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Higher Compression L28 Shortblock Build Questions


jknc90

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I've got an L24 in my 240 now. The previous owner did a lot of head work, but afaik didn't do anything to the shortblock. My plan is to build a somewhat inexpensive short block to replace the L24 block. After playing around with the engine calculator, I think I've decided on using an F54 (flat-top pistons) with the rods from my spare L24. According to th calculator I'd have a comp ratio of 11.455:1 with my E31 head and a 3mm headgasket (due to the -2.75 deck clearance)

 

The only big money item I would need to buy is the F54 shortblock. How much of a HP boost could I expect from this as opposed to my current 2.4l?

 

Thanks a lot everybody!

 

PS: Sorry if some of this doesn't make sense, I'm new to major internal engine work

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unless your wanting to run race fuel or a really high octane gas then id stay away from anything over 10:1 comp...most likely that e31 you have still has the smaller valves(not to mention the brass seats and valve sleeves which were made for leaded gas) which will just hurt proformance on a l28...id suggest going with any l28 block with flattops and a n42 head which already has the larger valves and a stock headgasket...which should give you somewhere around 10:1...

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So would that be too high of compression for 93 octane? That's what I was afraid of.

 

EDIT: And the head has already had the work done to use unleaded, I'm not sure about the valves though. The PO of the parts car I got the engine out of did all the head work.

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Find ANY L28 short block, rebuild it using a set of ITM flat top pistons. Use the L28 rods and crank. Use your E31 head and a stock head gasket for a 10.2:1 CR. This should be good for a 40-50HP gain. I'd say that setup would make 180-200WHP with the proper tune.

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unless your wanting to run race fuel or a really high octane gas then id stay away from anything over 10:1 comp...most likely that e31 you have still has the smaller valves(not to mention the brass seats and valve sleeves which were made for leaded gas) which will just hurt proformance on a l28...id suggest going with any l28 block with flattops and a n42 head which already has the larger valves and a stock headgasket...which should give you somewhere around 10:1...

 

Agreed

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Find ANY L28 short block, rebuild it using a set of ITM flat top pistons. Use the L28 rods and crank. Use your E31 head and a stock head gasket for a 10.2:1 CR. This should be good for a 40-50HP gain. I'd say that setup would make 180-200WHP with the proper tune.

 

And I'd be able to use pump gas for this setup right? And just to clarify, the ~11.5 comp ratio would be too high for pump gas?

 

Thanks for all the input everyone.

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I've got an L24 in my 240 now. The previous owner did a lot of head work, but afaik didn't do anything to the shortblock. My plan is to build a somewhat inexpensive short block to replace the L24 block. After playing around with the engine calculator, I think I've decided on using an F54 (flat-top pistons) with the rods from my spare L24. According to th calculator I'd have a comp ratio of 11.455:1 with my E31 head and a 3mm headgasket (due to the -2.75 deck clearance)

I think what you're missing here is pistons. You'd need custom pistons with the pin height higher so that the longer rods didn't put the pistons 3mm out of the deck. What you're talking about is a good idea, the bigger displacement with the better rod/stroke ratio, but it's not quite as cheap and easy that. I'd just go with a standard L28 bottom end.

 

As to the compression ratio being too high, I agree that it is. If you bought an engine with flat top pistons it would have a P79, which could be shaved .080 (relatively cheap machining) and then you could shim the cam towers, run N series valves in it and you have a nice little engine.

 

http://datsunzgarage.com/

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And I'd be able to use pump gas for this setup right? And just to clarify, the ~11.5 comp ratio would be too high for pump gas?

 

Thanks for all the input everyone.

 

11.5:1 is too high for street use. We here in the east have 93 octane, so 10:1 is streetable for sure.

 

Pete

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Jon, I see what you're saying. I think I would be better off with the stock, flat-top L28 bottom end. As for the head though, my E31 has already been shaved (not sure how much though), its been ported and polished, and has a crane cams camshaft (I can find the specs on it if it would help). So I'm really just looking to changed out my P30 bottom end.

 

Pete, thanks for clearing that up. I don't want to have to run race fuel as this car will see more street time then track time. And we've got the 93 octane here in NC too.

 

Thanks everybody

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You could run dished pistons. I did that for a while with my E31. The compression ratio ends up being pretty low, but it will still work. The other option is maybe sell the head to put something else together...

 

My low comp L28/E31, cam, Mikunis, exhaust, etc.:

http://videos.streetfire.net/video/2000-autox-indisde-and-outside-I-think-my_8051.htm

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I don't mean to steal the thread, but I have a similar question and I've been searching and haven't found the answer. I have a '72 240Z and a friend will sell me his spare l28. I don't know if I want to get involved swapping to fuel injection and I have a 240Z exhaust manifold that won't bolt onto the L28 head (if I'm correct on that). Is it possible/worth it to just swap the block from the l28 to my l24 just for the added displacement? Or is it going to be more work than it's worth?

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So is there a difference in the block between the L28 with the N42 and N47? I've been reading this thread http://forums.hybridz.org/showthread.php?t=129066 but I don't see the information I'm looking for. Do you know where can I find more info on the swap? I think I have an E88 head in my '72 right? I get the impression that it flows better than the N42. Anything else I should be aware of if attempting this. Does the L82 block have the place to put my mechanical fuel pump and everything? I really did look. Mostly when I search I see a lot of "That has already been answered. Use the search". I appreciate any help you can give me. Or just point me in the right direction so I can find what I need in other posts.

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I realized my last post was kinda hard to read. Here's a second shot at posting the same thing.

 

So is there a difference in the block between the L28 with the N42 and N47? I think I have an E88 head in my '72 right? I get the impression that it flows better than the N42.

 

I've been reading this thread http://forums.hybridz.org/showthread.php?t=129066 but I don't see the information I'm looking for.

 

Do you know where can I find more info on the swap? Anything else I should be aware of if attempting this. Does the L82 block have the place to put my mechanical fuel pump and everything?

 

I really did look. Mostly when I search I see a lot of "That has already been answered. Use the search". I appreciate any help you can give me. Or just point me in the right direction so I can find what I need in other posts.

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I realized my last post was kinda hard to read. Here's a second shot at posting the same thing.

 

So is there a difference in the block between the L28 with the N42 and N47? I think I have an E88 head in my '72 right? I get the impression that it flows better than the N42.

 

I've been reading this thread http://forums.hybridz.org/showthread.php?t=129066 but I don't see the information I'm looking for.

 

Do you know where can I find more info on the swap? Anything else I should be aware of if attempting this. Does the L82 block have the place to put my mechanical fuel pump and everything?

 

I really did look. Mostly when I search I see a lot of "That has already been answered. Use the search". I appreciate any help you can give me. Or just point me in the right direction so I can find what I need in other posts.

 

 

there is only 2 types of l28 block..the n42 block which was found on 75-78 280z's and ocasionally found in the early 280zx's...the other block is a f54 block which came on 79-83 280zx's...about the only difference in the 2 is that the f54 has semi sieced(sorry) water passages...its been up in the air for years which one is better (i think everyone has there own opinion) but if your not going with a stroker motor or going to bore it over...then either block will do...the only l28 that came with flattop pistons is the f54/p79 combo...

 

now as for the head..unless you want to drop a lot of money on head work...then i would get a stock l28 and go from there...they were designed for that block with the bigger valves and you can just bolt it on and go...and yes all the l28 heads have the whole to mount your fuel pump...

 

the easiest way to go about doing a high compression head...which has already been stated is to get a f54/p79 motor and shave the head and shim the cam towers...although the p79 is round port and you would have to get a header or the original exhuast manifold with the f54/p79 because the l24 is a square port head...

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