Trying to figure out how to determine where the real bottleneck is in the intake system (TPI). I have a 355 TPI with Trick Flow 175cc runner 56cc chamber heads (Flow bench shows 242/178 @.500 lift) Using an unported Edelbrock base (claim is flows 220cfm) with a comp 268xfi flat tappet cam with 1.6 rockers and AS&M big tube runners.

Im considering a full stage 4 port job on the edelbrock base moving the total cfm through the base close to 300. My question is whether the heads then become the limiting factor (goal is to extend peak HP as close to 5500rpm as possible) or are the bigger runners or even possibly the cam the largest restriction at that point? not sure how to make the right calculation other than throwing money at it by trial and error.

 

That's really a good question. And I don't have all the answers. But here's two things that hold true when you reach the level of sophistication where you think about the ENTIRE intake system:

1. What you see at the valve is the sum-total of the restrictions before it.

2. The closer you are to the valve the more important any potential restriction is.

I'd be looking for input from the guys who have done a max-effort TPI setup as to which of those parts can yield the biggest gains with porting. End of the day I think you'll find that to get the most out of it you really need to address EVERYTHING, not just one part here or there.

Remember, just because one part flows 250 and another flows 220 doesn't mean that you get "whichever is lowest". You get something lower than the lowest number, since the losses stack on eachother.

 

"Remember, just because one part flows 250 and another flows 220 doesn't mean that you get "whichever is lowest". You get something lower than the lowest number, since the losses stack on eachother. "

That is true. Generally what people do is take the intake port of the head and find the cross sectional area at the opening to the intake manifold. Make the intake manifold the same or slightly less and have a small taper increasing in CSA to where the runners bolt on.

Hopefully the runners are larger in CSA than the head opening. If not then things are a compromise. The cross sectional area of the runners should at a minimum be the same as what the heads are but a little bigger is better.

 

TPI's problem isn't so much the "restriction"; it's the "tuning".

The runners work like a musical instrument, with their length determining their resonant frequency. FOr those, it works out to somewhere near 3600 RPM, give or take, depending on what assumptions you can make about temperature and some other things. TPI gives this HUGE reinforcement from CONstructive interference at that RPM therefore. (which is its major strong point) The penalty of course is, it DEstructive interference at RPMs above that; which starts being significant at about 4200 RPM, and basically kills any hope of meaningful usefulness above about 4800.

So don't read too much into flow #s, they won't tell you much about what effect things will have, in that system.

 

Red- you had enough yet?

All above posts are full of good info. They do not conflict- they are complementary to eachother. They are all addressing different aspects of intake design from slightly different angles.

The flow aspect (relatively easy to understand) and the "resonance" or "tuning" aspect (less easy to understand until you realize that intake pulses are like the sound waves of a guitar string traveling through the intake port- VERY important in a long runner intake like the TPI that was designed specifically to take advantage of them).

So, maybe it would be better to boil this down to a simpler equation. What do you want to do with this motor? What are your goals and expectations? That would clear away the techno-clutter and aid greatly in avoiding a lot of idle theory discussion (which makes everyone's eyes glaze over) as well as avoiding the pitfall of "just throwing money at it," as you are rightly concerned about doing.

 

I'd match the intake to the CSA of the head entrance with slight taper as 1989GTA has mentioned. Getting 300 cfm out of the base requires a larger CSA than the head you are using. There would be a step in the flow path.

If you did have a 300 cfm lower intake, then head would be the restriction along with the runners and cam They all work together and have their own restrictions. Intake can flow but runners being long will limit the ability to tune power above a certain rpm, but then the cam is too small to support higher rpm anyway. Fun stuff.
So larger diameter and/or shorter runners will help tune for peak hp's higher than 4500-5000 rpm where most typical LTR TPI's peak, and the 175cc heads could support it but really should have a 195-200cc by 5500 rpm's and higher. Once tuned for higher rpms, the 268 cam is more of a 5800-6000 rpm shift cam in a 350-355 motor (depending). Next size up or 2 will support 6000+ rpm and match the heads and intake flow but need shorter runners to tune for that range. Results are alot more hp overall with that configuration. Read posts by 1989GTAtransam above. He shows how you can keep a longer runner setup but have it tuned for high rpm and big hp.


You can have various effects with intakes on head flow. Depends on the quality of flow in the head, there have been tests where more intake flow may hurt a head's ability to make power and then some lower flow intakes that helped a higher flow head make power. Just because a head port flows X cfm doesnt mean its going to make Y hp based on that cfm. Velocity profile through the port is very important. Making effective use of the cross sectional area the port provides will determine how good a head is at making power. THen applying a proper shaped intake runner to the head port to limit flow loss will keep HP potential up. There are alot of variables when testing combos. Each component will have effects on the entire system.

 

Let me add that you don't want the intake tract to get bigger, then smaller, than bigger etc. The air will speed up and slow down which takes energy out of it for cylinder filling. As stated at a minimum keep it the same size all the way but better if you can put a slight taper to it. With a slight taper the air will gradually speed up as it nears the cylinder head which promotes better cylinder filling.

 

i'd try to find a used TPIS Mini ram intake and ditch the stock style stuff altogether.

if you can't find one of them, then get an LT1 intake converted.

 

What youre thinking about doing wont get you close to 300cfm guarantee it.

Sound advice thats why getting a good job on those is a bitch ($$$) most make the runner entry way larger for oohs and ahs maybe the entrance to the head and the rest barely gets scuffed and smooth (where its hard to get to) thats where all the work is. Lotsa time.

 

Sounds to me like my current setup up will pull low 300's hp at the wheels and the best investment of time and money for a big jump up would be a Miniram and then better heads. just porting the edelbrock base as this point doesnt seem worth the bang for the buck.

 

You could cam up if needed absolute max hp and et's. Way to crutch a small head is to throw more cam at it. Will hold powerband out past peak allowing more rpms to turn in gear. Basically allows you to stay on or near peak hp longer which will get you quicker et's.

But basic good port job on base with good big tube runners will get you low 300's whp. Heavily hogged cut welded base plus matching huge tube or heavily siamesed runners can get 20-30 hp more and be closer to 350 whp on those heads. The cali guys are doing that and more with better heads and bigger cams.
Same thing with miniram or hsr but those will be easier to work with imo. Little to no port work needed. Not sure how much more the heads will support. Id say 360-370 whp, maybe tad more on optimized setup. They are similar to a lightly cleaned up lt1 head which does 350 whp stock and 370-380's mild port. 250 cfm but require a healthy cam and some rpm

 

Would a comp 280xfi be overkill in my setup? (I have no emmisions, long tubes, 2500 stall and 3.73's

 

Edit: the roller version wouldnt be but flat tappet version seems bigger than i remember for that duration, but it needs more stall to show best gains at the track. Even with those heads it will work ok but id rather see miniram or stealth ram with that cam. My friend ran that cam in roller version (230/236) with old afr 190's which flow 250-260 cfm advertised on a .060 over 350 with stealth ram. Made 330 whp before suffering valve float. Requires very stout double springs which he didnt have. Dyno curve went crazy after 5700. If it had control it would have made 350-360 by 6200-6400 rpm drawing the line out. Very nice cam. Sounded good and ran hard but on that motor it wanted 4000 stall. It had 3600 and still 60'd abit slower than i'd like to see

I notice the flat tappet versions have bit more duration for the same advertised spec compared to rollers. Should be abit rough, it will lope pretty hard even on a 113 lsa. Would require alot more compression and stall. Your 268 is a better match

 

Yeah, I had a 3500 stall in my SRT 8 300c and it was a bit annoying daily driving. I guess I will stick with the 268 cam for now and upgrade if i go Miniram etc. My tune is pretty good but I have not had a baseline dyno run yet or specific dyno tuning so I need to set the basline then max out the tuning and then decide if the results are good enough. Really, I would like to run mid to high 12's as the ultimate goal.

 

Yeah 268 with basic port job on lower intake to match heads best will run 12's easily. How heavy is car?

3400 ish lbs and 330whp even with valve float it ran 12.2 at 112 in texas on a mild spring day. Air was ok but not great. Still mid 12's in hot air. My 3680 lb pig went mid 12's with just 320whp 318 wtq

 

I think I have finally decided to go HSR and simply bolt in on to my current setup. My guess is it should be good for at least 30 more RWHP on top end than my current large tube TPI setup.