Secondary Mainjets

[Joe C.]

This question will hopefully be best answered by Clem, Duke, or any of our more renowned engine/carburation gurus.

My 1965 327/365 runs fine, and pulls strongly at the top end. There is no black smoke, or any smoke for that matter. There is no lean misfire or surging. The engine is built to original specs EXCEPT that the heads are massaged (pocket ported, polished, gasket matched, cc'd, valves reprofiled, further unshrouded chamber). The engine has never seen a dyno, but it "feels" strong, to me, which is totally subjective. So, assuming that I did everything right, then those heads should flow much better than "stock" at the top end, right? Should I increase the size of the 2818's secondary mains "as a matter of course", because of the mods to the heads? Does the absence of symptoms signal the adequacy of the stock jetting, or will larger jets provide even more performance?

Joe

[Clem Z.]

stock jetting in carbs from the factory

is set at sea level and you need lean out 2% in area for every 1500 feet you are above sea level. unless you are at sea level i do not think you need to anything to the jetting as you still may be slightly rich which is better than being lean..

[Joe C.]

Re: stock jetting in carbs from the factory

Thanks Clem. So, if I read you right, you're saying that at sea level (where I'm located), the carb will deliver optimum a/f ratio with stock jets, REGARDLESS of flowrate, right?
Also, thanks for the above pose Re: head flowrates. My heads are 461's, which are, I think, very similar (except for spark plug location and casting thickness in critical areas) to the 462's tested on the website. According to what's shown, my 30-30 camshaft, with .485 max lift intake/exhaust will need a set of open headers to notaceably benefit from the mods. Only about 5% improvement on the intake side, and almost 25% better flow on the exhaust.

Joe

[Duke W.]

Re: Secondary Mainjets

In the recent "327 LT-1" test results I posted here, the actual dyno chart is posted on the Corvette Forum link. It was tested with a wide band O2 sensor, and there were some "issues" with the A/F data, but the top end data was probably okay.

The average was about 13:1, and since it was very hot (CF=1.06) it will lean out about another 0.5 A/F ratio on a SAE air density day.

The easy fix would be to increase the secondary jets two sizes which will richen the secondary about 1.0, which should average out to about 0.5 on the engine assuming the additional secondary fuel is evenly distributed, and that will get it to an average of about 13:1 at SAE standard air density.

The other alternative is to richen the secondary one size and drill out the Power Valve Channel Restriction (PVCR) to achieve an increase of 0.5 A/F through the PVCR only. I am assuming the primary jet is okay, and I do not want to increase it as this will increase the cruise mixture, which is not necessary. Assuming the PVCR richens by 15 percent, flow through the PVCR needs to be increased by about 28 percent (which is about a 13 percent larger restriction) to richen the primary side by about 4 percent without touching the main jet.

Obviously the first choice of just adding the extra fuel in the secondary is a much easier solution and just fine as long as distribution is equal.

Overall the OE jetting isn't bad, and more flow should not lean out the mixture excessively because the carb will add fuel in proportion to flow over the useable flow range. Some carbs can lean out at the top end if you increase engine demand by a lot, and the other consideration is that because A/F will increase/decrease with decreasing/increasing air density, so you are constantly chasing a moving target.

I like to see the average WOT mixture in the range of 12.5-13.0:1, and the only way to know what's going on with a specific engine is to do dyno pulls with a wide band 02 sensor on a "typical day" for you driving environment.

Mark's mixture wasn't that bad, and if #78 jets in the secondaries yields an average WOT or no less than 13:1 at something near SAE standard air density (CF=1.0) I will deem it "good".

Duke

[Clem Z.]

if the carb is the limiting factor to air flow

the jetting does not need to be changed with the change in the engine. a 750 CFM carb on a 350 cu in engine uses the same jetting a the same carb with a 427 cu in engine. with open headers you could need a change but with closed exhaust i would not worry

[Duke W.]

Re: stock jetting in carbs from the factory

Dave McDufford's 461X 2.02/1.6 heads saw an improvment of about 10 percent on the inlet side and nearly 50 percent on the exhaust side with pocket porting and multiangle valve seats. Dave's "after" flow number were better percentagewise than most before/after tests I have seen, including Vizard. He had a very good machinist, and it's all in the details.

Though not flow tested, the chassis dyno data for the "327 LT-1" I reported on a couple of weeks ago must have been at least as good if not a little better to pull useable power to 7200 with 90 percent of peak torque at 2450.

Taking into account actual rocker ratio and .023" lash, the actual peak valve lift with a 30-30 cam is about .440", both sides. With the LT-1 cam peak actual inlet lift at .016" lash is about .423". Same as the 30-30 on the exhaust side because it's the 30-30 lobe.

Trust me, pocket porting, port matching, and multiangle valve seats with a top 30 degree cut off the unused seating area of the inlet valve is the single best modification to ANY vintage engine you can make, including the 300 HP engine. The improved valve seat geometry enhances low lift flow and elimination of the annular ridge in the pocket enhances higher lift flow. For any given configuration, you get more top end power and about 500 more useable revs with virtually no loss of low end torque.

Duke

[Clem Z.]

also cutting a back angle on the valve head

will help the low to midlift lift a lot.

[Joe C.]

Re: stock jetting in carbs from the factory

Duke,

Thanks for the reply. I followed Vizard's procedure, including recontouring all valves, and CAREFULLY decreasing the valve seat areas per his recomendations. I even went so far as polishing much or the inside surfaces of the original rams horns, and fairly extensively opening up the mating surfaces to match the exhaust ports. A lot of work and patience, especially since it was all done with a die grinder, and no other special equipment!

The GM/factory sidepipes certainly choke the exhaust with a BB, and I'm not certain how much (if any) of an effect it has on a SB, since I can find no test data for them. One of these days, I'll put the car on a chassis dyno, which will tell me how much power the engine is making.

It does get rubber in all 4 gears, with the 3.70 axle and bias ply 7.75-15's, though.

Joe

[Joe C.]

Re: also cutting a back angle on the valve head

Clem,

Thanks for the response......see my reply to Duke, in this thread.

Joe

[Duke W.]

Re: stock jetting in carbs from the factory

It will be interesting to compare your engine's performance to the "327 LT-1" since the basic difference appears to the the 30-30 cam in your engine versus the LT-1 cam. That should put to bed the discussions of "which cam is better".

Shops with rear wheel dynos are fairly common today in large metropolitan areas, and clubs organize "dyno days" where you get to do about three runs for say forty bucks. It's a good way to get a baseline.

Check around your area to see if there's a shop with a Dynojet chassis dyno. Since they are the most common, I prefer to compare data from them. Other makes tend to give somewhat different results. You can get a quote from the shop for some baseline pulls and find out if they do any club dyno days.

Testing on a cool day is best with plenty or external fans to hopefully keep the fan clutch fron tightening, and if they have a wide band O2 sensor you can get A/F data.

Please start the pulls at 1500. They're typically started at 3000, so you learn absolutely nothing about the low rev performance and we DO actually spend a lot of time below 3000.

Duke

[Joe C.]

Re: stock jetting in carbs from the factory

Duke,

If and when I decide to do some "pulls" you all will be the first to know! As you know, I have the stock GM rods installed, and I do not want to tear the engine down at this time, since it performs so well and I'm enjoying my car. It uses about 1 quart of oil/800 miles, which is more than I would like to see......but no smoke, no fouling, even with the AC44's installed. During engine assembly, I made the mistake of moderately lubricating the cylinder walls with assembly lube, and some opinions are that it might be restricting the rings' movement, or has even "coked". It could also be sucking oil thru the intake gaskets. I keep watching the oil, and when/if it starts to bother me, I'll investigate further. For now, the philosophy is "if it ain't broke, then don't muck with it". Another concern is detonation. An "expert" at Federal Mogul's customer service section stated that excess oil has the same effect as lowering the octane rating of the charge. It seems to me that the situation should be the exact opposite, since motor oil is harder to ignite than gasoline. Regardless, sometimes I think I hear slight detonation, but, with the sidepipes, it's very hard to tell. Probably the only way to know, for sure, is to connect an engine analyzer while under load. What DOES bother me, is the stock rods, and I no longer run the engine to redline for fear of a catastrophe. I'm not sure if I am willing to stress the engine on a dyno, for that reason.

What type of exhaust system has the 327/LT1? Were any mods done to its ramshorns? Does it have headers? Sidepipes? How were the heads reworked, and how was the method different from Vizard's. Were all ports/gaskets matched like I did?

Joe

[Duke W.]

Re: stock jetting in carbs from the factory

Oil can cause detonation, but not at the rate you report. The engine has to suck in a big gulp of it, like through the PCV system.

The 327 LT1 has the OE undercar exhaust system, but it might have replacement mufflers. The heads were pocket ported/port matched with a multianglseats and Manley "race flow" valves. I'm not sure it they really help, but that's what the owner wanted to use. I believe the heads ports were also matched to the exh. manifolds.

Everthing is completely original appearing. The only mods are inside as above, including Crower Sportsman rods.

Duke