I have are the two different runners for my TPI setup, stock tubes and one set of aluminum Siamese pairs with larger bores.

The stock tubes are 1.52 inches, the stock intake manifold is 1.45 (inside) so there is already a slight compression happening, so the aftermarket tubes are 1.62 will they increase this ramming effect of worsen it??

I wonder IF the large runners to the intake might produce more of a ram effect when they hit the stock manifold over the stock smaller tubes.

Could there be an efficiency loss as the air is going to compress and heat up as it does.

Then again that might work better, A) this compression could raise the pressure a little and B ) hot air is supposed to help cause more vaporization of the gasoline?

The cast aluminum Siamese large set has a light sand casting texture where the factory seems slightly smoother.

Also there is the opening between pairs at the face, more like a opened premium four barrel manifold...I wonder what effect that would have??

I would think that will allow cross breathing and perhaps lower the controlled force induction allowing them to share like that.

I would be of the opinion that the relieved port might actually be to benefit larger camshafts but I plan on running a stock or near stock cam..

In theory the slots would provide a smoothing of vacuum across the ports, I assume is a concern when the cam is aggressive and potentially not great for vacuum.

So as I will be using a smaller cam should I have them filled in??

The $64,000 question is will they (the larger tubes) harm or help my MPG running a small cam??

I am building a 350/383 for low ROM power for greater MPG.

Or will these throw off the whole idea of TUNED port injections air flow at low RPMs??

Rich

 

Now this is just hearsay and theoretical, so for what it's worth, free advice and all that....

As a general rule, the impression I get is that the runners aren't the real bottle neck in the TPI, that distinction falls upon the base manifold. From what I've seen over the years, SLP runners are pretty much wasted on an otherwise stock setup. The only big numbers coming from their use has been when they're combined with serious gutting and rework of the base. Otherwise, all you're really doing is something of a half-assed increase of plenum area available to adjacent runners, by opening up the space between the runners. Now plenum area has never really seemed to be a problem either, so that leaves the sorta kinda effect of kinda effectively shortening the runner length and changing the 'tuned' effect. Whether that is worth anything is open to your own interpretation. I'd expect that any positive effects gained by using a siamesed runner is going to be rendered moot unless the intake is dealt with, but **** dawg, I don't know. At least I know that I don't know, that has to count for something right? Carry on.

 

this. Siamesed runner acts like a shorter length runner. Pushes peak hp rpm up and should also shift torque curve some.

 

Here's the $128,000 question (BLM pun). If introducing more air into the combustion chamber, either by increasing the velocity of the air, or increasing the plenum volume, calls for the compensation of additional fuel to maintain a particular air/fuel ratio, then why would you think your mpg will not be effected? More air requires more fuel, which means more money spent at the pump. It may not be a drastic increase in terms of money spent, but a better breathing engine will most certainly harm your mpg, not help. Could always make up for it with gearing, and a leaner tune at idle and light/moderate throttle...

- Rob

 

In my experience a better breathing engine almost always gets better MPg provided you accelerate the same way as the less powerful engine. When you use the extra power that is when your MPG tanks.

The L31 based engine in my Express van is making 475 hp and I am running a 4L85E and 10.5" rear with 5.13 rear gear. My highway MPG is almost identical to what the 245 HP L31/4L60E/8.5" 3.42 gear did stock with a tuned 0411 pcm. I say almost because the thing sits nearly 4" taller now and only gets 1/2 MPG less despite pulling around another 500 lbs. The MPG drop was not the drivelines fault per say. I now turn 2,800 rpm @ 70 compared to 2,000 rpm @ 70 stock too. The difference is I almost never drop out of lean cruise and its 16.5:1 air/fuel ratio where it almost never went into lean cruise with the stock setup. Stock setup would only run in lean cruise between about 55 and 70 mph on level road or downhill and on days with no head or cross wind and not running the a/c. Now it doesn't seem to care. 80 mph, uphill, headwind it doesn't care. 18-19 mpg highway is what it has ALWAYS gotten with a few tanks almost getting 20 mpg.

 

I just finished a build with oversize runners, AS&M large tubes, not Siamese. I did port my base and plenum. That 1.48 opening after the runner pulse will do nothing but hurt you. At that point in the system you want flow. I also feel the plenum needs to be open to the 1.62 also. This may not be as critical as the base but I feel it is. The "venturi" is in your TB so after that the logic is get as much air in as smoothly and quickly as possible for max HP. My speed density system seems to be doing a good job after the influx of more air. I have not datalogged but I did a plug test and they looked good, if anything maybe a little lean. If I drive like I did before the mods I think the mileage remained the same. It is on the decrease the more you increase the throttle but the HP increase you feel so this is normal. On the highway I think the MPG is the same, maybe 1 or 2 MPH better? I have not driven alot on the freeway. The biggest difference I see is my RPM's have dropped @ cruising speed. I used to run about 3k to do 80mph on my speedo. Now it is about 2700-2800 to maintain the same speed.This is consistent at other MPH's. So I will assume that I will get a mileage increase when just cruising.

Doing the complete package is the only way to go IMHO, the runners are just a piece of the restrictive puzzle that is TPI and unless you address it all It is like you a robbing Peter but not paying Paul.

https://www.thirdgen.org/forums/tpi/...the-build.html

 

Vinny R, you did some good work on the porting and I'm sure there's a noticeable increase in power and throttle response. That said, RPM to MPH is what it is. There's only a few ways you could drop RPM and be going the same MPH, like a gear or tire change. The only other thing I can think of is going from a bad/slippiing stall to a properly functioning one, or a high stall converter to a more efficient low rpm stall. If no other changes, then maybe you're lock up function on the stall converter wasn't functioning properly before and is now (after the mods)?

Regarding the OP, there's not much benefit going with the SLP runners unless you open up the siamesed area, match up and port the plenum, and at the lease take out the lip/bump at the runner to base point, Vinny R took a really good shot showing the lip/bump right at the runner entry point (below). His link to the work he did should be very helpful.


So, IMHO, unless you're willing to do the work to the SLP runner, plenum, and base, you're probably better off with just sticking with the stock runners.

 

We don't drive around at WOT all day. Air volume is controlled by throttle.

An engine that has to work less to get air into it can output more torque/hp at any given throttle position with the same air/fuel due to less energy being used to suck air into the cylinders. Less loss equals more to the crank... better MPG.

This doesn't apply equally to WOT. While yes, working less will allow more output to the crank, but other dynamic effects have to be accounted for that matter more.

 

bigger runners shouldnt affect your mpg just go to town on the base...easy to over think things.
Can remember spending crazy amounts of hours on those..yikes.

 

Also just so folks know Arizona Speed and Marine still makes the big tube TPI runners... They do them to order so it takes a couple of months to get them but they are the same price as all the used ones I've seen for sale about $520 shipped.
Plus since they are made to order you can get them with or without the cold start hole ect....

 

Those are real nice good to see they are around. See used beat up ones asking more than new price lol