mike89-350

I have a 350 tpi and i'm gonna do a cam swap. while the intake is off i.m gonna port the plenum and gasket match everything. I've been seaching about siamezing the base but some people say it will lean out a cylender. Is this true or just a wives tale.

gta324

Quote from previous post about the subject.

Question:
-Would siamesing the base manifold show any gains on a superram(plenum, base, runners) engine? This engine would have been treated to cam, heads, headers, etc. I recall seeing a gentleman's website (Mike Davis) where he was using non siamese LTR's and a siamese base with incredible results.

MAX:
-Mike Davis blew that engine up twice and figures it was teh intakes fault. His theory was that 5 was stealing the fuel from 7 since they are next to each other in teh firing order. Both times the engine blew 5 was dark and rich looking and 7 is the one that popped (ringlands). His theory makes alot of sense. Extreme siameseing is probably a bad idea, especially if you are going to run the tune on the edge. He doesn't even own the intake anymore, another member here bought it.

Mike:
-Yep, MAX basically has the story.. My Exh valve on #7 was leaking and was blowing the intake charge back into the siamesed area of 5&7, which #5 was getting most of because of the firing order...

When the motor was rebuilt, I had to scrape 1/8" of carbon off of #5 because it was so rich, and #7 actually melted a forged piston it was so lean..

I don't tout siameseing the base any longer, but mostly because I hate pulling motors twice in 1 year.. 8-)

mike

Jimmy:
-My friends motor did the same thing after siameseing his base way into the intake

he siamesed the whole thing and i dont remember what cylinder went.....but he took it to the track and it didn't go any faster, said the car wasn't running right

then the car just "exploded" while cruising.....just driving along (lol, this sounds like a lie already) and KABOOM

I can ask him to post to this thread when i see him online

Answer#3
-I'm the guy Jimmy was talking about . #7 piston ate itself on mine . Honestly I did'nt give it a whole lot of though after it blew up other than the fact that a N/A stock shortblock , heads , cam runners and plenum car has no business blowing up like it did . That piston had a hole the size of half dollar in it . In the back of my head I was concered about air or fuel distribution problems and I guess that concer had mertit . Prior to the intake I never had a problem with the car . The engine wasnt a tired motor either , my Iroc had 55K miles at the time .

Interpret what u want from that . The intake ( siamesed almost al the way thru ) was on my car for one month before the carnage .
I improved from a 13.11 to a 13.10 and mph stayed the same right about 102 , which tells me I really want making anymore power .






I have a friend who has done it and hes rebuildning his engine now due to lean cylinder.

Guest

Still running mine. Still racing the car. Still no problems. Still no traction, either.

*sigh*

92HeritageZ28

ive been trying to research this as well. ive gathered that the people having this problem are the ones who siamese almost allthe way through to the head. it seems as though people who dont go as far into the base have less problems. i could very well be wrong though.

1989GTATransAm

iTrader: (3)

I'am siamesing the SLP runners on mine at both the plenum side and the intake side. One side on each set is already fully siamesed from the factory. I'am leaving the base alone except for the Extrude Hone and port match. This way the runner length will be close to the LS1. Allen

BowTieBear

I too have experimented with siamesing the lower manifold. It seemed to work well at hight rpm but out of the hole there was a huge tendancy for the car to fall on its face unless you back the throttle off and get into it gradually.

The first real track time with the car was at a Super Chevy show here in Pomona 2years ago, the first e.t. i logged was like 16.2 with a trap speed of 108!!! The car literally spent 3 to 4 seconds stumbling on the starting line.

I thing the reversion waves traveling back and forth in the runners are what contributed to the hole on the line. As the engines rpm increases the reversion wave gets shorter. I'm thinking that at off idle WOT the wave was traveling down the tubes to the area where the tubes and the lower join and this was breaking the wave up causing a severe amount of turbulance. Then as the intake valve opens theres a large vacuum thats pulling fuel from the next injector (as it is a batch fire system). My car ran super rich on the bottom end. but on the top end it made all the power in the world.

Before you ask, this is a heavily modified 358ci with ported merlin heads a custom grind cam and 12:1 pistons.

Guest

I ran into that on my 305, but adjusted it away with a timing change.

vertigo42

Hey i know im new here but i used to owna 85 z 28( sold it worst thing i've ever done) withthe 305 comp quadajunk in it,( now looking for an 88 irock with tpi 5.7) any way....

You guys that are toasting your #7 cylider because of 5 robbing the feul just befor e 7 fires on your siamesed intakes ever thought about puting in comp cams new cam
It reverses #4 and #7 firing order so that way it goes
1-8-7-3-6-5-4-2
the engine is suppose to run cooler and have less vibration and S**t
Comp says:
"Dyno test shows an exceptionally smooth torque curve and decreases in cranckshaft deflectionn when the 4 and 7 cylinders are revesed that mesans more horsepower, cooler temps and less wear and tear on your engine, no additional wengine mods are necessary"

Only for SBC and BBC

I have been doing a little bit of ready on this and the horsepower gains are nly like 5 to 15 in SBC to see bigger gains ou have to have like a 1000hp motor, any way they are in the new summit mag and run right at 400 the temp drop is noticeable like 10 to 20 degrees

This should stop #5 from robbing #7 from all it fuel due to it is not firing right after 5,

Hope this will help, I will try and read up some more and post what else i find on this
Thanx


Ok i did some searching and it seems some of you have already been discussing these, they make power on the upper rpm band , unsure about low rpm, anywya just do a search Ror:: Firing order swap:: and it comes up with a few links to it, the only other thing you have to do is switch the #4 and #7 plug wires on the distibuter.

Travis

Super_GTA

IMO those cams are junk. They don't solve the problem of fuel starvation between cylinders, they just move it. Look closely and you will see that cylinders #4 and #2 fire right next to eachother. They also share the same pair of runners. So you solve the 5 and 7 problem, but now 4 and 2 have the same problem. Waste of money, IMO.

Guest

I pulled the engine to swap heads and some other stuff (have 15k miles, most of those on the siamese intake), here's the inside picture of whats going on in my engine, 7 5 3 1 in that order.


I should mic the distance from the intake face at the head to the cut I made, I am still not having any lean problems.

BowTieBear

i'd like to know what that distance is. i think i cut in about 4", it would go rich at low throttle.....are you using the stock runners?

davel98

Just a little info from the Corvette forum on the same issue. A member cut 1" deep and 2" deep, and comapred it to stock. Now, with the siamesed base, it runs at around 14:1 A/F ratio instead of 12.5:1 A/F. So take that into consideration:

RPM......Stk/1"cut/2"cut
3,400....210/200/188
3,600....220/210/200
3,800....230/220/210
4,000....240/230/218
4,200....245/235/225
4,400....227/237/230
4,600....247/240/233
4,800....243/240/233
5,000....233/237/230
5,200....220/233/222
5,400....200/225/212
5,600....180/215/200
5,800....160/205/187
6,000....135/190/173

Now, it would seen that anyone who does serious siamesing may be loosing power. The depth of the cutout seems to kill the velocity of the airflow, and therefore may cause a loss of power, espically with a stock base.

BowTieBear

what are the values???....hp?

davel98

That's RWHP.

BowTieBear

no motor mods??..any idea what the cam is?

davel98

Stock Corvette L98 cam. No major internal mods (stock heads, displacement, pistons,etc)

Mods are:

- Custom Mufflex mandrel-bent SS front Y-pipe: 2 1/2" into 3".
- Random Technology hi-flow cat.
- TPIS mandrel-bent 3 1/2" into 3" rear Y-pipe with custom FlowMasters.
- SLP Ram Air intake w/ scoop opened an additional inch.
- Comp Cams 1.6:1 ProMagnums w/ 7/16" studs, springs w/ Ti retainers.
-AFPR
- HyperTech coil/cap/rotor

1989GTATransAm

iTrader: (3)

247RWHP is not bad at all for the mods. Shows what a good exhaust system can do. I bet the 1.6RR rockers helped some to.

Your right about the air fuel ratios. It's my understanding that a air fuel ratio of around 13:1 or so should be about right for a NA engine. Around 11.7 for forced induction.

As far as siameseing goes I will just stick with the runners and plenum. I Extrude Honed my Edelbrock base. I will be posting with pictures my further mods. I think they will be interesting to all. Allen

Edit: Forgot to add that the early factory SLP runners came with one set of runners fully siamesed from one end to the other. I have had a set on my car now for around 50,000 miles with no problems.

novass

iTrader: (1)

I have run my 305 siamesed for about 5000 miles, I went about half way in the lower intake manifold. I had to remove the drivers head this year and it looked fine. No signs of rich/lean.

BowTieBear

your force fed....changes the dymanic of whats going on

novass

iTrader: (1)

Only for about 200 of those 5000 miles.

Tibo

iTrader: (15)

If you are really interested in doing this the best thing to do is to siamese it and just watch how your car reacts, plugs, mileage, etc. You can always go deeper, not back though. start with 1" and go from there, make sure to do the other free mods as well.

QuickStyle

i dont see how siameseing your intake could rob fuel from a cylinder just because it fires right after it, the injectors shoot the fuel right in the head itself, so there is no fuel inside the intake, so how is it possible for #5 to take the fuel from #7

novass

iTrader: (1)

The fuel is injected into the intake manifold, pre siameseing, therefore it could draw fuel vapor from the other side.

You might be thinking of siamesed runners, these are before fuel.

QuickStyle

the fuel is injected into the manifold, but its on the side closest to the head, so the injectors are actually spraying the fuel onto the valves, im pretty sure that when you siamese the intake that you dont do it all the way through to the head, only about half way down the intake passages. so in order for it to take the fuel from the other cylinder, it would have to travel backward from the direction of flow, and around the point where you stop porting, and then into the other cylinder.

Guest

No it isnt.

KiLLJ0Y

uh... how would it draw mixed fuel/air all the way out of the cylinder back up the runner, make a right turn, go back down the other runner into the cylinder?? not to mention get out fast enough before the intake valve closes??

i dont get it..


am i misunderstanding the quote?

davel98

The injectors spray the fuel right where the manifold reaches the head. It also directs the nozzle toward the heads, so the only possible way I could see the motor being starved for fuel is if you siamesed the base completely, taking the entire center section all the way through.

This is also possible because the motors use a bank fire system, so when the one valve opens, it may not be the same time the fuel injectors have opened.

novass

iTrader: (1)

Aww ****..... I got my *** backwards. You guys of course are correct.

Tibo

iTrader: (15)

Extreme siamesing can rob AIR from one runner/cylinder, not fuel.

QuickStyle

explain that please, thats not possible either. siameseing shares two runners to one intake port, so while both are feeding the #5, 1/3 the way through it pulling air, the #7 opens up, and #5 and #7 are sharing the air from both runners, and 1/3 the way through that the #5 closes, and #7 takes all the air. the piston is only gonna take as much air as possible, so it wont be able to take all the air. hope this makes sense, its kind of hard to understand, but its the best i could explain it.

Tibo

iTrader: (15)

I think you need to reducate yourself with what happened to some of the peoples engines that have done the extreme siamesing. Cylinder numbers five and seven are the problem area. Since they fire right after each other and are sharing an intake passage way #5 will pull the most air, leaving #7 with not as much air to pull. #5 will pull part of the air in intake runner #7, leaving it empty in the middle. #7 Will then pull the air that is not as much. Some people have found this to happen by looking at plugs or their cylinder heads.

QuickStyle

when #5 is pulling the air from #5 and #7 runners, more air is right behind it, so it is never empty. and after #5 is finished pulling the air from both runners, #7 pulls airs from both of those runners. imagine this.... the plenum is to feed all runners, so how come all the runners arent starved for air? is #1 stealing air from #8, they fire right after each other? the answer is no, because after #1 pulls the air, more air comes in so #8 can get enough air. and as for other people haveing problems with theirs, maybe they did something wrong, i dont know. but i have had my intake siamesed for about a year and a half, and the only problem i have had with it is loss of traction. plugs are fine on ever tune up, and about 4 months ago when i pulled the heads off, i looked at all the pistons, they all look the same, no signs of damage.

Tibo

iTrader: (15)

Because runners #5 and #7 cylinders that will now share a runner and fire right after each other. You still need to keep in mind that the people who had trouble had their intakes done with only 1.5" left in the intake. not only 1.5" siemesed into the intake.

QuickStyle

all runners share the same plenum, so why aren't all the runners starved for air?

BowTieBear

it does just that....because of the reversion wave that tuned port intakes make. when the intake valve closes a high pressure wave travels back up hte runner. and because the injectors are batch fire and not sequential, the open valve gets too much fuel making a too rich condition untill the motor builds enough rpm to overcome this. this is why on some motors with siamesed lower manifolds fall on their face at launch. then you lift alittle bit and the air builds alittle velocity in the runners, then you can mash the gas again. now it seems that there is all the power in the world because the effective runner lenght is really short and the motor is drawing from a huge pleanum.

Tibo

iTrader: (15)

Because not all of the cylinders fire right after each other and can pull right from the other before it can recover.

Tibo

iTrader: (15)

Because not all of the cylinders fire right after each other and can pull right from the other before it can recover.

QuickStyle

no, what i mean is.... why isnt the #8 starved from the #1 firing right before it, because they all share the same plenum, and why isnt the the #4 starved from the the #8 firing right before it and so on....... it doesnt matter if they are right next to each other. i could understand your theroy if it was not a fuel injected vehicle, because their is fuel before the intake, but on a fuel injected vehicle the fuel goes directly into the head after the intake.

BowTieBear

its because these are batch fire and not sequential. all the injectors of one bank of cylinders are energized to fire. the thing that makes one injector spray more fuel tha thte others is the diffrential pressuer drop that occurs when one intake valve opens and hte others are closed