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Let's talk high compression and pump gas

Discussion in 'The Hokey Ass Message Board' started by 57JoeFoMoPar, Sep 5, 2008.

  1. 57JoeFoMoPar
    Joined: Sep 14, 2004
    Posts: 4,457


    I did a search for this and came up with zip.

    When it comes to building a high performance street motor, my thinking is to run as much compression as pump gas will allow for. It seems today's motors are capable of sustaining much higher compression on much lower octane fuel. For instance, my buddy's LT1 equipped Camaro has 11:1 compression and can run all day on 89 octane. Whereas my 383 Chrysler with factory flat-tops has 10.2:1, and will sometimes ping on 93 on a hot day.

    So, what are the factors in preventing detonation and sustaining high compression on the street? Obviously ignition has a lot to do with it. But what about aluminum heads? Combustion chamber design? Cam profile? I remember Scotch's thread years ago about a nasty SBC he built with ceramic coated pistons(?) cranking out 500+ hp on 87 octane. THAT'S the kind of stuff I'm talking about. Combine some new-fangled technology and some old it possible to have a carbed, 11:1 331 Caddy or 303 Olds running like a scalded dog on pump gas?
  2. First, I'm jealous you have 93.We have 91:mad:. Anyway, reduce total advance in the dist, cooler plugs, alum heads, and reverse cooling are some things that have worked for me. I ran 12-1 SBC and the reverse cooling's the key. I had Brodix intake and heads. This allowed me water ports on either side of the dizzy, carb, and thermostat area. I ran a Howard Stewart water pump that has big threaded ports on each leg right before the engine block. So, I put a shim between the pump and block so NO water entered the block. Instead, I plumbed each leg of the water pump to each of the 3 spots mentioned above with 1/2" alumimun tubing and AN fittings. Now freshly cooled water from the radiator goes right to the heads where combustion and heat originate. Water exits the block through a freeze plug, or drill into the side of the block and tap a fitting. I'll try and find a pic. Gimme a minute

  3. The new motors benefit from better combustion chamber shapes, aluminum heads etc - but more importantly Electronics that control fuel and spark maps with knock sensors to alleviate that dreaded death rattle. When you see old style motors running big compression on crap gas, they usually run lots of cam overlap which bleeds off compression at lower rpm where pinging mostly occurs. If you're willing to live with a big cammed motor that is soft in the lower rpm range, then yes it can be done, but something more street friendly in the cam department, I'd say not likely. I have run small block Chevy's on the street with 11.5:1 (and more) with big cams and a loose converter, but even old SBC Chevy's are less prone to pinging than earlier design motors.
  4. squirrel
    Joined: Sep 23, 2004
    Posts: 45,726


    LT1 chamber:


    383 chamber (stocker on right, aftermarket head on left):


    something about getting lots of movement, keeping the gasses stirred up as it burns, seems to prevent detonation.
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  5. Your 383 prob runs a piston .020"+ down the hole with a .040" head gasket for .060"+ of quench.

    Running a flat top or dished piston, .035" or .040" quench, closed chamber heads, decent duration @ .050" will bleed a little cylinder pressure off down low.

    Quench is ultra important. The old muscle car engines with open chamber design heads were'nt very good in that dept and combustion turbulence was pretty poor. Engines with good tight quench will require less timing and are very efficient with increased knock resistance on pump gas

    The old Cad or Olds engnes will likely need pisotns with a quench pads that will make an open chambered head spec out like a closed chambered one as far as quench is concerned. IF they have the right CR, quench, cam duration, good flowing heads etc then it should be not too difficult to make these engines run very strong on pump gas. Parts choice esp camshafts will most likely be the problem.

    This has been my experience.

  6. thunderbirdesq
    Joined: Feb 15, 2006
    Posts: 6,814


    What Rat Bastad said. The guy that taught me everything I know about engines said the key is keep the quench area nice and tight and a wedge shaped combustion chamber helps so the charge starts burning on one side and flows toward the other.
  7. dirty old man
    Joined: Feb 2, 2008
    Posts: 8,510

    dirty old man

    I'd be a bit concerned about detonation at higher rpms under heavy acceleration or load if I used a high overlap cam to bleed off cylinder pressure at low rpm. Then you would have the pipes rapping, plus wind and road and gear noise, etc. masking the noise and probably wouldn't hear the pinging and might get some engine damage.
    MSD now has a dist. that functions totally by electronics as far as controlling spark advance according to vacuum and rpm. Hopefully they or a competitor will soon figure out a way to add in knock sensors for an automatic retard when needed.
    If they ever market such a "better mousetrap", traditional or not it'll be the hot ticket.
    Till then, when I build an engine, it'll be around 10.0:1 at the highest, but future builds will do this with a thicker head gasket so that when and if such a system comes on the market I can increase comp. ratio just with thinner head gaskets.
  8. squirrel
    Joined: Sep 23, 2004
    Posts: 45,726


    I think that what the other guys just said about quench implies that you'd be better off running thin head gaskets because having tight quench lets you run more compression!
  9. SinisterCustom
    Joined: Feb 18, 2004
    Posts: 8,266


    Carbon buildup on pistons/head chambers can contribute to detonation on higher mileage engines as well.....
  10. dirty old man
    Joined: Feb 2, 2008
    Posts: 8,510

    dirty old man

    I totally agree about quench area helping. In fact back in the 70s when I was still active on dirt ovals I ran really tight deck clearance and the .016" chev. stainless steel gaskets and milled the heads for a higher CR then milled the piston popup .150" to keep CR within reason and speed flame front travel. clearance so tight that after a few races if you pulled the heads you could read the printing on the tops of the pistons in the carbon on the quench surface of the heads.
    What I'm talking about here is head gaskets in the .040-.050 range with .010 deck above piston @TDC, then you could drop to a thinner head gasket and still have adequate piston clearance when new technology allowed a higher CR.
    On edit: Carbon buildup can be retarded with a water/alcohol injection system such as the Edelbrock Vara-Jection ( do they still make it?). I ran one on a 5.0 Mustang to use 87 octane in a daily commute of 75 miles to work and back. Worked great with almost no carbon build up. I used methanol and water 50-50 mixture.
  11. Scotch
    Joined: May 4, 2001
    Posts: 1,488


    Quench helps a lot- consider what you're trying to do, by squeezing an air;/fuel mix to one tenth (on average) it's size, and then some of this stuff will make more sense.

    The chamber should be small, and offer the quench area we're talking about, but even a well-designed piston with a dish that reflects the shape of that chamber can be used to cheat a bit. This is especially true when it's compared to a traditional dished piston with a depression all the way across it, like a shallow saucer. A piston with a chamber-shaped dish and the rest of the surface flat will offer greater efficiency (which translates also into its ability to resist pinging and detonation longer, in addition to offering a better burn because the flame doesn't have to travel as far).

    The coatings I'm all geeked about just keep getting better (and cheaper). The 383 mentioned earlier has now been on the road for a year, and is fine-tuned to run hard on 87. Here's a link to that engine build for those who didn't see it:

    The 383 made 545 hp @ 6,400 rpm on 87 octane, and a twin 355 built with the exact same parts (except for the crank and pistons, of course) made 515 hp. This was done years ago and now others are offering engines with similar power.

    The key was to run 9.7:1 on the bad gas, but the theory is universal. By using a porcelain-like ceramic coating on the piston tops, chambers, valves, and port interiors, the heat from the chamber is spread out evenly over a larger area rather than being isolated into hot spots typical in castings. It's these hot spots that ignite the fuel before the spark plug can, and it's why polishing chambers has resulted in performance gains over the years. The coatings are the next level of chamber polishing, which has been shown to allow fuel to fall out of suspension in some cases- especially at lower rpm levels. It's why that Indian dude who cut grooves into his piston tops saw the improvements he did- by keeping the air/fuel mix moving while the piston is in the compression stroke, the fuel stays in suspension and burns more efficiently.

    My research showed that leaving the tool marks from CNC machining in the chambers had a similar effect. In the AFR heads I chose for the 383 and 355 builds linked above, the chambers were CNC-finished and then coated, getting the best of both worlds.

    A later story by Marlan Davis in Car Craft showed that coatings alone don't make much power- surely not enough to justify their cost. It was a simple before/after test, which wont' show much difference. What 'ol Marlan didn't do was take advantage of the coatings he added...addition of chamber coatings will allow for higher compression ratios while keeping detonation at bay, and if he'd have done the test differently, this benefit would have shown the true advantage of these performance coatings.

    If the test would have been done on a 10:1 compression engine, it would not have been able to run more than 30-32 degrees of timing without detonation. With coatings, it would have been able to run 36-38 degrees of advance (or whatever the engine liked to produce peak power) without detonation, and the additional power gained could have been credited to the coatings.

    The coatings don't make the power, they just make it possible to make more power.

    Anyway, teaming a super-efficient shallow chamber with a well-designed dished piston and adding coatings offers about the best-case scenario. To this, add a cam designed with some overlap to bleed off excess compression at low rpm and gain additional flow at higher rpm levels, and you can make some really good power.

    Another 'trick' I used was NOT to port-match the intake to the heads. I left the intake ports smaller than the intake ports in the heads on purpose, and if you've ever seen a stepped header, you'll soon figure out why.

    Remember when I mentioned the fuel falling out of suspension? That happens a lot in carbureted engines. Gravity pulls fuel out of suspension (especially at low rpm) and it settles on the port walls. By leaving a 'step' in the intake tract, the fuel is drawn back into supension right before it passes the valve. There's another reason this works, too.

    If you've ever seen a flow map of an intake port, you'll see how the air in the center of the port is moving much faster than the air along the edges. This is due to friction along the walls of the port, and while porting work can reduce this friction, nothing can completely eliminate it. But, by leaving a step between the intake and the cylinder head, the high-flow area of the cylinder head port encompasses a larger portion of the intake manifold port, and the quality of the flow into the head is improved.

    So, I'd advise NOT to port match your heads to your intake manifold, but rather, leave 1mm or so of a step between them so the larger cylinder head port can suck more effectively on the smaller intake manifold port.

    The plumbing of the cooling system mentioned earlier is also a great idea and is worth power, especially when higher compression is being supported. The increased cooling capability of the heads keeps temps under control, since too much heat in the chambers will cause preignition.

    Also- always run pure synthetic oils in newly rebuilt engines once they're broken in. It also will keep an engine running cooler, and it's ability to fight viscosity breakdown means it'll keep the inside of your engine cleaner for longer.

    There's more, of course. Solid roller cams make more power, but they require maintenance. EFI is more accurate and more efficient with regard to fuel economy too. A well-designed exhaust system offers benefits throughout the rpm range and will also contribute to higher fuel mileage numbers. Plumbing the engine to breathe fresh air helps.

    I used every trick I knew in the 383/355 builds, except EFI. Now I know more, and I still might add EFI to the engine someday. But, for what it's worth, the 383 is in my station wagon, and I have little doubt it'll run in the 11s when I get it down the quarter mile. The car also gets 20-plus mpg. It should pull 1G in the turns too, but that's a subject for another thread...!!

  12. Newer engines like the Gen III GM stuff (LS1 - L92) have great cylinder heads and that's one reason why LS1's run less timing than older SBC's. Much of the newer GM stuff uses aluminum blocks so there is less heat kept in the block. We've seen LSx engines make 500-525hp with a 348ci, 3.900 bore x 3.622 LS setup with like 27 degrees of timing, 11.5:1, on 93 pump. Car made 415rwhp thru 8" unlocked converter, so 450-460 rwhp locked or with stick car, and ran 10.9@123mph at 3300 raceweight.

    I had a 422ci (4.085 stroke x 4.060 bore) LS1 stroker with a truck block and it made 472rwhp locked converter, 93 pump, 25 degrees of timing, with 12.2:1. Car went 123mph at 3650 raceweight.

    I don't think you can do such things with older engines. The older heads flow like crap, and you have iron blocks.

    Every point increase in compression is worth 3% in total hp.

    So I'd go 10:1 with 93 pump gas with older car stuff, I'd try to run the best ported cylinder head possible, and keep the timing down.
  13. 57JoeFoMoPar
    Joined: Sep 14, 2004
    Posts: 4,457


    Scotch, thanks so much for chiming in on this thread.

    Great stuff guys, this is what I was looking for. Keep it coming!
  14. i think all the basics have been covered here, good stuff. my ranchero is my daily driver, & i run about 11:1 compression with no problems. i had to deck the block .036 to get closer to a zero deck height, most engine builders i've talked to say this is one of the main keys to no detonation. also, smoothed out any sharp edges, & my cam is pretty big so that helps too, also carb is on the bigger side. coming back from vegas i was forced to run 87 octane for about 100 miles cause thats all the station had, my car didnt ping at all, but it wasnt as snappy on the throttle response.
    for awhile we battled detonation on my friends blown hemi. what finally got rid of it was going from 600 edelbrock carbs to a pair of 650 double pumpers with the biggest jets i had stuffed in the secondaries.
  15. aerorocket
    Joined: Oct 25, 2007
    Posts: 488

    from N.E. P.A.

    Rickyracer1962 I found the same to be true on my stock F.I. 375h.p. 327. The engine is factory rated @ 11.0 and I run 42 total all in at 2800 with no pinging on 93 octane. The cure was to fatten up the jetting.
  16. Shifty Shifterton
    Joined: Oct 1, 2006
    Posts: 4,965

    Shifty Shifterton

    Your buddy's LT1 has a knock sensor and computer controlled ignition. And I guarantee if he was watching what it does with a scanmaster, he'd see the computer pulling a bunch of timing out, to the tune of a tenth or two in the quarter. Even with 91 octane, most LT1s will pull some timing WOT unless fitted with the less sensitive LT4 knock module.
  17. I agree
  18. SlowandLow63
    Joined: Sep 18, 2004
    Posts: 5,953

    from Central NJ

    Wow Scotch, thats probably the most informing post I have ever read. Thanks!
  19. btmatt
    Joined: Nov 15, 2006
    Posts: 227


    All things above will lead to succes with high compression on pump gas; good quench, cam overlap, heat control,timing, and fuel ratio.

    One contributor noted detonation at higher engine speeds and this tends not to be the case. Detonation will rear its ugly head at low rpm and high load. Typically at higher engine speeds the piston moves fast enough that there is not enough time for detonation to create the deathly pressure spike.

    One of the most important items not noted yet, and it really applies to vintage engines is oil control. If the intake charge is contaminated with engine oil in the slightest amount, it effectively lowers the knock resistance greatly. One must pay careful attention to piston fit, minimization of piston rock, ring material and fit, and most important valve seal type.

    There is no way that you can effectively control oil in a high performance engine using "vintage" type "o-rings" or "umbrella seals" You have to spend the bucks and machine the guides and install a quality PC type seal. In addition you must effectively vent the crankcase to prevent overpressure and oil blowby. This means good bye draft tubes and hello PCV valves and vacuum pumps.

    All things considered, there are some really good suggestions listed above. So is the gauntlet laid; highest compression on "dog piss" gasoline? I have a 400 flat top Chevy with fuelie heads and fuelie cam waiting to be installed in my truck. With a different cam, it might be a good candidate.
  20. A little extra cam shaft overlap helps too!
  21. RugBlaster
    Joined: Nov 12, 2006
    Posts: 563


    I don't know why you would build a street engine so close to the limit of compression. I personnally would'nt go much over 9.5 ......The key to max street power is the use of a roller cam......rollers offer a much more aggressive lift, curve capabilities compared to a flat tappet cam.....much more lift can be achieved without the needed duration that the flat tappet cam has to use. For a street motor, the use of a proper hyd. roller cam and appropriate cylinder heads will produce as much HP as you will need or want.

    What is the point of building a high compression motor if you have to fatten up the fuel mixture or take timing out to get it to run without pre-ignition.....normally, for pure horsepower considerations, you would lean it out and have an aggresssive timing curve.
    Last edited: Sep 6, 2008
  22. i run a vortec based 11:1 SBC in my 55 on 93 all day, every day. i keep the timing at 32 degrees at 3000 rpm, i'm running old style aluminum heads (not the centerbolt heads), aluminum intake, full msd ignition and 2 heat rannge colder plugs than factory spec for my motor.
    i'm doing this through a simple edelbrock water pump, desert cooler brass radiator and no shroud on the fan. 160-165 when moving, but it will creep up to about 180-190 in traffic. keeping it cool and keeping the heat out of the chambers is key. smoothing down rough surfaces and angles inside the chamber helps as well as getting the exhaust out as efficiently as possible.
  23. Mr. Creosote
    Joined: Feb 27, 2006
    Posts: 275

    Mr. Creosote

    It is important to realize that the engine sees three different compression ratios. One is the static ratio which we are all familiar with: clearance volume + swept volume, divided by the clearance volume. A number like 9:1 is a common static compression ratio.
    The second is the effective compression ratio, which the engine sees when the intake valve closes against the valve seat. A number like 7:1 is common. This is determined by the interactions of the static compression ratio, the rod ratio, and cam timing for closing the intake valve. (Wrist-pin offset has an additional but minor effect.)
    Third is the dynamic compression ratio which is when the engine is in the peak power range and the volumetric efficiency is above 100% then the cylinder pressure-compression when the intake valve closes is at its highest, example above 8:1.
    Building an engine for more performance often means raising the static compression ratio close to
    10:1, but keeping the effective compression ratio not much over 7:1. Anything lower gives up power. Anything much higher will not run at low speed with WOT on pump gas without detonating and destroying itself.

    Copied from an artical written by D. Elgin
  24. Great thread guys !! Some good info here.

    Hyd roller cams are great - they give you that area under the curve but asre easier on the valvetrain. Theres a reason new engines run em...frictional losses are lower therefore maximising efficiency !

  25. panic
    Joined: Jan 3, 2004
    Posts: 1,450


    "a high overlap cam to bleed off cylinder pressure at low rpm"

    Please: let me know how this is possible?
  26. BOTH intake and exhaust valves are open at the same time, for longer periods of time than stock. So, when the piston travels up during compression stroke, some of the pressure/compression is bled off through the open valves. A stock cam will shut the door/valve sooner allowing more cylinder pressure. This "effect" diminishes with higher RPM. Not to mention, the "bleed" reduces the effect of the higher compression. So, what's the point? It's not really the way to go
  27. Did you say your 56 was Yesterday's Child or something? It's a full page in the new Rod Kulture Magazine
  28. Mr. Creosote
    Joined: Feb 27, 2006
    Posts: 275

    Mr. Creosote

    Engine cycles are determined by the direction the piston is traveling and the timing of the openings and closings of the valves — collectively termed valve-timing events. Timing these events becomes complicated because a lot of compromise must be made in order to balance out all of engine operating cycles. We’ll walk through each cycle in turn, now from the viewpoint of determining how changing every valve-timing event affects other cycles, and how balance builds power.
    · The Power Cycle: TDC to Exhaust Valve Opening
    By the time the crankshaft reaches 90° ATDC, cylinder pressure has dropped greatly and most of the power that can be recovered from it already has been. So opening the exhaust valve well before BDC loses less power from the power cycle than it later gains across the following cycles. The lower the rod ratio, the faster cylinder pressure drops.
    · The Blowdown Cycle: Exhaust Valve Opening to BDC
    The blowdown cycle relieves excess (but unrecoverable) cylinder pressure and begins clearing exhaust gases off the energy of their own pressure. Otherwise the piston would have to push all the exhaust gases out of the cylinder on the next up-stroke, lowering horsepower from a pumping loss.
    The timing for Exhaust Opening is the least important of the four valve events. It can be anywhere between 50° and 90° BBDC, so its timing is easily adjusted to match the performance characteristics of that engine.
    With higher compression ratios the burn rate is faster, so the exhaust valve can be opened earlier, which aids in the cylinder blow-down. With lower compression ratio (static 8:1 or lower) you want to delay the exhaust opening as late as possible in order to utilize the last usable bit of pressure that is on top of the piston. But that hurts the top end horsepower, because the blow-down period is no longer as effective.
    · The Exhaust Cycle: BDC to Intake Valve Opening
    The piston reaches maximum velocity at about the same number of degrees BTDC as it did ATDC on the way down, or a degree or so sooner with offset wrist pins. The exhaust valve must be open sufficiently by this time so that spent gases in a hurry meet little resistance against being pushed out.
    How far the valve must be open is known from flow-bench data. The proper cam meets that need from a combination of timing, total lift, and its rate of lift (its “velocity”).
    · The Scavenge Cycle: Intake Valve Opening to Exhaust Valve Closed
    The scavenge cycle occurs during the overlap period, when intake and exhaust valves are both open at the same time. The intake valve is just opening. The exhaust is closing but not yet seated. Overlap is what the cam and valves are doing, dictated by the combination of total cam duration and the locations of lobe centers. Scavenging is what the engine is doing with that.
    A good number of engine processes (and a few unsolved mysteries) are going on now simultaneously. The most important are (1) scavenging the last of the exhaust gases as much as possible from the clearance volume, where the piston cannot reach to push them out, and (2) initiating intake flow into the cylinder without wasting very much of it out the open exhaust valve.
    Overlap duration increases as total duration increases, and it also increases as the lobe center decreases. Increasing the time for Overlap makes more time for scavenging at high rpms. Residual exhaust gases kill power twice over: they displace their volume in incoming charge, and later during combustion they absorb heat that should have gone into making power. At 5000 rpm an engine with a high-performance cam carrying 55 degrees of overlap must complete the entire scavenge cycle in less than two thousandths of a second.
    In standard engines, valves are open together for only 15-30 degrees of overlap. In a race engine operating between 5000 and 7000 rpm, the overlap period is more like 60-100 degrees. The penalty for so much overlap in a street engine is very poor running at lower rpms, when a lot of the intake charge has time to sidetrack directly out the open exhaust valve. Mileage goes South. Heads overheat from fuel burning in exhaust ports. The engine runs hot. The exhaust system gets fueled like a blowtorch. The tailpipe turns white. Catalytic converters fry. The buyer blames the cam grinder.
    Timing Exhaust Closing must be balanced against flow through the intake port. If the intake port flows poorly from being too small (or too large) then later Exhaust Closing might help to initiate intake flow. I consider this only as a last resort for kick-starting a lazy intake port. It always carries some charge out the exhaust valve, wasting fuel and all that.
    Make the overlap period as short as will complete the job of scavenging. Factor in the effects from the combustion chamber size and shape (including the shape of the piston top) and shrouding near valves. Balance power goals with other requirements for the intended usage, such as idle quality, low-speed throttle response, fuel economy, and smog test compliance.

    · The Intake Cycle: Exhaust Valve Closed to Intake Valve Closed
    I consider Intake Valve Opening the second most important valve timing event, because that does two important jobs. (1) It initiates the Scavenge Cycle and (2) it begins lifting the intake valve out of the way of the incoming charge. The air/fuel mixture began entering the cylinder during the Scavenge Cycle, builds to a maximum, tapers off, then packs in a final gulp.
    The intake valve is in a race with that pressure differential at maximum piston velocity that drives intake flow. The valve always loses this race, because max draw happens between 70° to 80° ATDC, yet the intake valve does not open fully until it reaches centerline, down around 105° to 115° ATDC.
    When you can’t win, do your best. Get the valve out of the way as far as possible by giving it a fast rate of lift, a “high velocity”. Much the same could be accomplished by more valve lift, but then the nose of the cam gets pointy and real stiff springs are needed for closing the valve – a combination not favorable to very long service life.
    The Intake Closed point – when the valve seals on the seat – is the most important valve-timing event. This event governs both the engine’s rpm range and its effective compression ratio. Closing the intake valve later optimizes intake flow for high rpm and allows inertia to pack in its last gasp of air. The drawback to that is back-flow at low rpm. But closing the valve earlier shuts down rpm. Pick your operating range.
    · The Compression Cycle
    The piston compresses the air/fuel mixture to a high enough pressure and temperature for it to be ignited efficiently by the spark. The effective compression ratio must be high enough to compress and pre-heat the air/fuel mixture for a fast, complete burn.
    But too much heat and pressure kick off the whole charge at once in the destructive explosion of Detonation. When pistons taken from a blown engine show ring lands melted as if by a cutting torch, that was by Detonation. (If a hole has been blasted through the center of the piston crown, that came from a hot spot in the chamber pre-igniting the mixture.)
    Joined: Oct 19, 2006
    Posts: 1,205

    from Ramona CA

    Three things help the octane issue. Timing, cooling and exhuast flow.
    I ran a small block Ford with 10.5:1 on 89 octane..if I had to.
    For awhile I had over heating problems and then pinging.
    I installed a cross flow radiator with a better than stock water pump. Trashed the points for electronic dizzy with a msd box and enlarged the exhuast flow.
    Which of course worked to the advantage when running 110 at the track.
    Also remember running aluminum heads will save you a point on compression.
  30. Dyce
    Joined: Sep 12, 2006
    Posts: 1,863


    A step or two colder on the plugs helps. Great advice here. I'm really thinking the ethonal fuels may be a good deal hear too. Maybe run less overlap on the cam and use e-85. This would allow a tighter convertor and a taller gear running less rpm's. I love high compression and big cams, but I also love driving my hotrods too.

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