What is the target DCR for a pump gas motor? Then how about a 104 octain motor? Just trying to get an idea of what to look for?


Michael

 

8-8.5:1 for pump gas and maybe one full point higher for 104.

Rich Krause

 

I've been told 8.5 - 9.0 with 93 octane.

The LTx engines, being reverse cooled, can be in this range without much problem. My engine is right on 9.0:1 fwiw.

I think you have to keep in mind that the efficiency of your cooling system is going to play a big part in how much you can get away with. That and the temps of your intake air.

-Mindgame

 

I will give you the other 1/2 point for a PERFECTLY set up motor. Thermal coatings may extend the range a little higher as well.

Rich

 

When I was at Ultra Pro last February they were working on combustion chamber analysis. Looking at flame rates and ways to increase combustion efficiency. The goal, as I understood it, was to increase dynamic compression to the point of inducing detonation and preignition. Then work to increase combustion efficiency through port and combustion chamber "modifications". "For fuel economy and power" as I understood it from Don.

Bringing that over to this discussion, I don't think you can lay a limit out so easily. As far as small block chevys are concerned you have over 40 years of combustion chamber development. Even the LS2 combustion chambers are supposedly better than the not-so-old LS1 and LS6. So who's to say the same limitations exist with variations in combustion efficiency?

The reverse-cooled LTx cylinder head is supposedly ~30º cooler than previous designs in critical areas along the combustion chamber. Combine that with a coated exhaust valve (one of the hottest areas in the combustion chamber) and a cooler spark plug and you have even more detonation resistance. So what's the new theoretical limit?
Are we now allowed to move DCR up beyond what a "perfect" setup is recommended to run at??

I'm not in disagreement or anything like that... just making a point that a proposed limit based on "perfect" setups is folly. Besides, what is the definition of perfect?? Perhaps we should ask Bill Clinton... I hear he's good with definitions.

We have cast blocks with small variances in coolant passages. Who's to say that flow around cylinders 1&3 are the same as 5&7 on the same engine.... now on different engines? How much are they going to vary from one engine to another??

What about intake port modifications and their effects on swirl?

I have yet to see a problem with setting LT1's up at 9.0:1 DCR but I've only built 2 others besides my own. No signs of detonation so far but my fingers are confidently crossed. Maybe those reading can just split us in the middle and go with 8.5:1 ±.2.

-Mindgame

 

Just a guideline for 2 valve engines with ~4" bore. Smaller bores are more detonation resistant and can tolerate higher DCR. Large bore motors are less tolerant of high DCR, all else being equal.

And so on.

Rich

 

Thanks MG and Rich

Rich, how accurate is the DCR calculator that Pat Kelley, I think I got the link to it from a previous post of yours. Also, do you mind if I send you a spreadsheet of some cams I have been looking at for my motor?

Michael Kelly

 

I believe I set the 9:1 guideline in one of our first threads on dynamic compression.
MG is correct. There are factors which give favor to lower or higher values. Nothing is written in stone here. Based on my studies and acquisition of data over the years, I still stand on a 9:1 limit, as a safe approximation. Although I've had good success with higher values there is no harm in erring to the safe side.

Take care.

 

It gives about the same results as the other methods I am familiar with. I'll be glad to look.

Rich

 

To make things a wee bit easier for others, I found the link to that thread and will post it here.

http://web.camaross.com/forums/showt...threadid=46128

 

Thanks Cheston.

Seems I missed out on some questions, conversations and explanations in that thread. Might revive some of it this weekend when I have more time.

Yes, I'm a neglectful one.

Take care.

 

Cooling and quench are what I have found to be the most critical on pump gas.Also taking the time to tune it right and not that,AHHH thats good enough attitude.Also get the same amount of fuel to all cyl's--That would come under good tuning though. May be Chuck could shead some more insite without giving away to many of his secrets.

 

I'm know there have been some good threads on coolant itself here by Chuck, but let's look at what the coolant passes thru.

Different manufactures but lots of time into coolant passage design, OTOH some don't. GM has always had excellent design here, one reason why LT1 castings and LT4 castings are good things to play with. You can also feel this in weight, a AFR is one heavy head, where a LT4 is quite a bit lighter. Not just because GM is stingy and they want to put aluminum where they need it and use as little as possible, but because they have a ton more volume for coolant in the water passages.

This shows up in HP too. Brodix and GM make similar head castings for 18 deg heads.

Here we can see EXTREMELY similar ports in the 921 Hutter CNC programs....
http://www.weldtech.com/graphics/Hut...dix_HUT921.gif
http://www.weldtech.com/graphics/Hut...M_Complete.gif

The big difference in those heads is the water passages, the GM head is better in this aspect and a change from the Brodix to a GM casting would yeild higher results with the GM casting. I'm sure you could run more DCR with these heads as well.

Then we get into revearse cool vs. standard block first cooling. There have been some interesting applications of this in things like the engine masters contest. One guy in particular had a pretty high compression BBC put out identical numbers to Jon Kaase's BBF. To me that says something with those rules. The reason being is this....

First start here http://www.popularhotrodding.com/eng...3/0405phr_emc/

Look what Kaase did to run 13.2:1 (incidentially it was also 9:1 DCR) on 92 Octane pump gas.

He had a short rod to reduce dwell, a revearse dome piston with a 29cc chamber! to produce faster combustion, along with a small bore.

Ok now to the BBC.....

http://www.popularhotrodding.com/eng.../0403phr_emc3/

Norm Grimes down that page got DQ'ed for a illegal muffler, dumb reason to be booted but oh well. Anyways he ran at the finals for fun with legal mufflers and put up a extremely good fight with his BBC. Let's just say if he was in it Kaase would not have run away with this like he did.

Now why is this so impressive? Well look at the rules....
Valve Angles within +/-1 deg of as cast. He was working with a head that was not 29cc here, but probably closer to 100cc. That and he was running a 4.500" bore. Completely opposite of Kaase, and since the numbers we so close he had to be in the 9:1 DCR range to do that. The answer on how he achived that DCR with a less efficent design lays in this picture...
http://www.popularhotrodding.com/eng...r_emc_33_z.jpg

Other than the GM castings (I think) he made the BBC a revearse cool motor within the rules. That I feel played a huge part in his setup runing what it did w/ the engine design he started with.

This water passage stuff goes deeper too, how about the new water pumps for NASCAR motors, better designs that you can run slower, eat less HP and use more tape on the front end of the cars. (It's in a Circle Track from the last year)

Anyways, that's all I can think of.

Well maybe not. How about LS1's? When are we going to see 12.5:1 SCR LS1's running on street gas? Small bore, good chambers...... Only EASY way to do it is with cubes since the chambers are a bit big for a stock cube motor.

Bret

 

Is there a DCR formula I can punch some #'s into?

 

There are links to a few in the link I posted.