Pistons/Compression Ratio for 87 convertible
Thread Starter
Racer
Joined: Aug 2002
Posts: 386
Likes: 0
From: Lancaster Lancashire
Pistons/Compression Ratio for 87 convertible
I am in the process of removing the engine in my 87 to overhaul it. I will be reinstalling the original 1st design alluminium (sorry about spelling, I'm British) heads which I am also porting, together with a Big Mouth manifold, Runners, and a Tpis ZZ-9 cam. The dilema is what pistons to use ? My first choice is a KB-Silvolite. flat top (7cc v/v reliefs), which should give approx. 10.6:1 comp ratio with my heads and a piston to deck of approx.0.040". Tpis recommend 10.5-10.8:1 with the ZZ-9, which puts this right in the "recommended" ball park.
However I have a worry that this is a touch high for pump gas. (nearly wrote petrol then). The second choice KB piston has a 12cc dish which would give me just under 10:1.
Does anyone out there have a similar combination or any experiences to share ? The last thing I want is an engine that knocks and pings.
UK pump gas comes in two flavours 95 & 98 octane ratings. (we use a slightly different method for measuring octane, I think it roughly translates to 93 & 95 in the US) its just unfortunate that the cheap one is also equivalent to $5US a gallon. :cry :D Any advice appreciated. [IMG][/IMG]
However I have a worry that this is a touch high for pump gas. (nearly wrote petrol then). The second choice KB piston has a 12cc dish which would give me just under 10:1.
Does anyone out there have a similar combination or any experiences to share ? The last thing I want is an engine that knocks and pings.
UK pump gas comes in two flavours 95 & 98 octane ratings. (we use a slightly different method for measuring octane, I think it roughly translates to 93 & 95 in the US) its just unfortunate that the cheap one is also equivalent to $5US a gallon. :cry :D Any advice appreciated. [IMG][/IMG]
Drifting
Joined: Dec 2002
Posts: 1,861
Likes: 4
From: Houston TX
Re: Pistons/Compression Ratio for 87 convertible (Davemc1963-87conv)
The ZZ9 seems like a big cam for your setup (going from memory on the cam specs), but the long ad short of it is that I'm running 10.4:1 on US 93 Octane, and if my timing is advanced too much by more than about 1 degree, I ping like crazy. Personally, I'd run the lower compression, or run the higher compression pistons and unshroud the valves on the stock heads. You should be able to remove enough material to bring the c/r back to something reasonable.
Safety Car
Joined: Dec 1999
Posts: 4,691
Likes: 1
From: Framingham MA
Re: Pistons/Compression Ratio for 87 convertible (Davemc1963-87conv)
I honestly don't think 10.6:1 is too much compression (as long as the engine is tuned properly) and I would use a zz9 cam for your setup as that is what TPIS pretty design it for (ported heads, Large Tube Runner and aftermarket TPI base) I would just take note that the advertised 58cc heads are closer 55-56cc. LPE use 11.0:1 in all their motors, and GM is using compersiion ratios of 10.5+ in the recent Corvette
Melting Slicks
Joined: Apr 2002
Posts: 3,019
Likes: 0
From: down that one road next to that big tree
Re: Pistons/Compression Ratio for 87 convertible (dgoodhue)
the ls1 heads also have much better swirl. however the zz9 might be big enough to kill some of the low end velocity that leads to detonation. also 10-6:1 isn't that bad. there was a fellow on ebay selling cast federal mogual pistons with moly rings for $85 a set. you might shop around before buying, some places sell there overstock ultra cheap.
Pro
Joined: Apr 2002
Posts: 629
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From: Rutland Ma
Re: Pistons/Compression Ratio for 87 convertible (Davemc1963-87conv)
its just unfortunate that the cheap one is also equivalent to $5US a gallon. :cry
Thread Starter
Racer
Joined: Aug 2002
Posts: 386
Likes: 0
From: Lancaster Lancashire
Re: Pistons/Compression Ratio for 87 convertible (dgoodhue)
Yeah, I incline to agree, but often wonder whether the compression ratios are actually calculated accurately. I have the heads off and am slowly porting them (methodically) using the guidance from David Vizards books. I have a high regard for his technical ability, based on his early work in the UK. I cc'd the combustion chambers when I removed them and they came out right on 58cc. I do intend to do some valve unshrouding & modifying of the machining index pads in the chambers, but suspect that this will only add a couple of cc's. Thanks for your advice :)
Thread Starter
Racer
Joined: Aug 2002
Posts: 386
Likes: 0
From: Lancaster Lancashire
Re: Pistons/Compression Ratio for 87 convertible (gtsyellow)
Thanks for that I'll have a look on E-Bay out of interest. Although some people don't want the hassle of exporting goods.
You don't have a Federal Mogul catalog do you? another piston that I was looking at was a Sterling H631P but can't find out info such as compression height. I notice that a lot of aftermarket pistons have a shorter compression height than the KB's meaning that you have to mill the block more to get a tighter piston to head clearance. The std deck is already relatively thin and milling more than necessary only compromises this more. Thanks again. :)
You don't have a Federal Mogul catalog do you? another piston that I was looking at was a Sterling H631P but can't find out info such as compression height. I notice that a lot of aftermarket pistons have a shorter compression height than the KB's meaning that you have to mill the block more to get a tighter piston to head clearance. The std deck is already relatively thin and milling more than necessary only compromises this more. Thanks again. :)
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Thread Starter
Racer
Joined: Aug 2002
Posts: 386
Likes: 0
From: Lancaster Lancashire
Re: Pistons/Compression Ratio for 87 convertible (Chefskeez6)
Have you ever wondered why Brits and Europeans drive silly little cars with silly little engines ? Now you know. I once had a Fiat with a 650cc 2 cyl. air cooled engine, but thats another story. Diesels are where its at in the UK I'm afraid, I have one that does 45mpg please don't give me too much stick. I love American iron (and glassfiber) and have owned my corvette nearly ten years and hope fully will for another ten. By the way its more than $100 to fill her up. :cry
Melting Slicks
Joined: Apr 2002
Posts: 3,019
Likes: 0
From: down that one road next to that big tree
Re: Pistons/Compression Ratio for 87 convertible (Davemc1963-87conv)
i COMPLETELY understand why the euros drive silly little cars and motorcycles everywhere. your petrol prices are crazy !! :crazy: i don't have the federal mogual catalog but i could find the part number if you want there high end forged piston since i used them in my engine. summit racing only wanted $230 a set. if your not making over 400hp a nice hyperutectic piston would probably fit the bill really well. another ebay guy had those for $87 a set. search words, chevy pistons and chevy rings bring up good results. as far as what to expect with your heads, i decked fifteen thousands off mine but polished and unshrouded the combustion chambers, wound up with 57cc's by the time i was done. i WISH i could remember what i decked off the block but i think it was somewhere around the same number. with a .039 head gasket and 4 valve relief flat top piston i was right at 11:1 compression. which should be ideal for the octane grades you have available over there.
