This is a ready reference for the before and after flow, hp, tq and 1/4 mile ET figures for the intake components and the best bang for the buck TPI intake mods. Stock TPI engine is assumed to be a L98 1987 370 cfm, 240 hp, 400 ftlb with an ET of 14.8 sec @94 mph. The stock heads/cam combo are more restrictive than the intake setup... which is why we see very little gain from upgrading the TPI pieces on a stock motor

Intake Pics For some good pics of different intakes see http://forums.corvetteforum.com/c4-g...painted-2.html and porting http://forums.corvetteforum.com/album.php?albumid=1682

,,,,,,,,,,,,,,,,,,,,,,,,,,,cfm Stock,,,,,,,cfm Mod,,,,,,,,,,Mod,,,,,,,,,,,,,,,,,,,,HP,, ,,,TQ,,,,ET,,,,,,,$
1. Air Box,,,,,,,,,,,,,,,,500,,,,,,,,,,,,,,,,50 0,,,,,,,,,,,,,,Cut Lid/K&N,,,,,,,,,1-4,,,,,,?,,,,,,,,,,,,,,100
2. MAF,,,,,,,,,,,,,,,,,,,518,,,,,,,,,,,,,,, ,658,,,,,,,,,,,,,,Screens Out,,,,,,,,,,,?,,,,,,?,,,,,,,,,,,,,,,,00
3. Ducting,,,,,,,,,,,,,,,,,?,,,,,,,,,,,,,,, ,,,,?,,,,,,,,,,,,,,,,Power Pipe,,,,,,,,,,,?,,,,,,,,?,,,,,,,,,,,,,,, 50
4. Throttle Body,,,,,,,783,,,,,,,,,,,,,,,,822 ,,,,,,,,,,,,,A/Foil/TB Bypass,,,,,?,,,,,,,?,,,,,,,,,,,,,,,,50
5. Plenum,,,,,,,,,,,,,,,,,,?,,,,,,,,,,,,,,, ,,,,?,,,,,,,,,,,,,,,,Porting,,,,,,,,,,,, ,,,,,?,,,,,,,?,,,,,,,,,,,,,,200
6. Runners,,,,,,,,,,,,,,,203,,,,,,,,,,,,,,, 242,,,,,,,,,,,,,,SLP,,,,,,,,,,,,,,,,,,,, ,,5,,,,,,,,5,,,,,,,,,,,,,,600
7. Intake Manifold,,,,,,185,,,,,,,,,,,,,,,222,,,,, ,,,,,,,,,Porting,,,,,,,,,,,,,,,,,10,,,,, 10,,,,,,,,,, ,,,300
Totals,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, ,,,,20,,,,,,20,,,,0.2,,,,,$1300

1. A/Box - Cut lid, better air flow. Use K&N cotton filter as this works better than a paper filter. The main TPI restriction is where the airbox narrows over the radiator, so the airbox will only flow 500 cfm and present a major bottleneck! The solution here is a cold air intake. K&N Air filters generally add 1-4 horsepower.

2. MAF - Removing screens on the MAF works, but do not grind the cooling fins away. For cfm figures refer http://www.thirdgen.org/techboard/al...ctor-info.html. For info about MAF / ecm ranges issues refer http://www.thirdgen.org/techboard/di...-5000-wot.html. MAF reads up to 255 g/s only in ecm. Resolution is limited by the 8 bit A/D converter in the ecm. You have 256 values between 0 and 5.12 volts, or 0.02 volts per step. The change in flow that corresponds to a 0.02 volt step is the effective resolution. The stock 1985-1989 BOSCH MAF sensor is not the major restriction in the induction system. It has been on 10 second cars prior.

3. Ducting - Power duct is no seat of the pants difference.

4. Throttle Body (48mm) - On a stock engine the airfoil will not come into play but it does smooth airflow at high flows. The TB bypass mod removes the coolant from the throttle body thus allowing cool inlet air for higher HP.

5. Plenum porting - Port the plenum especially the ridges just behind the butterfly valve in the throttlebody and on the side of the openings behind the throttlebody, but be aware that there is not much aluminium to take away on the sides. That's the idea, to provide a smooth transition from the plenum into the runners. Air doesn't like to make sharp bends; if the edge is a sharp right angle the air tends to be compressed into a smaller area near the center of the runner, thus reducing air flow.Think of air leaving a horn where the end has an exponential bend; in a SBC at 6,000rpm air is pulsing into the runners at about 50Hz, which is in the low frequency sound wave region. For the same reasons you do multi-angle valve jobs and back cut valves. Start the "rounding" operation inside the upper runners and then port match the openings into the plenum. http://www.gmtips.com/3rd-degree/dox...-port/port.htm

6. Runners - The existing 1.47" runner tubes can only be port matched. Port the output openings to the runners be it large tube runners or siamese so that they are the same size and ensure that the gaskets are the same size. Aftermarket runners from SLP or AS&M are quick bolt on solutions. http://forums.corvetteforum.com/c4-t...-remember.html

7. Intake manifold - Port the stock intake manifold with 3/8 inch radiused inlet
and port match to runners. Heavily ported Accel base is another option

General
More CFM equals more HP. The TPI is a great intake and does a very good job at what it is intended to do, which is make low-end torque and that's basically really all it does very well. TPI just isn't good at making power above 5000rpm. TPI is great for low-speed bursts in 1st and 2nd gear, but it's not a very good intake design for someone building an engine that will have a "wide" power band.

The TPI intake smoothed out to say 550 cfm can allow for good HP if worked right. Factory tpi maxes hp at 4000 rpm. The MAF goes static at about 345hp (no matter what RPM), at .45BSFC. A good rule of thumb for Naturally Aspirated applications is 15% more flow from the intake than the heads. A V8 motor has 4-cycles. With 8 cylinders, 2 cylinders must be on the intake stroke at any given time. So, multiply a single runner flow by 2 to see if you're common flow areas are big enough. For good measure, add 10-15% to allow for volumetric efficiency (VE) above 100%.

Tuned Port
The TPI will never make big horsepower because of the very long total length runners to the backside of the Intake valve. The TPI does one thing better than any other GM style EFI intake system. TPI makes tremendous low end and midrange torque. A 240hp TPI will flat out smoke a 350hp LT1 from the early 70's. A 340hp TPI can have torque over 475 ftlb and be over 400 ftlb for more than 3000 rpm.

TPI design. Uses "shock waves" (harmonics) with tuned length runners that 'push' more air into a cylinder than that cylinder can draw on its own. You can customize the total runner length to take advantage of "wave tuning". The 4th harmonic is worth 4%. The third harmonic worth 7% and the 2nd harmonic is worth 10%. If going for the 2nd harmonic you will use all three boost rpm ranges. The stock TPI uses the 3rd harmonic range for its boost. Most are well aware of the boost that creates. It is best to have the runner volume roughly equal to the volume of the cylinder. This helps in the added boost in flow from the kinetic energy from the ramming of the air mass into the cylinder. If done right and with a good exhaust you can create up to 5 pounds of boost. Based upon stock intake runner diameter, a 350 produces peak torque (400 ftlb)at 3,427 rpm. To be effective, the TPI to a point HAS TO be restrictive. The same restriction to flow is what speeds up the air in the runners which contributes to the wave effect. This means that extra 20 or 30 cfm you get from hogging it out huge can actually hurt performance down low by means of port velocity though it can help it up top. Ignore flow numbers in the intake, and concentrate on using the right size and length. Best place to worry about flow numbers is in the heads.
http://www.thirdgen.org/techboard/tp...culations.html





As the info comes to light I will update this 1st post.

 

Hi I have an 85 z-51.
To do the free ones are ust to do it because it does no harm.
Porting the plenum especially the ridges just behind the butterfly valve in the throttlebody and on the side of the openings behind the throttlebody, but be aware that there is not much aluminium to take away on the sides.
Do port the output openings to the runners be it large tube runners or siamese so that they are the same size and ensure that the gaskets are te same size.
Port the manifold after descriptions and so that the openings are the same size.
On a stock engine the airfoil is not a bang for the buck. The airfoil is what they say when it is the throttlebody limiting the airflow, but anyhow the air passes through easier but for the prize no seat of the pants difference.
Cut lid, yes something it will do and its free so why not. Do use for ex K6N filter as this works good for a longer time interval. Power duct is no seat of the pants difference , but if you feel like it?
Screens on the MAF works, but do not grind the cooling fins away.

The most efficient thing you can do is the exhaust and install long tube headers, there ou will get about 20 hp as the exhaust is hopelessly inefficient as stock.
Good luck

 

A lot of good comments. I have added them to the 1st post. Do you have any flow/hp numbers for the mods?

 

Good idea and follow on from the maf thread Steve,
Looking at the flow figures from the original link,
48mm throttle body 783 cfm
Bosch maf (no screens) 658 cfm
TPI airbox 500 cfm
the restriction is where the airbox narrows over the radiator, so effectively it doesnt matter what you do to your intake system, the airbox will only flow 500 cfm !
The only option is a cold air intake, does anyone have any real knowledge of flow using these systems ?

 

Thanks Sean. I have added your info to post #1. Good point on the a/box restriction. I'm sure the air filter plays its part in the restriction too. A lot of drag racers ditch the filter when racing to get a bit more hp.

 

Nice post
Does anyone have photos of porting the stock l98 aluminum heads to go along with plenum porting, slp runners and Edelbrook base? I would love to do some mild head porting if I had some photos for guidance.

 

Most people focus on the main bottle neck in any system. While doing so is important, remembering everything else is also important. The number of times I see someone say "don't do this, it's the tubes are the main bottle neck" is wrong in saying "don't do this"

We start at the most, 14psi trying to shove air into our engines (atmospheric pressure).. If the air filter and housing can flow say 700cfm but the intake only 450, don't think for a second getting the air box to flow 800 isn't going to help. If the restriction is very slight through the filter/box, then that 14psi outside the engine may be only 13psi to the snorkle, and for guys with MAF, maybe 11 psi past the MAF.. Now our crappy little tubes don't have 14psi trying to push air through them, just 11.

That said, those crappy little tubes restrict things enough, maybe only 5psi is hitting the intake port, and so on and so on..

Point is, replacing the intake with one that flows better will certainly make a big change, however removing even slight restrictions ahead of the intake will allow more atmospheric pressure at the intake, which means more air will go through it.

Next thing to watch when portin an intake, head or what ever is smooth air flow. An abrupt change (like a square edge entrance or a mismatch gasket) causes turbulance, just like little whirlpools of air. If for example, there is a 1/8" edge of gasket sticking into the opening, the air cause a whirlpool getting past that. In the area of that whirlpool there is no inbound air flow. That whirlpool can extend the obstruction that causes it even further. In this case, the 1.5" port with a 1/8" interferance all around it plus the 1/8" whirlpool is 1/4" all around, or a port that flows the same as 1" of smooth pipe.

I've seen some guys hog out the exit holes in their plenum, not good. Now the air hits a sharp square entry into the tube and slows down. Use that material in the plenum to make a rounded edge, a funnel shape and match it to the tube perfectly..


I went through a lot of this years ago and TPI is quite new to me, but having now ported my intake, it's no different than an old Holley single plane sitting on iron heads. Number 1 rule, air is heavy and takes effort to change directions quickly or get past a rough spot. Make everything smooth, curved and even and you'll get a lot more air through the same sized opening.
It's no different than the guys in the 50's putting velocity stacks on their carbs. They didn't know for sure why that smooth, funnel shape game more power, but it did.

 

Great information. Laminar and turbulent airflow is a fascinating subject. In Australia people talk about "porting and polishing". I presume porting and smoothing as per your post is the same thing. Polishing to make the inside flow surface as slippery (low friction) to the air as possible.

Good point about not just focussing on one aspect of the flow path restriction. Every little bit helps.

 

Exactly what it is, porting is opening up, removing casting ridges, perfectly matching mating surfaces. Polishing is removing sanding and grinder marks and making things even smoother to reduce the so called 'boundry layer' of reduced flow along a surface.

Best part compared to doing a new intake, free or nearly free! Guys that win on the track do install all the good parts, but also do all the little things to get an edge over the next guy.

 

I have a ported superram and ported accel base ready to go on my 87 (thanks to a cf member selling it to me). Not being in the trade I took the easier but more expensive way.

 

Bit of light reading for you:
http://forums.corvetteforum.com/c4-t...ng-school.html

 

Here's some great TPI porting videos on YouTube.

http://www.youtube.com/user/3859c1

 

Have pics of every type of TPI/LT1 intake head on albums pm me if I can help. Even welded/epoxied siamesed stuff.

Its not done with little dremels drills or hones (guys have done it but youll be there forever and pissed off) youll have to spend a little money up front on the right tools







Real old one little crude

 

If it were me just do the plenum get some used ASM runners and do a good base call it a day. All that siamesing with the SLPs and Accels (royal pain) imo isnt really worth all the work. ASM you just bolt on and done they are already larger and cant really be ported more

 

Just as a reference...I was told NOT to smooth the CNC tool marks out on my heads...on the intake side. Mine were done by CNC Machine in Florida.

My machinist said that inside the intake at the valve, you want some turbulence along the walls...it keeps the fuel being shot at the backside of the valve in suspension. I realize these are the exhaust ports...but I was just showing them as reference. We pulled the valves and smoothed the exhaust side on the second rebuild when I starved it for oil and spun 3 rod bearings.



I'm here to tell you that I was skeptical, but was quickly a believer after we got it tuned.

>400 FWHP and > 500 lb/ft from a 355 TPI...here is the build.

Of course like anything else it's the sum of the parts that did it...but I got/get more horsepower and more torque from my 355 than Lingenfelter was getting from their 383 SuperRam package in the day...I looked them up to entertain a build.

A TPI will still run out of air...but for short stuff like AutoX and such, mine has held its own against many comers and still gets around 25-30 MPG on the highway.

 

 

Best intake mod is a blower, even with 383 cubic inches she gets all the air it needs

Before that i got her down to low 13 sec quarters naturally aspirated with headers adjustable fuel pressure regulator cold air intake and cut air filter lid.