the bolt on parts parade posts.

 

Might I suggest the limitations of all the posters were due to their inexperience and inability to tune SU's properly and not due to any inherent "lack of airflow ability"---someone mentioned 30% larger engine, did anybody do the math on John C's example twisting to 8K instead if only 6500, and making 240HP instead if only 150?

 

Lack if airflow?

 

I think not.

[morbias]

It's funny how you jump in just to be condescending. I only made a suggestion based on SU recommendations, but apparently that means I don't know how to tune a carb?

 

[Tony D]

Yes, indeed we should follow the charts, they mesh well with the actual world testing and example as evidenced by John's Post.

 

If you are relying on a chart, then yes, that is not tuning a carb. So I'd say yes then if that is the point you want to press.

 

[morbias]

I still don't see how you can tell if it's definitely going to work without knowing the full specs of the engine being built. Surely the carb needed is going to have a lot to do with the cam being used.

 

If I was the OP I would try it and see, and if the standard carbs aren't up to it then look for something bigger.

[Tony D]

Built enough of them. They work just fine. Caveats John C gave were SPOT ON!

[steve260z]

I'm using the Stage 2 Schneider Cam (.480/.480, 274/274) Compression is 10.0. I'm certainly committed to using the SUs and not looking to Triples.

ss

Edited March 4, 2012 by steve260z

[BlueStag]

So, is it possible for a pair of 1 3/4" SUs to deliver enough fuel and air to maximize the potential of a somewhat "hot" 3.1 (lively cam and medium-high compression ratio)?

 

Apparently. What is the absolute upper potential of this pair of carbs? Apparently on the order of 250hp or there-abouts.

 

If I had had to guess based on my familiarity with Brit cars, I'd have said 200hp, but that would be entirely armchair experience.

 

Consider what sort of power Jaguar and Aston got off some straight sixes with SUs, how many SUs (twins, triplets) and the size.

 

Also assume that your stroker is going to be a few percent more volumetricly efficient than those old Brit engines.

 

Tony says you can do it, so I'd say do it. Lord knows you are not going to find cheaper induction than a pair of SUs.

 

Get them utterly rebuilt. You will get no power out of a carb that sucks air down the wrong path.

 

I'd still like to see SOMEONE ELSE mount three SUs on an L6. Hehehhehehehe.

Edited March 4, 2012 by BlueStag

[Tony D]

Define 'maximize the potential'? As well as 'lively' cam, and 'medium-high compression ratio'...

 

Some people would insist the only thing that would fit the bill with that phrase would be nothing less than 55 DCOE Webers.

 

Crappy definitions of performance goals result in discussions like this.

 

My point originally being, the stated example provided by John C is a 2.4 running at 8000rpms (what is the CFM Requirement there when making 240HP???? All I did was suggest people use their HEADS and do the calculations for what has ALREADY BEEN SHOWN as possible, and not rely on a 50 year old application chart or other STOCK example of under-carburettion or massive overcarbing for 'maximization' which would make a miserable street engine.

 

I can't count how many 3, 3.1, and 3.2 engines I've worked with, seen, or known built which work PERFECTLY FINE with SU's. Are they bolted on straight out of the junkyard? Hardly. When you got a couple grand in the engine, why slap junkyard parts on it and run it?

 

I just found it humorous (and now by extension somewhat sad) that people didn't heed the question and blindly followed recommendation charts and postulated it would somehow restrict the airflow on the larger engine. Exactly how high is this 3.X going to rev? Obviously they CAN support 240 HP in the real world, not on a piece of paper, or from a conservative engineering application data sheet.

 

We had an engineer do many calculations on his 3.0 thinking three SU's were the way to go because the stockers are undersized... Ultimately due to the way they work (Constant Velocity) adding more carbs is somewhat counter intuitive and counter productive. In reality to do a three-carb setup the 38mm SU's are what is called for airflow wise to keep proper velocity through them under most driving conditions.

 

SU's are NOT PHH Mikunis, DCOE Webers, or DHLA Dellortos... The flow bridge is a BIG hole and it's VARIABLE. A Bigger engine may draw slightly higher vacuum at full load, peak torque or rpm... but how often are you there on the street engine (as stated by John in the first go-round)?

 

Ultimately a lot of this stuff is more solely for LOOKS than a requirement from a performance standpoint. So that starts going down the Rice Road. Fine, it's your car do what you want. But insofar as people postulating that a pair of Twin 46 or 47 Hitachi SU's are somehow a major limiting factor or something that will 'strangle performance'---that's ludicrous!

 

Sure, for all-out racing you can run 2" units and get marginal performance improvement---at what cost? In the real world, the airflow provided WILL NOT seriously be impacted by running the stock carbs, you simply need to spend some time outside the paradigm of bolting on a Holley and running it to be rewarded with maximum reward possible.

 

You can get 80HP from an L24, and the same exact combination of parts can give you 120HP, 175HP or even more. The guy with the 80HP setup will SWEAR you NEED triples to get 175 from that engine, along with a Cam, porting, polishing, etc etc etc...

 

The guy with 175 says: "Uh, no you don't. It's a matter of tuning."

 

Referring to the question in the original post:

"Can the SU deliver the additional volume of air for the increased displacement? Does the SU have that much "headroom" so to speak?

 

The SM will certainly deliver more fuel at a higher RPM but can the SU provide the air? There's certainly a volumetric max for any carburetor design....."

 

In this instance the answers are, Yes (reference L2.4 8000 rpm vs 3.1 6500 is a 19% Larger vs 23% proposition--5% difference? Is 4% going to make any difference?) Yes, and Yes.

 

To address the volumetric max on a STANDARD carb, you size for 9" HG at WOT Peak Torque RPM+. On an SU, this is a matter of the bridge. This point could be (and generally is) occurring with the Suction Dome at various places at different times. It's not a fixed venturi carb. It changes, and that change is EXACTLY why it is not an issue in this application.

 

Note there are 1.75" and/or 2" duals on 3.5L V-8's as well. If they can fuel that at 4500rpms.... See where this is going?

[BlueStag]

Hey, Tony.

 

I'm agreeing with you.

 

You say that a pair of Hitachi SU's can supply an engine producing 250hp, I am willing to believe you.

 

Again, had I been forced to guess, I'd have said more like 200hp, but again, as I say, I only have these opinions based on my reading of old Brit cars.

 

You say you have succeeded repeatedly. I utterly take you at your word.

 

I think that I can say two things without fear of contradiction:

 

You will not find cheaper induction for this application.

 

You need to have the carbs utterly rebuilt, no cutting corners.

 

Other than that, I say go for it.

 

Hell, make an engine that produces 300hp on a pair of those carbs. I dig it.

 

Triples don't work well on a Z? OK Jag and Aston used them to effect. I have seen a Triumph TR6 with triples. Me? I think that guy needs to find better ways to spend his money....

 

Hell, get six tiny SUs from motorcycles.....

Edited March 5, 2012 by BlueStag

[steve260z]

Greg Ira's example provides compelling support in addition to Tony's last post. The SUs I'm using are not ebay carbs. Been on my 260 for 10 years/40,000 miles. They were rebuilt shortly prior to that by the original owner. These are round tops. They are in good condition. Can't say top condition but certainly not worn out. I'll probably get some new jets to be safe. The throttle shafts are in good condition. Frankly, the SUs are my last project on this 3.1L deal....So, I haven't decided yet what to replace there yet. I'm bolting up the flyweel and clutch now so carbs are next. I WANT these to pull hard to 6.5K...That will be awesome.

 

Feel free to toss in advice on the carbs and Thanks as always for the input.

ss

[johnc]

Greg runs the twin SUs because the SCCA EP rules require it. Would he run 44 Mikunis or 45 Webers if the rules allowed it? Absolutely. If I had a 3.1L stroker engine in my street car I would go FI without thinking twice.

 

You can run SUs on a 3.1L stroker just fine with proper tuning.

[reefedjib]

Ultimately due to the way they work (Constant Velocity) adding more carbs is somewhat counter intuitive and counter productive. In reality to do a three-carb setup the 38mm SU's are what is called for airflow wise to keep proper velocity through them under most driving conditions.

 

I want to make an observation regarding the Constant Velocity of SUs, as far as I see it. It is quite remarkable, yet another great feat of Japanese engineering. Twin SUs come with an intake such that the first SU feeds cylinders 1, 2 and 3, and the second SU feeds cylinders 4, 5 and 6. The distribution of intake strokes for the first SU (like the second SU) is #1, then a skipped stroke, then #2, then a skipped stroke, then #3 and finally a last skipped stroke before going back to #1. So it goes 1-skip-2-skip-3-skip-1... I do not know if it actually creates a standing wave of air flow, certainly the airflow will pull mixture into the manifold on a skip step. The result is constant velocity. Pretty neat, huh?

[letitsnow]

I want to make an observation regarding the Constant Velocity of SUs, as far as I see it. It is quite remarkable, yet another great feat of Japanese engineering. Twin SUs come with an intake such that the first SU feeds cylinders 1, 2 and 3, and the second SU feeds cylinders 4, 5 and 6. The distribution of intake strokes for the first SU (like the second SU) is #1, then a skipped stroke, then #2, then a skipped stroke, then #3 and finally a last skipped stroke before going back to #1. So it goes 1-skip-2-skip-3-skip-1... I do not know if it actually creates a standing wave of air flow, certainly the airflow will pull mixture into the manifold on a skip step. The result is constant velocity. Pretty neat, huh?

 

I'm not sure you're getting why they're called constant velocity, read the wiki article. http://en.wikipedia.org/wiki/SU_carburetter

 

Also, the SU was designed by a Brit. http://en.wikipedia.org/wiki/George_Herbert_Skinner

[Leon]

I want to make an observation regarding the Constant Velocity of SUs, as far as I see it. It is quite remarkable, yet another great feat of Japanese engineering. Twin SUs come with an intake such that the first SU feeds cylinders 1, 2 and 3, and the second SU feeds cylinders 4, 5 and 6. The distribution of intake strokes for the first SU (like the second SU) is #1, then a skipped stroke, then #2, then a skipped stroke, then #3 and finally a last skipped stroke before going back to #1. So it goes 1-skip-2-skip-3-skip-1... I do not know if it actually creates a standing wave of air flow, certainly the airflow will pull mixture into the manifold on a skip step. The result is constant velocity. Pretty neat, huh?

That's not the correct etymology of the "constant velocity" term.

 

Also, there is no "great feat of Japanese engineering" here. The natural firing order for any inline-6 is 1-5-3-6-2-4 and there were straight-sixes with twin SUs way before the 240Z, e.g. the Jaguar XK140, predating the first Z by 16 years.

 

FYI...

Edited March 5, 2012 by Leon