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

[conedodger]

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.

[Zedyone_kenobi]

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.

[leecheater]

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??

[Stanley]

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?

[30 Ounce]

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 August 10, 2013 by 30 Ounce

[LeonV]

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.

[Lazeum]

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 August 10, 2013 by Lazeum

[Zedyone_kenobi]

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!

[Carl Beck]

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.

[gnosez]

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.

[madkaw]

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.

[LeonV]

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.