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Have you tried radius-ing the bottom/outlet of the lid?
yes. the one I'm using now has a radius but took a lot of material off the underside of top to make more clearance above runner entrance. could have made the radius to shallow. was thinking radius needs to be more gradual with TB. going to radius another top and see what that does. really don't know. Trial and error. #1 is the worst flowing runner. the rest of the runners will flow 236-238cfm with TB's. going to make mold of runners. maybe that will show something
was thinking radius needs to be more gradual with TB.
When I did mine, I started the enlarging (of the bore) right below the throttle plate to allow for as much time/space for the air to spread out and slow down before entering the plenum. No objective testing on this design though. In my mind, having the air shoot out the fixed dia bore of the TB, it all just slams into the floor of the plenum. Then it's got to get moving again, perpendicular to the way it came in, and either one way, or the other.
So recently, this is what a guy that is local to me and sent me this from his stock(ish) 82 vette. 13psi on regulator. Not bad actually and he plans on doing some porting on his stock manifold.Things that he did so far are on there. I'm assuming you are close to his numbers stock? If not, post them please. Any news?
Last edited by Buccaneer; Feb 12, 2025 at 07:36 PM.
I'm assuming you are close to his numbers stock? If not, post them please. Any news?
I agree....I can't wait to see how this thing performs in the car!
But my god....180 RWHP!? That would be "good"/fine, normal for a stockity-stock, stocker...wheezing through crappy Y-pipe, single cat etc...but that guys got no cats, dual exhaust. I guess the converter and gears could be hurting the actual number, some, but I'd expect to see 200-210 RWHP for a good-exhaust, L83. That car has the exact same mods Jim's car had....plus some gear. What's he run in the 1/4 mile? Mid 15's I'd guess. IDK....I think that thing could use some tuning of some kind? Or something.
[QUOTE=Buccaneer;6553855]So recently, this is what a guy that is local to me and sent me this from his stock(ish) 82 vette. 13psi on regulator. Not bad actually and he plans on doing some porting on his stock manifold.Things that he did so far are on there. I'm assuming you are close to his numbers stock? If not, post them please. Any news?
[/QUOT
that's great results for stock. looks like 1982 has better exhaust manifold than my 1984 do.
no news. it's going to take a while. I'm old and slow. still learning how to tune.
I still think 182 at the rear is a good number for a 43 year old motor. 200 at the motor new and 182 at the rear 4 decades later is pretty good with only a exhaust change. I have seen over the years an 82 with 40k miles with only 175 which is more common than not. This is an original car.
As far as what you are doing on the top plate, are you saying that you are undercutting the TB hole and you saw an increase? Do you have a pic? That is awesome and have never heard of that before, so I learned something new.
this shows finished ports. not perfect, but good enough. no dyno test. could be good, could be a total dog. Has been a fun journey regardless of results.
235cfm with TB's. this is manifold porting job #5
intake that is on the car now. smaller runner csa, a little uneven build. 212cfm average with throttle bodies. If I'm remembering correctly cfm was the same with heads connected as well, been a while. this is manifold #3'
I was drawn to this project because the runners flow so little and wanted to know why. Was actually happy with stock performance. I'm not a racer. but very curious.
as small as these runners are on the ends, there's usually an even smaller section in the runner, uneven sidewalls, casting protrusions. Read that port matching helps. It does help, but not a lot on its own. also tried tapering entrance without port matching head side, didn't do as much. doing both is more effective. My theory of having a larger CSA opening to fill the runner more efficiently is roughly based on how bell mouth or velocity stacks work. What is a Velocity Stack
take all this with a grain of salt, has not been dyno tested. don't know if it's going to work.
going to make molds of runners now, because if it works it's not coming back off car. car will be done. can move on to another car. old school carburetor 2500lb car 400hp that I built years ago. get it running again. need to do this before I get too old to work on cars.
I learned so much from forum and I want to give back too forum, but this isn't dyno proven. take it for what it is.
my advice to anyone porting an intake, buy another intake to port so the option to go back to stock is always there. I still have all the stock parts in a box. advice if welding, don't weld to floor or sides where top mounts to. Can weld right up to it and not weld it. to thick, could cause warping. port runners before welding. thin walls are easier to weld.
Last edited by mike1111; Feb 13, 2025 at 01:01 PM.
I still think 182 at the rear is a good number for a 43 year old motor. 200 at the motor new and 182 at the rear 4 decades later is pretty good with only a exhaust change. I have seen over the years an 82 with 40k miles with only 175 which is more common than not. This is an original car.
isn't 182hp at wheels around 230hp at crank. better than stock for sure.
As far as what you are doing on the top plate, are you saying that you are undercutting the TB hole and you saw an increase? Do you have a pic? That is awesome and have never heard of that before, so I learned something new.
I wouldn't say that. Sorry if I gave you the wrong impression. what happened, modifying the underside of top to the point of losing flow. using another top it came back. It's not a gain at all. just another failed attempt at trying for more. another part for recycling. that's why I said less is more. I'm done now. I'll be happy with what it is. time to finish learning how to tune. however long that will take. Hope you can start producing your manifold again if that's what you want and work out ecm to sell.
Also have a lightweight old school v8 car to get going. not GM. I custom build a ton on that car years ago before the internet. books and speed shops back then. no forums for that car. on my own. can use some of what I learned here.
these GM forums are great! I'm trying to give back the best I can. never been part of a forum before. I've always worked on my own. Learning from books, now internet.
Mike
I still think 182 at the rear is a good number for a 43 year old motor. 200 at the motor new and 182 at the rear 4 decades later is pretty good with only a exhaust change. I have seen over the years an 82 with 40k miles with only 175 which is more common than not. This is an original car.
IDK what "43 years" has to do with anything. How it's cared for does, though -which we don't know. You might? I agree 200 crank...182 at the wheel is pretty good...w/NO mods. The exhaust that he's done = a pretty large increase in power. To wit my Kart engine -a 36 year old engine (that was poorly cared for by it's PO), did 241/342, wheel... with dual exhaust w/no cats and "free mods". We know that most late TPI's RWHP ~210ish, so...that's lookin' like ~30 hp from exhaust, to me.
I'd BET that the C3 single exhaust is similarly restrictive compared to the C4 single exhaust, on the L83's. What I'm getting at is the 180 number IS pretty respectable for a STOCK L83...but your buddy's ain't stock and therefore, I think he should be closer to 200 RWHP. Anyway, it' is what it is...180/236. I don't think it's that great, w/the exhaust/no cats. What's it trap in the 1/4?
Originally Posted by Buccaneer
As far as what you are doing on the top plate, are you saying that you are undercutting the TB hole and you saw an increase? Do you have a pic? That is awesome and have never heard of that before, so I learned something new.
Pretty sure I've brought this up on the forums decades ago. I did that mod and posted pics of it in early '00's. You're making the underside of the TB hole in the plenum, look somewhat like the mouth of the TB; radius'd inlet...in this case, outlet. In order to get the most effectiveness from this idea, the taper needs to start right below the throttle plate. I'll try to post a pic of my old lid, tomorrow. But it sounds like Mike found that the idea was fruitless or even detrimental.
Last edited by Tom 400 CFI; Feb 13, 2025 at 06:40 PM.
Are you by chance using a sf750 SuperFlow bench to check the numbers or something similar? We did a lot of testing way back on TBs on that bench I think it was without looking at my notes. Anyway, I'm sending my stuff out to a well known company in OK to have some work done. I'm only doing that because I don't feel like doing it and rather just pay a good company to do the work and match my ports on the Renegade to see how much you can get out of it. Also, when I get that back, I will be installing my new one off set of 2.21" TBs that we did for my motor. Should be interesting... I feel pretty confident that some more tuning may be in order to get me into the 11s with the new stuff. 11.99, here I come!
this is what mine looks like. Took even more material out then this picture shows. didn't work.
Originally Posted by Tom 400 CFI
IDK what "43 years" has to do with anything. How it's cared for does, though -which we don't know. You might? I agree 200 crank...182 at the wheel is pretty good...w/NO mods. The exhaust that he's done = a pretty large increase in power. To wit my Kart engine -a 36 year old engine (that was poorly cared for by it's PO), did 241/342, wheel... with dual exhaust w/no cats and "free mods". We know that most late TPI's RWHP ~210ish, so...that's lookin' like ~30 hp from exhaust, to me.
I'd BET that the C3 single exhaust is similarly restrictive compared to the C4 single exhaust, on the L83's. What I'm getting at is the 180 number IS pretty respectable for a STOCK L83...but your buddy's ain't stock and therefore, I think he should be closer to 200 RWHP. Anyway, it' is what it is...180/236. I don't think it's that great, w/the exhaust/no cats. What's it trap in the 1/4?
Pretty sure I've brought this up on the forums decades ago. I did that mod and posted pics of it in early '00's. You're making the underside of the TB hole in the plenum, look somewhat like the mouth of the TB; radius'd inlet...in this case, outlet. In order to get the most effectiveness from this idea, the taper needs to start right below the throttle plate. I'll try to post a pic of my old lid, tomorrow. But it sounds like Mike found that the idea was fruitless or even detrimental.
I believe this is your lid tom.
Last edited by mike1111; Feb 13, 2025 at 07:58 PM.
only runner #1 flows less and only a couple of cfm. going to retest. I don't get it. going to experiment with some clay.
your top would flow fine. for whatever reason (to thin) the one top I did loses cfm. I even filled the gaps with clay and that made it worse. taking small numbers. never notice the difference. for kicks, used 4 throttle gaskets to raise TBs gained 1 cfm. it was worth trying and good to know the results
made a mold. one small section of height is 1.5 instead of 1.53. I'll have to check that. dotted line basically shows the original port. mold show all my porting imperfections. the width is about 1.125, except the first 1.5 of entrance tapers from 1.2 to 1.125.
all tested with head. @.625 266 cfm @ .500 253 cfm. the runners on this head all flow different.
this is one of the outer runners. outer runners don't flow as much as the 4 inner runners. this represents the lower flowing runners. don't want to port head.
stock TB's @.625 222.16 cfm. @.500 214.4 cfm mildly modified top
50mm velocity stacks @ .625 226.28 cfm. @ .500 218.24 cfm mildly modified top
2.125" tube with clay entrance @ .625 231.74 cfm @ .500 222.56 cfm more aggressively modified top.
50mm velocity stacks cfm @ .625 228.89 @ .500 219 cfm more aggressively modified top.
this shows that bigger throttle bodies flow at a higher percentage.
270cfm head would most likely max out this intake
with no head. intake only
2.125" tube with clay entrance 236cfm
50mm velocity stack 234cfm
stock TB's 223.84
2" TB's should work good for my use
tested stock exhaust manifold today. for kicks. It's ported as much as possible.
CSA 1.43 x 115= 164cfm header pipe
tested CSA 1.049 x 115 = 120 cfm ported at collector. on a port that flowed 165cfm. gain 19cfm over stock. still not good
CSA .886 x 115 = 101 cfm stock at collector. this is the choke on the stock manifold
I still think the first modification should be exhaust.
starting to layout parts for modifications if anyone is crazy enough to want to do it. once it's on the car and it works. it's not coming back off
no more test to do. done as far as I'm concerned. no miracle here. good enough for me. what I have learned will help me with my next project. another car with no hood clearance.
IDK what "43 years" has to do with anything. How it's cared for does, though -which we don't know. You might? I agree 200 crank...182 at the wheel is pretty good...w/NO mods. The exhaust that he's done = a pretty large increase in power. To wit my Kart engine -a 36 year old engine (that was poorly cared for by it's PO), did 241/342, wheel... with dual exhaust w/no cats and "free mods". We know that most late TPI's RWHP ~210ish, so...that's lookin' like ~30 hp from exhaust, to me.
I'd BET that the C3 single exhaust is similarly restrictive compared to the C4 single exhaust, on the L83's. What I'm getting at is the 180 number IS pretty respectable for a STOCK L83...but your buddy's ain't stock and therefore, I think he should be closer to 200 RWHP. Anyway, it' is what it is...180/236. I don't think it's that great, w/the exhaust/no cats. What's it trap in the 1/4?
I agree with you oddly on this one. I swapped the failing flat tappet cam out of the most limp wristed 350 TBI ever built in a GM truck and used a good 305 peanut roller cam I had laying around in it. 8.75:1 compression, 810 swirl ports, GM 155 178/194 @ 0.050, 0.350/0.385 lift roller cam and some cheap Flowtech 1-1/2" primary headers, dual 2.25" pipes into a single 3" cat and had a flowmaster muffler at that. This one had a clutch fan on it and even a carrier bearing in the driveline. This engine was crank rated at 180 hp and 300 tq and that was with the larger 194/203 @ 0.050, 0.390/0.410 lift flat tappet cam. Like you said, is what it is.
all tested with head. @.625 266 cfm @ .500 253 cfm. the runners on this head all flow different.
this is one of the outer runners. outer runners don't flow as much as the 4 inner runners. this represents the lower flowing runners. don't want to port head.
stock TB's @.625 222.16 cfm. @.500 214.4 cfm mildly modified top
50mm velocity stacks @ .625 226.28 cfm. @ .500 218.24 cfm mildly modified top
2.125" tube with clay entrance @ .625 231.74 cfm @ .500 222.56 cfm more aggressively modified top.
50mm velocity stacks cfm @ .625 228.89 @ .500 219 cfm more aggressively modified top.
this shows that bigger throttle bodies flow at a higher percentage.
270cfm head would most likely max out this intake
with no head. intake only
2.125" tube with clay entrance 236cfm
50mm velocity stack 234cfm
stock TB's 223.84
2" TB's should work good for my use
tested stock exhaust manifold today. for kicks. It's ported as much as possible.
CSA 1.43 x 115= 164cfm header pipe
tested CSA 1.049 x 115 = 120 cfm ported at collector. on a port that flowed 165cfm. gain 19cfm over stock. still not good
CSA .886 x 115 = 101 cfm stock at collector. this is the choke on the stock manifold
I still think the first modification should be exhaust.
starting to layout parts for modifications if anyone is crazy enough to want to do it. once it's on the car and it works. it's not coming back off
no more test to do. done as far as I'm concerned. no miracle here. good enough for me. what I have learned will help me with my next project. another car with no hood clearance.
this would be for someone that wants to keep stock appearance.
top left hand opening has a 1.31" radius compared to the stock 1.06" radius.
stock opening flows 110 cfm.
I cut and welded a piece to open up pitch point.
.415 lift 145cfm
.500 lift 152 cfm
.625 lift 157cfm
plenty for stock heads
later model exhaust manifolds look to have same pinch. bigger gains with later heads.
this would be for someone that wants to keep stock appearance.
top left hand opening has a 1.31" radius compared to the stock 1.06" radius.
stock opening flows 110 cfm.
I cut and welded a piece to open up pitch point.
.415 lift 145cfm
.500 lift 152 cfm
.625 lift 157cfm
plenty for stock heads
later model exhaust manifolds look to have same pinch. bigger gains with later heads.
I get the desire for the stock looks, but you will never have even close to the power of 4-1 long tubes with those manifolds even ported. The primary pipe length is just way too short to create any real scavenging effect.
Is that upper right tube in the pic, rotted through?
That's good data...I've always thought that the crimp down to 1/4-of-a-pipe was a pretty big restriction. Same w/the Edelbrock "header". Good data. Thanks once again, for sharing.
Is that upper right tube in the pic, rotted through?
That's good data...I've always thought that the crimp down to 1/4-of-a-pipe was a pretty big restriction. Same w/the Edelbrock "header". Good data. Thanks once again, for sharing.
I did one tube before getting headers. so, I tested it.
not rotted through, I cut it, to see how much material has to be removed. just experimenting. didn't feel like welding it. could modify and put heat shields back on and nobody would know the difference. that was my original plan. but they didn't flow enough for aftermarket ported heads. my car has headers.
Stock header tubes have CSA of 1.39 X 115 = 160cfm max. 1.31" radius is 1.347 CSA x 115= 155cfm. the math is really close.
I believe any C4 with this type of ex manifold would benefit from this mod.
I also did some more testing on modified crossfire intake manifold. concerning radius the bottom side of the top TB's openings.
long story short. stock TB's work good with stock top and loss CFM with highly modified top. but gains cfm with larger TB's bores. highly modified top flows more with larger TB' bore vs stock top with larger bored tb's. I can't explain it.
Also, larger TB's flow a higher percentage of the head flow, when testing with head.
Found a box stock '84 'Vette on local classifieds about a week ago, clean car, 55k mi, 4+3....$6k. A great buy if it's the way it presented in the listing. Either way, I've long wanted to "re-do" the mod path on a CFI car to "re-confirm" what I already know, but document it way better than I ever did or could have with my last CFI car, back in the late '90's/early '00's. So, I was tempted to buy this car and document the **** out of it...and in MY mind, the key to that "mod path" would be running it into the 13's for $0.00. UNFORTUNATLEY, we don't have a drag track here anymore, and so the reality of my being able to accomplish that documentation is pretty far from reality.
My point is; your CFI posts are teasing me! You're making me want to take what you've learned and apply it to show the potential of a stock/stockish CFI even more than I did back then and/or could have today w/o your testing. Man...I wish we still had a strip!
Found a box stock '84 'Vette on local classifieds about a week ago, clean car, 55k mi, 4+3....$6k. A great buy if it's the way it presented in the listing. Either way, I've long wanted to "re-do" the mod path on a CFI car to "re-confirm" what I already know, but document it way better than I ever did or could have with my last CFI car, back in the late '90's/early '00's. So, I was tempted to buy this car and document the **** out of it...and in MY mind, the key to that "mod path" would be running it into the 13's for $0.00. UNFORTUNATLEY, we don't have a drag track here anymore, and so the reality of my being able to accomplish that documentation is pretty far from reality.
My point is; your CFI posts are teasing me! You're making me want to take what you've learned and apply it to show the potential of a stock/stockish CFI even more than I did back then and/or could have today w/o your testing. Man...I wish we still had a strip!
my test are only flow bench numbers. I'm still learning how to tune, not my strength.
knowing what I know now. the first thing I would do is full free flowing exhaust, upgrade to 85 fuel pump, set pressure to 13psi, mild porting of intake manifold. after that it starts costing more money.
knowing what I know now. the first thing I would do is full free flowing exhaust, upgrade to 85 fuel pump, set pressure to 13psi, mild porting of intake manifold
Yep. That'd get you pretty close to 300hp....which few people can believe.
Right (close) lane is a local guy who's running that '92 LT1 auto, with LT headers, Muffs, roller rockers, "C"AI, and a tune, IIRC. Left (far) lane is the late, great, CFI-EFI, who has an '84 auto with...
Originally Posted by mike1111
full free flowing exhaust, .... set pressure to 13psi, ....(heavy) porting of intake manifold.
...advanced timing and a converter. My god, look at the traps! Look at those E.T.'S!!!
Right (close) lane is a local guy who's running that '92 LT1 auto, with LT headers, Muffs, roller rockers, "C"AI, and a tune, IIRC. Left (far) lane is the late, great, CFI-EFI, who has an '84 auto with...
...advanced timing and a converter. My god, look at the traps! Look at those E.T.'S!!!
he beat a 92. that's so satisfying. Isn't a 92 a second faster in the 1/4. that was at high elevation 4200'. isn't it 3% lose in HP for every 1000'. that is fast.
he beat a 92. that's so satisfying. Isn't a 92 a second faster in the 1/4. that was at high elevation 4200'. isn't it 3% lose in HP for every 1000'. that is fast.
You got it....and you got it. That 4200' is ALWAYS on a 6000'+ DA. And yes, 3% per 1000'. Which is why that 14.7 is a pretty typical number that you'll see STOCK LT1's run at that track. In my decades living here, I've never seen a DA below 6000' when the track is open.
And yes, Jim (and all of us) were stoked with that particular race (Underdog FTW!!!). But that's a 300hp car against a 205hp, same car. "Should've" been a slaughter. I love that vid.
Last edited by Tom 400 CFI; May 1, 2025 at 01:42 PM.
tested ported intake manifold with stock heads and stock TB's
@ .500 178 cfm 91.5% of head flow
@ .403 175 cfm 95% of head flow
certainly better than stock intake, stock head and stock TB's. @ .500 140 cfm
exhaust port with no pipe @.500 137cfm
In my opinion this engine is begging for aftermarket heads, header, ported intake
revision for bar graph test. after test head fail. I think this is more useful. this intake flows 90%+- of the head flow, depending on TB bore size. maxes out around 238cfm I did get more on one runner, but I don't count that. A head like an AFR 180 would be ideal. I believe AFR 195 would be too much. 195 would be good for renegade intake manifold. make video next and time to install
finally, tested100% stock top. no head.
ported intake bottom with 100% stock top flowed 220 cfm with blade diffusers and stock TB's. no gains from bigger TB's.
without blade diffusers, stock TB's flowed 229 cfm
same set up with 50mm velocity stacks 237.5
no gain with 1.125 tube with clay entrance. holes in top plate are too small on stock top.
tested every port, 2 outer runners were lower with 100% stock top. with a modified top the numbers came back up. I believe it's the 7-degree taper on the TB's holes.
I'm surprised that 100% stock top with diffuser blades flows 220cfm.
so the only modifications that works on top is removing diffuser blades and tapering tb holes. removing diffuser blades gains 9cfm. other than that, not much to do on the top. gains are small with the 7-degree tapered holes.
I have tested modified blades. trim off sides, can get really close to flow numbers compared to no blades. still have lower flow on outer runners though. the blades are a little directional. good for some runners bad for others.
I believe the diffuser blades are for fuel mixing. thinking about making diffuser blades with tapered side and larger diameter. not sure it's worth the trouble.
the flow is slightly better with modified top, but hardly worth the trouble.
the CSA is now 1.767. 1.53 x 1.155 smallest part of runner.
revision for bar graph test. after test head fail. I think this is more useful. this intake flows 90%+- of the head flow, depending on TB bore size. maxes out around 238cfm I did get more on one runner, but I don't count that. A head like an AFR 180 would be ideal. I believe AFR 195 would be too much. 195 would be good for renegade intake manifold. make video next and time to install
this is what I settled on for mods for top. have 4 different tops, all different mods. from stock to removing all bumps and grinding top thinner thinner. thinner top plate lost cfm. stock is pretty good except for the bolt hole bump that restricted 2 outer runners. Was surprised to learn that that tear dropped shaped bump actually flows more than if it's removed. I don't understand. I believe these mods are unique to this design. Is what it is. took a while to learn.
remove earlier ported intake from engine and tested it again. 215cfm average flow. latest intake flows 237cfm average.
in the process of installing new intake now. still need one gasket to finish job. also want to change plumbing so the top plate tb's, fuel pressure regulator can all be removed as a unit.
1.375 x 1.06 = 1.4575 CSA 1.4575 x 103 = 140 to152cfm stock. ports vary in size
1.4 x 1.1= 1.54 CSA 1.5 x 123= 190cfm radius and tapered entrance 1.5 x1.2 +/- entrance size. 1.4 x 1.1= 1.54 CSA 1.54 x 139.6 = 215cfm average. round rod entrance or adding weld to entrance with radius and taper. welding is easier. 1.53 x 1.15= 1.759 CSA 1.759 x 134.7= 237cfm average with bell mouth entrance 1.53 x 1.3= 1.989 CSA 1.989 x 112 = 223 cfm no welding, thin walls. not practicable in my view. may not be achievable for every runner. adding weld could bring it up to bell mouth runners numbers. never pursued this because of thin walls. I'm sure I would have gone through and had to weld hole. could get water leak.
things that didn't work.
spacers for top and or spacers for throttle bodies
reducing top plate thickness
top plate hole mods other than radiusing top plate holes
removing all the bumps from underside of top. I showed the one mod that helped ,earlier post.
interesting 1.4 x 1.1 with added rod or weld ,radius and tapered entrance was most efficient. not by much though.
no welding required for 190cfm.
I still don't understand how air flows other than it wants to go in a straight line.
latest intake is installed now. have a unrelated problem holding me up now. typical,
I think everyone realizes that....which is why it's so weird that the thread got derailed, talking about boat intakes. (?)
The CFI tech in this thread is incredible, for any CFI owner. The other stuff might be good for the MasterCraft Forums.
I think 237cfm average is pretty good for a modified stock intake. it still doesn't have thin walls.
bigger tb's flow a higher % of head flow. I wonder what the downside is, for using larger tb's.
I think 237 CFM is absolutely fantastic, for what you're dealing with! You're talking ~80 CFM increase from your own work, alone.
No downside, other than off idle throttle tip in CAN be "too sensitive" -that is a personal thing. I bored mine to 53mm (the largest that you can go w/o "hole'ing" the bore, but I went through the bore walls anyway.
Anyway, I repaired the hole'd bores and ran them on that 400 I had in my car, and it was very throttle responsive off the throttle stop. *I* loved it, I thought it was sporty, ballsy feeling and appropriate for a sporty fun car. But the down side would be throttle response that's too "jumpy" or too much, off, coming off the throttle stop.
I think 237 CFM is absolutely fantastic, for what you're dealing with! You're talking ~80 CFM increase from your own work, alone.
No downside, other than off idle throttle tip in CAN be "too sensitive" -that is a personal thing. I bored mine to 53mm (the largest that you can go w/o "hole'ing" the bore, but I went through the bore walls anyway.
Anyway, I repaired the hole'd bores and ran them on that 400 I had in my car, and it was very throttle responsive off the throttle stop. *I* loved it, I thought it was sporty, ballsy feeling and appropriate for a sporty fun car. But the down side would be throttle response that's too "jumpy" or too much, off, coming off the throttle stop.
tested stock head more to get a clearer picture. stock head 196 cfm @. 500. at best. some ports flow up to 10 cfm less. some flow more @ .400 stock head, stock intake manifold, diffuser blades, stock TB's @.500 135 cfm. runners are different sizes. cfm varies. --------------------------------------same with no diffuser blades @ .500 140 cfm. 50mm velo stack 143cfm stock head, intake manifold with ported entrance 1.2 x 1.5, 1.06 x 1.375 main runner, port matched head side. no diffuser blades, stock TB's @ .500 159cfm. 50mm velocity stacks @.500 160.5 cfm
this is an easy porting job, a day or 2.
if a person wants to weld on entrance, main runner 1.15 x 1.53 and an entrance 1.2 x 1.85 should flow 220-225 without head.
bad video Mike - YouTube
Last edited by mike1111; Sep 18, 2025 at 12:37 AM.
More great CFI data and good vid! I love what you have done and what you're doing.
Not that I would do it, but I wonder how far you could get the heads w/basic/minimal port work?
I got 210cfm intake and 150cfm exhaust with minimal porting on stock head. bowl blend and gasket match. I wouldn't bother porting stock heads. for me, if the heads are off, I'm putting on better heads. after market heads don't cost that much and there so much better.
as far as porting crossfire intake. if the port has no tapered entrance, multiply the CSA x 112 or 110. example 1.375 x 1.06 straight runner will flow CSA 1.457 x 112 = 163 cfm. with tapered entrance its 190cfm. without head. my long bell mouth runner to straight comparison is, 237cfm vs197cfm. looking at a runner simulation, the air entering a straight runner is smaller than runner size and it looks like a velocity stack. My theory is that making entrance CSA bigger lessens that effect.
Last edited by mike1111; Sep 18, 2025 at 09:39 PM.
tested stock head more to get a clearer picture. stock head 196 cfm @. 500. at best. some ports flow up to 10 cfm less. some flow more @ .400 stock head, stock intake manifold, diffuser blades, stock TB's @.500 135 cfm. runners are different sizes. cfm varies. --------------------------------------same with no diffuser blades @ .500 140 cfm. 50mm velo stack 143cfm stock head, intake manifold with ported entrance 1.2 x 1.5, 1.06 x 1.375 main runner, port matched head side. no diffuser blades, stock TB's @ .500 159cfm. 50mm velocity stacks @.500 160.5 cfm
this is an easy porting job, a day or 2.
if a person wants to weld on entrance, main runner 1.15 x 1.53 and an entrance 1.2 x 1.85 should flow 220-225 without head.
bad video Mike - YouTube
did a little more work on stock easy porting
stock head 196cfm, stock TB's, stock intake manifold
with diffusers 135cfm. flow varies not all runners are the same size
without diffusers 140cfm
ported entrance and gasket matched. 1.53 x 1.2 entrance, 1.395 x 1.11 main runner 168cfm
without head
ported entrance and gasket matched. 1.53 x 1.2 entrance, 1.395 x 1.11 main runner 198cfm.
straight ported runner 1.53 x 1.15 197cfm
1.395 x 1.11= 1.548 CSA 198/1.548= 127.9 same flow with smaller main runner and easy to port. a fraction of the time
1.53 x 1.15= 1.759 CSA 197/1.759= 111.99
not the best picture showing the entrance. easy porting job.
takes a lot of time to max port runners. need to do some welding to make it worth it.
better picture, can see roof curving up. did some more work on entrance 1.6 x 1.3 entrance, 1.4 x 1.11 main runner. Small cfm gains. 201.6 with tb's. 204.5 cfm with 50mm velocity stacks. 1 cfm gain with clay on top of runner. not worth welding.
Last edited by mike1111; Sep 30, 2025 at 12:08 AM.
all number are with no head and 50mm velocity stacks
#1 runner design 1.4 x 1.1 main runner with 1.6 x 1.225 entrance. no radius, no bell mouth. 204.5 cfm
#2 runner design 1.53 x 1.3 main runner with 1.6 x 1.3 entrance plus radius added to top of runner = 235 cfm. Not possible for every runner. not practical. this same runner flows over 250cfm with bell mouth.
#3 runner design 1.53 x 1.15 main runner with 2.125 x 1.225 entrance. 237cfm. need to check wall thickness while porting
there are many variables. these are samples of results.
a straight runner like the stock runner is 1.31 x 1.0625 = 1.39 csa. 146cfm/ 1.39 = 105. csa x 105 for straight runner cfm.
port matching at head = csa x 112. 1.39 x 112 = 155.68 cfm.
port matching at head plus tapered entrance = csa x 133. 1.39 x 133 = 187.87 cfm. also 1.4 x 1.1 = 1.54. 1.54 x 133 = 204.5 cfm. there is a point where the cfm start going down as the main runner gets bigger and the taper gets smaller.
port matching at head, no tapered entrance, with radius on entrance = csa x 118.
port matching at head plus slightly tapered side entrance with bellmouth = csa x 134.
tapered entrance
radius on top of entrance
bell mouth. pretty sure knowing what I know now. bell mouth design is capable of 250cfm, with planned out porting
that looks very good -any details on the engine this manifold will go on ?
this manifold is on car now, 237cfm with 50mm velocity stacks, no heads. flows 225cfm with current heads and stock TB's. bigger TB bores will flow more.
heads are mildly ported dart/summit 165ss 253cfm intake, 190cfm exhaust @.500.
mild roller camshaft 213/217@.500 .495/.502 @.500 112 LSA. 1000- 5000 rpm range
9.4 CR
header with free flowing exhaust
external fuel regulator
in the process of learning how to tune now.
When I did mine, I started the enlarging (of the bore) right below the throttle plate to allow for as much time/space for the air to spread out and slow down before entering the plenum. No objective testing on this design though. In my mind, having the air shoot out the fixed dia bore of the TB, it all just slams into the floor of the plenum. Then it's got to get moving again, perpendicular to the way it came in, and either one way, or the other. 
-any details on the engine this manifold will go on ? 
