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I'm in the process of considering the purchase of a very clean, original, low-mileage 1987 Trans Am with the LB9 305 TPI engine and the MM5 5-speed manual transmission. I've been doing a bit of comparison of the horsepower/torque numbers claimed by Pontiac for that powertrain combination in 1987 vs. 1989, and have formed a theory based on observation about where some of the horsepower gains came from. Looking for some insight on how sound that theory may be.
Now, Pontiac's number for the '87 LB9/MM5 combination were reported as 205 Hp/285 lb-ft; in '89 the same combination was reported as 215 Hp/285 lb-ft (225 Hp/300 lb-ft with the N10 exhaust). My understanding is the LB9 engines during this period were equipped with the same cam when mated to the MM5 transmission; so my question became: where did the extra 10 horsepower come from in '89...?
Comparing the respective engine compartments, I've noticed a significant difference in the air intake assembly. The '87 Trans Am has a short, straight snorkel leading from a flat, rectangular airbox at the front of the engine compartment to the intake of the TPI plenum; my '89 Formula (and it looks like the other TPI-equipped models in the Firebird lineup) used a longer, "S" shaped snorkel that leads from a cylindrical airbox tucked down in a space at the front of the passenger's side fender up to the intake of the TPI plenum. So I'm speculating that Pontiac switched to the "S" shaped intake setup because (1) it breathes better, and (2) it pulls in cooler air -- resulting in a 10 Hp gain...?
If all that theory holds water, my next question became: how hard is it to retrofit the '89 TPI intake setup to the '87 Trans Am? Since both model years use a Mass Air Flow (MAF) sensor, I'm thinking all I'd need is all the cylindrical airbox, intake plumbing, and mounting hardware for the later intake setup; possibly the radiator shroud as well, if the "S" tubing mounts differently...? Also not sure if the MAF on the '87 setup will work in the '89 setup; they may have a different housing tailored to the plumbing of the respective model years.
But I imagine someone here can tell me these things! Thanks in advance!
It is very easy to put that intake on the earlier models. I believe the ten horsepower comes from the N10 dual cat exhaust more than the intake ducting though. However, the later intake is more aesthetically pleasing so that is a good upgrade anyways.
It may not be that simple. I personally doubt that changing from a factory 87 intake to a factory 89 intake would yield 10 HP.
I have no clue personally, but might want to check part numbers on pistons, etc. Could have been a compression ratio change..... The $6E tuning on the 89 may be responsible for some of the gains.
For sure the exhaust is a huge restriction.
I'm curious myself as I'm running an 86 LB9 rated at 195 HP. I've got it up to about 198 with 1.6 rockers. It's coming back from my exhaust guy soon with Dyno Don headers so we shall see what difference that makes.
in 1987 the 305 tpi 5 speed had 215 hp and in 1989 it had 220 horsepower without the n10 option .. the 88-89 had a little better cam which gave them the 5 horsepower increase ..
Thanks for the inputs so far! Glad to hear the swap isn't too hard, if I go that direction. Still haven't even talked price on the car yet, so I'm still a long way off. Not really looking to do the change unless (1) I can establish it's the reason for the difference in rated horsepower, and (2) it's easy to reverse to stock, should I want to; otherwise I'll just keep it stock, which is always my default preference.
Yeah, I hear folks on the N10 exhaust. But I'm trying to take the N10 exhaust out of the equation for the purposes of this discussion. The N10 bumped the horsepower up to 225 from 215 in '89. But in comparing Pontiac's published numbers for both model years (no N10), there's still a 10 Hp disparity in the LB9/MM5 combination that I still can't really explain...
Let's review what we have so far:
Improved air supply system in '89 (i.e., my original theory). Jury seems skeptical, but also not convinced it's outside the realm of possibility... I know that there's an entire sub-industry devoted to selling people on the notion that higher-flow intakes give measureable horsepower increases... Also, Chevrolet showed a 5 Hp bump for the based-model C5 Corvette in 2001, and it's my understanding that came from nothing more than an almost imperceptible change in the airbox. So is a 10-Hp bump on a Third-Generation Firebird possible out of an airbox enlargement/relocation...? I don't know, but Pontiac must have done it for some reason...
Several have suggested there's a more aggressive cam in the '89 LB9/MM5 pairing. Based on my limited web research, I was led to believe this was not the case, but my research certainly wasn't exhaustive; and I'm admittedly not an expert in this area...
Compression ratio higher in '89. Could explain the increase, but I've never heard such was the case... Again, I'm no expert...
Tuning improvements between '87 & '89. Very well could be the case... I hadn't ever heard there were significant changes, but still not an expert...
Pontiac was underrating the horsepower in one or more model years. Nah...
Originally Posted by Bob88GTA
The 87 TPI has the 9th Injector...the 89 does not. So programming it out may be needed.
I wouldn't think so, as long as the '89-style MAF sensor provides the same information as the '87-style sensor, right...? The ECU wouldn't really know any difference -- just that it might be seeing [theoretically] more and cooler air going past it with the '89 intake setup.
Originally Posted by ray jr
in 1987 the 305 tpi 5 speed had 215 hp and in 1989 it had 220 horsepower without the n10 option .. the 88-89 had a little better cam which gave them the 5 horsepower increase ..
Not saying you're wrong, but this is where things get confusing. The Pontiac literature (i.e., sales brochure) I found online claimed only 205 Hp in 1987 for the LB9/MM5; 190 Hp for the LB9/automatic. According to what I've read, that difference was due to the cam. But then I occasionally see a 215 Hp quote (so yours is not the only one I've seen) regarding the '87 LB9/MM5 combination -- which would put it the same as the '89 numbers (sans N10 exhaust). A lot of conflicting information to sift through on these engine specifications...
There were definitely significant changes to tuning in 89. In fact they completely re-wrote the code to run an entirely different set of tables. 89 was the only year for $6E where it still runs MAF, but has no ninth injector, has VATS enabled, etc. You can run $6E on previous years with some modification. I'm running tuned $6E on my 86 for example. It's akin to installing a newer version of Windows on your PC....
There were definitely significant changes to tuning in 89. In fact they completely re-wrote the code to run an entirely different set of tables. 89 was the only year for $6E where it still runs MAF, but has no ninth injector, has VATS enabled, etc. You can run $6E on previous years with some modification. I'm running tuned $6E on my 86 for example. It's akin to installing a newer version of Windows on your PC....
GD
Now that you point all of that out, I do recall being told at some point that '89 turned into a transition year on the Firebirds; some things (like the engine wiring harness) are even unique to that model year. So tuning could very well be the answer to the horsepower difference.
Well, my inclination remains to leave the car stock if it means different coding and such -- especially since I have no way to upload new software to the ECM. Not really looking to turn the car into an ultimate street performer; I just thought it might just be a simple matter to get a bit more "gitty-up" with minimal and easily-reversible physical modifications, but it sounds like it could be a bit more involved.
Yes, thank you! So the '87 LB9/MM5 cam is different than the '89 LB9/MM5 cam!
It's starting to look like there were several differences that all combined to give the '89 power train 10 Hp more than the '87: cam, ECM tuning -- and perhaps including the revised air intake system.
Looks like I'm going to leave well-enough alone on this one; the stock setup is sufficient for my purposes, should I elect to purchase the car.
12-21-2017, 07:44 AM
