My inline chevy 292 engine is almost done being machined at the local shop. They have the short block put together for me and they are currently machining the cylinder head. I called them yesterday to get an update on the progress and I asked the guy what kind of compression ratio it would have and he got back to me later that day after calculating it and said it was about 7.5 to 1. The stock compression ratio is supposed to be 8:1 according to what I found online and the engine would make about 165hp and 280 lb tq. I was initially hoping to end up with a ratio higher than stock like 8.5 or maybe 9. I talked to him again today and he said that if they shave off 25 thousandths from the head and if I buy a MLS gasket that is thinner than the stock fel pro type then I would be able to get it back to 8:1. Also, this is with a new cam installed that is longer duration and higher lift than stock. I was wanting to get right at 200 hp out of the motor and now I don't think it will happen since the ratio did not get bumped up high enough. I suppose it will have a nice lopey idle at least! This is the first engine that I have had the privelege to have rebuilt and so I'm asking how concerned should I be about the lower than stock ratio with the bigger cam? I asked him about other pistons and he told me he only found really expensive stock type pistons that were offered in the propane powered 292's. they have a shallower dish in them. It would cost another $400 for those alone and then the labor for them to disassemble and reassemble the rotating assembly again....
Have them dissasemble the motor and set it up at 0 deck (surface the block) then use the felpro gasket and mill the head if necessary - just check your pushrod length/valvetrain geometry if you take a lot off...you need the higher compression b/c your new cam has more overlap - that thing will be a dog with the low compression...
I can't recall exactly, but I think I remember that stock that had a steel shim head gasket, which was quite thin. All modern composite head gaskets are quite a bit thicker, resulting in lower compression ratio, if the head is not milled to accommodate. Have them mill the head, and surface he block. They should be able to accommodate your wishes.
.025 off the head doesn't seem like much, I did .050 off my 300 ford [I know apple and orange] and didn't have any problems..Be great if you can get it to 9:1..
7.5 to one is going to need a supercharger and prime for accepting one. Ask him what the quench distance is right now to get 7.5 :1 compression Right now he's wanting to get .024 ish out of the gasket And then 0.025 off the heads. Ill bet quench is at 0.100 or so. Should be around .040
that's what I was afraid of.... I have already racked up a bill around $2500 and to have the take the block back apart, machine the deck, and reassemble the block will be another couple hundred at least. I am already $500 over what I wanted to spend originally, but $300 of that is because I decided to have them go ahead assemble the short block for me since they have all the tool to check bearing clearances and I don't. I know I could use plastigage and that was my original intention but I decided not to. Well they have already done the machine work and assembled the short block. They just finished up the valve job today. They haven't assembled the head yet because of the talk we had yesterday and today. As of right now, I told him to mill another .025 off the head and to order the MLS gasket ($170) and he says it will be .026 thick instead of .040 like the fel pro and that will give me the 8:1 ratio at this point. If I want more then I have to pay for the disassembly of the block (1hr labor), further deck machining ($75), and reassembly (2 hr labor). sooo probably about 300 bucks more! If i remember correctly, he said he could do .050 total but he didn't recommend going more than that. The engine has never been rebuilt before and he told me that they only just took the minimum off the surfaces to make them straight. I just got off the phone with him and he said the max he would machine of the head is .050 and the block is about the same. He said he would crunch the number some more after they shave the .025 off the head and recalculate the chamber sizes and we'll go from there.
He said it's a comp cam grind and they don't show the spec card on their website but he said it has about .480 lift and 212/212 duration (at .050 i assume). I believe the stock cam lift was in the high 300s or low 400s but I couldn't verify that.
I will ask him about that tomorrow since they have closed now. I told him I would like to raise it higher than stock from the beginning and that was about 2 months ago when I dropped it off but I did not actually tell him a specific number like "9:1" because the block and head were really nasty and they just wanted to get it cleaned and magnafluxed first. I was really just hoping the block would be good enough to use at that point. The block did have a small crack in it which I had repaired.
292 advertised compression is 8-1. In reality it's always lower on almost all stock engines,so an actual 7.5 is accurate. With the hotter cam bleeding off some cylinder pressure at low rpm may not be the best way to go. What your shop should have done......done a mock up before assembly to check piston to deck height then called you at that time so you consider the options.
If you asked for better than stock, and now it is below stock, then you did not get what you asked for. Their job is to deliver what you specify. They did not. Correcting this should be at THEIR expense. It is not your job to manage their shop, and make sure that they do what they are asked to do. Remind them politely that you did ask for better than stock, and now it is less. They at-least owe you the portion of the work to get it back to equal to, or better than stock. Ask them for that, and offer to pay the difference to get it to 9.0-9.5:1.
I would be most concerned about the squish or piston to head clearance. If it's much over .050 assembled I would consider biting the bullet and correcting it. jm2c rh
Yes.. I do know on a 230 Chevy 6's the quench is quite large,about .060 with stock pistons and a steel shim head gasket.This is probably true with the 292 truck engines also. I checked and the maximum deck milling recommended is .030
Ok that squish or quench dimension is going to become very important. There's a stock speced dimension for this. " read must have" and it doesn't sound like its even close. And of he's looking to close it by 0.049 between the gasket and head milling. If the block was bored and fitted with oversize pistons - that increases "swept volume" and increasing swept volume increases the compression ratio. In other words all things equal your compression should have gone up on that alone. There's a set dimension that the piston should be down the hole at TDC. From what you say He's missed that mark.
In a /6 I often take 070 from the block and also the head. In a 250 build I beleive I have taken 060 from the head. (same head basically as a 292. ) A rebuild shop is often not familiar with hotrod engine work and what can or cant be done. that doesnt reflect on their skill. It just isnt their cup of tea so to speak. If it were mine or were here I would cc the head against stock specs (using a pair of 50cc or 50ml cattle syringes if necesary of nothing else is available.)Then at the very least make sure itwas as it was supposed to be. Many heads are cast way thick in production and are not machined anywhere near spec. A good example isthe 440 Mopar headss whichare supposed tobe 79.5 or 80cc depending on year.Most are90 ccs at least and they require sometimes removeal of 090 to get them in spec. Folks go beserk untill you explain that the head was not machined enough in production and while you are removing that much you are only taking it to its designed size. it is not some super duper exotic big cut. Same here . Find out where it is in relation to its stock specs. make sure it is at least there. A felpro or compsition gasket is typically .043 while a stock steel from back in the day are .020 more or less. (.019 to .022) so I always stole the difference at least to maintain at least original compression. Any head from back then can stand .030 or.040 as that was often done. I have stolen a lot more too without any complaint from the engine. Don
Turbo-charger time? That would boost (no pun intended) the power for sure. OR, find a head from a 194 six cylinder, casting number 3864883, with a combustion chamber size of 60-62 CC's. That alone will put you in the 9.0-9.5 to one zone. You could also mill it .060 to get you in the 10:1 range. The 250 six I built 40 some years ago was done this way, along with Clifford headers and intake, Holley 2 barrel carb, Mallory dual point with a Judson "magneto" coil (who knows why they called it a magneto because it was't), and a BIG cam/kit right out of J.C. Whitney. Pretty potent little six. Use an HEI distributor instead of a points unit; the stock points plate is "held" in position by a flimsy spring, and moves it around quite a bit, causing fluctuations in timing. I think you're being lead around too much by your machinist as far as the price of pistons go, just check the prices on that auction site we all know and love. IF you don't want to start over with a different head, then deck the block, and mill the head, like has already been mentioned. Butch/56sedandelivery.
Yeah they definitely did not stop and do a mock up and tell me options on the block, just assembled it and probably assumed that I would be okay with it just having the minimum shaved off the deck. I will try to remind him of our convesation we had two months ago. We originally spoke about how I would like a pretty stock type rebuild but I wanted a higher compression ratio so I could have it run on mid grade gasoline and my thinking was that it would give me a bump in HP. I guess he never wrote down any notes about what we talked about. All of our interaction has been over the phone from day one because I work later than their shop hours BUT I do have off tomorrow and I will go up there and look at this in person. I believe he told me that the piston to block deck is .050
I asked him about how much we could cut and he said something like a maximum of .050 on the head and .050 on the block but I will confirm that with him at some point this morning.
I do acually have another engine. it's a 250 and it does have a 194 head on it. I will have a price for them to redo the block and mill the head some more when I talk to him later this morning and hopefully it will not be too painful to accept and the problem will be solved. The turbo option is definitely something I have thought of but I believe I would want to run naturally aspirated for now and do the turbo in a couple years or so simply because of my budget and i'm trying to accomplish other main components of the build first to get it on the road. It would be cool!
inline sixes can be done with pretty simple plumbing http://www.homemadeturbo.com/member...evy-inline-6-141-picture-feb-2012-166-615.jpg
If you get the block to zero and the gasket to .041 and .050 off the head you should be good to go. We used to do .100 off the head but we put posts through the water jacket to support the deck, not a good idea for a street combo. Remember to check the pushrod length, rocker tips on the valve. Hoop
That would be cool but it won't fit under my hood or clear the firewall. Even if I got it to 8.5 or 9 compression, would it still be pretty safe to boost and how much horesepower do you think I would gain? Do I not have to have an air to air intercooler? I would prefer not to use one if that's possible. The machinist told me that I would be able to use stock length pushrods and reuse the rockers. Do you think they might actually need to be longer or shorter potentially with the cam change?
The cam change and the milling could balance out. Smaller base circle needs longer pushrod, milling needs shorter. Just be sure when you are on the base circle that you are on one side of the center and as the valve opens the contact crosses the center. Stamped steel rockers are forgiving, just be sure you are in the ball park. http://www.aera.org/ep/downloads/ep10/EP04-2010_20-30.pdf
I just got off the phone with him. The piston to block deck is .050 these are factory dished pistons. The MLS gasket is .030 A fel pro gasket is .040 or thicker They will start the .025 head cut today and remeasure the chambers. He said that if they can get the chambers to 64cc and use the MLS gasket and do nothing to the block then the final static compression ratio would be 8.13:1 The additional block work would cost another $325 and that would be to disassemble, cut the block deck .050, and reassemble and then my ratio would be around 8.5:1 according to him. It doesn't seem really worth it to me at this point. I will just have to be happy with it and I also spoke to him about the turbo option later and he said that it wouldn't be any problem to turbo this motor if i stop right here with the comp ratio. I could use around 5-7 psi of boost safely and that would be pretty fun. The MLS gasket would already be on there to protect from a blown gasket and I wouldn't have to change anything else on the motor as far as I can tell.
It's not the compression as much as having a minimum of .080 -.090 squish/quench. We have learned as an industry the importance of closing that up under .050. This helps prevent detonation and increases turbulence. It's not my money but I really think you will be glad to get that clearance down to .045 or so. I am assuming your pistons have the standard 292 style squish pads; Hoop
Having just wrapped up a hopped up 292 build, I thought I'd add my two cents here...maybe I'll get some change back? 1. Typical compression ratios on stock 292's is around 7.8, so your 7.5 doesn't surprise me. But, given the deep dish and very heavy pistons, using these in a performance application doesn't make a ton of sense to me anyway. For my build, I'm using a smaller dish, forged piston with low friction rings...lighter, stronger pistons, less friction, and a compression ratio aiming right at 9.5:1 for today's pump gas. 2. It's pretty routine to take .030 off the head and not have a problem at all. Given your stock pistons, I think you're still in good shape, but with a bigger cam, just make sure the shop checks valve to piston clearance - .100 minimum! 3. If you haven't, I'd make sure to go check out inliners.org, grab Leo Santucci's book "Chevrolet Inline six power manual", and look up Tom Lowe (my builder, and very knowledgable about Chevy 6 builds). 4. From my personal experience, sometimes it's best to take a step back and relax for a moment to really think things through. I know you have a bunch of cash in the machine work already, and its a pain in the ass to not move forward sometimes, but it would also suck to go through all the work and not have the motor you want. Been there, done that. Hope some of this is useful. If you want to know more details on my build, I'm happy to let you know. Posted using the Full Custom H.A.M.B. App!
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