Well, number one, this is not an engine that will ever see any street time, as such. It is just a drag race vehicle and driveability issues are not pertinent to what we are trying to accomplish. The hole in the torque curve (if, in fact, it even exits; I am not 100% positive about that,) only manifests itself when we are trying to stall the car for a launch. I think it probably has to do with opening the secondries (during the stalling process) on a carb that is probably too large for the engine (750 Holley double pumper on a 234 cube motor.)

The guys who had built their 500hp motors that we tried to copy, both said that 5500 rpm was the absolute upper limit of the effectiveness of their power production. I, in my infinite wisdom (riiiight,) had concluded privately, that the reason their engines didn't continue to pull hard up to and past 6,000 rpm, was that they were running out of carb capacity. They both had carbs in the 650 range.

So, I bought a 750, in an effort to address that issue.

The jury is still out on that "solution," but one thing is for sure; that little motor has no affection for that big carb at stall; mixture velocity is probably too low... so, we'll see if I can get past this glitch.

Insofar as spark advance is concerned, I am trying to do my best to see to it that this motor never has to endure detonation.

One way to help ensure that, is with an ignition system that will never allow sspark advance to even come close to what it would take to create a dynamic cylinder situation that involved spark advance that could get into an area that racing gas could construe as having cylinder pressure that was beyond the extant octane to effectively control.

From all I can learn by reading multiple articles on spark advance in boosted engines , and the velocity that a flame front travels across the top of the piston in such engines, the concensus seems to be that significant boost makes possible conditions that ensure that a flame front travels much faster than the flame front in a naturally-aspirated engine.

Given that infoirmation (if, indeed, it's true and I believe it is,) the apark advance requirements of a significantly boosted engine are much less than a naturally-aspirated one. It just doesn't need as much time to create the necessary cylinder pressure for effective power production.

Hence, the 18 degrees instead of 32, or so.

If I were building a car for the street, and was concerned with driveability issues and fuel economy, spark advance could get a lot more complicated, but for what we are trying to do with this race car, I think a locked plate and 18 degrees is probably the most advantageous thing to do. It's "safe" and runs well at that number (if I can get this stall issue fixed.)

I wasn't sure if you were asking why I gapped the plugs at .025", but the reason for that is, they are really cold plugs (to help stave off detonation,) and, as such, may show a proclivity toward fouling, and the narrow plug gap logically, should be harder to foul.


Anyway, that is my thinking on this mechanical puzzle.

I DO appreciate your input and interest in our car! Please keep the comments coming; who knows, I may be proven 100% wrong about this... It's happened before, believe me!

Thanks, and have a good week!!!

Bill, in Conway, Arkansas

That makes two of us, Mike... LOL!

Lock of the secondaries so you're only running on 2 bbls and see if you get the bog.


You may need to change the accelerator pump shot and delay the secondaries opening point.

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

I have used a 750 mech sec on my high comp engine and the smaller blown engine. the 11.4 to 1 engine ran 25degs timing. shifted at 6500rpms and 6800 over the line best 12.30et@109mph. the small blower motor did the same but the 60fts were lower. with 5psi boost. my larger 471 engine has 2x450 mech sec holleys with 12psi boost and 20degs timing has run 11.15ets@120mph.

My guess is the RPM limit might be due to head flow. My 68 pulls right along through 5500 and has bounced off 5800 rev limit several times with no sign of nosing over. I have a well ported Mike Jeffrey head on there with 1.76/1.50 valves. Stock rebuilt 170k bottom end, so not wild about spinning above 5500...

Lou

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Home of Slant6-powered fun machines since 1988

Of course I know you have a race engine, Bill, I just used the street engine example to explain what I meant, it has nothing to do with driveability, but power. But I gave an incorrect number, the intake mixture actually burn fastest at an air/fuel ratio of app. 17:1, but a mix at that ratio is of no use.

Even if the flame front travels faster with boost, the same laws of physics apply to the speed of the flame front, if the air/fuel mixture is not constant/ideal over an rpm range. And you still want the pressure to peak at ~15° ATDC, or alternatively a uniform CA50 at 7-8° ATDC, where CA50 indicates that 50% of the mixture mass has burnt at that point, as equal as possible for each cylinder.

Terrylittlejohn, I seem to remember that you used some small vacuum advance on a locked mech. advance distributor, on one of your blown engines?

Olaf

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Aspenized

Olaf,

I am confused. I have a recollection that Tom Wolfe's car and Ryan Peterson's cars both of which make approximately 500 horsepower (calculated from their real-world drag racing performances,) utilize spark advance of around 20 degrees BTDC.

Are you contending that they could actually make more horsepower, safely (as regards detonation,) on gasoline, if they ran a significantly increased amount of spark advance?

Maybe I don't understand exactly what you mean with these notes, but I would like to understand this.

In my ignorance, based on my lack of experience with forced induction, it would seem to me that both of these cars are probably running right on the ragged edge of detonation with the spark advance they are currently running (given their engines considerable outputs.)

But I could be wrong, and so could they, I suppose.

Making 500+ horsepower, as both of those engines do, it would seem to me that their spark advance total isn't very far off the optimim... and they both run right around 20 degrees, I think.


What do you think?

Thanks for any information... I'm learning as I go, here...

Bill, in Conway, Arkansas

Lou, I don't have any information RE: Ryan's engine, but I think Tom's ported head flows pretty well.

I bought this head that is on our car, from a guy in Las Vegas (on ebay,) who said it was on a junkyard motor in a van... but it turnrd out that WAS ported, in fact, and had the popular-size of oversize valves, so until I get it flow-tested (not likely, very soon,) its flow-potential in its current state, shall remain a mystery.

I am a lifelong drag-racing addict, having been a tech inspector at three different, local, drag strips over many years, and have had several race cars during that time (1956-to-currently,) but I have never had a six of any kind (for racing) and never had a turbo on any car I ever owned, so, I'm really ignorant about these engines and what they like. This is an education for me... and I'm really starting out at ground zero, so I do appreciate any information regarding the technical side of this operation.

At the outset, I was conssidering spending a litttle more money on this thing and running NHRA's Competition Eliminator, but after reading the rulebook pretty carefully, I can't find a class for it; I don't think that NHRA has one... and, that's a shame. All those classes, and not ONE for a blown (turbochsrged) slant six... so, a Bracket car it is...

I can only guess what the engine in our car is making, in terms of horsepower, as we have yet to get it to a strip with clocks, but it FEELS like it might run some low 13's, at worst. That is with the boost gauge showing ten pounds, and a clean launch, The car weighs 2,900 pounds with my fat arse in the driver's seat, ready-to-race.

Do you have any idea what kind of power this thing is likely making at ten pounds?

And, how that might change, with 15, or even 20 pounds of boost?


I can't see us needing to go beyond 20 pounds, as I think it would be just begging for trouble, as neither our heads nor block are O-ringed.

Your car sounds like it might be pretty fast; have you had it on a set of clocks, when you have been running significant boost??? Just curious...


Thanks for any and all information!!!

Bill, in central Arkansas

You cant really compare apples to apples until you know all the comparative data between the two of them. And since you know relatively nothing about your cylinder head in terms of flow,chamber shape/size, etc, you aren't going to be able to compare your build to anyone else's that you do know about their specs. You might find that you will have to run 5 lbs more boost to equal the same HP as others that have a better cylinder head than yours.....You just don't have enough data from your own combo to be able to make accurate comparisons to what others are doing. The cylinder head info is a big piece of your puzzle thats missing, and you need to know that before you can compare yours to theirs.

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There's no such thing as too much cam....only not enough engine!

You're not o-ringed? What head gasket are you using?

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1965 Plymouth Belvedere II Turbo Project - SOLD
2002 Subaru OBS
1995 Lexus LS400 Race Car

I wholeheartedly believe 750 CFM is too much flow at too low of air speed and that it will be tough to get the car to run anywhere close to optimum with such a large carburetor. Terry Littlejohn can run much more CFM as his carburetors are on the atmospheric side of the compressor (blower) and the compressor is mechanically driven. I would have used an Edelbrock 500 carb with solid Thermoquad floats grafted onto the original float arms. The Edelbrock AVS would potentially provide easier secondary tuning than the Performer (AFB) type.

18° initial timing might be fine at idle and under considerable boost, but even a drag engine operates in between these extremes. How much manifold vacuum do you have? You might do well to connect vacuum advance directly to the intake manifold. The vacuum advance will disappear under load, but will ignite the less dense part-throttle mixture at a more advantageous time and increase part-throttle torque. You'll probably still have to put some mechanical advance in the distributor so that you can use less initial timing to offset the vacuum advance at idle, but I feel the effort would pay handsomely. Better yet would be a full normal timing curve for a low compression engine with a relatively large cam (lots of initial timing and restricted to 28° total mechanical and generous vacuum advance) with boost retard. This is why I bought a Mallory ignition box with boost retard for my turbo build.

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Joshua

Well as the CNC dude pointed out, without data on this mystery cylinder head we have, it's really hard to pinpoint ANYTHING with regard to tuning this engine... Too many unknown variables... Your comment, Josh, about the carb being too large may well be right. If I downsized to a smaller carb, it would probably be a 600 Holley 4150 series, because I have some experience (and, parts.) relative to tuning that one.

In a week or two, I am hoping the weather will cooperate and we can get some definitive data from a real drag strip with accurate clocks, so we can evaluate some of this.

This is a brand new car and we are obviously flying blind as regards tuning, so we'll know a lot more in a couple of weeks.

To the poster who asked about the head gasket, it is one I got from Charrlie Schmid, and is a stamped steel gasket that is pretty thin... maybe .022"-inch... and yes we are running without O-rings, so our boost will be limited, dunno how much, but I'd like to try 20 pounds (max) down the road a piece, if it doesn't blow at 10 pounds....

Time will tell....

Thanks again, to all who posted the technical stuff, 'cause knowledge IS power, and my opinion, you can never have too much of either!!! I appreciate it!!!!

Bill, in Conway, Arkansas

I think Joshua Skinner covered my thoughts very good in his post above, I also think you may benefit from some advance when applied at the right time. After all, I presume you are going to drive back in the return lane and move around in the pit area, without being towed or pushed, so your engine will not run at a constant 10, 15 or 20 psi boost. Whitout knowing the details for your specific engine build, the SL6 in general has combustion chambers that provide little squish area to make high turbulence, and little quench area to control the heat, so your engine's sensitivity to increased pressure may be very different from the next guy's SL6 engine.
They say that 15 psi of boost doubles your HP. Only a dyno can tell exactly how much, but it depends on what you started with.
I use a stock engine example again: After all, it is not uncommon on a stock engine to raise the cylinder cranking pressure from a compression test reading 150 psi, to 160-165 psi, by timing the cam and doing a good valve adjustment job. That's equal to 10-15 psi of boost, but it can still benefit from standard ignition timing advance. If you give the same engine 10-15 psi boost after doing the valve job, it may be right in the detonation zone.

Will the design of your cam lobes give valve bounce or flutter at high rpm?

Does your carb atomize the fuel mixture best for your engine/boost?

As you say, Bill, there is a long list of little details to consider. You are still in the beginning of a long journey, enjoy the ride!

Olaf

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Aspenized

yes olfa I have that dist in the car now, it`s mech adv is locked and I have the vac pod adj tight to keep adv too 10degs. I am running 20degs total now with water injection set for 450hp, 2 x #5 injectors. I also ran a steel stock gasket until Charlie had the group buy here on site, I had no problems up to 7psi with the steel, then I o-ringed the block and went to a .043"copper gasket. I am at 12psi right now.