Compression Ratio With Pump Fuel

[agius]

hey guys

I'm currently in the process of building a 3tc which will be going in my beloved ke30

 

I took my cam in to crowcams a few days ago and was talking to the bloke there about my head..

 

He rekons not to go any higher than 10.1 compression he said being a cast iron head it will start pinging any higher than that..

 

I told the bloke thats doing my head what crowcams said and he told me that he totaly agrees.

 

Now the question is what do you guys rekon? Is this true, is there anyway around it? Reason why I'm asking for advice is because theres a few people running 11.5 with there k motors on this forum with pump fuel, I'm just geting confused now.

 

Details about the motor, std pistons rings and bearings twin mikini solex side drafts, port polished shaved and portmatched head, reground cam, recurved and reco dizzy msd digital box, msd coil and fully balanced bottom end, thats all i can think of from the top of my head

 

Thanks in advance guys :)

[Trev]

I take it that you are increasing the C/R via shaving the head?

 

It all seems a bit strange since you can have over 11.5:1 C/R in an old cast v8 on pump fuel with no problems.

[bogstock ke35]

I'm pretty sure they work on the amount of octane fuel your going to run and divide it by ten to get the compression ratio you should run.

So if it was 98 octane fuel then 98/10 = 9.8:1

If that makes sense

[TRD ke70]

You have a cast iron head on a 3TC, i'd like to see that, should be an Alloy head.

[Trev]

You have a cast iron head on a 3TC, i'd like to see that, should be an Alloy head.

 

Well that just proves my theory of the guy at crow knowing SFA, and no I didin't know what type of head a 3tc had.

[TRD ke70]

The other thing would be the cam over lap will effect static compression ratio as well.

So if you have a reasonable sized cam, 11-11.5 should be ok on 98 octane.

[styler]

When people talk about compression ratios it's just figures ie static to make sure it ends up at an useable ie dynamic ratio.

When bigger cams are used they reduce compression as they are open longer and you need to compensate.

 

Eg Rough example:

 

8:1 static with std cams = 8:1 dynamic

 

10:1 static with mild cams = 8:1 dynamic

 

12:1 static with big cams = 8:1 dynamic

 

Static numbers are easier to work with and are usually recommended with the cam spec sheet.

Edited June 8, 2012 by styler

[agius]

You guys are the best I'm drunk as at the moment let me shoot across at you my new cam, regound specs to youse in the mornin

 

Btw the best advertised octane pump fuel i can provide is 100 octane at them weird but well to known liberty pstrol sations

[agius]

alright guys sorry for the late reply ive been busy out of my mind

 

here is the cam sheet:

[bruce]

With that 280/286 duration cam, people normally go around 10:1 to 11:1 static compression. Probably 10.5:1 would be good:

 

http://www.3tcgarage...hp?f=23&t=10121

 

http://www.3tcgarage...php?f=23&t=5746

 

I've got a 3TC with a 2TC head shaved 1mm, with the stock cam. Not sure on the compression ratio but it pings on standard fuel during acceleration (crackling noise coming from the engine). Timing is set on 10 degrees BTDC. If I use 98 octane fuel like BP ultimate, back the timing down to 8 degrees, and put in colder spark plugs, two heat ranges down, it goes away. But it still has slight pinging when I floor it hard, or accelerate hard up hills. Also, I put in a smaller idle or main jet in the weber carby and that reduced the pinging a bit. It was running a bit rich.

 

From what I've read hemi engines are more prone to pinging due to their chamber squish area and pistons. The pistons are domed with edges so the heat is not easily dissipated, leading to pinging more easily. Where as K engines and 4ages have flat top pistons, and with their type of squish area, they can get away with higher compression. 2t/3t heads have a 63cc chamber where as k engines and 4ages have chamber volumes a little bit more than half of that.

Edited June 19, 2012 by bruce

[bruce]

When I enquired around for a mild 3tc camshaft, Crow Cams recommended me the profile you have (603740). But I ended up getting a regrind from WadeCams, 282 duration with 0.449" lift.

 

I'm putting another 2T head on, but I'm being a bit more conservative with shaving it. Though when I was taking the head off I notice the hose to the vacuum advance on the distributor had a hole in it, so that could of caused problems :hmm: With the reduced dynamic compression ratio with the longer duration cam, you should be able to run 10.5:1 static compression. NA 3tc Wiseco pistons have a 10.2:1-10.9 compression ratio when used with 63cc chamber head, so that's probably targeted for mild NA 3tc engines.

Edited June 19, 2012 by bruce

[agius]

Thanks so much for your advice, ive already sent my head in but they said they rather shave it off my block, so at the moment there just polishing and matchporting

 

so when i drop my block off ill ask them about how hemi heads are more prone to ping and ill chang it to 10:5.1. Last thing i want is a pinging motor haha!

 

What did wade offer that crow didnt, more lift? I'm not the best at reading all theese cam measurements haha

 

 

[bruce]

Yeah ask them about the pinging, get a different opinion, it's only from what I've read. Your engine builders would have a better idea on compression for your engine, there are different variables to consider. They probably recommend 10:1 to be safe.

 

When I was looking for a 3tc camshaft I was looking at profiles from the USA; companies such as Crower, Schneider, and Engle Cams. I figured that people over in the USA deal with these engine a bit more and that they have a good selection of cams that have been tried with good results. I tried Camtech and Crane Cams but they didn't have any profiles designed for the 2t/3t engine. Camtech did say they could use a profile from a different engine and I could specify the duration and lift that I wanted. I didn't get a reply from Tighe cams.

So I just looked at the specs from Crower and Schneider for a 280 duration camshaft and matched one close to one in the Wade cams catalog. Crower and Schneider wanted $100-$120 to do a regrind, but charged too much for postage, so I went local. Wade cams and Crow seem to be the the main ones that have a selection of 2T (ta22) cams. Wade cams have a lot of profiles in their catalog, but they seem generic. You will see the same profile number for Datsun, Ford etc. just slighly different duration and lift. I decided to go with Wade cams with $100 a regrind, or $150 for regrind, resurface lifters and registered post.

 

Are you putting in larger valves or keeping them stock size?

Edited June 20, 2012 by bruce

[ke70dave]

coudlnt you get an approximate dynamic compression ratio from the info you already have?

 

i don't have enough time ATM to have a crack at it.

 

but if you worked out at what position your piston was in when both exhaust and intake are completely closed (which you can do from bore stroke and that camshaft sheet(, which would be Xmm above bottom dead centre, you would know aproximately how much air is being compressed?

 

the static compression ratio just assumes that the whole cylinder is filled with air, which i woudlnt think is the case even on a standard camshaft.

 

all these measuremnts need to be taken with a grain of salt though, as they don't take into account the volumetric efficiency of the engine.

 

id take whatever value you get from the above calc and assume 80-85% VE perhaps as a guess. (big guess...but its an old engine after all)

 

or have i gone made? its quite possible.

[agius]

Yeah ask them about the pinging, get a different opinion, it's only from what I've read. Your engine builders would have a better idea on compression for your engine, there are different variables to consider. They probably recommend 10:1 to be safe.

 

When I was looking for a 3tc camshaft I was looking at profiles from the USA; companies such as Crower, Schneider, and Engle Cams. I figured that people over in the USA deal with these engine a bit more and that they have a good selection of cams that have been tried with good results. I tried Camtech and Crane Cams but they didn't have any profiles designed for the 2t/3t engine. Camtech did say they could use a profile from a different engine and I could specify the duration and lift that I wanted. I didn't get a reply from Tighe cams.

So I just looked at the specs from Crower and Schneider for a 280 duration camshaft and matched one close to one in the Wade cams catalog. Crower and Schneider wanted $100-$120 to do a regrind, but charged too much for postage, so I went local. Wade cams and Crow seem to be the the main ones that have a selection of 2T (ta22) cams. Wade cams have a lot of profiles in their catalog, but they seem generic. You will see the same profile number for Datsun, Ford etc. just slighly different duration and lift. I decided to go with Wade cams with $100 a regrind, or $150 for regrind, resurface lifters and registered post.

 

Are you putting in larger valves or keeping them stock size?

 

I'm leaving the valves stock..for now anyway haha

 

I droped off the block earlier today and spoke to them, they said its true in some points he even said most of the big v8 heads they do they don't go that much on them.

 

10:5.1 sounds good, but they said not to stress theyve got the block and the head and theyll measure up everything and give me a call on what they can do and whats the best option is for my application..

 

Thanks so much for ur advice and thanks for those links, they paid so much attention to detail to there engines its awsome how well single cam 3t goes with a bit of work!