Jump to content

NA 3.1L=>head & camshaft questions. No shortcuts, max

Recommended Posts

Any thoughts on why my rwhp is 50 hp < similarly built engines? Valve lift and carb tuning are issues....but 50hp?


Don't start down this path of confusion, you'll just screw your head up! Don't compare chassis dyno runs from different shops with different dyno operators on different days. The numbers ARE NOT COMPARABLE despite all the BS you read in SCC, Hot Rod, etc.!


As Keith said, tune your car and compare it against others on a race track. Nobody races on dynos.

Share this post

Link to post
Share on other sites

Your A/F ratios are a bit wacky, hence the power drop if you call it that. I'd guess that you can eek out a few more hp once you correct the A/F ratios a bit. Your peak power was ~6Krpm and you're running at a not so power friendly 13.6-13.8 A/F ratio at the same rpm. Maximum power is achieved at a ratio between 12.5-13.0 so you'll need to richen things up a bit.


Based on your graph, it looks like you go super rich once you hit the throttle and then lean out as the engine builds rpm. Since you're running triples, this indicates two major things.


1) Your accelerator jet is huge. Although, I have found with cams of this duration that a super rich condition is required to reduce the stumble. There just isn't enough air velocity at these lower rpms with high duration and large valves. Adjusting the timing in conjunction with trying a slightly leaner jet should clean this area up a bit.


2) Your car gets progressively lean and then richens back up at high rpms. This indicates to me that your main jet and/or emulsion tube could use some richening. I would recommend bumping the main jet up a size or two and then give it another run. If the A/F ratio isn't down to a healthy 12.5-13.0 from 4K rpm up to redline then you may have to richen it further via emulsion tubes and/or richer main jets.

Share this post

Link to post
Share on other sites

First off, CONGRATS on gaining 40hp!!!

Secondy, ditto on the non-validity of comparing dyno results from different dynos at different shops in different states under different environmental conditions.


You're still running 40mm carbs, no? 44s or 45s, with 38 or 39mm main venturis should give you a broader torque peak, more peak power, and more power under the curve.


What's your compression ratio? Torque seems low, I'd expect 200+ lb-ft peak, over a fairly broad range. With that cam you should be able to run ~10.8:1 on 93 octane pump (I am at 11:1, with ~305/.550" cam).

Share this post

Link to post
Share on other sites

As usual, you guys never cease to amaze me. I get several replies the SAME day, and give me hope in tuning my investment...


Yes, my Webers are still 40mm. They have 34mm chokes in them.


as for compression ratio, i don't know. it's probably fairly high, its an E31 head, and it looks like sunbelt shaved it some. how do you measure compression? i did the dyno pull tuesday using 100 octane gas. I haven't tried pump gas since I dialed in the timing curve.


Dave: thanks a lot for your input. I'm a little thrown off: I don't remember coming across said "accelerator jet." I pulled out my weber book, and again, can't find a jet with that name. Do you mean the idle jet? I have found the idle jet controls how the car transitions between light pedal and heavy pedal.



180 air correcion jet

65.F8 idle jet

130 main jet

F11 emulsion tube


according to a website i found online,

http://members.aol.com/dvandrews/webers.htm i can calculate all the jet sizes i need based on the main venturi size and cylinder displacement. here are the numbers i got from these calculations.


185 air correcion jet

135 main jet

F2 emulsion tube

60.F8 idle jet


would it be a reasonable guestimate that the numbers my car would like might be slightly higher than the calculated on the basis that my head probably flows better cam is a little more aggressive than the "average car" that they used for their formulas?


here are my thoughts for the first jets to play with...I'll get the 135 main jets and the 185 air correction jets. I'll only make one change at a time of course. I'll probably see how each jet affects the car independently...and then I'll see how they work together.


or am i missing this completely?


thanks again.

Share this post

Link to post
Share on other sites

I was trying to figure out your compression ratio, and looking at the How to Modify book. Be CAREFUL when you try to figure this out. It looks like your pistons from the Big Bore kit should have a positive deck height of .025 (the book suggests machining the pistons), where most 3.1 builds have the KA pistons -.37 mm below the deck (from Lengine program). That's a big difference. I suppose that Nissan could have eventually changed the pistons in the kit, but I was surprised when I saw that in the book and I've never heard anyone else mention a positive deck height with the pistons.


Did Sunbelt give you a build sheet? You'll need to know how big the chambers are post shave too.


I would think you should be able to get more out of it too, I'm wondering how much carbs and possibly exhaust are restricting you. You pretty much have everything else covered by the looks of it.


EDIT--You've seen this right? http://hybridz.org/nuke/index.php?name=PNphpBB2&file=viewtopic&t=34844

Share this post

Link to post
Share on other sites

thanks, jon.


yes, i definately have a positive deck height of .025" -- i measured it myself. i pulled out my old nissan competition catalog (circa '99) which is where i bought my stroker kit. it has a table of compression ratios. using a 2mm gasket, the ratio is 10.29:1. using a 1mm gasket, the ratio is 11.54:1. i have the 2mm gasket on my block, but with the head shaved...

10.3 < my car < 11.5 is probably the case...


(those are the numbers for an E31 head)


yes, i saw that weber post, and have been participating in it. unfortunately it hasnt been helping me as much as i had hoped...everyone contradicts each other. which is why i put my info in this thread since this group has always seemed to know what they are talking about.


as for exhaust...i have headers, 2.5" pipe, and a flowmaster. not much restriction there. i even have the headers wrapped and a heat shield under the carbs. i also have an airbox and 'cold' air routed from in front of the radiator. ill put up a pic soon. hell, i might just update a website like ive been meaning to and put the link up....


oh, and i found 'pump jets' in my weber book...ill look at the numbers in my carbs tomorrow. i dont think ive ever pulled em out before...

Share this post

Link to post
Share on other sites

Well I'd ditch the flowmaster and get a better flowing muffler like a Super Turbo or even better an UltraFlow. Mandrel bent? I've got a friend with press bent 2 1/4" and I swear it probably flows like mandrel 1 3/4" from the looks of it. I suppose it depends on who does it, but mandrel makes a big difference regardless.

Share this post

Link to post
Share on other sites

What total ignition advance did you end up with for max power? Do you have the curves to compare run to run at different advance levels? That could give us some insight as to whether your compression ratio is too high for the fuel your using, though if you're using race fuel you should be OK even at 11.5:1. Agreed you're probably *somewhere* between ~10.5:1 and 11.5:1, I suspect it's over 11:1 as I think they usually set up the valves to sit somewhat proud of the chamber (appears that way in my case). It'd be good if you can get cc volume.


I think *most* 3.1 builds have the pistons above deck (unshaved KA pistons). My original build they were unshaved (.022" above deck). For the rebuild last year, they were shaved => .003-.004 below deck.

LEngine builder is only a tool, doesn't really reflect real-world engine builds, I don't know of any 3.1 builds with -.37mm deck clearance (i.e., KA pistons shaved 1mm). And it's erroneous in it's CR estimation when you shave dished pistons (like the KAs). Doesn't account for the fact that your not removing as much material due to the dish. Just so you guys know.



Can't offer you anything on accelerator pump jets or emulsion tubes. But judging from your lean condition 5000-6500, you clearly want bigger mains. And based on the rate at which it goes richer up top, I *think* you want to make a BIGGER jump with the air jets. I'd try going from 130/180 to maybe 135/190 or 140/195 or 200. Hopefully you know someone with a mess of Weber jets to try out on the dyno!


You might try 36mm chokes and even BIGGER mains and air jets, maybe 150/210 (based on your current A/F and scaling up the guess to "correct" it by 36/34 squared). But I think you're ultimately going to want bigger carbs.

Share this post

Link to post
Share on other sites

Here's my advance curve:




Do you have the curves to compare run to run at different advance levels?


I'm not quite sure what you're asking for, Dan. Are you asking for the above plots with it's respective power curve for each run? Regardless, no...the curves on this post are all I have. I just did run after run playing with the settings on my ignition to change its advance plot. If power went up, I'd make the same change again and kept doing it until it didn't like any more changes.


As for carburetor changes...here's what I think I am going to do for next step. I am reasonably confident because following that guide I found (link further up this post) produced a lot of numbers that were recommended to me on this post:


34-->36mm chokes

130-->145 main jets

180-->195 air corrector jets

F11-->F2 emulsion tube

65.F8 idle jet? *

40 accell. pump jet **


* idle jet will have to experimented with all of these other changes.

** this is what's in the car. should i go smaller, dave?


We've talked about main choke sizes...but what about the aux. venturis? Do they have various sizes?


So...before I invest in the expensive chokes and yet another set of jets...and possibly another dyno run, I have a few questions/concerns for those following this post if you won't mind:


I am concerned about changing too much at once. Should I start with some **recommended** jet sizes to go with bigger chokes, or should I just get the chokes and see how it responds? Does anyone know if the above changes will mean I might not need a smaller pump jet?


Dave240z recommends smaller accel. pump jet, bigger mains, richer emulsion tube. Dan recommends actual numbers:


I'd try going from 130/180 to maybe 135/190 or 140/195 or 200. Hopefully you know someone with a mess of Weber jets to try out on the dyno!

You might try 36mm chokes and even BIGGER mains and air jets, maybe 150/210 (based on your current A/F and scaling up the guess to "correct" it by 36/34 squared).


I understand that a lot of this is guestimation, but the guesses follow a similar trend, and my on paper calculations agree that I need bigger mains, air correctors, emulsion tubes, and main chokes.


So...I assume that changing all of this will require changes with my timing? Or will that timing plot I have stay relatively the same? I am not willing to spend $150-200 on a dyno refinement every time I want to see where a jet moved my mixture and/or timing. If the timing doesn't require a lot of change, will my butt dyno be able to refine any further corrections? I do have one of those G-meter accellerometers, but I can't get a curve from it, just peak hp and peak G forces under accelleration (it comes within 5hp of the dyno).


Thoughts? Comments?


What I really need is a dyno shop with a butt ton of jets. Any ideas?


Thanks yet again, fellas. After your responses I think I'll put whatever my then current inclination is on the Weber post and ask that NapaBill guy what he thinks with the jetting...

Share this post

Link to post
Share on other sites

Take my jetting recommendations with a HUGE grain of salt. Just giving you the first guesses *I* would try, and I am NO expert!

Aux. or "booster" venturi size doesn't directly affect power potential and main jetting like main venturi size does. The main venturi size is the total cross-sectional area a cylinder is breathing through, so it directly affects flow velocity: larger => less velocity => less vacuum signal => bigger jets required.

It's gonna be kinda tough to get consistent reliable data with a GTech. Best bet is to be good at predicting what you'll need jettingwise and go to the dyno, prepared to swap 'em out quickly between runs.


Regarding advance,

I was wondering if you had torque curves vs. ignition advance. Better to compare curves against each other than absolute peak torque and hp. My approach is to go with the LEAST ignition advance that doesn't hurt the torque curve in the desired range of operation (for me, 4k-7k) to give some leeway for a tankful of lower-octane fuel. It sounds like you might have done the opposite, going with the MOST advance that didn't hurt you.

Agree w/ jmort :D , 23 initial/43.5 all-in seems very high. Which would seem to imply that compression ratio isn't too high for the fuel you're using (oh yeah, race gas). FWIW, race gas required an additional 4deg advance over pump 93-octane (38deg vs. 34deg) when I tried 'em both the same day at the dyno. And I got similar results over ~6deg range on pump fuel, which means I *might* have seen as much as ~44 not hurting my power on race gas. Which may or may not mean something regarding your setup...

Share this post

Link to post
Share on other sites

I apologize for the late (and lengthy) reply, but my free time is consumed by my home projects. Nonetheless, here are my ramblings.


This fine tuning is what makes Webers great, but at the same time gives them such a bad rep for being too complicated as I'm sure you are now finding out. Keep in mind that any advice offered in this forum should be taken with a few grains of salt and some pepper as well. The fact that we're giving this advice over the internet without actually seeing your car makes that even more important.


The absolute best way to tune your car would be to go back to the dyno and have someone with knowledge of Webers and a full range of jets so you could make changes on the dyno and note the effects on both power and the A/F ratio. Of course, finding someone like this is next to impossible these days since most "tuners" use a laptop as their tool of choice rather than a screwdriver and wouldn't know a carb from an apple. Your best chance finding someone like this is probably through some old MG or Alfa user group. Old school VWs and Porsche guys may know someone as well, but they're usually tuned into IDA type carbs, not DCOEs.


In the likely event that the above scenario proves impossible, then you're going to be on your own. You will have to make a decision as to how you are going to move forward; stay with the carbs or switch to EFI.


If you choose the Weber route, you are going to have to invest in some jets and more than likely will end up wasting some money on jets you ultimately won't need. You will also be on your own for the most part since people who know and tune carbs are becoming harder and harder to find. Investing in a good wide-band oxygen sensor and A/F ratio meter would be good. Remember, you're basically tuning the A/F ratio across the entire rev range at different throttle positions. Having actual A/F ratio readings will go a long way towards telling you where you're at.


Your other alternative is to swap the carbs for some type of EFI system. Either the TWM ITB setup or a more stock type could be used with the good news being that you will have more luck finding someone to help tune things. The downside of course is a much higher initial investment and probably some more work on the car to make it EFI ready (i.e. fuel tank, pump, fuel lines, etc...)


All that said, here are my weber recommendations:


1) Your timing appears to be a bit high as others have mentioned. Something like 16 initial and 34-36 all-in by 2000rpm is a good baseline number. Anything higher and you may be limiting the ultimate hp and make the engine prone to pinging. Couple related questions: Is your dizzy recurved and did you disconnect the vacuum advance?


2) For webers, if you want a richer jet, you get a larger number. i.e. a 150 main is richer than a 130 main. The number refers to the diameter of the fuel orifice in the jet.


3) I think your setup is a bit going to be a bit more difficult to fine tune because it jumps around too much. For example, your 40 accel. jets are a bit small (I would expect 45-50 with this cam) but your 65.F8 idle jet is huge (most people go with a 55.F9).


It's hard to give actual numbers, but if I did have to recommend something, I'd go with the following:


36mm venturis (chokes)

4.5 Aux venturis

150 Main Jets

190 Air Corrector

F11 Emulsion Tube

50 Accel Jet

60F9 Idle Jet


In closing, here are some basic things to keep in mind.


Main chokes affect the amount of air that can be ingested by the engine. Smaller sizes tend to help the low-end since air velocity is naturally increased but they will restrict (hence the term choke) the ultimate amount of airflow that can be provided to the engine at high rpm. On the contrary, using a large choke will allow plenty of airflow at high rpm but will sacrifice the low speed air velocity which will make the car difficult to drive at low rpms. Ever notice how vintage race cars tend to sputter and buck at low rpms but sing at high rpms? It's because of the large chokes to achieve maximum hp at the expense of low speed operation. The trick is finding a good balance between the two extremes. Easily said, but not easily done.


Idle jets have a great effect on overall drivability. These are the jets that are used to transition from idle to ~2-3K rpm which is the most common rev range during normal driving so these jets have a great deal to do with the subjective "drivability" factor.


Accel jets affect the transition from some throttle position to WOT. That's it.


Main Jets affect upper rpm and cruise. Richening these ends up richening the rpm range from 2Krpm up to redline.


Air Jets affect high rpm (5K+rpm). These merely control the air that mixes with the fuel in the emulsion tube. Since they flow air, you want to size these opposite of the fuel jets. For example, a smaller air jet will RICHEN the mixture at the top end and vice versa.


Emulsion tubes affect the transition between all of the various jet stages. These are tricky to fine tune, but most Datsun guys are using the F11. These are a rather expensive to replace so I wouldn't bother with them unless you were way off base.


Here is some good general info regarding webers. Might be worth the read.


Share this post

Link to post
Share on other sites



No worries for the "delays." Wasn't long anyway! I very much appreciate the detailed response. It cleared up a few things I was very close to grasping.


As for my distributor, I don't have one. My ignition is direct, using three coils to send spark based on crank position. This is why I can plot a curve like you saw in that graph, and also why I don't have a vacuum advance.


As for finding someone to tune it, I'd like to learn to tune them myself. I used to think that one day I might switch over to a TWM 3x2 throttle body setup, but now I think I might just go for the 45 DCOEs. Especially since I finally am starting to understand what the different jets are controlling.


I like your idea of an O2 sensor. Do they mount in the intake manifold? The exhaust?


Why the F9 idle jet as opposed to the F8?


One last question: suppose I swap some jets and the car runs a lot better. Does that also shift where my timing should be, or is that strictly dependent on the cam?


Thanks again,


Share this post

Link to post
Share on other sites

im from South Africa and me plus a mate have stroker 3.1 L28's,not sure what my mate is running but my previous car with the stroker motor had 500lift with 290 degrees.Its what we call R77 and L9 cams (different makes) over here for good bottom and mid range performance with the torque of the motor.Very satisfied.....

Share this post

Link to post
Share on other sites



yes, i am alive...but my engine has seen better days.


not sure what the cause was (possibly bad tune over time coupled with pulling vacuum off of the one cyl?), but i've been having a 5500 rpm and up hesitation/float that i thought was my ignition system. finally pulled the plugs and did a leakdown test.


turns out cyl#5 has an 88% static pressure drop. yeah. you heard right. i'm pissing all that air through the intake valve. the engine upset has caused cyl #6 to leak 16% past the rings.


so yeah. looks like it's time to open it all up. the good news is, i've aquired a friend that is a master mechanic and owns his own shop (my new BEST friend!), and he wants to learn me on how to do all this stuff. all i gotta do is buy him some diet coke! (score! :D) the other good news here is that i'll get a real tune this time and come back with another round of dyno #s...


back to the thread.... i'm thinking hey, since it's open, i might as well think about those valve reliefs! at the very minimum i'll drop the 2mm gasket to 1mm and gain the ~+1 to my CR. what i'd like is to cut them deep enough to allow for a higher lift cam, as i'll likely be putting a new one in...


so... after googling and searching all morning, i've found zero guidance on cutting valve reliefs. here are my questions:


do i need to buy new/custom pistons for this or can i cut the reliefs in the pistons i own? if the answer is maybe, then what do i need to look for on the pistons to know if they are thick enough or not? how deep is too deep? i'd like to be able to clear any cam (up to i dunno, .600" lift? .625?) (havent called sunbelt yet to see what their lift recommendation is, i just remember them telling me "real power isnt made until lift exceeds .550"...")


any recommendation on lift if unrestricted? the application is still autocross, so i want as wide a power band as possible. at the moment, i have tons of torque available immediately and power kicks in between 3 and 3.5k and last to about 7k. (the car has evolved to slicks with camber plates and still has the strut braces.) no longer driven on the street, trailered to the races, 100LL fuel, etc.


any experience in this matter would be greatly appreciated.


thanks in advance!

Share this post

Link to post
Share on other sites

No personal experience, but BRAAP turned me onto this tool on p17 of the ISKY catalog when I asked a while back: http://www.iskycams.com/onlinecatalog.html


It is a valve shaped cutter that you operate with a hand drill. Just stick it in the valve guide, put the head on the block and cut away. I think the rule of thumb is that you want .050" clearance between the valve head and the piston, I've seen a machinist figuring out clearances with modeling clay. I think that is the way to go if you're not sure whether or not you need to cut the pistons. Put a dollop of clay on the pistons, set the head on the block with a headgasket, turn it over, cut the clay down the middle and measure the clearance.

Share this post

Link to post
Share on other sites

ordered the sunbelt cam #2:


"micro polished rifle-drilled billet ground cast iron." whatever that means. lol.


.565" lift

326 deg intake, 315 deg exhaust


(from .520" lift, 290/275 in/ex)


with this aggressive of a cam, i was concerned about losing some low end torque, but jim was adamant that through experimenting with johnc's cams and one other customer's, the more cam they added, the more power they got and the wider and smoother the power band was. the other customer went with this cam. this cam would also give me 500 more rpm to work with.


the way i calculate it, with the difference in lift, reducing gasket thickness from 2mm to 1mm, and allowing a .08 clearance, i need to cut .170 into the pistons for exhaust and .155 for intake. (jim advised more clearance on the exhaust valve) sounds like a lot. has anyone cut this much?


cherry pickin and opening the engine this weekend. more updates to come.

Share this post

Link to post
Share on other sites

That sounds like the cam I ran in the Rusty Old Datsun. Is this a track only car or do you plan on driving it on the street?


EDIT: Never mind.


My experience with that cam:


1. Reasonable idle is around 1,500 rpm.

2. Power band is from 4,000 to 7,250.

3. Lots of torque with a little bump in power delivery aorund 5,600.

4. Reinforce your alternator mount.

5. If you're running regular fan belts, upgrade to deep groove pulleys and/or a wider belt.

6. You'll need the 1 3/4" primary Nissan Motorsports header or the same size Stahl header (preferred).

7. 3" exhaust at a minimum and you'll need a good Y pipe with a real merge collector.

8. The car will be loud under power.


Be prepared to re-gear the car especially for autocross use. You'll need to top out at 75 mph in second and you will have to use first gear if the rpms in second get below 4,000. Find a gear calculator and do some figuring.

Share this post

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...