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Tech / General EngineIs your car making a strange sound or won't start? Thinking of adding power with a new combination? Need other technical information or engine specific advice? Don't see another board for your problem? Post it here!
I am from Switzerland (French speaking side actually), so I will do my best with english, but it aint really good... sorry!
I am the happy owner of a 89 formula LO3 305 TBI targa.
I know it aint the formula spoiler and my "hot" air filter is absolutly not good for perf.
I am about to:
-Swap TBI <=> TPI
-Burn new chips (thanks to Moats.com hardwares and wideband!)
-Swap stock camshaft <=> LT1 stock camshaft (hydraulic)
-Increase compression ratio to 10,1:1 if possible (for 95 octane gazoline) (I hope I could reach it with a 0,015 gasket, but i think I will have to mill my 14102187 heads (58cc I guess)).
2) I am very worried of smashing my valves against the pistons if I manage to mill my stock heads (14102187)... I don't know if you understand what I mean.... Lets draw:
Any ideas of what is the stock clearance between piston and valves? (blue arrows) (with the stock LO3 camshaft)
I promise to send Swiss choclate to the one who help me figurate everything out!
You have no need to worry about piston-to-valve clearance. The valves are at their max opening when the piston is about halfway down the bore. They are always nearly closed when the piston is at the top. But even then, a stock small block Chevy is a "non-interference" engine; when the timing set fails and the valves just stop wherever, the pistons still don't hit them.
Only way you get 9.3:1 there is with a pretty thin head gasket- which is what the factory did. If you go dropping some fat FelPro Blue .039" "rebuilder" gasket on the motor it'll be down in the high 8s every time.
The 87-up 305s used "small dish" pistons which were nearly flattops, but not quite. Even the 86-down L69 HO 305s that used actual flattops (just valve reliefs) were only rated 9.5:1
Hitting 10:1 won't happen without some serious milling on the heads or a set of small domed pistons (at which point you might as well just start with a 350 and be done with it).
Don't worry about piston-to-valve clearance. No chance of that being a problem for you with a cam that mild. Tight LSA and big duration is when you start having those problems (the piston tries to kiss one or both valves during the "overlap" period between the exhaust and intake stroke) and the LT1 cam isn't tight LSA or big duration.
Still, low 9s for compression with the LT1 cam and a properly tuned TPI setup will still be a well-matched combination.
Factory steel shim head gaskets are around 0.018" thick. You now have a composite 0.040" head gaskets. That's going to make a big change.
For low octane pump gas, what you have should work just fine. These cars don't have the high tech fuel management that modern cars now have to allow higher compression and low octane.
a stock small block Chevy is a "non-interference" engine; when the timing set fails and the valves just stop wherever, the pistons still don't hit them.
Thanks a lot for the correct specs! Now I have my 9.3!!!
I was worried the LT1 camshaft shape would smashed the valves at the exhaust. But since it is a "non-interference" engine...
Originally Posted by Damon
Hitting 10:1 won't happen without some serious milling on the heads or a set of small domed pistons (at which point you might as well just start with a 350 and be done with it).
Since I have a milling machine at home, and sadly no "american engine shops" whithin 2000 miles... I will not buy the pistons... I am dreaming of a 350! Hard to find in my country... Swiss people are only running very small engines like 60cid... The only things I have found is a spare pair of stock heads.
Originally Posted by AlkyIROC
For low octane pump gas, what you have should work just fine. These cars don't have the high tech fuel management that modern cars now have to allow higher compression and low octane.
According to your performance in your signature, I am taking you very seriously. Do you think it is a mistake to try to reach 10.1?
With your 0.018, it will increase to 9.7:1. In Switzerland we commonly found 95 and 98 gazoline (RON). Do you think it is high enough with such fuel?
Search and read up on "Dynamic Compression Ratio". The discussion here is the easy-to-measure Static Compression Ratio. SCR + cam timing = DCR. Bigger cams = more cam timing = later intake valve closing = lowers the DCR. That is why bigger cams need higher SCR, yet can still tolerate low(er) octane.
Target DCR is 8.0-8.5 to 1. Anything less is poor response. Anything more needs higher octane.
The mild stock/L03/L98/LT1 cams have relatively early intake valve closing point, and do not drop DCR too much. So, 10.0:1 SCR with such a mild cam is likely to build too much pressure, and need high-octane. And a bigger cam (longer duration) will lower the DCR and the octane requirement... And you get more power!
That's basically same same as mid grade and high grade in north america
How much octane do you really need? There's no more BTU energy in 87 octane as there is in 94 octane. Octane is the fuels resistance to detonation. It keeps the fuel from preigniting under compression.
If your car does not ping/knock under load with 87 octane then putting in a higher grade won't increase performance. Trouble with modern cars is the knock sensor. When it detects that knock, it will retard the timing until the knock does away. You'll never hear it. You need a scan tool that can display knock counts to know how bad the knock is.
Drivers will then claim that putting in a higher octane increased their performance. Well it does because if the ECM doesn't detect and knock, it won't retard the timing and you'll have more performance. How much energy in a gallon of fuel never changed, just the way the engine uses it does.
My race engine is around 14:1 compression but I also run methanol for fuel.
For your piston to valve clearance, contact is made from incorrect valve timing more than lift/duration. On the intake stroke, the piston is moving down into the cylinder. At the same time the intake valve is opening and is chasing the piston. Same thing but backwards on the exhaust stroke. The exhaust valve is closing as the piston is coming up to push the exhaust gas out. You can have a huge amount of lift/duration and still not make contact if the valve timing is adjusted properly but there comes a point when valves cutouts are required in the pistons to allow more clearance.
Under normal conditions the minimum clearance should be 0.060" on the intake and 0.100" on the exhaust. The best way to check is with some modeling clay on top of the piston then bar the engine over 2 revolutions. Pull the head back off and measure how thin the clay gets.
OEM camshafts have far more clearance than when the minimum should be.
Well Dave the rule of thumb for compression ratio on pump gas is 9.5:1 max with iron heads and 10.5:1 max with aluminum heads. But there is much more to this. Better quench will help but there are alot of stock pistons that dont have good quench area regardless of deck height. Also altitude the car runs at can affect detonation as high altitudes thinner air dont fill the cylinder as well and will resist detonation better. Of course air temperature affects this also.
To bad shipping and import taxes add so much cost. What we here in the states we really take for granted as the small block chevy is cheap everywhere here.
To bad shipping and import taxes add so much cost. What we here in the states we really take for granted as the small block chevy is cheap everywhere here.
Yes you're right! Last time I have ordered some U-pipes to weld my exhaust. The U-pipes cost me around 80$, but the shipping cost with the custom: 359$ !!!!!!!
Imagine how much will cost the shipping of a such heavy engine...
Target DCR is 8.0-8.5 to 1. Anything less is poor response. Anything more needs higher octane.
Thanks for the tip! I had never heard about DCR before since I am no car-mechanic...
So, I have read a lot about DCR these days and now I try to compute it.
The probleme is there is no way to find the ABDC of my LT1 camshaft...(205/207 duration @ 50*, .447/.459" lift, 117LSA) I have spend hours but nothing. It seams this value was removed from chevy tables... Very weird: (seems to be last one, 12551705)
Any Ideas where I can find it? Otherwise I will have to mesure it directly on the engine...
Last edited by davidmoret; 02-18-2016 at 03:19 AM.
Since #2 (14060655) and #3 (14060651) have 294-296 duration, and the LT1 is @ 293, I would guesstimate the LT1's intake closing is ~90* ABDC. Google didn't find a definitive answer, but some other threads match up with my guess.
ETA: Running 90* through a DCR calculator end with ~5 or 6:1, and it takes plugging in 60*-ish to get DCR up to 8:1, which is a REALLY short cam @0.006". I suspect many of these calculators are expecting the @050 measurement and add 20+ degrees.
For dynamic compression ratio calculators, use the .006" tappet lift timing. Here's the approximate numbers for your LT1 cam: 262/266/116+5. IVC 62 deg ABDC.
That results in a DCR of about 7.6:1 .. not so good.
I think you should use .015" shim head gaskets to get that compression up to better utilize the LT1 cam. That would give you a 9.9 SCR and 8.0 DCR.
Since neither of those are rubberized (I don't know why I can't find those anymore), spray both sides with copper form-a-gasket spray, and let them dry for about 15 minutes before installation.
No reason they shouldn't be cheap. Just a single layer piece of embossed sheetmetal.
How much octane do you really need? There's no more BTU energy in 87 octane as there is in 94 octane. Octane is the fuels resistance to detonation. It keeps the fuel from preigniting under compression.
If your car does not ping/knock under load with 87 octane then putting in a higher grade won't increase performance. Trouble with modern cars is the knock sensor. When it detects that knock, it will retard the timing until the knock does away. You'll never hear it. You need a scan tool that can display knock counts to know how bad the knock is.
Drivers will then claim that putting in a higher octane increased their performance. Well it does because if the ECM doesn't detect and knock, it won't retard the timing and you'll have more performance. How much energy in a gallon of fuel never changed, just the way the engine uses it does.
Ok so if I have well understand what you said, the higger octane, the less the engine will knock thus less timing retard and thus better perf. So why are gas station selling lower octane? Because high octane rate = high prices?
Thanks to my moates+protuner kit, I can check the knock count. So if there is no knock with the thiner gasket (according to the new DCR it should be ok), I can still use 95 octane gasoline. Otherwise I will have to go to 98 or more or change my DCR?
There's nothing to be gained from milling those swirl-port heads, and the LT1 cam is a bad mis-match. For a cam, the best match to the heads you have would be this, and it'd work with the stock TBI: http://www.summitracing.com/parts/cc...make/chevrolet
The best solution to the lack of 350s is a Chinese turbo, once you figure out the fuel delivery. 600 HP is safe if you can keep it from detonation. Then you'll be breaking transmissions and rear axle parts instead
There's nothing to be gained from milling those swirl-port heads, and the LT1 cam is a bad mis-match. For a cam, the best match to the heads you have would be this, and it'd work with the stock TBI: http://www.summitracing.com/parts/cc...make/chevrolet
The best solution to the lack of 350s is a Chinese turbo, once you figure out the fuel delivery. 600 HP is safe if you can keep it from detonation. Then you'll be breaking transmissions and rear axle parts instead
Your name cosmick is very appropriate, because it appears you just dropped in here from another planet. Wtf.....did you even read this thread?
Ok thanks everyone! Thanks to you all I think I am done with the SCR/DCR!
By the way... any ideas of a thread speaking of a mod for my cylinder heads? I really hesitate to drill intake and exhaust holes to wide them a little bit. (manifold as well of course). Or at least make them smoother (polish). It is worth it? Did anyone already try such mod?
Start with simple: look for any bumps and casting flash and smooth that out. Smooth=good, sharp=bad. Don't make any large change to cross section (height x width), which will cause the air to speed up or slow down due to restriction. Remember, whatever you do, you will have to do 8x.
Basic tip: If you make the intake passage larger than in the head , you'll have a step blocking the airflow... worse than what you have now. Better to have the head slightly larger than the intake. Best to have intake=head.
So why are gas station selling lower octane? Because high octane rate = high prices?
The majority of the cars on the road are older low performance cars or modern cars with electronic fuel management systems. Both perform just fine with a lower octane fuel.
The handful of production cars with higher compression engines or some sort of boost system with a blower or a turbo normally need the higher octane to reduce detonation under load.
Like I said, you only need enough octane to reduce or eliminate detonation. If the fuel doesn't produce a knock under load then higher octane will not make more power.
Ok thanks everyone! Thanks to you all I think I am done with the SCR/DCR!
By the way... any ideas of a thread speaking of a mod for my cylinder heads? I really hesitate to drill intake and exhaust holes to wide them a little bit. (manifold as well of course). Or at least make them smoother (polish). It is worth it? Did anyone already try such mod?
Well good head work is the same in Europe as the US im sure. What i mean u should be able to find a competent performance machine shop that does head porting. As far as doing head porting myself i have read and researched it enough that while an amateur can do some improvements its more likely i would make more mistakes than improvements. There are so many dos and donts and results can only be verified w/flow bench that i would seek professional work for this. And the cost of the cutting tools can be significant.
Basic operations that should help even w/o porting the runner areas are larger vlvs, 3 angle or better vlv job, unshrouding the vlvs, pocket porting.
I have heard my stock 14102187 heads are one of the worst chevy has ever made. Since they are really heavy as well I am considering to buy used alu heads.
Well those are rebuildable cores. Thats kinda the hard way to do it as they are not ready to use after purchase and will needs new parts and machining. Also i cant verify they will fit your 305" bore/motor. U need a veriry reliable source to confirm they work on a small bore 305".
I guess what im saying is good quality heads that are ready to bolt on may seem more expensive up front but could save you a lot of $$ and headaches to install and use.
02-13-2016, 02:02 PM
Thanks!