Why we have lean cylinders

WyoSyclone

WyoSyclone

Active member
Okay, so I'm really anal, and I've been studying our cylinder heads for about three weeks now :D

I got to wondering why we have lean cylinders (#1 and #6 from what I remember people saying, and what my sparkplugs tell me). Everyone seems to think it's our intake manifold that is the culprit.

I think I've found the real reason... just pistol whip me if this has been discussed previously....See this picture, and see the red arrow. Look at that damn recessed intake port divider wall.

So, we have a manifold that seals up tightly against 1 and 6, and the rest of the cylinders are left to struggle with 'leakage' between intake ports... who's the brainiac that thought of this design?? #1 and #6 get more air.. run leaner, the rest have to deal with the disturbances in flow and volume.


This isn't a very good pic, but those of you who have seen one in person know about this 1/8" gap between the manifold and the intake ports.
14098768pic2-1.jpg


14079248pic1.jpg
 
BoostedSUV

BoostedSUV

Active member
Re: Why we have lean cylinders

Interesting theory and one that certainly has some basis to go by. Maybe someone will chime in with some relevance to the gap in between the ports but I can't make it make any sense? None of my race engines are like that, they all seal around each and every intake port
 
J

jpalmer

New member
Re: Why we have lean cylinders

are sbc's the same way then??? I don't have a set of heads around right now cause my buddy bought them off me. I think i'll have to give him a shout.
 
SyTyAddict

SyTyAddict

Member
Re: Why we have lean cylinders

That's interesting, i've noticed that before, but never put much thought into it. If that really turns out to be an issue, it wouldn't be that bad to have fixed if the heads are off. Just some welding, grinding, and milling; though cast iron can be a pain to weld on.
 
WyoSyclone

WyoSyclone

Active member
Re: Why we have lean cylinders

jpalmer said:
are sbc's the same way then??? I don't have a set of heads around right now cause my buddy bought them off me. I think i'll have to give him a shout.
Click to expand...

If they're the right year/casting number, then they would be the same. I've got a 2-page spreadsheet built that gives all the relevant information for the LB4/L35 head designs (there were some real surprises in there for me).

Naturally aspirated motors wouldn't see the issue as severely as we do with boosted motors.
 
It's just a six

It's just a six

Super Member
Re: Why we have lean cylinders

When siamessed ports like on my inline 1969 6 250 Chevy gets fuel injected,the intake port next to the actual port injesting the A/F mixture it takes fuel from the injector next to it,causing it to be rich,when guys have devided the siamessed ports completely (& left the fueling the same) ,they need to pull fuel out ,because they become too rich,kinda confusing,but when the guys made them individual ports,the intake port window gets really small & draws in less air because of this.

For sure, I would devide the ports completely,so they will get close to the same fuel mixture.
 
J

jpalmer

New member
Re: Why we have lean cylinders

Ok, my Vortec's have no gap, and the sbc's ii had have no gap, but i am sure that the same/similar year sbc could be the same way. More fuel on the fire for Vortec conversion. ;) or just weld up and machine off the excess.
 
WyoSyclone

WyoSyclone

Active member
Re: Why we have lean cylinders

jpalmer said:
Ok, my Vortec's have no gap, and the sbc's ii had have no gap, but i am sure that the same/similar year sbc could be the same way. More fuel on the fire for Vortec conversion. ;) or just weld up and machine off the excess.
Click to expand...

Yup, Vortecs don't have the gap... and neither do the 'good' L35's :D
 
the baPhoon

the baPhoon

Active member
Re: Why we have lean cylinders

Are the "good" L35s the ones discussed a while ago with a combustion chamber similar to Vortecs?
 
M

MikeRenz

not stock
Re: Why we have lean cylinders

Based on your theory of the siamese ports fighting each others fuel intake...it would be these cylinders showing signs of fuel starvation/lean conditions. Its the single port that ends up showing lean when running.

I'm no fluid/air dynamics expert...but...fuel/air isn't going to make a sharp turn easily from the intake into the head like that. I can see under pressure equalization that it could back flow into the adjacent port minimally..but not drastically. Imo it doesn't explain anythying happening to the single port, which is the culprit.
 
Foot Performance

Foot Performance

Donating Member
Re: Why we have lean cylinders

jpalmer said:
are sbc's the same way then??? I don't have a set of heads around right now cause my buddy bought them off me. I think i'll have to give him a shout.
Click to expand...

I have never ran a cross a small block head like this. I have never had our intakes off to see if they are all like this. I have recently had 3 v6 engine in my shop 93-95 all were counter shaft engines and only 1 had the recessed intake port. I have also seen this same recess on some v6 intakes.

The only way I could see 1 and 6 running lean is because 2 3 4 5 basically get to get fuel from 2 injectors in our batch fire setup. so those 4 "could" run rich (especially since no cylinder fire one right after the other like a small block chevy) when 1 and 6 were at a normal afr. compensating for the other four cylinder makes number 1 and 6 lean out

Do the vortex guys have any issue with 1 and 6???
 
Last edited:
WyoSyclone

WyoSyclone

Active member
Re: Why we have lean cylinders

MikeMuffins said:
Based on your theory of the siamese ports fighting each others fuel intake...it would be these cylinders showing signs of fuel starvation/lean conditions. Its the single port that ends up showing lean when running.

I'm no fluid/air dynamics expert...but...fuel/air isn't going to make a sharp turn easily from the intake into the head like that. I can see under pressure equalization that it could back flow into the adjacent port minimally..but not drastically. Imo it doesn't explain anythying happening to the single port, which is the culprit.
Click to expand...

The single ports are completely sealed to the intake, with no 'leakage' via the recessed divider wall. More air flowing due to the sealed path equals a leaner mixture in the single ports.
 
WyoSyclone

WyoSyclone

Active member
Re: Why we have lean cylinders

It's just a six said:
What is the PN for the good L35's?
Click to expand...

I'm still working on it. I've discovered that the ones that everyone thought were the good ones, were actually made with a few pretty major design differences. It's going to take me another week to get to the bottom of it.
 
Windedv6

Windedv6

Ty n 10s
Re: Why we have lean cylinders

The lean issues come from the poor intake design at higher boost/higher flow situations. Flow Tech, the company that originally ported my vortec heads and stock intake setup (and supposedly did the head intake work for the Stokes motors) explained to me years ago that the syty intakes were not designed to flow air for higher hp applications. They were originally side loaded in the front corner to only accommodate the intercooler location. This original design never was a major problem under stock conditions and the design worked correctly for emissions which were a major factor to get these motor approved.

The problem starts to happen when the motor needs higher air flow. The intake is not designed to distribute equitably at higher flows. Flow Tech went as far as to port differently on several ports to help compensate for this. Although it helped, I would still have #1 and #3 go the leanest. We even tried a little fatter injectors and colder plugs in those cylinders. I know ppl that cut out the center “hump” to help with the air flow. The center hump is only there to help accommodate the bolt installation. Again its not that the design was poor for the original application, it just didn’t need to be designed beyond the original application. Once the designers got the runner sizing and the volume correct for idling, emissions, drivability, the rest was for fitment and serviceability.

Think about it, in 99% of cars, the oem intakes are changed out for after market intakes when looking to improve performance, changing heads are close behind. GM, Ford, etc has the capabilities to design more performance in their intakes and heads, they just have no reason to. They anticipate what the market will absorb, what the government regulations will allow and then value/engineer for profit.

Years ago before cometic gaskets came on the scene it was common tfor us to blow the felpro gaskets when running 22 -26 lbs of boost. The air fuel was ok as an average (even fat), but some cylinders were obviously lean and others rich. Most engine builders for our trucks compensated for this by recommending a low 11/high 10 AFR and low 20s on timing. When I went to the single plane intake we could get the plugs fairly consistent. With those intakes we had the cylinders running fairly even and were able to run 28-30 lbs of boost in the 11.8-12.2 afr range with 26 -24 degree of timing. We weren’t blowing head gasket anymore either.

As to the original question about the recessed port wall. A lot of Siamese runner setups, in oem motors had open gaskets between the two cylinders which is similar.at recessing the wall. It may have been to control the ports’ pulse wave between the two cylinders. By allowing it to bleed across mid port it may be less problematic than at the entrance of the intake port as wave (being side by side) could affect the air entering in to the neighboring port. Intake runners have a pulse wave caused by the intake opening and closing of the valve.

John
 
Quickstop [UK]

Quickstop [UK]

Combating adversyty.
Re: Why we have lean cylinders

Are there any setups that allow you to control the injectors on a per cylinder basis to combat this effect?
 
Windedv6

Windedv6

Ty n 10s
Re: Why we have lean cylinders

Quickstop [UK] said:
Are there any setups that allow you to control the injectors on a per cylinder basis to combat this effect?
Click to expand...

FAST and BS3 both have sequential modes that allow timing and injector control for individual cylinders. I use it on my TT motor. You have to be setup for a crank trigger ignition.

John
 
WyoSyclone

WyoSyclone

Active member
Re: Why we have lean cylinders

Windedv6 said:
The lean issues come from the poor intake design at higher boost/higher flow situations. Flow Tech, the company that originally ported my vortec heads and stock intake setup (and supposedly did the head intake work for the Stokes motors) explained to me years ago that the syty intakes were not designed to flow air for higher hp applications. They were originally side loaded in the front corner to only accommodate the intercooler location. This original design never was a major problem under stock conditions and the design worked correctly for emissions which were a major factor to get these motor approved.

The problem starts to happen when the motor needs higher air flow. The intake is not designed to distribute equitably at higher flows. Flow Tech went as far as to port differently on several ports to help compensate for this. Although it helped, I would still have #1 and #3 go the leanest. We even tried a little fatter injectors and colder plugs in those cylinders. I know ppl that cut out the center “hump” to help with the air flow. The center hump is only there to help accommodate the bolt installation. Again its not that the design was poor for the original application, it just didn’t need to be designed beyond the original application. Once the designers got the runner sizing and the volume correct for idling, emissions, drivability, the rest was for fitment and serviceability.

Think about it, in 99% of cars, the oem intakes are changed out for after market intakes when looking to improve performance, changing heads are close behind. GM, Ford, etc has the capabilities to design more performance in their intakes and heads, they just have no reason to. They anticipate what the market will absorb, what the government regulations will allow and then value/engineer for profit.

Years ago before cometic gaskets came on the scene it was common tfor us to blow the felpro gaskets when running 22 -26 lbs of boost. The air fuel was ok as an average (even fat), but some cylinders were obviously lean and others rich. Most engine builders for our trucks compensated for this by recommending a low 11/high 10 AFR and low 20s on timing. When I went to the single plane intake we could get the plugs fairly consistent. With those intakes we had the cylinders running fairly even and were able to run 28-30 lbs of boost in the 11.8-12.2 afr range with 26 -24 degree of timing. We weren’t blowing head gasket anymore either.

As to the original question about the recessed port wall. A lot of Siamese runner setups, in oem motors had open gaskets between the two cylinders which is similar.at recessing the wall. It may have been to control the ports’ pulse wave between the two cylinders. By allowing it to bleed across mid port it may be less problematic than at the entrance of the intake port as wave (being side by side) could affect the air entering in to the neighboring port. Intake runners have a pulse wave caused by the intake opening and closing of the valve.

John
Click to expand...

Thanks for that info, John.... I appreciate the fact that you're willing to share this 'been there/done that' knowledge :tup:

So to summarize, you were seeing uneven flow with your ported Vortecs and stock intake, and some things that you did helped but not entirely, until you went with the single plane.

I'm very familiar with intact tract reversion, standing waves, etc. that little gap between 3/5, and 2/4 actually creates kind of a helmholtz resonator... but probably only effective in N/A applications.

L35's and Vortecs all did away with the recessed divider wall, so I kind of assume that we may be seeing two issues - the 'compromised' factory intake design, and the recessed divider wall that probably served a purpose on the originally-designed-for N/A engines.

I started wondering about all this because of the almost 3 years of tuning I've been doing on my Sy (stock except for Stage1, 40lb injectors, IC upgrade)... running 15lbs boost/15 degrees timing. My 1 and 6 plugs always seemed a tad lean compared to the others, so I started looking for answers. I also wonder if that recessed divider contributes to fuel puddling in those cylinders?

Once again, thanks for your valued input. I've studied Mike's posts regarding the intake manifold work he has done, and appreciate that as well.

...Rob
 
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