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Raising exhaust roof on gt40p's

5K views 45 replies 12 participants last post by  damage 
#1 ·
I purchased a set of gt40p heads, then tinmans thread came out. So of course I had to get a grinder, burrs, and crossbuffs. I have read Vizard's book on porting and flow testing. In the book he discusses raising the roof(by essentially lowering the floor). Does anyone know how much can u raise the roof(grind the floor)? Tinmans pic of his exhaust port looks like he was able to raise the roof quite a bit. Thank you
 
#6 ·
Ive never messed with them, but if the exhaust flow now has to make a sharp turn downward and to the rear as it goes into the header I don't see how that could be good...
 
#7 ·
i want to pose a question. Beyond normal porting techniques (not to be confused with simply going and making things bigger), why would you try to really crazily increase the exhaust flow on a lower flowing head?

What happens on a typical street type exhaust system is the upper lift flow gets short circuited to the max flow capability of the exhaust system itself. So, the 180 cfm exhaust port (flowed naked or with unrealistically short stub) will only be capable of moving about 135 cfm on a 1 5/8 header to a 2 1/2 system.

A crazy flowing port, in a situation like this will drive the seat timing need downward, while still needing the good valve activity on the mid and upper lifts. This means the exh lobe need becomes incredibly steep, impractically steep.

For example, the low lift exh/int ratio may be 90%, but mid lift exh/int may 65%. If you do put a decreased lobe ramp into this combination, the seat duration has to increase, and it takes away from cylinder pressure. To Recoup, the static compression has to be increased. A Similar effect to putting a larger exhaust on a motor and losing low/mid output.

Running a 180 cfm port on a 1 3/4 header to a 3" system is a whole other situation and can really take advantage of the port AND work properly with the intake and also make lobe needs a whole lot more practical.

The valve event needs of lower flowing inlet paired up,with a very high exh/int ratio drive towards very delayed and shortened timing, something that is not available in any ots cams have ever seen.

The cylinder sees the entire inlet path and exh path capability, not just what gets measured on a flow vench with a naked head.

Just throwing info out for to think about.

With all my thoughts on the valve events, i forgot to add a critical part of exhaust porting. The flow path has to be directed into the header , the flow hitting the header at an angle kills all the port work effort.

Case in point, i have some old crane fireball heads that flow crazily on the exhaust side, But the flow exits the port at a crazy angle. On the last set of 351 heads i did, we got 150 and it directed perfectly out of the face at a nearly perfect angle (90 deg to the exit face (header flange).

In the old days, we ported to the mr gasket dimensions and the roof of the port always angled up. Looked great, flowed great, until we got older and noticed where the flow was really going
 
#46 ·
What happens on a typical street type exhaust system is the upper lift flow gets short circuited to the max flow capability of the exhaust system itself. So, the 180 cfm exhaust port (flowed naked or with unrealistically short stub) will only be capable of moving about 135 cfm on a 1 5/8 header to a 2 1/2 system.
Where do you get this information? And I'm asking this in the nicest way.

Reason I ask is theoretically a 1 5/8th straight pipe can flow 200cfm. Mandrel bends aren't going to lower that down to 135cfm. 2.5" exhaust can flow 500cfm. Considering we are dealing with 8 pulses and not constant flow, that isn't an issue.
Not even to mention scavenging and that fact that the air is under pressure but cooling as it leaves the chamber.
 
#8 ·
Damn Buddy, fingers sore? ;)

Your post is so on the money. That's why people have to consider the entire exhaust tract, same as you have to consider the entire inlet tract.

From exhaust valve seat and chamber to collector and muffler is just as important as throttle blade to intake valve.

We've gone as far when testing exhaust setups in one heads-up combination, just moving the X pipe location around to see where we could gain some power.

There "IS" power in testing the little things!


>:)
 
#10 ·
Thanks for all the responses. The reason I wanted to work with the 40p's is to really just work on a motor combination that I contributed too, good or bad. I also wanted to see if I could push some horsepower out of these heads.
Buddy I have read some of your previous posts about header and exhaust size issues. After reading the posts. I realized hooker supercomp 1 3/4 headers and a 3" exhaust dumped exhaust, were in my future.
I was also planning on using a custom cam. For the price of a cam from one of the big companies. A custom cam is in the same ball park price wise. To not use a custom cam would be crime.
For the induction. I was going to wait until the cam was ground but, will most likely consist of a single plane intake and a aftermarket dp carb or Holley hp carb.
One other question I have. Would the cam sbfnut used on his 436hp build have any sort of drive ability or just used for racing?
The cam specs are
238/248@.050"
288/298@.006"
.584"/576"
109LSA installed @ 106 ICL
 
#11 ·
Would the cam sbfnut used on his 436hp build have any sort of drive ability or just used for racing?
The cam specs are
238/248@.050"
288/298@.006"
.584"/576"
109LSA installed @ 106 ICL
I daily that engine with a Holley Systemax. Can cruise around town at 1,600rpm, doesn't buck or surge, idles nicely at 850-900rpm, cold starts in 30 degree weather with the first turn of the key.

All due to a very good tune from a fellow ford nut who also dailies a stout 306" with GT40p heads.

With Rotella 10w-40 oil, mechanical fan, BBK 1 5/8" longtubes, VRS X pipe, Vibrant 2.5" resonators, lightly ported Holley Systemax, 30# injectors, Accufab race 75mm, AFM 4" power pipe, and 90mm lightning MAF, it made 326/321 on a chassis dyno. Torque curve was 1,000rpm lower with the Holley vs the Victor Jr. In the coming months I am going to send my Victor Jr to Aaron Price for an EFI conversion, and order his stock hood 90 degree elbow. This coupled with light weight synthetic oil, and an electric fan should hopefully shift the torque curve back up to where it was on the engine dyno, crossing my fingers to pick up another 25-30hp and break the 350rwhp mark with good ole iron heads. >:)
 
#13 ·
Sbfnut I don't know what's more impressive the hp/tq you achieve, or the drivability of the whole combo. The thread on your build is really what inspired me.

When you do the Vic jr conversion, would you please post a thread on the process? I'd be really interested.

Did you use a efi for increased drive ability and street manners? do you think a well tuned carb would work as well on the street?

I am torn b/t going efi or staying carbed. I have a 79 coupe, and have all the needed pieces for a efi conversion, except a harness, gas tank, and fuel lines. The efi conversion will cost a little more, but is really the route I want to take. Finding a harness for the conversion is proving to be the most difficult item to find. All of the conversion harnesses, articles and threads, that I can find are for later 4 cyl conversions.

The car will be a daily also, but Numidia Dragway is only 15 mins away. A forum member tripleblackvert use to race there, really nice guy with a lot of knowledge. I believe he moved, haven't seen or head of him in a year or 2.
 
#16 ·
What a gem of a post this has been to read. Nice reply earlier buddy! That is exactly some info I was after and now I'll have a look through your other posts. I am currently working on a 306 build for myself and have taken into account intake and cam requirements but realised I haven't looked at the exh side. I actually came on to create a topic about what exh people were using etc for input but your reply above was fantastic.
 
#18 ·
You should be able to, I made 315 or so at the wheels with ported GT40's, ported truck lower, and SMII upper, get compression up, and if all that fails, put a bag of ice on the intake for 20 minutes or better, always good for 10-15HP.
 
#19 ·
Sbfnut, I really don't think you'll rest until you get 350rwhp. I have to agree being able to tune a efi system has its advantages and definitely seems superior. Being able to tune the dashpot as you described is just plan amazing.
Thank you for everyone's posts. I really value and appreciate everyone's response.
Buddy I have read your post numerous times. When you talk about cam timing, A little more sinks in each time I read it.
 
#20 ·
Sbfnut, I really don't think you'll rest until you get 350rwhp.
I will not!! Unfortunately I heard back from Aaron and he no longer does EFI conversions or elbows for stock hoods...Going to get the new EFI 4150 Victor Jr manifold when it is released to the market, and see if I can find a fabricator to help me come up with an elbow that will fit under the stock hood.
 
#30 ·
FWIW, I went through the P head exercise earlier this year despite protests from other to not waste my time. I still don't think it was a complete waste of time as I learned a lot, but my results were slightly disappointing considering the time I invested.



You can dig around and find my thread here if you like, but the most helpful information I can give you are the pictures of the head I cut up for you to make your own decisions.





 
#35 ·
Engine dyno had much better headers and exhaust system, zero accessories, dyno controlled water temps, fresh cold out coming from air vent, and the single plane intake manifold.

Chassis dyno has old BBK 1 5/8" headers, mechanical fan, power steering, water pump, and alternator, air coming from fender, and the long runner style holley intake manifold. Drivetrain was a 14# chromoly flywheel, spec clutch, aluminum pressure plate, T5, aluminum driveshaft, and stock 8.8 with 3.73s.

I will re-dyno the car once I switch to electric fan, delete power steering, and convert to a Victor Jr intake manifold.
 
#38 ·
Hi Ed, I found this a n another site under gt40p porting help
3 main areas of any port
1-pushrods pinch area
2-area over short turn
3-throat area
Decent rules of thumb
A)for 45* valve job, valve size minus 0.200" is a good throat diameter
B)pushrods pinch area should be about 80-90% of throat area
C)short turn area should be about 125-133% of pushrods pinch area
D) exhaust likes to expand, so try to get the opening to be 133% of throats area ( minus valve stem area)
E) air like wider vs taller to help turn,but can't make side to side width diverge or open too fast because the air won't be able to stick to the walls. If you can get 90-95% of intake valve diameter in width over the short turn that's good, that's good get the rest in height.
Where someone can get into trouble is if they make one area (i.e. Throat) bigger and flow a bunch more but don't match the area over the turn bigger
Now the air is being pulled harder(faster) over the turn and can't stay attached. Turbulence, noisy and you'll see the flow go backwards.or manometers bounce around as the air struggles to stay attached.

I've been trying to gather as much info as I can, and take my time.
I really don't want to raise the roof of the runners, but instead increase the distance between the roof and floor by, lowering the floor after the SSR, creating a smooth transition. As sbfnut stated and hoopty5.0 confirmed, the floor of the exhaust is quite thin in areas. The intake SSR and floor is a little thicker and may benefit if the radius was worked as on. I have another pic that shows a vertical crosscut of a gt40p head. The outside wall has quite a bit of material that can be removed(if desired)Luckily, I have 2 sets of heads. One being a practice and experimental set.
One other question I have is if anyone has lowered the valve stem boss in the bowl? I was planning on lowering the valve stem boss by 0.125-0.1875", on the exhaust port. The intake port, I don't think would benefit from this because the flow is the opposite and doesn't create any restrictions.
Would love to hear anyone experiences and/or advice.
 
#40 ·
Thx, man. If you lived closer I'd come pick em up. The pics are more than enough. The helped me out a lot. I know they've helped others out too. Because many other threads talked about them. Took me awhile, but I finally found them. Thanks for the advice. It is well taken. I live kinda in a rural area and at the moment I don't know if anyone does flow testing. If there's none. I am really going to consider building my own. There are some threads and articles on how to do this. Because you are absolutely right. Cut some test some. Right now I'm just copying pics and measuring ports. Not exactly scientific.
 
#42 ·
...Cut some test some. Right now I'm just copying pics and measuring ports. Not exactly scientific...
the bigger question exists when copying these pics and ports, do they really work well on a motor, or simply flow well on a bench? Stick a full length header tube on the exhaust port with similar transition angles and bends, then see how well all that effort panned out.

the exhaust port makes up about 10% of the port length if you say valve seat to collector (30" primary tube). It is critical that the flow is directed into the header properly.

++++++DONT think of the exhaust port as an entity unto itself. think of it as the doorway or entrance to the total exhaust system (or atleast to the full primary length to collector). the header primary tube is what really defines the port flow capability. the exhaust port is simply the opening into the primary tube. IT has to work as an entire unit++++++

Just that thought perspective may change your opinion on exhaust port work.
 
#41 ·
while its not a P head, My latest ported early 351W heads got 158 cfm (28") on the exhaust. We kept the 1.54 valve, and the port outlet was squared up to match an old exhaust manifold (metal sandwich gasket) I have used for years. I will revise this post when I get home and measure the actual template, but from memory, I think its 1 1/8 wide and 1 1/4 tall. If the top of the port outlet is raised, its on the order of 1/8 inch, or less. My template basically squares up the mis-shaped factory port outlet to a nice rectangular shape with symmetry and widens it slightly.

typically when you port match the exhaust on a factory casting head to a gasket such as a MrG 253 (or similar), you have to really increase the height of the port outlet. But the port itself is not being raised. So basically the flow path comes up at some horrendously steep angle, and the flow is not clean either. You get some nice velocity at the port roof but its dirty flow, then its hits the tube at a 30-40 degree angle. Its better to keep the port shape intact and work the contours and casting irregularities.

By going with a smaller outlet dimension, non raised too, you end up with a very effective reversion flow 'dam' around the perimeter. the flow needs to move from the seat to the exhaust tube as 'nicely' as possible.
 
#43 ·
Buddy your post couldn't have came at a better time. After, posting earlier today. I went to start on the exhaust ports. I really thought I was prepared and ready. But, looking at the exhaust ports. I thought, the port leading into the header is the place where the least amount of material can be removed without changing the shape of the port and dramatically affecting the flow the into the header. Yes, your earlier post did sink in. It's really amazing but, before starting this thread I never gave a thought to what happened after the exhaust left the exhaust port.
Thank you Buddy, and everyone for all the help.
 
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