Exhaust Gas Temps.

[PGCobra]

Hello,
my supercharged 383 cuins SBC has come off a Dyno and left me with a number of questions, one of which is why do the RH bank of cylinders ( #'s 1,3,5 & 7 ) run hotter than the LH bank ( #'s 2,4,6 & 8 ).
The temp range is between 50 to 250 degF on average.
Thanks.

 

[Lou Scannon]

"The temp range is between 50 to 250 degF on average"
These are not exhaust gaa temperatures at the ports.
Your links did not work for me.
Try to clarify your information, and we will try to help.

 

[patprimmer]

It seems that 1,3,5,7 is lean. To even guess why we need info on your induction and fuel systems as obviously you have more air or less fuel on one side.

Did you make fuel system adjustments on the dyno. If not, why did you bother to dyno.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[PGCobra]

to answer your question re trying various jetting - yes of course we did, but please read on re this particular subject.
Should have given more tech info initially. As a 'newby' to this site I was not sure how much info to burden you with.
The engine is a small block chevy with an 8v-92 Blower (nom 436 cuins/rev - like an 8v-71 ).Initially set at 10% overdrive and for the last test 6% under. Fueling is by 2 off modified 750 Holleys with vac secondaries. One mod was to the Power Valve restrictions, increased by 0.010" firstly before considering larger jet increases. Jet sizes ended up nearly standard at 72 primary and 76 secondary. Other mods were larger needles at 0.120" plus larger shooters at 0.035". Power Valves at 6.5 each. 30 cc pumps. Purple vacuum diaphragm springs. Vacuum balance tubes between the two vac secondaries.
Heads are Edelbrock Victor Juniors modified to give a 7.7:1 compression ratio.
Cam is a solid roller lifter with timing of 244 deg duration Inlet and 252 deg duration Exhaust @ 0.050".
1.5:1 rocker ratio Inlet and 1.6:1 rocker ratio Exhaust.
112 deg LCA.
Valve lifts are 0.543" Inlet & 0.579" Exhaust.
Cam installed at 4 degrees advanced.
This engine spec was based on a previous engine. It did not have such bank to bank exhaust temp variations.
Tried various timing:- starting at 40 deg all-in made the engine 'pink'( detonate ) at only 4,900 rpm. Timing re- set to 30 deg ( as per my previous engine ) and again 'pinking', but higher at 6,000 rpm. Next was a safe 26 deg and this was fine upto 6,500 rpm. Positive 28 deg at 6% under would have been OK, but ran out of Dyno time.
I use an MSD Flying Magnet Crank Trigger.
Exhaust temperature readings I gave previously were at the 26 deg timing runs, with very little difference from 10% over and 6% under.
Max, indicated, boost at 6,500 rpm, at 10% over was 12.3 psi and 12.1 psi at 6% underdriven.
Inlet temps varied between 79 and 83 deg F ( 26.1 and 28.3 deg C ).
I noticed in this test session that the Blower was mad hot - I could not even touch it ( heat = pressure ? ). It was built to original specification clearances. The blower on the previous engine ran quite warm, but you could touch it.
BSFC figures :-
At 10% overdriven. At 6% underdriven.
3,400 rpm:- 0.068 lb/hph. 0.677 lb/hph.
3,500 rpm:- 0.646 lb/hph. 0.557 lb/hph.
4,000 rpm:- 0.599 lb/hph. 0.562 lb/hph.
4,500 rpm:- 0.568 lb/hph. 0.546 lb/hph.
5,000 rpm:- 0.508 lb/hph. 0.612 lb/hph.
5,500 rpm:- 0.653 lb/hph. 0.619 lb/hph.
6,000 rpm:- 0.616 lb/hph. 0.609 lb/hph.
6,500 rpm:- 0.642 lb/hph 0.607 lb/hph.

I can give the A/F ratio's, but how much info is required ?
One thing I can say is that after 6,000 rpm in both the 10% overdrive and the 6% underdrive the A/F ratio's went into lean between 13.2 & 14.4. The majority of time from 3,400 rpm to 6,000 rpm range the figures stayed in safer, but richer, 10.2 to 12.77 numbers respectively.
Comments on fitting a Methanol Injection system, injecting into the Blower inlet, under the Carbs, to try to cool the intake charge. Or maybe Water, or a combination of the two.
Previous use of Nitrous and C16, on a blown motor, brought the initial high inlet temperatures down to acceptable levels.
This car is used for Drag Racing as well as the road.
Power is 630 HP at 6,500 rpm and climbing before shut-off, with max Torque 600 lbsft occuring at 4,850 rpm.
Hope all this helps and if you need me to be more specific/clarify some aspects, please ask.
Many thanks.

 

[patprimmer]

Are the carbies mounted in line or crossways

What jets are in each barrel of the carby and how does that barrel relate to EGTs.

Do you have the same head CCs, deck clearance, gasket thickness, piston dome cc on both sides of the motor.

Do you have the same exhaust on both sides.

Are the centre of the carbies directly over the centre of the blower.

Is the blower directly over the centre of the manifold

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MikeHalloran]

The links you include still don't work. Do something differently.

The few numbers you report are not credible for EGT. Please describe exactly how you made the measurement.



Mike Halloran
Pembroke Pines, FL, USA

 

[IceStationZebra]

First, engines don't "pink," - they ping, technically called pre-detonation.

Second, you didn't mention the intake manifold which has the biggest influence on fuel distribution. Also, does you blower have straight or helical rotors? (I would think that helical would slightly favor one side.) Could you have different exhaust restrictions?

I was thinking of different ignition timing bank to bank due to your ignition trigger - but that can't be it. Each magnet fires one cylinder from both banks.

See this page regarding your power valve.

http://www.holley.com/TechService/FAQ.asp?category=Blower

Try this forum - they seem pretty savvy with all things engine.

http://speedtalk.com/

Hope this helps. ISZ

 

[patprimmer]

Helical rotors favour one end, not one side.

As far as I know, all Roots blowers used in automotive applications have some level of helix.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[PGCobra]

Thanks guys for the replies & questions.
Firstly let me rectify my typing error re the exhaust gas temps.
I should have entered " the exhaust temp range between the 2 banks of cylinders are between 50 ( lower rpm ) to 250 ( higher rpm ) deg F ". ie the LH side ran cooler by this temp range compared to the RH side. An average total temp difference over an rpm range of 3,200 to 6,500, for two opposing cylinders, in this case #'s 1 & 2, is 1,033 deg F for #1 and 822 for #2. A difference over the rpm range of 211 deg F ( 99.4 deg C ).
To address each reply :-
Patprimmer - 1) yes the Carbs are mounted crossways.
2) I cc'd the combustion chambers within 0.5% of each other. I always deem this sufficient.
Yes the deck clearances/gasket thicknesses/piston inverted domes are all the same sizes/cc's.
3) yes the same 2" od exhaust manifold design on both sides.
4) yes the Carb centre lines are directly over the Blower inlet centre lines.
5) yes the Blower is directly over the centre of the manifold. As I used an 8V type Blower I made an adaptor plate, 2" thick, to adapt the Blower to the standard 8-71 Supercharger Inlet Manifold. I made the Adaptor multi-purpose by incorporating 1/2" od tubes into the Adaptor to act as an ' intercooler ' (tubes installed lengthways ) which I can use as an Air- to-Air type, or a Water-to-Air type.
Thanks Patprimmer.

MikeHalloran - The above should answer your questions Mike, however re the EGT these were from individual sensors in the Exhaust Manifold installed about 2" from the Manifold Flange.
Thanks Mike.

IceStationZebra - Engines always make 'PINKING' noises in England.
Submarines 'PING ' - like the one in the film of your handle. Anyway it still means the same - impending doom !
Yes 8V-91 Blowers have 3 lobe Helical Rotors.
Helical Rotors are not known to favour one side more than another and I have never found this on other blown engines.
The only problem with a Helical Rotor Blower is that the charge is, in fact, pushed forwards. With Drag Racing the inertia, when accelerating from the line nullifies this trait. Never had any problems on the road.
Intake Manifold. Unless the design is terrible the Blown Engine is not as fussy about the design as normally aspirated engines were the design is critical.
In my case I had to use what was commercially available and what I could make myself. I had to make an Adaptor ( see Patprimmer above ). The 'Heat Exchanger ' tubing part would help conduct unwanted heat away.
Basically I have, in effect, a large Plenum Chamber of 365.5 cuins ( Adaptor and Manifold )volume. I found that Plenums' of around 2.5 times the engine size work best on smaller 4 cylinder engines, with straight rotor Roots Blowers as it dampens out the pulsing inherent with this rotor design. However having a Roots type Blower with Helical rotors negates this pulsing to a greater extent. The plenum volume equates to 95.4% of my engine size which I have found that the larger the blown engine the smaller the Plenum volume needs to be.
Thanks IceStationZebra and for the sites.
Looking forward to further input.

 

[PGCobra]

I agree Patprimmer, all helical rotor Blowers push the charge to one end - usually to the front casing ( gear ) end.
All Roots type Blowers such as these only have efficiencies of 35 to 60%.
Later Superchargers such as made by Eaton and used by Jaguar, Aston Martin for example, use 3 lobe high helix rotors of 60 degs. They have the Inlet at the rear and exit at the front, presumably using the natural tendency of the rotors to push the charge to the front.
The Screw type Blower is the best of the breed so far, as efficiencies of between 70 to 85% are being achieved and they are without any pulsing problems beset of so many 3 lobe Roots Blowers in varying degrees.

 

[MikeHalloran]

My guess is that if you swap the carbs side for side, the EGT asymmetry will follow.



Mike Halloran
Pembroke Pines, FL, USA

 

[IceStationZebra]

My mistake on the helical rotor comment - for some reason I had a "moment" and was thinking that the charge is discharged between the rotors. But of course we all know that the charge goes between the rotors and the case.

I'm leaning towards fuel distribution problems. It could be the 2" spacer under the blower since that charges how the airflow enters the manifold, but more likely it is the crosswise mounted cards. Were the secondaries opening when you were at the upper RPM's? Since the primaries are on the left it makes sense that the right side is running lean. Poor atomization could cause the fuel to drop down and favor the left lobe. One way to test it would be to put a spacer under the carbs which either allows you to shift the primaries more towards the center or elongate the opening so you can mount them front-to-back. Either way you should make sure that the fuel is being properly atomized as it leaves the carbs.

As far as "pinking" goes I just learned something new.
ISZ

 

[patprimmer]

Mount your carbies length ways and your problems will disappear.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[PGCobra]

Hi Mike,ISZ and Pat. Just not possible to swap the Carbs side by side, or inline. I did look into this but it's just not on - I'd get in all sorts of linkage/fuel line problems. BUT please read on as The End is Nigh.
I agree that I have a problem on the Primary circuit and I have given it thought and looked at this in greater detail.
Lets look at the premis that the Vac Secondaries discharge ok and the fact that the 2" Spacer/Intercooler does not have any adverse effect on the fuel(distribution) the bank of cylinders are receiving from the Vac Secs, the problem must lie inside the Carbs Primary circuits.
As these are Carbs set up for a Blower, I looked at the specs I had previously modified these Carbs to ( used them on another engine some time ago ).The Power Valve Channel restrictions had been enlarged by 0.015" each, to cure a 'bog ' I had when accelerating off the line.
The guys at the Dyno centre reduced my Jets, finally arriving at #72 Jets in the Primary circuits (which is standard for a normally aspirated engine !? ) as they found the Primary side running too rich at my previously installed # 79 Jets.
At the start of the 12 Dyno runs I undertook, the Dyno Printer threw a wobbly ( broke ), so I only had, until now, my own notes to rely on.
The print-outs show that on the 1st run the Carbs were overly rich at the lower rpm range - the A/F Ratio figures were 5.91 to a max of 7.08 - well, yes quite rich. When the final (12th) run was undertaken the Carbs had been reduced to #72 Primary Jets/#76 Secondaries, the lowest A/F Ratio achieved was 10.28 at 3,400 rpm but rising to 14.2 at 6,400 rpm!
It looks like they attributed the initial higher than normal richness to the Primary #79 Jets - remember the PVR mods, which not only reduced a previous 'bog', but will also give extra fuel when demanded and then reduces as demand is reduced.
Many WOT runs were undertaken and yes the Secondaries were fully open - this bothered me too so I made sure this was checked at each run.
It appears the Intercooler tube fins did prevent any chance of mixing of the charges, from each Carb section, i.e the charges must have taken a vertical flow down each fin face. If this is, as it seems, the case, then at least I know the hot charge from the Blower is in full contact with the Intercooler section and therefore should be temperature influenced by it.
When I get the car on the road I can check this, as I will fit temperature probes before and after the Intercooler.
I now know in which direction to go.
This has been a very valuable lesson - even after 40 + years building engines/racing cars etc you can still be suprised.
Many thanks guys for your invaluable assistance.

 

[patprimmer]

With Holleys, straight out of the box is a VERY good starting point.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[PGCobra]

Hi Pat,
yes I've heard of that ( was from Holley?), but when you get to the level of tune I'm into you have to do modifications not only as decribed before but :-
Vacuum Balance between the Secondary diaphragm housings.
Discard the 3/16" check in each diaphragm housing.
Pump Nozzles changed,as a start, to a 0.035"versions.
Needle and Seats changed, as a start, to 0.120" versions.
Obviously remove and throw away the bronze filters in the float bowls.
Change the vacuum diaphragm springs to, in my case, to purple types.
After all this then the fun starts by if you find an off-idle hesitation start looking at the idle feed restrictors and open them up a couple of thou at a time ( tedious or what !)until it vanishes.
If the fuel in the in the Carb idle slots dry up and give another hesitation then look at drilling small holes, about 0.090" and go upto 0.0125" ( going off memory on this one).
Cut-off/mill-off the choke towers and mix an epoxy resin up and then blend in all the air flow routes into both the Primary and Secondaries.
If you have a Secondary Metering Plate, throw it away and fit a Secondary Block.
Mess around with the Power Valves until you find one that operates at the correct vacuum reading from the Carb base.
AND that is only getting you started on serious Blower/Carb usage.
Thanks for your continued interest.

I have questions on propshaft designs if you are up to it, but I'll leave that for tomorrow - it's now 12.30 am in the UK and I must get my beauty sleep.