engine upgrades

[Guest Recian]

I'm gona be going to the junkyard again saturday. I'm thinking of small things i can do to the engine to make it better. Coming from the import side of the automotive world I know polished intakes, open exhaust (in some cases), bigger t-body, stuff like that to add more power without forced induction at the cost of nearly nothing from junkyard runs. Im looking to take stock GM parts to modify for use on the reatta. Im just curious before I go if anyone's done anything to that effect yet?

[Barney Eaton]

The firewall exhaust header has a restriction. The picture posted is a 1991 but all years have the same problem. On the one pictured, I calculated that over 20% of the down-pipe is restricted. It looks like they had a rat nibble the hole in the header where the down-pipe attaches. Some owners have used a hole saw, but most of us do not have one that large. A long burr in an air grinder will also work. If you have the time and skills, the manifold could be modified for better flow.

This restriction would not affect 95% of Reatta owners because they don't demand that much performance from their cars. But if you are looking for cheap increases, this one is easy.

[Guest Recian]

That was one of my ideas. I'll find another manifold since one stud is so rusty its become one with the manifold. That isn't a problem it just makes removing it in the car a pain. I'm going to get the intake with the 3800 stamp for the looks and polish it then match it to my intake ports thenglass blast and paint it. I was thinking of a big throttle body upgrade. I've done it on imports and getting more air in at less throttle makes a difference. With that other intake ill probably remove the coolant lines for the t_body making swapping for experiment easier and it also allows cooler air into the engine. I don't have to worry bout it freezing here in nc. Besides its by a hot crossover pipe so I'm sure it gets too much warm air.

[2seater]

If you can weld, this is a modified rear manifold in progress. This was done to help the exhaust flow from the front mounted turbo, which it does. It does not help the poor flow from #4, which would require the same mod. on the other side. Just removing the restriction as Barney suggested does help, the flowbench confirms that. The whole stock system actually is pretty evenly matched although #4 is the worst due to the poor design. Easy to see why when you look at the location and angles used.

I have a ported version of the stock throttle body, 2mm larger, with a new throttle blade. The biggest hurdle is finding a MAF and throttle body combination that is less restricted, but uses the same frequency response as the stock system, 2kHz - 10.4kHz, at the same flow rate. The 3" inline MAF from a Chevy 3.4 (I think that's what it fits), has the correct frequency range, and flows about 1.55 times the mass at the same frequency, requiring chip recalibration. The stock throttle body and MAF flow pretty much the same wether connected to each other or separately, so GM did a good job matching them. I am presently using the MAF/Throttle body (a one piece unit), from the aluminum version of the TPI manifold, and while it looks like it flows better, it really doesn't. I am using it for convenience and location of the TPS and IAC. As a matter of fact, the aluminum TPI manifold is slightly worse flowing than the LN3, but better balanced. I have not tried the plastic version, which is supposed to be better all around.

I have fiddled with changes and "improvements" for about fifteen years (off and on), and while incremental improvements are possible, maybe 15- 20hp or so, boost, or a more aggressive cam, are really needed to make a big difference.

[Guest Recian]

wierd that small mods like that wouldnt effect anything, not even slight MPG improvements? I have access to most parts from the junkyard

[2seater]

They do make small improvements, just not a dramatic difference. Mileage is pretty much unaffected, unless of course you "use" the improvements regularly. High flow catalytic converters really do flow more, about 40%-50% more. A gutted stock cat. only picks up flow by 30% or so. A smooth cone filter type intake helps on the intake side. All of these things really do allow a greater flow potential, however, much potential is limited by what the engine will actually ingest and expel, and the cam determines most of that. Aside from the butt dyno, a g-tech or racetrack testing, the one onboard way to see if you are actually gaining airflow through the engine is to watch the MAF reading under the same ambiant conditions. The best stock cam is the 1988 version of you can find one.

[handmedownreatta]

high flow cats flow better than empty cats?

[Guest Mc_Reatta]

Thought we decided that a cat that flowed more than stock decreased the ability of the O2 sensor to properly controll the mixture and was counterproductive.

[KDirk]

MC-

Not sure that was ever agreed on, but the theory makes some kind of sense in my mind. Of course, I am not an expert on internal combustion engines. I know enough to maintain and repair them. I do not possess the detailed knowledge to design proper performance mods.

I would like a definitive answer to the issue of car flow as it relates to the proper functioning of the O2 sensor, and thereby mixture, as it may dictate whether I should put a cat back on my 91.

KDirk

[2seater]

I have never heard of lack of backpressure affecting the O2 reading? An air leak, either upstream of the O2, or close to it downstream can certainly give false readings. I have been told that the O2 sensor should not be located upstream of a turbocharger as the high pressure does have a negative effect on the accuracy. I have seen many examples of O2 sensors located in the exhaust headers of engines on a dyno for testing (probably wideband) but I would believe that would be a low pressure application? I ran without a cat for more than ten years without discernable problem? Conventional wisdom states 2.2 cfm per horsepower @ 28"wc is required for a zero loss exhaust system.

I have posted similar information before but this is a synopsis of what I have found over the years. Please do not take the absolute flow numbers as gospel, but the relationship of one to the other should be accurate. All tests @ 28" wc. unless noted.

Stock cat.: 236.1 cfm

Gutted stock cat: 322.96 (turbulence?)

Walker 2.5" universal cat: 354.64

Straight 2.25" pipe: 495.0

Stock TB/MAF w/o screen: 470.24

TPI TB/MAF w/o screen: 452.15

Install screen in MAF = -25cfm

Rear manifold flowed with everything plugged except the front crossover pipe:

Stock: 241.52

Stock ported (also ceramic coated): 326.81

Modified w/funnel (per photo): 364.21 @ 20" of water. Actual flow exceeded capacity of 2" pitot sensor scale so test pressure was reduced.

EDIT: I only reinstalled a cat. because I tired of the smelly exhaust but then discovered a blown trans. modulator (probably from boost) which allowed the engine to burn ATF.

The replacement cat in my example is not sold as a high flow, but it is the actual area of matrix inside that will limit the flow. I suspect a 2.25" through 3" cat. with similar size bodies will actually have similar flow numbers. I used a 2.5" simply for a possible future exhaust upgrade. I still use the stock 2.25" complete exhaust system save for the cat. and a 2.5" Dynomax Ultraflow straight through muffler.

Edited February 17, 2012 by 2seater (see edit history)

[Guest Recian]

I had decided and those numbers lock in that decision. I'm going to take my parts car exhaust cut out the cat and have the muffler shop weld in a straight pipe that length. If they ask I'm just extending my exhaust. They don't know it had a cat that will give the best flow rate and then I can just swap over exhaust systems which may sound tough to most but its easiest way for me.

In regards to o2s. First off all an o2 does is switch from rich to lean based on the combustion how efficiently it burns the fuel. Knowing that flow will have no effect. As more gas passes by there's more air but also more hydrocarbons. The ratio of air to unburnt fuel will still be the same just having more flow out allows it to suck more in. I've gutted cats on all my cars with more mpg and more HP gain in addition to a deer sound when coupled with the right exhaust and more responsiveness in acceleration.

[J3Studio]

They do make small improvements, just not a dramatic difference. Mileage is pretty much unaffected, unless of course you "use" the improvements regularly.

Ain't that the truth!

Cool thread.

[2seater]

Recian, if your exhaust is still stock, a chain type exhaust cutter works well. The exhaust is expanded somewhat on each end of the cat. and there is a resistance weld in that expanded area. If you cut right next to the weld, on the side away from the cat., the cat will slide right out. That will leave a short section of expanded exhaust on each side and a simple straight piece of 2.25" exhaust tubing will slide right in and can be muffler clamped in place, or, the cat can be reinstalled if desired. I used s/s pipe from Speedway Motors.

[Guest Recian]

im going to take your pics and advice to a buddy's shop and have him do those rear header mods to increase it's flow and cut out my cat and have the muffler shop weld in a straight pipe in it's place so it's just smooth high flow. If i can make that series1 supercharger work then we should be looking at a pretty beefy upgrade to stock with less than $100. The junkyard offers "intakes" for $25 so all i've gota do is tell em it's an intake and they'll believe me. From what you've showed me there's potential to get almost 400 CFM out of the exhaust but the restriction is the at most 300 CFM t-body/intake. The only way to put out more CFM when you can only take in x amount is forced injection. So making the supercharger upgrade work on my fully overhauled engine (so i know it's strong enough) with higher flow exhaust is my plan now.