I am going to be rebuilding the motor for my project car and wanted to ask for advice on what compression ratio to run. The motor is a '67 302 that will be used for a cruise night car that (hopefully) runs on pump gas. I had another 302 with 11:1 pistons and it ran on and pinged like crazy on 93-94 octane (that's all I have access to) and I'd like to eliminate that as much as possible without completely losing the performance capacity of the motor. Any suggestions? Thanks, Mark.
-
-
Re: High Compression Rebuild?
Mark-----
A 302 is just about the only engine that I hate to see a compression reduction in when it's rebuild time. This is an engine which really needs to be rebuilt to its original specs. However, detonation is destructive and you've got to eliminate it. Here are some steps that I'd try:
1) First, make sure that the distributor is performing correctly and that the advance curve is right at factory specs. You may have to send the distributor out to a specialist in "old time" distributors to have this done, but you need to make sure that your advance characteristics are where they should be;
2) Second, try reducing the initial timing a little bit. Your engine specs call for 6 degrees initial advance. Try 4 degrees or, even, 2 degrees. I wouldn't go less than 2 degrees, though;
3) For your sort of usage, an octane booster might be practical. I don't consider these practical for every day usage, frequent usage, or long trip type usage. But, for the kind of driving you describe, they can be quite practical. Try them at various concentrations in combination with the 93-94 octane pump gasoline. Use the lowest concentration that will eliminate the pinging;
4) If all the above fails, then you're going to have to reduce the compression ratio when you rebuild the engine. I'd recommend going to 9.5 to 10.0 MAXIMUM if you do need to go this way. No sense in doing this by "littles"; it's hard to re-do it if you get it wrong, so take a good whack at it.In Appreciation of John Hinckley -
Re: High Compression Rebuild?
Mark:
That other 302 either had problems, or had a non original camshaft installed. I believe that the Z28 302 motor used the same 30-30 cam as the '64-'65 L76/L84. That cam will easily tolerate the stock 11.0:1 compression, factory timing setup, and run on today's hi test gasoline.
If you are going to build the engine yourself, just pay close attention to detail. If you are going to hire an engine builder, then stay with him, every step. If he measures everything, you will be able to calculate your ACTUAL c/r. It will probably come up somewhere between 10.25 - 10.75, depending on deck/head surfacing, head gasket thickness, and chamber volume.
JoeComment
-
Re: High Compression Rebuild?
The amount of static compression an engine can take on today's fuel is also considerably influence by valve timing - primarily the inlet closing point. The later the inlet valve closes the lower the dynamic compression ratio, and the lower the dynamic compression ratio the higher static compression you can run for a given fuel octane quality. Many are running original 11:1 engines on unleaded premium with no major detonation, but most of these engines were assembled with two shim head gaskets so their compression is below the advertised 11-11.25:1 value.
I assume you're talking about a '67 Z-28 engine. With the OE 30-30 cam, which has the latest closing inlet valve of any OE SHP cam, it will probably run detonation free on current premium, but may require some ignition map juggling to obtain detonation free operation.
Actual CRs average about half a point lower than advertised due to both double gasketing and typically high OE machined block deck heights. Using the OE pistons with a composition gasket and the OE machined deck height probably won't result in more than 10.5:1.
When rebuilding a SHP engine you need to target a CR of no more than 10.5:1 and then make all the measurements required to compute CR. This starts with measuring the piston crown to deck clearance on teardown, so you can make an initial calculation of CR using the piston dome volume data, nominal head chamber volume, and available gasket thicknesses.
A '67 Z-28 engine starts with a small bearing block, so I would recommend "upgrading" to a SHP 327 crankshaft with the 327 SHP pistons. Also replace the original 30-30 cam with a LT-1 cam. The combination of longer stroke and LT-1 cam will considerably improve low end torque and if you pocket port/port match the OE heads with a three angle valve job the power may still be climbing at the prudent 6500 redline.
Target a true compression ratio in the range of 10-10.5:1.
DukeComment
-
Re: High Compression Rebuild?
Thanks guys, I really appreciate you taking the time to respond so thoroughly. Yes, the motor is for my '67 Z-28 (you can see it at www.67z28network.com under "My Cars" if you care too). I'd like to keep it as close to factory specs as possible, so your answers lead me to believe this should not be a problem. The "other" 302 I had may have had the 140 off-road cam as it had a very "lumpy" idle (I loved listening to that car idle!). Jerry McNeish will be rebuilding my distributor, so that should be OK. I have an NOS 822 crankshaft that I plan on using and I had considered using the LT-1 cam to help with low-end torque. Is the idle sound much different with the LT-1 cam versus the 30-30 cam? Thanks again, Mark.Comment
-
Re: High Compression Rebuild?
The LT-1 cam pulls about 2" more manifold vacuum at 900 idle speed due to its less effective overlap. You might be able to tell the difference in idle quality with two identical engines idling side by side, one with the LT-1 cam and one with the 30-30. The LT-1 exhaust lobe is the same as the the 30-30, but phased four degrees earlier, so it should have a sharper exhaust note.
The 140 cam is a pure racing cam with very high overlap designed to be used with headers and open enhaust, and in this configuration the 80 percent torque bandwidth starts at about 3500. With manifolds and mufflers the overlap will absolutely kill the torque curve across the rev range.
In some cases it comes down to chosing between an engine that will turn heads idling through a parking lot or an engine that actually provides good street performance.
DukeComment
-
Re: High Compression Rebuild?
I am surprised that in Toronto you cannot get 110 octane racing gas(leaded). I have an L72(427-425HP) with 11:1 and I run the above race gas cut 50/50 with premium unleaded( 91/92 octane) and the engine runs very fine. Racing gas is far more expensive ($4-$5/gal.-US). You can't pump it into your car legally, but you can pump it into gas cans and take it home. Just something else to maybe look into.Comment
-
Re: High Compression Rebuild?
Mark:
Those are really 2 beauties! I am partial to red and black.
Yes, the 30-30 does have a decidedly lumpier idle, and if you measure the manifold vacuum, the 30-30 should only register about 10 in-hg if the lash is set properly, whereas the LT1 will pull about 2 more inches. The 30-30 actually stands for .030" lash intake and exhaust. As Duke kindly points out, the actual lash is closer to .027". The LT1 calls for .030"/.024", which will also slightly mute the singing of the lifters as compared to the 30-30.
Duke prefers the LT1. I prefer the 30-30. The LT1 will perform marginally better when installed in a 327 or a 302. The 30-30 will sound better. Another thing to remember, is that the LT1 torque/horsepower ratings are with A 350 CUBIC INCH ENGINE, whereas the 30-30 ratings are for a 327 inch motor.
John Hinckely has a 302 Z28 motor in original configuration. He must be in Windsor right now, or he might have chimed in already. It might be worthwhile getting a third opinion before you decide.
JoeComment
-
Re: High Compression Rebuild?
Thanks John,
Yes, we can buy racing gas up here, but I live in the city and our garage is below some bedrooms in our home. So, I don't want to store a drum of racing gas in the garage and to buy it in 5 gallon containers is both expensive and a pain in the butt! Thanks again, MarkComment
-
Re: High Compression Rebuild?
Thanks Joe, Yes, the red/black cars are pretty neat. The first project car I had was a red/red RS which I sold when I went back to school. Of the 4 that I've owned, I regret selling the red/red car the most. The new one is Mountain green with a black vinyl top, which I think will look really nice when done.
I appreciate hearing everyone's opinion and look forward to others "chiming" in. I am always torn between keeping things in their original configuration and taking advantage of improvements that were made over the years. With only 188 '67 Z's having been located, I am leaning towards keeping it as original as possible while still being able to drive it sparingly. Thanks again, Mark.Comment
-
Re: High Compression Rebuild?
Thanks Duke, I appreciate your comments. I could listen to my old motor idle for hours - yes, my wife thinks I'm crazy too!
I have a set of reproduction factory headers that I am going to out on the car, but feel the 140 cam is way too radical for my purposes. I just want the car to be as close to its original configuration while still being able to take it to a cruise night of two so others can enjoy it also. Thanks again, Mark.Comment
-
Re: High Compression Rebuild?
I agree with Joe on the 30-30 cam. I've installed many LT-1 cams in engines in place of the correct original 30-30 and the owners were very disappointed in both the exhaust sound and the flat depressing torque curve. The "kick in the pants" that was typical of the original 30-30 cam was gone and the cars never performed near the same as they did with the correct cam. I could go into details about all the testing that was done but that's another story. We wrote off that LT-1 cam in about 1972. As Joe mentioned, a big part of the increase in low/mid range torque of the LT-1 came from the added 23 cubic inches that the longer stroke 350" had.
I bought a new 68 Z28 and it had the 30-30 cam. That car ran VERY well and even had a decent amount of mid range power for a small inch motor. (I suppose the factory installed 4.88 helped) That was probably a bit more cam than was needed on the street with closed exhaust but was still very strong. I also well remember the late production 69 Z28's that came in with the new 178 cam. Those cars were hopelessl slow compared to an earlier production that had the good 30-30 cam.
I would think the 30-30 is an excellent choice for a car that has to use todays fuel. If the cam were installed two to four degrees advanced, you would have decent mid range and still have that power surge at the top. (4 would be the max and I would probably stay closer to 2 to 3)
As Duke pointed out, the 140 "off road" cam is hopeless for ANY street application. Ask me how I know. It worked extremely well with the 2x4 setup and open exhaust but other than that, the low/mid torque was non existant.
Good luck with your project. Let us know how it turns out.
MichaelComment
-
Re: High Compression Rebuild?
Duke: I have used/experienced the DZ140 cam in my '70 LT-1. I am running exhaust headers, MSD ignition, and port matched head/intake, along with the correct 850cfm Holley dual-feed. I developed a flat spot and am looking to replace the DZ140 cam. Can you recommend a replacement supplier? Tks, Jeff BurkeyComment
-
Re: High Compression Rebuild?
My untouched/unrestored original '69 Z/28 (just turned over 34,000 miles last week) runs just fine on 93-94 octane pump premium; there's no sound like a "30-30", but it needs all the gear you can stand to spool it up in a hurry with the little 302. Runs like gangbusters above 3500, not much torque below 3000.Comment
-
Re: High Compression Rebuild?
GMPP offers a clone 140 cam and lifter kit - 12364053 - but you are making a big mistake running that cam on the street. It may sound mean idling through a parking lot but has lousy torque bandwidth - headers or not. It's a racing cam designed for 4500-7500 operation, and is a stone below.
DukeComment
Comment