As someone who has gone the route of ported stock TPI, large cast runners on TPIS Big Mouth base, to SR, to MRII, then to a converted C4SP intake, I completely agree with your statements above. Save frustration, time, and money and just move directly to a short runner intake (i.e. MRII).

 

The stock TPI may be 24" including the intake port. I can't recall TBH. I saw one quote about the FIRST intake having shorter runners. I am very, very skeptical. I don't know how you make wider tubes negotiate that turn with 4" less length than a stock TPI. I have the feeling that whoever got that spec only measured the short-side length, which isn't the measure that matters. It's the average length that matters, and I don't see how it can possibly be significantly shorter. The problem with TPI and similar intakes isn't steady-current restriction - it's the anti-phase blockage that happens in the air columb that starts and stops at half the crankshaft RPM frequency. The larger cross section of the runners will help a little (see the Helmholtz equation), but it won't raise the RPM at which the torque curve crashes very much. On a stock TPI that resonance happens around 3500rpm (look at Tom's graph), and after that RPM it's that resonance that dominates the flow restriction. You can't port your way out of that. You have to get shorter runners to eliminate that problem. That's why the LT5 intake had two sets of runners and switched to the short runners at higher RPM.

And I'm saying that the FIRST intake will stunt any gains you would have otherwise gotten from those heads. And it will be even worse with a cam upgrade.

The FIRST still has runners that are equally long and cause resonant restrictions. It's a bad idea for an intake. There are far better choices.

You don't need to pull the whole engine to swap the cam, though. I'm not saying you shouldn't do those other things, but you could do the cam with heads and intake and not have to yank the engine if you wanted to.

At the wheels? I doubt it. Maybe you can provide some citations for those, because I'd be interested to see the cam they are using in particular. I think 400hp at the crank at 4500rpm is pretty common: But that's like 350rwhp and it's leaving at least 50rwhp on the table because of that ridiculous intake. I'm not saying it's impossible to get 400rwhp with that FIRST intake on a 383, but you're going to need a considerably more radical cam than with a short-runner intake on an otherwise identical engine. That means you'll have a worse idle, worse low-speed drivability, worse low-RPM torque/power, and it will still cut off at an artificially low RPM (compared to what the heads and cam would do with a better intake). It's a much narrower power band.

Peak horsepower shouldn't be anybody's goal. You're looking for best average power over the RPM range you want to use, and/or the flattest and widest torque curve possible. TPI-style intakes make that impossible because they create a torque bump at 2500-3000rpm and then cut everything off about 50% higher in RPM (yes, even the FIRST). So you have a much narrower range of high torque values. IOW, a long-runner intake like that makes the engine more peaky, not less. Please think that through, because it's important.

OTOH, my 396 with heads very similar AFR 195 Comps and a ported LT4 intake had a smallish cam (218/224 at .050" and .570/.565 with my 1.7 rockers on a 110 LSA) and pulled hard from 2000-6000rpm. You could lug it at 1500rpm on the freeway with no surging and no need to downshift to accelerate, and you could spin it to the 6400rpm rev limiter (still pulling). It made about 420rwhp, it would pull on a stock C6Z (which runs mid 11s at 127mph), it was totally street drivable, and still got 26mpg on the two long trips I took it on. I'm thinking that's far more like what you are trying to accomplish.

 

I agree. I've got a larger base plus runners that are both larger and siamesed. The siameseing runs halfway down the runners, about 3"-4". The torque still peaks at 3,500 rpm (though with a very flat curve thanks to constant boost pressure.) Horsepower plateaus 5,200-5,300 rpm even with a cam that should run closer to 5,800 rpm. As much as I love the look of my intake, I'm changing it to a Miniram as soon as it is delivered.

 

The FIRST base and runners measure 13.5” straight line (shortest path) and right at 16” centerline. With most SBC heads being around 5”, you end up with about 21” total intake tract length. That is about the same length as the GM TPI (don't know where the 24" number I see so much comes from). However, in order for a FIRST to model the same power curves in the EA simulation as the actual dyno results, interestingly enough I have to enter 13.5” for the intake length (head’s runner length added separately).

I hate being the FIRST intake poster boy, but it’s frustrating to see people that have never run one dog on it - especially repeatedly. Instead of me typing another reply with the same stuff, just click here for the posts I made back in November explaining that the FIRST is not a "typical" TPI.

https://www.corvetteforum.com/forums...post1602545769

 

Bump because this topic is still on my mind.

I am trying to get an idea for how much of a restriction the factory bottom end on the L98 is. For example in the "stage 3" setup with factory L98 short block (including cam) but with aftermarket heads like AFR 195s and either a FIRST TPI intake or an extrude honed factory TPI setup, plus the supporting stuff like fuel pump, fuel pressure regulator, injectors, long tubes, true dual exhaust, and a good dynamic tune, what kind of power might be achieved?

Similarly I am curious what the 1989 L98 Long block is worth with the factory heads and cam. Say just upgrading the intake to either the FIRST TPI or Extrude Hone the factory TPI, plus fuel pump, regulator, and injectors, full dual exhaust long tubes, cat delete, and a good dyno tune.

A cam swap is basically off the table for me as if I were to put that much labor/time into the short block, I would pull the motor and stroke it.

 

Check out PipeMax software and you can do all the simulations you want. Not very expensive, and perhaps free if you just use it for a limited time. They have an SBC file in there that's close... input exact cam information, you can try different intakes and headers, and it will tell you where restrictions are.

 

What do you mean by "restriction" regarding the factory short block? The short block won't directly limit your power, except for its displacement vs a stroker. Do you mean durability with RPM? If you put different heads and FIRST intake on the engine but keep the stock cam, you are never going to need to go above 5000-5500rpm, because peak power will be below that (and it won't be a very high number either). The factory short block alone is probably good to at least 6000rpm. The stock valvetrain is almost always the first thing to fail at RPM limitations, and that factory cam is so ridiculously small that it won't stress out any set of springs and rockers that will come with any aftermarket heads.

It's worth what Tom showed you in his tests. The FIRST will probably perform as well or less well as his T-ram on an otherwise stock engine. You won't need a different fuel pump, regulator, or injectors. The problem with this plan is that if/when you ever decide to get good heads or even just upgrade the cam, you'll be stuck with an intake that will severely limit your RPM potential.

Then I wouldn't spend money on heads or anything else. Just do the intake and exhaust and be done with it. That's cheap and easy. All the other iterations of your plan make zero sense without upgrading the cam. You'll gain a few hundred RPM in power band and maybe 25-30hp total. The only question I'd have is whether the stock springs are okay for that little extra RPM. I'm pretty sure they are because the cam is so teeny. Tom, did you have any issues with the stock springs and the T-ram or MiniRam?

 

I mean upper limits on power/tq production. I don't want a high RPM engine, want to keep it as a street machine for the most part, but trying to extend the RPM range a bit for pulls 60-100

It makes sense to just do exhaust and intake setup and leave the long block alone, if there isn't much power gain by swapping the heads. I might be mistaken but to me it seems, swapping heads is a good bit less time/labor effort than swapping heads and cam. I say this because I've pulled heads off a motor in a truck before, but I've never take a cam out of a motor without the entire motor pulled.

 

That doesn't matter at all, because you won't come close to making the kind of cylinder pressure (torque) that blows these engines up with the stock cam and heads...or even with the stock cam and aftermarket heads. The only thing that will eventually blow up a naturally aspirated short block is RPM, and again you won't get anywhere near the stock short block's limits with the stock cam. "Hi-RPM engine" doesn't mean an engine is good for street use or not. That's old wives tale BS. What makes an engine bad for street use, generally, is a narrow useful RPM range and a cam that gives it bad manners at idle and low RPMs. Essentially, the best street engine is the one with the widest, flattest torque curve. And unless you have some form of boost or more displacement or compression, your cylinder pressures (torque) are mostly fixed and you will need to extend the torque curve to a higher RPM to make more power. You can gain a little power with an intake swap only because the stock TPI intake is so unbelievably shitty that it's a massive choke point. The FIRST might be a decent upgrade, but only because the stock cam is so unbelievably shitty that you still won't be going above 5000rpm. Otherwise, the FIRST would still be a massive choke point compared to a intake with shorter runners.

With the stock cam and any TPI-based intake, you might gain 20hp with aftermarket heads. Maybe. It's a huge expenditure for not much gain. If you insist on keeping the stock cam, there's just no point spending money on heads. Just get some intake that will let the engine breathe decently to 5000rpm and call it good. As a bonus, you probably get to save money by not needing a tune or injectors or fuel pump or anything else: just slap that intake on and maybe do exhaust and you're done. There's nothing wrong with that plan. You'll be up 30-40hp over stock and the car will be more fun and a little faster.

OTOH, if you are going to pull the intake and heads anyway, then you're probably at least halfway through the labor of swapping the cam in the car. I think you just need to pull the radiator and timing cover at that point, and you're there. I haven't done it, but lot's of people have. If you are comfortable swapping heads, there should be zero issues doing the cam. But again, if you are going to do heads and cam then you really need to rethink your intake choices. Anything with runners that long is going to kill the power potential of good heads and a cam.

 

Yeah that is exactly what I was curious about; how much extra power upgraded heads might provide with the factory cam still being used, in conjunction with an upgraded intake and exhaust. 20hp really is not enough for me to justify the added labor and cost of swapping heads. However if that 20hp would be 50hp with supporting fuel and tune, and if these would be heads that would work well on a long term 383 build, then that would be worth it to me.

Silly question perhaps but would higher ratio roller rockers in the proposed aftermarket heads potentially net some of the power gains that a cam swap would realize?

I have heard in car cam swap potentially requires things like loosening motor mounts, and jacking up the motor, not to mention radiator removal, while heads just means un bolting the accessories up front.

 

do you know stock cam specs

 

I swapped the cam in my 85 with the engine in the car. Did it while I was changing heads. Almost all of the work that you need to do for a cam swap is already done to change the heads The radiator is already drained, not much more work to pull it to get it out of the way for the cam swap. You can move the AC condenser out of the way well enough with the lines still connected.
I didn't have to do anything like removing/loosening engine mounts to do it. You can remove the harmonic balancer with the engine and power steering rack still installed.

 

They are easy enough to Google, but:

GM Stock Cams
Model Number **.050 Duration**1.5:1 Lift***1.6:1 Lift**Lobe Sep***Notes
14093643********202/207****404/.415***431/.443***114.5******87 350,305man
10088155********179/194****350/.384***373/.410***109********87 305 auto
10066049********207/213****415/.430***443/.459***117********88-89 350,305
10111773********202/207****413/.428***441/.457***114.5******90-92 350,305

Just in case anyone doesn't believe me when I say that spending lots of money on other mods while keeping the stock cam is a waste of time and money.

 

Okay so cams themselves are cheap. If I don't have to tilt the motor or loosen motor mounts, just pull the radiator, then I'm open to it. I would upgrade the radiator at the time anyway.

Another question, and maybe the answer is more technical than I'm ready to digest, but is there a good cam option will work well in both builds:

1) 1989 L98 short block 350 with AFR 195 heads, FIRST TPI intake setup, requisite fuel injector, pump, and FPR upgrades, long tube headers, true dual exhaust
2) 1989 L98 block 383 stroked, AFR 195 heads, FIRST TPI intake setup, requisite fuel system upgrades, LT headers, true dual exhaust etc.

Dyno tune for both. What kinda power might one expect from the first build?

Thanks all!

 

I do not understand going through all of this effort and staying with a long runner intake (FIRST). This just makes no sense to me. I would be swapping to a much better intake (MRII).

 

Well, sort of. You already know I agree with KSA Aaron: it's silly to spend all the money and time on all these upgrades and then hobble it with a FIRST. Depending on exactly how radical you want to go, you're going to leave 40-50hp on the table...or more. I'm just going to tell you that the car you see in my avatar was a 396 with heads very similar to AFR 195 "Race" (Competition) heads and a ported stock intake (like a MiniRam, basically). It had a still-small XFI cam (but with tighter lobe centers so a lot more overlap). Nobody - and I mean nobody - who ever rode in it would describe it as a "high-RPM" engine that was no good for street and autocross duty. People always thought that car was faster than it really was, precisely because it pulled so hard in the midrange. A friend of mine had a C5 with a cammed LS1 (not sure what else was done to it) that made significantly more peak power but always felt broken to me because you have to get up to 4000rpm or more before it would pull hard.

If you just insist on spending 4-digit money for an intake that's going to limit you to 5000rpm or less, then you're going to want a very mild cam that probably has a lot of LSA (114 or so), in order to maximize your low-end. Otherwise, you're going to have an engine that's only happy in a very narrow range of RPM. The Comp Cams XFI 210/218 is probably the most I'd suggest. With good AFR heads and that intake, you can probably make 3300rwhp with the 350 and 355rwhp with a 383 and you'll still have a decently civilized idle and low-RPM grunt. Anymore cam than that you'll be compromising low-RPM power/response but running into the limitations of that intake on the top end.

 

This is a datalog of some random 1/4 mile run with my 1990 6-speed 383 using FIRST TPI. As you can see I’m shifting from 2nd to 3rd at 6225 rpm. However, the optimum change point would have been approximately 500 rpm lower. My rev limiter is set to 6375 rpm and it can be achieved with no problem.

 

Whether an engine can reach a certain RPM is not the question. The question is where is peak power made and how much can be made. If you're shifting a 6225 but the engine made peak power at 5225, then you'd go faster by shifting at a lower RPM. Which is exactly what you indicated. I don't know what rwhp your car made, but let's say it made 350rwhp at 5200rpm, peak. What I'm saying about the FIRST and any other intake with runners that long, is that if you put a shorter-runner intake on the engine, built the higher compression it would now accept, and tuned for that: it would extend the torque curve up and the engine would make more power and go faster, all else being equal. What if a Miniram would make the same torque at 6000rpm and therefore give you a 404hp peak? Why would anyone choose not to do that?

If someone has such an intake sitting on a shelf and doesn't want to spend money on a new intake, I understand that completely. But if someone is planning to buy a new intake anyway and expects to install new heads and maybe even build a stroker with aftermarket cam under those heads, then I can't for the life of me understand why he would spend big money on a FIRST intake that is, by design, going to seriously choke the engine, make it peakier, and limit power.

 

It looks cool.

That's the only reason I can think of that people want to stick to TPI and force it to work in ways it can't work (well). And to that end, I don't even really thing the FIRST looks that great; I don't like the blockish mono TB and I don't like the runners cast together (from a looks perspective). If I HAD to have TPI "looks"...HAD TO...I'd to a TPIS Bigmouth, TPIS or AS&M runners and then go around bragging about how much low end TORK I had!

 

The look of the FIRST TPI is absolutely part of my attraction to it, this I will admit. Additionally however I am interested in a motor build that has its peak in power and torque lower in the RPM range, even if that comes at expense of total area under the curve and or peak number itself.

My understanding is that the FIRST TPI does this.

I may be totally mistaken about the following, but I am under the impression that using tuned runner length and the subsequent "packing" a motor can achieve a higher torque at a lower RPM than it could with a shorter RPM design. This statement I don't mean to be equivalent to saying a longer runner design can result in a higher peak torque overall (max on the curve), just a higher peak torque at a lower RPM.

That being said, the discussion of longer runner vs short runner intakes really is aside from my intended purpose of the thread.

I more just wanted to get an idea from the more experienced on here what sort of power numbers I could achieve, and how I could achieve them, ASSUMING I had already made the potentially incorrect decision to go with a long runner intake setup!