just curious if anyone has ever tested a l-98 with tbi and better flowing exhaust than the tbi motors had. I'm wondering how they compare.

 

Do a search for posts by me and my father Dyno Don. We converted his old 92 RS from 305 TBI to 305 TPI and went from 158 HP and 220 TQ to 175 HP and 290 TQ. The car already had a stock L98 2.75 catback on it, which previously gained 8 HP and 20 TQ over the stock 2.25" TBI exhaust.
Hopefully some others will chime in with 350 TBI dyno tests.

Hi Fast355!

 

Here's some of the relevant threads on TBI-vs-TPI:

https://www.thirdgen.org/forums/tpi/...no+don+tpi+tbi

https://www.thirdgen.org/forums/tbi/...ml#post2976055

https://www.thirdgen.org/forums/tech...ht=dad+TBI+TPI

In both the dyno testing cases of Fast355 and Dyno Don, the TPI had better peak torque --- as it should because of the equal-length intake runners on the TPI. On the TBI, the runners are in different lengths and in separate planes (upper, lower). Fast355's tests shows TBI has better torque at low rpms than TPI.

The main point of the debate is what happens above (say) 4000 rpm? Fast's test shows TBI was slightly better than TPI -- while Don's showed the opposite (because the TPI had slightly higher peak power).

As for

GM sort-of did that test.

1. iron head L98 in an Fcar was rated at 225-230 fwhp and 340 fwftlbs, using the "773" cam and 083 casting suffix iron heads and obviously TPI. 6 deg BTC ignition base timing IIRC.

2. 1992-1993 copcar (Caprice 9C1) used the same "773" cam, 193 swirl port heads, and TBI; rated at 205 fwhp and 300 fwftlbs. 0 deg BTC ignition base timing due to the higher swirl faster-burn heads.

3. up to 1995 LO5 TBI truck, 210 fwhp and I think 310 fwftlbs; this engine did not used the 773 cam; IIRC the specs were something like 194/202 deg i/e flat tappet (Fast355 will correct me if I'm wrong), a less restrictive exhaust than the copcar, and the same TBI and hswirl port heads as the copcar.

The above three GM engines would still show at least +10 fwhp gain and +25 fwftlbs gain for the TPI over TBI, even if you accounted for the differences in the exhaust and for the lack of roller cam in the truck TBI.

Then there is the debate of head flow -- there isn't that much of a difference in the airflow on stock heads, TBI swirl port vs TPI non-swirl. The TPI heads appear to flow slightly better, but one could argue that the combustion in the swirl port head is more efficient which makes up for any difference in head flow.

I don't think that conclusion, using a comparison of three GM engines, is out-of-line with the tests of Dyno Don and Fast355. The fwhp differences were either small in favor of TPI, or a wash. The torque difference clearly showed TPI having the better peak number -- as expected. We can't easily compare the broad rpm effect of torque vs rpm, because Fast355 posted his dyno plots and Dyno Don didn't (I think his son Kevin linked the plots to TGO... but that link is no good now).

TPI owners like to think that their engines are much better than TBI -- and that's probably not true for anything other than how the engine looks under the hood. In stock-vs-stock, or even in similarly modified comparisons, the TPI seems to have a very slight power edge, and it has a large (but-narrow-rpm) torque gain. Both types have reported excellent fuel economy, so the port injection vs throttle injected also seems to be moot. TPI is also very expensive to modify, while TBI can use any of the carb intake manifolds + an adapter. Both types have compromises when you want to move a LOT if air, because they both affect air and fuel.

TPI and TBI are different, in looks and in output, but the differences in output are a lot less than people think. IMO.

 

I can see slight gains in RWHP/RWTQ when going stock TBI to stock TPI. I can also see gains in RWTQ with TPI over even a tuned TBI setup. The difference is where the torque will show up. On a stock TBI 350 longblock the HP peak only 4,000 RPM and the torque peak 2,800 rpm. In this scenario, it is possible to gain power at both peaks. Unfortunately, most dyno operaters start at around 3,000+ rpm in 3rd gear and the losses rarely show up on the low-end. They are there though. You can also build a TBI long-block that will easily pull to 5,500-6,000 rpm (stock intake & TBI) then swap it to TPI, it will struggle to reach 5,500 with the stock TPI on it. I saw this in my dyno test of the 305 TBI.

EDIT-
Below is me playing with desktop dyno on an otherwise stock TBI truck engine, both assume ideal timing and a/f ratio. TBI nor TPI achieve this due to emissions. TBI truck ECMs dump fuel and retard the timing way back at the first hint of load, which kills torque in the low-mid range. Then when the timing and fuel finally get closer to normal it makes OK peak HP for a stock engine. The TPI gives a slightly leaner air fuel mixture and more timing advance, which makes for more TQ/HP. This is above what the graph here shows. I am going with 400 CFM @ 1.5 in/hg, 566 CFM @ 3.0 in/hg (what I calculated from a flowbench test, 470 CFM @ 28 in/h20) for the TBI intake tract and 550 CFM @ 3.0 in/hg, also converted from flowbench numbers) for the TPI.

#1 is the TBI 350



#2 is the TBI 350 longblock with TPI intake



#3 is the closeup graph with both



#4 is a mildly cammed TBI 350 with headers



#5 is a mildly cammed TBI 350 with TPI intake and headers



#6 is a comparison of the mildly cammed engines. (262/262 @ .006, 218/218 @ .050, .462/.462" lift, 107 LSA, 106* ICL, its one that I spec'd out and works VERY well with stock TBI heads and EFI)

 

It'll reach 5500 rpm BUT the hp curve will flatten (plateau) around 4500 rpm and stay there. That's the classic problem with TPI: you get awesome torque near one mid-rpm engine speed but you pay for it elsewhere. Power should go up linearly with rpm so long as the breathing and cam aren't limiting the output, and that seldom (if ever) happens on any TPI engine.

Even in a 383 sbc with very good heads and a very hot cam, as shown in the huge TPI comparison done here:

https://www.thirdgen.org/forums/tpi/...feb-super.html

IIRC they were all port injected so there is no port vs TB injected issue to debate (though that one has been discussed on yet another thread). Note also that there is one "conventional" (single plane Holley) intake manifold (that's TBI-like) tested against 9 other TPI and TPI-like intakes. The items that stands out among the TPIs is that they all plateau in output power at a certain rpm, and that they all make outstanding mid-rpm torque. The single plane Holley does well overall.

That's why I'm not surprised by the dyno tests of Fast355 -- they look a lot like the TPI vs single plane Holley above, except on a smaller engine and using a much smaller cam.