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Zedyone_kenobi

A converstion about how much compression is too much for pump gas

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Let us all have a talk about compression, and timing. How much is too much, how to deal with it. And how to achieve the max amount of advance you can while not pinging.

Enter the subject. My Datsun Spirit STR2.8 liter engine.

L28 N42 Engine Block, N42 Cylinder Head base

87mm (bore) x 79mm (stroke) = 2817.8cc

Combustion Chamber Volume: appx 43.5cc

Piston above deck: appx 0.25~0.30mm

Compression ratio: 10.5 : 1

Camshaft: STR274F

Duration 274

Max Valve lift: 12.00~12.19mm

Lobe: IN: 106, EX: 110, Lobe Center: 108

I have driven this engine hard for 2000 miles or so since it was installed. It has run beautifully. During 1000 mile rally the car ran without a hitch, but I purposely dialed back my timing to take into account for iffy gas as small mom and pop gas stations I may encounter. After the rally I crept back up to 34 deg BTDC at 2800 rpm and drove her. At low RPM in 3rd and 4th when the engine is under heavy load I noticed copious amounts of pinging. I run 93 octane only locally.

Long story short, I cannot run 34 deg of max advance without dropping a bottle of good quality octane boost in my car. With the mileage I drive, that means about 5 bottles a year.

Correct timing is so important to getting the max HP out of your engine. Every degree of timing can yield substantial HP changes. So what is the max timing a street engine can run. Naturally I have cast my stone. And I find with my particular set up, 34 degrees is about all I can run with 93 octane and a bottle of NOS octane booster (don’t laugh, it was ranked 3rd best out of the top 10 in an independent study, plus I like the blue bottle)

Can the veterans of the Z world chime in on this. What steps can you do after the engine is built to help avoid pinging. I have not tried some BP7ES plugs which are colder ( I have them in my garage though) . I currently run a 190 deg thermostat. Would a 160 help?

I love how my car runs and the extra 65 dollars a year in octane boost really does not bug me that much considering how well the car responds to it, but for others building an engine, what should be the max limit on93 octane gas. I would have to say 10.5 is too much.

Granted the camshaft over lap and lift all have an effect on this. But lets get a good conversation going.

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Two very important things need to be mentioned here:

1. There is no specific compression ratio-to octane conversion, since knock-resistance is dependent upon much more than just static compression. Some key factors are combustion chamber design (including spark plug location), RPM, AFR, and valve timing profile.

2. Max timing DOES NOT equal max power. The optimal timing map for an engine is found during controlled dyno testing.

There are a many reasons that can cause engine knock, as well as different types of knock. Fundamentally, knock is when a part of the mixture ignites before the flame front from the spark plug gets there. In your case, I'd be thinking about things getting too hot in the combustion chamber. This usually comes from three things: spark plug, exhaust valve, glowing deposits. Going to a cooler plug and/or dropping coolant temperatures could help, but not guaranteed.

Spark plugs are easy enough to try out. However, if I were you, I'd pour in some 100 octane fuel and go to the dyno to see what YOUR engine really needs as far as timing goes. Come up with a "good" dyno testing strategy, i.e. minimize variables.

Just keep in mind, knock happens for many reasons and is also dependent on ambient conditions. This is why there's a safety net built in on factory spark maps.

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Might want to add piston to head clearance and HG specs also. The get ready for more opinions then raw facts;)

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Leon, well said!!

I agree with your assessment. My BP7ES are going in this weekend, but I am not sure it will make that big a difference, but it certainly is in the direction of goodness. I have been wanting to get the engine on a dyno for a while, since it is totally broken in. I just need to find some time.

I also echo'ed your opinion about all the variables and knobs that can be turned. Right now the problem is not really that bad. But I am liking the idea of dropping some 1000 octane and playing with timing until I get 'the ideal settings', then try to figure out how much I am losing with 93. According to independent lab reports NOS octane booster increases the octane by right at 1.8-2 points per tank. So figure i start with 93 at about 95 all my pinging stops and the car runs very well with 14 degrees at 900 rpm, and 34 degrees max at 2800 rpm. I have not advanced it beyond that as it starts up, runs and keeps AFR dead nuts on all the time. In fact it runs perfect.

This is where my engine build got a bit off track. We wanted to use OEM Datsun L28 flat tops in the build. But we could not get any .005" over bore. OR rather Eiji could not find any and I came up empty as well. But being the impatient person I am I ordered some ROSS forged pistons (completely overkill) that were 0.005" overbore. However they were a few thousands taller. We had to choose smaller lift cam because of this as we had valve clearance, but not the recommended amount. So we put a slightly smaller lift cam in. This is also the source of my extra compression. I think we were shooting for around high 9's or 10:1. Add a taller piston and blame, 10.5+:1 and the need for uber fuel now exists.

I do not want to come across as angry or mad. In fact I LOVE how my car runs. It is strooooong. In fact my only real complaint is that I should have bought 45 DCOE's instead of 40's.

The purpose of my post was to warn others and to ask now that I have taken this path of high compression, is there anything I can do other than add octane boost or retard timing to ease the situation?

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Thanks Madkaw, the opinions form you fine fellows are always welcome!

I have thought about finding another head a P79 perhaps or something with a bigger combustion chamber and having some serious port work done to it, and then one day swap out my hardware with it. But that would be a bit of an undertaking. I would however get me down to closer to 10:1 and at the same time get me some FLOW...

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Squish of a Maxima N47 head is nice. When I looked at the Maxima head, it looked like an E31 all welded up and shaped like the racers did in the 70's. I think the racer experiments were adapted in the Maxima N47 head.

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The purpose of my post was to warn others and to ask now that I have taken this path of high compression, is there anything I can do other than add octane boost or retard timing to ease the situation?

Thank You. I've got the F54 turbo block with dished pistons & an E31 head with hopes of running pump gas.

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The purpose of my post was to warn others and to ask now that I have taken this path of high compression, is there anything I can do other than add octane boost or retard timing to ease the situation?

If colder plugs and cooler coolant doesn't help at all, I'd really consider going with a more aggressive cam or at the least retarding your cam. My hypothesis would be that the high heat and pressure caused by your current valve timing profile and static compression is causing engine knock. Knock is more likely to happen at lower RPM because it has more time to develop while waiting for the flame front to reach the hot spot. Therefore, you want to decrease cylinder pressure within the zone where knock happens. As far as valve timing goes, this is mainly done by delaying the intake valve closing point.

I have thought about finding another head a P79 perhaps or something with a bigger combustion chamber and having some serious port work done to it, and then one day swap out my hardware with it. But that would be a bit of an undertaking. I would however get me down to closer to 10:1 and at the same time get me some FLOW...

That's a good plan for the future, IMO. An unshaven P79 or P90/P90A with flat-top pistons will get you to ~8.5-9:1. The shaved P90 that's going on my flat-top L28 has 44cc chambers and should give me approximately 10:1 compression. I was hoping for a bit lower but I'll probably be close to the edge of what I can run on pump gas, especially in CA where we get up to 91 at most pumps. I do have access to 100 octane if I need it, but I REALLY don't want to resort to that...

Thank You. I've got the F54 turbo block with dished pistons & an E31 head with hopes of running pump gas.

You'll be fine, probably even on regular gas. That combo is ~8.5:1.

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So running a tigher lash on the intake valve would keep it open a hair longer.

That is something to think about. I will try the cooler plugs this weekend. And I have to find a good quality 160 thermostat.

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The Nissan parts man in Tucson had the 160's but talked me into a 180 thermostat.

My engine seems to make more power when it's well warmed up. Always check my revs vs. mph on the freeway, it consistently goes faster at 3700 on a warm afternoon than in the cool morning. A 160 might run too cold in the winter.

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So running a tigher lash on the intake valve would keep it open a hair longer.

That is something to think about. I will try the cooler plugs this weekend. And I have to find a good quality 160 thermostat.

Sure, tighter lash would leave it open a bit longer but it's not exactly what I was recommending. You can try but I'd go with the lash settings given by the cam grinder. I was recommending retarding the cam (as long as you have ample valve to piston clearance) to drop low-rpm cylinder pressure. You may need an adjustable cam sprocket for this depending on where your cam is currently set. If it's on the #2 hole, set it to #1. That will retard the cam by 4 degrees and may be enough to help. You'll lose a bit of low-end but make up for it in the high-end, if you notice anything at all.

Always check my revs vs. mph on the freeway, it consistently goes faster at 3700 on a warm afternoon than in the cool morning. A 160 might run too cold in the winter.

That makes absolutely no sense to me. Engine RPM to MPH is controlled by gear ratios, i.e. solid meshing gears that don't have magical temperature-sensitive gear ratios. ;)

Could you clarify what you mean here?

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On the engine I am building, Dave Rebello does not seem concerned about even higher static CR than that. F54 block, flat top pistons, Gerolomy modified E31 head. I am assuming Dave isn't concerned because the dynamic compression will be within reason. It has been my experience that raising the compression on an L2X really wakes it up.

By the way, I have a very nice P79 turbo head from an automatic transmission car sitting on a shelf. Appears to very low mileage. Dave says E31 so I will not be using it.

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Well I have the E312 from my orginal L24 still on the original engine. I hate to take those apart. I still want to refresh the OEM engine that came with my car. It was running strong when I pulled it.

If you can get the overlap up due to a big cam you can alleviate some of those high pressures for sure. I would not be surprised if you have Zero issues. But let us know if you do.

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This is my first post on this forum. I have a high compression L-28, it's the n-42 block with the E-88 head. The engine was built by previous owner. This is my first Z-car. He wasn't very helpful on giving me a list of the mods done to the engine. It has Comp 280 cams. Venolia pistons. Arzona Racing headers, I also believe ported head and ? larger valves.

I wanted to make sure he had the Carb set correctly (Holley 390). So I asked the local import shop to put her on the wideband and to please do a compression check. The average compression was 260 lbs per cylinder. He recommended running high octane fuel because she was pinging. I filled her up with 111 octane gas, and she loves the stuff. No obvious pinging, but expensive to run.

I now realize many here think that having a Holley is in bad taste. (didn't know that until I met a couple of the local Z guys). The Holley is very easy to adjust and with the car running so well, I am not in a hurry to change it. I am thinking strongly about changing her to E-85 though, due to cost of race gas.

How high of compression ratio is this? Any else this high. The numbers really surprised me and my mechanic. E-85 anyone??

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I noticed 79 mph at 3700 rpm in the morning, 84 mph at 3700 rpm in the afternoon, seems to be consistent. It's an AT, maybe that makes a difference. Don't understand it at all. Maybe tire pressure?

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I have a 3.0 liter F54 block built with Rebello's 3.0 liter kit. It has 2 mm longer stroke and forged JE pistons and Eagle 138 mm H beam rods. The rod stroke ratio is 1.7 which gives good dwell at TDC for my .6mm quench. Head is E88 with welded chambers (I think they were 39cc), porting, bigger valves and Isky's 490 lift 290 duration cam. We spoke with both Dave (Rebello) and Ron (at Isky) about compression ratio before we finished the head Dave thought 10:1 should be ok but Ron said 11.5:1 with my altitude and well designed combustion chambers should work well. We did 11.5:1 and I can run 34 degrees timing with 93 octane and no pinging. I run B8ES NGK and 160 degree thermostat. I do run 100 octane at the track because I'm pushing 7000 rpm and am a little worried about combustion chamber temps after 20 minute sessions. Last dyno was 203 hp at the wheels. Still running Rebello modified SU's.

Edited by 30 Ounce

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Well I have the E312 from my orginal L24 still on the original engine. I hate to take those apart. I still want to refresh the OEM engine that came with my car. It was running strong when I pulled it.

If you can get the overlap up due to a big cam you can alleviate some of those high pressures for sure. I would not be surprised if you have Zero issues. But let us know if you do.

It's not the overlap, but the intake valve closure point that's the important bit.

This is my first post on this forum. I have a high compression L-28, it's the n-42 block with the E-88 head. The engine was built by previous owner. This is my first Z-car. He wasn't very helpful on giving me a list of the mods done to the engine. It has Comp 280 cams. Venolia pistons. Arzona Racing headers, I also believe ported head and ? larger valves.

I wanted to make sure he had the Carb set correctly (Holley 390). So I asked the local import shop to put her on the wideband and to please do a compression check. The average compression was 260 lbs per cylinder. He recommended running high octane fuel because she was pinging. I filled her up with 111 octane gas, and she loves the stuff. No obvious pinging, but expensive to run.

I now realize many here think that having a Holley is in bad taste. (didn't know that until I met a couple of the local Z guys). The Holley is very easy to adjust and with the car running so well, I am not in a hurry to change it. I am thinking strongly about changing her to E-85 though, due to cost of race gas.

How high of compression ratio is this? Any else this high. The numbers really surprised me and my mechanic. E-85 anyone??

No way to tell your compression ratio from that data. You need to know bore size, compressed head gasket thickness, and combusion chamber volume. The Holley can be part of the issue because of uneven mixture distribution. All it takes is one cylinder, one weak link, to cause knock. The wideband shows average AFR, not cylinder-to-cylinder differences. That engine is begging for triple carbs or standalone EFI. The more control you have, the better chance of alleviating your issues.

In the meantime try colder plugs, cooler thermostat, and a slightly richer mixture, in that order.

I noticed 79 mph at 3700 rpm in the morning, 84 mph at 3700 rpm in the afternoon, seems to be consistent. It's an AT, maybe that makes a difference. Don't understand it at all. Maybe tire pressure?

That makes a bit more sense. Sounds like your torque converter lockup changes depending on heat, since the torque converter is a fluid coupling. This has nothing to do with how much power your engine is making.

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As Leon said, we need to shoot for MBT, not always max timing. this is true at every engine state.

Bumping up the compression ratio often makes the engine knock limited which makes MBT timing = timing before knock for max power. This is also true for turbo application where max timing often means MBT.

This is true statement only at certain conditions related to rpm, engine load, temp, intake pressure, etc.

We are talking about timing but not about how to achieve it. Once more, timing management is the key here.

I use to run F54 block with N42 head combo with flat top pistons. It was knocking with Mallory dizzy and 34° max timing at around 2500/3000rpm while cruising & going at WOT whereas I could start at 0 speed and going WOT through all gears at full load without knock. So only option available at that time to get rid of knock was to lower max timing to lower timing at 2500rpm. I went to 29° total. I lost some power, for sure.

I've played after with springs inside dizzy, it helped a bit but I wasn't able to run 34°.

With good timing management, I could have lowered timing at 2500/3000rpm and keep 34° at high rpm with no knock.

So with same engine, one could say "I can run only 29°" and the other could have said "34° - no problem".

Since, I've changed the head for the reasons involved above:

I have thought about finding another head a P79 perhaps or something with a bigger combustion chamber and having some serious port work done to it, and then one day swap out my hardware with it. But that would be a bit of an undertaking. I would however get me down to closer to 10:1 and at the same time get me some FLOW...

This is exactly what I've done :)

I've delt with Paul Ruschman (Braap on hybridZ) and he's built for me a P79 head for street (torque at low rpm while still having decent power, not all power head with no relevant torque before 4000rpm).

So he improved flow for low rpm application, basically ports haven't been increased in size at lot - they were just raised a little, valve seats area has been ground, comb chambers CC'ed & modified, valve unshrouded, valve seat chamfered, head milled a little (1mm only). Valves size remained unchanged. We expected power to max at 6500rpm so there was no need for "as big as possible". Bottom end would have needed some serious modifications also that I did not need for street.

Paul and Dave Rebello have worked together to provide me a cam that matches head work so I've got a "street" cam grind from Dave with relatively high lift (+0.49") & asymmetrical lobes.

The torque increase is dramatic between both setups whereas compression ratio has been lowered (9.5:1 if not lower). With same dizzy, I was able to run 34° with no knock.

Next step, this winter, was to change ignition management. I went with Megajolt + EDIS with load control (with TPS) instead of regular dizzy. I can now shoot for MBT at every engine condition & it makes a big difference.

Change is amazing: engine runs cooler & a lot smoother, torque has increased at lot everywhere, total timing is set at 32° since I cannot feel any difference in power ( I need a dyno session too!) and fuel consumption is much lower since engine efficiency is better.

Big change was also timing at same rpm but different load. This is something that happens when you accelerate on the street. Engine feels much stronger during throttle transition. With acceleration starting at 3000rpm steady, timing goes from 49° to 32°.

For race engine where they run at WOT most of the time, load control is not so important. Good dizzy system well setup could probably be enough. MSD makes some programmable 6-AL2 box that should work great.

Edited by Lazeum

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This thread is all I could hope for! Such great information for myself and all others that may be looking to build an engine

Bravo guys for keeping such splendid useful information here! What a great collection of thoughts!

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Hi Zedyone:

I think your assessment of what when wrong is most likely correct. But I'm a little confused by the numbers. 0.005" overbore? Did you really mean 0.5mm or 0.020 inch overbore?

If that is the case, then your bore is 86.5mm and you would have 461.9cc per cylinder - - yet you list 459.6cc per cylinder {2817.8cc / 6}. Were the pistons built to a custom size?

Combustion Chamber volume - 43.5 cc Is that an "as measured number", or as figured based on how much was milled?

Be all that as it may. Because of the extreme deck height of the pistons you have -

Option 1. Find someone that still has a 2mm HKS Head Gasket - and install that.

Option 2. Pull the head - and increase the combustion chamber volume by 3 or 4 cc's. Most guys reported an increase in combustion chamber volume of 3.3cc's by un-shrouding the valves. So I've used that number to recalculate CR.

Mixing my numbers with some of yours... I get.

if you actually overbored it .5mm thus 86.5mm x 79mm:

deck= -1.763 cc {using your .30 mm worse case}

gasket = 7.6cc

combustion chamber 43.5cc {your number}

Sub Total = 49.337cc

Cylinder Volume = 461.49

Sup Total -

461.49+49.337 = 510.827 / 49.337 = 10.35:1 CR

Increasing the Combustion Chamber Volume by 3.3cc

deck= -1.763 cc

gasket = 7.6cc

combustion chamber 46.8cc

Sub Total = 52.637.cc

Cylinder Volume = 461.49

Sup Total -

461.49+52.637 = 514.127 / 52.637 = 9.77:1 CR

If you over-bored it a full 1mm and have 87x79 then Cylinder Volume = 469.62 cc

469.62 + 52.637 = 522.257 / 52.637 = 9.92:1 CR

in that case - you might have to find another couple cc's of volume to take out of the combustion chambers... get them up to 48.8 cc

As general advice, I usually say that you want to keep CR at about 9.5:1 for a street engine on pump gas. {93 octane with 10% Ethanol is normal today}. As others have stated, you can run higher CR's but the complexities go up...

FWIW,

Carl B.

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Okay, trusting your octane rating to a can of boost bought at say Autozone is not something I would be doing. But don't take my word for it, ask Dave Rebello. You want to increase the rating by 2-5 not 0.2 - 0.5 and you need lead to do that. Without the lead all you get is a bottle of 99+% toluene.

My current 3.2L needs 98 octane to meet the requirements of the 12.2:1 CR I have in the my street 240Z. I run straight 110, a mix of 110 & 93 or 93 and 2.5 oz/gal of Octane Supreme ($24/32oz). The math breakdown for a full tank would be close to the whole 32oz can plus the 93 octane gas. Since my mileage varies with the amount of pressure I apply via my right foot from a high of 20 to a low of 8 let's assume I get an average of some 220+ miles per tank. The dyno says 287 rwhp@ 6500rpm.

How much is too much CR? BSR ran some engines at 15:1 but they had a very, very short bottom end life - a weekend of racing.

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I'm with Carl, your numbers seem weird. I remember reading about your build and thought it curious that the builder had to reduce the cam lift to increase clearance. Reading countless threads of other builder's( Braap, 1fastZ, and others on Hybridz), the guys that do many of these motors, piston height above deck is usually positive in stock configuration. I believe Braap said he measured and almost all motors were .019 +. That's more then what your forged pistons gave you. So what HG thickness are you running? Trying to figure your head to piston distance.

My point being is the squish distance is very important on these motors to fight detonation. Getting outside this distance makes these motors more prone to detonation. Now from what I have read you want to stay less then .035 . Some run as close as .022. Their claim is it makes a profound difference in power in achieving MBT.

Also head cooling is a issue with these motors the more power you extract from them. This is most prevelant in track conditions or hard driving. Maybe you should go with a cooler thermostat.

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Steve makes good points. The cam I'm putting in the L28 I'm currently building is .540" lift and the pistons pop out of the bore .021". I'll be doing piston-head and piston-valve clearances soon and can report back. Pistons are flat-tops and compression should be close to 10:1.

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