DarkKnight

Can somebody answer a few questions about the TPI system?

First, was it truly designed for the 305?

Second, is it true that this system does not really provide enough airflow for what the 350 needs?

Third, where is the bottle neck in this system for the 350? The plenum, the runners or the intake manifold? All three?

MdFormula350

yes 305 tpi was first out for thirdgens.

i dont know about that, the 350 tpi's seem to do very well, TORQUE

i think its the intake manifold, not sure though.

305sbc

iTrader: (3)

TPI first came out in 1984 on the 350 engine (corvette).

All parts are a restriction on a healthy engine. The most restrictive point in the induction is the area right around the intake valve seat in the heads.

TKOPerformance

The TPIs biggest drawback is its inability to make power beyond about 5,000 RPM. This is caused by the extreemly long runners of the TPI manifold. Everything else in a TPI engine is designed to work with this intake, so the heads and cam are also tapped out by 5,000 too.

f-crazy

iTrader: (-1)

but its great up to those 5000 rpms hahahaha

then it gets old when ****in maximas start givin you crap to 60 lol

DarkKnight

So the runners are the biggest restriction on the TPI system? The plenum and manifold flow pretty well?

91wtROH17's

The manifold (or TPI base) is the biggest restriction, the runners come second, the plenum is OK but had a few minor problem areas that can be fixed easily with a dremel or die grinder. The TPI was designed for the 305 so it is quite restrictive on the 350!!

Ohhh and 305 is right, the stock heads and cam suck aswell, but that's another ball game alltogether from the intake itself.

kdrolt

TPI (in L83 form for the 350, and later LB9 on the 305) was designed as the replacement for the Crossfire (L-83 engine on Vettes, LU-5 on Fcars) by Roy Midgley (and his group) at GM.

You need the Crossfire background before the TPI can be explained.

The Crossfire manifold was intentionally designed, by Midgley, to be a flat-as-possible single plane manifold so that the hood line on the (then) new Vette for 1983 could be kept as low as possible. The manifold also looked cool - resembling the Cross-Ram manifold used on the 1968-1969 Z/28 Trans Am series racing engines. Looking cool is especially important with Vette hardware, so GM management was sold on the concept.

As the manifold was developed, they discovered mixture distribution problems.... and didn't learn (until too late) that GM previously had the same problem on the Cross Ram manifolds in the late 1960s -- a problem solved by running rich.

Mixture distribution among the cylinders was the main problem on the Crossfire so they needed to run it fairly rich, they needed a lot of manifold heat to vaporize it as much as possible (especially in cold weather), and they had to add swirl plates to the undersides of each throttle body to help spread the fuel around. High velocity in the runners also helped keep the fuel suspended and so the runners were not made very large in cross sectional area. All of these things hindered the performance, in terms of power and acceleration, but it didn't matter that much at the time because the performance boost from the carb engines (1981) to the fuel injection (TBI) engines (1982+) was significant both in terms of numbers and in terms of track performance. So for 1982 (when the Crossfire came out in the Corvette) and through the 1984 model year, it was good enough.

During the Crossfire development, Midgley realized the fuel problems they were having so the manifold got the above compromises in design -- and none of these helped airflow... but that couldn't be helped. The Crossfire design had to be used because they had already "sold" the idea to GM management based on leveraging TBI technology obtained from the Cadillac 8-6-4 v8 engine program (See L.L. Bowler, SAE 800164, 2-1-1980), but the fuel economy still wasn't exactly as good as they wanted. The Corvette, meanwhile, got delayed so that the Crossfire was used in 1982 and in 1984 (no 1983 Corvette). The Crossfire manifold, being a Corvette item, was an easy insertion into the Fcar platforms because it allowed the manufacturing and development costs to be spread out over many more cars than just the Corvette, and the lower-cost Camaro/Firebird could be had with a Corvette-type engine.

CAFE and emissions regulations were only going to get worse so Midgley got the ok from GM to pursue a replacement for the Crossfire, and that's how the TPI happened --- as a evolution of the Crossfire manifold for use in the Corvette.

So I don't think it is true that the Crossfire was designed for the 305 because the 305 never got serious attention for a Corvette engine, and the Crossfire was born to give the Corvette low-cost fuel injection, and a big torque boost (even if the hp was lacking) in an era of sh*tty engine performance. I'll add that the TPI wasn't designed based on a 305 either, because the TPI development (also by Midgley et al) was done to fix the problems on the Crossfire, and apply it first to the Vette, and then (later) sell the TPI in the Fcars (again, to reduce the unit cost).

TPI uses the same exact acoustic resonance tuning concept that the Crossfire had, except that the TPI had port injection (which overcame the fuel distribution problem), and that the runners were curved and longer than used in the Crossfire engine. The longer runners moved the rpm for peak torque down, but the TPI runners themselves got a slightly larger cross sectional area (for better flow). TPI still used small area runners so that the mid-speed torque from the engine would be superb, at the cost of upper rpm performance. I believe that there was also a move afoot to design a much higher-end engine so that they had no great need to make the L98 a bigger power performer than it already was. The higher-end engine later turned into the LT5 engine used in the ZR-1 Vette.

Midgely, meanwhile, and his group got shut out of the LT5 engine development -- which they probably didn't much like, so they went to GM management and sold them on an idea to replace the L98, and that they had enough skill in-house to do a really good job by leveraging the engine work done in the Bowtie racing heads. The development in those heads went directly into the first Vette LT1 engine that went into the 1992 Vette, with the LT1 engine replacing the L98. If you look in D. Vizard's Chevy cylinder head porting book, you'll find pix of the Bowtie head combustion chambers from the late 80s --- they're clones of what later appeared in the LT1, L99, LT4 and L31 heads. In this engine (LT1), Midgley was able to have excellent mid-speed torque (from superior head breathing) without sacraficing the upper rpm power. Later on, and I'm not sure if Midgley was involved at that point, the TPI (tuned port runners along with port injection) came back in GM practice in the Northstar intakes, and later the intakes used on LS1 and other GM v6 and v8 engines.

I am surprised that most of what I just wrote doesn't appear here on TGO. My sources for most of the material above come from reading Dave McLellan's book Corvette from the Inside as well as from reading on the Crossfire web sources. I also review papers for the SAE and I work in acoustics and have an engineering background, so it's a little easier for me to survey the Crossfire/TPI landscape. I still get a kick of out seeing TPI owners bash the Crossfire, because the two designs have more in common that most people realize, they were designed by the same people, and they have some of the same limitations.

As far as the TPI itself -- I've never found an SAE paper talking about the engine and it's innovation. Ditto for the Crossfire. Paper(s) were given on the LT1 however, so the LT1 was probably considered innovative at GM, while the TPI and CFI both were not.

TPI certainly wins the beauty contest though among the three, even today, but there is next to nothing written on it unless you sink your teeth into an acoustics paper in an obscure journal -- and if you do, you won't see terms like TPI or L98.

Midgley would probably be a very interesting person to interview, to get the full story directly from him, as opposed to having to cull it all together.

EDIT:

IMO, no. For the reasons stated above.

That's the wrong question. It does do exactly what it was designed for: give improved emissions, excellent fuel distribution, give awesome low- and mid-speed torque for a 350, and enough upper rpm power to satisfy the Corvette owner for 1985.

If the present-day owner wants more than 245 hp, then no, it probably doesn't provide enough airflow.

The runners and the base together because the cross sectional area in each is the same. I treat the runners as the bolt-on tubes plus the manifold base together. IOW if you bolt on big tubes, you still have the small runners in the base to deal with. So in your terms, the tubes + base are the bottleneck.

The Mach Index for them isn't that much better than the Crossfire manifold; I've done the math for the Crossfire to see where it stalls (Mach Index reaches 0.5, using C.F. Taylor's nomenclature -- search on the CF Vault for that post) but I haven't done it on the TPI. On a 350, the Crossfire manifold starts to stall above 3000 rpm (that doesn't mean it doesn't make power there, it just means that the rate of power increase starts to fall off prematurely).

The TPI runners+manifolds have slightly larger runners, and flow bench test (again, as posted on the CF Vault website) show that they flow approx 190+ cfm bone stock vs 155 cfm bone stock for the Crossfire, so that suggests that the TPI will start stalling (Mach Index -wise) above 3700 rpm (3000*190/155).

C.F. Taylor, FWIW, wrote a 2-volume book on Internal Combustion Engines, and it's considered the bible among engine designers. It's published by the MIT Press.

Another really good reference to dig deeper into is Scientific Design of Exhaust & Intake Systems by John C. Morrison and Philip H. Smith.

Long, but HTH.

D Stroy H8

Niiiiice. Very informative

DarkKnight

kdrolt, that was very interesting and informative...

So your saying that bolting on larger runners, as most people do, wont really give a significant gain?

Let me also ask this... what is your opinion on putting an LT1 intake onto a TPI engine?

I also have a 1994 Z28, I thought about porting out the manifold but everyone tells me there would be no gain out all on an LT1 engine? Whats your opinion?

D's89IROCZ

sweeet . I enjoyed reading that .

kdrolt

You will get gains from using larger tubes but the runners in the manifold base are still going to be a bottleneck... so you don't get LARGE gains -- you get small gains. I think there are enough track runs & dyno tests on TPI mods, especially to the intake manifold, to support what the modifications give you for track & power improvement. You have to sift through the large volume of TPI posts though to find them. I know that's a lot of back reading - but there's no substitute for it, and you also get the benefit of the discussion that follows a really good (or bad) post. The truth usually reveals itself along the way.

I also like the siamesing work done on the stock L98 intakes -- it borrows directly from the change made to the Crossfire intake in 1984 (siamese holes between two pairs of runners); small torque boost loss to gain better breathing at high(er) rpms.

I love the use of the LT1 intake on the L98. It a miniram gain without the miniram cost. It, or rather they, give up some torque in favor of breathing at higher rpms. And it makes that old L98 manifold available to truck and station wagon owners who want eye candy under the hood. HTH.