ozconnection

You're welcome....borrow, test, then buy!

Were the old lash pads all the same thickness? Yeah, I've done a few cam swaps myself. The 'supplied' lash pads were never the right ones. They're only guessing on what you need. Reconditioned/reworked heads are a different KOF compared with stockers. I'd go to somewhere like SWR and ask Stuart for a range of lashpad thicknesses. Ask if you can use them to see which one(s) will match your rocker/cam setup. Then you know what you need, how many and what thicknesses. Easy. I borrowed then bought several sizes because the dumbhead that did my seats didn't equalize valve seat depth in the head, so I had to use about 4 different sizes all the way along the valves to get the correct wipe patterns on the rockers. What a pain in the rear that was!! Now I'm sure to mention this whenever I get head work done (and the seats need attention for whatever reason). Be sure to try some thinner lash pads my friend. Cheers.

What thickness lash pad did you use?

This may seem obvious, but have you actually installed a rocker arm to see how it is in its proper working position? Without doing this, looks can be deceptive. Also, get the thinnest lash pad you can (0.150), instal it and go from there.

This is my car, walkaround, on the dyno and down the road. Enjoy! All of the clips are with the Arizona intake and a Holley 465 vac sec. I might point out that the Clifford intake made heaps more top end power but the Arizona is the choice for low rpm torque, something that's important to me and my machine. Having said that, the thing still pulls pretty hard at the top end. And the sound inside the car? Quite quiet just cruising around with just a burble out back. Great on warm nights with the windows down or driving your favourite stretch of winding road with the exhaust bouncing back off the walls. No one see's what you've got under the hood until you stop and show them, that's your choice.....the difference is when you drive past they know its a Datsun.......but somehow it 'sounds' different.

All that I can tell you is that I've been a solid user of the 4 barrel carb setup for probably 10 years now. I have an L28 and have used 3 different manifolds and 4 different types of Holley 4 barrel carbs. Why? It's easy to bang on a Holley and manifold onto your car and get it to run reasonably well straight outa the box. Few people really take the time to really 'tune' it up well and soon give up saying that the setup is crap. I spent many hours on the road testing all of my combinations because it was 'fun'. I mean it....when that 4 barrel is cracked open it gives our engines a unique sound which I love. Others enjoyed it too judging by the number of people who'd turn their heads when you'd roar past 'em! Then, if you don't like tuning a carb, there's always this as an option. That should give 'em a giggle! I'd say 'Go for it!'

Crumbs, that noise is pretty loud! The idle speed is very very low and your engine is shaking around on the verge of stalling. It's not a vibrating bracket or something obvious like that is it? Get the engine at a verified 750 rpm and see what she runs like then. Otherwise, pull the camcover off again and do an inspection. Of everything! After that, it's someone else's call. Good luck.

Good solid advice is what we need....and get, thanks!

I think you need to do some research on the subject. Have you? There is a massive amount of info here that is simply a few keystrokes away. Search, read, absorb and apply. If you're still not sure you can ask a question but we're more inclined to take you and your question more seriously if its obvious that you've made an effort to help yourself first. Cheers.

Thanks for offering up some specs on those headers but I seriously think there'd be no hope of getting them between my rails, shame though, they are lovely. Lucky boy!

Jesus, they are 'wide' aren't they! Is it alright to be dumping those two pipes like that into the collector? Nice set of pipes man! I don't think they'd fit the 'C' otherwise I might be interested. Cheers

I'm also happy that your machine is running well

Thank you very much John, Cheers mate!

Are there any books written (to your knowledge) that would be considered 'updates' to Smith and Morrisons' book? I wouldn't mind upating my library if one existed.

Like you Shawn, I have read a few books on the topic too. The best book I have found on the topic is Smith and Morrisons book, 'Scientific design of exhaust and intake systems'. The chapter on 'pressure phenomena' , 'Sud-division of manifolds' on page 101 says "There is absolutely no evidence to support this theory." On the other point of backpressure, Bell's book 'Four-stroke performance tuning' suggests on page 157 in Chapter 5 'The exhaust system' that the lower the exhaust backpressure the less restriction, or pumping loss, the engine will see and the greater the potential for power production, combined with appropriately timed camshafts and well designed intake systems. Volumetric efficiencies greater than 100% are possible in some instances. Neither book suggest that six cylinder engines with a firing order like ours are to merge into one pipe. They are to be split into two separate groups of three and remain that way. There are two types of wave action taking place in the exhaust system. The exhaust travels throught the pipes between 200-300 ft/second. The other, more powerful and influential, are the pressure waves that move at 1500 to 1700ft/second. These later waves are the ones that help to evacuate the cylinders at overlap if setup correctly. They work particularly well in exhaust setups that are split into two groups, again due to cam timing and the firing order split by 120 degrees of crank rotation ie.(1,3,2 and 5,6,4). I do think that Roberts' exhaust designed for the racing/hot street L powered Z cars are built that way for good reason with sound scientific research and evidence to back it up.

What do you mean? Please explain.

From memory, vacuum advance, once engine has warmed up sufficiently, valve opens allowing proper vac. advance to the dizzy.

Are you seriously suggesting to this guy that he run a carb like this for 'street' performance? Heck, its for a 460 big block etc and small blocks that rev to 8.5K rpm!!! GetZ, you haven't told us whether its a single or dual plane setup? Engines equipped with dual plane manifolds can accept larger carbs than ones with single plane manifolds. I still think that an 825cfm carb is too big though. Just my 2 cents.

That's very interesting...I wouldn't have thought that be the case at all. I'm certain that the companies that produced most of the 'other' headers wouldn't like to read things like this. I know I don't as I'm one of those smo's that bought one! Is there any dyno proof to back up these claims? It's not that I don't believe you, but I do rely on certain 'proof' if that be the case. I have read in Bell's book "Four Stroke Performance Tuning" and others that its a waste of time to run a camshaft with more than 240 degrees duration with an exhaust manifold. A six cylinder engine fires every 120 degrees, so if one were to group the cylinders together in such a way, you could avoid the pressurisation of an adjoining cylinder during the blowdown period. If done this way you could in fact cause a negative pressure in the pipe, 'extracting' the residual cylinder exhaust gas from out of the cylinder. So strong is this effect, it can literally suck some of the intake charge straight out of the cylinder during the overlap period! I really need to see those dyno tests......

13 AFR is not quite rich enough for WOT and not lean enough for cruise. I would be looking for 12.5 WOT and 14.5/15 for cruise. Accel should be around 13.5 and idle should be stable at 12.5 but your cam plays a part in this too, so I don't feel as comfortable suggesting the idle mixtures. You need to play around with this yourself to make it idle properly. Reving the engine without load will never give you the info that you need to tune properly. I think you know this now. You can do the tuning yourself on the road or on a dyno. I did mine on the road over several weeks of weekend testing. The AFM you have is an excellent tool to help you do this. Remember the old hot rodders adage; Change only one thing at a time and run it...see what effect it has and record it. It's too hard sometimes to remember everything and whether it helps/hurts performance. Cheers mate.

Very few factory exhaust manifolds are what I would call 'performance items', especially those that were penned 30+ years ago. The way they merge the exhaust gasses together indicates that performance wasn't a high design priority and to be able to improve on this design wouldn't be hard. You've made an interesting point about testing the stock exhaust manifold. I think this is where the definitions become blurred. I agree that a totally stock engine may not flow more exhaust than the manifold can handle but add a cam or do some other flow enhancing mod and at what point will it be realised that the stock exhaust manifold isn't up to the task?? How would you know? Would you bother to drill and tap the manifold and attach a pressure guage to it to see how much backpressure pressure there was? There's always a 'weak link' somewhere in the system and what I'm saying is the sooner you get rid of it, the better. You'd be potentially limiting the performance gained by those mods until the manifold was replaced by a header. It wouldn't be hard to 'run out of flow' with the stock item.....I'd say lose it now!

What do you mean by "streetish" setups? I can't agree with this part of your response. The rest of it was on the money though. I spent some money a while ago and 'improved' the exhaust system on my L28. Stock N42 head and compression, bottom end, automatic trans with just an MSD and 280ZX ignition upgrade and an Arizona Manifold and a 4bbl Holley. Then I put the exhaust on last. Did dyno figures before and after the change. WOW, big difference to the curves, the exhaust ended up being a set of non tuned length headers and a 2.5 inch single pipe with two 2.5 inch mufflers to keep it 'quiet'. The graph below is of this setup. However, my next system will be a twice pipes setup. I've had a look at the 'science' behind this sort of thing. It seems to be able to offer the best in terms of power and torque production by the way this setup works with the exhaust pulses and resonant tuning. This won't happen unless you have headers. The big single system will reduce backpressure, that why some guys here have said that they felt the motor run heaps better with a change in muffler or no muffler at all. But to get the full advantage of low backpressure and resonant exhaust extraction you need to be running the twin system.

If you want 4 corner idle and downleg boosters on a Holley, may I suggest you have a look at the 4150 HP series carb. The 0-080528-1 have the features you mentioned. The calibration on these Holley carbs is for performance so they may be a little on the rich side for purely street use. You mention sluggish performance after you adjusted your carb using an AFM. What were your target AFR's? Wide open throttle, Cruise, Light accel, heavy accel etc? Getting this wrong or at least not quite where they should be will have a significant effect on performance, be it economy or power. You've probably helped the sluggish nature by leaning things out. Now, in conjunction with the leaner mixtures, did you bump up your ignition timing? Very rich and very lean mixtures can afford to have increased ignition advance because the fuel tends to burn more slowly at these extremes! Give that a try, adjust your ignition slowly and see how you go. It's a combination thing you always gotta be aware of. "If I change that, how will if effect this" is how I work with my engines. I would hold off buying a new carb for a while. Tune up what ya got and see how you get on. Good luck PS, I don't have a Chevy but I do have a Holley...lots of them!!

I don't think that if the L24 HG isn't perfectly round it would make any difference to what you're trying to achieve. I wonder if someone you know can let you have an decent old L24 gasket to muck around with. Why don't you go out and see if you can buy an el-cheapo L24 gasket and play with that? "ozconnection - I think Tony D means that the notches should not touch the gasket when installed." Yes, that's good advice, I just wasn't sure if that's what he meant.

I'm not sure that I know what you mean. "Never touch the gasket"? The idea behind using a gasket that closely represents the block for which the head is to be used allows the user to see where there is contact between the valves on the head and block. You're expecting contact, that's why using the smaller L24 gasket is a great template for this sort of thing. I went into the garage just now to measure the diameter of the two valves in an N42 (I44mm/E35mm) and it was pretty much spot on 83mm's, the same as the L24 bore. I used an l24 gasket on an L20 once without any dramas whatsoever because I couldn't get an L20 HG. That's why I don't see any problem using an L28 gasket on an L24. Where is your collegue staying in Sydney Tony? PM me with some details if you like and I can ask him what you're like to work with!