Lets say you are designing a new LS1 motor combo (I am!). What route would you choose between these two, keep in mind that I will be running 93 octane pump gas 90% of the time:

A. 11.0:1 Compression ratio, and higher timing values

B. 12.5:1 Compression ratio, and less timing

Both should run fine on pump gas when tuned correctly, but which is the better method of making the power, and WHY?

Tony

 

I would say go with the higher compression and make sure the tuning is right to prevent any problems. There is about a 4% increase in power per point of compression as long as the car is tuned right. I think with the LS1Edit out now MTI can get your new motor Tuned perfectly even with that high of a compression ration.

You might have already seen this link about compression but here it is again if you havent.

Compression

[ February 04, 2002: Message edited by: Bad Habit Bird ]</p>

 

More CR less timing IMO.
I wont get into why I think this is the way to go other than to say the LS1 doesnt really need alot of timing to begin with since it has such a great combustion chamber.

JS

 

[quote]Originally posted by Nine Ball:
<strong>Lets say you are designing a new LS1 motor combo (I am!). What route would you choose between these two, keep in mind that I will be running 93 octane pump gas 90% of the time:

A. 11.0:1 Compression ratio, and higher timing values

B. 12.5:1 Compression ratio, and less timing

Both should run fine on pump gas when tuned correctly, but which is the better method of making the power, and WHY?

Tony</strong><hr></blockquote>I would say the 2nd one because with higher compression you would make more power and have better throttle response and the car would really respon to higher octane fuel better than the first one.Thats just me though I like high compression and I think you should get as much as you can on NA or NA/NOS.But thats just me some people say its crazy to have high compression and juice.

 

How about an alcohol fed 14:1 motor? <img src="gr_grin.gif" border="0">

I would go with the compression and a HUGE for MTI solid roller setup. <img src="gr_stretch.gif" border="0"> Anybody know the compression & timing of a top fuel dragster?

 

[quote]Originally posted by Jantzer98SS:
<strong> Anybody know the compression & timing of a top fuel dragster?</strong><hr></blockquote>

somewhere around 6 to 1 give or take, they have lots of different pistons to use for tuning. But they also run lots of boost, can get upwards of 40 PSI. dont know what timing is for them though... Did you know that Nitromethane has a negative octane rating

 

Tony,

I'm going to be in the minority here, but an engine will make its maximum potential power with the best ignition timing curve. Let's say you can safely run 30-32 degrees of timing on an 11-1 motor (which is pretty close to ideal with the cam & heads you're talking about running). If you had to drop the timing down to 24-26 degrees on a 12.5-1 motor to keep it from rattling, it may not make as much power as the 11-1 motor with full timing. I've seen this phenomemon for years in non-LS1 applications...can't see why it wouldn't apply here too.

Remember, you're talking pump gas here. I think you'll make more power with the 11-1 motor and max timing personally. I'd even bet money on it.

 

I would personally vote for more compression. You may remember when I took my T/A to MTI for the cam swap to the R1. I had Jayson get a PCM tuned for me so I could use the HPP3 again, and the non-RFG tuning added 4 degrees of timing at WOT. I picked up 4 RWHP with the added timing. Also, Jayson said that it's harder to get more timing with the larger cid engines and you typically don't need as much timing. I think that the higher compression will less timing will make more power (no detonation assumed).

 

12.5 is ridiculous for pump gas. You'll have to back the timing way down from the optimum with race gas. If you get any bad pump gas which is very likely to happen your in trouble.
I side with PatrickG.
Top fuel runs 50' lead and maybe more.
Nitro burns so slow that's why you see it still flaming out the pipes.

You want to do something different Tony?
Ok, have MTI put in aluminum rods, go with a 7-1 piston, fill the block, put in the biggest damm exhaust valve you can ,Bearing clearance at .0035" and open up your ring gaps to .040" , maybe start out with 40' degrees timing, buy the largest injectors you can find and let's see a "fuel" ls1!
<img src="gr_eek2.gif" border="0">

Steve

 

The 12.5 motor will make more hp and tq but will leave you with little to no margin for error or you will detonate. I would never recommend 12.5 with pump gas on hot summer days. Its just too close for comfort. It would make more power for sure. 11-1 would be safe and only about 25 hp less but safe on pump gas.

 

Well Pro Stock runs more compression and less timing, but you aren't building one of those. I'd go with less compression and more timing. Why? Because its a street motor and its not worth losing a motor over a bad tank of gas IMO. You also live in a pretty hot climate (for the US), and will be running a lot of 92-93 octane I suppose? If you lived in upstate Michigan and only ran gas out of a 55 gallon drum it'd be a different story.

J.

 

the Colonel runs 12.2:1 in his 422 and it seems to be working out pretty well for him, I think the SAM SS has even more compression than that. Both of these cars are running 9's NA.

They must be doing something right <img src="gr_grin.gif" border="0">

 

True, how much static compression you can get away with depends a lot on how high you're going to rev it, cam specs, bore size, etc. A larger bore is going to be more prone to detonation.

edit - I've got a feeling Tony's new setup will see a lot more street miles than the SAM SS or Stephen's car though. Even though he's got a truck now, he will still probably drive it a lot on the street.

J.

[ February 05, 2002: Message edited by: Crazyquik ]</p>

 

Assuming you run a large cam, 12.5:1 with less timing will make more power in my opinion. The best part is on race gas you can bump the timing up and make even more power. <img src="gr_images/icons/wink.gif" border="0">

 

I think a LOT depends on cam choice Tony.
If you are going with a 422+ motor with a Big *** hyd cam I would run the 12.5:1 comp with a LARGE duration cam on 110LSA. That will help bleed off compression at higher rpms due to the increase overlap. However it wouldnt be very streetable. I would hold off until you see the results of JPR's EWP on a high compression motor. If you can get away with 28 degrees timing on a 12.5:1 motor with his EWP on 94 octane I would definitely look at that route.
You haven't provided enough details.
I am going to ruin 11.1-11.3:1 comp in my new combo and a 226/234 112LSA cam. That should work well on 94 octane and 28 degrees advance.
Cheers
Chris

 

Chris, wouldn't he want a 114 instead of a 110 LSA? The 110 is going to increase cylinder filling and be more prone to detonation, especially at the torque peak.

J.

 

While my 12:2 motor does work ok on pump gas, I race it on unleaded race gas so that I can run the timing high and the mixture lean.

12.2 is really too high for pump gas but it does allow you to take advantage of racing fuel at the track. <img src="gr_images/icons/cool.gif" border="0">

 

Here in Texas we have plenty of 93 octane premium gas. The oil companies, at least the majors, meet the octane specs very closely. They should always hit the 93 octane mark and then a little more. Very little. Extra octane is expensive and they monitor the gas they ship out in their knock labs to make sure they have as little octane give away as possible. Premium usually runs around 93.1/93.3. Not much more. Different times of the year, the formulation varies a little but the octane is still there.
After saying all of that, I've seen street cars run 12.5:1 in the past but I think I would rather err on the side of safety. You know it is a hot SOB down here what with Global Warming and everything. <img src="gr_grin.gif" border="0">

Les

 

I say go with the higher compression and get your own LS1Edit so that you can run different programs at different times.

not very much timing for street use
a lot of timing and race gas for the track

that is the way to do it my friend <img src="gr_grin.gif" border="0">

 

There are quite a few different factors here - I don't think the question provides enough information.


First off more timing isn't always better, etc. There is going to be a certain timing value that is optimum at a given rpm point based on piston velocity and flame speed. Flame speed will be changed by things like chamber configuration, fuel qualities, compression ratio, charge quality, dyanmic compression, etc.

Once you can achieve the optimum advance for your setup more will only slow you down/kill engine components. But many times in the real world we can't achieve that number. It generally *doesn't* work well to have to retard the crap out of your timing to crutch ignition related problems (detonation/pre-ignition).

Will 12.5:1 work with pump gas on a stock motor? No way.

What if we throw in a huge cam and end up bleeding away a good bit of that compression - well, now we are getting somewhere.

So how can Colonel run 12.2:1? He has to run 12.2:1 with his setup to make power. That LS6 intake is still pretty restrictive, believe it or not. That doesn't mean you can't make power, but you have to work around that intake. It's cutting down his VE so much that he isn't getting a full intake charge. so his 422" of displacement aren't sucking in as much air as they can. This acts to reduce the actual dynamic compression because we aren't compressing a full intake stroke of air, but rather only what can be pulled in through the intake.

This is how nascars work - the run ungodly amounts of compression, but because they are pulling it in through that little tiny restrictor plate their dynamic compression is nothing like the calculated static.


All in all it depends on the setup, but I would rather err half a point on the conservative side and be down a few hp than half a point the other way and have to run race gas.


Chris