M K

iTrader: (9)

Opions please?

To low? Nice and safe etc I'm new to all this


Seen some cosworth racing pistons on 7.4:1 ratio with valve cut outs and that have done dyno time only & wondered if they would be ok for my engine

200 10 studded block
Shot peened rods
Arp rod bolts
Fully worked crank, knife edged, L&B etc
High torque cams (not 100% sure on spec at this moment as info not with me)
Gtx3071r turbo
2wd manifold with tial 44mm external gate
Hart inlet with 4x4 throttle body

Power guesstimate for the above spec also please, I'd like to see a genuine 500bhp

Any info appreciated

Thanks
Marc

Chip

iTrader: (3)

I cant see any point in dropping that low unless you are planning to run it on shit fuel.

J1mbo

iTrader: (1)

The problem is, they were 7.4:1 cr in that engine they came out of, they may have stopped the piston 20thou down the bore. If you block has a low deck heighgt you could be nearer to 8:1 again. Also are they 7.4 on a 4x4 head or 2wd? Cosworth pistons are a good piston, may be worth getting them and dummy building your bottom end to work out the cr.

M K

iTrader: (9)

I will have to look into it a bit more,

Is there a way i can measure the block height on the bench? the block is bare, where would i measure from if you can, deck face to ?

Is it worthwhile cc'ing my head then while im bored working nights then i may get a better idea whats what

Chip

iTrader: (3)

You just use a feeler gauge from top of piston to top of block, but it will need to be a complete bottom end when you do so, ie crank rods and pistons in situ, you cant really measure just the block in isolation.

Yes its worth cc'ing everything if you want an accurate CR figure.

Martin-Hadland

iTrader: (2)

You can get a good idea if the block height is near std by measuring from the top of the main bearing to the deck face, its not exact but you'll know straight away if there's loads been removed from the deck face. 6973 thou is what most of mine seem to be.

Chip

iTrader: (3)

Better than nothing, but you'll struggle to measure with the accuracy you need (ie to within a few thou) over that distance.
What measurement for a std one should he be using as a reference though if he does?

Martin-Hadland

iTrader: (2)

I edited it above Chip, I checked 3 and got a consistant 6973 thou ... having said that all of mine could have been machined 27 thou

I have verniers that go that big Chip so measuring is reasonably easy.

Beetlejuice

I wouldn't consider putting second hand pistons in any engine, let alone in a build at the level of yours.
Not worth the risk to save a few quid.

Chip

iTrader: (3)

Martin you just quoted 0.673 inch and 673 inch, neither of which sound likely to be correct

Martin-Hadland

iTrader: (2)

Haha! Now that's a tall and short block I know what I meant!

Chip

iTrader: (3)

Ah, 6673, I was thinking that maybe you meant 6.73" TBH which is obviously quite a bit different to 6.673" so its a good job I asked for clarification or if MK had interpreted it the same way I did he would have thought he'd had loads skimmed off, lol

Martin-Hadland

iTrader: (2)

It is the other figure, I'm working and typing.... Fuck this being helpful lark, I will leave it to you chip! All corrected now, lets just say thats its about 25thou less than 7". Need metric verniers!

Chip

iTrader: (3)

FPMSL.

At least we got there in the end then mate

750hp escos

The head is prob the biggest factor and what the pistons were specced for..

If the pistons were 4x4 and you run a 2wd head the cr will go even lower but if the pistons were 2wd spec and you run a 4x4 head then the cr will be around 7.7.1 if the head and block were of near std origin,ie block deck hieght and head depth..

Basically you need to know what the pistons were specced for and then measure your block and head to get a better idea..

Also you need to know how old the pistons are because if theyre old 2wd spec ie no offset gudgeon pin it will sound like an orchestra on warm up under your bonnet!

Check out ebay mate as julian godfrey has some nice piston sets for sale at a reasonable price and theyre new old stock..

Ps

With your spec engine mate the cams are pretty vital and cr wise i would aim between 7.8.1 at lowest and up to about 8.3-8.4.1 at highest..somewhere in this region with the right cams will be awesome and on a gtx3071.82 should see around the 550hp 500 lb ft region fairly easy


cheers danny

M K

iTrader: (9)

Thanks for all the info

General concensus is obvious an as said although I'm up for saving as much money as I can it's obviously not worth saving here so il buy new and spec properly to suit etc

Thanks
Marc

M K

iTrader: (9)

Went down to FMS today with the engine and handed it over for the full build to commence, let the pro's do what they do best


Tommy had a set of cosworth racing pistons on the shelf as well, brand new, valve cut outs, coated skirts, rings pins and are perfect for the compression ratio he is gonna go with on the engine so happy days

Best start saving

M K

iTrader: (9)

Went with the brand new cosworth items, was mad to consider 2nd hand on a new build lol

Bling bling

vroooom ptssssh

They look fairly low comp!?

I don't understand why you wouldn't go low comp? can run more boost, more ignition advance and make more power....???

High comp means less boost, more retard unless on some fancy fuel...and less room for error and increased chance of det.

All the high powered cars i've ever saw (not read about, or heard about, I mean SAW) have been low comp!

stevieturbo

Do you see things with blinkers on ?

You do misunderstand how it all works. Maybe if he was aiming for 1000bhp, 7.4:1 might be justified.
For only 500bhp it would be a totally ridiculous CR to aim for unless he had no intercooler or some other huge failing.

vroooom ptssssh

Clearly you do, away and read some more stuff on the net Stevie


*edit, out of interest...seeing as you feel so strongly about it*

What's "totally ridiculous" about going low comp
If it's not that big an issue...why do manufacturors lower CR on turbocharged cars vs same engine/NA?
What is the problem you think occurs when going low compression?

stevieturbo

I think I'll base my info on engines Ive built and tuned rather than bullshit Ive found on the internet You can keep reading that stuff yourself.

vroooom ptssssh

Funny that, 'cos that's exactly what I done....I tend not to listen to anything I read on the net

stevieturbo

Show me a decent production engine that's as low as 7.4:1 ?

Hmmm nope, cant even find any even on the internet

The only problem is you end up with a soggy uneconomical piece of shit that doesnt make the power it could because it is an inefficient package.. I can think of few circumstances where I'd build turbo engine with less than 8.5:1. And if there was any, they would be extreme and for a specific reason.
500bhp is neither extreme or anything of that kind.

And manufacturers maybe drop from 10 or 11:1 to 9:1 these days. Some may venture into the 8's, but even these are becoming less common now.

7's...that shit is 70's and maybe 80's technology. This is 2012 believe it or not. Get with the times.

vroooom ptssssh

Did I say manufacturors go as low as 7.4:1...nope.

I'll let you carry on, you're clearly the man here!

toodles.

stevieturbo

Sorted

And thats with never building any engine as low as that !

If you've never built and tuned any engines as seems to be the case with you...I guess you just wouldnt understand.

vroooom ptssssh

750hp escos

i was running 8.1 with 539 hp in 96 and that engine was the bollox,my last 642 hp engine was running 8.4 cr with no probs and my new 700+hp engine will be around the 8.1-8.4 cr(i havnt got the exact rod length yet)so i agree that 7.4.1 these days is pointless!!


cheers danny

vroooom ptssssh

Im not saying it can't be done, Im just pointing out that by less CR, you can run more advance...less heat, less chance of det...more power for less boost.

"70's technology" good one that!

Anyway my breath is clearly wasted here, carry on!

750hp escos

It only makes for a horrible of boost car and really isnt needed these days


cheers danny

J1mbo

iTrader: (1)

What manafacturers spec their engines to run 32psi of boost?

What you have to keep in mind is the yb is a bit of an old boat anchor, they don't like massive compression ratios. for big boost applications 8.0:1 seems to suit them perfectly, for a road engine that is going to do lots of miles with big boost then a touch lower is good IMO.

I do have to agree that 7.4:1 days are gone, I don't think we should be seeing under 7.8:1 any more.

M K

iTrader: (9)

Just to make clear I'm not going 7:4.1 Or using second hand pistons so the original questions irrelevant now lol

Ended up 8:2.1 using new cosworth pistons

750hp escos

I take it you have a 4x4 head thats had a bit of a skim then?

Them bowls look quite big in the pics and with valve pockets too it looks more like a 7.8.1 cr? obviously all depends on head etc and 8.2.1 would be awesome any way..


cheers danny

Rod-Tarry

With Stevie on this.

The YB loves 8.5:1 thats for sure & 2.9 bar of boost as well. Go to a Tuner thats thinks in 2012 not in the 1990's.

T28 RST

O deary me, vroompish strikes again

Stop listening to Alan's vague answers advice and move on, his way of doing things is way off the mark.

T28 RST

What was Euans, 7.0:1 ?

Wat dd that make at max boost

MAD Ade

iTrader: (1)

Mine's 7.4:1, drives like an aspirated engine off boost, gives 30mpg on a run and is over 700 brake....

stevieturbo

That's because it is a normally aspirated engine off boost lol just a very low compression one.

MAD Ade

iTrader: (1)

ie with N/A level of compression ratio.....