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I know this isn't a popular topic and that there is a strong contingent of well educated members here who have little to no interest and perhaps even some disdain for the motivations behind this question, but nonetheless this is the best place to ask it.
I also know that this question has been asked in slightly different forms before, but there are some different considerations I would like to have addressed.
I am curious what sort of crank and whp numbers one could expect only doing "light" or "stage 1" modifications to an L98 motor. By this I mean the following sorts of things:
Long tube headers, cat delete, muffler delete, true dual exhaust with H or X pipe, high flow air filter (my understanding is that dedicated intake systems don't really gain much), mild throttle body, plenum, and runner porting, Smog provision deletes, and of course a good tune/chip...
Perhaps somewhere around 300whp?
I am also curious what sort of numbers one might expect from doing the following "medium" or "stage 2" modifications on top of the above described "stage 1" mods:
Aluminum heads like the AFR 195, FIRST TPI intake kit with their electronics, and a good tune.
Finally I am curious what numbers might be achieved with a "stage 3" build, pulling the motor, 383 stroking, intelligent cam decision, as well as all of the above "stage 1" and "stage 2" mods.
I am trying to plan out several years of L98 ownership and modification.
I had spent a lot of time a few years back to try and figure out the head flow and the build parameters of the 10xTQ, 383 engine so that when I entered the parameters for the different intakes they would all be within reason. I also spent some time to figure out what the best cam would be for the 10xTQ engine with a stock FIRST intake. I had that already prepared and ready to go (so to speak) and was only anticipating upping the compression to 11:1 and entering a cam for the “MiniRam” engine simulation and plugging those numbers into the ET simulation.
However, I went ahead and changed the compression, head flow, and cam in the simulation to those provided by Matt. The intake parameters used are the same used for the previous MiniRam simulation. There are a few assumptions here and they’re just going to have to be what they are because I really don’t have the time to keep this going on and on. I used the same cross-sectional area and volume for “Matt’s heads” as the Competition ported AFR195s. If the specs are actually smaller or larger for either, it is what it is.
The increased compression (10.8:1), head flow, and smaller cam with the 383 MiniRam ended up making about 20 HP more than the simulated 10:1, 383 MiniRam with the TFS heads and big cam (included for reference). That pretty much shouldn’t be a surprise to anyone. What may be surprising to some is the simulated ET for the 10:1, 10xTQ engine with the TFS heads, FIRST (same length as the TPiS with the increased CSA and airflow), and a 228/228-112 cam (estimated to make 1” less vacuum than the 218/224-110 cam). It is what it is – I just entered the numbers into the program and pressed “calculate”. I will say I believe the results and you either do or you don’t – no skin off my nose either way.
Just for additional clarification, I kept the gear ratio constant at 3.54:1 and entered the numbers generated by the engine simulation into the ET simulation – feel free to double check and make sure I didn’t make an entry error. The shift points for each combination are optimized as well as the launch “Slip RPM”. Most would not want to ride the clutch like that to get the best ET, but that’s what it took to get the best ET from each combinations.
I’m not running any more simulations - yes the Miniram needs more gear to run a better time, but the FIRST could benefit from more compression and head flow as well. It could go on and on, and on,,, and is another reason I don’t like bench racing and posting simulations. Not to be a dick, but I usually don’t put this kind of time into an internet post and really can’t keep this going much, if any, longer. I don’t mind dropping in from time to time and trying to help someone out, but I have a life and a job and really need to get my own ride up and running.
An increase in displacement from 350 to 383 with no other changes won't make a huge difference. Its just under a 10% increase in displacement.
On a 400 HP engine, that equates to about 40 HP assuming the heads, intake, and exhaust don't become restricted by the additional demand.
Are you dead set on keeping the TPI? That intake is really going to limit your power potential, ported or not. A Stealth ram, Super ram, or Miniram would be a much better choice. Your "stage 1" is a good foundation to get you started. However, I wouldn't bother with a bigger throttle body (at all) or porting/siameseing the runners and plenum. Long tubes, heads, and a cam (properly tuned) are your best path to more power. You might also consider regearing the rear end.
If you want big power, boost is going to be your best bet. (But not cheap.)
Are you dead set on keeping the TPI? That intake is really going to limit your power potential, ported or not. A Stealth ram, Super ram, or Miniram would be a much better choice. Your "stage 1" is a good foundation to get you started. However, I wouldn't bother with a bigger throttle body (at all) or porting/siameseing the runners and plenum. Long tubes, heads, and a cam (properly tuned) are your best path to more power. You might also consider regearing the rear end.
If you want big power, boost is going to be your best bet. (But not cheap.)
I'm not dead set on keeping the factory TPI, except for the "stage 1" mods. If I'm replacing the factory TPI it would be for the FIRST TPI setup, which would be "stage 2". I also don't think I would swap cams except as a part of the 383 stroker build of "stage 3".
I have to balance labor/time with cost. Boost is off the table because I just want an NA car.
Any idea what the "stage 1" power might be like? Thanks!
I wouldn't have much other than a wild guess on power numbers as my 85 is turbocharged. If the FIRST is definitely what you're going to use, consider that it will have approximately the same RPM limitations that the stock TPI has when picking your camshaft. The FIRST just flows more and the RPM range is more influenced by that long runner length.
Awesome video. Thank you! I like the power curve and values given by the Extrude Hone port/polish of the factory TPI base, TPIS big tube runners, and the port matched plenum plus larger TPI throttle body. I also like the curve and numbers of the big runner complete TPIS system. I wonder how the FIRST TPI would compare?
Last edited by yakmastermax; Jan 5, 2022 at 01:55 AM.
I did an 89 last year that started essentially stock. The owner did not want to spend lots of money to gain ~50CHP.
Ultimately we went with a NOS 5150 package. This is the easiest and cheapest way to add a decent amount of CHP to the L98 engine. Stage 3 can be had with simply a pill adjustment.
I don't think you're going to get 300whp without touching heads or cam, regardless of the other mods you listed. Maybe 250-265 as best case scenario.
Even with your medium mods, you're looking at 300whp as a best case without a camshaft.
I think you can do a little better than that, but agree that getting to 300 WHEEL would be pretty hard w/o a cam.
OP: I just did your "Stage 1" essentially; I took an '89, fabbed a 2.5", true dual exhaust with no cats (still using stock manifolds). Did the "Free mods"; timing, no air filter, smog pump delete and 160 stat. Car made 240 RWHP, which is about 265 crank or so.
Next, I changed the intake manifold and made 276 RWHP, which is a bit over 300 crank hp...maybe 305 or so?
All of this was on a stock tune. No tuning done at all. I think if I added long tubes, 1.6 rockers and a proper tune, it could have kissed 300 wheel....maybe.
As to the "value" or gains form tunes....I've seen a HUGE range. I once saw a mid '90's LT1 Firebird gain 45 RWHP with a tune....watched it happen on the rollers, first pull, tuning, subsequent pulls and tuning with a final, 45hp gain. Later that same day, my own car, an '06 C6, gained a paltry 6 RWHP with a tune on the same dyno, same day, same tuner. I'd BET that the LT4 is tuned pretty well with not a lot of low hanging fruit, but maybe a little. Just a guess there.
300rw is going to be a bit of a stretch, I figure you could barely get close, but I don't see it as likely without at least a camshaft change. Needs to be 88 or later to have the '113 heads, which don't have to be ported but if you have them off you should.
Basically port out and port match everything. Siamesed runners, ported out plenum, bigger base intake manifold, headers, no cats, 1.6RR.
Stock base, I don't see it happening, that part has to go aftermarket, would be better if you can find larger AS&M runners and port match everything.
Then a tune of course.
But youre looking at some money that might not go along well with a second stage of mods, so you'd have to know what the end goal would be. I might not spend the coin for the base manifold or larger runners just to switch to FIRST later.
I think you can do a little better than that, but agree that getting to 300 WHEEL would be pretty hard w/o a cam. OP: I just did your "Stage 1" essentially; I took an '89, fabbed a 2.5", true dual exhaust with no cats (still using stock manifolds). Did the "Free mods"; timing, no air filter, smog pump delete and 160 stat. Car made 240 RWHP, which is about 265 crank or so.
Next, I changed the intake manifold and made 276 RWHP, which is a bit over 300 crank hp...maybe 305 or so?
All of this was on a stock tune. No tuning done at all. I think if I added long tubes, 1.6 rockers and a proper tune, it could have kissed 300 wheel....maybe.
As to the "value" or gains form tunes....I've seen a HUGE range. I once saw a mid '90's LT1 Firebird gain 45 RWHP with a tune....watched it happen on the rollers, first pull, tuning, subsequent pulls and tuning with a final, 45hp gain. Later that same day, my own car, an '06 C6, gained a paltry 6 RWHP with a tune on the same dyno, same day, same tuner. I'd BET that the LT4 is tuned pretty well with not a lot of low hanging fruit, but maybe a little. Just a guess there.
240 RWHP or 265 Crank is pretty solid! I wonder how much more long tube headers would have helped. My understanding is that the factory exhaust manifolds aren't terrible, but aren't great either. Maybe another 10RWHP?
Which intake did you change to?
Originally Posted by vader86
300rw is going to be a bit of a stretch, I figure you could barely get close, but I don't see it as likely without at least a camshaft change. Needs to be 88 or later to have the '113 heads, which don't have to be ported but if you have them off you should.
Basically port out and port match everything. Siamesed runners, ported out plenum, bigger base intake manifold, headers, no cats, 1.6RR.
Stock base, I don't see it happening, that part has to go aftermarket, would be better if you can find larger AS&M runners and port match everything.
Then a tune of course.
But youre looking at some money that might not go along well with a second stage of mods, so you'd have to know what the end goal would be. I might not spend the coin for the base manifold or larger runners just to switch to FIRST later.
Yeah I wouldn't spend any money on modding the factory TPI intake because I plan to go FIRST TPI down the road. I was just thinking of port matching, porting, and maybe a bit of polishing myself by hand.
Porting an polishing any part of that engine with stock heads, cam, and intake are pointless - total waste of time. Out of all your "stage 1" mods, the headers and good exhaust are about the only worthwhile parts. You'll get around 240rwhp, as Tom mentioned. There is no point in doing the rest.
For the "stage 2" options, you can't get a good return on your investment (dollars:hp) unless you're willing to forget about the TPI and any other long-runner intake. I don't know why anyone would spend the ~$3000+ to get AFR heads and a fancy new intake and then choke it with 24" runners and the stock cam. If you want the heads in prep for future "stage 3" mods, I get it. But don't waste money on an intake that is always going to limit you to power at or below ~5500rpm. You need an intake with shorter runners that has real power potential, or those AFR heads and all the stage 3 stuff are mostly wasted. There's no way around that. The stock cam is also going to break your heart, so don't bother with the heads unless you're ready to get a real intake and cam.
If you build a stroker and select a good cam along with the heads and a good shorter-runner intake, you can start making real power: well over 400rwhp are easy to get, depending on how radical you're willing to go with the street manners. 99% of people here have no idea how fast a C4 with an honest 400-450rwhp really is. It's fun. If you insist on a FIRST intake, you can count on being limited to about 350-375rwhp with all the same other equipment and same street manners (mostly a function of the cam). At this point, you're talking about spending $6-7k for this build, so IMO you should be aiming for at least 400rwhp.
ETA: I don't know what ECM you have, but you need to make sure you have a way to tune it for your stage 3 level of mods. You probably don't need to do that for stage 1. Your stage 2 proposal...I don't know: with a stock cam and a long-runner intake (FIRST), it might not even need a tune...which is also an indication of how little power it will add.
Porting an polishing any part of that engine with stock heads, cam, and intake are pointless - total waste of time. Out of all your "stage 1" mods, the headers and good exhaust are about the only worthwhile parts. You'll get around 240rwhp, as Tom mentioned. There is no point in doing the rest.
For the "stage 2" options, you can't get a good return on your investment (dollars:hp) unless you're willing to forget about the TPI and any other long-runner intake. I don't know why anyone would spend the ~$3000+ to get AFR heads and a fancy new intake and then choke it with 24" runners and the stock cam. If you want the heads in prep for future "stage 3" mods, I get it. But don't waste money on an intake that is always going to limit you to power at or below ~5500rpm. You need an intake with shorter runners that has real power potential, or those AFR heads and all the stage 3 stuff are mostly wasted. There's no way around that. The stock cam is also going to break your heart, so don't bother with the heads unless you're ready to get a real intake and cam.
If you build a stroker and select a good cam along with the heads and a good shorter-runner intake, you can start making real power: well over 400rwhp are easy to get, depending on how radical you're willing to go with the street manners. 99% of people here have no idea how fast a C4 with an honest 400-450rwhp really is. It's fun. If you insist on a FIRST intake, you can count on being limited to about 350-375rwhp with all the same other equipment and same street manners (mostly a function of the cam). At this point, you're talking about spending $6-7k for this build, so IMO you should be aiming for at least 400rwhp.
ETA: I don't know what ECM you have, but you need to make sure you have a way to tune it for your stage 3 level of mods. You probably don't need to do that for stage 1. Your stage 2 proposal...I don't know: with a stock cam and a long-runner intake (FIRST), it might not even need a tune...which is also an indication of how little power it will add.
Thanks for the input. I've read that the stock TPI runners are ~19" in length while the FIRST TPI runners are ~15" in length with significantly more cross section.
I specified that I would not upgrade the heads without also upgrading the intake to the FIRST. Both are part of "Stage 2".
"I don't know why anyone would spend the ~$3000+ to get AFR heads and a fancy new intake and then choke it with 24" runners and the stock cam"
That doesn't quite make sense to me. How would I utilize the fancy new intake (FIRST TPI) and simultaneously choke things with the 24" (presumably 19" stock TPI" runners), when I am utilizing the fancy new intake?
Regarding upgrading the heads and intake (by this I meaner intake manifold, runners, and plenum), but NOT the cam, this is a matter of cost/time/benefit. Pulling the shortblock to swap cams, I might as well go stroker at that point, which is why stage 3 includes BOTH cam and stroking.
ALSO there are a lot of people with 383s, FIRST TPI setups making well over 350-375hp. Additionally, peak HP is not my only goal here.
Thanks for the input.
240 RWHP or 265 Crank is pretty solid! I wonder how much more long tube headers would have helped. My understanding is that the factory exhaust manifolds aren't terrible, but aren't great either. Maybe another 10RWHP?
Which intake did you change to?
SLP T-Ram
Later I switched to the TPIS MiniRam, and lost ~18hp, I think?
I agree with you about the manifolds; not as bad as 3rd gen F-bodies.... but not great either. The cross sectional area of each primary at the collector is tiny.
Most power from L98 TPI
but not great either. The cross sectional area of each primary at the collector is tiny.