The Stovebolt Site Forums The Shop Area Engine Shop Chevy 292 build questions -

Fellow bolters . . .

I read the various comments on the 261 boring post and it is clear that many of you guys have more than a casual interest in these old engines. I am building a 292 now and the plan is to run a Whipple supercharger with between 8 and 12 pounds of boost. I am wondering if you can comment on compression ratio as I have been putting off ordering a set of Ross pistons and Crower sportsman rods due to a nagging doubt in the back of my mind regarding a couple of unconventional ideas I have about compression ratio.

Here are my thoughts and goals.

Engine is going to run pump gas and drive through a 700R4 tranny / 3.73 rear, tall stock looking tires in a 51 suburban. I want to have choices down the road if the Whipple doesn't work out so while I want a low compression engine for supercharging purposes . . . if I yank the supercharger I don't want a total dog of a motor with a huge investment in forged pistons. Combustion chambers are 76 cc's and I have been told that in order to run 12 pounds boost on pump gas that I should shoot for 7:1 or at most 7.5:1 CR.

Because I want my engine to run with or without a supercharger and only want to buy one set of pistons . . . I am considering running a Miller Cycle cam (delayed closing of intake valves till 30 - 70 degrees past bottom dead center) - Isky will grind it to whatever I want . . . but I am not really sure what I want to do here. The delayed closing of the intake valve provides a lower mechanical compression ratio thus allowing a higher "real" compression ratio when running a traditional cam.

My thoughts are that I could run with a traditional cam with higher compression ratio and get this rig on the road and then later when I am ready to put a supercharger on it I could put the miller cycle cam in and bolt on the whipple and get the thing going under boost.

Do you guys have any thoughts on this?

Thanks in advance,

Ken

Ken, changing the cilinder head in a L6 is pretty simple. You stated that your engine has 76cc chambers, so what about smaller chambered heads? You can use the desired compression ratio ti run the supercharger with the 76 cc heads, and have the possibility of swapping latter a small chamber head if You quit the supercharger.
I don't know what sizes are available there but as long as there are smaller engines that uses the same head(194, 230 and 250), for sure may have one. Down here we have a head for alcohol engines with 55cc chambers and hardened valve seats. They have the same size on the ports and valves.

Alexandre - good point, but I only want to build one head. Cost of new valves, springs, porting, polishing, screw in rocker studs, new guides, etc. bring the cost of running two heads much higher than a $140 cam change.

Ross Pistons are waiting for me to make up my mind . . . any thoughts on the miller cycle cam? Does anybody know of anyone who has run one of these?

Shall I just go for 9:1 and call it good / use a miller cycle cam to keep it from detonating when I run the supercharger? How high of a compression ratio can you go on pump gas and not worry about detonation?

Ken, with all the money you have, build two engines. Then when you decide which one you don't want, ship it to me and I will pay the freight.

Ken,

Never used such a cam. Dunno about machining costs there, but for sure are high. But also is much easier to change a head than a cam in a L6, so I'd opt for the head. Have in mind that I do most of my job and machining here is nickel and dime stuff. The cam/lifter stuff ain't for free either. It's up to Ya.
If it were mine, would go for a 8/1 cr on the big chamber head, 76cc or even a li'l more, let's say, 80cc. You must see that this is not possible with a flat top zero deck, it must be at around .160" down the hole with a flat top. So if You can get let's say a 60cc chamber with the very same piston, You'd be with a 9,5/1 cr. Get a cam with a bigger lda than You'd pick for a 250, use the charger. No big pressure too, unless a specif charger cam is to be used. Tired of it, get the small head and go. I use to improve this heads using the 350 heads valves, the 1.94 intake seems right to me, more and it's too close to the cilinder, 1.60 ex. is ok. I'd say that using a supercharger in an engine with such a long stroke and a usable(1,63) rod/stroke ratio is wise. Better than trying to spin it high to get good power numbers. Would opt for the charger rather than the screamer on this one. And to avoid the stress on the crank snout I'd consider a hair drier instead. Seen some turbocharged L6 and they are nice.
As always, just my 0,02.

Ken, isn't the Miller Cycle engine dependent on the forced induction for proper performance?Maybe you might think along the lines of 5 lbs of boost maximum and 9-1 compression and for sure the intercooler you have already planned.The combustion chamber of the 230-292 engine looks like a typical wedge chamber that was on many V-8's which is prone to moderate detonation levels.Maybe an investment in electronic knock sensors and a ignition retard would be a good idea.An aluminum head is better at controling wayward combustion , but none is available for street 292's.Above all a rich WOT fuel mixture is additional insurance.

Ken: Never seen a "Miller Cycle cam". Sounds like this cam is a wide LDA design, like Tony said, that is dependent on the induction pressure of the boost, to raise or "stabilize" the CR. This can be done, but in cam selection it is a balancing act between late intake closing for "maximum" top end power, without the problems associated with spark knock or detonation.

It all comes back to the octane thing. You want an engine that will run decently, idle OK, and yet get up an GO! Naturally aspirated, or blown, an engine needs to be manageable, yet maintain decent/compatable design to octane sensitivity.

The octane demand goes down with -

1.cam timing retarded (later intake closing)

2.cam ground with wider LDA

3.ignition timing is retarded


This is the hard part of engine building, these judgement calls, when it comes to cam choices on an engine, like the one you have.Cam choices are based upon: the cam design parameters, and, head flow CFM volumes, ( both intake and exhaust), and, shape of the combustion chamber, and, the type of induction system used on your engine, (FI or carb).

I realize that as air density rises (supercharging), the demand for octane will increase. The static compression ratio: 8.5 to 1 as an example, might produce a dynamic compression of say 150 PSI, with a compression gauge, and also might be comfortable to run on 87 octane gas. But with the correct cam design, it will change/influnce, and balance, against flow, velocity, and pressure differentials, to actually bleed off some intake pressure so as not to make it ultra octane sensitive, maybe not combatable with 87, but would run on 92 octane pump gas.(I'm talking about a blown 8.5 to 1 example, but not running the high pressure boost you are talking about, maybe something in the range of 4 to 6 psi)

I am of the opinion that if that was my 292, I would build up the bottom end, basically stock. Spend the bucks on the forged slugs. You can't go wrong there. Stick with stock rods, be sure everything is magged and resize rods. Spend your money on the HEAD!! Lots of porting and flow work to get that baby breathing! New valves, new seats, blend the short sweeps to the runners. Open up the exhaust side a bunch. Smooth out that combustion chamber. Weld up, and smooth all the sharp corners of the chambers also. You will gain much more for your dollars here, than in anything else you do, guaranteed! Proper porting and flow work to the chambers can gain 30% to 50% increase over stock CFM flowrates, no kidding.

I do not know if you already have the Whipple, or not? Repost and let me know, and I will give you imput/responces, my 0.02 cents.

Think Chevy! :rolleyes: :p

Chevyman, I think the idea is trying to avoid the higher RPM levels needed to boost power using non supercharged tuning.As you know ,the stresses on the engine increase as the square of the rpm. 300 hp at 4500 rpm with a supercharger is less stressful on the crank and rods than the same HP devoloped at 6200 rpm naturally aspirated.That long crank shaft get some serious harmonics at higher rpm levels.

Ken,
If you are going to intercool your intake, have you considered a roots blower? Eaton M-62's and M-90's work well with 3.5-4.5 l engines and can handle up to 10psi. They are cheap and plentiful, as they were used on the Buick 3800 (231) Grand National and Phorde 3.8 supercharged V6's (T-bird). They make plenty of boost at low RPM because they are positive displacement. It is pretty easy to make an air-to liquid intercooler yourself, or you can use a volvo air-to air intercooler. I am considering either this or a big turbo for eventual use on a 235 or 261. I am going to plumb in an oxygen sensor in each exhaust manifold for monitoring the mixture. I have a design for a cheap a/f mixture meter based on a 1 wire O2 sensor and a cheap LED bargraph driver chip.

Chevyman, haven't bought the blower yet, I ordered the pistons, but then called them back to put the order on hold as I thought it would be good to get more feedback. I like the whipple because it has a much higher adiabatic efficiency than a roots style blower - lowering intake charge temps considerably. Bottom end is going to be stock, I had one bad stock rod and figured that getting a lighter set of sportsman rods and Ross pistons would cut way down on the inertial forces on the bottom end. Crank is being smoothed, lightened a little and balanced.

Tony has the right idea, I want to pump some fuel through this thing at reasonably slow rpms and have a strong low end torque engine that is a steady puller through 4500 rpm. I have talked to Isky about the Miller cycle cam and they seem to be up to speed on it.

Guess I am really looking for options to be able to run the motor with/without a blower. . . . what do you think a realistic CR would be to run premium unleaded with a moderately lumpy cam designed for decent performance with a naturally aspirated carburetor'ed intake?

Once this is nailed down, then I can have the guys at Isky recommend a Miller Cycle grind that allows the level of boost the Whipple can develop without creating big problems with detonation.

If I can nail this down, I will be able to pull the trigger on the piston order and finish machining of the block.

As many of the guys who have played with turbos know, when you port and polish your head, you will actually decrease boost, but, you will not decrease power, you are simply making it easier for the turbo to do its job by giving a smoother path in/out of the cylinder. polishing also helps keep the fuel charge dense, as it does not get "smacked" around and broken up by the rough castings, and it makes it harder for the charge to pick up heat from the head, and also helps exhaust heat not soak into the head.

Ken: When I mentioned the re-working of the head, that was a very real and valid point, that has a direct cause/effect relationship on modifications and the power levels that you are trying to obtain from your engine. Because you are wanting much "better" than stock, naturally aspirated engine performance, as well as the capability of, in the near future to be supercharged. Kind of a "dual" role, in engine design, and certainly a "paradox", from a building standpoint. Because of the performance "parameters" of high torque at lower RPM's, that you want, while at the same time dealing with the limiting factors of the block/head design, long stroke, RPM limitations, and rod ratio.

Moderate head/port preparation will considerably improve the intake and exhaust flow rates of the production 292 head. Making the runners taller and wider is a real plus. You can remove much more material from the floors and roof, than from the walls. Do not be conservative with the combustion chambers. Build it up with weld, as needed in the low spots and smooth it out, lots of blending. These chambers are very thick and rough, and almost anything you do, will improve flow. Smooth all the edges around seats, and blend. The chamber is tapered, so you want to open up the small side a little, and more smoothing to help prevent any secondary combustion (detonation), from this squish area. In the valve area, bowl blending is all important, make sure to form a smooth radius from bowl sweep to floor of runner.

All this head work is important, because it has direct relationship on flame-front, cam selection and cam timing, flowrates, velocity, and backpressures.

You also need the head prep done first, so as to ascertain the CR. Need to know: bore, stroke, head gasket volume (thickness), head combustion chamber volume, piston dome volume, piston deck height. I feel you would not want to go more than about 150/160 PSI static compression maximum! Not knowing all your variables, the basic formula goes something like this:

EFFECTIVE COMPRESSION RATIO

static CR= x2psi x4 x6 x8 x10 x12psi

7.0-1 8.0 8.9 9.9 10.8 11.8 12.7

7.5-1 8.5 9.5 10.6 11.6 12.6 13.6

8.0-1 9.1 10.2 11.3 12.4 13.4 14.5

8.5-1 9.7 10.8 12.0 13.1 14.3 15.4

9.0-1 10.2 11.4 12.7 13.9 15.1 16.3

9.5-1 10.8 12.1 13.4 14.7 16.0 17.3

This chart shows the various levels of boost (PSI) and the effective compression ratio. It is based on averages, so is only approximate estimates. Can be used for 4, 6, or V-8 engines. For most "street" engines, the effective compression should be kept at/or below 12.1 CR maximum, when using 92 octane pump gas. When using 100 octane "blended racing gasoline", the CR should be kept at/or below 15.1 . In both cases, even if using "lead" additives, DO NOT exceed these compression limits. It is recommended that a "boost-sensitive" timing retard
system device be used to prevent "DETONATION"!

The dynamics of airflow sure does change things. It will be difficult for you to have a high torque NA engine, unless you jack up that CR. But if you do that, you will have to drop it right back down to run a blower. I would suggest you stick with a CR something around 8/8.5 . Any more than this and it will be too high for blower. Any lower and you won't have any power NA!
Just a decent compromise in the middle.

For a NA, you can find cam with a narrower LDA to give you better throttle responce, and better low speed pull. With the blower, then you can go with the wider LDA split duration cam to assist in clearing the additional exhaust volume. Got to get rid of all that extra air you jammed in there!
What you need is an adjustable cam drive on that 292. Then you could run a dual pattern cam and retard timing with the blower use. Something like a"Jesel" set-up for the Chevy BB, but mounted on the front of that 292. That would be MOST cool!

Don't know what type fuel delivery system you plan on using, carb or injection? If using carb on the blower, you may need 2 of them because of 50% more airflow into engine/carb with blower, you can see that soon run out of carb CFM flow and might require 2 carb's.

If going the injection route, you should not have the problem. A cool set-up that I have seen on blower motors is multiport injection by Electromotive. It uses its own computer module which is programmable. The unit replaces the carburetor, distributor, and needs no other ignition sources. It has its own crank fired direct ignition system built in. Check it out at: www.electromotive-inc.com

Have fun and good luck!

Think Chevy! :rolleyes: :p