Another piston option for the 273.

[Assiemick]

I have a 1965 Barracuda with a 273. Yes im strange im one of those guys who is going to keep this motor. However i have a real nice Scat crank. 3.58 If i use an oversize of .040 that will give me 302ci. The 273 rods are the correct length but i will probably go with scat rods for the strength plus depending on which way i go i may need to have either the rods re bushed or the pistons wrist pin bores honed out. Their is a company in Australia called ACL.
THEY DO NOT MAKE PISTONS FOR THE 273. But in Australia GM Holden did produce a v8 motor the 253. It was a smaller of two motors the other a 308. The 253 has the identical bore to the 273. Now ACL make race pistons for these little gems. They are cast in high silica alloy which negates the need for forged slugs, they are also 30% lighter than the stock units. Im sorry i cant remember exact figures but the wrist pins are slightly smaller in Dia than the mopar. I will use the speedway semi finished piston so i can machine the head as it is nearly 1/2" thick creating a slightly lumpy piston bringing up the compression. I will be making the pins fully floating. For you guys in the USA/Canada the prices should work out good as 75cents get $1.00 Aud. The standard GM pistons would be a bit short but the raised head racing pistons overcome that problem. There is a hugh selection of oversizes for these pistons.
Its worth a look at the catalogue shown in the photo here you can get the web address i think.
Good luck

 

[273]

I thought of the exact same setup when i was gonna do my 273, some of those pistons come with Chevy pin size. Good thing about making a 302 you can spin about 6500 to hit 400 hp.

What you gonna use for top end ?

I was leaning towards notch block and 1.94 EQ's or heavily ported stock with ford 1.80 1.55 valves and Comps solid roller 268 gave nice curves on my desktop dyno. don't forget to make a small displacement to work you need probably at least 20% more gear than a similar built 360 or at least no smaller than 3.91.

 

[Assiemick]

I thought of the exact same setup when i was gonna do my 273, some of those pistons come with Chevy pin size. Good thing about making a 302 you can spin about 6500 to hit 400 hp.

What you gonna use for top end ?

I was leaning towards notch block and 1.94 EQ's or heavily ported stock with ford 1.80 1.55 valves and Comps solid roller 268 gave nice curves on my desktop dyno. don't forget to make a small displacement to work you need probably at least 20% more gear than a similar built 360 or at least no smaller than 3.91.

 

[Assiemick]

Im still looking at options, being a machinist my options are varied outside of the box so to speak.
The heads have the early angled manifold bolts. I believe later 273 heads are the same as all the rest of the LA mills. I will point out for those reading.
I have an Edlebrock performer manifold with the use of a 1" spacer the result is a performer RPM.
Now Edlebrock have given a real thick 5/8" thick flange on its inlet ports. Also the holes could be as large as 1/2", the bolts for the manifold are smaller. So the bolts can still be screwed in.
The solution is either make up some angled washers or have the manifold bolt holes spot faced to suit the early bolt angles. For the valves i was going with the 1.88 in, and 1.6 ex. Once the block is bored and i get a better feel for clearances, if i must i will reduce the outside diameter of the valves to get my 80th clearance from the cylinder wall. I like the voodoo Lunati range of cams not going too big as its still a small motor the 1500-5500 solid street strip unit looks good. Have a look at Nicks garage on youtube. I will be porting the original heads. I measured the combustion chamber and was surprised it was at 68cc. The piston at present sits about 60th below the deck. With the race pistons the catalogue says 12:1 but thats in the GM block the raised head pistons are a bit taller than the mopar originals. With deck machining and facing the heads i will be aiming for 10 to 10.5:1.
I may go with roller tip rockers but the originals should be ok. The Scat cranks usually need weight added but im hopeing the 30% lighter racing pistons might help..
Im considering a 5 or 6 speed gearbox so final ratio around the 3's '3.55' should be fine. First gear being pretty low but 5 and 6 are overdrives.

I recently fitted a SC14 super charger on my slant 6 blowing thru a FITECH injection system running 7lb boost water injection with bypass valve mainly for pressure control but to help on idle also i have retained the Toyota electric clutch so when the throttle is right back at its rest the SC stops pumping. The car has a celica 5 speed ive run down and passed some impressive V8's with this old girl. That car has its original diff of 2.78. Actually the silver car is in my gallery but its not finished when they were shot.

 

[nm9stheham]

Interesting and thanks! What are you going to do on wrist pins? Bush the rods or bore the piston holes? Have you considered offsetting the pin upwards in this process to help move the piston top up?

Have you looked at the bobweights and where it will end up? It sounds like you are going to be taking a fair amount or material to get this balanced.

The 68 cc chambers sound like the open chamber ones. Maybe you could save yourself some work if you ran down some closed chamber ones. But, since you are a machinist, it may be no real barrier to mod the block & present heads instead.

 

[273]

If the piston are Chevy pin size can't just use aftermarket Chev rods 6.125 and machine the crank and rods to fit ?

 

[rumblefish360]

If the piston are Chevy pin size can't just use aftermarket Chev rods 6.125 and machine the crank and rods to fit ?

I would say, the use of "Generic" rods comes into play here. Some of the cranks these days are already a reduced size for the Chevy sized journal. If not, machining the crank for the Chevy size journal comes into play. Aftermarket rods can be purchased with the MoPar journal size and Chevy piston pin size.
Lengths vary as well.i have seen 6.250. I do not remember if the small blocks had longer specs to the rods after that.

His plan is very much so a do-able one. The piston ether needs to be machined by the manufacturer or by him for the valves size and angle.

Once he or they know what height is needed, ether the manufacturer can modify the height or he can adjust it himself or change the rod length to accommodate.

I like this re-engineering a lot Mick.
Can you post pictures of this build?

 

[Assiemick]

It's early days the motor has not been fully pulled down yet.
My thought on the rods is I'm going to either try and order the scat rods with the smaller pin bushings or have them rebushed.
The piston catalogue lists clips so I'm guessing these are for a full floating wrist pin.
I will go full floating as it will make it easier for me whilst I'm working out what needs to be done machining wise.
The first piston will be the lab rat and pattern for the rest so fitting and removing a few times is a certainty.
I actually own a GM253 that I worked in a 1979 Holden HZ Kingswood. "A body size car"
The crank dimensions are very similar to the LA but 253 has a shorter stroke I hope the clearance for piston to weights will be fine. If not I will trim the piston skirts.

I'm at work but I will get a comparison of the GM to LA dimensions, to post.

Where I said the piston is 60thou below the deck that is the original piston on the original crank.
Just putting the rod and piston on the new crank will push the piston up to 40th below the deck.
I would be ok with that as a small quench area to be machined is no drama and the rest going into the chamber compensates for the big cc's
Because I need to rebore going straight to 40over gives me 302. 60over would give me just about a perfect 5litre. (305)
I'm happy to post pictures but it's a bit early for that.
But when I'm matching pistons and or doing the heads I will take photos. Don't hold your breath I have a lot on and this is a low priority job.
I really was just trying to awaken guys to a light at the end of the 273 tunnel.
There is another company that will cast pistons here but at over $100 each I rather use racing pistons. The manufacturer does say these racing pistons are just fine for road use.

 

[toolmanmike]

This is going to be a interesting thread.

 

[nm9stheham]

513 grams for the unmachined 9115 pistons and 95 grams for the pin = 608 grams total. That compares dang light versus the standard piston+pin weight of around 750 grams. Drop another maybe 120 grams with SCAT rods.... and probably 10 grams or so off the rings.

Bobweight then becomes around 1791 grams before machining anything on the piston head....nice. (Stock 273 bobweight is around 2147 grams.)

 

[65Val]

This os going to be a interesting thread.

X2....subscribed!

 

[273]

Here's some ideas I had when designing my 273 that never happened. I wanted a street car plus able to do time attack. So I wanted around 400 hp with a nice flat powerband.

The reason I was gonna use the Ford valves is you get a decent aftermarket selection. I wanted the under cut ones to gain a little extra flow plus there smaller stems in the first place. And want valves big enough so the use a new part of the seat but not to much bigger than stock since its pretty tight in there even with notches. I also want to use beehive springs with tool steel retainers if I could get away way with it. I was trying to get away with the least spring pressure for the rpms I needed 7000-7500 But the car was also to be a street engine.

As for heads I used onewildcrazyguys@ flow numbers. I was gonna go custom cam. But found comps 268 solid roller work the best on my dyno program, probably the custom cam would be close to those specs. It had a pretty good powerband it peak hp around 420 @ 6700 rpm and 350 @ 5500 for torque. And started around 2500 rpm and didnt fall off much hp wise until 7500+ rpm. I know those are just desk top dyno but even if it came within 10% of those numbers I would of been happy.

As for your engine since your not gonna rpm so high you don't need solid but I'd still go with hydraulic roller. I think the trick to keeping a decent bottom end is to let the heads do most of the work with the least overlap you can get away with. So with roller you can get the lift and even 1.6 rockers might be a good idea to keep duraration down. And if you go with Rhoads roller lifters you can squeeze out a few more degrees may around 225-235 with or 215-225 without.

If not willing to port out I'd just notch and go with 1.94 magnum or EQ heads. Stock head will strangle the power unless your happy with under 300 hp. I'd be shooting for 350+ with your engine.

 

[Assiemick]

Because im running a stroker crank the 6.123 rods will work in my project.
Even thou the Holden 253racing pistons are 2.92mm lower than the Mopar.
For those not going to stroke thier motor maybe rods with around the length of 6.250 as suggested by Rumblefish360. For the racing piston to achive the same position as the LA the rod would need a length of 6.238. Im still looking i may even go for longer rods myself.

Here is a bit of a comparison chart between the GM253 and LA273

Spec. LA273. GM253 Difference
Bore 3.625 3.625 0
Stroke. 3.31/my app3.58*. 3.062 -0.248
Comp ht. 1.80 1.685race piston -0.115 piston to be at LA HT rod=6.238
Piston wt 530-569gm. 513-550gm =
Piston lgth stock 3.19 3.050 -0.140
Rod lgth 6.123 5.621 -0.502
Pin dia. Full float 0.9842 Pressed 0.9272. -0.057
Pin lgth. 2.810-2.826 2.705-2.735. -0.091
Skirt below
Centre line of pin. 1.39. 1.365. -0.025

 

[Assiemick]

Sorry that didnt turn out how i typed it but i think if you take the time you can make it out.
The first figures on each line is LA the last is the difference maybe write it out on paper.

 

[rumblefish360]

LOL, longer the rod the better ... or so she said....

 

[Assiemick]

Here's some ideas I had when designing my 273 that never happened. I wanted a street car plus able to do time attack. So I wanted around 400 hp with a nice flat powerband.

The reason I was gonna use the Ford valves is you get a decent aftermarket selection. I wanted the under cut ones to gain a little extra flow plus there smaller stems in the first place. And want valves big enough so the use a new part of the seat but not to much bigger than stock since its pretty tight in there even with notches. I also want to use beehive springs with tool steel retainers if I could get away way with it. I was trying to get away with the least spring pressure for the rpms I needed 7000-7500 But the car was also to be a street engine.

As for heads I used onewildcrazyguys@ flow numbers. I was gonna go custom cam. But found comps 268 solid roller work the best on my dyno program, probably the custom cam would be close to those specs. It had a pretty good powerband it peak hp around 420 @ 6700 rpm and 350 @ 5500 for torque. And started around 2500 rpm and didnt fall off much hp wise until 7500+ rpm. I know those are just desk top dyno but even if it came within 10% of those numbers I would of been happy.

As for your engine since your not gonna rpm so high you don't need solid but I'd still go with hydraulic roller. I think the trick to keeping a decent bottom end is to let the heads do most of the work with the least overlap you can get away with. So with roller you can get the lift and even 1.6 rockers might be a good idea to keep duraration down. And if you go with Rhoads roller lifters you can squeeze out a few more degrees may around 225-235 with or 215-225 without.

If not willing to port out I'd just notch and go with 1.94 magnum or EQ heads. Stock head will strangle the power unless your happy with under 300 hp. I'd be shooting for 350+ with your engine.

I was looking at the alloy heads around at the moment.
They all seem to come with 2.02 intake
I would be prepared to machine custom seats for the 1.8 intake.
My intention is to port out as far as i can go.
Remembering its a small engine ports and valves dont have to be as big as on a 360 to get the result.

 

[Assiemick]

I have a pair of bonnet scoops off a Dart i think. Be a bit of a **** stir for ford dudes seeing 302 badges on those scoops

 

[Assiemick]

Ive been doing a bit more number crunching.
With the 3.58 crank and 6.123 rod holding the speedway racing piston.
The piston will actually be 0.040th or 1mm below the deck.
To gain a zero deck its just a .040 thou cut.
this will give me a 9:1 comp with the 68cc heads i have,
63cc heads will give me 9.8

 

[408 swinger]

Could offset grind rod journals on crank to gain .020 more stroke when using 6.125 Chevy rod. Have to turn journals down anyway to use that rod, just as well gain some stroke doing it.

 

[Assiemick]

Could offset grind rod journals on crank to gain .020 more stroke when using 6.125 Chevy rod. Have to turn journals down anyway to use that rod, just as well gain some stroke doing it.

Yeah your right ive been on the scat web sight.
They do a 6.200 and a 6.250. For Chev. The big end are a bit wider but the BE pin is .024 smaller.
I could get a push up centre of pin to top of 6.220- 6.270. I think thats a good idea.
6.238 is what i needed to get the race piston to the position of the LA piston rod combination.
Its late for me but i will do a bit more number crunching to see how far the pistons push out.
Very possible Chev rods are cheaper as well.

 

[408 swinger]

That's right. Forgot about the width, but, then again "while you're in there". Lol. Doing this on a 360 right now. Also doing a 318 with the scat 360 stroke/ 318 main crank. The 1.74 compression distance piston stuck out .037. With the Magnum head gasket, piston to head clearance was a little tight. Guy got scared and milled a few thousand off the top.

 

[Assiemick]

That's right. Forgot about the width, but, then again "while you're in there". Lol. Doing this on a 360 right now. Also doing a 318 with the scat 360 stroke/ 318 main crank. The 1.74 compression distance piston stuck out .037. With the Magnum head gasket, piston to head clearance was a little tight. Guy got scared and milled a few thousand off the top.

Best to keep 40th on the quench area so zero deck 40th head gasket perfect.
On my heads only a real small quench area.
Considering removing it all together, create a kind of hemi chamber, push the piston up into the chamber to get the 10's. Those race pistons are 12mm thick on the face some heavy machining is possible. I'm considering a very shallow taper from the edge to the crown following the shape of the chamber,

the valve pocket can go right in 8mm. That leaves 3/16" under the pocket. Just need to make sure the top ring is about 1/4 below the deck.

 

[Assiemick]

Upon further thought I will leave the quench in the chambers untouched.
I will be removing sharp edges and polishing the chambers, this will only make the chambers bigger but I will get the comp back with the piston. One valve has rust hoping its just a case of water got in whilst sitting around, cracked valve seat ? I will get the heads checked. Exhaust valve seats will be replaced anyway.
I have put up a few photos of the chambers.

 

[273]

Have you checked out onewildcrazyguy@ how to port 318 (273) heads he got around 205 cfm with 1.75 valves. Which should get you at least 1.7 hp per cfm with street cam and cr (220/9:1) and around 1.15 lbs-ft per cid. The smaller bore might lower the cfm somewhat but 1.7 is conservative.

 

[Assiemick]

Than

Have you checked out onewildcrazyguy@ how to port 318 (273) heads he got around 205 cfm with 1.75 valves. Which should get you at least 1.7 hp per cfm with street cam and cr (220/9:1) and around 1.15 lbs-ft per cid. The smaller bore might lower the cfm somewhat but 1.7 is conservative.

thanks for that address.
I just took a photo and width of the two valves. I know the bores to head won't all perfectly line up but assuming the valves and bores are all central, the best will be an average clearance to bore.
The photo where the readout is white should have been the standard bore of 3.625.
I will work out the existing clearance then using the over bore and the larger valves of 1.88 and 1.6 this will give me an idea I'm open to overbore 60th if that gives me clearance. Then I will truely have a 5litre or 305ci.