| Author | Message | ||
| Philip Airey (Pma1010)
Junior Member Username: Pma1010 Post Number: 74 Registered: 7-2002 |
John Great data, thanks for sharing the graph. Certainly looks like the 140s provide a LOT more power (20+ BHP). Paul I was surprised by the results too. I'll look in Croft's book and see if there are relevant graphs and if so I'll try and scan. The only point I can reference at this stage is the credibility of the author - many consider his to be the finest [Ferrari] engine tuning book. Philip | ||
| John_Miles (John_miles)
New member Username: John_miles Post Number: 20 Registered: 7-2001 |
<img> OK, well, I guess img src links don't work... Try this one. | ||
| John_Miles (John_miles)
New member Username: John_miles Post Number: 19 Registered: 7-2001 |
Paul, for what it's worth this dyno graph was taken to compare 140/220 main/AC jets with 135/220s in my car. This is a '76 308GTB with a Tubi exhaust and a stock air filter (I hadn't switched to K&N at this time). Pretty big difference between richer (~14:1 A:F) and leaner (~15:1) ratios, although it doesn't tell us what might have happened at even richer settings. <img> This graph is over a year old, and to make a long story short, at the time it was taken the richer main jets caused progression/drivability glitches. So I went back to 135/220 (and eventually 130/200) to fix the latter problems. A recent carb rebuild seems to have cured those issues, enabling me to go to 135/200 without problems. I need to go back to the dyno to see if I still see a benefit to 140 mains, and what AC jets I should actually be running in either case. Because there were unknown carburetion issues at this time this graph was taken, I don't know how applicable it would be to other cars. But there ya have it... | ||
| Paul Newman (Newman)
Member Username: Newman Post Number: 886 Registered: 12-2001 |
Philip, the reason is not so much HP but a cushion. High rpm and lean mixtures mean holes in pistons or burning the edge off a piston above the rings. I prefer the saftey that comes with a slightly rich mixture at the top end. The factory PCM controlled cars like GM's do this for just that reason. I would like to see dyno numbers though that show more HP at 14:1 than 12:1 at or near red line. Just curious. | ||
| Philip Airey (Pma1010)
Junior Member Username: Pma1010 Post Number: 72 Registered: 7-2002 |
Paul IN my "prior life" with a turbo'd car, I too was told 12:1 was the target AF under full boost. However, as I indicated, I recently read Guy Croft's book (subject of another thread) where he advocates 14.2:1 as optimal (in a NA car). Philip | ||
| John_Miles (John_miles)
New member Username: John_miles Post Number: 18 Registered: 7-2001 |
Addendum: the spirit moved me to swap the 130 mains out for 135s tonight. The car runs decidedly better now, with no trace of the hitch around 3K that caused me to drop down to 130 in the first place. I probably will not put the 140s back until I get it on a dyno, but... for now, 135/55/200 is a very serviceable combo for a car with a K&N air filter element and a Tubi exhaust. | ||
| John_Miles (John_miles)
New member Username: John_miles Post Number: 15 Registered: 7-2001 |
In my '76, I'm currently running 130 mains (stock was 135), 200 AC (stock was 220), and 55 (stock idle jets, with the stock emulsion tubes. ("Stock"=whatever came with the car; I don't have the OEM specs handy). Back when I was running the 220 AC jets, I saw a ~20 RWHP boost from 175 to 195 by going to 140 main jets... but at the cost of a drivability hitch around 3K RPM. I seemed to get the best overall drivability by going to leaner mains and richer AC jets, but was still plagued by periodic starvation-ish problems. Had the carbs rebuilt a couple of months ago and the latter problem went away... so it's possible that I can get away with the richer jets now. I don't have any complaints about the current performance level, though, so it's unlikely I'll mess with the jets (much) before I can check my current power level at the next Seattle club dyno day. | ||
| Hans E. Hansen (4re_gt4)
Member Username: 4re_gt4 Post Number: 684 Registered: 4-2002 |
I had my '75 'GT4 dyno'd, and made sure that they recorded A/F ratio as well as HP. At 3500rpm (WOT), A/F was about 13:1, and leaned out to 14:1 at 7500rpm. From this, and consultation with a couple of legitimate "experts" on the scene, they suggested that stock jetting was optimum, *except* the AC jets. I needed to go smaller. My '75 has 135 mains and 220 AC. I'll be doing a rebuild shortly, and try some 210 AC. | ||
| david handa (Davehanda)
Member Username: Davehanda Post Number: 330 Registered: 5-2001 |
Sorry, I don't have any of the graphs anymore, I gave them to the new owner of my car. As I recall, the AF ratio was actually a touch under the ideal 14.1 to 1, but the shop owner was afraid if he went to 145 mains, it would push things too rich, at least from his previous experience... | ||
| Paul Newman (Newman)
Member Username: Newman Post Number: 795 Registered: 12-2001 |
Actually, at WOT, a ratio closer to 12:1 would probably make more power. Ive seen this ratio used on late model gm's in the pcm fuel map. 14.1:1 would be an ideal ratio for the converters to work properly under normal driving conditions. | ||
| Philip Airey (Pma1010)
Junior Member Username: Pma1010 Post Number: 63 Registered: 7-2002 |
Dave Do you have a pic or can you describe the shop's download of the AF ratio? I was reading Guy Croft's tuning book (see another thread on the topic) and he was advocating a 14.2:1 AF ratio for maximum power on the 2 cam Fiats (and I assume the Ferrari's would be not far different). Anyway, would love to see a comparison of before versus after. BTW, I assume the shop also measured with a wideband AF meter. Philp | ||
| '75 308 GT4 (Peter)
Intermediate Member Username: Peter Post Number: 2269 Registered: 12-2000 |
Thanks Matt for the tip, I'll look into that the next time I'm in these things again. P.S. Did you get the 408 stuff I emailed to you? | ||
| Matt Morgan (Kermit)
Junior Member Username: Kermit Post Number: 87 Registered: 8-2001 |
Peter, one observation I have made in tuning Webers is an off idle problem is often due to the throttle plate position. The idle speed is often set by turning the screws, which is fine , but be watchful that the plate is in the correct position. The air bypass screws, usually used to balance must be taken into consideration. If the throttle plates are open too far from adjusting the idle speed screws alone, it allows them to feed from the intermediate circut (series of small holes). This can drive one crazy trying to jet. | ||
| '75 308 GT4 (Peter)
Intermediate Member Username: Peter Post Number: 2265 Registered: 12-2000 |
Don, I'm gonna try some changes this winter, nothing drastic. Hopefully next year, I'll get the car down to the Seattle Dyno Day and see what it'll do. I find that my tail-pipes are kinda bleached-looking after high-speed driving, considering most that have passed through that dyno have bumped up to 140 mains (like Dave Handa's and many other 308's) I'll do that. I'll also try 210 AC's from the 220's, just to fatten that up a bit. And I still get the odd stumble when pulling away from stops, so I'll try 57 idles (instead of the current 55's). I'd also like to see the current buzz about hot plugs (NGK 5's) and how it affects driving. | ||
| david handa (Davehanda)
Member Username: Davehanda Post Number: 329 Registered: 5-2001 |
The set up described on my 78 308 GTS was done through several dyno runs with exhaust analysis... The shop had done a couple other 308's and always found the 135 or 140 mains to work well. FWIW | ||
| Don McCormick (Dandy_don)
New member Username: Dandy_don Post Number: 42 Registered: 2-2002 |
Peter, Is there anything in these cars that you did not take a picture of? I appreciate your pictures as I am sure that everyone else does. They are a great resource. Have you done anything in changing the jetting/chokes etc with your carbs? Don | ||
| Paul Newman (Newman)
Member Username: Newman Post Number: 793 Registered: 12-2001 |
I believe a rule of thumb in drag racing is 1HP loss per 10 degrees the air temp is increased. It wont be an improvement that is for sure, regardless of airflow increase. Has anyone done an airflow test on a carbed aircleaner box using a flow bench? Just curious. | ||
| Bruno (Originalsinner)
Member Username: Originalsinner Post Number: 779 Registered: 5-2002 |
Dont remove the airbox.You will lose hp. There is a formula for hp loss after a certain temp per degree. I have a good ideal of what it is off the top of my head but am not 100% sure and dont want to get slammed if I am wrong with it. | ||
| '75 308 GT4 (Peter)
Intermediate Member Username: Peter Post Number: 2263 Registered: 12-2000 |
This is what the "AC" or Air-Correctors look like (on the left side of the pic, they are the top pieces of the main fuel supply system - AC's/emulsion TUBES/main jets):  | ||
| Paul Newman (Newman)
Member Username: Newman Post Number: 790 Registered: 12-2001 |
As far as black art or science goes, it aint. a carb airflow meter and time is all it takes. | ||
| Paul Newman (Newman)
Member Username: Newman Post Number: 789 Registered: 12-2001 |
The air corrector jets or AC jets are what you see when you look down your carb. They are at the very top in the center. They will be stamped with a number like 200 or 220. You unscrew them to access your emulsion tubes and your main jets are pressed into the ends of the tubes. They come out as an assembly. I will be going to 195 AC jets. I saw a noticable gain with the main jet change. You want to lower the AC number to reduce the air mixing with the fuel. They work backwards compared to other jets that meter fuel not air. My engine is not stock but other 308's are running 140 mains on a stock never rebuilt engine. On my application, I bet I could go to a 145 main but I will test on a dyno at that time. The AC jets only come into play when you are flat out above say 6000rpm for arguement sake. | ||
| Philip Airey (Pma1010)
Junior Member Username: Pma1010 Post Number: 60 Registered: 7-2002 |
Don Everything I have read suggests tuning Weber's is something between a black art and a science. Enlarging the jets to richen up the mixture may help, but remember while the jets on the earlier 308s (76, 77) had larger main/idle/AC jets, they also had lumpier cams. While the 78, 79 cars had smaller jetting, they also had a milder cam to compensate, at least in part. Clearly, the best solution, if you have the time and access, is to dyno test with exhaust gas analysis from the ex port tubes, and having a tech rejet the Webers while you are there. On the AC jets -- mine (1977) are 200s, for example, others used 220s and 190s [and as I recall smaller number equates to more fuel] -- these enable more fuel to be added at higher levels of venturi velocity and will help change the mixture at say 6000 rpm. Good luck. Philip | ||
| Don McCormick (Dandy_don)
New member Username: Dandy_don Post Number: 39 Registered: 2-2002 |
Still in the dark as to what AC jets are. Can anyone shed light on this? Don | ||
| david handa (Davehanda)
Member Username: Davehanda Post Number: 328 Registered: 5-2001 |
My carbs were rebuilt, but stock in configuration, other than the increase in main jets and the K&N drop in filter. The ignition was refreshed with new factory wires and spark plug extenders. The caps and rotors surprisingly, were the original ones with 30k miles on the clock, but still looked fine according to my mechanic. The ignition was further upgraded with the Crane Cams XR700 and new Bosch blue coils...that's all. | ||
| Don McCormick (Dandy_don)
New member Username: Dandy_don Post Number: 35 Registered: 2-2002 |
Thank you for the responses. Magoo, Steve and David, I have seen the discussions about not taking off the stock air box before and I understand the theory. However, does anyone have any testing to support this theory? If ducting in outside air was so important then why don't you see this sort of arrangement on other performance cars like Porsche for example. Virtually all Porsches (certainly of the 1980-1990 vintage) draw their combustion air from the engine compartment, through a small aperture usually. I understand intercoolers on Porsche turbos cool the air down so it is denser etc but even they have no ducting from the outside. Also I have seen some postings here where people remove the stock air box top when they want to get the most performance from the car. Clearly the thought there is to get past the air cleaner restriction but the net result is that the engine is drawing lots of air from the engine bay and not from the outside air duct. Why do they not suffer the loss of power from this heated air? Anyway perhaps I might just try a before and after dyno test. I got from the responses that I should try a main jet size of 1.40 instead of 1.35. Sounds fine to me. Also it seems likely I have a 5 bolt carb top as my carbs are completely stock (pretty sure anyway as the PO's were hands off guys who took the car in to have the oil changed etc, etc.) Dave Handa or Paul Newman, I do not have a clue as to what an AC jet is. The closest I can come is an accelerator pump jet but the sizes listed at the website don't seem to match the 200's or 195's mentioned. Also Dave did you change any of the other parts I listed above, like chokes or aux venturis? I am interested in knowing just what you did to the carbs if you would not mind sharing it. I too have Nick's bypass pipes and a stock exhaust. Putting on a K+N would seem to be pretty easy or the 4 separate filters discussed above. What sort of ignition upgrade did you install? Obviously if money were no object then it would make sense to do Nicks direct fire system but $1500 is a bunch of money. I have read about the costs of replacing the stock dist caps, rotors, and points but for now they are working fine. Any thoughts here? Thanx again all. Don | ||
| Paul Newman (Newman)
Member Username: Newman Post Number: 788 Registered: 12-2001 |
220's are stock, fatten up the ultra high rpm mixture with smaller AC jets. | ||
| david handa (Davehanda)
Member Username: Davehanda Post Number: 325 Registered: 5-2001 |
I had the stock AC jets, whatever that was for 78. | ||
| Paul Newman (Newman)
Member Username: Newman Post Number: 787 Registered: 12-2001 |
I dont see any mention of AC jets. I would go to a set of 200's or 195's. I went with 140 mains as well. | ||
| david handa (Davehanda)
Member Username: Davehanda Post Number: 322 Registered: 5-2001 |
I had a K&N, cat bypass pipes from Nick on my 78 309 gts (stock exhaust) and changing the stock mains from 125 to 140 bumped the hp about 19 hp or so...but the car was way lean before this change (with a recent major service)and was below the stock 205 hp figures...finishing at 214 hp at the crank when all was said and done....car ran really well. | ||
| David Jones (Dave)
Member Username: Dave Post Number: 454 Registered: 4-2001 |
I see both Steve and Magoo beat me to it... I type slowwww... | ||
| David Jones (Dave)
Member Username: Dave Post Number: 453 Registered: 4-2001 |
Hi Don, My 77 308 has 5 bolt tops... What are the specs on your carbs now? I wouldn't rejet the carbs unless you have opened up the breathing a bit with say a K&N filter on the intake, and something on the lines of a Tubi on the exhaust... I also wouldn't recommend putting 4 separate air cleaners on as you will get better performance using the stock air box to feed cool air into your intake instead of hot air form your engine bay. | ||
| Steve Magnusson (91tr)
Intermediate Member Username: 91tr Post Number: 1224 Registered: 1-2001 |
Don -- the stock 308 40DCNF are the 5-bolt top cover (from Peter R.):  | ||
| magoo (Magoo)
Advanced Member Username: Magoo Post Number: 3671 Registered: 2-2001 |
Don, This has nothing to do with the purists and I know you don't want to hear this and I agree that those filters look great sitting on there but it definetly will affect the performance of the engine. The airbox provides cool forced outside air to the carbs. Without it you will be taking in heated engine bay air and the engine will not perform as well. See archives for other opinions. Regards, | ||
| Don McCormick (Dandy_don)
New member Username: Dandy_don Post Number: 31 Registered: 2-2002 |
Hello all, I am planning on rebuilding the carbs this winter and will probably buy the parts from Weber Carbs Direct 1 800.871.3619 as their prices seem the most reasonable. Any one have experience with these guys? Do our cars have a 4 or 5 bolt carburetor tops as the website lists a rebuild kit for both types? I plan on replacing the accelerator pump diaphrams also. I also plan on rejetting the carbs and am going to try the jet upgrade program outline by Dina on a post dated June 22, 2001. I talked with the guys at Pierce Manifolds (whose prices seem a bit higher than the supplier above) and they said that while a jet upgrade is common I should not plan to: change the progression holes(too easy to screw up) modify the delivery valves and nozzles-same reason In keeping with Dina's post I do plan on the following increases: Main jet 1.35-- 2 each/carb Aux venturi 4.5-- 2 each/carb Idle jet .6-- 2 each/carb emulsion jet no change needle valves no change starter jet 2.2-- 2 each/carb Acc pump jet .40-- 1 each/carb Chokes 34mm-- 2 each/carb Any thoughts or experience with this type of upgrade? I think that it will cost about $100/carb including the rebuild kits. I have removed the cats from the car as well as the entire air injection system (rails, pump etc) and plugged the holes with metric plugs. Stock exhaust system will remain. The ignition is stock and set to 6 degrees BTDC at idle-plan to upgrade this as well after the carbs project but don't know to what just yet. Also planning on removing the air box and putting 4 separate air cleaner/filter units on it. I know that purists will shudder but I am looking forward to much better access to engine V, cold start mechanism, chokes connection, fuel connections to the carbs (fuel leaks, etc),not to mention the fact that the engine won't look so innocuous-it will be very clear that there are 4 carbs on this motor. Of course I will keep the airbox and all pieces safely stored away for the next owner (if I ever sell) Your thoughts please. |
