saw on here a while ago the best specs and bump stick for keeping the tpi but i cant find it again? I am gonna tear my 350 down this winter and just refresh the block re ring it and put new seals in it since it has sat for a while and figured might as well put a bigger bump stick in it for some lope. any help appreciated.

 

Not sure what thread you're talking about, but in general "lope" and "TPI" don't belong in the same discussion.

The TPI forum would probably be a better source for usable information.

 

The problem is that "lope" comes largely from large amounts of overlap in the cam profile. This causes there to be a lot of airflow variations in the intake tract. Air being pushed back through the intake valves because they are opening before the exhaust cycle has completed. ThIs has scavenging benefits at higher RPM but is very detrimental to the accurate operation of the MAF or MAP sensor. Causing wild fluctuations of the readings and totally confusing the computer.

Comp makes some cams in it's "XFI" line that are the closest thing I've seen to what you want. Go on YouTube and listen to the Comp 280XFI.

Rick

 

THIS is the fundamental cause of "lope": REVERSION into the intake manifold. It is NOT caused by the exh and int valves "overlapping". It is caused PURELY AND SOLELY by the int opening while there is still pressure in the cyl, and has NOTHING WHATSOEVER to do with "overlap". In fact it could be MADE WORSE by eliminating "overlap" completely: if the exh were closed sooner, there would still be EVEN MORE pressure left in the cyl at the instant the int opened, producing EVEN MORE reversion and therefore EVEN MORE "lope". (of course no real-world cam would ever be made that way, because it would run like total crap; it's just a thought experiment to illuminate what's REALLY going on) "Overlap" DOES NOT "cause" lope; rather, it is just a convenient PROXY (something representative that stands in for the real thing) for "how big" the cam is. A handy way of quickly evaluating "how big" the cam is, without actually having to work out THE REAL situation in detail.

That said, the comments by all above about putting too big of a cam, or an otherwise inappropriate one, under TPI, are all correct. TPI works by way of its TUNED (the "T" in "TPI") effect, whereby it uses the effect of all the air rushing down into a runner and smacking off the back of that cyl's int valve when it closes, to generate a large pulse of pressure, which then travels BACK up the runner and down THE NEXT one, reinforcing that next cyl's fill. Due to the simple physics involved (speed of sound) the effect of the particular length runners (22" more or less) results in this effect being maximized at 3600 RPM, which as anyone who has ever driven one knows, is TPI's toque peak. The down side is, once the RPM gets past around 4500 or so, the pulse effect begins to work AGAINST cyl fill, thus HARMING high-RPM performance.

The key to picking a cam for TPI, apart from AVOIDING "LOPE" AT ALL COSTS, is to choose one designed specifically for the behavior of TPI. It also pays to avoid extremely aggressive ones like the XFI cams since those tend to require careful attention to valve train setup and relatively expensive springs and other hardware.

A cam well and widely known to work well under stock TPI is the Comp "502" grind https://www.summitracing.com/parts/cca-08-502-8. If you have headers and a bit of tuning ability, you could get away with the "503" https://www.summitracing.com/parts/cca-08-503-8.

No way would I recommend a 280 XFI either for a stock TPI, or for someone who doesn't have significant tuning capability. Which, no disrespect intended, given the level of knowledge your post would seem to display, you more than likely lack.

Now, on the other hand, if you do away with TPI, and put some other intake system on it such as the Stealth Ram or MiniRam, the situation becomes altogether different. THEN the XFI cams can do what they do to full effect, and might be a GREAT choice. Of course you'll still have to spend A SHIPLOAD of money, relatively speaking, on the rest of the valve train, just to reduce the chances of parts breakage, let alone achieve the full optimum performance that the cam is capable of delivering; but that's a bit beyond "put new seals". And of course, headers, and TUNING. Tuning will always be a make-or-break consideration with big cams and EFI.

 

I have headmann headers on the car already no offense taken Im not familiar with gm tuning im a Chrysler technician I am learning about the older stuff like this ive only been swinging wrenches for about four years now so im still green thats why i asked id like to keep the fuel injection although the thought has run my mind to do an efi setup from holly perhaps still debating that I may also go to a carbed setup I really haven't decided yet I wish i knew more on tuning these old cars so any input is appreciated!

 

Don't change from TPI to carbed.

 

Indeed...

TPI to some other EFI intake, yes; to a carb, not so much.

 

Welcome to the vast options available for the small block chevy. Choices are nearly unlimited. You can run the OEM TPI or get fancy with a EFI converted single plane intake using a Nitrous Outlet intake elbow for better breathing. It can be difficult to decide but your bank account is a good reference.

But before you get started are you sure you really need an overhaul? Do you have compression numbers that say so? Leakdown test numbers? If it's a low mileage engine you may really only need to replace dried up seals.

IMHO I would focus my time and money getting the motor a good tune first. Once you have sorted out how to tune a TPI engine then performance mods will be much easier to optimize. Everything you change/add will need a new/different tune. Obtain good tuning software and devices to tune with it whether it's a chip burner or Ostrich cable along with the correct laptop for this. Then find a dyno you can work with for tuning.

If don't want to jump onto tuning the car but want a rebuild for peace of mind then I suggest EFI compatible cam and I know most cam mfr's sell them. Keep the cam duration short and stay with stock manifolding should keep you out of trouble with the computer/ECM.

Good luck.

 

I know valve seals are dried up and shot its smoking bad after it gets warmed up. iI know you can do em on the car but I hate doing it that way because if you make a mistake your yanking the head anyway. I am at least gonna pull the heads and re work the heads new springs have new seats and valve put in and new seals. I am probably gonna wind up just going to a 1.5 roller rocker the car has a cam in it already, so im debating on just reworking the upper half and leaving the rotating assembly but when I leak down the cylinders that gonna determine that i just have been lazy and not done compression or leak down yet. I will probably pull the timing cover to see if I can figure the cam out the guy I bought it from had no idea what cam is in it thats part of why I wanted to just replace it so i can get the cam degreed in and get the timing set perfectly. The timing is close now and had a light on it but I just wanna make sure it doesn't have its own specs off the stock timing thats where its set right now.