Why kind of flow numbers should i shoot for when porting LT1 heads for a SC?? Not sure what the limit of the SC is but lets say its going to be no more then 9lbs. IM going to assume that a larger valves with a boosted setup are even more benificial then with a NA setup. What is the max safe size. Dont want to push the boundries of the head.

I did a search on the subject and found alot of referance to welding meterial in the porting process.
Where in the head do you weld, what does this do to flow,what kind of increases would you see.

Sorry for the 897 questions, just an inqusitive mind. Thanks.

 

I know Total Engine Airflow is getting high 290's out of their LT1 heads. I would say that is pretty much the max from a pro.

 

well I may have to eat my words, I heard from a friend TEA was getting 290's out of the LT1. According the the graph on their website its 265@600.

 

2.00" and 1.56" in/ex are fairly normal for an LT1 head. Flow numbers vary a bit, but I think good home ported numbers would be 265+ in and 200+ ex. These are peak number though and do not tell the whole story. Low and mid lift numbers are very important on street engine IMHO, but I do not run a blower, so hopefully others will chime in.

 

Cool, i think i read those valve sizes somewhere.

I know peak flow is only a part of the story. Im going to go out on a limb and guess that low and mid flow numbers are less important in a boosted application. Anyone with hands on experiance please chime and and agree or deney that theory.

Thanks guys

 

Hello all,
I know I'm a new guy here, but I am an avid head porter and have several cars out their running very good numbers. Flow numbers are very important for a few things... one is selecting a camshaft, one is for the person performing the work to read the ports and figure out what changes the ports are responding most too, and one is for guys on here to brag about... not saying it is impossible, but i think 295cfm at 28" of water on a superflow 600 bench or equivalent is kinda wishy washy. I have seen numbers massaged by the flow bench operator to show some very impressive figures.. does that mean the end user is getting a better set of heads?

295cfm, there would have to be a good bit of welding done to the port roofs, the valve seats would need to be machined out and larger seats to accept bigger valves would need to be installed, the chambers would have to be seriously worked to unshroud the newer larger valves or else i say no its not a real world figure being reflected and to do some extensive research on what it takes on the LT1 casting to achieve these numbers. There is simply not enough material to work with in the port to achieve the volume needed for those numbers and if you took one out this far and attempted to run them on the street, the owner would be less then happy with anything under 5k RPM.

The numbers reflected between .200"-.450" are probably the most critical numbers to study when choosing heads. The peak number is reached once during a valve event... the others are seen twice.. Once on the valve opening and again on its return to seat. This accounts for a very large portion of intake event. Dwell of the cam lobe is responsible for how long the port sees max lift.. So for a std street cam, the dwell is not overly long in duration. On a radical solid cam designed see upwards of 6500 RPM, a huge intake port is best suited with great max lift flow numbers.. Compare the lobes of a hydraulic roller to a big solid roller.. The ramps of the lobe look parallel to each other indicating a rapid ramp rise and fall rate meaning that the cam is designed for max dwell at peak lift. This setup is notoriously used for drag racing where the engine generally does not see operating RPM ranges under 5000 RPM. Also, extremely large stalls or 4.56 rears or better are used to handicap the torque loss at lower RPM's.

For home porting, a solid suggestions would be to work the bowls and blend them to flow smoothly... don't concentrate terribly on the valve guide boss besides smoothing it and giving it a gentle taper towards the valve opening, as it actually serves a purpose...as air nears the back of the valve, you’ll notice the port widens... this is to compensate for the guide boss and valve stem. Removing the boss or too much of it gives a larger volume closer to the valve.. You want the port volume to remain at a minimal but constant decline as you near the valve throat. Large changes in volume will have a negative effect on port flow velocity and will create erratic air movement. Street heads will benefit most from velocity rather then all out flow as the velocity at lower RPM is what fills the cylinders. As the RPM increases, the demand for air increases until the port cannot support the airflow (choke).. Which on street cars is generally well within the characteristics of the selected camshaft. Smooth the port walls; port matching although not agreed upon by all porters, in my opinion and experience yields positive results. If volume is desired in the port, gain it from removing material from the roof and NOT the floor.

The short side radius is actually not bad on the LT1 castings, a very light touch up can be beneficial, keep in mind a longer sweeping radius will prove better then a short blunt turn.

The valve job..... Very important.. Its a shame that so often heads are brought to me that have very promising port work only to have a second rate valve job and throat work performed by a run of the mill repair shop machinist. All that work you've done on the ports needs to be complimented by the valves, valve throat, and valve back cuts. The throat.. From the point the valve seat is set in the casting, this can be opened up at an angle that tapers open toward the valve seat mating surface... this gives the air an opportunity to clear the valve lips at lower lifts gaining more flow sooner in the intake event, and lasting longer upon the valves closing. The back cuts on the valves and the valve job performed on the valve seats is something of a mystery to most machinists...some things work better for some and others better for others.. A good 3 angle valve job can do wonders... most high end valve manufacturers have very acknowledgeable back cuts that can be used to obtain high goals.. The more serious racer can benefit from a differed back cut and a 5 angle valve job.. But these add a premium to the performed task.

Head porting is different for engine to engine and from goal to goal.. a good port job will still yield very good gains in forced induction. The volume required for forced induction is not as critical as N/A applications as the engine is working at VE's higher then 100% and port velocities will be incredibly higher.

When porting the main ports of the intake side... be wary of the pushrod slots as the material here varies in thickness and punching through it will require an experienced welder to repair.

The rocker arm stud hole on the LT1's should never be an issue of puncturing as if your removing that much material from the port sides your heads will be severely handicapped for good flow. LT4 castings have extra material in the port in this area as the rocker stud hole comes very close to the port. In most cases this is removed and the hole filled and blended smooth with the port wall to remove the port hiccup. The stud bottom is well above the puncture location when removing the aluminum here so don't be terrified when you break through.

Puncturing a water jacket in the head is when you can almost certainly in most cases use the casting as a paper weight as these are the most difficult to repair. No need to remove that much material unless you simply feel the need to ground holes in the heads for personal reasons.

On polishing the ports.... some companies claim that CNC machines leave a ridge from the machining process that helps flow by giving the air a cushion to ride on rather then contacting the port walls and creating drag... I disagree.. A polished wall although to the naked eye looks slick and shiny but magnified still has ridges. if a cushion of air is created you are essentially creating a smaller port. The less turbulence created on the outermost edge of the flow charger... the more charge your moving through the port.. Its that simple. Ports in the past were not polished highly because carbureted engines needed the turbulence to completely atomize the droplets of fuel siphoning from the jets in the venturies. The turbulence gave the fuel a chance to effectively combine with the air charge to form an adequate combustible mixture. On fuel injected engines, good injectors do a majority of the work for us, and properly formed valve throats that create a swirl effect effectively complete the task at hand to give this mixture in the chamber. so polishing the ports is a desirable method on fuel injected engines.

Ok ..blah blah blah.. I have just written a TSM on head porting.. if you’re reading this then you successfully made it through without drooling on the keyboard... I hope this helps.. and one last tip... a lazy person makes the best head porter.. be patient.. Work slowly.. and just because there are big nasty porting bits out there.. it doesn't mean to use them until you have fully grasped where the material is supposed to be removed.. my suggestion is to work with the lighter finer grit cartridge rolls through your first head porting experience... remember its always easier to remove a little here and there then have to put alot back somewhere you went too far on. Good luck and have fun!

Chris
Next Generation Motorsports, LLC

 

hahah..........great info chris......welcome to the board!.......experienced input on porting is always welcome here.

 

WOW, thats alot of amazig info. thank. Deffentally going to have re-read that a few time to get some more info. I still have a few qustions though.

How do flow numbers for a SC engine differ then and NA engine. Would losing a little low lift flow to gain flow alittle high be ok with the SC on the street. ie would the boost make that feel ok at lower RPMs? IM doing these heads for my friend for a project at school. I dont want my porting to be the weak link in his engine.

In regards to welding, im not planning on welding these heads but id still like to know something about it. Where would you weld, and what does it do for flow??

How important would going with bigger valves be for power. How much would I pay for bigger valves and seats and a valve job. I dont think we can do the valves at school.

Thanks again

 

Hello everyone from Brian Tooley here at TEA. Just so that everyone knows the deal, the head flows 290 cfm @ .600" and does have weaker mid lift than the smaller port. The smaller port is around 185cc port volume and the new one is around 200cc port volume. To give some midlift info our small port goes 200 cfm @ .300" the bigger one more like 190-195, but is still better then a GTP head that went 185 @ .300". I did not do this port, one of my employees that has a LT1 car (not Brent) did it, his name is Tony. They say that even a blind hog can find an acorn every once in a while, and those that know Tony know what I mean. I have been porting SBC heads since 1993 with a flowbench and I can not get the heads to go over 280 cfm. So it is real, believe it, it is not a mystery how a 200 cc port can flow 290 cfm. FWIW, we have flowed a cylinder head manufactuers Competition level CNC ported 195 head that only flowed 270 cfm @ .600". So I hope this helps dispell the disbelief, but in all honesty, if I didn't see it myself I wouldn't believe it, but then once we checked the port volume I knew why it would flow that. We don't have to weld the regular casting except sometimes on one pushrod pinch, but not everytime. There is a different casting out there that has a exhaust flange that looks like a LT4 and that casting does not work with this program. That is the beauty of CNC porting something, a hand porter could not consistently port these heads to this size without durability issues, but we can.

 

In reply to the questions unanswered, the difference between porting for supercharger or naturally aspirated engines:

both flow characteristcs should be alike... your changing the way the air is introduced with a SC setup.. it is not relying on atmospheric pressure to fill the cylinder.. however... if you have the most efficient port available naturally aspirated.. it means that the restrictions have been minimized and this in turn will prove beneficial for flow with a supercharged application as well....

now on the exhaust side... increasing the port volume is beneficial as your going to have an over abundance of spent charge to remove from the cylinder as to what it was typically designed for.. this is why exhaust systems and long tube headers yield such high gains over stock with SC applications.. generally much more then on a naturally aspirated. both NA and SC have the same difficulty removing the spent gas, the Super Charger only helps on the intake side... so the main difference is the amount to get rid of. most of the time camshaft selection here will show a significant degree of duration above the intake side on a split duration sltyle cam. this help to balance the intake and exhaust events in terms of flow.

an example of stock valves vs. aftermarket larger.. I have built a hotcammed car with stock valves that made 372/368 at the wheels using LT1 castings and a larger cam (GTP 6) with LT4 castings and stock 2.00 valves that made 410 RWHP, they were both GM stock LT1 bottom ends. valves could be a big plus running boost as temp will be much higher and SS valves can handle the abuse much better the the stock valves. you see on the top fuel motors inconel valves which help prevent melting and burngin of the valve do to the fact that they run nitromethane which actually releases oxygen as it burns creating a ton of heat and in the end they generally see 6000+ hp with numbers like this many things change but to err on the side of caution, we generally use the lighter SS undercut valves.

the welding question..
the welding refered to would be done on top of the head, you are essentially adding material to the top side of the head so that porting and removing material above the breakthrough point of stock castings can be achieved. a taller port tends to be a better flowing design as you can well see from the differences in flow capabilities between LTx and LSx motors. notice the ports on lsx engines are narrower and much taller.. this gives a few benefits to flow.. one it allows the angle the air flows towards the back of the valve to be less then stock and helps flow at both the long and short side radius. early versions of better heads for the stock 350 would be your 18 degree heads where the intake ports are noticeably higher then then on the 23 degree heads. GM made the heads similar to this layout on the LSx models and it has been a vast improvement in these recent years.


As for the 290 CFM, TEA.. that is very impressive... 200 cc port volume is quite large on the LT1 casting... and with that in mind yes I agree it could be done. that is a full 30cc of port volume.. he must have been very lucky to get that without finding some neat new caverns lying behind the port material.. but hey if he got it and it worked that is a notable acheivment! well done! where are you guys located, i want to say i heard in NC somewhere.. we are down here in wilmingotn/Souhtport area and will be heading to Aiken SC for the vette get together on Feb, 21/22 if your headed that way would be great to meet you guys! always fun meeting everyone in your neck of the woods! hope to hear from ya!

Chris

 

We're located in Bowling Green, KY......home of the Vette! Sorry, but we won't be making it to the get together.